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

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

Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator

Lasers based on Cr$^{2+}$-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with super-octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr$^{2+}$-doped II-VI oscillator pumped by a single InP diode, providing average powers of over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 $\mu$m. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step towards a more accessible alternative to synchrotron-like infrared radiation, and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.Comment: 8 pages, 5 figure

Resonant Tunnelling Optoelectronic Circuits

Nowadays, most communication networks such as local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs) have replaced or are about to replace coaxial cable or twisted copper wire with fiber optical cables. Light-wave communication systems comprise a transmitter based on a visible or near-infrared light source, whose carrier is modulated by the information signal to be transmitted, a transmission media such as an optical fiber, eventually utilizing in-line optical amplification, and a receiver based on a photo-detector that recovers the information signal (Liu, 1996)(Einarsson, 1996). The transmitter consists of a driver circuit along a semiconductor laser or a light emitting diode (LED). The receiver is a signal processing circuit coupled to a photo-detector such as a photodiode, an avalanche photodiode (APD), a phototransistor or a high speed photoconductor that processes the photo-detected signal and recovers the primitive information signa

Improvements to Optical Communication Capabilities Achieved through the Optical Injection of Semiconductor Lasers

Optically injection locked lasers have shown significant improvement in the modulation capabilities of directly modulated lasers. This research creates a direct-modulated optical communications system to investigate the bit-rate distance improvements achievable using optically injected Fabry-Pérot laser diodes. The injection strength and detuning frequency of the injection signal was varied to determine their impact on the optical communication link\u27s characteristics. This research measured a 25 fold increase in bit-rate distance product using optical injection locking as compared to the injected laser\u27s free-running capability. A 57 fold increase was measured in the bit-rate distance product when signal power is considered in a power-penalty measurement. This increased performance is attributed to the injected signals tolerance to dispersion given its reduced linewidth and chirp. This work also investigates the suitability of optical injection for radio over fiber applications using the period-one dynamic of optical injection. The all-optically generated, widely tunable microwave subcarrier frequency, well above the 3-dB cutoff frequency of the laser\u27s packaging electronics, was modulated with the same baseband electronics. This optically carried, ultra-wide spread spectrum signal was transported over 50km of standard-single-mode fiber. After detection at a high-speed photo- detector and the baseband modulation component was removed, the resultant signal was found to be suitable for broadcasting with an antenna or added to a frequency division multiplexed channel

Nonlinear Dynamics of Semiconductor Lasers and Their Applications

Semiconductor lasers are key components in many optical systems due to their advantages, including their small size, low cost, high efficiency, and low power consumption. It is well-known that semiconductor lasers under external perturbations, such as optical injection, optical feedback, or delayed coupling can exhibit a large variety of complex dynamical behaviors. Nowadays, cutting-edge engineering applications based on the complex dynamics of diode lasers are being conducted in areas, such as optical communications, optical signal processing, encoded communications, neuro-inspired ultra-fast optical computing devices, microwave signal generation, RADAR and LIDAR applications, biomedical imaging, and broadband spectroscopy. The prospects for these applications are even more exciting with the advent of photonic integrated circuits. This Special Issue focuses on theoretical and experimental advances in the nonlinear dynamics of semiconductor lasers subject to different types of external perturbations

Collisional Dynamics, Lasing and Stimulated Raman Scattering in Optically Pumped Cesium and Potassium Vapors

This dissertation explores collisional dynamics, lasing and stimulated Raman scattering in cesium and potassium vapors. The spin orbit mixing and quenching cross sections of several buffer gas species are measured. The ne-structure mixing cross sections for He, CH4 and C2H6 are 14±3, 35±6 and 73±10 °A2, respectively. The 2P3/2 state is quenched more rapidly than the 2P1/2 state. A pulsed blue laser operating via direct excitation of the cesium 72P3/2 state is demonstrated. A theoretical model is extended and compared with experimental results. The maximum output energy was 3.3 microJoules with a threshold of 10 microJoules/pulse and a slope efficiency of 0.45%. A tuneable hyper Raman potassium vapor laser, tuneable over 4 nm is demonstrated and characterized. The output was tunable from 766-770 nm. The threshold for the hyper-Raman process was 60 mW. The maximum slope efficiency (10.4%) and output power (12 mW) are comparable to previously demonstrated potassium DPAL systems that used several atmospheres of buffer gas