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Frontiers of Optoelectronics

Front Optoelec Chin    2011, Vol. 4 Issue (3) : 292-297     DOI: 10.1007/s12200-011-0142-0
Generation of 40 GHz phase stable optical short pulses using intensity modulator and two cascaded phase modulators
Yu JI(), Yan LI, Wei LI, Xiaobing HONG, Hongxiang GUO, Yong ZUO, Kun XU, Jian WU, Jintong LIN
State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
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Pulse sources based on lithium niobate modulators are very attractive for optical time division multiplexing (OTDM) transmission systems because the modulators are now commercially available, qualified for system use, and can operate up to very high speeds and over a wide wavelength range. In this paper, we describe the principles of operation and performance of the pulse source based on lithium niobate modulators. The pulse source is based on a Mach-Zehnder intensity modulator (IM) and two phase modulators (PMs). The continuous-wave (CW) light is modulated in an IM and then strongly phase modulated in two cascaded PMs. The chirped pulses are subsequently compressed to desired width using dispersion compensation technology. This method has the advantage of acquiring larger chirp using normal PM rather than that special designed PM of very low Vπ. It can also generate shorter pulses than conventional methods incorporating only one PM driving by a radio frequency (RF) signal with the power larger than 1 W which may damage the device. Generation of 40 GHz optical pulses shorter than 2 ps is theoretically illustrated, simulated and experimentally verified. Experimental results show that 40 GHz phase stable optical pulses with pulse-width of 1.88 ps, extinction ratio (ER) larger than 20 dB, the timing jitter of 57 fs and signal-to-noise ratio (SNR) of 32.8 dB can be achieved. This is also a cavity-less pulse source whose timing jitter is determined only by the RF source rather than by the actively controlled cavity. In the experiment, the phase noise of the RF source we used is as low as -98.13 dBc/Hz at a 10 kHz offset frequency which resulting very low timing jitter of generated pulses. The pulses are then modulated at 40 Gbaud/s with an inphase/quadrature (I/Q) modulator and multiplexed to 160 Gbaud/s with less interference between each other. After back-to-back demultiplexing by an electro-absorption modulator (EAM) to 40 Gbaud/s and demodulation by a delay interferometer (DI), clear and opened eye diagrams of 40 Gbaud/s I and Q tributary signals are obtained which verify the good performance of generated pulses in the 160 Gbaud/s differential quadrature phase shift keying (DQPSK) OTDM system and further prove the phase stability and high quality of generated pulses.

Keywords short pulse      intensity modulator      phase modulator      pulse compression      phase stable      differential quadrature phase shift keying (DQPSK)     
Corresponding Authors: JI Yu,   
Issue Date: 05 September 2011
 Cite this article:   
Yu JI,Yan LI,Wei LI, et al. Generation of 40 GHz phase stable optical short pulses using intensity modulator and two cascaded phase modulators[J]. Front Optoelec Chin, 2011, 4(3): 292-297.
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Yan LI
Wei LI
Xiaobing HONG
Hongxiang GUO
Yong ZUO
Kun XU
Jian WU
Jintong LIN
Fig.1  Principle of pulses generated by IM
Fig.2  Relationship of intensity and phase modulation
Fig.3  Pulses generated from IM
Fig.4  Intensity and phase properties of pulses after PMs
Fig.5  Pulse width versus DCF length
Fig.6  Generated pulses after DCF
Fig.7  Experimental setup of proposed short pulse source
Fig.8  Generated pulse train and its spectrum after IM. (a) Waveform of pulse train; (b) corresponding spectrum
Fig.9  Generated pulse train and its spectrum after DCF. (a) Waveform of pulse train; (b) corresponding spectrum
Fig.10  Eye diagram of 160 Gbaud/s OTDM signal
Fig.11  Demodulated eye diagrams. (a) 40 Gbaud/s I tributary signals; (b) 40 Gbaud/s Q tributary signals
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