The Study On Control Of Monolithic Tunable Semiconductor Lasers

Tunable semiconductor lasers have been considered as one of the key devices for fiber-optic communication systems and next-generation optical networks. They can be used in dense wavelength division multiplexing (DWDM) systems to provide timely and effective inventory management and rapid channel establishment with great reduction in backup cost. Moreover, they can be used in next-generation reconfigurable optical networks to provide automatic wavelength configuation, wavelength conversion and wavelength routing, which extremely enhance the flexibility and the efficiency of bandwidth utilization in networks. Tunable semiconductor lasers have attracted much attention due to their wide applications. In this dissertation, the control techniques for tunable semiconductor lasers have been explored and investigated. The main contents are as follows:(1) The basic theory of tunable semiconductor lasers, including gain characteristics of semiconductor lasers, wavelength selecting mechanism of distributed Bragg reflector (DBR) gratings and Vernier tuning principle of sampled grating DBR (SGDBR) lasers, have been described. This can be considered as the theoretical basis of the research for control of SGDBR lasers and SGDBR lasers monolithically integrated with semiconductor optical amplifiers (SOAs) (SGDBR-SOA integrated devices).(2) Based on the. wavelength tuning principle, an automatic wavelength test system has been designed and established for wavelength scanning of our SGDBR lasers. This automatic test system can be used for accurate and rapid wavelength calibration of SGDBR lasers, which is the preparation for dynamic performance test of the devices.(3) The control of the thermoelectric cooler (TEC) inside the laser has been studied. Two different control methods of TEC have been performed and compared on an SGDBR laser. The impact of the TEC control circuits on the wavelength switching dynamics of the SGDBR laser has been analyzed and the improving design has been proposed according to the experimental results.(4) Wavelength switching dynamics of SGDBR lasers and SGDBR-SOA integrated devices have been experimentally studied. Experimental results of wavelength switching transients have been analyzed. Appropriate shuttering operation of SOA section during the wavelength switching process has been implemented to eliminate the impact of transient emitting modes. Great improvement on dynamic performance of the SGDBR laser can be observed.(5) A small-signal equivalent microwave circuit model of the gain section of the SGDBR-SOA integrated device with direct current modulation has been derived from the rate equations of carrier and photon density. Our model takes into account electrical parasitic effect and series cascade action of SGDBR laser and SOA. The circuit parameters have been extracted by a synthetical approach, which combines the globle numerical optimization technique with the analytical extraction method using the measured S-parameters (reflection coefficient S11 and transmission coefficient S21) of the device for different bias currents of the gain section and different operating wavelength of the device. The impact of the different test conditions can be embodied in different parameter values. Finally the responses of the device for small-signal current modulation on the gain section have been simulated hierarchically. The simulation results indicate the electrical parasitic effect and the integrated SOA have depressed the response of the SGDBR laser.(6) A small-signal equivalent microwave circuit model for direct current modulation on the SOA section of the SGDBR-SOA integrated device has been derived from the rate equations of carrier and photon density. To take into account the wavelength dependence of the circuit parameters of the model, the parameter extraction has been performed by fitting the circuit model including parasitic effect with the measured S-parameters of the integrated device for different operating wavelengths of the SGDBR laser. The obtained circuit model has been used to simulate the frequency modulation response of the device. The simulation results show the frequency modulation response of the device is related with the bias current of the SOA section, which can be discriminated by the gain saturation condition of the integrated SOA to cause contrary variation trends of the frequency modulation responses. The optical frequency chirp caused by the current modulation on the SOA section has also been simulated. Simulated results of the optical frequency chirp are comparable to that caused by the direct current modulation on the gain section of the SGDBR laser and coincide with the measured results in some published documentation.(7) The optical gain of the SOA section of the SGDBR-SOA integrated device has been first simulated in time domain with a microwave equivalent circuit model of direct current modulation of the SOA section. Simulated results show a resonance behavior indicating the possibility to generate ultra-wideband (UWB) signals with complex shapes in the time domain. The UWB pulse generation is then experimentally demonstrated for different selected wavelength channels with an SGDBR-SOA integrated device. The generated spectral envelopes in the frequency domain are compliant with the Federal Communications Commission (FCC) requirements. Our method has introduced the wavelength tunability for the photonic UWB generation.(8) According to the wavelength tuning characteristics of SGDBR lasers, a precise and rapid control method has been introduced for wavelength locking of SGDBR lasers. With the help of the proposed idea, a practical wavelength locking system for our lasers has been designed.