U.S. patent application number 12/427694 was filed with the patent office on 2009-08-27 for measurements of polarization-dependent loss (pdl) and degree of polarization (dop) using optical polarization controllers.
This patent application is currently assigned to General Photonics Corporation. Invention is credited to X. Steve Yao.
Application Number | 20090213453 12/427694 |
Document ID | / |
Family ID | 36844409 |
Filed Date | 2009-08-27 |
United States Patent
Application |
20090213453 |
Kind Code |
A1 |
Yao; X. Steve |
August 27, 2009 |
Measurements of polarization-dependent loss (PDL) and degree of
polarization (DOP) using optical polarization controllers
Abstract
Devices and techniques that use a polarization controller and a
feedback control to the polarization controller to systematically
control the polarization of light output from the polarization
controller in measuring the polarization dependent loss (PDL) of an
optical device or material that receives the light from the
polarization controller or the degree of polarization (DOP) of a
light beam directed into the polarization controller.
Inventors: |
Yao; X. Steve; (Diamond Bar,
CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
General Photonics
Corporation
|
Family ID: |
36844409 |
Appl. No.: |
12/427694 |
Filed: |
April 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11291585 |
Nov 30, 2005 |
7522785 |
|
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12427694 |
|
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60632579 |
Dec 1, 2004 |
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Current U.S.
Class: |
359/301 |
Current CPC
Class: |
G01M 11/337
20130101 |
Class at
Publication: |
359/301 |
International
Class: |
G02F 1/29 20060101
G02F001/29 |
Claims
1. A device, comprising: a polarization unit to control a
polarization of light received by the polarization unit in response
to a control signal, wherein the light from the polarization unit
passes through an optical medium; an optical detector to receive
light transmitted through the optical medium and to produce a
detector output; and a feedback unit that receives the detector
output and, in response to the received detector output, produces
the control signal to control the polarization unit to adjust the
polarization of light to the optical medium to measure a maximum
transmission through the optical medium and a minimum transmission
through the optical medium at the optical detector.
2. The device as in claim 1, wherein the feedback unit comprises an
electronic circuit that converts the detector output into a
feedback signal, and a processor which processes the feedback
signal and produces the control signal.
3. The device as in claim 1, wherein the polarization controller
comprises a plurality of adjustable polarization elements, and
wherein the feedback unit is configured to adjust one adjustable
polarization element at a time while keeping other adjustable
polarization elements at fixed settings to adjust each of the
adjustable polarization elements to search for the maximum
transmission and the minimum transmission at the optical
detector.
4. The device as in claim 3, wherein the adjustable polarization
elements comprise an electrically controlled electro-optic
element.
5. The device as in claim 3, wherein the adjustable polarization
elements comprise a fiber and a plurality of adjustable fiber
squeezers.
6. The device as in claim 3, wherein the adjustable polarization
elements comprise a rotatable waveplate.
7. The device as in claim 3, wherein the adjustable polarization
elements comprise a fiber coil which is rotatable.
8. The device as in claim 3, wherein the polarization controller
further comprises (1) a feed-forward control responsive to an input
polarization to the polarization controller to change the
adjustable polarization elements to produce a desired output
polarization and (2) a feedback control responsive to an output
polarization to control the adjustable polarization elements to
reduce a deviation of the output polarization from the desired
output polarization.
9. A method, comprising: separating a plurality of WDM channels
into separated signals; directing the separated WDM channels, one
at a time, into a device that measures a degree of polarization of
light; and measuring the degree of polarization of light of each
separated WDM channel by using the device.
10. The method as in claim 9, wherein the device comprises: a
polarization unit to receive an input beam of light and to control
a polarization of the received input beam in response to a control
signal; an optical polarizer placed to receive light output from
the polarization unit to produce an optical output; an optical
detector to receive the optical output from the optical polarizer
and to produce a detector output; and a feedback unit that receives
the detector output and, in response to the received detector
output, produces the control signal to control the polarization
unit to adjust the polarization of light to the optical polarizer
to measure a maximum transmission and a minimum transmission at the
optical detector, and the method further comprising computing the
degree of polarization using the measured maximum transmission and
minimum transmission at the optical detector.
11. A method, comprising: directing light from a light source
through an optical bandpass filter to produce a reference light
beam; measuring a first degree of polarization of the reference
light beam output from the optical bandpass filter; using the first
degree of polarization to compute a first optical signal to noise
ratio; directing the light from the light source through an optical
amplifier first and then through the optical bandpass filter;
measuring a second degree of polarization of light output from the
optical bandpass filter; using the second degree of polarization to
compute a second optical signal to noise ratio; and using the first
degree of polarization and the second degree of polarization to
determine a noise figure of the optical amplifier.
Description
[0001] This application is a divisional application of U.S. patent
application Ser. No. 11/291,585, filed Nov. 30, 2005, which claims
the benefit of U.S. Provisional Patent Application No. 60/632,579,
filed Dec. 1, 2004, and entitled "Measurements of
Polarization-Dependent Loss (PDL) and Degree of Polarization (DOP)
Using Optical Polarization controllers." The disclosures of the
prior applications are considered part of (and are incorporated by
reference in) the disclosure of this application.
BACKGROUND
[0002] This application relates to optical polarization devices and
measurements of polarization-dependent loss (PDL) and degree of
polarization (DOP).
[0003] Optical polarization is an important parameter of a light
beam or an optical signal. Polarization-dependent effects in fibers
and other devices, such as polarization-dependent loss (PDL) and
polarization-mode dispersion (PMD), can have significant impacts on
performance and proper operations of optical devices or systems.
Hence, it may be desirable to measure and monitor the state of the
polarization (SOP), the PDL and the DOP of an optical signal.
SUMMARY
[0004] This application describes, among others, various
implementations and examples of devices and techniques that use a
polarization controller and a feedback control to the polarization
controller to systematically control the polarization of light
output from the polarization controller in measuring the
polarization dependent loss (PDL) of an optical device or material
that receives the light from the polarization controller or the
degree of polarization (DOP) of a light beam directed into the
polarization controller.
[0005] One example of devices described here includes a
polarization unit to control a polarization of light received by
the polarization unit in response to a control signal, wherein the
light from the polarization unit passes through an optical medium;
an optical detector to receive light transmitted through the
optical medium and to produce a detector output; and a feedback
unit that receives the detector output and, in response to the
received detector output, produces the control signal to control
the polarization unit to adjust the polarization of light to the
optical medium to measure a maximum transmission through the
optical medium and a minimum transmission through the optical
medium at the optical detector.
[0006] As another example, this application describes a device
which includes a polarization unit to receive an input beam of
light and to control a polarization of the received input beam in
response to a control signal; an optical polarizer placed to
receive light output from the polarization unit to produce an
optical output; an optical detector to receive the optical output
from the optical polarizer and to produce a detector output; and a
feedback unit that receives the detector output and, in response to
the received detector output, produces the control signal to
control the polarization unit to adjust the polarization of light
to the optical polarizer to measure a maximum transmission and a
minimum transmission at the optical detector.
[0007] This application also describes methods of optical
measurements. In one example, a method is described to include
separating a plurality of WDM channels into separated signals;
directing the separated WDM channels, one at a time, into a device
that measures a degree of polarization of light; and measuring the
degree of polarization of light of each separated WDM channel by
using the device.
[0008] In another example, a method is described to include
directing light from a light source through an optical bandpass
filter to produce a reference light beam; measuring a first degree
of polarization of the reference light beam output from the optical
bandpass filter; and using the first degree of polarization to
compute a first optical signal to noise ratio. This method also
includes directing the light from the light source through an
optical amplifier first and then through the optical bandpass
filter; and measuring a second degree of polarization of light
output from the optical bandpass filter; using the second degree of
polarization to compute a second optical signal to noise ratio.
This method further includes using the first degree of polarization
and the second degree of polarization to determine a noise figure
of the optical amplifier.
[0009] These and other implementations and applications are
described in greater detail in the attached drawings, the detailed
description, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a detection device which measures PDL of an
optical medium such as an optical device or an optical
material.
[0011] FIG. 2 shows examples of PDL measurements of two samples
using polarization scrambling.
[0012] FIG. 3 shows one example of a PDL measurement with the
Muller matrix method over a spectral range from 1520 nm to 1620
nm.
[0013] FIG. 4 shows one implementation of a device which measures
the PDL of an optical medium using a feedback control of a
polarization controller based on a maximum-and-minimum search
described in this application.
[0014] FIGS. 5A, 5B, and 5C show examples of PDL measurements for
different sample materials using the system in FIG. 4 based on the
maximum and minimum search using the feedback control.
[0015] FIGS. 6A and 6B show two exemplary methods to depolarize
laser light for pumping a Raman amplifier.
[0016] FIG. 7 shows one implementation of a device which measures
the DOP of an optical beam using a feedback control of a
polarization controller based on a maximum-and-minimum search
described in this application.
[0017] FIGS. 8A and 8B show two exemplary applications of the
device in FIG. 7.
DETAILED DESCRIPTION
[0018] Polarization dependent loss (PDL) is the total range of
insertion loss of light after transmitting through an optical
medium (e.g., a device or a material) due to changes in
polarization from the original SOP before entry of the optical
medium (the launch state). Hence, PDL can be expressed as:
PDL=10 log.sub.10(Pmax/Pmin) (1)
where Pmax and Pmin are maximum and minimum transmission powers of
light through the device or medium for all possible input
polarizations.
[0019] FIG. 1 illustrates a typical system for measuring the PDL of
an optical material or device under test (DUT). The SOP of the
input light is changed through all possible SOPs and the power of
the optical transmission through the DUT is measured to obtain the
maximum transmission power (Pmax) and the minimum transmission
power (Pmin). Based on the measured Pmax and Pmin, the PDL can be
computed from Eq. (1). TABLE I lists PDL values for some fiber
optic components.
[0020] Various techniques were developed to implement the system in
FIG. 1. For example, an optical polarization scrambler may be
placed in the optical path of the input light before the DUT to
scramble the input SOP to cover as many SOPs as possible. An
optical detector is used to receive the transmitted light and
measure the transmission power. This process can be slow because
the polarization scrambler needs to operate at various settings to
generate all possible SOPs and the accuracy of the measurement
generally increases with the measurement time. Notably, many
polarization scramblers have the uncertainty whether the SOP is
truly scrambled so that all possible SOPS on the Poincare Sphere
are covered. Such polarization scramblers may have uncovered SOP
areas on the Poincare Sphere. This uncertainty compromises the
trustworthiness of the measured maximum and minimum transmission
power Pmax and Pmin. In addition, the polarization scrambling
method is known for its inaccuracy for measuring devices or media
with high PDL values.
[0021] FIG. 2 shows 100 measurements of two PDL samples using the
polarization scrambling method. The measured PDL values fluctuate
from measurement to measurement. The minimum measured PDL deviates
from the averaged PDL by 6.6% and 4.4% in the two samples tested.
Such inaccuracies may be unacceptable in certain applications.
TABLE-US-00001 TABLE I PDL values of Some Fiber Optic Components
Component PDL (dB) Single-mode fiber: (1.0 m) <0.02 (10 km)
<0.05 Optical connector: (Straight) 0.005-0.02 (Angled)
0.02-0.06 50/50 coupler: (Single wavelength) 0.1-0.2 (1300/1500 nm)
0.15-0.3 90/10 coupler: (through path) 0.02 (-10 dB path) 0.1
Directional Isolator 0.05-0.3 Three port circulator 0.1-0.2 DWDM
multiplexer 0.05-0.1
[0022] Another method for measuring PDL of a device or medium
measures the elements of the Muller polarization matrix by changing
the input SOP to 4 different SOPs, such as a set of 4 SOPs of
linear polarization states at 0, 45, and 90 degrees, and one
circularly polarized state. The PDL is then computed from the
Muller matrix elements. To compute the Muller matrix elements, the
optical power is first measured without the medium or device under
test to obtain a reference for the incident power. Next, the
incident polarization is changed to the four selected SOPs and the
optical transmission power levels through the medium or device
under tests are measured for each of the four selected SOPs. Based
on the above measurements, the four Muller matrix elements can be
computed.
[0023] The above Muller matrix method for measuring PDL is slow and
may take about several seconds per power measurement. The Muller
matrix elements vary with the optical wavelength and hence
wavelength calibration is needed. In addition, the Muller matrix
method tends to be inaccurate. FIG. 3 shows one example of a PDL
measurement with the Muller matrix method over a spectral range
from 1520 nm to 1620 nm. The measured PDL changes with the
wavelength with a uncertainty of about 35 mdB. This magnitude of
measurement uncertainty may be unacceptable in some
applications.
[0024] The technique for the described implementations for
measuring PDL is in part based on the recognition of the above and
other limitations of the aforementioned techniques using the
polarization scrambler and the Muller matrix. Different from the
above techniques where the control of the input SOP to the medium
or device under test is entirely independent from the light
received by the detector that measures the optical transmission
from the medium or device under test, the present technique adjusts
the input SOP in response to the optical power of the optical
transmission through the device or medium that is received by the
optical detector. A feedback control is implemented to adjust the
incident SOP in search for the maximum and the minimum optical
transmission power levels through the optical medium (a material or
device) under test. This feedback controlled search is systematic
and deterministic and removes the uncertainty in the polarization
scrambling method and the wavelength calibration in the Muller
matrix method. The present technique further provides high speed
measurements with high accuracy.
[0025] FIG. 4 shows a system 400 for measuring PDL of an optical
medium or device under test 420. A sample holder may be used to
hold the medium or device 420 in place along the optical path of
the system 400. A light source 401, such as a laser (e.g., a diode
laser) is used to produce a probe beam 402. An optical polarization
controller 410 with multiple adjustable polarization elements may
be used to control the polarization of the probe beam 402 and
produce a probe beam 412 that reaches and transmits through the
medium or device 420. The adjustable polarization elements in the
polarization controller 410 are adjusted to produce the desired
incident SOP in response to the power of the optical transmission
through the medium or device 420. An optical detector 430, such as
a photodiode, may be used to collect the optical transmission
through the medium or device 420 and produce a detector output
indicative of the power of the optical transmission. An detector
circuit 440 may be used to condition and process the detector
output from the detector 430 and produce a feedback signal 442. A
feedback controller 450, which may include a microprocessor (".mu.
processor") or a digital control circuit, is provided to receive
the feedback signal 442 and to control the polarization controller
410 based on the measured power level of the optical transmission
for the previous incident SOP to adjust the SOP in order to search
for the Pmax and Pmin. In this example, the feedback control for
the device 400 includes the units 440, 442 and 450.
[0026] In implementations, the feedback controller 450 may be
programmed to carry out the search by controlling the polarization
controller 410 based on the received power at the optical detector
430. As one example, a maximum-and-minimum search algorithm may be
used to control the search. First, the polarization controller 410
is set to produce a selected initial SOP in the probe beam 412. The
power of the optical transmission is measured. Next, the
polarization controller 410 is controlled to change the SOP along a
path on the Poincare Sphere to a new SOP. The power of the optical
transmission at the new SOP is measured. The power either decreases
or increases at the new SOP. As a specific example, assuming that
the optical power decreases at the new SOP, the polarization
controller 410 is controlled to continue to change the SOP along
that path on the Pincare Sphere until the measured optical power no
long decreases and begins to increase. This step finds the first
minimum optical transmission. Next, the polarization controller 410
is controlled to continue along the same path to find the SOP with
the first maximum optical transmission. Such search continues and
the largest optical transmission is used as the Pmax and smallest
optical transmission is used as the Pmin. The search steps may be
reduced and the search path may be modified or altered to improve
the search for Pmax and Pmin.
[0027] The polarization controller 410 may be include multiple
adjustable polarization elements that change the polarization of
light. One way of adjusting the polarization elements is to
repetitively adjust one element at a time while fixing other
elements to search the local maximum or minimum in a iterative
manner until a global maximum or minimum is found. As an example,
consider a polarization controller 410 with 3 adjustable
polarization elements A, B and C. First, the elements A, B and C
are set to some initial settings and the search is initiated by
adjusting element A while keeping the settings of elements B and C
fixed. Assume the element A is adjusted along a path on the
Poincare Sphere to increase the optical transmission at the optical
detector 430. The element A is adjusted until the optical
transmission at the detector 430 begins to decrease. Hence, a local
maximum is found. Next, the element B is adjusted to change the
polarization of the light on the Poincare Sphere while keeping the
settings of elements A and C fixed to find the maximum at the
detector 430. After the maximum is found, the element C is adjusted
while keeping elements A and B fixed. After the maximum at the
detector 430 is found by adjusting the element C, the above process
repeated until the newest maximum at the detector 430 no longer
increases. At this time, the maximum is said to be found. A similar
search process is used to search for the minimum at the detector
430.
[0028] FIGS. 5A, 5B, and 5C show examples of PDL measurements for
different sample materials using the system in FIG. 4 based on the
maximum and minimum search using the feedback control. FIGS. 5A and
5B show PDL measurements of a low PDL sample and a high PDL sample,
respectively. The PDL accuracy of the present technique in one
implementation is, e.g., about 0.005 dB, a significant improvement
over the accuracy of the polarization scrambling and Muller matrix
methods. FIG. 5C shows the repeatability of the present PDL
measurement technique over a wavelength range from 1520 nm to 1620
nm.
[0029] The above maximum-and-minimum search technique may also be
used to measure the degree of polarization (DOP) of a light beam.
The degree of Polarization (DOP) is an important property of light
sources. DOP directly relates to the accuracy of optical component
characterization, the sensitivity of sensor systems, and the
quality of optical signals in optical communication systems.
Therefore, the accurate and fast characterization of DOP is
important in various applications.
[0030] In optics, DOP is used to describe how much in the total
light power is polarized and is defined as the power in the total
polarized portion of the light beam divided by the total optical
power (sum of the total polarized portion and the total unpolarized
portion):
DOP=P.sub.polarized/P.sub.total=P.sub.polarized/(P.sub.polarized+P.sub.u-
npolarized) (1)
For totally polarized light, DOP is unity. For completely
unpolarized light, DOP is zero. DOP of different light sources
ranges from 0 to 1. High DOP sources include DFB lasers and
external cavity lasers. Such lasers can be incorporated in laser
transmitters in telecommunication systems and used as light sources
in interferometers and other devices. On the other hand, amplified
spontaneous emission (ASE) sources, light emitting diodes (LED),
and super-luminescence light emitting diodes (SLED) represent
sources with low DOP. Such low DOP sources are important for sensor
applications to minimize polarization sensitivity. For example,
SLED and ASE sources can be used in a fiber gyro, a rotation sensor
for measuring the rate and degree of rotation of an object. Low DOP
sources are also attractive for accurate characterization of
optical components to remove PDL effects in the measurement system,
including polarization sensitivity of photodetectors. Therefore,
accurate characterization of DOP of these light sources is
extremely important for both optical component manufacturers and
users.
[0031] Optical amplifiers are critical devices for fiber optic
communication and sensing system. One of the important parameter of
the amplifiers is the low polarization sensitivity. Unfortunately,
both Er+ doped amplifiers and Raman amplifiers have polarization
dependent gain (PDG). In particular, if not properly implemented,
the PDG of Raman amplifiers can be much stronger than that of Er+
amplifiers.
[0032] Raman amplifiers are based on stimulated Raman scattering of
optical signals by optical phonons excited by a pump laser in an
optical fiber. A weak optical signal is amplified by stimulating
the excited phonons to release energy into the signal. This process
is called stimulated Raman scattering. PDG is particularly strong
for Raman amplifiers because, in stimulated Raman scattering, an
incident photon can only stimulate phonon contributed from a pump
photon of the same polarization. The Raman gain is the strongest
when the polarization of the signal is aligned with that of the
pump, but is negligible if the polarization of the signal is
orthogonal from that of the pump.
[0033] One effective method to minimize the PDG or polarization
sensitivity of a Raman amplifier is to pump it with depolarized
laser sources. A depolarizer may be used to convert a polarized
pump laser from a laser into a depolarized source with a DOP close
to zero, as shown in FIG. 6A. The output beam from the depolarizer
is then used to pump the Raman amplifier. Such a depolarizer can be
made with birefringent crystals, PM fibers, or other methods.
[0034] Alternatively, FIG. 6B shows a system where a polarization
combiner is used to obtain a nearly unpolarized light beam by
combining two laser sources of a similar or identical frequency but
with orthogonal polarizations. Because the DOPs of the two laser
pump sources directly relate to the polarization sensitivity of the
Raman amplifier, accurate characterization of their DOPs is of
paramount importance for Raman amplifier manufacturers. For
example, the DOP of the combined pump source critically depends on
the power balance between the two orthogonally polarized pump
lasers and a fast and low cost DOP meter is desirable for the live
adjustment of the pump lasers on the manufacturing floor.
[0035] DOP may be measured with traditional polarimeter which
measures four Stokes parameters S0, S1, S2, and S3. The polarimeter
approach is less accurate in measuring low DOP light sources and
may be expensive. In addition, the polarimeter approach exhibits
wavelength sensitivity, has cumbersome calibration requirements,
and can be complicated to operate.
[0036] Another commonly used method is the polarization scrambling
method in which a polarization scrambler is placed in front of a
polarizer and a photodetector to scramble the polarization.
Ideally, at some points during the scrambling cycle, the polarized
portion of the signal will either be aligned with or orthogonal to
the polarizer's passing axis. When aligned, all of the polarized
portion passes through and a maximum power level is detected at the
photodetector. When orthogonal, the polarized portion is blocked by
the polarizer, assuming that the extinction ratio of the polarizer
is sufficiently high. Thus, a minimum power level is detected at
the photodetector. The difference of (P.sub.max-P.sub.min) equals
to P.sub.polarized in Eq. (1). On the other hand, the unpolarized
portion is not affected by the scrambler. The contribution of this
unpolarized portion to the total detected power is constant, but is
reduced to one half by the polarizer. Because the contribution of
the polarized portion is zero at P.sub.min,
P.sub.min=P.sub.unpolarized/2. The DOP from Eq. (1) then can be
calculated as:
DOP=(P.sub.max-P.sub.min)/(P.sub.maxP.sub.min) (2)
[0037] Therefore, the maximum and minimum power levels at the
photodetector are measured while scrambling the SOP of the incoming
signal. Based on such measurements, the DOP of the signal can be
determined. However, in order for the method to be practical, the
scrambler must be sufficiently fast to cover the whole Poincare
Sphere in a short period of time. Second, the scrambler itself must
have negligible activation loss (the maximum insertion loss
variation during scrambling). In addition, the detection
electronics must be fast and accurate enough to faithfully detect
the maximum and minimum power levels.
[0038] However, no matter how fast and uniform the scrambler is, it
is generally difficult to completely cover the Poincare Sphere
within a finite time. The uncovered areas on the Poincare Sphere
contribute to uncertainty in the DOP measurement. The faster the
measurement requires, the larger the uncertainty is. Such
uncertainty makes the scrambling method especially less accurate
for measuring the light sources of high DOP. However, for low DOP
values, the requirement for the coverage on the Poincare Sphere is
less stringent and therefore it is more accurate and faster than
the polarimeter. In addition, compared with the polarimeter method,
the polarization scrambling method has the advantages of wavelength
insensitivity, calibration free operation, high power capability,
simplicity, and low cost.
[0039] FIG. 7 shows a DOP meter 700 according to one implementation
based on the feedback from the optical detector and the
maximum-and-minimum search. A light source such as a laser diode
401 is used to generate the input light beam whose DOP is to be
determined. A polarization controller 410 receives the input light
and produces an output light beam after controlling the
polarization of the light. An optical polarizer 710 is placed in
the path of the output light beam from the polarization controller
and an optical detector 430 is used to receive the light
transmitted through the polarizer 710. A feedback circuit is used
to direct the polarization controller 410 to adjust for the maximum
and minimum power levels received by the detector 430.
[0040] In the device 700, instead of trying to hit the right
polarization by luck, as in the scrambling method, the
maximum/minimum search method is implemented in the control 450 and
assures the instrument to unmistakably find the P.sub.max and
P.sub.min for the DOP calculation in Eq. (2). Because only two
points on the Poincare Sphere are required and can be found
deterministically and precisely, the measurement speed and accuracy
are essentially guaranteed for both low and high DOP sources.
Consequently, implementations of such an approach can be used to
overcome the shortcomings of both the polarimeter method (less
accurate for low DOP values) and the polarization scrambling method
(less accurate for high DOP values).
[0041] To a certain extent, the maximum/minimum search method is
essentially a closed loop polarization scrambling method. This
search method substantially eliminates the inaccuracies, but
inherits all the advantages of the scrambling methods, including
wavelength insensitivity, calibration free operation, high power
capability, easy operation, simple construction, and low cost. In
addition, this search can also be implemented in a way that
achieves a high measurement speed, e.g., less than 0.2 seconds in
some implementations.
[0042] The optical powers of different light sources can vary
dramatically from one to another, ranging from, e.g., microwatts
for LEDs to watts for pump lasers. However, typical DOP meters have
a dynamic range on the order of 30 dB. Users may specify their
intended power range, e.g., from -30 dBm to 0 dBm, or from -10 dBm
to 20 dBm. For high power light sources, such as Raman pump lasers
with power up to 500 mW, fixed attenuators may be used. To preserve
the DOP accuracy, these attenuators must have low PDL, because the
PDL generally repolarize the light source. As a good estimation,
the DOP error induced by a PDL source when measuring an unpolarized
source is
DOP(%)=12PDL(dB) (3)
[0043] For example, for an attenuator with a PDL of 0.1 dB, the
induced DOP error is 1.2%.
[0044] In addition to measurement of the DOP value, an accurate and
fast DOP meter is also important in the manufacturing floors for
tuning DOP values of the light sources. For example, by monitoring
DOP in real time while adjusting the power balance of the two pump
lasers in FIG. 6B, extremely low DOP value for the Raman pump can
be achieved. Polarimeters are generally too expensive to be used in
production stations, too complicated to operate for less
sophisticate production personnel, and not accurate enough for such
demanding an application. On the other hand, due to its low cost,
simplicity to operate, and high accuracy, the DOP meter based on
the present maximum-and-minimum search can be used for such
applications.
[0045] A high-speed DOP meter can also be used in optical networks
to monitor PMD or optical signal to noise ratio (OSNR). FIG. 8A
illustrates one example where an optical switch, such as a
1.times.N switch, may be used to connect the DOP meter to the N
output channels from a WDM demultiplexer in a fiber network to
measure the DOP of the WDM channels, one WDM channel at a time.
Because PMD in an optical system degrades the DOP of the optical
signal, monitoring of the DOP can directly reveal the influence of
PMD on the optical signals. As illustrated, the DOP meter 700 shown
in FIG. 8A is the DOP meter 700 in FIG. 7. However, a DOP meter
different from the DOP meter 700 in FIG. 7 may be used.
[0046] On the other hand, in absence of PMD influence, OSNR can
also be obtained from the DOP measurement by the following
equation:
OSNR=10 log [(P.sub.max-P.sub.min)/(2P.sub.min)]=10 log
[DOP/(1-DOP)] (4)
where it is assumed that the signal is totally polarized and the
noise is totally unpolarized.
[0047] An accurate DOP meter can also be used to measure the noise
figure of an amplifier. FIG. 8B shows one exemplary system for
measuring the noise figure of an amplifier 830. A laser 810 is used
to provide the laser beam for the measurement and an optical
bandpass filter 820 is placed before the DOP meter 700 to select
the spectral band within which the noise figure is measured. The
OSNR of the signal source (i.e., the laser 810) may be first
measured without the amplifier 830 by inserting a fiber jumper 850
in the place of the amplifier 830. The result of this measurement
based on Eq. (4) is OSNR.sub.o and is used as a reference OSNR. The
light that transmits through the optical filter 820 is a reference
beam and the OSNR of this reference beam is measured as OSNR.sub.o.
Two optical connectors 841 and 842 may be used to connect either
the fiber jumper 850 or the amplifier 830. Next, the amplifier 830
is inserted in the optical path between the laser 810 and the
filter 820 to obtain the measurement of OSNR for the optical signal
output from the amplifier 830. This measurement of
OSNR.sub.amplifier is also based on Eq. (4). The noise figure of
the amplifier 830 is the difference between the two OSNR
measurements in dB: (OSNR.sub.amplifier-OSNR.sub.o). The optical
filter 830 can be selected to limit the bandwidth. In practice, the
noise figure can be expressed in a 0.1-nm bandwidth and therefore
the effects of the bandwidth and shape of the filter need to be
taken into account for the final OSNR value.
[0048] The laser 810 may be fixed at a particular laser wavelength
and the corresponding filter 820 should have a passband centered at
the laser wavelength. Alternatively, as illustrated in FIG. 8B, the
laser 810 may be a tunable laser and the filter 820 may be a
tunable filter so that both the laser 810 and the filter 820 may be
tuned in synchronization with each other to measure the noise
figure at different wavelengths, e.g., the noise figures for
different WDM channels of the amplifier 830. With the aid of a
computer ("PC") or a digital processor 860, the wavelength
dependence and power dependence of the noise figure can also be
determined. The DOP meter used in FIG. 8B may be the DOP meter 700
in FIG. 7 or a different DOP meter.
[0049] A DOP meter is an important instrument for accurate
characterization of the DOP values of different light sources for
communication, manufacturing, testing and sensing applications.
Implementations of the DOP meter based on the maximum and minimum
power search method can be configured to achieve one or more
advantages such as low cost, simple operation, high speed, low
wavelength sensitivity, and calibration free. Such a DOP meter can
be implemented to offer high accuracy for both high and low DOP
values with less cost, less effort, and less measurement time.
[0050] In the above described devices and their variations or
modifications, the polarization controller 410 may be implemented
in various configurations. Multi-element adjustable polarization
controllers described in U.S. Pat. No. 6,576,866 issued to Yao on
Jun. 10, 2003 may be used to implement the controller 410. The
entire disclosure of U.S. Pat. No. 6,576,866 is incorporated by
reference as part of the specification of this application.
[0051] For example, the polarization controller may include
multiple adjustable polarization elements in various
configurations. In one example, three cascaded rotatable waveplates
of fixed phase retardation values of 90 degrees, 180 degrees, and
90 degrees, respectively, may be used to form the polarization
controller. In a more specific implementation, a half waveplate
(HWP) can be placed between two quarter waveplates (QWP) in free
space to form the controller. In another implementation, three or
more fiber coils which are optically birefringent are used to
produce the fixed retardation values of 90 degrees, 180 degrees,
and 90 degrees, respectively. Bending of the fiber in each coil
introduces stress and thus causes birefringence. The number of
turns (length of the light propagation) and the diameter of the
each fiber coil (degree of bending) can be set to produce the
corresponding fixed phase retardation. The fiber coils may be
rotatable to change their relative orientations in their principal
axes to adjust the output polarization. In yet another alternative,
three electrically controlled electro-optic materials may be used
to produce the fixed retardation values of 90 degrees, 180 degrees,
and 90 degrees, respectively, without physical motion.
Electro-optic crystals such as LiNbO.sub.3 may be used. Each
electro-optic polarization element can be applied with two control
voltages to control and rotate the orientations of their optic axes
via the electro-optic effect without physical rotations of the
elements.
[0052] In other implementations, a Babinet-Soleil compensator can
be used as an adjustable polarization element to produce both
adjustable retardation and adjustable orientation in a polarization
controller. Two movable birefringent wedges can be positioned
relative to each other so that their hypotenuse surfaces face each
other. The input optical beam is directed to transmit through the
hypotenuse surfaces of two wedges. The total optical path length
through the wedges and thus the total retardation of the system may
be varied by moving two wedges relative to each other. The two
wedges may also be rotated together about the direction of the
input optical beam to provide the adjustable orientation by a
rotation mechanism.
[0053] A fiber polarization controller based on the basic mechanism
of the Babinet-Soleil compensator can be built by using a rotatable
fiber squeezer which is rotatably engaged to the fiber so that the
direction at which the squeezer squeezes the fiber can be adjusted.
The squeezing produces birefringence in the fiber to control the
light polarization. A pressure-applying transducer, such as a
piezo-electric transducer, may be engaged to the squeezer to
produce a variable pressure and hence a variable birefringence in
the fiber.
[0054] In yet other implementations of the polarization controller,
four or more adjustable polarization elements with fixed relative
orientations and variable birefringences may be used. For example,
the principal polarizations of two adjacent elements are at about
45 degrees relative to each other. Electro-optic materials and
liquid crystals may be used. In an all-fiber implementation, a
fiber can be engaged to four fiber squeezers whose squeezing
directions are fixed at angles of 0 degree, 45 degree, 0 degree,
and 45 degree, respectively. The pressure on each squeezer may be
adjusted to change the retardation produced thereby. Such an
all-fiber design may be used to reduce the optical insertion loss
as compared to other designs and may be used to operate on light of
different wavelengths.
[0055] A polarization controller with multiple adjustable
polarization elements may use a control mechanism to dynamically
control the multiple polarization elements by implementing two
control mechanisms: a feed-forward control and a feedback control.
In one embodiment, the feed-forward control measures the input
polarization of the input signal and adjusts the multiple
polarization elements to pre-determined settings for producing the
desired output polarization. The feedback control measures the
output polarization and, in response to the measured output
polarization, adjusts the multiple polarization elements around the
settings initially set by the feed-forward control to reduce the
deviation of the output polarization from the desired output
polarization. In another embodiment, the feed-forward control is
engaged to control at least two polarization elements while the
feedback control is engaged to control at least two polarization
elements that are not engaged to be controlled by the feed-forward
control. To certain extent, the feed-forward control essentially
provides a fast, coarse control of some or all of the polarization
elements in response to the input polarization and the feedback
control essentially fine tunes the settings of some or all of the
polarization elements to reduce the deviation of the output
polarization from the desired output polarization. See U.S. Pat.
No. 6,576,866.
[0056] Only a few examples and implementations are described.
However, other implementations, variations, modifications, and
enhancements are possible.
* * * * *