Phase-locked Optical Signal Recovery

Abstract

Abstract : Phase-locked receivers have long been used for the recovery of signals in low signal to noise ratio (SNR) environments, such as space applications. A loop for use with semiconductor laser generated signals would require a bandwidth of >500 MHz for low phase error variance (<0.01 rad2) tracking, constraining the propagation delay to <0.3 ns. Since this requires an equivalent path length of <100 mm, conventional realizations in optical fiber technology are not possible, requiring special micro-optical solutions. An alternative technique is to use optical injection locking. This avoids loop propagation limitations, but stable locking ranges are typically <1 GHz, requiring precision (<10 mK) control of the receiver laser and continuous adjustment to track drift in the incoming signal. At University College London (UCL), we have developed a technique that overcomes the limitations described above-the Optical Injection Phase Lock Loop (OIPLL) in which a narrow bandwidth optical phase lock loop (OPLL) is used to control the free-running frequency of an optically injection locked laser to compensate for thermal drift, drift in the incoming signal and low-frequency noise. If successful this would enable low SNR optical signals to be recovered without the need for constant skilled adjustment of the receiver system. For the feasibility study we propose to investigate two possible detection schemes. The first is an homodyne OIPLL. Although this appears simple in principle there are significant challenges in implementation. The second scheme is based on heterodyne detection, with the heterodyne frequency chosen to be remote from data modulation interference. In this approach, the incoming signal passes through a modulator, where it is sinusoidally intensity modulated at microwave frequency by the offset source. The slave laser is injection locked to one of these side frequencies through an optical circulator.