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JPH11262093

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DESCRIPTION JPH11262093
[0001]
FIELD OF THE INVENTION The present invention relates generally to methods and systems for
suppressing polarization hole-burning in rare-earth doped fiber amplifiers. More particularly, the
present invention relates to methods and systems for varying the polarization state of an input
signal using acousto-optic modulation.
[0002]
BACKGROUND OF THE INVENTION Long distance optical communication systems are known to
suffer from various polarization dependent effects that cause a reduction in the signal to noise
ratio (SNR) of the system. Polarization hole burning (PHB) is one of the polarization dependent
phenomena that greatly impairs the performance of erbium doped fiber amplifiers (EDFAs)
placed in optical fiber communication systems. PHB occurs when a strongly polarized light signal
is input into the EDFA, causing anisotropic saturation of this amplifier. This effect, which is
related to the dynamics of population inversion of the EDFA, reduces the gain of the EDFA for
light having the same polarization as the saturation signal. Thus, for PHB, a signal having a
polarization state (SOP) orthogonal to the saturation signal will have a larger gain than the
saturation signal.
[0003]
In a saturated EDFA column, amplified spontaneous emission (ASE) noise is more for polarized
light orthogonal to the saturated information signal than for polarized light parallel to the signal.
05-05-2019
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It can be accumulated quickly.
[0004]
ASEs that are orthogonal to the saturation signal are integrated in each amplifier stage of the
transmission line.
The accumulation of orthogonal ASEs reduces the SNR of the optical transmission system, thus
creating the possibility of errors in the received data stream. Thus, in order to maintain a system
with good SNR characteristics, it is desirable to reduce the impact of PHB in the amplified system.
[0005]
Operating the EDFA in the gain compression state causes an undesirable PHB effect. The gain
compression Cp is the value of the optical signal in the operating power level (G) state with
respect to the value obtained by the optical signal having low optical power (i.e. a non-saturating
signal to obtain a maximum gain called Go) of the propagating signal The difference in gain of
the amplifier in the operating state is shown. The gain of the operating state of the amplifier in
decibels can be measured using the saturation signal of the input power Si according to the
following equation:
[0006]
G = So-Si (1) where, So is the saturated output power. Thus, the amount of gain compression is:
[0007]
Cp = Go-G (2) On the other hand, the gain in orthogonal polarization can be measured using a
probe signal having an input polarization orthogonal to the saturation signal according to the
following equation.
[0008]
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2
Where Pi and Po are the input and output powers of the probe signal.
In equation (3), ΔG corresponds to the PHB value.
[0009]
Furthermore, the amount of PHB increases as the amplifier goes deeper into gain compression.
FIG. 1 is a graph of experimental measurements showing the relationship between the amount of
gain compression and the amount of PHB in an EDFA. As shown in this graph, the amount of PHB
is only 0.08 dB for a single EDFA operating at 3 dB gain compression. However, as gain
compression increases, PHB also increases. When the EDFA operates in saturation with Cp equal
to about 9-10 dB, PHB is more pronounced and can be quantified to as low as 0.2 dB per EDFA.
[0010]
Furthermore, the amount of PHB in the EDFA depends on the degree of polarization (DOP) of the
saturated signal passing through the amplifier. FIG. 2 is a graph of experimental results for an
EDFA operating at 10 dB gain compression. As can be seen from this graph of FIG. 2, as the
degree of polarization of the saturated signal decreases from 100%, the gain variation caused by
PHB also decreases. This fact suggests that the adverse effects caused by PHB can be mitigated
by changing the polarization state. The PHB can be reduced by scrambling the SOP of the
transmitted optical signal at a rate much higher than 1 / ts. Here, ts is anisotropic saturation
time. Since the EDFA takes about 0.5 msec to reach gain steady state after SOP variation of the
signal, the SOP of the signal must be scrambled at about 10 kHz or higher to overcome the PHB
phenomenon. You must.
[0011]
Several proposals have been made in the literature for reducing the PHB effect in optical
transmission systems. In European Patent No. 615356 and US Patent No. 5,491,576, by
simultaneously inputting two optical signals having different wavelengths and similar power
levels and polarizations substantially orthogonal to each other into the same transmission path
05-05-2019
3
Techniques for reducing non-linear signal degradation are disclosed. Thus, the resulting
transmitted signal as a whole is essentially unpolarized and the effects of deleterious
polarization-dependent effects in the transmission system are reported to be minimized. The
combined signal is modulated by a polarization independent light modulator, whereby both
wavelength components of the combined signal carry the same data, ie, the respective
wavelength paths are combined before being combined, Modulated separately. Similar
disclosures for systems using two signals of different wavelengths can be found in Bergano et al.,
"Polarization Hole-Burning in Erbium-Doped Fiber-Amplifier Transmission Systems," ECOC '94,
pp. 621It can be seen at-628.
[0012]
U.S. Pat. No. 5,107,358 describes a method and apparatus for transmitting information by means
of a coherent light detector and detecting it after propagating through a waveguide. In particular,
FIG. 3 shows a transmitter with a light source producing a single carrier signal applied to the
modulator. An optical splitter produces two versions of the modulated signal. The first version is
provided to a first polarization controller, while the second version is provided to a second
polarization controller via a frequency shift circuit. The polarization of this signal is adjusted by
the second controller to be orthogonal to the polarization of the signal from the first controller.
Both signals, polarized orthogonally, are then combined by a polarization selective coupler to be
transmitted.
[0013]
It should be understood that in all the examples described in the '358 patent, the two optical
carrier frequencies are typically separated by two to three times the bit rate in hertz. Applicants
have achieved the same magnitude (2 bit rates) by superposing an optical signal with an optical
signal having orthogonal polarizations and whose frequency is shifted by 2 to 3 times the bit
rate. It has been found that an optical signal having a bandwidth of double to triple is obtained.
The bandwidth of the filter used at the receiver should be greater than or equal to the bandwidth
of the signal. Due to this large filter bandwidth, the noise at the receiver will be very high in the
case of long-haul amplified optical communication systems, so that, in particular, the bit rate is
better than 1 Gbit / s. Signal reception becomes impossible.
[0014]
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4
For example, US Pat. No. 5,327,511 or Heismann et al., "Electro-optic polarization scrambler for
optically amplified long-haul transmission systems," ECOC '94, pp. 629Generate a carrier signal
having a single wavelength, modulate the carrier signal with data, and transmit the modulated
carrier signal through the polarization modulator or scrambler as described in It is known to help
eliminate the effects of polarized hole burning. These documents use a lithium-niobate-based
electro-optic modulator with a single path to pass the carrier wavelength and modulate its
polarization at modulation frequencies of eg 40 kHz and 10.68 GHz Is disclosed. These
polarization modulators or scramblers produce highly randomized polarization states for the
signal. These devices produce output polarization according to the control signal and use
relatively high levels of power.
[0015]
Electronics Letters, Vol. 30, No. 18, p. 1500As described in -1501, a transducer is placed at one
third of the interaction length, whereby tuning of the appropriate wavelength to suppress
polarization hole burning in the EDFA An acousto-optic Ti: LiNbO3 device is known in which a
polarization-independent depolarizer is formed, which consists of two or more sections of a TETM converter capable of The author of this paper proposes a two-stage depolarizer with a
residual polarization of less than 0.03.
[0016]
Similarly, an acousto-optic device is also known which provides polarization rotation to an input
optical signal and modulates that signal using sound waves from a modulation source. Related
publications include, for example, European Patent No. 737880, European Patent No. 757276,
M. Rehage et al., "Wavelength-Selective Polarization Analyzer with Integrated Ti: LiNbO3
Acousto-Optical TE-TM Converter," Electronic Letters , vol. 30, no. 14, July 7, 1994.
[0017]
Applicants have found that known techniques for minimizing polarization hole burning by
rotating the polarization of the carrier signal with an electro-optic modulator require high power
levels. I found it too. We have also discovered that known techniques for providing polarization
rotation signals to erbium-doped fiber amplifiers require much wider bandwidth than is
05-05-2019
5
practically possible at the receiver of the optical transmission system. Furthermore, systems
using two sources of different wavelengths are difficult to implement due to the difficulties in
selecting the sources and stabilizing their wavelengths. Thus, WDM transmission by this system
is very complicated and expensive.
[0018]
SUMMARY OF THE INVENTION In accordance with the present invention, an optical signal useful
for reducing polarization hole burning in a rare earth doped fiber amplifier by converting an
optical carrier signal having a characteristic wavelength into a polarization rotating optical
carrier. A transmission system is developed. This system uses an acousto-optic modulator that
modulates a portion of the optical carrier. This acousto-optic modulator causes orthogonal
rotation of the polarization of that part of the optical carrier. A polarization beam combiner then
combines the modulated quadrature signal from the acousto-optic modulator with the remainder
of the original optical carrier signal to produce a polarization rotating optical carrier. Polarization
rotating optical carriers are inserted into the optical communication system and consequently
used inside of the rare earth doped fiber amplifier.
[0019]
In order to achieve the effects of the present invention and in accordance with the object of the
present invention, as described herein by way of example and as outlined, converting an optical
carrier having a characteristic wavelength and an initial state of polarization to a polarization
rotating optical carrier An apparatus for reducing polarization hole burning in a rare earth-doped
fiber amplifier in an optical communication system includes an acousto-optic modulator and a
polarization beam combiner. An acousto-optic modulator comprises a carrier input optically
coupled to receive a first portion of a polarized light carrier, a modulation input electrically
coupled to receive an RF modulation frequency, and a modulator output. And. Furthermore, the
acousto-optic modulator comprises a circuit for orthogonally converting the polarization of the
polarized light carrier and shifting the frequency of the polarized light carrier by the modulation
frequency. A polarization beam combiner orthogonally transforms the SOP (polarization state)
and receives a frequency shifted polarization signal, a first input optically coupled to a second
portion of the polarized optical carrier. A second input optically coupled to receive and an output
optically coupled to the rare earth doped fiber amplifier downstream of the optical
communication system.
05-05-2019
6
[0020]
In another aspect, the invention provides a light source transmitting an optical carrier having an
initial state of polarization, a splitter, a modulation source providing a modulation signal, an
acousto-optic modulator, an attenuator, a polarization beam combiner An optical transmitter for
reducing polarization hole burning in a rare earth-doped fiber amplifier in an optical
communication system comprising: A splitter is disposed downstream of the light source and has
an input, a first output, and a second output, and the optical carrier received at the input has a
first output and a second output. Split between. The acousto-optic modulator has a carrier input
optically coupled to the first output of the splitter, a modulation input electrically coupled to the
RF modulation source, and a modulation output. The acousto-optic modulator further includes a
circuit for orthogonally converting the polarization of the optical carrier and frequency shifting
the optical carrier by the frequency of the modulation signal. A polarized beam combiner
includes a first input optically coupled to receive an orthogonally polarized transformed
frequency shifted optical signal, a second input optically coupled to an attenuator, and an optical
communication system. And an output optically coupled to the rare earth-doped fiber amplifier
downstream of
[0021]
In another aspect, according to the present invention, there is provided a method of suppressing
polarization hole burning in a rare earth doped fiber amplifier in an optical communication
system, the method comprising: transmitting an optical carrier signal to a first subcarrier signal
and a second sub signal. The step of separating into the carrier signal and the polarization of the
first subcarrier signal are orthogonally rotated, and the first subcarrier signal is modulated using
the RF modulation frequency to create the quadrature modulated subcarrier signal. And
providing a method. The method further comprises the steps of combining the orthogonally
modulated subcarrier signal and the second subcarrier signal to produce a polarization rotation
carrier signal, and the polarization rotation carrier signal downstream of the optical
communication system, rare earth doped. Sending to the fiber amplifier.
[0022]
In yet another aspect, the invention provides an acousto-optic modulator for rotating the
polarization of an optical carrier signal, comprising: a substrate of birefringence and photo-elastic
material; A splitter having an optical carrier signal from the first port, a splitter having an input
coupled to the first port, a first output, and a second output, and an end coupled to the first
output of the splitter A first optical waveguide branch, a second optical waveguide branch having
05-05-2019
7
one end coupled to the second output of the splitter, and at least a first optical waveguide branch
on the substrate A portion of the acoustic waveguide, a sound generator on the substrate and
disposed on at least a portion of the acoustic waveguide, a first input coupled to the other end of
the first optical waveguide branch, and A second coupled to the other end of the second
waveguide branch An acousto-optic modulator is provided that includes a polarization splitter
having two inputs and an output.
[0023]
It is to be understood that both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not restrictive of the scope of the
invention as defined by the appended claims. .
The following description, as well as the practice of the present invention, set forth and suggest
additional advantages and purposes of the present invention.
[0024]
Reference will now be made to various embodiments in accordance with the present invention,
examples of which are illustrated in the accompanying drawings and will be apparent from the
description of the invention. In the drawings, the same or similar components are denoted by the
same reference numerals as much as possible, even if the drawings are different.
[0025]
An optical communication system according to the invention, generally designated by the
reference numeral 300 in FIG. 3, includes a polarization modulator that reduces polarization hole
burning in a rare earth doped fiber amplifier. The optical communication system 300 includes a
light source (OS) 305 for transmitting an optical carrier, a polarization-fixing device (PC) 310,
and a polarization modulator 312. Polarization modulator 312 includes a splitter 315, an RF
modulation source 325 that provides a modulation signal, an acousto-optic modulator 320, an
attenuator 330, and a polarization beam combiner 335.
05-05-2019
8
[0026]
A light source transmitting an optical carrier, shown at 305 in FIG. 3, is comprised of a laser
diode or similar element that produces an optical signal having a relatively fixed wavelength. The
light source 305 generates a relatively fixed wavelength as a carrier signal that may be
modulated by various techniques within the optical communication system 300, as described in
more detail below. For example, the light source 305 is a model no. An AT & T / DFB
semiconductor laser with 246 AH, operating at a nominal wavelength of 1556.7 nm in vacuum
and having a line bandwidth of less than 100 MHz.
[0027]
Downstream of light source 305, optical communication system 300 includes an electro-optic
modulator (EOM) 304 that modulates the information signal onto the carrier signal generated by
light source 305. As will be readily appreciated by those skilled in the art, the electro-optic
modulator (or data modulator) 304 is a Mach-Zehnder interferometer or equivalent device, RF
source 345. Provides amplitude modulation on the optical carrier according to the
electromagnetic signal introduced by The electromagnetic signal is, for example, an RF signal
including data to be transmitted to the optical communication system 300. The use of data
modulator 304, although optional for the implementation of the present invention, provides the
feature of inserting information onto the carrier signal. Instead of the data modulator 304, it is
also possible to modulate the light source 305 directly. For wavelength division multiplexed
(WDM) transmission, multiple light sources 305 of different emission wavelengths or multiple
wavelength sources can also be used.
[0028]
Downstream of the light source 305, and in some cases further downstream of the data
modulator 304, a polarization fixing device 310 is fixed, which corresponds to the optical carrier
from the light source 305, to the preferred input SOP of the polarization modulator 312. It is
optically coupled to convert it to an optical carrier with SOP. Of course, if a data modulator 340
is used in this optical communication system 300, the polarization fixing device 310 will convert
the optical carrier modulated with data by the data modulator 340 to a certain SOP optical
carrier. Convert. The polarization fixing device 310 is preferably a polarization controller, which
has a series of loops of optical fibers having angular adjustment and providing selected and fixed
polarization to the signal output from the polarization controller. Have. Polarization controllers of
05-05-2019
9
this type are already known in the art but are commercially available or can be made if the
skilled person desires. Alternatives to the polarization fixing device 310 include polarization
maintaining fibers, polarization maintaining splitters, polarization stabilizers, and the like. It is
also possible to select another configuration, which is not mentioned here, as a polarization fixing
device 310, whereby the output of the device 310 provides an optical signal with a fixed
polarization.
[0029]
The optical communication system 300 for reducing polarization hole burning further includes a
polarization modulator indicated by reference numeral 312 in FIG. The polarization modulator
312 includes a splitter 315 located downstream from the polarization fixing device 310. Splitter
315 has an input 317, a first output 318, and a second output 319, for example, resulting in first
and second subcarrier signals. Preferably, the splitter 315 is a 3 dB coupler consisting of a fused
fiber variety that splits the polarized optical carrier received at the input 317 from the
polarization fixing device 310 between the output 318 and the output 319. is there.
[0030]
Furthermore, the polarization modulator 312 according to the invention further comprises an
acousto-optic modulator (AOM) 320 arranged downstream of the splitter 315. Acousto-optic
modulator 320 has carrier input 321 optically coupled to the first output 318 of splitter 315. In
this way, part of the polarized light carrier sent from the polarization fixing device 310 is
received by the acousto-optic modulator 320 via the input port 321. Acousto-optic modulator
320 also includes a modulation input 322, a modulator output 323, and an additional output
324. The modulation input 322 is optically coupled to a modulation source (RF) 325 that
provides a relatively fixed electromagnetic frequency to the acousto-optic modulator 320. AOM
320 is preferably a waveguide device fabricated on LiNbO 3, for example, S. Schmid et al., Post
Deadline Paper ThP1, pp. 21-24, Proceedings of the 7th European Conference on Integrated
Optics, Delft, The Netherlands, April 3-6, 1995. For a waveguide AOM fabricated on a LiNbO3
substrate, the frequency ν of the RF signal is, for example, about 172.6 MHz for an optical
signal having a wavelength λ = 1556.7 mm. The change in RF frequency Δν (tuning slope)
required to tune the AOM after the wavelength change Δλ of the optical signal is Δν / Δλ ≒
-120 kHz / nm in the above example. If multiple optical signals of different wavelengths are input
to AOM 320, modulation source 325 effectively provides a corresponding number of modulated
signals, each tuned to one optical signal.
05-05-2019
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[0031]
As explained more fully below, the acousto-optic modulator 320 modulates the carrier polarized
by the modulated signal received at the modulation input 322, thereby orthogonally converting
the polarization of the polarized carrier . That is, the acousto-optic modulator 320 performs TE to
TM or TM to TE conversion of the received polarized carrier signal. When the polarization fixing
device 310 sets the polarization of the carrier signal to the TE (transverse electric) mode, the
acousto-optic modulator 320 orthogonally rotates the TE mode to the TM (transverse magnetic)
mode or vice versa. . Also, the acousto-optic modulator shifts the optical frequency of the
polarized carrier signal having the frequency of the RF modulation signal.
[0032]
Coupled to attenuator 330 is a second output 319 of splitter 315. Attenuator 330 comprises an
adjustable attenuator or a fixed attenuator, depending on the preferred design implementation.
Attenuator 330 adjusts the size of the portion of the polarized optical carrier received from the
second output 319 of splitter 315, whereby this second portion is transmitted from acousto-optic
modulator 320 via output 323. It has a function of making it substantially equal to the
magnitude of the output quadrature modulated signal. Consequently, the polarized beam
combiner (PBC) 335 of FIG. 3 receives the orthogonally shifted and modulated polarized signal
from the acousto-optic modulator 320 and a portion of the original polarized optical carrier from
the attenuator 330 Receive The two signals received by the polarized beam combiner 335 have
approximately the same magnitude. As mentioned above, attenuator 330 is used to equalize the
magnitudes of the two signals received by polarized beam combiner 335. Also, the splitter 315 is
an imbalance splitter or coupler specifically designed with the ratio between the first output 318
and the second output 319, whereby it is eventually received by the polarization beam combiner
335 Can be made to have substantially the same magnitude.
[0033]
As already mentioned, the polarization beam combiner 335 is located downstream of both the
acousto-optic modulator 320 and the optional attenuator 330. The polarized beam combiner 335
has a first input 336 optically coupled to receive an orthogonally shifted modulated polarized
signal from the output 323 of the acousto-optic modulator 320. Similarly, polarized beam
combiner 335 has a second input 337 optically coupled to receive a portion of the polarized
05-05-2019
11
optical carrier from splitter 315, which may be routed through attenuator 330. . In a known
manner, the polarization beam combiner 335 is a quadrature polarization converted frequency
shifted polarized signal received from the acousto-optic modulator 320 and a portion of the
original polarized light signal received from the splitter 315. Are combined to produce a
polarized rotational carrier signal. The polarization rotation carrier signal has substantially the
same wavelength as the original carrier signal generated by the light source 305, but has a
polarization state that fluctuates at a rate proportional to the modulation frequency generated by
the modulation source 325. There is. In the preferred embodiment, this modulation frequency is
approximately 172.6 MHz. As a result, the overall polarization modulator of the present
invention, defined by the splitter 315, the acousto-optic modulator 320, the attenuator 330 and
the polarization beam combiner 335 Change at high speed. This rate of change of polarization
state exceeds the response time of the erbium-doped fiber amplifier defined by 1 / ts. Here, ts is
an anisotropic saturation time. Typically, for erbium-doped fiber amplifiers, tsts0.5 μs.
[0034]
The polarized beam combiner 335 is, for example, a model PB100-1L-1S-FP according to JDSFITEL. The polarized beam combiner 335 also has an output 338 optically coupled to at least one
rare earth doped fiber amplifier 340 located downstream of the optical communication system
300. The rare earth doped fiber amplifier is preferably an erbium doped fiber amplifier. One, two
or more amplifier stages can be used. Multiple amplifiers separated from one another by links of
long distance transmission fibers (not shown) can be used. In an experimental setup, a polarizing
filter (Glen-Thomson prism) was placed downstream of the polarizing beam combiner 335 to
detect the rotation of the signal's polarization. However, polarizing filters are generally not
included in the presently disclosed apparatus for reducing polarized hole burning.
[0035]
As in prior art optical communication systems such as 300, a receiver system 350 is located at
the end of the communication system 300 to receive and detect information transmitted along
the optical path. Receiver 350 includes demultiplexing circuitry for wavelength division
multiplexer applications and functions to detect and demodulate an optical carrier signal
containing data modulated by data modulator 304 upstream of optical communication system
300.
[0036]
05-05-2019
12
A preferred embodiment for polarization modulator 312 is illustrated in FIG. Although an
integrated acousto-optic device as shown at 312 in FIG. 4 is known, its operation is to propagate
light in a waveguide obtained on a substrate of birefringent photoelastic material Based on the
interaction between the signal and the sound waves propagating on the surface of the substrate
which originates via a suitable transducer. The interaction between the polarized light signal and
the sound wave causes a conversion of the polarization of the signal, ie a rotation of the
polarization of the TE and TM components of the light signal.
[0037]
The polarization modulator 312 of FIG. 4 generally comprises a substrate 410, an optical coupler
315 formed using optical waveguides in the substrate 410, an acoustic waveguide 420 on the
substrate 410, an electroacoustic transducer 430, and , A second optical waveguide branch 450,
an acoustic cladding 460, and a polarized beam combiner 335.
[0038]
The substrate 410 is preferably a crystal of lithium niobate (LiNbO3), cut perpendicular to the xaxis, and the optical waveguide branches 440 and 450 are oriented along the y-axis of the
crystal.
Other birefringent and photoelastic piezoelectric materials such as LiTaO 3, TeO 2, CaMoO 4 can
also be used.
[0039]
The coupler 315 is formed by an optical waveguide in the substrate 410 and has an input 317
that can be connected to an optical fiber (not shown) from the upstream component of the
optical communication system 300, such as a polarization locking device. The output polarization
of the polarization fixing device 310 is preferably selected to match the TE or TM propagation
mode of the optical waveguides 440, 450 of the polarization modulator 312. The coupler 315
splits its light path into a first light branch 440 at a first output 318 and a second light branch
450 at a second output 319. The coupler 315 is substantially independent of polarization.
05-05-2019
13
[0040]
The first light branch 440 passes through the acoustic waveguide 420 to form an acousto-optic
modulator. The second optical waveguide branch 450 bypasses the acousto-optic modulator and
recombines with the first optical waveguide branch 440 in the polarization beam combiner 335.
[0041]
An electroacoustic transducer 430 is disposed within the acoustic waveguide 420 and in
communication with the first optical waveguide branch 440, thus forming an acoustic converter.
The electroacoustic transducer 430 is formed by interdigital electrodes capable of generating
radio frequency (RF) surface acoustic waves. An optical signal received at the input port 317 of
the coupler 315 and propagating along the first optical waveguide branch 440 interacts with the
acoustic wave propagating through the acoustic waveguide 420. The acoustic waves in the
acoustic waveguide 420 are created such that the intensity profile of the surface acoustic wave
has a peak at the central portion of the acoustic waveguide 420 and two troughs at the end of
the same waveguide. The optical signal propagating along the first acoustic waveguide branch
440 interacts with the acoustic wave, but this acoustic wave is increased by half along the path
to an area with a preselected interaction length. The other half has decreasing strength. The
acoustic waveguide 420 is circumscribed by acoustic cladding, but the velocity of the acoustic
wave generated by the electroacoustic transducer 430 is higher than in the acoustic waveguide
420.
[0042]
Polarized beam combiner 335 is preferably formed by an evanescent wave polarization splitter
or directional coupler comprising a central optical waveguide having a pair of input waveguides
336 and 337. The operation of the polarization splitter / combiner 335 is described in columns
12-14 of EP 737 880. This document is incorporated herein by reference. Outputs 338 and 339
provide orthogonally separated signals for output from modulator 312 and to output optical
fibers for transmission downstream of optical transmission system 300.
[0043]
05-05-2019
14
The operation of the polarization modulator 312 of FIG. 4 according to the invention is as
follows. For example, if a suitable modulation signal, such as a 172.6 MHz RF signal, is provided
from the modulation source 325 to the electroacoustic transducer 430, then the transducer 430
may have a driving acoustic frequency corresponding to an optical resonant wavelength,
Generating an RF surface acoustic wave. At this resonant wavelength, a conversion of
polarization from TE to TM or from TM to TE takes place. An optical signal enters the
polarization modulator 312 from the polarization fixing device 310 with a fixed change of TE or
TM. The received optical signal is converted into a corresponding quadrature component as it
propagates through the first optical waveguide branch 440. That is, if the received signal has TE
polarization, that polarization is rotated to the orthogonal component TM. Or the opposite is true.
Also, these signals are subject to a frequency shift whose absolute value is equal to the frequency
of the RF signal.
[0044]
A polarization splitter or combiner 335 combines the modulated optical signal from the first
optical waveguide branch 440 with the unmodulated optical signal from the second optical
waveguide branch 450. Outputs 338 and 339 provide signals that are orthogonally separated. As
a result of combining the polarization-modulated and frequency-shifted optical signal and the
unmodulated signal, an output 338 produces an optical signal having a rotational state of
polarization. As already mentioned, this rotational state of polarization occurs, in part, at a rate
determined by the RF source 325, but preferably at a rate greater than 1 / ts. Where ts is the
anisotropic saturation time of a fiber amplifier such as the amplifier 340 of FIG. 3 downstream of
the polarization modulator 312. Consequently, the polarization modulator 312 illustrated in FIG.
3 is a simplified and effective implementation of an acousto-optic modulator that generates an
optical signal that rotates the polarization to help suppress hole burning in the EDFA. provide.
[0045]
In the following, the experiments concerning the present invention described above and the
results thereof will be described. FIG. 5 illustrates a test apparatus 600 for experimentally
analyzing the optical communication system and transmitter of FIG. As shown in FIG. 5, a light
source 610 in the form of a laser diode (AT & T model 246AH) operating at a nominal
wavelength of 1556.7 nm in vacuum provided the optical carrier signal to the polarization
controller 620. The polarization controller 620 is a series of fiber optic loops and is configured to
have an angular displacement that controls the polarization of the optical carrier received from
05-05-2019
15
the laser source 610. The output from polarization controller 620 is optically coupled to
polarization modulator 630, which consists of the components of polarization modulator 312.
[0046]
At the output of the polarization modulator 630, a test apparatus, indicated generally by the
reference numeral 600 in FIG. 5, divided the output signal from the polarization modulator 630
at a ratio of 90:10. Ten percent (%) of the output of the polarization modulator is separated by
the coupler 640 and provided via the polarization filter 650 and the photodiode 660 so that it
can be detected and analyzed in the oscilloscope 670 It has become possible. Coupler 640
directed 90% of the output from polarization modulator 630 to attenuator 675. Another coupler
680 with a split ratio of 90:10 was placed downstream of attenuator 675 and separated 10% of
the output signal from attenuator 675 to power meter 675 for detection. Coupler 680 splits 90%
of the output signal from attenuator 675 and sends it to erbium-doped fiber amplifier 690. The
amplifier 690 was a two-stage EDFA pumped with a 1480 nm laser diode. For an input power of 15 dBm, the amplifier produces an output power of 9 dBm, gain G = 24 dB, Gc = 12 dB, Nf = 6.5
dB, γ = 0.92 and Psat = 0.94 μW. Had. After amplifying the received polarization rotation
carrier signal from coupler 680, the EDFA sent this signal via the polarization controller 697 to
the optical signal analyzer 698 for detection and analysis. A polarization controller 697 has set
the polarization of the amplified signal received from the EDFA 690 to be acceptable by the
optical signal analyzer 698.
[0047]
FIG. 6 graphically illustrates the test results obtained by the optical signal analyzer 698 under
various test conditions and is generally labeled 700. Signal trace 710 shows the spectrum
received at the output of the EDFA if modulation and rotation of the signal polarization were not
realized. In other words, signal 710 in FIG. 6 illustrates the output from EDFA 690 when the
input signal to the amplifier has a degree of polarization equal to 100%. Signal trace 720 shows
the output from EDFA 690 when the degree of polarization of the input signal is 36%. Signal
trace 730 shows the output signal of the EDFA when the degree of polarization of the input
signal received downstream of polarization modulator 630 was 6%.
[0048]
05-05-2019
16
As shown in FIG. 6, amplified spontaneous emission (ASE) noise is greatest at the signal 710 with
100% polarization and at the signal trace 730 with the input signal equal to 6% polarization.
small. Comparing the two results in signal traces 710 and 730, it can be seen that the ASE noise
is reduced by about 0.24 dB when polarization modulator 630 is used. This reduction in ASE
noise approximately corresponds to the amount of gain variation caused by polarized hole
burning present in an EDFA with a polarized saturation signal. In particular, FIG. 2 shows that for
10 dB gain compression, the polarization hole burning in decibel units corresponds to about 0.2
dB for a degree of polarization equal to 100%. Thus, polarization modulator 630 significantly
reduces signal degradation caused by polarization hole burning.
[0049]
Applicants have also determined that the invention described above is effective in reducing
polarization hole burning in wavelength division multiplexed (WDM) optical transmission
systems. As will be readily appreciated by those skilled in the art, in WDM systems, multiple light
sources generate carrier frequencies for channels in a transmission system. One or more of the
channels are modulated by the information, and the channels are multiplexed and transmitted
over a common optical fiber. Repeaters or optical amplifiers along the transmission path raise
channel levels for transmission over long distances. At the receiver end, a demultiplexer
separates the channels into their respective paths, and a receiver obtains modulated information
from a particular channel. For such WDM systems, polarization hole burning can be reduced by
using one polarization modulator as described above downstream of the multiplexer. In this way,
the polarization of all the channels in the WDM system can be rotated. Also, multiple modulators
that rotate the polarization may be used before multiplexing all channels, so that the polarization
of a group of adjacent or interleaved channels is individually rotated. it can. In addition,
Applicants rotate polarized hole burning in a WDM system by rotating the polarization of not all
channels in the WDM system, even if it rotates the polarization of only one channel in the WDM
channel. I believe it can be reduced.
[0050]
It will be apparent to those skilled in the art that various modifications and alterations can be
made to the systems and methods according to the present invention without departing from the
spirit and scope of the present invention. It is intended that the present invention cover the
modifications and variations of this invention provided that they come within the scope of this
invention as defined by the claims and their equivalents.
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