The quantum dot optical frequency comb laser (QD-CML) is an attractive photonic device for generating a stable emission of fine multiple-wavelength peaks. In the present paper, 1.0-GHz and 10-ps-order short optical pulsation is successfully demonstrated from a hybrid mode-locked QD-CML with an ultrabroadband wavelength tuning range in the T+O band. In addition, 10-GHz high-repetition intensity-stable short optical pulse generation with a high S/N ratio is successfully demonstrated using an external-cavity QD-CML with a 10th-harmonic mode-locking technique.

Optimization of InGaAs/InAlAs multiple quantum well structures for high-speed and low-driving modulation, as well as polarization insensitivity and low chirp, was investigated as a function of well thickness and strain magnitude. As a result, very short optical pulses with 4-6 ps was obtained using a low driving-voltage (<2. 0 Vpp) electroabsorption modulator module operating at a 40-GHz large signal modulation. Small chirp operation for low insertion loss (<8 dB from fiber-to-fiber) with prebias was also demonstrated and the product of the pulse width and the spectral width was estimated to be 0. 39 for a 5 ps pulse width that is nearly transform-limited.

We investigate the relaxation oscillation characteristics of an actively mode-locked fiber laser and a novel stabilizing method of the laser theoretically and experimentally. The stabilizing method controls cavity length to suppress the rf power of the relaxation oscillation frequency of the laser output, and can directly monitor the stability of the laser to ensure the most stable operation. With this method, the rf power ratio between mode-locking frequency and the background noise can be kept to more than 70 dB, and highly stable transform-limited pulse generation is achieved. Bit-error-free operation at 6. 3 GHz over 10 hours is successfully demonstrated. The stability of the center wavelength of the laser output and the required accuracy of cavity control for high-speed laser operation are also discussed.

This paper reviews very high-speed optical signal processing technology based on the instantaneous characteristic of optical nonlinearities. Focus is placed on 100-Gbit/s optical time-division multiplexing (TDM) transmission systems. The key technologies including ultrashort optical pulse generation, all-optical multiplexing/demultiplexing and optical timing extraction techniques are alse described together with their major issues and future prospects.

A new device consisting of an optical pulse generation section and pulse coding section monolithically integrated on a single-chip has been developed. The pulse generation section consists of a multiple quantum well (MQW) electroabsorption modulator integrated with an MQW DFB laser. The modulator operates at large-signal modulation and low voltage (from 2 to 3-V DC bias with a 3.2-V peak-to-peak RF signal). The second modulator is operated independently as a pulse encoder. An approximately transform-limited optical pulse train, whose full width at half maximum (FWHM) in the time domain is less than 17-ps and spectral FWHM is 28-GHz, is obtained with a repetition frequency of 10-GHz. Compressive strain is introduced in both InGaAsP quantum wells in order to obtain efficient device characteristics. These include a low threshold current (18-mA) for the laser, and low driving voltage (30-dB for 3-V swing) and high-speed operation (over 12-GHz for a 3-dB bandwidth) for the modulators. Demonstrations show that this new device generates short optical pulses encoded by a pseudo-random signal at a rate of 10 Gbit/s. This is the first time 10 Gbit/s modulation has been achieved with a multi-section electroabsorption modulator/DFB laser integrated light source. This monolithic device is expected to be applied to optical soliton transmitters.