Picosecond optical pulses are widely used in optical communication systems, such as the optical time division multiplexing (OTDM) and photonic analog-to-digital converter (ADC). We have proposed and demonstrated a simple method to generate picosecond optical pulse using the mach-zehnder modulator (MZM), phase modulator (PM) and single model fiber (SMF). The phase modulator is used to generate a frequency chirp which varies periodically with time. The MZM is used to suppress the pedestal of the pulse and improve the performance of the pulse. The SMF is used to compensate the frequency chirp. We have carried out theoretical analysis and numerical simulation for the generation process of the picosecond optical pulse. The influence of phase shift between the modulation signals loaded on the MZM and PM is analyzed by numerical simulation and the conditions for the generation of picosecond optical pulse are given. The formula for calculating the optimum length of SMF which is used to compensate the linear chirp is given. The optical pulses with a repetition frequency of 10 GHz and a pulse width of 8.5 ps were obtained. The time-bandwidth product was as small as 1.09 and the timing jitter is as low as 83 fs.
In microwave photonic radar systems, the generation and transmission of linear frequency modulated wave (LFMW) are influenced by dispersion in the optical systems and devices. As the bandwidth of LFMW used in radar systems becomes greater, the effect of dispersion on wideband optical signal cannot be ignored and should be well compensated. Traditional compensation methods of dispersion in optical systems are facing difficulties when dealing with high order dispersion and wideband signals with demand of precise frequency control. This paper proposed a method of dispersion compensation for wideband LFMW transmission in optical systems with dispersion, based on single-side band (SSB) modulation and pre-distortion, and the linear mapping from time to frequency of LFMW. Dispersion of the transmission systems is measured to calculate the pre-distortion of LFMW. Then the single frequency laser is SSB modulated by microwave LFMW in amplitude to remove the influence from dispersion on the envelope, and the LFMW is predistorted with the calculated results in generation. In the proof-of-concept experiment, an LFMW with period of 10 us, pulse width of 8 us and instantaneous frequency from 8 GHz to 12 GHz is modulated on the laser with wave length of 1550 nm, and transmitted in dispersion fiber or devices. Second order dispersion of about 1713 ps/nm introduced by dispersion fiber is compensated in experiments. Third and fifth orders dispersion introduced by an equivalent electronic filter are compensated, and 44% improvement of the linearity of frequency modulation after compensation is obtained in the experiment.
A tunable ultraflat optical frequency comb generator based on the optoelectronic oscillator (OEO) using a dual-parallel Mach–Zehnder modulator (DPMZM) is proposed and experimentally demonstrated. By incorporating a tunable DPMZM-OEO, five comb lines were generated with frequency spacing from 5 to 12 GHz under a wide central wavelength tuning from 1530 to 1560 nm, and a comb flatness of 0.3 dB is obtained. The corresponding signal generated by the DPMZM-OEO is also measured, and the phase noise of the frequency tunable signals is as low as −125 dBc/Hz at 10-kHz frequency offset.
Nonlinear wave mixing in optical microresonators offers a route to chip-level optical frequency combs with many promising applications. The properties of the combs generated depend crucially on the interaction between nonlinearity and dispersion. This paper will discuss our research on Kerr comb generation in silicon nitride chip-scale microresonators, with an emphasis on distinct features observed in the normal and anomalous dispersion regimes. The topics covered include comb initiation, comb coherence and mode-locking, power conversion efficiency, and second-harmonic involved comb generation.