Timing-jitter measurements in optically generated subpicosecond pulses by a quantum-dash passively mode-locked semiconductor laser as a function of the bias current are reported. All the measurements are retrieved from a second-harmonic-generation frequency-resolved optical gating system. A decreasing trend in the pulse width and the associated timing jitter is found with the bias current. Additionally, the optical and mode-beating linewidths are analyzed in terms of both the mode wavelength and the bias current. From our results, we can conclude that once the optical modes are phase locked, the optical linewidth associated to every individual longitudinal mode of the device under test does not have a significant impact on the mode-beating signal on neither the pulse width nor its respective timing jitter.

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We present a linear self-referenced measurement of the spectral amplitude and phase of a free-running quantum-dash modelocked laser diode. The technique is suitable for measuring optical signals with repetition rates up to 100 GHz. In contrast to many other linear techniques it requires no external electronic clock synchronized to the signal under test. Using this method we are able to compensate for the intracavity dispersion of the diode to demonstrate 500 fs pulses at a repetition rate of 39.8 GHz. We also use the technique to characterize the dependence of the diode's intracavity dispersion on the applied current.

A narrowband terahertz signal generated by a unitravelling carrier photodiode ͑UTC-PD͒ interfaced with a dual-mode Fabry-Pérot laser diode is demonstrated. A beat tone corresponding to the free spectral range is generated on the UTC-PD, and radiated by a transverse-electromagnetic-horn antenna. A terahertz signal at a frequency of 372 GHz, featuring a linewidth of 17 MHz is recorded by a subharmonic mixer coupled to an electrical spectrum analyzer. All components involved in this experiment operate at room temperature. The linewidth and the frequency of the emitted terahertz wave are analyzed, along with their dependency on dc-bias conditions applied to laser diode.

Wavelength tunability of an all-optical clock recovery operation based on a quantum dash mode-locked Fabry-Pérot laser diode is experimentally investigated. Synchronization of the device to the injection of 40 Gb/s NRZ incoming data is assessed by analyzing both the carrier-to-noise ratio and the linewidth of the 40 GHz beat-tones measured at the mode-locked laser output. Under optical injection, beat-tone linewidths below 10 Hz are measured. Recovered clock pulses featuring a width of 1.6 ps are obtained irrespective of the wavelength detuning between the laser spectra and the optical carrier of the incoming data stream. Index Terms-All-optical clock recovery, non-return-to-zero (NRZ), mode-locked laser, quantum dash, Fabry-Pérot laser. I. INTRODUCTIONll-optical clock recovery (CR) might be an essential technology in future optical networks and optical signal processing as the information is kept in its optical format. Clock recovery operations have been investigated by using mode-locked semiconductor laser diodes (MLLD) [1]-[6]. Such devices have attracted significant attention because of their stable operation, cost effectiveness, low energy consumption and small size, holding possibilities for monolithic integration. Among the different approaches of clock recovery with MLLDs, those based on quantum dot/dash (QDot/QDash) lasers have demonstrated good performance in terms of frequency stability and low timing jitter [5] because of their narrow spectral linewidth and small associated phase noise [5], [7].