Yuanlong Fan

Improving the measurement performance for a self-mixing interferometry-based displacement sensing system

Approaches that are, to our knowledge, novel, are proposed in this paper to improve the accuracy performance of self-mixing interferometry (SMI) for displacement measurement. First, the characteristics associated with signals observed in SMI systems are studied, based on which a new procedure is proposed for achieving accurate estimation of the laser phase. The studies also revealed the reasons for the inherent errors associated with the existing SMI-based techniques for displacement measurement. Then, this paper presents a new method for estimating the optical feedback level factor (denoted by C) in real time. Combining the new algorithms for estimating the laser phase and updating C value, the paper finally presents a novel technique for displacement measurement with improved accuracy performance in contrast to existing techniques. The proposed technique is verified by both simulation and experimental data.

Dynamic stability analysis for a self-mixing interferometry system

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 (ϕ₀) 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, ϕ₀), from which a critical C (denoted as C(critical)) 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, ϕ₀) 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 re-modeling the system by considering the bandwidth of the detection components.

Measuring Young's modulus using a self-mixing laser diode

This paper presents a novel approach for determining the Young’s modulus by using a self-mixing laser diode (SMLD). An SMLD system consists of a laser diode (LD), a microlens and an external target. With a small portion of light backscatterd or reflected by the target re-entering the LD inside cavity, both the amplitude and frequency of the LD power are modulated. This modulated LD power is referred as a self-mixing signal (SMS) which is detected by the photodiode (PD) packaged in the rear of the LD. The external target is the tested sample which is in damping vibration excited by a singular elastic strike with an impulse tool. The vibration information from the tested sample is carried in the SMS. Advanced data processing in frequency-domain is applied on the SMS, from which the resonant frequency of the vibration can be retrieved, and hence Young’s modulus is calculated. The proposed method has been verified by simulations.

Stability Limit of a Semiconductor Laser With Optical Feedback

This paper presents comprehensive studies on the stability of a semiconductor laser (SL) with external optical feedback. In particular, based on numerical computation on the Lang and Kobayashi equations, the stability limit under the condition of minimum linewidth is investigated, revealing how it is influenced by three major parameters associated with SL operation conditions, including the feedback strength (k), the external cavity length (L), and the injection current (J). In contrast to existing work in literature, the presented removed all the approximations and assumptions, hence resulting in relatively complete description of the stability limit. This paper presented leads to a number of important and interesting discoveries: 1) the possible stable area is broader than what is described by the existing work; 2) for long external cavities, the stability limit can be described by a linear relationship between k and L; and 3) on the stability limit, the critical feedback strength (kc) and critical external cavity length (Lc) are discovered, respectively, being proportional and inversely proportional to J and k.

All-Optical Comparator With a Step-Like Transfer Function

We propose a scheme for an all-optical comparator which embeds a nonlinear fiber ring resonator in a Mach–Zehnder configuration. Such a resonator accumulates the circulating power of the light traversing the ring, so that the effective nonlinear phase shift in fiber can be significantly enhanced. Numerical results demonstrate that with appropriate choice of the parameters of the Mach–Zehnder configuration and the power of the probe light, the all-optical comparator can exhibit an excellent step-like transfer function. Simultaneously, the configuration reduces the switching threshold by 3 or more orders of magnitude to the level of milliwatt when conventional highly nonlinear fibers are adopted. In addition, our simulations show that this all-optical threshold comparator also has a multiperiodic transfer characteristic and thus may be used to construct multibit analog-to-digital convertors.

Features of a Self-Mixing Laser Diode Operating Near Relaxation Oscillation

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.

Analysis on the transient of a self-mixing interferometry sensing system

The transient of a self-mixing interferometry (SMI) sensing system is investigated in this paper by solving the well known Lang and Kobayashi (LK) equations. Specifically, the influence of two important system parameters, i.e., the feedback level factor and the linewidth enhancement factor, on the transient is discussed, showing that the decay time and the oscillation period of the transient respectively increases and decreases with the increase of the feedback level factor, and both increase with the increase of linewidth enhancement factor. The results presented in this paper are useful for designing a stable SMI system.

An estimation method for feedback level factor C of a self-mixing interferometry system

This paper presents a fast estimation method for feedback level factor C of a self-mixing interferometry (SMI) system. The reconstruction of a displacement waveform using a SMI signal needs to know a C value. However, it is difficult to maintain a constant C value during the reconstruction process. We study the features of the reconstructed displacement waveforms incorporating different pre-set C values and classify waveforms into two types. Bisection method is introduced in our method for fast estimating C value. The effectiveness of our proposed method has been verified by both simulation and experimental data.

A displacement reconstruction algorithm used for optical feedback self mixing interferometry system under different feedback levels

Displacement information of a moving target can be detected using an optical feedback self-mixing interferometry (OFSMI) system. A sensing signal observed from the OFSMI system is called self-mixing signal (SMS). The paper studies the waveform features of the SMSs and proposes an algorithm for reconstructing the displacement of a moving target. The reconstruction accuracy of the algorithm mainly depends on the locating accuracy for those characteristic points on a SMS. A set of rules for identifying those characteristic points are described in the paper. The proposed algorithm is verified by simulation signals firstly, and then applied on extensive SMSs which are obtained from the experimental set-up. The results show that the displacement of the external moving target can be reconstructed under different feedback levels.

Self-balanced real-time photonic scheme for ultrafast random number generation

We propose a real-time self-balanced photonic method for extracting ultrafast random numbers from broadband randomness sources. In place of electronic analog-to-digital converters (ADCs), the balanced photo-detection technology is used to directly quantize optically sampled chaotic pulses into a continuous random number stream. Benefitting from ultrafast photo-detection, our method can efficiently eliminate the generation rate bottleneck from electronic ADCs which are required in nearly all the available fast physical random number generators. A proof-of-principle experiment demonstrates that using our approach 10 Gb/s real-time and statistically unbiased random numbers are successfully extracted from a bandwidth-enhanced chaotic source. The generation rate achieved experimentally here is being limited by the bandwidth of the chaotic source. The method described has the potential to attain a real-time rate of 100 Gb/s.