Semiconductor laser dynamics induced by optical feedback for photonic microwave sensing

As one of the most widely used light sources today, semiconductor lasers (SLs) are an important part of many optical systems, especially for sensing, communications, metrology, and storage applications. SLs have the advantages of small size, easy integration, and miniaturization. The massification of electronic devices has furthered this agenda, allowing the creation of portable systems capable of supporting optical sensing systems. Essentially, SLs are inherently nonlinear devices, in nonlinear systems, the folding and stretching behaviors of variables result in di↵erent dynamical routes. It is worth noting that under the conditions of a stable operation, an SL biased by constant current usually emits laser light with a constant intensity. However, with the introduction of external optical feedback (OF), the laser light can become unstable. SL will undergo from steady state, switching status, to period-one (P1) oscillation by crossing Hopf-bifurcation. In the P1 state, the system produces a modulation of the laser optical output power for the generation of microwave photonics (MWP) signals. In this thesis, we operate SL with OF scheme in P1 dynamics, and found that the proposed system has the great capability to achieve both displacement and absolute distance sensing applications with high resolution and wide measurement range, by using time-frequency information, relaxation oscillation information, and nonlinear dynamic characteristics carried in that SLs emit signals. The contributions of each chapter in this thesis are described in the following: In Chapter 3, we propose an SL with OF set at the P1 dynamics to generate the MWP signal for displacement sensing. Di↵erent from the traditional MWP generation method, the designed laser nonlinear dynamics are used by slightly perturbing the SL source with the help of external feedback light to make the system work in the P1 dynamic state, thereby generating regular microwave oscillation. By using the fourth-order Runge-Kutta method to numerically solve the famous Lang-Kobayashi differential equation, the boundary of di↵erent laser dynamic states is delimited, so that the system can generate stable and sustainable MWP signals in P1 dynamics. A set of parameter selection rules for designing an SL based MWP displacement sensing system is obtained. In addition, a measurement algorithm for recovering the displacement from an MWP sensing signal is developed. By making full use of the sensing information carried in both amplitude and frequency of the MWP signal, displacement sensing with high resolution and high sensitivity can be achieved. Both simulations and experiments are conducted to verify the proposed method and show it is capable of realizing high measurement sensitivity, and high resolution for displacement sensing. In Chapter 4, utilizing the rich nonlinear dynamics of an SL with OF, under the proper controllable system parameters, the system enters the P1 dynamics through Hopf-bifurcation. In the P1 state, the detailed relationship between the relaxation oscillation frequency of MWP signals and external cavity length is studied through solving the Lang-Kobayashi delayed di↵erential equations. The displacement measurement formula is thus obtained. In addition, the relevant signal processing algorithm is developed by considering mode-hopping, frequency-hopping, and sawtooth-like phenomena that occurred in the relaxation oscillation. The displacement measurement can be enhanced in a wider sensing range by fully using the relaxation oscillation frequency relationship. Verification results in simulation and experiment show that the proposed MWP displacement sensing system based on SL with OF contributes to designing a prototype of a compact displacement sensor with wide measurement range and high resolution. In Chapter 5, OF induced switching status between two nonlinear dynamic states (stable and P1 states) is observed in the SL with OF system. Without the need for any electronic or optical modulation devices, the laser intensity can be modulated in a square wave form due to the switching via utilizing the inherent SL dynamics near Hopf-bifurcation boundary. The periodicity in the switching enables us to develop a new approach for long-distance sensing compared to other SL with OF based absolute distance measurement systems and lift the relevant restrictions that existed in the systems. Moreover, the impact of system controllable parameters on the duty cycle of the square wave signals generated was investigated as well, aiming to maintain the proposed system robustly operating at the switching status

Period-one Microwave Photonic Sensing by a Laser Diode with Optical Feedback

With external optical feedback (EOF), a laser diode (LD) can operate at different dynamic states. In this work, an LD with EOF is set at period-one (P1) oscillation state to generate microwave photonic (MWP) signal for sensing. Firstly, the P1 state boundary of the LD is determined and then the influence of the LD controllable parameters on the boundary is studied by solving the well-known Lang-Kobayashi equations. A set of parameters selection rule for designing an LD based MWP sensing system is obtained. In addition, a measurement algorithm for recovering the displacement from an MWP sensing signal is developed. By making full use of the sensing information carried in both amplitude and frequency of the MWP signal, displacement sensing with high resolution and large range can be achieved. Both simulations and experiments are conducted to verify the proposed method and show it is capable of realizing wide measurable range, high measurement sensitivity, and high resolution sensing

Unsupervised Learning Composite Network to Reduce Training Cost of Deep Learning Model for Colorectal Cancer Diagnosis

Deep learning facilitates complex medical data analysis and is increasingly being explored in colorectal cancer diagnostics. However, the training cost of the deep learning model limits its real-world medical utility. In this study, we present a composite network that combines deep learning and unsupervised K-means clustering algorithm (RK-net) for automatic processing of medical images. RK-net was more efficient in image refinement compared with manual screening and annotation. The training of a deep learning model for colorectal cancer diagnosis was accelerated by two times with utilization of RK-net-processed images. Better performance was observed in training loss and accuracy achievement as well. RK-net could be useful to refine medical images of the ever-expanding quantity and assist in subsequent construction of the artificial intelligence model

Achieving high resolution measurement using laser diode operating at period one

Copyright 2019 SPIE. When a part of light emitted by a laser is back-reflected or back-scattered from an external target and re-enters the laser cavity, both the laser intensity and its wavelength can be modulated. This is so-called self-mixing effect (SME), the optical feedback interferometry (OFI) utilizes such effect in an LD developed various applications. In this paper, we use a dualcavity OFI system that operating in period one state, the laser intensity from this system exhibits an oscillation with its amplitude modulated by a traditional single cavity OFI signal. The dual-cavity OFI system has the same measurement resolution as the single cavity which is half laser wavelength. This paper developed a method to improve the resolution by using fringe subdivision. Our simulation result shows that this method can achieve subnanometer resolution

Sensing using dynamics of a laser diode with dual-cavity

© 2019 The Author(s). waveform analysis is conducted on a sensing signal generated by a laser diode with dual-cavity operating at period-one state. The proposed design can achieve high measurement resolution and sensitivity for a moving object

Measuring Linewidth Enhancement Factor by Laser Dynamics

© 2019 The Author(s). A new method for measuring linewidth enhancement factor of a semiconductor laser (SL) is proposed by using laser dynamics. This method can work when the SL suffer very strong optical feedback

Measuring linewidth enhancement factor by laser dynamics

© 2019 The Author (s). A new method for measuring linewidth enhancement factor of a semiconductor laser (SL) is proposed by using laser dynamics. This method can work when the SL suffer very strong optical feedback

State boundaries in a laser diode with optical feedback and its sensing application

© 2019 The Author(s). By studying the influence of system parameters on the dynamical state boundaries in a laser diode with optical feedback, a sensing system working at period-one state for achieving displacement measurement with high resolution is designed