extracted from wipo
description
FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. Title of the Invention
METHOD AND SYSTEM FOR INTERROGATING A BIREFRINGENT FIBER BRAGG
GRATING SENSOR, EMPLOYING HETERODYNE OPTICAL DETECTION
2. Applicant(s)
Name Nationality Address
BREMBO S.P.A. Italian
Via Brembo, 25 I-24035 Curno,
Bergamo, Italy
3. Preamble to the description
The following specification particularly describes the invention and the manner in which it is to be performed
2
DESCRIPTION
[0001]. Field of the invention
[0002]. The invention relates, in general, to a method and system for measuring physical
parameters using sensors of the birefringent Fiber Bragg Grating type.
5 [0003]. More specifically, the present invention relates to a method and a system for querying a
sensor of the birefringent Fiber Bragg Grating (FBG) type (e.g., in birefringent fiber), employing
heterodyne optical detection and integrated photonic technology.
[0004]. Background art
[0005]. Optical fiber sensors of the Fiber Bragg Grating type (FBG sensors) are becoming more
10 frequently used for measuring physical magnitudes, such as strain or temperature, by virtue of its
features of simplicity and accuracy. Such sensors are passive, meaning that they need to be illuminated
by optical radiation and the spectrum reflected or transmitted thereby must be analyzed to obtain the
measurement of the desired physical magnitude.
[0006]. In this context, the sensors of the birefringent Fiber Bragg Grating (Bi-FBG) have the
15 advantage of providing more information, compared to the standard fiber FBG sensors, because they
generate by reflection an optical signal which can be seen as the combination of two partially
independent optical signal components, having a different optical polarization, each of which is biased
in its own peculiar and predictable manner by one or more physical magnitudes detectable by the
sensor.
20 [0007]. Since it is possible to independently detect components of the spectrum reflected by the
sensor having optical polarizations which are mutually orthogonal (e.g., associated with so-called "fast
polarization axis" and "slow polarization axis"), it is then possible to detect the wavelength deviation of
each of these components from a nominal operating wavelength of the sensor.
[0008]. Therefore, a Bi-FBG sensor made of birefringent fiber makes it possible to obtain more
25 information than a conventional fiber FBG sensor for measuring physical parameters or physical
magnitudes to be detected.
[0009]. On the other hand, the birefringent fiber FBG sensors require much more complex, and
therefore more expensive and bulkier, interrogating/querying and processing methods and systems than
standard fiber FBG sensors.
30 [0010]. Indeed, the interrogation/querying must occur by optical excitation on at least two different
channels, i.e., one channel corresponding to the fast axis polarization and one channel corresponding to
the slow axis polarization (because there are at least two reflected optical signals, with different
polarization, each acting around its own frequency/wavelength), which is usually carried out, in known
solutions, by tunable laser sources.
35 [0011]. Furthermore, the entire receiving, filtering, and electro-optical processing part of the optical
signals reflected by the birefringent Bi-FBG sensor is at least duplicated, compared to non-birefringent
3
FBG sensors.
[0012]. The problems of complexity, cost, and bulk, which already afflict the standard fiber FBG
sensor interrogation/querying systems, are felt even more critically in birefringent fiber Bi-FBG sensor
interrogation/querying systems and are at least partially unsolved to date.
5 [0013]. In light of the above, the need is strongly felt for systems and methods for interrogating
birefringent fiber Bi-FBG sensors which can mitigate the above technical drawbacks, and meet the
following criteria: (i) compactness and simplicity in structure and use; (ii) effectiveness in
performance.
[0014]. Such needs are felt especially in many technical fields in which birefringent fiber Bi-FBG
10 sensors, from the point of view of detection capabilities, potentially offer significant advantages, which
in practice can be frustrated by the fact that it is difficult, if not impossible, to install the birefringent
fiber Bi-FBG sensor interrogation systems (which are, of course, essential for practical applicability)
made available in the prior art, due to complexity and bulk.
[0015]. Solution
15 [0016]. It is the object of the present invention to provide a method for interrogating a sensor of the
birefringent Fiber Bragg Grating type which can at least in part solve the drawbacks described above
with reference to the prior art and respond to the aforesaid needs particularly felt in the considered
technical sector.
[0017]. This and other objects are achieved by a method for interrogating the at least one sensor of
20 the birefringent Fiber Bragg Grating type according to the claim 1.
[0018]. Some advantageous embodiments of such method are the subject of dependent claims 2-
12.
[0019]. It is a further object of the present invention to provide a corresponding system for
interrogating at least one sensor of the birefringent Fiber Bragg Grating (Bi-FBG) type.
25 [0020]. This object is achieved by a process according to claim 16.
[0021]. Some advantageous embodiments of such a system are the subject of dependent claims
17-29.
[0022]. It is another object of the present invention to provide a method for determining at least two
physical magnitudes detectable by a sensor of the birefringent Fiber Bragg Grating (Bi-FBG) type
30 employing the aforementioned interrogation method.
[0023]. Such an object is achieved by a method according to claim 13.
[0024]. Some advantageous embodiments of such a method are the subject of dependent claims
14-15.
[0025]. It is a further object of the present invention to provide a corresponding system for
35 determining at least two physical magnitudes detectable by a sensor of the birefringent Fiber Bragg
Grating (Bi-FBG) type.
4
[0026]. This object is achieved by a system according to claim 30.
[0027]. Some advantageous embodiments of such a system are the subject of dependent claims
31-33.
[0028]. Brief description of the drawings
5 [0029]. Further features and advantages of the method and system according to the invention
will be apparent from the description below of preferred embodiments thereof, provided by way of
non-limiting explanation, with reference to the accompanying drawings, in which:
- figure 1-6 illustrate respective different embodiments of a system for interrogating a
sensor of the birefringent Fiber Bragg Grating (Bi-FBG) type by means of functional block charts;
10 - figures 7 and 8 illustrate, in a simplified manner, some known optical heterodyne
detection/reception techniques.
[0030]. Detailed description of the invention
[0031]. With reference to figures 1-6, it is now described a method for interrogating at least one
sensor of the birefringent Fiber Bragg Grating (FBG) type (hereafter also sometimes named
15 birefringent Bi-FBG sensor for the sake of brevity), e.g., a Bi-FBG type sensor obtained in a
birefringent fiber or in an optical fiber zone made birefringent.
[0032]. Such a method firstly comprises the steps of illuminating the aforesaid at least one sensor
of the birefringent Fiber Bragg Grating Bi-FBG type with a broadband optical excitation radiation OA,
and conveying the reflected optical spectrum OR, reflected by the at least one sensor of the birefringent
20 Fiber Bragg Grating Bi-FBG type in an integrated photonic PIC detection circuit (or device).
[0033]. The method then includes separating a first component of the aforesaid reflected optical
spectrum OR1, characterized by a first optical polarization generated by the birefringence and centered
around a first frequency ω1, from a second component of the aforesaid reflected optical spectrum OR2,
characterized by a second optical polarization generated by the birefringence and centered around a
25 second frequency ω2, by means of a polarization optical beam splitter comprised in the detection
photonic integrated circuit PIC.
[0034]. The method further comprises providing the aforesaid broadband optical excitation OA to
the detection photonic integrated circuit PIC, and obtaining at least two narrowband optical signals
(LO1, LO2), on the basis of at least one narrowband optical filtering of the aforesaid broadband optical
30 excitation radiation OA carried out in the detection photonic integrated circuit PIC.
[0035]. The aforesaid at least two narrowband optical signals (LO1, LO2) comprise a first local
oscillator optical signal LO1, centered around a first local oscillator frequency ωLO1, and a second local
oscillator optical signal LO2, centered around a second local oscillator frequency ωLO2.
[0036]. The method then includes providing the aforesaid first component of the reflected optical
35 spectrum OR1 and the aforesaid first local oscillator optical signal LO1 to first optical heterodyne
detection means, integrated in the detection photonic integrated circuit PIC, to carry out a heterodyne
5
detection and obtain a first electrical signal E1 at a first intermediate frequency ωIFs, equal to the
difference between the first local oscillator frequency ωLO1 and said first frequency ω1 of the first
component of the reflected optical spectrum OR1.
[0037]. Similarly, the method further includes providing the aforesaid second component of the
5 reflected optical spectrum OR2 and the aforesaid second local oscillator optical signal LO2 to second
optical heterodyne detection means, also integrated into said detection photonic integrated circuit PIC,
to carry out a heterodyne detection and obtain a second electrical signal E2 at a second intermediate
frequency ωIFf, equal to the difference between the second local oscillator frequency ωLO2 and the
aforesaid second frequency ω2 of the second component of the reflected optical spectrum OR2.
[0038]. The method finally comprises the step of determining the first intermediate frequency ωIFs 10 ,
indicative of a first wavelength shift ∆λ1 of the first component of the reflected optical spectrum OR1,
having the first polarization, with respect to a first reference wavelength λref1; and, furthermore, the step
of determining the aforesaid second intermediate frequency ωIFf, indicative of a second wavelength
shift ∆λ2 of the second component of the reflected optical spectrum OR2, having the second
15 polarization, with respect to a second reference wavelength λref2.
[0039]. The aforesaid first wavelength shift ∆λ1 and second wavelength shift ∆λ2 (of which the
determined intermediate frequencies are indicative ) are representative of at least one physical
magnitude measured by the optical fiber sensor Bi-FBG.
[0040]. According to an embodiment of the method, shown in figure 1, the aforesaid step of
20 obtaining at least two narrowband optical signals (LO1, LO2) comprises the steps of:
- narrowband filtering the aforesaid broadband optical excitation radiation OA, by means of a
narrowband band-pass optical tunable filter OTF integrated into the detection photonic integrated
circuit PIC, to obtain a narrowband optical signal centered around a local oscillator frequency ωLO
adapted to act as a local oscillator signal LO;
25 - splitting the aforesaid narrowband optical signal by means of an optical beam splitter OSPL,
configured to make two attenuated replicas of the same narrowband optical signal, received as input,
available to two output ports.
[0041]. In such a case, the first local oscillator signal LO1 and the second local oscillator signal
LO2 are the two signals, identical to each other, present at the two output ports of the optical beam
30 splitter.
[0042]. According to another embodiment of the method, shown in figure 2, the aforesaid step of
obtaining at least two narrowband optical signals (LO1, LO2) comprises the steps of:
- splitting the aforesaid broadband optical excitation radiation OA by means of an optical beam
splitter to obtain a first replica of the broadband optical excitation radiation and a second replica of the
35 broadband optical excitation radiation;
- narrowband filtering the aforesaid first replica of the broadband optical excitation radiation, by
6
means of a first narrowband band-pass optical tunable filter OTF1 integrated in the detection photonic
integrated circuit PIC to obtain the first narrowband optical signal centered around the first local
oscillator frequency ωLO1;
- narrowband filtering the aforesaid second replica of the broadband optical excitation radiation,
5 by means of a second narrowband band-pass optical tunable filter OTF2 integrated in the detection
photonic integrated circuit PIC to obtain the second narrowband optical signal centered around the
second local oscillator frequency ωLO2.
[0043]. According to an embodiment of the method, e.g. illustrated in figures 1-4, the step of
carrying out a heterodyne detection and obtaining a first electrical signal E1 comprises combining the
10 first component of the reflected optical spectrum OR1 and the first local oscillator optical signal LO1
in an optical waveguide of a first optical coupler OC1 of the first optical heterodyne detection means,
and further comprises converting the optical signal obtained at the output of the first optical coupler
into a respective first electrical signal E1, by means of a first opto-electronic receiver PD1 of the first
optical heterodyne detection means.
15 [0044]. In such an embodiment, the step of carrying out a heterodyne detection and obtaining a
second electrical signal E2 comprises combining the second component of the reflected optical
spectrum OR2 and the second local oscillator optical signal LO2 in an optical waveguide of a second
optical coupler OC2 of the second optical heterodyne detection means, and further comprises
converting the optical signal obtained at the output of the second optical coupler into a respective
20 second electrical signal E2, by means of a second opto-electronic receiver PD2 of the second optical
heterodyne detection means.
[0045]. According to an implementation option (shown in figure 3), each of the steps of carrying
out a heterodyne detection to obtain a first electrical signal E1 and a second electrical signal E2
comprises carrying out a balanced detection, using a respective 2x2 optical coupler, configured to
25 provide as output two optical signals, for each heterodyne detection, which are detected by two
respective photodiodes for balanced detection, for each heterodyne detection, wherein each of the first
electrical signal E1 and the second electrical signal E2 is obtained as a subtraction of the currents
output from the respective photodiodes.
[0046]. According to another implementation option of the method (shown in figure 4), the step of
30 carrying out the first heterodyne detection further comprises shifting, in a controlled manner, the phase
of the first local oscillator optical signal LO1 by means of a first optical phase shifter OPS1, comprised
in the photonic integrated circuit PIC, before the input in the first optical coupler OC1.
[0047]. Similarly, the step of carrying out the second heterodyne detection further comprises
shifting, in a controlled manner, the phase of the second local oscillator optical signal LO2 by means of
35 a second optical phase shifter OPS2, comprised in the photonic integrated circuit PIC, before the input
in the second optical coupler OC2.
7
[0048]. According to another implementation option of the method (shown in figure 5), the step of
carrying out a heterodyne detection comprises injecting the first component of the reflected optical
spectrum OR1 and the second component of the reflected optical spectrum OR2 into a single 2x1
optical coupler OC, configured to generate as output an optical signal at an intermediate frequency ωIF
5 representative of the difference between the frequency deviations of the first component of the
reflected optical spectrum OR1 and the second component of the reflected optical spectrum OR2.
[0049]. The heterodyne detection, which can also be referred to as coherent optical detection (or
reception) using a heterodyne technique, is a general known technique. In the following description of
the corresponding system according to the invention, further details will be provided in this regard,
10 with reference to the embodiments shown in figures 1-6.
[0050]. According to an embodiment, shown in figure 6, the method is capable of interrogating a
plurality of sensors of the birefringent Fiber Bragg Grating type (Bi-FBG1 - Bi-FBGn) in cascade,
each characterized by a respective nominal operating wavelength (λ1-λn).
[0051]. In such an embodiment, the step of conveying comprises conveying the overall reflected
15 optical spectrum ORT, reflected from the cascade of sensors of the birefringent Fiber Bragg Grating
type (Bi-FBG1 - Bi-FBGn), into a detection photonic integrated circuit PIC; and the step of separating
comprises separating a first component ORT1 and a second component of the aforesaid overall
reflected optical spectrum ORT1.
[0052]. The first component of the overall reflected optical spectrum ORT1 comprises the
20 superposition of the first components (OR11-OR1n) with first optical polarization, each centered
around a respective first frequency (ω11-ω1n).
[0053]. The second component of the overall reflected optical spectrum ORT2 comprises the
superposition of the second components (OR21-OR2n) with second optical polarization, each centered
around a respective second frequency (ω21-ω2n).
25 [0054]. In this case, the method comprises the further steps of:
- spectrally separating the first components (OR11-OR1n) from one another by means of first
frequency discrimination or demultiplexing means AWG1;
- spectrally separating the second components (OR21-OR2n) from one another by means of
second frequency discrimination or demultiplexing means AWG2;
30 - carrying out the aforesaid heterodyne detection steps on each of the first components (OR11-
OR1n) and on each of the second components (OR21-OR2n), to obtain a respective plurality of first
electrical signals E1k and second electrical signals E2k;
- carrying out the aforesaid steps of determining the first intermediate frequency ωIFs,k and the
second intermediate frequency ωIFf,k for each pair of first electrical signal E1k and second electrical
35 signal E2k corresponding to a respective sensor of the birefringent Fiber Bragg Grating type Bi-FBGk.
[0055]. According to an embodiment of the method, the aforesaid first optical polarization
8
corresponds to the polarization on a "slow polarization axis" ("slow axis" for the sake of brevity) and
the first birefringence peak frequency ω1 corresponds to the slow axis birefringence peak frequency
ωs; the aforesaid second optical polarization is orthogonal to the first optical polarization and
corresponds to the polarization on a "fast polarization axis" ("fast axis" for the sake of brevity),
5 orthogonal to the aforesaid "slow polarization axis", and the second birefringence peak frequency ω2
corresponds to the fast axis birefringence peak frequency ωf.
[0056]. According to an embodiment of the method, the aforesaid first reference wavelength λref1
and the aforesaid second reference wavelength λref2 correspond to two respective nominal operating
wavelengths of the sensor of the birefringent Fiber Bragg Grating type Bi-FBG, on the two fast and
10 slow channels, determined by means of an initial calibration.
[0057]. According to another embodiment of the method, the aforesaid first reference wavelength
λref1 and second reference wavelength λref2 coincide and correspond to a reference wavelength λi
identified by the tuning of the narrowband band-pass optical tunable filter OTF.
[0058]. According to an implementation option of the method, the aforesaid first reference
15 wavelength λref1 and second reference wavelength λref2 respectively correspond to two reference
wavelengths λi1 and λi2 identified by the tuning of the two narrowband band-pass optical tunable
filters (OTF1, OTF2).
[0059]. According to another implementation option, the aforesaid first reference wavelength λref1
and the aforesaid second reference wavelength λref2 coincide and correspond to two respective nominal
20 operating wavelengths of the sensor of the birefringent Fiber Bragg Grating Bi-FBG type, on the two
fast and slow channels.
[0060]. A method for determining at least two physical magnitudes detectable by a sensor of the
birefringent Fiber Bragg Grating type Bi-FBG is now described.
[0061]. Such a method provides performing a method for interrogating at least one sensor of the
25 birefringent Fiber Bragg Grating Bi-FBG type according to any one of the embodiments described
above and then determining at least two physical magnitudes on the basis of processing of the aforesaid
first intermediate frequency ωIFs and second intermediate frequency ωIFf.
[0062]. According to an implementation option of such a method, the two determined physical
magnitudes are longitudinal strain and transverse strain.
30 [0063]. According to another implementation option of such a method, the two determined physical
magnitudes are strain and temperature.
[0064]. In this regard, further explanations are provided below, referring to the aforesaid
embodiments concerning a method for measuring two orthogonally polarized Bragg wavelengths
reflected by Fiber Bragg Grating Bi-FBG sensors characterized by birefringence.
35 [0065]. A periodic and uniform change in the refractive index of the optical fiber "core" is the
simplest form of the Bi-FBG structure. The fundamental characteristic of the Bi-FBG sensor is the
9
presence of a resonant condition, which reflects light at a particular wavelength, named Bragg
wavelength (λB), defined by the following relationship:
λB =2neff · 
where neff is the effective refractive index of the fiber and  is the grating period, also called pitch of
5 the grating. The Bragg wavelength depends on the grating pitch () and the effective refractive index
of the fiber core (neff), and these parameters are sensitive to changes in temperature and strain. Because
of this, Bi-FBG sensors can be directly exploited as strain and temperature sensors.
[0066]. If the fiber is birefringent, the effective refractive indices experienced by light propagating
in two orthogonal polarizations are different and are typically defined denoted as neff-s and neff-f
.
10 [0067]. Thus, two orthogonally polarized spectra, reflected by the Bi-FBG sensor in birefringent
fiber, are observed with two different wavelengths, having a peak wavelength separation of = s-f
=2 (neff-s - neff-f) .
[0068]. Similar considerations can be made by referring, instead of the wavelength, to the
corresponding frequency parameter ω.
15 [0069]. Examples of birefringent fibers used in the manufacturing of birefringent Bi-FBG sensors
include Panda and TruePhase fibers, bow-tie fibers, fibers with D cladding, and elliptical core,
elliptical cladding fibers, and micro-structured high-birefringence optical fibers (MOFs).
[0070]. Birefringent Bragg gratings can also be induced in an optical fiber by femtosecond writing;
in such a case, the birefringence is inscribed only in the limited area of fiber where the Bi-FBG sensor
20 is obtained.
[0071]. The birefringent Bi-FBG sensor advantageously allows simultaneous strain and
temperature detection by measuring Bragg wavelength offsets corresponding to the optical polarization
fast and slow axes.
[0072]. Assuming a linear dependence, the correlation between the Bragg wavelengths s and f
25 and the temperature and strain variations can be expressed through the following equations expressed
in matrix form:
where  and  are the Bragg wavelength variations of the slow axis and the fast axis,

and

are the temperature sensitivities for the slow axis and the fast axis,

and

30 are the strain
sensitivities,  is the temperature variation, and  is the strain variation.
[0073]. Therefore, the temperature change  and the strain change at  can be simultaneously
calculated through the inverse matrix:
𝑠
𝑓
=

𝑠
𝑠

𝑓

𝑓



10
where

is the determinant of the matrix.
[0074]. The birefringent Bi-FBG sensor also allows for the distinction between transverse and
5 longitudinal strain.
[0075]. Indeed, the Bi-FBG sensor experiences maximum transverse strain dependence if the
birefringence axes are positioned to be orthogonal and parallel to the surface. When a transverse load is
applied along with one of the two birefringence axes, the corresponding reflected peak exhibits the
maximum transverse strain sensitivity.
10 [0076]. Instead, if the axes are positioned at 45° with respect to the surface, the sensitivity of the
Bi-FBG sensor to transverse strain is reduced and the wavelength deviation depends primarily on the
longitudinal strain.
[0077]. This property of birefringent Bi-FBG sensors allows the distinction between the two types
of strain and, advantageously, enables a shear strain measurement; for example, the article by S.
15 Sulejmani, et al, "Shear stress sensing with Bragg grating-based sensors in microstructured optical
fibers," Opt. Express 21, 20404-20416 (2013) shows that a maximum shear strain sensitivity of
wavelength separation deviation is obtained when the birefringence axes are positioned at 45°.
[0078]. Therefore, it is apparent the advantage of the Bi-FBG birefringent sensor, which allows, by
virtue of the information provided by the two spectral components of different polarization, to detect
20 two different magnitudes simultaneously.
[0079]. With reference to figures 1-6, it is now described a system 1 for interrogating at least
one sensor of the birefringent Fiber Bragg Grating FBG type (hereafter also sometimes named
birefringent Bi-FBG sensor for the sake of brevity), e.g., a Bi-FBG type sensor obtained in a
birefringent fiber, or in an optical fiber zone made birefringent.
25 [0080]. Such a system comprises a broadband optical radiation source BS, an integrated photonic
detection device PIC, and electronic processing means 2 operatively connected with said integrated
photonic device PIC.
[0081]. The broadband optical radiation source BS is configured to illuminate the aforesaid at least
one sensor of the birefringent Fiber Bragg Grating Bi-FBG type with a broadband optical excitation
30 radiation OA.
[0082]. The integrated photonic detection device PIC comprises a first input port C1, operatively
connectable to the aforesaid at least one optical fiber sensor of the birefringent Fiber Bragg Grating BiFBG type to receive the reflected optical spectrum OR from the sensor, and a second input port C2,
operatively connected to said broadband optical radiation source BS to receive the aforesaid broadband
11
optical excitation radiation OA.
[0083]. The integrated photonic detection device PIC comprises a polarization optical beam splitter
PS, local oscillator signal generation means, first heterodyne optical detection means, second
heterodyne optical detection means, a first output port U1, and a second output port U2.
5 [0084]. The polarization optical beam splitter PS is configured to separate a first component of the
aforesaid reflected optical spectrum OR1, characterized by a first optical polarization generated by the
birefringence and centered around a first frequency ω1, from a second component of the aforesaid
reflected optical spectrum OR2 by a second optical polarization generated by the birefringence and
centered around a second frequency ω2.
10 [0085]. The local oscillator signal generation means are configured to obtain at least two
narrowband optical signals (LO1, LO2) comprising a first local oscillator optical signal LO1, centered
around a first local oscillator frequency ωLO1, and a second local oscillator optical signal LO2, centered
around a second local oscillator frequency ωLO2.
[0086]. Such local oscillator signal generation means comprise at least one tunable narrowband
15 optical bandpass filter OTF, configured to perform narrowband optical filtering of said broadband
optical excitation radiation OA.
[0087]. The first optical heterodyne detection means are configured to receive the aforesaid first
component of the reflected optical spectrum OR1 and the aforesaid first local oscillator optical signal
LO1 and generate, by means of heterodyne detection or reception techniques, on the basis of the first
20 component of the reflected optical spectrum OR1 and of the first local oscillator optical signal LO1, a
first electrical signal E1 at a first intermediate frequency ωIFs, equal to the difference between the first
local oscillator frequency ωLO1 and the aforesaid first frequency ω1 of the first component of the
reflected optical spectrum OR1.
[0088]. The second optical heterodyne detection means are configured to receive the aforesaid
25 second component of the reflected optical spectrum OR2 and the aforesaid second local oscillator
optical signal LO2 and generate, by means of heterodyne detection techniques, on the basis of the
second component of the reflected optical spectrum OR2 and of the second local oscillator optical
signal LO2, a second electrical signal E2 at a second intermediate frequency ωIFf, equal to the
difference between the second local oscillator frequency ωLO2 and the aforesaid second frequency ω2
30 of the second component of the reflected optical spectrum OR2.
[0089]. The first output port U1, configured to make the first electrical signal E1 available, and a
second output port U2, configured to make the second electrical signal E2 available.
[0090]. The electronic processing means are operatively connected with the integrated PIC
photonic device to receive the aforesaid first electrical signal E1 and second electrical signal E2 and are
35 configured to determine the aforesaid first intermediate frequency ωIFs, indicative of a first wavelength
offset ∆λ1 of the first component of the reflected optical spectrum OR1, having the first polarization,
12
with respect to a first reference wavelength λref1.
[0091]. The electronic processing means are further configured to determine the aforesaid second
intermediate frequency ωIF2, indicative of a second wavelength shift ∆λ2 of the second component of
the reflected optical spectrum OR2, having the second polarization, with respect to a second reference
5 wavelength λref2.
[0092]. The aforesaid first wavelength shift ∆λ1 and second wavelength shift ∆λ2 are
representative of at least one physical magnitude measured by the optical fiber sensor Bi-FBG.
[0093]. According to an embodiment of the system 1 (shown in figure 1), the aforesaid means of
generating local oscillator signals comprise a tunable narrowband OTF optical filter and an optical
10 beam splitter.
[0094]. The narrowband optical tunable OTF is configured to narrowband filter the aforesaid
broadband optical excitation radiation OA, and to generate a narrowband optical signal centered
around a local oscillator frequency ωLO adapted to act as a local oscillator signal LO.
[0095]. The optical beam splitter is configured to split the aforesaid narrowband optical signal and
15 to make two attenuated replicas of the narrowband optical signal itself received as input corresponding,
respectively, to the first local oscillator signal LO1 and the second local oscillator signal LO2 available
to two output ports of the optical beam splitter.
[0096]. Some additional details of the aforesaid embodiment will be discussed below with
reference to figure 1.
20 [0097]. As can be seen, the system comprises a device for coupling and a device for splitting (or
divide) the two orthogonal polarizations, corresponding to the fast axis and the slow axis, into two
optical waveguides.
[0098]. The device for splitting (or dividing) the two orthogonal polarizations, in several possible
implementation options, is for example a two-dimensional 2D grating coupler or an edge coupler
25 combined with a polarization rotator and splitter (PSR).
[0099]. The system further comprises two different integrated photonic circuits which implement
heterodyne detection schemes.
[00100]. The two orthogonal polarizations at different frequencies/wavelengths, reflected by the
birefringent Bi-FBG, are separated and coupled to two single-mode waveguides, in which the two
30 signals at different frequencies/wavelengths are analyzed separately.
[00101]. The two signals are separately combined, by means of two integrated optical couplers, with
a local oscillator signal, and then mixed in integrated photodiodes to detect the frequency deviation of
the two individual peaks, through heterodyne detection.
[00102]. As shown in the example shown in figure 1, the light coming from a broadband source BS
35 is divided by an optical splitter ("splitter") OS and sent to both the first port of an optical circulator 3
and an input port of the photonic integrated circuit PIC.
13
[00103]. The birefringent Bi-FBG sensor is interrogated by the broadband source BS through the
second port of the optical circulator, and the reflected optical power from the birefringent Bi-FBG
sensor is collected by the third port of the optical circulator and coupled to the chip implementing the
PIC integrated photonic circuit.
5 [00104]. The integrated PIC photonic circuit comprises an optical coupler C1 and a signal
polarization splitter PS (wherein the polarization splitter, or polarization optical beam splitter may
comprise, as noted above, a 2D grating coupler or a PSR), with respect to the fast and slow axis
polarizations reflected by the birefringent Bi-FBG sensor.
[00105]. The integrated photonic circuit PIC further comprises an additional optical coupler C2 (e.g.,
10 a one-dimensional grating coupler or an "edge" coupler) configured to couple broadband light to the
integrated photonic circuit.
[00106]. The photonic integrated circuit PIC further comprises:
- at least one tunable optical bandpass OTF filter (for example, but not limited to, a ring
resonator filter) to select a desired frequency/wavelength, which is used as a local oscillator signal;
15 - at least two integrated optical couplers configured to combine the optical signal and the local
oscillator signal in the same waveguide;
- at least two integrated photodiodes for heterodyne detection.
[00107]. The coupler couples the light reflected from the third port of the optical circulator to the
PIC optical chip, and the PS polarization splitter separates the light by sending it into two different
20 optical waveguides; the two orthogonal polarizations reflected from the birefringent Bi-FBG sensor at
the frequencies of the fast axis ωs and the slow axis ωf are coupled and sent into two different
waveguides of the PIC integrated photonic circuit.
[00108]. According to an implementation option, the scheme further comprises a polarization rotator
or a 2D grating coupler to couple the light separated by the splitter into two waveguides TE.
25 [00109]. The broadband light coupled and supplied as input to another input port on the PIC chip is
filtered by a tunable optical band-pass filter OTF to select a specific wavelength.
[00110]. According to an implementation option, the tunable filter is based on an extraction (drop)
port of a tunable micro-ring resonator.
[00111]. The filtered light at a selected wavelength operates as a local LO oscillator, and is split and
30 sent to two different integrated optical couplers OC1, OC2 and combined with light at the frequencies
of the slow polarization axis ωs and the fast polarization axis ωf.
[00112]. Each coupler combines the frequency of the local LO oscillator and a respective one of the
two frequencies ωs and ωf, and the output signal of the optical coupler is sent to an integrated
photodiode according to a heterodyne configuration.
35 [00113]. The two optical signals and the local oscillator propagate in the waveguide with the same
cross-section and polarization, providing the maximum polarization match for heterodyne detection.
14
[00114]. The terms resulting from the mixing of the two heterodyne detections at the two
intermediate frequencies ωIFs = ωs - ωLO and on the other side ωIFf = ωf - ωLO carry the information
about the wavelengths of the reflection peaks of the birefringent Bi-FBG sensor and can be used to
detect the wavelength offset of the birefringent Bi-FBG sensor and the wavelength separation between
5 the two peaks corresponding to the slow axis and the fast axis, respectively.
[00115]. According to another embodiment of the system (shown in figure 2), the aforesaid
generating means of local oscillator signals comprise an optical beam splitter OSPL, a first tunable
narrow-band-pass optical filter OTF1, and a second tunable narrow-band-pass optical filter OTF2.
[00116]. The optical beam splitter OSPL is configured to split the aforesaid broadband optical
10 excitation radiation OA, to obtain a first replica of the broadband optical excitation radiation and a
second replica of the broadband optical excitation radiation.
[00117]. The first narrowband optical tunable OTF1 is configured to narrowband filter the aforesaid
first replica of the broadband optical excitation radiation, and to generate the first narrowband optical
signal LO1 centered around the first local oscillator frequency ωLO1.
15 [00118]. The second narrowband optical tunable OTF2 is configured to narrowband filter the
aforesaid second replica of the broadband optical excitation radiation, and to generate the second
narrowband optical signal LO2 centered around the second local oscillator frequency ωLO2.
[00119]. Some additional details of the aforesaid embodiment will be discussed below with
reference to figure 2.
20 [00120]. In this case, the PIC device comprises two different tunable narrowband optical filters,
OTF1, OTF2, configured to select two independent local oscillators at two different frequencies LO1
and LO2.
[00121]. The two intermediate frequencies for detecting the two peaks in wavelength, ωIFs = ωs -
ωLO and ωIFf = ωf - ωLO, are independent. According to an of implementation option, such frequencies
25 are finely tuned to facilitate and make the subsequent detection phase more accurate.
[00122]. Advantageously, this embodiment, which uses two independent local oscillators, offers
greater flexibility in the heterodyne detection scheme, because it allows for independent control of
intermediate frequencies, i.e., the beat frequencies between the local oscillator and the "slow" and
"fast" polarized optical signals. Such flexibility, in turn, leads to additional advantages, for example, it
30 makes it possible to reduce the bands required to the photodiodes, thus reducing the noise at their input
and thus improving the signal-to-noise ratio SNR of the measurement.
[00123]. According to an embodiment of the system 1 (illustrated for example in figures 1-3), the
first heterodyne detection means include first optical coupler OC1 comprising a respective optical
waveguide, configured to combine the first component of the reflected optical spectrum OR1 and the
35 first local oscillator optical signal LO1, and include a first opto-electronic receiver PD1 configured to
receive the output optical signal from the first optical coupler OC1 and convert it into a respective first
15
electrical signal E1.
[00124]. The second heterodyne detection means include a second optical coupler OC2 comprising a
respective optical waveguide, configured to combine the second component of the reflected optical
spectrum OR2 and the second local oscillator optical signal LO2; and include a second opto-electronic
5 receiver PD2 configured to receive the output optical signal from the second optical coupler OC2 and
convert it into a respective second electrical signal E2.
[00125]. According to various possible implementation options, the first and second means of
heterodyne detection or reception are implemented by means of heterodyne detection or reception
devices known in themselves.
10 [00126]. Reference may be made, for example, to Rongqing Hui “Introduction to Fiber-Optic
Communications” - 1st Edition, 13th June 2019 (DOI: 10.1016/B978-0-12-805345-4.00009-3), from
which the illustrations in figures 7 and 8 were excerpted and will be briefly discussed below.
[00127]. Figure 7 shows a well-known example of a heterodyne mixing technique, which allows to
lower the frequency of an optical frequency signal obtaining a corresponding intermediate frequency
15 signal which can be easily detected by electronic reception.
[00128]. In particular, a mixing between an optical signal and a local oscillator can be achieved by
means of a photodiode, i.e., a detector with a quadratic detection law in which two electromagnetic
fields, corresponding respectively to the incoming signal Es(t) and the local oscillator ELO(t) are mixed
providing a photocurrent i(t) which is proportional to the square of the two input electromagnetic
20 fields.
[00129]. The incoming optical signal Es(t) and the local oscillator optical signal ELO(t) can be
expressed as:
25 where As(t) and ALO(t) are the amplitudes, ωs and ωLO are the angular frequencies or pulsations, and φs
and φLO are the phases of the incoming signal and the local oscillator, respectively.
Apart from the direct detection components of the optical signal and the local oscillator, and
considering that the optical power of the local oscillator is typically (and anyway can be made) much
greater than the power of the incoming optical signal, the most significant photocurrent component i(t)
30 is observed at the intermediate frequency IF = S - LO and can be written as:
where  is the responsiveness of the photodiode, ε is the power coupling coefficient of the
𝐸
𝑠(𝑡) = 𝐴

𝑠 𝑡 𝑒𝑥𝑝⁡(𝑗𝜔𝑠
𝑡 + 𝑗𝜑𝑠
𝑡)
𝐸
𝑙𝑜(𝑡) = 𝐴

𝑙𝑜 𝑡 exp⁡(𝑗𝜔𝐿𝑂𝑡 + 𝑗𝜑𝐿𝑂𝑡)
𝑖 𝑡 ≈ 2 𝜖 1 − 𝜖 𝑃𝑠𝑃𝐿𝑂𝑐𝑜𝑠𝜃 cos⁡(𝜔𝐼𝐹𝑡 + ∆𝜑)
16
optical coupler (shown in figure 7),  is the polarization state difference angle between the optical
signal and the local oscillator,  is the relative phase difference.
[00130]. According to another embodiment of the system, shown in figure 3, the aforementioned
first optical coupler OC1 is a 2x2 optical coupler, configured to output two optical beat signals,
5 resulting from the combination of the first reflected optical spectrum component OR1 and the first
local oscillator optical signal LO1.
[00131]. Furthermore, said first opto-electronic receiver B-PD1 comprises two photodiodes,
configured to perform a balanced detection, wherein the first electrical signal E1 is obtained as a
subtraction of the outgoing currents from the two photodiodes of the first opto-electronic receiver B10 PD1.
[00132]. Similarly, the aforesaid second optical coupler OC2 is a 2x2 optical coupler, configured to
output two optical beat signals, resulting from the combination of the second reflected optical spectrum
component OR2 and the second local oscillator optical signal LO2.
[00133]. Furthermore, said second opto-electronic receiver B-PD2 comprises two photodiodes,
15 configured to perform a balanced detection, in which the second electrical signal E2 is obtained as a
subtraction of the outgoing currents from the two photodiodes of the second opto-electronic receiver BPD2.
[00134]. According to an implementation option, the balanced heterodyne detection is performed
using balanced detection techniques known in themselves, such as the one shown in figure 8.
20 [00135]. In this case, the 1x2 optical couplers are replaced by 2x2 optical couplers and the two
output ports of the optical couplers are connected to a BPD balanced photodetector.
[00136]. Performing a balanced coherent heterodyne detection improves the signal-to-noise ratio
SNR of the detected signal and avoids unwanted effects of local oscillator intensity noise.
[00137]. Specifically, the balanced coherent heterodyne detection shown in figure 2 employs a 2x2
25 optical coupler and two photodiodes in parallel. The difference between the two photocurrents
provides only one component, namely the intermediate frequency component, which can be expressed
as:
30 [00138]. In this configuration, advantageously, the DC components of the optical signal and the
local oscillator signal are canceled, and such a scheme makes it possible to increase the signal-to-noise
ratio SNR and reduce the undesirable effects of excess laser noise (intensity noise, etc.) and non-local
effects in the reflected response.
[00139]. According to an embodiment of the system, shown in figure 4, the first heterodyne
𝑖(𝑡)1 − 𝑖(𝑡)2 ≈ 2 𝑃𝑠𝑃𝐿𝑂𝑐𝑜𝑠𝜃 cos⁡(𝜔𝐼𝐹𝑡 + ∆𝜑)
17
detection means further comprise a first optical phase shifter OPS1, configured to shift, in a controlled
manner, the phase of the first local oscillator optical signal LO1, before the input in the first optical
coupler OC1. The second heterodyne detection means further comprise a second optical phase shifter
OPS2, configured to shift a controlled manner, the phase of the second local oscillator optical signal
5 LO2, before the input in the second optical coupler OC2.
[00140]. Thus, in this case, the PIC device integrates two optical phase shifters which can control
and modulate the phase of the local optical oscillator signal.
[00141]. The phase shifters add a phase control of the local oscillator; the phase of the local
oscillator can be modulated by the phase shifter, which advantageously makes it possible to increase
10 the sensitivity of the measurement by a phase control.
[00142]. According to another embodiment of the system, shown in figure 5, the optical signals at
the two frequencies ωs and ωf are coupled together in a directional coupler. Again, the intermediate
frequency of heterodyne detection is given by ωIFs= ωs- ωf.
[00143]. According to an embodiment (shown in figure 6) the system is configured for interrogating
15 a plurality of sensors of the birefringent Fiber Bragg Grating type (Bi-FBG 1- Bi-FBGn) in cascade,
each characterized by a respective nominal operating wavelength (λ1-λn).
[00144]. In such an embodiment, the PS-polarized optical beam splitter is configured to separate a
first component and a second component of the overall ORT reflected optical spectrum from the
birefringent Fiber Bragg Grating (Bi-FBG1 - Bi-FBGn) sensor cascade.
20 [00145]. The first component of the overall reflected optical spectrum ORT1 comprises the
superposition of the first components (OR11-OR1n) with the first optical polarization centered around
a respective first frequency (ω11-ω1n), and the second component of the overall reflected optical
spectrum ORT2 comprises the superposition of the second components (OR21-OR2n) with second
optical polarization centered around a respective second frequency (ω21-ω2n).
25 [00146]. In this case, the system further comprises first and second frequency discrimination or
demultiplexing means, and a plurality of first heterodyne detection means, and a plurality of second
heterodyne detection means.
[00147]. The first frequency discrimination or demultiplexing means AWG1 are configured to
spectrally separate the first components (OR11-OR1n) from one another.
30 [00148]. The second frequency discrimination or demultiplexing means AWG2 are configured to
spectrally separate the second components (OR21-OR2n) from one another.
[00149]. The first heterodyne detection means are configured to operate on respective first
components (OR11-OR1n) to obtain a respective plurality of first electrical signals E1n.
[00150]. The second heterodyne detection means, configured to operate on respective second
35 components (OR21-OR2n) to obtain a respective plurality of second electrical signals E2n.
[00151]. According to an implementation option (referred to in the example shown in figure 6), both
18
the first and second means of frequency discrimination or demultiplexing are made by means of
respective integrated "array waveguide gratings" (AWG) type devices, placed at the output of the
polarization splitter, which enable measurements based on WDM - Wavelength Division Multiplexing.
[00152]. The AWG device is an optical device for distributing an optical signal comprising a
5 plurality of N wavelengths, present at a single waveguide input, and making it available at the input of
N different output waveguides.
[00153]. The wavelength peaks reflected from the N birefringent Bi-FBG sensors are coupled to the
PIC device; the N wavelengths each corresponding to the fast axis and slow axis polarized optical
signals are separated and sent to two different waveguides, and each individual wavelength is filtered
10 and selected to a respective different output port of the AWG device. Each individual wavelength at a
respective output port of the AWG device can be detected by any of the previously illustrated schemes
with reference to querying a single birefringent Bi-FBG sensor.
[00154]. According to another option of implementation, the first and/or second means of frequency
discrimination or demultiplexing are made by other types of respective WDM demultiplexing devices,
15 known in themselves.
[00155]. According to an implementation option, already mentioned above, the aforementioned
tunable narrowband optical bandpass filter OTF is a micro-ring optical resonator filter, known in itself.
[00156]. For example, the ring resonator is a structure in which the fiber or optical waveguide is
enclosed in a "loop" configuration; when light, at a particular resonant wavelength, passes through the
20 ring (or loop) in constructive interference conditions, the intensity of the light increases in the structure,
and light can be extracted/observed at the monitor port of the extraction port at the given resonant
wavelength.
[00157]. The micro-ring resonator can be designed and integrated into the integrated photonic circuit
PIC and can be used as a bandpass filter, or optical switch, or optical intensity modulator.
25 [00158]. The resonant wavelength of the micro-ring resonator depends on the refractive index and
geometry of the device (e.g., the size of the optical waveguide and the ring diameter), and the resonant
wavelength can be tuned by a small change in the refractive index of the optical waveguide, e.g., by
thermal tuning based on a local micro-heater.
[00159]. According to an implementation option of the system, the aforesaid polarization optical
30 beam splitter PS is a polarization optical beam splitter made by means of integrated photonics
technique of two-dimensional grating coupler type.
[00160]. According to another implementation option of the system, the aforesaid polarization
optical beam splitter PS is a polarization optical beam splitter made by means of the integrated
photonics technique of polarization splitter and rotator - PSR type.
35 [00161]. With reference to the polarization optical beam splitter, it is worth noting that there are
different strategies to couple and propagate orthogonal polarizations of the same light beam in the
19
silicon optical waveguide. The most popular known solutions are the two-dimensional grating coupler
and the edge coupler combined with a polarization splitter and rotator (PSR).
[00162]. The two-dimensional grating coupler (2D GC) can be considered as the superposition of
two one-dimensional grating couplers (1D GC), in which the two orthogonal polarizations coming
5 from the fiber, at the input, are coupled to two polarized optical waveguides TE in the orthogonal
direction. The two orthogonal polarizations are coupled to the optical waveguide in which the light
propagates with the same polarization.
[00163]. In a PSR polarization rotator and splitter, the signal comprising two orthogonal
polarizations is sent to an integrated polarization optical beam splitter, which divides the light into two
10 separate signals with the two orthogonal polarizations TE and TM [see for example; M. R. Watts, H.
A. Haus, E. P. Ippen "Integrated mode-evolution-based polarization splitter" Opt. Lett. 30, 967-969
(2005)]; the separated, TM-polarized signal is sent to a polarization beam rotator, where the
polarization is rotated 90° into a TE-polarized signal [M. R. Watts, H. A. Haus, "Integrated modeevolution-based polarization rotators," Opt. Lett. 30, 138-140 (2005)].
15 [00164]. According to an implementation option of the system, each of the aforesaid first optoelectronic receiver PD1 and/or said second opto-electronic receiver PD2 comprises at least a respective
semiconductor photodiode configured to detect and convert optical signals, at the wavelengths
considered, into electrical signals.
[00165]. According to an embodiment, the system further comprises an optical circulator 3 having a
20 first circulator port connected to the broadband optical radiation source BS, a second circulator port
connected to a birefringent optical fiber containing the sensor of the birefringent Fiber Bragg Grating
Bi-FBG type, and a third circulator port connected to an optical input port of the photonic integrated
device PIC.
[00166]. Such an optical circulator 3 is configured to transmit the broadband optical radiation OA,
25 received from the first circulator port, to the birefringent optical fiber containing the sensor of the Fiber
Bragg Grating Bi-FBG type, through the second circulator port, and it is further configured to convey
the spectrum reflected by the sensor of the birefringent Fiber Bragg Grating Bi-FBG type, received
from the second circulator port to the optical input port of the photonic integrated device PIC, through
the third circulator port.
30 [00167]. According to an embodiment of the system, the aforesaid first reference wavelength λref1
and said second reference wavelength λref2 correspond to two respective nominal operating
wavelengths of the sensor of the birefringent Fiber Bragg Grating Bi-FBG type, on the two fast and
slow channels, determined by means of an initial calibration
[00168]. According to another embodiment of the system, the aforesaid first reference wavelength
35 λref1 and said second reference wavelength λref2 coincide and correspond to a reference wavelength λi
identified by the tuning of the narrowband band-pass optical tunable filter OTF.
20
[00169]. According to an implementation option of the system, the aforesaid first reference
wavelength λref1 and second reference wavelength λref2 correspond, respectively, to two reference
wavelengths λi1 and λi2 identified by the tuning of the two narrowband band-pass optical tunable
filters (OTF1, OTF2).
5 [00170]. According to another implementation option of the method, the aforesaid first reference
wavelength λref1 and the aforesaid said second reference wavelength λref2 coincide and correspond to
two respective nominal operating wavelengths of the sensor of the birefringent Fiber Bragg Grating BiFBG type, on the two fast and slow channels.
[00171]. A method for determining at least two physical magnitudes detectable by a sensor of the
10 birefringent Fiber Bragg Grating Bi-FBG type is now described.
[00172]. Such a system comprises a sensor of the birefringent Fiber Bragg Grating Bi-FBG type, a
system for interrogating at least one birefringent Fiber Bragg Grating Bi-FBG type according to any
one of the embodiments described above, wherein the electronic processing means are further
configured to determine the at least two physical magnitudes on the basis of a processing of said first
15 intermediate frequency ωIF1 and second intermediate frequency ωIF2 detected.
[00173]. According to an implementation option, the two determined physical magnitudes are a
longitudinal strain and a transverse strain.
[00174]. According to another implementation option, the two determined physical magnitudes are a
strain and a temperature.
20 [00175]. According to an embodiment of such a system, the birefringent Fiber Bragg Grating BiFBG type sensor is configured to operate in a brake pad or embedded in or attached to a brake caliper,
or embedded in a washer device adapted to be placed between a brake caliper bracket and a brake
caliper.
[00176]. The at least two physical magnitudes detected are a longitudinal strain and a transverse
25 strain, which are present at the point where the birefringent Fiber Bragg Grating (Bi-FBG) sensor is
located and which, as a whole, represent a clamping force and/or braking torque acting on the brake
caliper.
[00177]. It is worth noting that the object of the present invention is fully achieved by the method
and system illustrated above by virtue of the functional and structural features thereof.
30 [00178]. Indeed, with reference to the technical problems stated in the description part of the prior
art, the system according to the invention is a simple and compact system, in which the essential
components are integrated (e.g., in photonic integrated circuits in PIC technology).
[00179]. The system and method for interrogating Bi-FBG sensors made of birefringent fiber,
according to the invention, meets the criteria of (i) compactness and simplicity in structure and in use,
35 (ii) effectiveness in performance.
[00180]. This is because such a method and system are based on photonic integrated circuits, which
21
is made possible in that the solution of the present invention includes optical division based on
polarization of the spectrum reflected by the birefringent Bi-FBG sensor, and subsequent double
heterodyne coherent detection (functions that can be accomplished by components that can be
integrated into a PIC).
5 [00181]. Furthermore, the need for local oscillators (which would not be integrable into the PIC) is
avoided, because signals substantially similar to those generated by local oscillators are obtained in the
PIC circuit through narrowband optical filtering of a replica of the same broadband optical querying
radiation.
[00182]. Based on the foregoing, the interrogating system provided by the present invention is
10 sufficiently small and simple, while remaining effective, to be installed in technical contexts, such as
strain and temperature sensing in brake calipers, and very suitable for brake pads.
[00183]. In such contexts, the advantages, already mentioned above, offered by the birefringent BiFBG sensor are particularly apparent, including in particular the ability to simultaneously detect at least
two magnitudes or physical parameters (e.g., transverse strain and longitudinal strain, or strain and
15 temperature).
[00184]. To meet contingent and specific needs, the person skilled in the art may make several
changes and adaptations to the above-described embodiments and may replace elements with others
which are functionally equivalent, without however departing from the scope of the following claims.
All the features described above as belonging to one possible embodiment may be implemented
20 independently from the other described embodiments.
22
WE CLAIM:
1. A method for interrogating at least one sensor of the birefringent Fiber Bragg Grating type
(Bi-FBG), comprising the steps of:
5 - illuminating said at least one sensor of the birefringent Fiber Bragg Grating type (BiFBG) with a broadband optical excitation radiation (OA);
- conveying the reflected optical spectrum (OR), reflected by the at least one sensor of the
birefringent Fiber Bragg Grating type (Bi-FBG), into a detection photonic integrated circuit (PIC);
- separating a first component of said reflected optical spectrum (OR1), characterized by a
10 first optical polarization generated by the birefringence and centered around a first frequency (ω1),
from a second component of said reflected optical spectrum (OR2) characterized by a second
optical polarization generated by the birefringence and centered around a second frequency (ω2),
by means of a polarization optical beam splitter (PS) comprised in the detection photonic integrated
circuit (PIC);
15 - providing said broadband optical excitation radiation (OA) to the detection photonic
integrated circuit (PIC);
- obtaining at least two narrowband optical signals (LO1, LO2), on the basis of at least one
narrowband optical filtering of said broadband optical excitation radiation (OA) carried out in the
detection photonic integrated circuit (PIC), wherein said at least two narrowband optical signals
20 (LO1, LO2) comprise a first local oscillator optical signal (LO1), centered around a first local
oscillator frequency (ωLO1), and a second local oscillator optical signal (LO2), centered around a
second local oscillator frequency (ωLO2);
- providing said first component of the reflected optical spectrum (OR1) and said first local
oscillator optical signal (LO1) to first optical heterodyne detection means, integrated into said
25 detection photonic integrated circuit (PIC), to carry out a heterodyne detection and obtain a first
electrical signal (E1) at a first intermediate frequency (ωIFs), equal to the difference between the
first local oscillator frequency (ωLO1) and said first frequency (ω1) of the first component of the
reflected optical spectrum (OR1);
- providing said second component of the reflected optical spectrum (OR2) and said
30 second local oscillator optical signal (LO2) to second optical heterodyne detection means,
integrated into said detection photonic integrated circuit (PIC), to carry out a heterodyne detection
and obtain a second electrical signal (E2) at a second intermediate frequency (ωIFf), equal to the
difference between the second local oscillator frequency (ωLO2) and said second frequency (ω2) of
the second component of the reflected optical spectrum (OR2);
35 - determining said first intermediate frequency (ωIF1), indicative of a first wavelength shift
(∆λ1) of the first component of the reflected optical spectrum (OR1), having the first polarization,
23
with respect to a first reference wavelength (λref1; λi);
- determining said second intermediate frequency (ωIF2), indicative of a second wavelength
shift (∆λ2) of the second component of the reflected optical spectrum (OR2), having the second
polarization, with respect to a second reference wavelength (λref2) of the Bragg grating of the sensor
5 of the birefringent Fiber Bragg Grating type (Bi-FBG);
wherein said first wavelength shift (∆λ1) and said second wavelength shift (∆λ2) are
representative of at least one physical magnitude measured by the sensor of the birefringent Fiber
Bragg Grating type (Bi-FBG).
10 2. A method according to claim 1, wherein said step of obtaining at least two narrowband
optical signals (LO1, LO2) comprises:
- narrowband filtering said broadband optical excitation radiation (OA), by means of a
narrowband band-pass optical tunable filter (OTF) integrated into the detection photonic integrated
circuit (PIC), to obtain a narrowband optical signal centered around a local oscillator frequency
15 (ωLO) adapted to act as a local oscillator signal (LO);
- splitting said narrowband optical signal by means of an optical beam splitter (OSPL),
configured to make two attenuated replicas of the same narrowband optical signal, received as
input, available to two output ports,
wherein the first local oscillator signal (LO1) and the second local oscillator signal (LO2)
20 are the two signals, identical to each other, present at the two output ports of the optical beam
splitter.
3. A method according to claim 1, wherein said step of obtaining at least two narrowband
optical signals (LO1, LO2) comprises:
25 - splitting said broadband optical excitation radiation (OA) by means of an optical beam
splitter (OSPL), to obtain a first replica of the broadband optical excitation radiation and a second
replica of the broadband optical excitation radiation;
- narrowband filtering said first replica of the broadband optical excitation radiation, by
means of a first narrowband band-pass optical tunable filter (OTF1) integrated into the detection
30 photonic integrated circuit (PIC), to obtain the first narrowband optical signal centered around the
first local oscillator frequency (ωLO1);
- narrowband filtering said second replica of the broadband optical excitation radiation, by
means of a second narrowband band-pass optical tunable filter (OTF2) integrated into the detection
photonic integrated circuit (PIC), to obtain the second narrowband optical signal centered around
35 the second local oscillator frequency (ωLO2).
24
4. A method according to any one of claims 1 to 3, wherein the step of carrying out a
heterodyne detection and obtaining a first electrical signal (E1) comprises combining the first
component of the reflected optical spectrum (OR1) and the first local oscillator optical signal (LO1)
in an optical waveguide of a first optical coupler (OC1) of the first optical heterodyne detection
5 means and further comprises converting the optical signal obtained at the output of the first optical
coupler into a respective first electrical signal (E1), by means of a first opto-electronic receiver
(PD1) of the first optical heterodyne detection means;
and wherein the step of carrying out a heterodyne detection and obtaining a second
electrical signal (E2) comprises combining the second component of the reflected optical spectrum
10 (OR2) and the second local oscillator optical signal (LO2) in an optical waveguide of a second
optical coupler (OC2) of the second optical heterodyne detection means, and further comprises
converting the optical signal obtained at the output of the second optical coupler into a respective
second electrical signal (E2), by means of a second opto-electronic receiver (PD2) of the second
optical heterodyne detection means.
15
5. A method according to claim 4, wherein each of the steps of carrying out a heterodyne
detection to obtain a first electrical signal (E1) and a second electrical signal (E2) comprises
carrying out a balanced detection, using a respective 2x2 optical coupler, configured to provide as
output two optical signals, for each heterodyne detection, which are detected by two respective
20 photodiodes for balanced detection, for each heterodyne detection, wherein each of the first
electrical signal (E1) and the second electrical signal (E2) is obtained as a subtraction of the
currents output from the respective photodiodes.
6. A method according to claim 4, wherein carrying out the first heterodyne detection further
25 comprises shifting, in a controlled manner, the phase of the first local oscillator optical signal
(LO1) by means of a first optical phase shifter (OPS1), comprised in the photonic integrated circuit
(PIC), before the input in the first optical coupler (OC1),
and wherein carrying out the second heterodyne detection further comprises shifting, in a
controlled manner, the phase of the second local oscillator optical signal (LO2) by means of a
30 second optical phase shifter (OPS2), comprised in the photonic integrated circuit (PIC), before the
input in the second optical coupler (OC2).
7. A method according to any one of the preceding claims, wherein the step of carrying out a
heterodyne detection comprises injecting the first component of the reflected optical spectrum
35 (OR1) and the second component of the reflected optical spectrum (OR2) into a single 2x1 optical
coupler (OC), configured to generate as output an optical signal at an intermediate frequency (ωIF)
25
representative of the difference between the frequency deviations of the first component of the
reflected optical spectrum (OR1) and the second component of the reflected optical spectrum
(OR2).
5 8. A method according to any one of the preceding claims configured to interrogate a
plurality of sensors of the birefringent Fiber Bragg Grating type (Bi-FBG1 - Bi-FBGn) in cascade,
each characterized by a respective nominal operating wavelength (λ1-λn), wherein
- the step of conveying comprises conveying the overall reflected optical spectrum (ORT),
reflected by the cascade of sensors of the birefringent Fiber Bragg Grating type (Bi-FBG1 - Bi10 FBGn), into a detection photonic integrated circuit (PIC);
- the step of separating comprises separating a first component of said overall reflected
optical spectrum (ORT1) and a second component of said overall reflected optical spectrum
(ORT2),
wherein the first component of the overall reflected optical spectrum (ORT1) comprises
15 the superposition of the first components (OR11-OR1n) with first optical polarization, each
centered around a respective first frequency (ω11-ω1n),
and the second component of the overall reflected optical spectrum (ORT2) comprises the
superposition of the second components (OR21-OR2n) with second optical polarization, each
centered around a respective second frequency (ω21-ω2n);
20 wherein the method comprises the further steps of:
- spectrally separating the first components (OR11-OR1n) from one another by means of
first frequency discrimination or demultiplexing means (AWG1);
- spectrally separating the second components (OR21-OR2n) from one another by means
of second frequency discrimination or demultiplexing means (AWG2);
25 - carrying out said heterodyne detection steps on each of the first components (OR11-
OR1n) and each of the second components (OR21-OR2n), to obtain a respective plurality of first
electrical signals (E1k) and second electrical signals (E2k);
- carrying out said steps of determining the first intermediate frequency (ωIFs,k) and the
second intermediate frequency (ωIFf,k) for each pair of first electrical signal (E1k) and second
30 electrical signal (E2k) corresponding to a respective sensor of the birefringent Fiber Bragg Grating
type (Bi-FBGk).
9. A method according to any one of the preceding claims, wherein said first optical
polarization corresponds to the polarization on a "slow polarization axis" and the first birefringence
35 peak frequency (ω1) corresponds to the slow axis birefringence peak frequency (ωs),
and wherein said second optical polarization is orthogonal to the first optical polarization
26
and corresponds to the polarization on a "fast polarization axis", orthogonal to said "slow axis", and
the second birefringence peak frequency (ω2) corresponds to the fast axis birefringence peak
frequency (ωf).
5 10. A method according to any one of the preceding claims, wherein said first reference
wavelength (λref1) and said second reference wavelength (λref2) correspond to two respective
nominal operating wavelengths of the sensor of the birefringent Fiber Bragg Grating type (BiFBG), on the two fast and slow channels, determined by means of an initial calibration.
10 11. A method according to any one of claims 2 to 9, wherein said first reference wavelength
(λref1) and said second reference wavelength (λref2) coincide and correspond to a reference
wavelength (λi) identified by the tuning of the narrowband band-pass optical tunable filter (OTF).
12. A method according to any one of claims 2 to 9, wherein said first reference wavelength
15 (λref1) and said second reference wavelength (λref2) correspond, respectively, to two reference
wavelengths (λi1, λi2) identified by the tuning, respectively, of the two narrowband band-pass
optical tunable filters (OTF1, OTF2).
13. A method for determining at least two physical magnitudes detectable by a sensor of the
20 birefringent Fiber Bragg Grating type (Bi-Bi-FBG) comprising:
- carrying out a method for interrogating at least one sensor of the birefringent Fiber Bragg
Grating type (Bi-FBG), according to any one of claims 1 to 12;
- determining the at least two physical magnitudes based on a processing of said first
intermediate frequency (ωIFs) and second intermediate frequency (ωIFf).
25
14. A method according to claim 13, wherein the two physical magnitudes determined are
longitudinal strain and a transverse strain.
15. A method according to claim 13, wherein the two physical magnitudes determined are a
30 strain and a temperature.
16. A system (1) for interrogating at least one sensor of the birefringent Fiber Bragg Grating
type (Bi-FBG), comprising:
- a broadband optical radiation source (BS), configured to illuminate said at least one sensor of the
35 birefringent Fiber Bragg Grating type (Bi-FBG) with a broadband optical excitation radiation (OA);
- a detection photonic integrated device (PIC) having a first input port (C1), operatively
27
connectable to said at least one sensor of the birefringent Fiber Bragg Grating type (Bi-FBG) to
receive the reflected optical spectrum (OR) from said sensor, and a second input port (C2),
operatively connected with said broadband optical radiation source (BS) to receive said broadband
optical excitation radiation (OA);
5 wherein said detection photonic integrated device (PIC) comprises:
- a polarization optical beam splitter (PS), configured to separate a first component of said
reflected optical spectrum (OR1), characterized by a first optical polarization generated by the
birefringence and centered around a first frequency (ω1), from a second component of said
reflected optical spectrum (OR2) characterized by a second optical polarization generated by the
10 birefringence and centered around a second frequency (ω2);
- means for generating local oscillator signals, configured to obtain at least two
narrowband optical signals (LO1, LO2), comprising a first local oscillator optical signal (LO1),
centered around a first local oscillator frequency (ωLO; ωLO1), and a second local oscillator optical
signal (LO2), centered around a second local oscillator frequency (ωLO; ωLO2), wherein said means
15 for generating local oscillator signals comprise at least one narrowband band-pass optical tunable
filter (OTF), configured to perform a narrowband optical filtering of said broadband optical
excitation radiation (OA);
- first optical heterodyne detection means (11) configured to receive said first component
of the reflected optical spectrum (OR1) and said first local oscillator optical signal (LO1) and
20 generate, by means of heterodyne detection techniques, on the basis of the first component of the
reflected optical spectrum (OR1) and of the first local oscillator optical signal (LO1), a first
electrical signal (E1) at a first intermediate frequency (ωIFs), equal to the difference between the
first local oscillator frequency (ωLO1) and said first frequency (ω1) of the first component of the
reflected optical spectrum (OR1);
25 - second optical heterodyne detection means (12) configured to receive said second
component of the reflected optical spectrum (OR2) and said second local oscillator optical signal
(LO2) and generate, by means of heterodyne detection techniques, on the basis of the second
component of the reflected optical spectrum (OR2) and of the second local oscillator optical signal
(LO2), a second electrical signal (E2) at a second intermediate frequency (ωIFf), equal to the
30 difference between the second local oscillator frequency (ωLO2) and said second frequency (ω2) of
the second component of the reflected optical spectrum (OR2);
- a first output port (U1), configured to make said first electrical signal (E1) available, and
a second output port (U2), configured to make said second electrical signal (E2) available;
and wherein the system further comprises:
35 - electronic processing means (2), operatively connected to said photonic integrated device (PIC) to
receive said first electrical signal (E1) and second electrical signal (E2), and configured to
28
determine said first intermediate frequency (ωIFs), indicative of a first wavelength shift (∆λ1) of the
first component of the reflected optical spectrum (OR1), having the first polarization, with respect
to a first reference wavelength (λref1; λi) of the Bragg grating of the sensor of the birefringent Fiber
Bragg Grating type (Bi-FBG), and also configured to determine said second intermediate frequency
5 (ωIFf), indicative of a second wavelength shift (∆λ2) of the second component of the reflected
optical spectrum (OR2), having the second polarization, with respect to a second reference
wavelength (λref2) of the Bragg grating of the sensor of the birefringent Fiber Bragg Grating type
(Bi-FBG),
wherein said first wavelength shift (∆λ1) and said second wavelength shift (∆λ2) are
10 representative of at least one physical magnitude measured by the optical fiber sensor (Bi-FBG).
17. A system (1) according to claim 16, wherein said means for generating local oscillator
signals comprise:
- a narrowband band-pass optical tunable filter (OTF), configured to narrowband filter said
15 broadband optical excitation radiation (OA), and to generate a narrowband optical signal centered
around a local oscillator frequency (ωLO) adapted to act as a local oscillator signal (LO);
- an optical beam splitter (OSPL) configured to split said narrowband optical signal (LO)
and to make two attenuated replicas of the same narrowband optical signal available to two output
ports of the optical beam splitter, said attenuated replicas being attenuated replicas of the same
20 narrowband optical signal received as input, corresponding, respectively, to the first local oscillator
signal (LO1) and the second local oscillator signal (LO2).
18. A system (1) according to claim 16, wherein said means for generating local oscillator
signals comprise:
25 - an optical beam splitter (OSPL) configured to split said broadband optical excitation
radiation (OA), to obtain a first replica of the broadband optical excitation radiation and a second
replica of the broadband optical excitation radiation;
- a first narrowband band-pass optical tunable filter (OTF1), configured to narrowband
filter said first replica of the broadband optical excitation radiation, and to generate the first
30 narrowband optical signal (LO1) centered around the first local oscillator frequency (ωLO1);
- a second narrowband band-pass optical tunable filter (OTF2), configured to narrowband
filter said second replica of the broadband optical excitation radiation, and to generate the second
narrowband optical signal (LO2) centered around the second local oscillator frequency (ωLO2).
35 19. A system (1) according to any one of claims 16 to 18, wherein:
- the first heterodyne detection means comprise:
29
- a first optical coupler (OC1) comprising a respective optical waveguide, configured to
combine the first component of the reflected optical spectrum (OR1) and the first local oscillator
optical signal (LO1);
- a first opto-electronic receiver (PD1) configured to receive the output optical signal from
5 the first optical coupler (OC1) and convert it into a respective first electrical signal (E1);
- the second heterodyne detection means comprise:
- a second optical coupler (OC2) comprising a respective optical waveguide, configured to
combine the second component of the reflected optical spectrum (OR2) and the second local
oscillator optical signal (LO2);
10 - a second opto-electronic receiver (PD2) configured to receive the output optical signal
from the second optical coupler (OC2) and convert it into a respective second electrical signal (E2).
20. A system (1) according to claim 19, wherein:
- said first optical coupler (OC1) is a 2x2 optical coupler, configured to provide as output
15 two optical beat signals, deriving from the combination of the first component of the reflected
optical spectrum (OR1) and the first local oscillator optical signal (LO1), and wherein said first
opto-electronic receiver (B-PD1) comprises two photodiodes, configured to perform a balanced
detection, wherein the first electrical signal (E1) is obtained as a subtraction of the currents output
from the two photodiodes of the first opto-electronic receiver (PD1);
20 - said second optical coupler (OC2) is a 2x2 optical coupler, configured to provide as
output two optical beat signals, deriving from the combination of the second component of the
reflected optical spectrum (OR2) and the second local oscillator optical signal (LO2), and wherein
said second opto-electronic receiver (B-PD2) comprises two photodiodes, configured to perform a
balanced detection, wherein the second electrical signal (E2) is obtained as a subtraction of the
25 currents output from the two photodiodes of the second opto-electronic receiver (PD2).
21. A system (1) according to claim 19, wherein the first heterodyne detection means further
comprise a first optical phase shifter (OPS1), configured to shift, in a controlled manner, the phase
of the first local oscillator optical signal (LO1), before the input in the first optical coupler (OC1),
30 wherein the second heterodyne detection means further comprise a second optical phase
shifter (OPS2), configured to shift, in a controlled manner, the phase of the second local oscillator
optical signal (LO2), before the input in the second optical coupler (OC2).
22. A system (1) according to any one of claims 16 to 21, configured to query a plurality of
35 sensors of the birefringent Fiber Bragg Grating type (Bi-FBG1 - Bi-FBGn) in cascade, each
characterized by a respective nominal operating wavelength (λ1-λn), wherein:
30
- the polarization optical beam splitter (PS) is configured to separate a first component
(ORT1) and a second component (ORT2) of the overall reflected optical spectrum (ORT) from the
cascade of sensors of the birefringent Fiber Bragg Grating type (Bi-FBG1 - Bi-FBGn), wherein the
first component of the overall reflected optical spectrum (ORT1) comprises the superposition of the
5 first components (OR11-OR1n) with first optical polarization, centered around a respective first
frequency (ω11-ω1n), and the second component of the overall reflected optical spectrum (ORT2)
comprises the superposition of the second components (OR21-OR2n) with second optical
polarization centered around a respective second frequency (ω21- ω2n);
and wherein the system further comprises:
10 - first frequency discrimination or demultiplexing means (AWG1) configured to spectrally
separate said first components (OR11-OR1n) from one another;
- second frequency discrimination or demultiplexing means(AWG2) configured to
spectrally separate said second components (OR21-OR2n) from one another;
- a plurality of first heterodyne detection means, configured to operate on respective first
15 components (OR11-OR1n) to obtain a respective plurality of first electrical signals (E1n);
- a plurality of second heterodyne detection means, configured to operate on respective
second components (OR21-OR2n) to obtain a respective plurality of second electrical signals
(E2n).
20 23. A system (1) according to any one of claims 16 to 22, wherein said narrowband band-pass
optical tunable filter (OTF) is an optical micro-ring resonator filter.
24. A system (1) according to any one of claims 16 to 23, wherein said polarization optical
beam splitter (PS) is a polarization optical beam splitter made by means of an integrated photonics
25 technique of the "two-dimensional grating coupler" type, or by means of an integrated photonics
technique of the "polarization splitter and rotator - PSR" type.
25. A system according to any one of claims 16 to 23, wherein each of said first optoelectronic receiver (PD1) and/or said second opto-electronic receiver (PD2) comprises at least a
30 respective semiconductor photodiode configured to detect and convert optical signals, at the
wavelengths considered, into electrical signals.
26. A system (1) according to any one of claims 16 to 25, further comprising an optical
circulator (3) having a first circulator port connected to the broadband optical radiation source (BS),
35 a second circulator port connected to a birefringent optical fiber containing the sensor of the Fiber
Bragg Grating type (Bi-FBG), and a third circulator port connected to an optical input port of the
31
photonic integrated device (PIC),
wherein the optical circulator (3) is configured to transmit the broadband optical radiation
(OA), received from the first circulator port, to the birefringent optical fiber containing the sensor
of the Fiber Bragg Grating type (Bi-FBG), through the second circulator port, and it is further
5 configured to convey the spectrum reflected by the sensor of the Fiber Bragg Grating type (BiFBG), received from the second circulator port to the optical input port of the photonic integrated
device (PIC), through the third circulator port.
27. A system (1) according to any one of claims 16 to 26, wherein said first reference
10 wavelength (λref1) and said second reference wavelength (λref2) correspond to two respective
nominal operating wavelengths of the sensor of the birefringent Fiber Bragg Grating type (BiFBG), on the two fast and slow channels, determined by means of an initial calibration.
28. A system (1) according to any one of claims 17 to 26, wherein said first reference
15 wavelength (λref1) and said second reference wavelength (λref2) coincide and correspond to a
reference wavelength (λi) identified by the tuning of the narrowband band-pass optical tunable
filter (OTF).
29. A system (1) according to any one of claims 17 to 26, wherein said first reference
20 wavelength (λref1) and said second reference wavelength (λref2) correspond, respectively, to two
reference wavelengths (λi1, λi2) identified by the tuning, respectively, of the two narrowband bandpass optical tunable filters (OTF1, OTF2).
30. A system for determining at least two physical magnitudes detectable by a sensor of the
25 birefringent Fiber Bragg Grating type (Bi-FBG) comprising:
- a sensor of the birefringent Fiber Bragg Grating type (Bi-FBG);
- a system (1) for interrogating at least one sensor of the birefringent Fiber Bragg Grating
type (Bi-FBG) according to any one of claims 16 to 29, wherein the electronic processing means
(2) are further configured to determine the at least two physical magnitudes based on a processing
30 of said first intermediate frequency (ωIFs) and second intermediate frequency (ωIFf) detected.
31. A system according to claim 30, wherein the two physical magnitudes determined are a
longitudinal strain and a transverse strain.
35 32. A system according to claim 31, wherein the two physical magnitudes determined are a
strain and a temperature.
32
33. A system according to claim 31, wherein the sensor of the birefringent Fiber Bragg
Grating type (Bi-FBG) is configured to operate within a brake pad or incorporated in or coupled
with a brake caliper, or incorporated in a washer device adapted to be arranged between a brake
5 caliper support and a brake caliper, and wherein the at least two physical magnitudes detected are a
longitudinal strain and a transverse strain, present in the point where the sensor of the birefringent
Fiber Bragg Grating type (Bi-FBG) is placed and generally representative of a tightening force
and/or braking torque acting on the brake caliper.