Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2003-05-22
2004-09-28
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S199200, C359S199200
Reexamination Certificate
active
06798557
ABSTRACT:
TECHNICAL FIELD
This invention relates to optical transmission systems and, more particularly, to phase shift keying of optical signals.
BACKGROUND OF THE INVENTION
Recently, there has been growing interest in alternate optical modulation formats for use in high bit-rate optical communications systems. To this end, optical differential phase shift keying (ODPSK) is a promising candidate to increase system capacity, reach and spectral efficiency. However, up to now only optical differential binary phase shift keying (ODBPSK), as described in an article entitled “2.5 Tb/s (64×42.7 Gb/s) Transmission Over 40×100 km NZDSF Using RZ-DPSK Format and All-Raman-Amplified Spans”, by A. H. Gnauk et al., OFC 2002, and optical differential quadrature phase shift keying (ODQPSK), as described in an article entitled “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission Using GaAs/AlGaAs Integration”, by R. A. Griffin et al., OFC 2002, have been successfully demonstrated at higher bit-rates.
In these systems, there is a need for an improved optical encoder. Indeed, one problem that has not been addressed is that prior OPSK arrangements could only perform a phase shift related directly to &pgr;, and at best could only realize a phase shift of &pgr;/2.
SUMMARY OF THE INVENTION
These and other problems and limitations of prior known optical PSK optical transmission arrangements are overcome by employing a unique optical encoder that utilizes an optical phase modulator, in which an arbitrary phase shift can be realized. The phase modulator is driven by an electrical signal, where the voltage amplitude is proportional to a desired arbitrary phase shift. However, any amplitude noise or poor rise and fall times of the driving voltage translates directly into phase errors. These problems are eliminated by employing a digital phase switch.
One encoder, in accordance with the invention, employs an optical phase modulator that can switch the optical phase in a binary manner, wherein the two phase states of the modulator can be chosen to differ by any desired arbitrary angle depending on the splitting ratio of the optical signal to be modulated.
Specifically, applicant's unique encoder invention is employed in PSK systems that implement optical N-state phase shift keying, for example, where N=4, 8, 16, . . . . These systems typically include a serial array of encoders each including a log
2
N optical binary phase shift modulator. Each of the optical modulators produces a binary phase shift modulated optical signal. The two output states of a modulator (e.g., binary +1 and −1) have a phase shift different than &pgr;. In preferred embodiments, the phase shift is &pgr;/N for the two output states of one of the modulators in an encoder in the serial array.
REFERENCES:
patent: 6594055 (2003-07-01), Snawerdt
patent: 2003/0058504 (2003-03-01), Cho et al.
patent: 2003/0133174 (2003-07-01), Salehi et al.
A.H. Gnauck, et al “2.4 Tb/s (64×42.7 Gb/s) Transmission Over 40×100 km NZDSF Using RZ-DPSK Format and All-Raman-Amplified Spans”, 3 pages, Mar. 2002,OFC 2002.
R. A. Griffin et al., “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission Using GaAs/AlGaAs Integration” 3 pages, Mar. 2002,OFC 2002.
Mack Ricky
Thomas Brandi N
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