A MODE CONVERTER
The present invention relates to optical mode conversion in an optical fiber.
A conventional optical fiber comprises an optical core that has the function of
transmitting, and where applicable amplifying, an optical signal, and it is surrounded
by optical cladding that has the function of confining the optical signal in the core.
To this end, the refractive index n^ of the core is greater than the refractive index n2
of the cladding.
The shape of the graph plotting the refractive index of an optical fiber as a function
of the radius of the fiber is generally referred to as the index profile. By convention,
the distance r to the centre of the fiber is plotted along the abscissa axis and the
difference between the refractive index of the core and the refractive index of the
cladding of the fiber is plotted up the ordinate axis. The expressions "step",
"trapezium", and "triangle" are used in respect of graphs that have stepped,
trapezoidal, and triangular shapes, respectively. These curves generally represent
a theoretical or set point profile for the fiber, and fiber fabrication constraints may
lead to a profile that is significantly different.
An optical signal transmitted in an optical fiber may be a single mode signal or a
multimode signal, depending on the size of the core and the nature of the light
waveguide. The propagation mode(s) transmitted in an optical fiber have specific
transverse intensity distributions. A longitudinal direction is defined as the axis of
optical propagation of the signal in the fiber and a cross section is defined as a
plane perpendicular to the longitudinal axis of the fiber.
In new high bit rate wavelength division multiplex (WDM) transmission networks it is
advantageous to manage chromatic dispersion, especially for bit rates equal to or
greater than 40 Gbit/s or 160 Gbit/s. The object is to obtain cumulative chromatic
dispersion over the connection that is substantially zero for all of the wavelength
values of the multiplex, in order to limit spreading of the pulses. The expression
"cumulative chromatic dispersion" refers to the integral of the chromatic dispersion
over the length of the fiber; at constant chromatic dispersion, the cumulative
chromatic dispersion is equal to the product of the chromatic dispersion multiplied
by the length of the fiber. It is also beneficial to limit the cumulative chromatic
dispersion slope over the wavelength range of the multiplex in order to prevent or to
limit distortion between the channels of the multiplex. By convention, this slope is
the derivative of chromatic dispersion with respect to wavelength.
Single mode fibers (SMF) and non-zero dispersion shifted fibers (NZ-DSF*) are
used as the line fiber in conventional optical fiber transmission systems. Those
fibers have positive chromatic dispersion and a positive chromatic dispersion slope.
It is known in the art to use short lengths of dispersion compensating fiber (DCF) to
compensate the chromatic dispersion and the chromatic dispersion slope in single
mode fibers or NZ-DSF* fibers used as line fibers.
Generally speaking, optical transmission systems use line fibers constituted by
single mode fibers in which only the fundamental LP01 mode is guided.
Nevertheless, the properties of higher order modes can be exploited for certain
optical purposes, for example for flattening gain or for compensating chromatic
dispersion. It is then necessary to convert the fundamental mode into a spatial
mode of higher order. For example, higher order mode (HOM) fibers have high
negative chromatic dispersion and large effective area.
There are two main optical mode conversion methods. A first method, called the
longitudinal method, introduces a periodical disturbance along the axis of a fiber
portion over a given signal propagation distance z. Such a disturbance, for example
a long period grating (LPG), leads to coupling between the fundamental mode and
a higher order mode. However, that method cannot achieve 100% mode coupling.
Although attenuated, the fundamental mode continues to be guided in the HOM
fiber and generates noise in a signal propagating in a higher order mode in the
HOM fiber.
The second method, called the transverse method, modifies the transverse intensity
distribution of the signal at a given point along the propagation of the signal by
means of a delay element adapted to introduce controlled phase jumps. A
technique of that kind can theoretically provide coupling of 100% between a first
mode and a second mode, providing the phase shifting elements are perfectly
aligned with the signal propagation axis.
Thus international patent application WO 99/49342 proposes to effect transverse
optical mode conversion by inserting phase selection elements between two light
waveguides, which phase selection elements can comprise diffractive or reflective
elements, such as lenses, mirrors, gratings, electro-optical components, etc.
However, a mode converter of that kind is complex, inflexible, and relatively bulky.
Moreover, a free space converter of that kind is sensitive to the environment, and it
is therefore difficult to operate in the long-term and to adjust.
European patent application EP 1 343 031 discloses an in-fiber transverse mode
converter, i.e. a transverse mode converter that is implemented entirely within a
fiber. A converter of that kind comprises a light waveguide including a
photosensitive region inducing a specific index profile along a section of the guide,
which profile simultaneously defines a longitudinal adiabatic transition and a
transverse guided mode distribution. It is therefore possible to modify the refractive
index of the waveguide locally by controlled irradiation of the photosensitive region
of the waveguide. The radial variation of the index profile effects mode conversion
and precise control of the irradiation along the waveguide ensures in particular an
adiabatic longitudinal transition of the radial variation. However, that kind of control
is difficult, and doping the fiber to obtain that particular photosensitive profile is
complicated.
The invention aims to mitigate the drawbacks of the prior art. It proposes a
simplified mode converter that achieves transverse conversion more efficiently.
The invention thus provides an in-fiber mode converter comprising:
- fiber portions constituting an input microlens adapted to expand the wavefront of
said optical signals,
- a fiber portion constituting a phase shifting region adapted to convert said
propagation mode of said optical signal into another propagation mode, and
- fiber portions constituting an output microlens.
According to a feature, the fiber portions constituting an input microlens are spliced
and the fiber portions constituting an output microlens are spliced.
According to a feature, the fiber portion constituting the phase shifting region has a
predetermined length, a predetermined refractive index profile, and a predetermined
core diameter.
According to a feature, the index profile of the fiber portion constituting the phase
shifting region comprises one or more jumps.
According to a feature, the length of the fiber portion constituting the phase shifting
region is from 25 micrometers (urn) to 1000 |jm.
According to a feature, the core diameter of the portion constituting the phase
shifting region is from 0.5 urn to 100 urn.
Thus the invention relates to an optical system comprising:
- a first optical fiber adapted to transmit an optical signal propagating in a first
propagation mode,
- a mode converter of the invention adapted to convert said first propagation mode
into a second propagation mode, and
- a second optical fiber adapted to transmit an optical signal propagating in said
second propagation mode.
According to a feature, the first and second propagation modes are circularly
symmetrical modes LPOm.
According to a feature, one of the optical fibers is a multimode fiber.
According to a feature, one of the optical fibers is a higher order mode fiber.
In an application, one of the optical fibers is a chromatic dispersion compensating
fiber.
In an application, one of the optical fibers is a Bragg grating component fiber.
In an application, one of the optical fibers is an amplifying fiber.
In an application, the amplifying fiber is a laser emission fiber.
In an application, the amplifying fiber is a Raman amplification fiber.
According to a feature, the optical system further comprises a second mode
converter of the invention adapted to convert said second transmission mode into
said first transmission mode.
The features and advantages of the invention emerge more clearly on reading the
following description, which is given by way of illustrative and non-limiting example
and with reference to the appended drawings, in which:
- Figure 1 is a diagram of an optical transmission system including mode
converters of the invention,
- Figure 2 is a diagram of a mode converter of the invention,
- Figure 3 shows an input microlens of the converter of the invention,
- Figure 4 is a simulated curve of the angular distribution of the far field intensity
for propagation modes LP01 and LP02 respectively in a single mode fiber and in
an HOM fiber,
- Figure 5 shows an arrangement in the Fourier plane equivalent to the converter
of the invention, and
- Figure 6 is a diagram showing the aperture ratio of the mode converter of the
invention.
The mode converter of the invention comprises spliced fiber portions. The first two
portions constitute an input microlens adapted to cause an input optical signal to
diverge and to collimate it. The next portion constitutes a phase shifting region
adapted to introduce an appropriate phase shift to modify the transverse distribution
of intensity in order to convert the propagation mode of the input signal into another
propagation mode. The next two portions constitute an output microlens adapted to
focus the output signal into an appropriate guide.
Since the fiber portions are spliced together, the converter is compact and losses
are minimized compared with a free space solution.
Figure 1 is a diagram of an optical transmission system including a mode converter
of the invention. An optical signal, which may be a WDM signal, is transmitted in a
transmission fiber, which is generally a single mode fiber, for example. In this case
the optical signal is transmitted in fundamental LP01 mode.
As explained above, it is generally necessary to compensate chromatic dispersion
on single mode fiber transmission lines. Such compensation can be made more
efficient by using a higher order mode compensation fiber, i.e. if the chromatic
dispersion of the signal is compensated in a higher order propagation mode, for
example in LP02 mode.
A certain length of higher order mode compensating fiber is therefore inserted into
the transmission line between two mode converters respectively adapted to convert
LP01 mode into a higher order mode accepted by the HOM fiber, and to convert the
higher order mode into LP01 mode, that is re-injected into a single mode fiber in
which chromatic dispersion is compensated.
As stated above, mode conversion must be efficient, in other words the overlap with
the converted mode must be as close as possible to 100%, in order to limit the
noise induced in the HOM fiber, which may limit its chromatic dispersion
compensation efficiency. Moreover, this double conversion must minimize the
optical power losses of the signal.
Figure 2 is a diagram of the mode converter of the invention. This in-fiber type of
converter is entirely implemented within a fiber, which strongly limits the optical
power losses of the signal.
The converter comprises fiber portions 10 constituting an input microlens, a fiber
portion 20 constituting a phase shifting region, and fiber portions 30 constituting an
output microlens. The microlenses 10 and 30 are described in more detail later with
reference to Figure 3. The phase shifting region is a fiber portion 20 having an index
profile, a core diameter e and a length L that are all predetermined. The expression
"phase shifting" means introducing at least one phase jump of specific value at a
given spatial position of the field.
Accordingly, the wavefront of an optical signal guided in a fiber 40 in a given first
propagation mode is expanded by the input microlens 10. The optical signal
propagating in the phase shifting region 20 is therefore no longer guided and may
be subjected to phase shifts that lead to optical mode conversion. The wavefront of
the converted mode is then focused by an output microlens 30 to match the
numerical aperture of the output fiber 50. An optical signal propagating in a second
given propagation mode may then be guided in the output fiber 50.
For example, a signal propagating in LP01 mode in a single mode transmission
fiber 40 is presented to the input of the converter. The wavefront of LP01 mode is
expanded and collimated by the input microlens 10. The wave propagating in the
phase shifting portion 20 is not guided, even though this portion has a core and
optical cladding that could serve as a waveguide. The input microlens 10 has
expanded the field of the incoming mode beyond the numerical aperture of the fiber
portion 20. The phase shifting region 20 therefore introduces appropriate phase
jumps, as described below, for example to convert LP01 mode to LP02 mode.
However, LP02 mode is no longer guided in the phase shifting region 20 and has a
mode diameter much greater than the numerical aperture of the HOM fiber 50. The
output microlens 30 then focuses the LP02 mode propagation to match the aperture
of the HOM fiber.
It must be understood that the same routing of the optical signal applies to the
opposite conversion; an HOM fiber 40 introduces a higher order propagation mode
signal, for example an LP02 mode signal, into the input microlens 10. The LP02
mode signal is then converted into LP01 mode in the phase shifting region 20. The
LP01 mode signal is then focused by the output microlens 30 onto a single mode
fiber 50.
Figure 3 shows an input microlens 10 of the mode converter of the invention. This
figure also shows (shaded) the expansion of the optical mode by the microlens.
The theory and properties of microlenses are described in "Design and
Performance of Expanded Mode Fiber Using Microoptics" by P. Chanciou et al. A
microlens consists of a fiber portion comprising a pure silica region 11 which
behaves like a divergent lens welded to a graded index region 12 which behaves
like a convergent lens. A component of this kind is available from Optogone.
One function of an input microlens is to cause the wavefront of the incoming optical
signal to diverge. The pure silica portion has no optical core and the optical mode
of the incoming signal is no longer guided in this portion. Another function of the
input microlens is to use the graded index region to collimate the divergent beam.
The function of an output microlens is to cause the wavefront of the output signal of
the phase shifting portion to converge and to focus it onto a fiber, maximizing the
overlap between the converted mode and the selected propagation mode in the
output fiber.
A microlens therefore has two bidirectional functions and behaves like a double
optical lens.
Accordingly, an input signal guided in a fiber is caused to diverge by the divergent
lens consisting of the pure silica region of the input microlens. The divergent beam
is then collimated by the convergent lens consisting of the graded index region of
the input microlens.
Similarly, an unguided but collimated optical beam may be caused to converge by a
first lens consisting of the graded index region of the output microlens. The beam is
then collimated by a second lens consisting of the pure silica region of the output
microlens in order to be introduced into a fiber with an appropriate mode field
diameter.
The length Ls of the pure silica region and the length LGRIN of the graded index
region are selected as a function of the input and output mode field diameters, i.e.
as a function of the core diameters of the fibers at the input and at the output of the
microlenses. These lengths Ls and LGRIN determine the focal lengths of the
equivalent lenses described above.
Accordingly, from a guided and confined input signal, for example a single mode
signal propagating in LP01 mode in a single mode fiber with a mode field diameter
of 9 urn, a microlens can supply an expanded collimated signal with a diameter from
5 Mm to 80 um.
Microlenses are used to convert the propagation mode of an expanded signal
propagating in the form of parallel beams in the phase shifting region 20. The
phase shifting region therefore behaves like a phase plate introducing spatially
appropriate phase jumps.
The fiber portion constituting the phase shifting region has a stepped refractive
index profile that may be rectangular, step-shaped, or of any other shape adapted
to produce a phase shift of more complex shape. Accordingly, depending on the
index profile, it is possible to introduce one or more phase jumps of different values
at different places in the signal propagation mode field.
For example, a phase jump of n can be introduced into the phase of the propagation
mode of an optical signal propagating in the above kind of fiber satisfying the
following equation, in which X, is the wavelength of the optical signal, L is the length
of the phase shifting portion and An is the index jump of the phase shifting portion:
X/2 = (An)L
For example, for a 1550 nanometers (nm) transmitted optical signal and a fiber
portion with an index jump of 10"2, the length of the phase shifting region must be
77.5 um. Depending on the profile of the fiber used for the phase shifting region,
the length L of the fiber may be from 25 um to 1000 um.
The position of the phase jump to be introduced is determined by the radial position
of the index jump of the fiber portion constituting the phase shifting region. If a
single phase jump is to be introduced, for example, to convert LP01 mode into LP02
mode, a fiber with a rectangular index profile with an appropriate core diameter e
may be used.
Thus Figure 4 shows the intensity of the far field as a function of the angle 9 for
various propagation modes in single mode fibers or HOM fibers.
The transfer function between LP01 mode and the required mode, which is LP02
mode in the present example, determines an angle 61 at which it is necessary to
introduce a phase jump of n, for example, to cause the far field to correspond to
LP02 mode. In the Figure 4 example, this angle 61 is 5.68°. Other phase jump
profiles may be envisaged to achieve mode conversion with an enhanced overlap.
Figure 5 shows an optical arrangement equivalent to the mode converter of the
invention. A transmitter 4 corresponds to a fiber 40 transmitting an input optical
signal propagating in a first mode. A lens 1 of focal length f corresponds to the
input microlens, a phase plate 2 corresponds to the phase shifting region 20, and a
lens 3 of focal length f corresponds to the output microlens. The phase plate 2 has
a central region of diameter e with a refractive index jump relative to the
surrounding region, which corresponds to the core of the fiber 20. The diameter e
of this region determines the angular position of the phase jump to be introduced.
Thus the diameter e of the core of the fiber constituting the phase shifting region is
linked to the focal lengths f and f of the first and second microlenses. This
expansion of the incoming mode in the converter must be controlled so that the
phase jump introduced by the index jump is correctly positioned relative to the
angular distribution of the incoming mode.
Figure 6 shows the aperture ratio of the mode converter of the invention. The
phase plate is in the Fourier space, the input fiber (for example a single mode fiber)
is at the point A and the output fiber (for example an HOM fiber) is at the point B1.
The respective half-angles 0 and 0' at which the phase plate is seen by the fibers at
the points A and B' are determined. The mode coupling efficiency is maximized if
the focal lengths of the two microlenses 10 and 30 are optimized.
The ratio a = sin(0)/sin(9') is then defined. Maximum coupling is observed when a is
a linear function of 0. By varying the parameter a as a function of the efficiency of
coupling between LP01 and LP02 modes for a fiber corresponding to the Figure 4
field profiles, the equation a = a0 + b, with a = 0.13 and b = 0.09 has been
established.
Coupling LP01 mode of a 1550 nm optical signal to LP02 mode with a converter of
the invention has been simulated. The angles 0' and 0! were both set to
approximately 5.68°, which gave 9 = 2°. A 775 urn portion of fiber with a
rectangular index profile and an index jump of 10~3 was used for the phase shifting
region. The diameter e of the core of the fiber constituting the phase shifting
portion was a function of the focal length f of the output microlens 30 defined by the
equation e = 2f'*tan 6'.
It was calculated that 84% of the power of the LP01 mode signal in the single mode
fiber was coupled into the HOM fiber.
It must be understood that the parameters of the fiber portions constituting the
mode converter of the invention must be determined in accordance with the
required application. Thus the converter of the invention for converting LP01 mode
into LP02 mode for a chromatic dispersion compensation application in an HOM
fiber will not have the input and output microlenses with the same parameters (L$
and LQRIN) or the same index jump profile of the phase shifting region fiber as a
converter of the same type intended to convert from LP02 mode to LP01 mode at
the output of an HOM fiber or to convert from LP01 mode to LP03 mode, or to effect
any other mode conversion that may be required.
Conversion to LP03 mode may be required for an application to higher order mode
pumping or to chromatic dispersion compensation in LP03 mode, for example.
In particular, the fiber portion constituting the phase shifting region may have a
stepped refractive index profile or any other profile adapted to introduce an
appropriate shift in the phase of the propagation mode to be converted.
The mode converter of the invention has circular symmetry. It is consequently
adapted to effect mode conversion on any circularly symmetrical mode LPOm
whatsoever.
The person skilled in the art will be able, given the information provided herein by
way of example, to determine parameters of the input and output microlenses and
the index profile of the fiber to be used for the phase shifting region. Certain
parameters may be fixed by commercial constraints, such as the commercial
availability of a particular microlens with a particular focal length or by the use of
fiber portions with a fixed core diameter e and a fixed index jump An.
Figure 1 is described above with reference to an example of using the mode
converter of the invention in an application to chromatic dispersion compensation
using an HOM fiber. However, other applications may be envisaged, for example
gain flattening in a higher order mode by means of a slant Bragg grating.
he mode converter of the invention may equally be used for an application to
pumping in amplifying fibers or laser emission fibers with good amplification
efficiencies.

CLAIMS
1. An in-fiber mode converter for converting the propagation mode of a received
optical signal, said converter comprising:
fiber portions (10) constituting an input microlens adapted to expand the
wavefront of said optical signals,
a fiber portion (20) constituting a phase shifting region adapted to convert
said propagation mode of said optical signal into another propagation
mode, and
fiber portions (30) constituting an output microlens adapted to transmit
said optical signal in said other propagation mode.
2. A mode converter according to claim 1, characterized in that said fiber
portions constituting an input microlens are spliced and said fiber portions
constituting an output microlens are spliced.
3. A mode converter according to claim 1 or claim 2, characterized in that the
fiber portion constituting the phase shifting region has a predetermined length
(L), a predetermined refractive index profile and a predetermined core
diameter (e).
4. A mode converter according to any preceding claim, characterized in that the
index profile of said fiber portion constituting the phase shifting region
comprises one or more jumps (An).
5. A mode converter according to any preceding claim, characterized in that the
length (L) of said fiber portion constituting the phase shifting region is from
25 umto 1000 Mm.
6. A mode converter according to any preceding claim, characterized in that the
core diameter (e) of said fiber portion constituting the phase shifting region is
from 0.5 umto 100 urn.
7. An optical system comprising:
a first optical fiber (40) adapted to transmit an optical signal propagating
in a first propagation mode,
a mode converter according to any one of claims 1 to 6 adapted to
convert said first propagation mode into a second propagation mode, and
a second optical fiber (50) adapted to transmit an optical signal
propagating in said second propagation mode.
8. An optical system according to claim 7, characterized in that the first and
second propagation modes are different circularly symmetrical modes LPOm.
9. An optical system according to claim 7 or claim 8, characterized in that one of
the optical fibers (40, 50) is a multimode fiber.
10. An optical system according to any one of claims 7 to 9, characterized in that
one of the optical fibers (40, 50) is a higher order mode (HOM) fiber.
11. An optical system according to any one of claims 7 to 10, characterized in that
one of the optical fibers (40, 50) is a chromatic dispersion compensating fiber
(DCF).
12. A system according to any one of claims 7 to 10, characterized in that one of
the optical fibers (40, 50) is a Bragg grating component fiber.
13. An optical system according to any one of claims 7 to 10, characterized in that
one of the optical fibers (40, 50) is an amplifying fiber.
14. An optical system according to claim 13, characterized in that the amplifying
fiber is a laser emission fiber.
15. An optical system according to claim 13 or claim 14, characterized in that the
amplifying fiber is a Raman amplification fiber.
16. An optical system according to any one of claims 7 to 15, characterized in that
it further comprises a second mode converter according to any one of claims
1 to 6 adapted to convert said second transmission mode into said first
transmission mode.