Study Of Grating-coupled External Cavity Tunable Semiconductor Laser Based On GaN And Its Actively Mode-locked Research

Semiconductor lasers are restricted in many applications because of their broad linewidth, poor coherence, and untunable wavelength. By using the external cavity (EC) optical feedback technology, we can obtain the laser beam with the characteristics of stable frequency, narrow linewidth and tunable wavelength, which has broad application prospects in optical telecommunication and other areas. Especially, due to the short emission wavelength, the blue-violet EC laser has potential applications in many areas, such as high resolution laser spectroscopy, gas monitoring sensor, and high capacity holographic data storage. Besides, ultra-short pulse laser can be realized by using actively mode-locked technology. It’s an ideal light source for high-speed switching and time-resolved spectroscopy. This paper mainly devotes to the research of GaN-based grating-coupled EC tunable semiconductor laser and its actively mode-locked technology. The main meaningful results of this article are as follows:1. The Littrow EC laser system was constructed. The semiconductor gain device, collimating lens, and blazed grating were selected to constitute the Littrow configuration. Furthermore, the main mechanical structure was designed.2. The output characteristics of the grating-coupled EC laser were given and discussed in detail. Firstly,600-lines/mm blazed grating was used as the mode selection component. The spectral characteristics, tuning bandwidth, threshold, and output power of EC laser were analyzed. The results show that the threshold current of the semiconductor laser diode decreases by27%, indicating high coupling efficiency between the external cavity and inner cavity. Besides, by changing the angle of the blazed grating, the total wavelength tuning range can reach up to7nm. Furthermore, the output power was improved by using the1800-lines/mm blazed grating. And then, the performance of EC laser under large injection current was studied. By changing the angle of the blazed grating along with the injection current, the EC laser with the characteristics of high power, narrow linewidth and tunable wavelength was achieved. 3. The experiment of actively mode-locked GaN-based EC laser was conducted. DC bias current and radio frequency modulation current were added on the gain device simultaneously in the experiment. Rely on the effect of gain switch modulation induced by the radio frequency sinusoidal modulation signal, a mode-locked EC laser with a variable repetition rate was realized.