This invention relates to optical pulse sources, and in particular to a return-to-zero or carrier suppressed return-to-zero optical pulse source particularly useful in long haul optical communication systems.
Pulses in the return-to-zero (RZ) modulation format are important in long haul and ultra long haul optical fiber communication systems. RZ pulses are optical pulses where the light level reaches or xe2x80x9creturns toxe2x80x9d zero for some predetermined time interval between pulses. An RZ pulse train is a continuous stream of same shaped pulses.
RZ pulses can be shaped to mitigate the impact of fiber chromatic dispersion, nonlinearity, and polarization mode dispersion as the optical signal travels over long fiber lengths. And, by varying the shape, duty cycle, or optical phase of an RZ pulse train, the power spectrum can be tailored to a specific system channel bandwidth and efficiency requirement. For example, RZ pulses can be formed with alternate IT phase shift, and such RZ pulses are known as carrier suppressed RZ (CSRZ) pulses.
When data is encoded onto an RZ pulse stream, data xe2x80x981xe2x80x99s are represented by pulses. The absence of pulses (intervals of no light of one bit duration) represent data xe2x80x980xe2x80x99s. An important aspect of the RZ modulation format is that where two data xe2x80x981xe2x80x99s come in sequence, the light returns to zero momentarily between successive xe2x80x981xe2x80x99s.
Early optical RZ pulse generators simply switched on and off to generate the optical pulses. This direct modulation scheme proved ineffective because intermittent powering of the laser produced an undesirable variation of the carrier wavelength on a time scale of individual pulses (chirp). Chirp causes the pulse power spectrum to vary from the optimum spectrum of an undistorted RZ shaped pulse. This variation can adversely affect bandwidth and produces signal distortion.
More recent schemes for producing RZ pulses have minimized chirp to tolerable levels. One such scheme was described in U.S. Pat. No. 5,477,375, Optical Soliton Generator to Korotky, et al. The ""375 patent describes a CW laser followed by an amplitude modulator and a phase modulator, and finally a pulse compressor to generate an optical pulse train. The same principle was used for optical pulse generation with integrated Mach-Zehnder modulators based on GaAs and enabled integration of the pulse generator and the data modulator (Griffin, et al, Integrated 10 Gb/s Chirped Return to Zero Transmitter using GaAs/AlGaAs Modulators, OFC 2001, PD-15).
To date, proposed solutions for the generation of chirp-free RZ or CSRZ pulse trains are still complex and costly. Accordingly, there is a need for a low cost method and apparatus for generating RZ and CSRZ pulses.
In accordance with the invention, a train of RZ or CSRZ pulses is produced by passing phase modulated laser light through a delay interferometer. The parameters of the phase modulation and the delay interferometer are calculated from the desired pulse train characteristics (e.g. repetition rate, RZ or CSRZ, duty cycle). A directly modulated CW laser, or a CW laser followed by a phase modulator, produces the constant amplitude, phase modulated light. The phase modulated signal is split into two paths. One signal path is delayed with respect to the other by the calculated delay. The signals are recombined in an optical coupler to produce an RZ pulse train and/or a CSRZ pulse train.