Abstract
When a fraction of the light reflected by an external cavity re-enters the laser cavity, both the amplitude and the frequency of the lasing field can be modulated. This phenomenon is called the self-mixing effect (SME). A self-mixing laser diode (SM-LD) is a sensor using the SME. Usually, such LDs operate below the stability boundary where no relaxation oscillation happens. The boundary is determined by the operation condition including the injection current, optical feedback strength and external cavity length. This paper discovers the features of an SM-LD where the LD operates beyond the stability boundary, that is, near the relaxation oscillation (RO) status. We call the signals from such a SM-LD as RO-SM signals to differentiate them from the conventional SM signals reported in the literature. Firstly, simulations are made based on the well-known Lang and Kobayashi (L-K) equations. Then the experiments are conducted on different LDs to verify the simulation results. It shows that a RO-SM signal exhibits high frequency oscillation with its amplitude modulated by a slow time varying envelop which corresponds to the movement of the external target. The envelope has same fringe structure (half-wavelength displacement resolution) with the conventional SM signals. However, the amplitudes of the RO-SM signals are much higher compared to conventional SM signals. The results presented reveal that an SM-LD operating near the RO has potential for achieving sensing with improved sensitivity.
Highlights
In 1968, it was reported that when a fraction of the light back-reflected or backscattered by a remote target is allowed to re-enter the laser cavity, both the amplitude and the frequency of the lasing field can be modulated [1]
The discovery presented in this paper is very important, as we found that, by both simulations and experiments, a SMI system can and sometimes inevitably enter into the semi-stable region, e.g., for the case when the feedback level C is around 2.5, which is quite commonly reported for the moderate feedback level in the literature
The results show that relaxation oscillation (RO)-SM signals are highly sensitive to a moving target in contrast to conventional SM signals and have potential sensing applications
Summary
In 1968, it was reported that when a fraction of the light back-reflected or backscattered by a remote target is allowed to re-enter the laser cavity, both the amplitude and the frequency of the lasing field can be modulated [1]. When an LD operates in the semi-stable region (with the value of C above the stability boundary but below the chaotic level), the waveform of the laser intensity E2 (t) exhibits a stable oscillation and may be capable of sensing the movement of the external target. The discovery presented in this paper is very important, as we found that, by both simulations and experiments, a SMI system can and sometimes inevitably enter into the semi-stable region, e.g., for the case when the feedback level C is around 2.5, which is quite commonly reported for the moderate feedback level in the literature In this case, the usual detection approach by employing the PD packaged at the rear of an LD is not able to fully detect a RO-SM signal due to its limited bandwidth. The results show that RO-SM signals are highly sensitive to a moving target in contrast to conventional SM signals and have potential sensing applications
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