Research On Linewidth Reduction For Semiconductor Laser

Semiconductor laser is widely used on the optical fiber sensing, optical communications for its high reliability, long life, low cost, small in size, frequency tunable and so on. Generally semiconductor laser's linewidth reaches a few GHz and its frequency noise and phase noise is very high, which limits its application in some fields. Narrow, ultra-stable semiconductor lasers have a variety of important applications such as optical atomic clocks, optical communications, high-resolution laser spectroscopy, measurements of fundamental physical constants and tests of fundamental physics. This article introduce the developments, applications and research of narrow linewidth semiconductor laser, and introduce the methods of linewidth reduction and frequency stabilization for semiconductor laser, and a experimental system used to reduce the linewidth of semiconductor laser with external folded cavity optical feedback here. The work of this article mainly contains two parts: theoretical and experimental parts.In theoretical research, firstly, we recommend the fundament of optical resonant cavity and the principle of mode matching. Secondly, we calculate the principle of linewidth reduction and frequency stabilization with optical feedback based on the rate equations. The results show that a low level of optical feedback can efficiently reduce semiconductor laser's linewidth and also automatically stabilize the laser's oscillation frequency, which is relative to the cavity finesse and the feedback ratio. Finally, we analyze the effect on phase of feedback light to cavity field theoretically, results show that the best experimental conclusion can be obtained when the phase of feedback light is on phase with the light field in the laser cavity.In experimental part, we design a 640 nm narrow linewidth semiconductor laser based on the external folded cavity optical feedback. Specified contents are shown as follows:(1) We design a driving circuit to control the current and temperature of the laser diode. The accomplished circuit is tested using the 640 nm semiconductor laser, results show that the controlled frequency stability of this driving circuit can be 2.5×10-4nm in a long time, so the results meet the requirement for the study on linewidth reduction and frequency stabilization for semiconductor laser.(2) We design two Fabry-Perot cavity with the finesse of 4200 and 28200 separately. By changing the cavity length we can shift the resonant frequency of the cavity to match the laser frequency, when the mode matching is obtained, resonant optical feedback occurs and the laser frequency locked to the cavity resonance, the feedback light can suppress the oscillation of side-mode and narrow the linewidth of semiconductor laser.(3) We design a circuit to automatically control the phase of feedback light, when the feedback phase is on phase with the cavity light phase of laser diode, the system acquire a 0.39 MHz linewidth semiconductor laser output with single-mode operation.Comparing with the multi-mode operation and GHz level frequency excursion, the linewidth is efficiently narrowed and the frequency stability is improved effectively. Results also show that the intensity noise is suppressed more than 30 d B in low frequency range, this work will broaden the application for this kind of semiconductor laser.