Effect of Injection Current on Signal Strength in a Single-Mode Laser Diode Optical Feedback Interferometer Subject to Weak Feedback
Résumé
Optical feedback interferometry (OFI) sensing is widely deployed in industrial and laboratory environments, due to its simple and low-cost optical setup compared to conventional interferometry techniques, in many sensing applications such as: the measurement of displacement, distance, velocity and vibration [1]. OFI signals can be acquired either through the observation of the power fluctuations with a monitoring photodiode implemented on the rear facet of the laser diode (PD signals) or by the amplification of the variations in the laser diode voltage (LV signals) [2]. In both methods, maximum signal-to-noise-ratio (SNR) should be maintained to achieve the best performance of the sensor. In this paper, we present a simple analytical model based on Lang and Kobayashi rate equations [3]. The model suggests a significant difference in injection current dependence of the strengths of the two interferometric signals. The PD signal is linearly dependent on injection current while the LV signal depends on the slope efficiency and the temperature. For an ideal laser diode, where slope efficiency is constant, the LV signal should remain constant. However, in practice slope efficiency decreases with the increase of injection current, as shown in Fig. 1a, due to thermal effects [4], resulting in the decrease of the LV signal with injection current.
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