Search

Optical frequency comb sources (OFCS) have recently attracted a lot of interest due to wide ranging applications such as optical arbitrary waveform generation [1], photonic microwave signal generation [2], optical signal processing [3] and multi-carrier spectrally efficient transmission techniques with the sub-channel spacing equal to the symbol rate of each sub-channel [4]. OFCS with good spectral flatness, stability and low linewidth are highly desirable for such applications. One of the conventional approaches used in realizing a comb source is based on mode-locked semiconductor and fibre lasers [5]. Although this technique can generate multi-carrier signals spanning over a wide bandwidth, it inherently suffers from cavity complexity and does not offer the free spectral range (FSR) tunability since the comb line spacing is fixed by the cavity length of the laser. Moreover, the optical linewidth of the individual comb lines can be relatively large (several MHz) preventing higher order (or low baud rate) advanced modulation formats to be imposed. Another technique that has been reported entails the use of a single, cascaded or dual drive Mach-Zehnder modulators (MZM) to generate the phase correlated optical frequency comb [6]. Although this technique provides a relatively flat optical comb, the large insertion loss of the modulator coupled with the modulation efficiency can prove prohibitive. The extra optical component also adds to the cost and complexity of the transmitter, rendering this technique unsuitable for short reach applications.

Synchronization of a 40 GHz quantum-dash mode-locked Fabry-Perot laser diode with optically injected pulse streams is experimentally studied by analyzing the linewidth of the associated 40 GHz beat-tones. Injected signals consist of trains of 1.6 ps pulses at various repetition rates, spanning from 10 to 160 Gb/s of encoded or not with pseudo-random on-off key data sequence. Data injections at the fundamental bit-rate (40 Gb/s), harmonic at 80 and 160 Gb/s and sub-harmonic at 10 and 20 Gb/s are used to synchronize the mode-locked laser. The response is a stable 40 GHz clock pulse train featuring a width of 1.8 ps. Moreover, the 40 GHz beat-tones associated to the recovered clock pulses exhibit a full-width at half maximum smaller than 8 Hz regardless of the injected data rate. It is observed that frequency components at 10 and 20 GHz do not create any amplitude modulation on the recovered 40 GHz clock pulses by the injection of data signals at 10 and 20 Gb/s. Similarly, synchronization of the quantum-dash laser with pulse streams at 80 and 160 Gb/s is sustained in spite of negligible components at and below 40 GHz are injected in these two data rates.