This paper presents a new method for measuring the linewidth enhancement factor (alpha factor) by the relaxation oscillation (RO) frequency of a laser with external optical feedback (EOF). A measurement formula for alpha is derived which shows the alpha can be determined by only using the RO frequencies and no need to know any other parameters related to the internal or external parameters associated to the laser. Unlike the existing EOF based alpha measurement methods which require an external target has a symmetric reciprocate movement. The proposed method only needs to move the target to be in a few different positions along the light beam. Furthermore, this method also suits for the case with alpha less than 1. Both simulation and experiment are performed to verify the proposed method.

A self-mixing interferometry (SMI) system is a laser diode (LD) with an external cavity formed by a moving external target. The behavior of an SMI system is governed by the injection current J to the LD and the parameters associated with the external cavity mainly including optical feedback factor C , the initial external cavity length ( L0 ) and the light phase (∅0) which is mapped to the movement of the target. In this paper, we investigate the dynamic behavior of an SMI system by using the Lang-Kobayashi model. The stability boundary of such system is presented in the plane of (C , ∅0), from which a critical C (denoted as Ccritical) is derived. Both simulations and experiments show that the stability can be enhanced by increasing either L0 or J . Furthermore, three regions on the plane of (C , ∅0) are proposed to characterize the behavior of an SMI system, including stable, semi-stable and unstable regions. We found that the existing SMI model is only valid for the stable region, and the semi-stable region has potential applications on sensing and measurement but needs remodeling the system by considering the bandwidth of the detection components. Abstract: A self-mixing interferometry (SMI) system is a laser diode (LD) with an external cavity formed by a moving external target. The behavior of an SMI system is governed by the injection current J to the LD and the parameters associated with the external cavity mainly including optical feedback factor C , the initial external cavity length ( 0 L ) and the light phase ( 0  ) which is mapped to the movement of the target. In this paper, we investigate the dynamic behavior of an SMI system by using the LangKobayashi model. The stability boundary of such system is presented in the plane of ( C , 0  ), from which a critical C (denoted as critical C ) is derived. Both simulations and experiments show that the stability can be enhanced by increasing either 0 L or J . Furthermore, three regions on the plane of ( C , 0  ) are proposed to characterize the behavior of an SMI system, including stable, semi-stable and unstable regions. We found that the existing SMI model is only valid for the stable region, and the semistable region has potential applications on sensing and measurement but needs re-modeling the system by considering the bandwidth of the detection components. Disciplines Engineering | Science and Technology Studies

A laser diode (LD) with external optical feedback can generate undamped relaxation oscillation (RO) under certain operational conditions. The RO frequency can be modified by the external cavity length of the LD and is highly sensitive to the variation of the cavity length (ΔL). This work first investigates the relationship between the RO frequency and ΔL by solving the well-known Lang-Kobayashi (L-K) equations and then verifies the relationship by experiments. Both theory and experiment show that the RO frequency changes in a sawtooth-like quasi-periodic manner with respect to ΔL. The fundamental period is half laser wavelength. This sawtooth feature enables us to achieve period unwrapping and thus extend the measurement range up to a few micrometers. This work shows a possible new solution for achieving high-resolution, large-range displacement measurements.

This paper presents the design of a fiber-coupled self-mixing laser diode (SMLD) for non-contact and non-destructive measurement of Young’s modulus. By the presented measuring system, the Young’s modulus of aluminum 6061 and brass are measured as 70.0 GPa and 116.7 GPa, respectively, showing a good agreement within the standards in the literature and yielding a much smaller deviation and a higher repeatability compared with traditional tensile testing. Its fiber-coupled characteristics make the system quite easy to be installed in many application cases.

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.