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This thesis presents the comprehensive design, fabrication, and demonstration of advanced high-power, high-efficiency single-mode semiconductor lasers operating at a wavelength of 9xxnm. We begin with the design of the laser epitaxial structure, serving as the cornerstone for achieving high-power high-efficiency lasers. Our methodology integrates a semi-analytical calculation model, which accounts for Longitudinal Spatial Hole Burning (LSHB) and Two-Photon Absorption (TPA) effects, facilitating a thorough exploration of how design parameters influence output power and conversion efficiency. This approach offers an effective and time-efficient epitaxial structure optimization strategy compared to conventional full 3D simulation models. Subsequently, we demonstrate high-power, high-efficiency ridge waveguide (RW) lasers leveraging the optimized epitaxial structure. The RW lasers, with ridge width of 7μm and cavity length of 4mm, exhibit kink-free output and near-Gaussian far-field profiles with the maximum power outputs surpassing 1.6W. Notably, their efficiency demonstrates minimal decline with increasing current, maintaining a remarkable efficiency of 60.2% at 1.52W. Additionally, we introduce narrower RW lasers fabricated with an optimized epitaxial structure featuring reduced aluminum content in the p-cladding layer. The RW lasers, with ridge width of 5μm and cavity length of 5mm, exhibit kink-free near diffraction limit single-mode laser output up to 1W, accompanied by a high efficiency of 59.3%. To achieve single-transverse-mode laser output in wider emission aperture for higher maximum achievable output power, we propose the triple ridge waveguide (TRW) laser structure, utilizing the principle of unbroken supersymmetry (SUSY). This innovative design incorporates lossy auxiliary waveguides to effectively suppress unwanted higher-order modes within the laser cavity. By precisely engineering the structural parameters of the TRW, we achieve significant modal discrimination, ensuring stable single-mode lasing. Experimental results demonstrate near-diffraction-limited output in the proposed TRW laser, validating the efficacy of our mode filtering mechanism. To further enhance mode discrimination in practical applications, we propose the introduction of trapezoidal deep-etched trenches in the auxiliary waveguide. These trenches serve to convert guided modes into leaky modes, effectively increasing propagation loss for undesired higher-order modes. This enhancement facilitates more stable single-mode operation, particularly at high power outputs. Moreover, in response to the increasing demand for highly coherent, low phase noise on-chip laser beams within integrated optics, we employ the self-injection locking technique to combine our demonstrated high-power ridge waveguide Fabry–Pérot (FP) lasers with silicon nitride photonic integrated circuits (PICs). Our experiments yield a stable narrow-linewidth single-frequency laser output with a fiber-coupled power of up to 3.5mW, accompanied by an impressive SMSR exceeding 48dB and an intrinsic linewidth smaller than 25kHz. Our research provides valuable insights into the advancement of high-power, high-efficiency, single-mode semiconductor lasers, with promising applications across the field of photonics and telecommunications.

Large mode area (LMA) has been a cornerstone of high power fibre amplifiers and lasers thanks to the power scaling capability without exceeding the intensity threshold for nonlinear effects and optical damage. Many applications require single-mode operation, and conventionally, these fibres are designed to operate with the fundamental mode. Alternatively, a single higher order mode (HOM) with the intensity distribution of a zeroth order Bessel function LP0m (usually m ≥ 4) can propagate stably in a highly multimode step-index fibre. These HOMs are also reported to be more resistant to bending induced distortions and mode coupling than the fundamental mode. Fibre amplifiers for these modes may have higher mode area scaling capability and can then offer higher power threshold for nonlinearities. So far, rare-earth (RE) doped fibre amplifiers for these HOMs have been demonstrated, while fibre Raman amplifiers (FRAs) for such modes have not been reported prior to my thesis work. FRAs are unique in that they are wavelength agile. More importantly for my thesis work, they may preserve the target HOM better than a RE-doped fibre amplifier does thanks to not only better control of fibre perturbations, but also to the absence of local gain saturation. Additionally, FRAs for HOMs can be better than cladding pumped FRAs in that they fulfil the area ratio requirement for efficient first order Stokes conversion while allowing stable single mode operation. This thesis is dedicated to the demonstration of the mode preservation and brightness enhancement capability of FRAs for HOMs. I start with investigations of signal mode purity degradation via numerical simulations and compare the performance of typical Yb-doped fibre amplifiers (YDFAs) and FRAs both with the same HOM. Results show that the mode purity is preserved better in the FRA than in the YDFA for incoherent signal and pump. My experimental demonstrations are restricted to FRAs and start with pulsed Raman amplification of a single HOM in large core step index fibres with lengths of ~10 m. The signal output sees 18 dB gain and is ~14 dB brighter in terms of maximum power in a single mode than the launched multimode pump, with good mode purity preservation and weighted conversion efficiency. Then a pulsed FRA in 335 m of large core step index fibre is demonstrated with over 23 dB amplification to the signal and ~16 dB brightness enhancement from the launched pump. The purity degradation is modest and is likely to be the result of the emergence of 2nd order Stokes. The low pump brightness also points towards direct diode laser pumping, which I attempted with an 877-m large core step index fibre and spectrally combined pumping from five diode lasers. A 6 dB Raman gain of the signal is achieved with quasi-CW pump with ~170 W combined peak power, which is expected to be improved with further optimizations. These results successfully demonstrate the capability and potential of power scaling and brightness enhancement of FRAs with HOMs. The advantages of FRAs are also utilized with fibre modes carrying orbital angular momentum (OAM). These OAM modes (OAMM) can be viewed as pairs of spatially coherent degenerate modes, in which the angular momentum improves the stability to some perturbations. Although in this case, the saturation of RE-doped fibre amplifiers may aid in suppressing the growth of degenerate parasitic modes, notably those of conjugate charges, FRAs are still preserving the target OAMM well, which is evident by the experimental results presented in the thesis. I first describe a pulsed FRA for a charge-2 OAMM in a 5-m multimode step index fibre with 15 dB maximum gain with an amplified signal mode purity of 83.2% dropping from 95.8% when unamplified. The purity degradation is attributed to coupling into the conjugate charge which I expect to decrease for higher charges. The subsequent experiment of Raman amplification of a charge-15 OAMM in a 30-m large core step index fibre confirms this. The signal gain reached 9.2 dB, with only modest purity degradation from 84.3% to 78.4% and negligible coupling into the conjugate charges. The results of both HOMs and OAMMs have proven the feasibility of FRAs for power scaling of these modes with great signal mode preservation, making FRAs an attractive alternative to RE-doped fibre amplifiers.

The interest in semiconductor ring lasers (SRLs) has been steadily growing in the last few years because of several unique properties such as ultrafast directional bistability, stable single mode operation and potential for integration. However, most of the mode dynamical behavior as well as the optimum device design are still far from a complete understanding. This thesis reports on the design, technological development and characterization of SRLs emitting at 1.55 um, which are monolithically integrated with a number of other optical elements such as tunable couplers, optical amplifiers, Bragg reflectors and distributed feedback lasers (DFBs). A detailed analysis on the device design is presented with particular emphasis on its robustness with respect to fabrication tolerances and to the optical feedback from the output waveguides. The complete processing technology is developed with a focus on selective dry etching to achieve very accurate control of the waveguide bending losses. Three completely novel and monolithically integrated SRL devices are fabricated and characterized. The first is a master-slave device based on the monolithic integration of an SRL with a DFB that shows highly efficient cavity enhanced four-wave mixing up to detuning frequencies of 1.5 THz. In a second geometry, a Bragg reflector defined on one of the output waveguides selects the lasing mode of the SRL. The device shows world-record wavelength switching speeds as low as 450 ps and strong immunity to thermal fluctuations of the grating. The third device is an SRL with tunable couplers for active Q-switching applications. Pulses as short as 120 ps at a repetition rate of 1.8 GHz are obtained by injecting only a few mA of current into the tuning section.

The work reported in this thesis is concerned with the fabrication and characterisation of laterally coupled (LC) distributed feedback (DFB) lasers, both single-and dual-mode, based on InAlGaAs-InP material emitting at 1.35 mum. The aim of the research is to realise a compact device, able to emit two wavelengths simultaneously, to be used for the generation of gigahertz (GHZ) and terahertz (THz) signals by photo-mixing. The motivation comes from the need for portable sources of radiation in the so-called THz region. The first part of the work involved the implementation of a laterally coupled grating that is intended to become a building block for the dual-wavelength device. To prove the effectiveness of the grating structure, single-mode DFBs and distributed Bragg reflector (DBH) lasers were designed and fabricated in the InAlGaAs-InP material, aiming to optimise the necessary technological processing. Assessment of these devices showed excellent performance: stable single-mode emission, with low threshold currents of about 55 mA and high sidemode suppression ratio (SMSR) over 40 dB, were demonstrated. Ridge-waveguide lasers (RWGL) were also fabricated alongside the single-mode devices, to allow comparison of the operating characteristics. The second part of the work was concerned with the implementation of a novel dual-wavelength laser, based on the lateral grating structure. The final device realised was a DFB laser having two different grating periods, one on each sidewall, fabricated in the InAlGaAs-InP material using the technology previously implemented. This source was thoroughly assessed in terms of lasing characteristics, mode behaviour, and suitability to generate microwave signals. Stable dual-mode operation, with low threshold currents around 50 mA and SMSR approaching 30 dB, was successfully demonstrated in continuous wave (CW) operating mode, at room temperature. The dual-mode behaviour was characterised by inspection of the modal transverse distribution and by injection locking experiments. Generation of microwave signals by photo-mixing was performed at about 23 GHz, and the linewidths of each of the individual modes and of the beat mode were measured as a function of the injection current. It was observed that, while the linewidth of the separate modes has a modest dependence on the injection level, the linewidth of the beat mode experiences a strong decrease as the injected current increases.

The mid-infrared is a frequency range where many greenhouse and pollutant gases lie their fundamental roto-vibrational absorption lines. Because of this, the mid-infrared region is particularly interested for many applications, for example, gas sensing and spectroscopy. Due to the broad wavelength range and the robust lasing performance, quantum cascade lasers (QCLs) become the major choice of mid-infrared laser sources. Although the first QCL only emitted several milliwatts in pulsed operation at cryogenic temperature, nowadays people have shown QCLs with up to several watts continuous-wave output power at room temperature and devices with above 20% wall-plug efficiency. Since the first demonstration in 1994, the rapid progress of QCLs has greatly opened its applications in many aspects of modern society, such as production control, environmental protection, medical detection and military purposes. This work can be separated in two parts. The first part is focused on a special type of QCLs with the properties including room temperature surface emission, high peak power, single mode operation and narrow beam divergence. These features are enabled by a photonic structure which is in a close reminiscence to the atomic crystals in the solid-state physics, the photonic crystal (PhC). Like electrons in ionic lattices, the motion of electromagnetic waves in a PhC is affected by the periodic refractive index modulation. The PhC behaves as a special laser cavity which allows large-area single-mode coherent oscillation and the surface emission, leading to these mentioned properties of interest. This presented work involves systematic efforts on our mid-infrared PhC QCLs, including the theoretical modeling, the fabrication process and the characterization methods. With the buried-heterostructure fabrication techniques and our extended model that is capable to analyze finite-size PhC structures, we realized mid-infrared large-area (> 1.1 × 1.1 mm2) PhC QCLs with many appealing features, such as stable single-mode operation at room temperature, up to 5 W surface-emitting peak power and below 1° divergence. In addition, our theoretical framework and experimental platform also allowed other physical discoveries. For example, we proposed and proved the influence of the envelope functions on far-field polarizations of finite-size PhC modes. The second part of this work is focused on QCLs with a low dissipation power. The consumption power of QCLs is typically up to several watts due to the short upper state lifetime inherent to intersubband transitions. Low dissipation QCLs are favored for many portable and battery-driven applications. In principle, they can be achieved by greatly reducing the device volume, but it is crucial to significantly suppress the mirror losses and especially the diffraction losses in such tiny-footprint devices. In this work, the impact of the diffraction loss is systematically analyzed in several high-reflection designs. Experimentally, we showed that the diffraction loss of a metallic coating can be alleviated by opening a subwavelength aperture, leading to a simultaneous enhancement in both its model reflectivity and the transmissivity. Implementation of this approach on both facets of a QCL results in a reduction of threshold power by 25%, leading to a record-low threshold dissipation power of 143 mW in continuous-wave at room temperature. In addition, watt-class continuous-wave output power with single-digit mA threshold current at -20°C is demonstrated on a QCL with dielectric high-reflection coatings. We also proposed the potential designs of low dissipation QCLs that are compatible with massive parallel productions. These works are of particular interest to both the academic and industrial communities.

In recent years, non-Hermitian degeneracies also known as exceptional points (EPs) have emerged as a new paradigm for engineering the response of optical systems. EPs can appear in a wide class of open non-Hermitian configurations. Among different types of non-conservative photonic systems, parity-time (PT) symmetric arrangements are of particular interest since they provide an excellent platform to explore the physics of exceptional points. In this work, the intriguing properties of exceptional points are utilized to address two of the long standing challenges in the field of integrated photonics- enforcing single mode lasing in intrinsically multimode cavities and enhancing the sensitivity of micro-resonators. In the first part of this work, I will describe how stable single mode lasing can be readily achieved in longitudinally and transversely multi-moded microring cavities through the systematic utilization of abrupt phase transitions at exceptional points. This technique will be first demonstrated in a parity-time laser that is comprised of a gain cavity coupled to an identical but lossy counterpart. A detailed study of the behavior of this system around the exceptional point will be presented. Furthermore, we report the first experimental realization of a dark state laser in which by strategically designing the spectral locations of exceptional points, widely tunable single-mode lasing can be attained even at high pump levels. Despite the presence of loss in such open laser systems, the slope efficiency remains virtually intact. Our results demonstrate the potential of exceptional points as a versatile design tool for mode management in on-chip laser configurations. In the second part of my dissertation, I will show how the exceptional points and their underlying degeneracies can be used to significantly boost the intrinsic sensitivity of microcavities. I will demonstrate the enhanced sensitivity in a binary PT-symmetric coupled cavity arrangement that is biased at an exceptional point. Then I will report the first observation of higher-order exceptional points in a ternary parity-time symmetric microring laser system with a judiciously tailored gain-loss distribution. The enhanced response associated with this ternary system follows a cubic root dependence on externally introduced perturbation, which can in turn be detected in the spectral domain. Using such arrangements, more than one order of magnitude enhancement in the sensitivity is observed experimentally. These results can pave the way towards improving the performance of current on-chip micro-cavity sensors.