Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1995-12-29
2001-10-30
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06310709
ABSTRACT:
TECHNICAL FIELD
The invention relates to the optical transmission of information and, more particularly, to synchronous polarization and phase modulation using a periodic waveform with complex harmonics for improved performance of optical transmission systems.
BACKGROUND OF THE INVENTION
Very long optical fiber transmission paths, such as those employed in undersea or transcontinental terrestrial lightwave transmission systems which employ optical amplifier repeaters, are subject to decreased performance due to a host of impairments that accumulate along the length of the optical fiber comprising the transmission path. Typically, these impairments vary with time and cause a random fluctuation in the signal-to-noise ratio of the received signal. These impairments can arise from accumulated noise effects caused, for example, by polarization hole-burning (“PHB”) in the erbium-doped fiber used in the optical amplifiers and waveform distortions caused by chromatic dispersion and optical nonlinearities throughout the transmission path. Scrambling the state-of-polarization of the launched optical signal to depolarize the optical carrier can improve the received signal-to-noise by reducing PHB. In my co-pending U.S. patent application Ser. No. 08/312,848, an arrangement is disclosed where the polarization scrambling frequency is chosen as the clock frequency that defines the bit rate of the transmitter. Such a technique may provide for more efficient use of optical bandwidth in wavelength division multiplexed (“WDM”) systems. This bit-synchronous polarization scrambling is also a particularly advantageous trade-off between the two regimes of low-speed and high-speed scrambling. In addition to the synchronous polarization scrambling, superimposed phase modulation (“PM”) can dramatically increase the eye opening of the received data pattern. The increase in eye opening results from the conversion of PM into bit-synchronous amplitude modulation (“AM”) through chromatic dispersion and nonlinear effects in the fiber.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and apparatus is provided that yields improved performance by modulating the optical phase and polarization of an optical signal with a periodic waveform having harmonic content that is more complex than that associated with a simple sinusoidal waveform. A phase modulator receives an optical signal onto which data has been modulated at a predetermined frequency. The phase modulator modulates the phase of the optical signal in a continuous manner with the periodic waveform with complex harmonics where the fundamental phase modulation frequency is equal to the same predetermined frequency at which the data is modulated onto the optical signal. In another illustrative embodiment of the invention, a polarization modulator further processes the optical signal by modulating the state-of-polarization of the signal in a continuous fashion with the periodic waveform with complex harmonics where the fundamental polarization modulation frequency is equal to the same predetermined frequency at which the data is modulated onto the optical signal. In addition to being continuously modulated, the polarization modulation is performed so that the average value of the state of polarization over each modulation cycle is substantially equal to zero.
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“Dual-Wavelength Data Transmitter for Reducing Facing in an Optical Transmission System”, N. S. Bergano 10, Ser. No. 08/173288, filed Dec. 23, 1993.
“Synchronous Polarization and Phase Modulation for Improved Performance of Optical Transmission Systems”, N. S. Bergano 11, Ser. No. 08/312848, filed Sep. 27, 1994.
“Spectral and Polarization Hole Burning in Neodymium Glass Lasers”, D. W. Hall et al., IEEE Journal of Quantum Electronics, vol. QE-19, No. 11, Nov. 1983.
Mayer Stuart H.
Pascal Leslie
Tyco Submarine Systems Ltd.
Young Mark K.
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