| Precise optical phase control of light sources is very important for coherent optical transmission with heterodyne detection and coherent optical measurement. With such applications, the optical frequency difference between a transmitter and a local oscillator (LO) must be kept constant in order to obtain a stable intermediate frequency (IF) signal. In a heterodyne detection system the use of an Optical Phase Locked Loop (OPLL) is a key technique for automatic frequency control. The problem of phase locking can be minimized by heterodyne OPLL scheme which gives the ability to control of the optical phase of lasers by varying the phase of the RF offset signal. As a feedback system, the main goal of my research is to design an OPLL to lock the two lasers to a stable difference frequency to make it more prone to instability with heterodyning two independent lasers by choosing an appropriate loop gain and dynamic response of the OPLL. Reduction of phase noise of the laser and long-term frequency stability of OPLL is an important factor in order to realize a precise OPLL. Only making the loop delay smaller enough or lasers having sufficiently small linewidth makes the system interesting. The lock-in range and increase of operation frequency of the OPLL also need further investigation. |
