Claims:1. A system (100) for interrogating one or more parameters at a plurality of locations in a sample (108), comprising:
a first ultrafast laser source (102) configured to provide a first plurality of pulses in a picosecond time domain or lower, wherein the first ultrafast laser source (102) is configured to provide comb frequencies having a first repetition rate, and wherein the first plurality of pulses interacts with the sample (108) at the plurality of locations in the sample (108) to provide processed pulses;
a second ultrafast laser source (104) configured to provide a second plurality of pulses in a picosecond time domain or lower, wherein the second ultrafast laser source (104) is configured to provide comb frequencies having a second repetition rate that is different from the first repetition rate;
a reference device (110) configured to provide referenced pulses having a variable time delay, a variable phase delay, a variable path length difference, or combinations thereof;
a detector unit (124) configured to detect at least a portion of the processed pulses from the first ultrafast laser source (102), the second plurality of pulses from the second ultrafast laser source (104), and the referenced pulses; and
a processor unit (128) configured to process the detected pulses and provide measurements of the one or more parameters for the plurality of locations in the sample (108).

2. The system (100) of claim 1, wherein the first ultrafast laser source, the second ultrafast laser source, or both comprise a femtosecond laser source.

3. The system (100) of claim 1, wherein the reference device (110) comprises a time delay device, a phase difference generator, or a path length difference generator, or combinations thereof.

4. The system (100) of claim 3, wherein the time delay device is a mirror.

5. The system (100) of claim 1, further comprising a third ultrafast laser source (306) configured to provide a third plurality of pulses.

6. The system (100) of claim 5, further comrpising a clock (310) operatively coupled to the first and third ultrafast laser sources (302, 306) to synchronize the first and third ultrafast sources.

7. The system (100) of claim 6, further comprising a phase difference generator operatively coupled to the third ultrafast laser source (306).

8. The system (100) of claim 5, further comprising a circulator (312) operatively coupled to the first ultrafast laser source (102) to direct at least a portion of the interacted pulses such that the interacted pulses are combined with the third plurality of pulses.

9. The system (100) of claim 1, further comprising an optical fiber in operative communication with the first ultrafast laser source (102) such that at least a portion of the first plurality of pulses traverses through at least a portion of the optical fiber.

10. The system (100) of claim 1, further comprising an optical fiber in operative communication with the second ultrafast laser source (104) such that at least a portion of the second plurality of pulses traverses through at least a portion of the optical fiber.

11. The system (100) of claim 1, wherein the reference device (110) comprises a liquid crystal, a micro array, a micro electro mechanical system (MEMS), or an optical structure that is configured to introduce a time delay, a phase delay, or an optical path length difference.

12. A system (100) for interrogating one or more parameters at a plurality of locations in a sample (108), comprising:
a first femtosecond laser source (102) configured to provide a first plurality of pulses in a picosecond time domain or lower, wherein the first femtosecond laser source(102) is configured to provide comb frequencies having a first repetition rate; and wherein the first plurality of pulses interacts with the sample (108) at the plurality of locations in the sample (108) to provide processed pulses;
a second femtosecond laser source (104) configured to provide a second plurality of pulses in a picosecond time domain or lower, wherein the second femtosecond laser source (104) is configured to provide comb frequencies having a second repetition rate that is different from the first repetition rate;
a reference device (110) configured to provide referenced pulses having a variable time delay, a variable phase delay, a variable path length difference, or combinations thereof;
a detector unit (124) configured to detect at least a portion of the processed pulses from the first femtosecond laser source (102), the second plurality of pulses from the second femtosecond laser source (104), and the referenced pulses; and
a processor unit (128) configured to process the detected pulses and provide measurements of the one or more parameters for the plurality of locations in the sample.

13. The system (100) of claim 12, further comprising a third femtosecond laser source.

14. The system (100) of claim 12, wherein the reference device (110) is operatively coupled to the first femtosecond laser source.

15. The system (100) of claim 12, further comprising a collimator disposed between the first femtosecond laser source and at least a portion of the sample.

16. The system (100) of claim 12, further comprising an optical fiber in operative communication with the first femtosecond laser source such that at least a portion of the first plurality of pulses traverses through at least a portion of the optical fiber.

17. A method (400) for interrogating one or more parameters at a plurality of locations in a sample, comprising:
providing a first plurality of pulses in a picosecond time domain or lower, wherein the first plurality of pulses comprises a first repetition rate (402);
interacting a portion of the first plurality of pulses at the plurality of locations in a sample to provide processed pulses (404);
introducing a variable time delay, a variable phase delay, a variable path length difference, or combinations thereof in another portion of the first plurality of pulses to provide referenced pulses (406);
providing a second plurality of pulses in a picosecond time domain or lower, wherein the second plurality of pulses comprise a second repetition rate that is different from the first repetition rate (408);
detecting at least a portion of the processed pulses from the first ultrafast laser source, the second plurality of pulses from the second ultrafast laser source, and the referenced pulses (410); and
processing the detected pulses to provide measurements of the one or more parameters for the plurality of locations in the sample (412).
, Description:BACKGROUND
[0001] The embodiments of the specification relate to systems and methods for distributed
measurements of one or more parameters. In particular, the systems and methods relate to the
distributed measurements using optical techniques.
[0002] Typically, in large gas chambers or in optically transparent samples absorption of
incident optical radiation is measured as a bulk quantity by passing the optical radiation through
the sample. It is desirable to develop optical techniques that can provide distribution of multiple
parameters such as temperature of gases, concentration of gases, and/or pressure of gases inside a
chamber, or measure spatially distributed high resolution spectrum.
[0003] Optical frequency pulsing is an optical technique that is employed to encode
information pertaining to transmission across fiber optic lines, determine physical properties of
molecules in samples, and the like. Identification of sample properties using frequency pulses is
based on width and stability of the pulses at desirable frequencies to obtain specific resolution of
the sample. Typically, interferometric measurements are used for optical analysis of samples.
However, usually, interferometric measurements result in decreased resolution in space and time
due to limitations on pulse widths, repetition rate of the pulses, range of spectra, and the like.
The optical frequency pulsing for transmission of information employs relatively wide
bandwidth of individual frequency pulses, resulting in overlapping pulses over large transmission
distances. As will be appreciated, as frequency pulses travel along a fiberoptic line, pulse width
increases. After determined distance, the overlap of frequency lines due to increase in the pulse
widths results in a loss of digital information content. Further, it is difficult to generate multiple
different, closely spaced frequencies, thereby limiting signal resolution in optical frequency
pulsing.
BRIEF DESCRIPTION
[0004] In one embodiment, a system for interrogating one or more parameters at a plurality of
locations in a sample is provided. The system includes a first ultrafast laser source and a second
ultrafast source configured to provide a first plurality of pulses and a first plurality of pulses,
respectively in a picosecond time domain or lower. Further, the first and second ultrafast laser
sources are configured to provide comb frequencies having first and second repetition rates,
respectively, where the second repetition rate is different from the first repetition rate. Further,
the first plurality of pulses interacts with the sample at the plurality of locations in the sample to
provide processed pulses. Moreover, the system includes a reference device configured to
provide referenced pulses having a variable time delay, a variable phase delay, a variable path
length difference, or combinations thereof. Additionally, the system includes a detector unit
configured to detect at least a portion of the processed pulses from the first ultrafast laser source,
the second plurality of pulses from the second ultrafast laser source, and the referenced pulses.
The system also includes a processor unit configured to process the detected pulses and provide
measurements of the one or more parameters for the plurality of locations in the sample.
[0005] In another embodiment, a system for interrogating one or more parameters at a
plurality of locations in a sample is provided. The system includes a first femtosecond laser
source and a second femtosecond laser source configured to provide a first plurality of pulses
and a first plurality of pulses, respectively, in a picosecond time domain or lower. Further, the
first and second femtosecond laser sources are configured to provide comb frequencies having
first and second repetition rates, respectively, where the second repetition rate is different from
the first repetition rate. Further, the first plurality of pulses interacts with the sample at the
plurality of locations in the sample to provide processed pulses. The system also includes a
reference device configured to provide referenced pulses having a variable time delay, a variable
phase delay, a variable path length difference, or combinations thereof. Further, the system
includes a detector unit configured to detect at least a portion of the processed pulses from the
first femtosecond laser source, the second plurality of pulses from the second femtosecond laser
source, and the referenced pulses. Moreover, the system includes a processor unit configured to
process the detected pulses and provide measurements of the one or more parameters for the
plurality of locations in the sample.
[0006] In yet another embodiment, a method includes providing a first plurality of pulses in a
picosecond time domain or lower, and comb frequencies having a first repetition rate. Further,
the method includes interacting a portion of the first plurality of pulses at a plurality of locations
in a sample to provide processed pulses. The method also includes introducing a variable time
delay, a variable phase delay, a variable path length difference, or combinations thereof in
another portion of the first plurality of pulses to provide referenced pulses, and providing a
second plurality of pulses in a picosecond time domain or lower, and comb frequencies having a
second repetition rate that is different from the first repetition rate. Additionally, the method
includes detecting at least a portion of the processed pulses from the first ultrafast laser source,
the second plurality of pulses from the second ultrafast laser source, and the referenced pulses.
The method also includes processing the detected pulses to provide measurements of one or
more parameters for the plurality of locations in the sample.
DRAWINGS
[0007] These and other features and aspects of embodiments of the invention will become
better understood when the following detailed description is read with reference to the
accompanying drawings in which like characters represent like parts throughout the drawings,
wherein:
[0008] FIG. 1 is a schematic representation of a system for distributed measurement of a
sample to interrogate one or more parameters at a plurality of locations in the sample, in
accordance with aspects of the specification;
[0009] FIGs. 2(a)-2(i) are graphical illustrations of distributed measurements using the system
of FIG. 1, in accordance with aspects of the specification;
[0010] FIG. 3 is a schematic representation of a system for long distance distributed
measurements at a plurality of locations in a sample, in accordance with aspects of the
specification; and
[0011] FIG. 4 is an example flow chart of a method for distributed measurements for a
plurality of locations in a sample.
DETAILED DESCRIPTION
[0012] In certain embodiments of the specification, systems and methods for distributed
measurement of one or more parameters of a sample are provided. In certain other embodiments
of the specification, distributed measurement of absorption, transmission, reflection, phase
spectrum, or combinations may be used to determine one or more parameters, such as, but not
limited to, concentration, pressure, temperature, and chemical composition at one or more
locations in the sample. Further, in some embodiments, the systems may be configured to
conduct short distance distributed measurements, long distance distributed measurements, or
both. In one example, the distributed measurement may be performed in the sample to determine
chemical composition of the sample at two or more locations in the sample. In another
embodiment, the distributed measurement may be carried out to determine temperature of the
sample at two or more locations in the sample.
[0013] In some embodiments, the systems and methods may be used for high spatial
resolution multi-parameter measurement along with wavelength resolution. For example, the
systems and methods may be used for spatial profiling of gas, oil, water, or combinations thereof
in down hole systems, spatial gas concentration measurements, such as in a gas turbine exhaust,
health monitoring of coatings, such as, but not limited to, thermal barrier coatings in turbine
blades.
[0014] In certain embodiments, a system for distributed measurement of a sample to
interrogate or probe one or more parameters at a plurality of locations in the sample is provided.
The system includes a first ultrafast laser source configured to provide comb frequencies having
a first repetition rate The first ultrafast laser source is configured to provide a first plurality of
pulses in a time domain that is equal to or lower than a picosecond time domain to interact with
one or more locations of the plurality of locations of the sample to provide processed pulses.
The system also includes a second ultrafast laser source that is configured to provide comb
frequencies at a second repetition rate that is different from the first repetition rate. The second
ultrafast laser source is configured to provide a second plurality of pulses in a time domain equal
to or lower than the picosecond time domain The difference between the repetition rates of the
first and second ultrafast laser sources may be based on a system parameter. Further, the
difference between the repetition rates of the first and second ultrafast laser sources determines a
range of spectrum that can be measured. By way of example, if the difference in the repetition
rates is below a determined value, the range of spectrum that is measured may be narrower than a
range of spectrum that is measured if the difference in the repetition rates is above the
determined value.
[0015] The system also includes a detector unit that is configured to detect at least a portion
of the processed pulses and at least a portion of the second plurality of pulses. Further, the
system includes a processor unit configured to process the detected pulses and provide
measurements of one or more parameters corresponding to the plurality of locations in the
sample.
[0016] In certain embodiments, the methods and systems may be used to provide distributed
measurement of the plurality of parameters to facilitate optimization of operation, such as, but
not limited to, an operation of a gas turbine. For example, the distributed measurement of the
plurality of parameters facilitate identifying hot spots in the gas turbine, reducing emissions from
the gas turbine, detecting impurities in materials in thermal barrier coatings of turbine blades,
and the like, or combinations thereof. Further, the methods and systems may provide
measurements for samples having one or more phases, such as two or more of solid, liquid, and
gaseous phases. The systems and methods are configured to measure broadband high resolution
spectrum and have applications in environmental studies where the samples may be in one or
more phases such as solid, liquid and gaseous phases. Moreover, the methods and systems use
synchronized ultrafast laser sources that facilitate long distance distributed measurements.
[0017] FIG. 1 illustrates an example of a system for distributed measurement of a sample to
interrogate one or more parameters at a plurality of locations in the sample. In the illustrated
embodiment, the system 100 includes an interferometric configuration, which uses a sample arm
and a reference arm. The term “sample arm” is used to refer to an arm or portion of the system
100 that includes a sample, and the term “reference arm” is used to refer to an arm or a portion of
the system 100 that includes a reference device.
[0018] In the illustrated embodiment, the system 100 includes a first ultrafast laser source
102, a second ultrafast laser source 104, and a reference device 110. The first and second
ultrafast laser sources 102 and 104 are configured to provide first and second plurality of pulses,
respectively. The first and second ultrafast laser sources 102 and 104 are defined by their
individual frequency combs and repetition rates. The reference device 110 is operatively
coupled to the first laser source 102 and configured to provide referenced pulses that have a time
delay, a phase delay, or an optical path length difference with respect to the pulses of the first
laser source 102.
[0019] In one embodiment, the difference in frequencies of the first and second ultrafast laser
sources 102 and 104 may be greater than a line width of any given line of the frequency comb of
the ultrafast laser sources 102 and 104. Further, the difference in frequencies of the first and
second ultrafast laser sources 102 and 104 may be less than half of the repetition rate of the
ultrafast laser sources 102 and 104.
[0020] In some examples, the ultrafast laser sources 102 and 104 may be pico or femtosecond
laser sources. Advantageously, use of pico or femtosecond laser sources in the system 100
results in relatively higher spatial resolution as compared to use of laser sources that emit optical
pulses at time durations that are greater than pico seconds. The repetition rates of the first and
second ultrafast laser sources 102 and 104 are different. In one example, the repetition rate of
the first ultrafast laser source 102 is referred to as the “first repetition rate,” fr, and the repetition
rate of the second ultrafast laser source 104 is referred to as the “second repetition rate,” (fr +/-
df). In particular, the first and second repetition rates are different. The difference in repetition
rates of the first and second ultrafast laser sources 102 and 104 is represented by df, where df is a
ratio that represents spatial distribution of length of laser pulses for a particular repetition rate of
the ultrafast laser source. The repetition rate may be decided based on the desirable spectral
resolution. In one embodiment, the value of df may be represented by Equation (1) as:
df ?? ??
???? Equation (1)
where, c represents speed of light, and fr represents repetition rate of the first ultrafast
laser source 102.
[0021] It may be noted that optical paths for the plurality of pulses from the ultrafast laser
sources 102 and 104 may be formed with or without optical fibers. In one embodiment, an
optical fiber may be in operative communication with the first ultrafast laser source 102 such that
at least a portion of the first plurality of pulses traverses through at least a portion of the optical
fiber. In same or different embodiments, an optical fiber may be in operative communication
with the second ultrafast laser source 104 such that at least a portion of the second plurality of
pulses traverses through at least a portion of the optical fiber. In certain embodiments, in the
optical configuration of FIGS. 1 and 3 of the present application, the first, second, and/or third
plurality of pulses from the first, second, and/or third ultrafast laser sources 302, 304, and 306,
respectively, may traverse through ambient atmosphere (e.g., air) or optical fiber. In
embodiments where the different plurality of pulses traverse using the optical fiber, connecting
lines between various components of the systems 100 and 300 may represent optical fibers.
[0022] The first ultrafast laser source 102 is operatively coupled to the reference device 110.
In a non-limiting example, the reference device 110 may be a time delay device, such as a
mirror. Other non-limiting examples of reference devices may include phase difference or delay
generators, path length difference generators, liquid crystals, micro arrays, micro electro
mechanical system (MEMS), or any other optical structures that are configured to introduce a
time delay, a phase delay or phase difference, or an optical path length difference in a path of
optical radiation in the system 100, for example. The reference arm, generally represented by
reference numeral 112 includes the reference device 110, such as the reference mirror. Further,
the sample arm, generally represented by reference numeral 114, includes a sample 108 and an
optical path that leads to the sample 108. The reference device 110 is configured to provide a
time delay between the optical pulses travelling in the reference arm 112 and the optical pulses
travelling in the sample arm 114 to create a desirable interference between the pulses in the two
arms 112 and 114.
[0023] In certain embodiments, the reference device may be configured to introduce a time
delay, phase delay or difference, or optical path length difference, in a portion of the first
plurality of pulses provided by the first ultrafast laser source 102. Further, the time delay, phase
delay, or optical path length difference may be varied with time. In particular, the time delay,
phase delay, path length difference, introduced by the reference device 110 in a portion of the
first plurality of pulses are such that these delays and differences may be varied according to a
location in the sample that is to be interrogated. By way of example, in the illustrated
embodiment, if the reference device 110 is a time delay device, such as a mirror, the mirror may
be configured to move along an optical path, represented by reference numeral 111, to introduce
a variable time delay in the path of the portion of the first plurality of pulses from the first
ultrafast laser source 102. This portion of the first plurality of pulses is referred to as referenced
pulses. Further, the speed or rate of movement of the time delay device may be adjusted based
on a desirable scanning rate at which different locations of the sample 108 need to be scanned.
Further, the time delay, phase delay, or path length difference introduced by the reference
device110 may be adjusted to correspond to spatial resolution of the sample 108. It may be
noted that the minimal spatial resolution for scanning a sample 108 may be similar to a pulse
width of the laser sources 102 and 104.
[0024] The system further includes splitters 116 and 117. In a non-limiting example, the
splitter 116 may be a 50:50 splitter, for example. The splitter 116 is configured to split the
radiation travelling from the first ultrafast laser source 102 into at least 2 portions such that a first
portion of the radiation travels towards the reference device 110, and the second portion of the
radiation travels towards the sample 108. Additionally, both the reference and sample arms 112
and 114 may employ collimators 120 and 122. The collimators 120 and 122 may be similar to
one another in function and structure or may be different. The collimator 120 collimates and
directs the radiation towards the reference device 110, the delayed radiation from the reference
device 110 is received back by the collimator 120. Similarly, the collimator 122 collimates and
directs the radiation towards the sample 108 to facilitate interaction of the radiation with at least
a portion of the sample 108, and to collect the processed pulses or interacted pulses.
[0025] In operation, a portion of the referenced pulses, such as time delayed pulses, from the
first ultrafast laser source 102 are allowed to interact with one or more locations of a sample at a
given instant in time. The sample locations 130, 132, 134, and 136 are interrogated or probed
based on a refractive index of a medium of the sample 108. In one example, where the reference
device 110 is a time delay device, such as a reference mirror, the distance moved by the
reference mirror along the optical path, which is along the direction 111 may determine an
amount of optical delay or path length difference introduced in the sample arm, or both. In one
embodiment, the reference mirror may be coupled to a motor (not shown in FIG. 1) to facilitate
movement of the mirror. It other examples where a piezo electric device or a MEMS mirror is
used as a reference device 110, electrical current may be provided to the reference device to
facilitate desirable movement of the reference device 110 to provide suitable time delay or phase
delay at a particular instance in time.
[0026] The interacted pulses from the sample 108 in the sample arm 114, and the referenced
(such as time delayed) pulses from the reference arm 112 are allowed to interfere with the pulses
from the second ultrafast laser source 104. This interference of the pulses is detected using the
detector unit 124. In one example, the detector unit 124 may be a high frequency detector.
[0027] Further, the system employs a processor unit 128 for processing the combined
radiation received at the detector unit 124. As used herein, the term “processor unit” refers to a
processing unit having integrated circuits as being included in a computer, as well as a controller,
a microcontroller, a microcomputer, a programmable logic controller (PLC), an application
specific integrated circuit, application-specific processors, digital signal processors (DSPs),
application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and/or
any other programmable circuits. In certain embodiments, the processor unit 128 may be
coupled to, or may include a memory device(s) (not shown in FIG. 1). The memory device(s)
may generally include memory element(s) including, but are not limited to, computer readable
medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a
flash memory), one or more hard disk drives, a floppy disk, a compact disc-read only memory
(CD-ROM), compact disk-read/write (CD-R/W) drives, a magneto-optical disk (MOD), a digital
versatile disc (DVD), flash drives, optical drives, solid-state storage devices, and/or other
suitable memory elements. The detected radiation processed by the processor unit 128 may
provide data that represents distributed measurements of one or more parameters of a sample as
described further in FIGS. 2(a)-2(h). Further, the method of operation of the system 100 of FIG.
1 will be described with respect to FIGS. 2(a)-2(h).
[0028] Referring to FIGS. 2(a)-2(h), interference of pulses from a reference arm, for example,
the reference arm 112 of FIG. 1 and a sample arm, for example, the sample arm 114 of FIG. 1 is
illustrated. In FIGS. 2(a)-2(f), ordinate 202 represents optical intensity of the pulses. Further, in
FIGS. 2(a)-2(f) abscissa 204 represents time, in FIGS. 2(g)-2(h) abscissa 206 represents
frequency of the pulses, and ordinate 208 represents amplitude of the pulses. In particular, FIG.
2(a) represents referenced pulses (IR) or pulses from the reference arm 112. At a given instant in
time, a position of the reference device 110, such as a location of the time delay device along the
direction 111, is selected such that one or more sample locations at a particular location (or
depth) along the optical path length are selected. FIG. 2(b) represents pulses (IS) from the
sample arm 114 that correspond to sample locations X1, X2, X3, and X4 that are represented in
FIG. 1 by reference numerals 130, 132, 134, and 136 respectively. In particular, pulses that are
returned after interacting with the sample position X1 130 are represented by reference numerals
212, and pulses returned from the sample location X2 132 are represented by reference numerals
214. Similarly, pulses returned from the sample locations X3 134 and X4 136 are represented by
reference numerals 216 and 218. It should be noted that the sample 108 may have fewer or more
than 4 locations that need to be detected for distributed measurement.
[0029] FIG. 2(c) represents the second plurality of pulses 210 that are from the second
ultrafast laser source 104. FIG. 2(d) represents combined pulses from the reference device 110,
the sample 108, and the pulses from the second ultrafast laser source 104 at the detector unit 124.
In particular, FIG. 2(d) represents a summation of referenced pulses (IR) from the reference arm
112, the second plurality of pulses from the second ultrafast laser source 104, and processed or
interacted pulses from the plurality of locations, such as locations X1, X2, X3, X4 130-136 in the
sample 108 are allowed to interfere. In one embodiment, the detector unit 124 is configured to
perform nonlinear operations to provide an interferogram signal (ID), as represented in Equation
(2) below:
???? ?? ???????????? Equation (2)
where ID represents interferogram signal, IR represents referenced pulses from the
reference arm, IS represents pulses from the sample arm, and IL represents the second plurality of
pulses from the second ultrafast laser source 104.
[0030] The multiplication of pulses from different locations X1, X2, X3, X4 130-136, and the
pulses (IR) from the reference arm 112 causes the signals other than ones having same path
length as that of the pulses from the reference arm 112 to reduce to zero. Accordingly, as
represented by FIG. 2(e), when the reference is moved such that the path length of the reference
overlaps with signals from X1 130, the interferogram signal (ID) includes the interferogram of
the signal from locationsX1 130 in the sample 108. Similarly generated interferogram signals
for X2 132 are illustrated in FIG. 2(f). Further, interferogram signals from other locations, X3,
and X4 134, and 136 may be obtained by correspondingly adjusting the reference device 110, for
example by moving the reference mirror.
[0031] FIGS. 2(g)-2(h) illustrate Fourier transform of the interferogram signals corresponding
to positions X1 130 and X2 132. Accordingly, the pulses represented by reference numerals 230
and 232 represent Fourier transforms of signals from positions X1 and X2 of the sample. The
Fourier transforms, such as the Fourier transforms 230 and 232, are used to form an absorption
profile of the sample 108 corresponding to corresponding sample locations as represented in
FIG. 2(i). Equation (3) represents absorption profile at location X2 as:
????z?? ?? ?????????????????????
?? Equation (3)
where, A(z) represents absolute absorption value at a location z in the sample, ?(z)
represents cumulative absorption value at the location z, and ?(z-?z) represents cumulative
absorption at a location z-?z, where the location z-?z is immediately before the location n in the
path of the optical pulses. For example, if z represents location X2, z-?z may represent location
X1. The absorption profile as represented in FIG. 2(i) includes a peak 244 and has abscissa 240
that represents wavelength, and ordinate 242 that represents absorption values. In certain
embodiments, height, width, and position of a peak, such as the peak 244 in the absorption
profile may be used to measure various parameters, such as, but not limited to, temperature,
pressure, concentration, composition, and the like. In the illustrated example, a height 246 of the
peak 244 is representative of the concentration of a particular species or concentration of the
sample 108 in general. Further, a width 248 and a position 250 of the peak 244 are
representative of temperature and pressure, respectively, of the sample 108 at that particular
location. Distributed measurements can be carried out at several locations in the sample with a
spatial resolution that is proportional to the pulse width of the ultrafast laser sources 102 and
104.
[0032] In some embodiments, the system 100 is configured for short distance measurements
in a range from about 100 microns to few cms, and long distance measurements in a range from
about few cms to few kms. Further, in some other embodiments, the system 100 is conductive
for upto about c/fr of the laser, where c is the speed of light and fr is repetition rate of the
ultrafast laser source. In certain other embodiments, the systems and methods of the present
application may be configured for long distance distributed measurements, such as in oil and gas
pipelines.
[0033] FIG. 3 illustrates another example of the system 300 for distributed measurements of a
plurality of locations in a sample. Further, the system 300 is configured for long distance
measurements, such as, but not limited to, down hole applications, spatial profiling of gas, oil,
water in down hole systems, spatial gas concentration measurements in gas turbine exhaust, and
health monitoring of coatings, such as, but not limited to, thermal barrier coatings on turbine
blades. The system 300 includes first, second, and third ultrafast laser sources 302, 304, and 306
configured to provide first, second and third plurality of pulses. In a non-limiting example, the
first, second, and third ultrafast laser sources 302, 304, and 306 are femtosecond laser sources.
In some embodiments, the ultrafast laser sources 302, 304, and 306 may be in operative
communication with other components of the system 300 via one or more optical fibers. In one
embodiment, an optical fiber may be in operative communication with the first ultrafast laser
source 302 such that at least a portion of the first plurality of pulses traverses through at least a
portion of the optical fiber.
[0034] Further, the ultrafast laser sources 302 and 306 are operatively coupled to a reference
clock 308 to synchronize the two sources 302 and 306 with respect to one another. Moreover, a
reference device, such as a phase delay generator 310, is coupled to one of the first and third
ultrafast laser sources 302 and 306. In the presently contemplated example, where the phase
delay generator 310 is coupled to the third ultrafast laser source 306, the phase delay introduced
by the phase delay generator 310 to the third plurality of pulses of the third ultrafast laser source
306 in turn introduce a phase delay in the clock 308. Further, the phase delay introduced in the
third plurality of pulses by the phase delay generator 310 results in a time delay being introduced
in the third plurality of pulses with respect to the first plurality of pulses from the first ultrafast
laser source 302. The third ultrafast laser source 306 along with the reference clock 308 acts as
the reference arm to provide referenced pulses.
[0035] In operation, the first plurality of pulses from the first ultrafast laser source 302 are
passed through a circulator 312 and a portion 315 of the first plurality of pulses that are passed
through the circulator 312 is used to interrogate the sample 314. A collimator 316 is used to
collimate the pulses 315 passing through the circulator 312 towards the sample 314. Various
sample locations in the direction 326 of the pulses from the collimator 316 are represented as X1
318, X2 320, X3 322, and X4 324. After interaction with the sample 314, the interacted pulses
are combined with the third plurality of pulses using a combinator 328. The combined pulses are
then further combined with the second plurality of pulses from the second ultrafast laser source
304 using another combinator 330, to form resultant pulses. The repetition rates of the first and
third ultrafast laser sources 302 and 306 are same. Further, the repetition rates of the first and
third ultrafast laser sources 302 and 306 are relatively different from a repetition rate of the
second source 304.
[0036] The resultant pulses are detected by a detector unit 332 and processed using a
processor unit 334. Results, such as shown in FIG.s. 2(a)-2(i) may be displayed on a display
device 336, such as a monitor, a touch screen, or the like.
[0037] FIG. 4 illustrates an example flow chart 400 for a method for distributed measurement
at a plurality of locations in a sample to detect one or more parameters of the sample. At block
402, the method commences by providing a first plurality of pulses in a picosecond time domain
or lower, and comb frequencies having a first repetition rate. At block 404, a portion of the first
plurality of pulses is allowed to interact at the plurality of locations in the sample to provide
processed pulses. At block 406, a variable time delay, a variable phase delay, a variable path
length difference, or combinations thereof, is introduced in another portion of the first plurality
of pulses to provide referenced pulses. At block 408, a second plurality of pulses in a
picosecond time domain or lower, and comb frequencies having a second repetition rate is
provided. The second repetition rate is different from the first repetition rate.
[0038] At block 410, at least a portion of the processed pulses from the first ultrafast laser
source, the second plurality of pulses from the second ultrafast laser source, and the referenced
pulses are detected by a detector. At bock 412, the detected pulses are processed to provide
measurements of one or more parameters for the plurality of locations in the sample.
[0039] In some embodiments, a third plurality of pulses may also be provided. In these
embodiments, the time delay, phase delay, or path length difference may be introduced in one of
the first or third plurality of pulses. Further, the first and third plurality of pulses may be
synchronized. Moreover, the processed pulses may be combined with the third plurality of
pulses.
[0040] The systems 100 and 300 of FIGS. 1 and 3 and methods of FIG. 2(a)-2(i) and FIG. 4
are configured for multispecies determination. By way of example, when interrogating a
gaseous sample, different gases present in the gaseous sample may be detected using the systems
and methods of the present application. Advantageously, the systems and methods of the present
application provide high resolution and faster measurement times. High resolution sensing
allows identification of multiple materials simultaneously. Also, the systems and methods of the
present application facilitate complete optical spectrum to determine multiple parameters instead
of measuring these parameters individually. Moreover, the broadband and coherent output of the
frequency combs also facilitates high signal-to-noise ratios. Also short pulse width of ultrafast
lasers provide high spatial resolution.
[0041] While only certain features of the invention have been illustrated and described herein,
many modifications and changes will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such modifications and changes as
fall within the scope of the invention.