Optical: systems and elements – Optical modulator
Reexamination Certificate
2002-03-15
2004-04-06
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
C359S250000, C359S281000, C359S316000, C359S484010, C359S494010, C356S367000, C356S453000, C385S011000, C385S015000, C385S050000, C372S703000, C348S742000, C348S751000, C324S1540PB, C398S205000
Reexamination Certificate
active
06717706
ABSTRACT:
BACKGROUND
This invention relates to optical systems for monitoring the state of polarization (SOP) of an optical beam.
The transmission of information over optical fibers is pervasive in modern communication networks. Optical fibers are often favored over electrical cable because optical fiber offers much larger bandwidths than cable. Moreover, optical fiber can connect nodes over larger distances and transmit optical information between such nodes at the speed of light. Among factors limiting transmission rates and distances in high-speed fiber systems, however, are polarization effects such as polarization mode dispersion (PMD) in optical systems such as optical fibers.
Polarization mode dispersion arises from small, random birefringences in optical fibers. For sufficiently short sections of fiber, birefringence may be considered uniform, and light traveling along the fast and slow axes experience different propagation delays. For longer sections of fiber, however, the orientations and amplitudes of the birefringence varies, leading to a phenomenon called polarization mode coupling. The polarization mode coupling eventually randomizes the polarization state of the propagating optical signal. PMD also results in pulse broadening, which reduces the available bandwidth of the optical fiber.
It is therefore desirable to reduce the effects of PMD. This can be accomplished by compensating for PMD by detecting or analyzing the state of polarization (SOP) of the optical signal, and passing the optical signal through a polarization modulator (e.g., a variable retarder stack) to reduce such PMD effects in response to the detected state of polarization of the signal. For example, the polarization modulator can impart a retardance that is exactly opposite to that experienced by the optical signal in the fiber. An example of a prior art polarization compensation system is described by T. Chiba et al., in “Polarization Stabilizer Using Liquid Crystal Rotatable Waveplates,” in
Journal of Lightwave Technology
, Vol. 17, No. 5, May 1999.
Referring to
FIG. 1
, an example of a polarization analyzer
10
, disclosed by Chiba et al., includes a pair of beamsplitters
12
and
13
position in the path of a beam
11
of unknown SOP. Beam splitters
12
and
13
direct sample beams
22
and
23
toward polarizing beamsplitters
30
and
32
. Prior to contacting polarizing beamsplitter
32
, sample beam
23
passes through a quarter wave plate
25
. Polarizing beamsplitters
30
and
32
split sample beams
22
and
23
into orthogonal X and Y polarization components, where the X polarization component is the linear component in the plane of FIG.
1
and the Y polarization component is the linear component orthogonal to the plane of FIG.
1
. The intensity of each component is measured by photodiodes
35
-
38
. Quarter waveplate
25
is oriented at 45° with respect to the nominal X and Y directions, therefore if dectectors
35
and
36
measure the relative linear x and y components of beam
11
, then detectors
37
and
38
measure the right-hand and left hand circular components. Accordingly, the SOP of beam
11
can be determined from these two sets of orthogonal components.
SUMMARY
The invention features an integrated optical assembly for providing information about the state of polarization (SOP) of an input beam passing through the assembly. Hereinafter, the assembly is also referred to as an SOP detector. The assembly includes multiple polarization-sensitive interfaces each providing a sample beam having an intensity providing information about the SOP of the input beam. For example, the assembly may provide four or more sample beams, the intensities of which are sufficient to uniquely determine the SOP of the input beam. Alternatively, the optical assembly may provide fewer than four sample beams, where the intensities of the sample are sufficient to determine the SOP of the input beam when combined with some a priori knowledge about the nominal SOP of the input beam. Furthermore, the optical assembly may provide multiple sample beams (e.g., 2 or 3 or more beams) whose intensities indicate a deviation of the SOP of the input beam from a desired SOP. The measured deviation can be used to provide a feed-forward or feed-back signal to a polarization modulator that alters the SOP of the beam.
At least two, and preferably all, of the polarization-sensitive interfaces in the optical assembly are oriented to direct the sample beams in a similar direction and to allow a compact integration of the optical assembly components. For example, the polarization-sensitive interfaces can be oriented substantially parallel to one another. Because the sample beams propagate in a similar direction, a single detector array can be used to monitor the intensities of the sample beams and add to the compactness of the overall optical system.
The optical assembly also includes a retardation layer positioned between each pair of similarly-oriented polarization-sensitive interfaces. The retardation layer(s) alter the polarization state of the beam to allow the polarization-sensitive interfaces to sample different polarization components of the input beam, and cause the intensity of each sample beam to provide different information about the SOP of the input beam. In preferred embodiments, such retardation layers are oriented similarly to the polarization-sensitive interfaces to improve the compactness of the optical assembly. For example, they can be oriented substantially parallel to the polarization-sensitive interfaces. Such a construction can be accomplished by forming each polarization-sensitive interface adjacent an optical window used to support a retardation layer. This can result in a monolithic and compact integration of the optical assembly components.
To separate the sample beams from the input beam, the optical assembly defines an optical beam path that contacts each of the polarization-sensitive interfaces at a non-normal angle (e.g., an angle of about 45°). In preferred embodiments, the optical assembly may further include an input prism and/or an output prism having a surface oriented substantially normal to the optical beam path to increase the coupling efficiency of the input beam into and out of the assembly. The assembly may also include a pre-compensation retarder to adjust the polarization state of the input beam prior to it contacting any of the polarization-sensitive interfaces and/or a post-compensation retarder to adjust the polarization state of the beam upon exiting the assembly. For example, the post-compensation retarder can be selected to cancel or minimize any change in the SOP of the input beam caused by passing through any of the intermediate retarders and/or polarization-sensitive interfaces.
The polarization-sensitive interfaces are constructed to sample only a small fraction of the input beam energy. For example, the optical assembly can have an insertion loss of less than 1 dB, or even less than 0.5 dB, or even less than 0.2 dB. Thus, the optical assembly can be positioned in the path of optical beam (e.g., a beam carrying optical telecommunication information) without significantly degrading the beam emerging from the assembly. Furthermore, the sample beams produced by the optical assembly have intensities that directly provide information about the SOP of the input beam. In other words, additional optical polarization processing of the sample beams is not necessary, thereby eliminating additional optical components and further adding to the compactness of the overall system.
The SOP detector can be used in a polarization control system that further includes a polarization modulator that adjusts the polarization of the input beam in response to a feed-back or feed-forward signal generated from the intensities of the sample beams. The polarization control system can be used to stabilize a varying SOP in an optical signal beam caused by effects such as PMD. Thus, the beam enters the polarization control system with an unknown (or only a nominally known) time-varying SOP and emerg
Cronin Paul J.
Miller Peter J.
Ben Loha
Cambridge Research and Instrumentation, Inc.
Dinh Jack
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