Optical waveguides – Polarization without modulation
Reexamination Certificate
2001-01-29
2002-11-19
Bovernick, Rodney (Department: 2874)
Optical waveguides
Polarization without modulation
C385S024000, C385S039000, C359S199200
Reexamination Certificate
active
06483957
ABSTRACT:
FIELD OF THE INVENTION
The present polarization mode dispersion compensator is based on delay generators that use thermally actuated, rotating micro-mirrors to add well-defined increments of delay to polarized portions of an optical signal.
BACKGROUND OF THE INVENTION
Fiber optics technology is revolutionizing the telecommunications field. The main driving force is the promise of extremely high communications bandwidth. At high bandwidths, a single beam of modulated laser light can carry vast amounts of information equal to hundreds of thousands of phone calls or hundreds of video channels. However, pulse broadening limits the effective bandwidth and propagation distance of an optical communication signal. Because of the inherent dispersive nature of an optical fiber medium, all portions of a light pulse do not travel the same distance through an optical fiber causing pulse broadening.
FIG. 1
illustrates how pulse broadening arises from varying light propagation delays, which eventually distorts light output. Digital input pulses
10
are input to an optical fiber medium
11
. The amplitude-modulated pulses are generated by a modulated laser source, such as a direct-modulated laser or an externally-modulated laser.
Different portions of a light pulse encounter varying propagation delays arising from the varying lengths of reflected paths within optical fiber
11
. For clarity, three paths are illustrated which correspond to a relatively straight, short path
10
a
, a reflected, intermediate length path
10
b
, and a relatively long, reflected path
10
c
. Due to the varying propagation delays (see, e.g., the &Dgr;t delay in arrival time between
10
a
and
10
b
), the combined optical output is distorted. Thus, a photoreceptor detecting the output pulses
10
a
-
10
c
will generate a distorted output
12
.
As shown in
FIG. 2
, such pulse broadening can lead to intersymbol confusion. “Pulse broadening” is called “dispersion” or “spreading” because of the non-uniform way in which parts of the incident signal
20
propagate through a dispersive fiber medium. In a mild form of dispersion, the transitions between ON and OFF states observed at a receiver are not as abrupt and distinct as the transitions that were originated by a transmitting laser. More severe blurring in the time domain limits the useful bandwidth of the path.
In
FIG. 2
, dispersion effects have broadened two closely spaced pulses to the extent that they are almost indistinguishable, as indicated by a question mark in the output signal
22
. This will cause an information bit to be received erroneously, with perhaps disastrous results on network communication and customer dissatisfaction.
Several refinements have been made to reduce dispersion and increase the useful bandwidth. First, single-mode fiber was developed having a slender core such that there is essentially only a single light path through the fiber. Secondly, the distributed feedback (DFB) laser was developed with an extremely narrow distribution of output wavelengths. This technique minimizes chromatic dispersion caused by the fact that different wavelengths traverse the length of the fiber over different periods of time. Finally, a dispersion-shifted fiber material was produced to minimize the increased time vs. wavelength dependency at a specific wavelength of fifteen hundred and fifty nanometers common in telecommunication applications.
Cumulatively, recent improvements in fiber materials and transmitter devices have reduced pulse dispersion and increased working bandwidth. Lightwave technology has advanced at such a pace that the bandwidth capabilities have more than doubled every two years. As a result, working bandwidths, expressed in terms of digital bit-per-second rates, have escalated from 500 Million bits per second (Mbps) to 10 Billion bits per second (Gbps).
These progressively more exotic refinements have brought the technology to a new bandwidth barrier: Polarization-Mode Dispersion (PMD). Previously, PMD was insignificant in magnitude relative to other dispersive effects, but now it is a limiting factor. It is well known that light can be polarized and that, for a given beam of light, this polarization can be expressed in terms of two orthogonal axes that are normal to the axis of propagation. As a beam of light propagates through a fiber, the light energy present along one such polarization may leak into the other polarization.
This leakage would normally be of little consequence (lightwave receivers will detect both polarizations), except that real-world fibers carry different polarizations at slightly different time delays due to reflection. This effect can be on the order of 10-20 picoseconds (ps) in a 100 km fiber and becomes important when the modulating pulses are 50-100 picoseconds in width. To complicate matters, the polarization dispersion within a given fiber changes as a function of time and temperature. Therefore, an effective PMD compensation mechanism must monitor and adapt to the changes so as to keep PMD to a minimum.
To nullify the effects of PMD, researchers have suggested application of an adaptive compensation device in an optical path at the receiving end just before the receiving transducer. These compensators typically employ a detector for analyzing the relative partitioning and delay of the incoming signal along two orthogonal polarizations. The compensators correct a data signal by purposefully adding delay selectively to one polarization or another. A controller interprets the findings of the delay analyzer and manipulates adjustable delay elements so as to compensate for the polarization-dependent delay differences caused by the imperfect fiber transmission path. However, these techniques are not practical in telecommunication applications, such as, long-haul optical fiber communication.
The variable delay elements are usually optical fibers that are either heated or squeezed to alter their propagation characteristics. While these elements are adaptable to laboratory electronic control techniques, they are inadequate in terms of reproducibility and predictability of response. They are also impractical for use in a commercial traffic-bearing fiber network wherein recovery time following an equipment or power failure should be minimized. (See, e.g., Ozeki, et al., “Polarization-mode-dispersion equalization experiment using a variable equalizing optical circuit controlled by a pulse-waveform-comparison algorithm,” OFC'94 Technical Digest, paper TuN4, pp. 62-64; Ono, et al., “Polarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systems”, Journal of Lightwave Technology, Vol. 12, No. 5, May 1994, pp. 89-91; Takahasi, et al., “Automatic Compensation Technique for Timewise Fluctuating Polarization Mode Dispersion in In-line Amplifier Systems”, Electronics Letters, Vol. 30, No. 4, February 1994, pp. 348-49; and WO 93/09454, Rockwell, Marshall A.; Liquid Crystal Optical Waveguide Display System).
U.S. Pat. No. 5,859,939 (Fee et al.) discloses a polarization beam splitter that separates the optical data signal into first and second orthogonally polarized optical signals. A first variable time delay element provides a first incremental propagation delay for the first polarized optical signal. A second variable time delay element provides a second incremental propagation delay for the second polarized optical signal. The first and second variable time delay elements consist of a series of optical switches optically interconnected by different incremental lengths of optical fiber. For example, 2×2 optical switches are provided for switching between a reference fiber segment and a respective delay fiber segment to provide a relative incremental propagation delay. A controller controls optical switches in the first and second variable switching delay elements to set first and second incremental propagation delays. The transition to and from the optical switches is a source of signal loss.
What is needed is a PMD compensation method and system that is relia
Hamerly Mike E.
Smith Robert G.
Smith Terry L.
Theiss Silva K.
Weaver Billy L.
3M Innovative Properties Company
Bardell Scott A.
Bovernick Rodney
Pa Sung
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