The Effect of Injection Current and Temperature on Signal Strength in a Laser Diode Optical Feedback Interferometer

International audienceWe present a simple analytical model that describes the injection current and temperature dependence of optical feedback interferometry signal strength for a single-mode laser diode. The model is derived from the Lang and Kobayashi rate equations, and is developed both for signals acquired from the monitoring photodiode (proportional to the variations in optical power) and for those obtained by amplification of the corresponding variations in laser voltage. The model shows that both the photodiode and the voltage signal strengths are dependent on the laser slope efficiency, which itself is a function of the injection current and the temperature. Moreover, the model predicts that photodiode and voltage signal strengths depend differently on injection current and temperature. This important model prediction was proven experimentally for a near-infrared distributed feedback laser by measuring both types of signals over a wide range of injection currents and temperatures. Therefore, this simple model provides important insight into the radically different biasing strategies required to achieve optimal sensor sensitivity for both interferometric signal acquisition schemes. 1. Introduction Optical feedback interferometry (OFI) is a promising sensing technique for both industrial and laboratory environments due to its simple optical setup relative to other interferometric techniques. Typical sensing applications of OFI are the measurement of displacement, distance, and velocity [1]. OFI signals can be acquired by two different means: (i) by observing power fluctuations in the light emitted by the rear facet of the laser diode with a monitoring photodiode (denoted here as the PD signal), or (ii) by amplifying voltage variations across the laser terminals (denoted here as the LV signal) [2]. The latter is the only viable measurement approach when a monitoring photodiode is not included in the laser diode package, for example when an array of laser diodes is used [3]. For both methods of signal acquisition , in order to achieve the best performance of the OFI sensor it is essential to maintain the signal-to-noise ratio (SNR) at a maximum. The dependence of OFI signal strength on bias current has been investigated previously in experiment [4–6], with earlier models providing important insight into the genesis of the interferometric signal [2, 7, 8]. Experimental results reported to date suggest that the relationship between OFI signal strength and bias current depend