This PDF file contains the front matter associated with SPIE Proceedings Volume 11672, including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Photonics West LASE conference 11672: Laser Resonators, Microresonators, and Beam Control XXIII

Photonic crystal surface-emitting lasers (PCSELs) are an unprecedented type of semiconductor laser that can achieve single longitudinal- and lateral-mode oscillation even over areas of millimeters in diameter. Their brightness, defined as power per unit area per unit solid angle, is expected to be increased up to the range of 1～10GWcm-2sr-1, which is comparable to those of gas lasers and fiber lasers. Also, PCSELs have additional functions, including the generation of beams with various patterns and polarizations, and they can even achieve electric two-dimensional beam scanning. Material systems for PCSELs are currently being expanded to not only InGaAs/GaAs (900-1000nm), but also InGaAsP/InP (1.3-1.55um) and even InGaN/GaN (400-530nm). LiDAR is discussed as one example of the applications of PCSELs. PCSELs will become a key light source in the forthcoming ultra-smart Society 5.0, which includes smart mobility and smart processing.

Over the past decade, the development of CMOS-compatible silicon technology has provided a novel platform for integrated nonlinear photonics, offering a path towards chip-scale devices for applications including spectroscopy, frequency metrology, and optical computing. The high optical confinement achieved in the silicon-based platform allows for large effective nonlinearities along with the ability to tailor the group-velocity dispersion of the device, which is essential for phase-matched parametric nonlinear interactions such as four-wave mixing. Here, we discuss nonlinear photonics applications based on silicon-nitride microresonators, including Kerr optical frequency combs and degenerate optical parametric oscillators for photonic spin glass.

Non-linear wave mixing in optical microresonators offers new perspectives for the realization of compact optical frequency ‘microcombs’, holding many promising applications. These typically rely on dissipative soliton formation in driven nonlinear passive cavities with anomalous dispersion, yielding a sech pulse shape. Here, we use a genetic algorithm to ‘invert’ the Lugiato-Lefever equation that models these systems, in order to find the optimum arbitrary dispersion profile needed to achieve a microcomb with a targeted spectral shape. We consider several use cases, such as generating near gaussian pulses, or a telecom-optimized microcomb, as well as optimize a dispersive wave position and power.

We investigate frequency comb generation in silicon nitride ring resonators by using a pump subject to a weak amplitude modulation. We show that a partial locking is obtained when the external modulation frequency differs from the resonator free-spectral-range by up to hundreds of MHz.

Ultra-silicon-rich nitride devices possess large Kerr nonlinearity and absence of two-photon absorption at telecommunications wavelengths. We report recent results in high gain amplifiers and soliton-effects in USRN devices. On-chip optical parametric amplifiers with 42.5dB gain, and slow-light enhanced optical parametric amplifiers with 333dB/cm gain are demonstrated. Nonlinear USRN Bragg gratings are designed and demonstrated to possess three orders of magnitude larger group velocity dispersion compared to photonic waveguides. Time-resolved measurements reveal high-order Bragg soliton dynamics as well as observation of compression and fission triggered by large third-order dispersion. We further demonstrate close to ten-fold soliton-effect temporal compression in USRN waveguides.

Turing patterns and solitons are a common occurrence in systems that are out of equilibrium. For example, they regulate the emergence of organised structures in biology and chemistry. In optics, the study of temporal Turing rolls and cavity-solitons in nonlinear micro-cavity resonators has been key to the understanding of optical frequency comb formation in these structures. Over a decade, the impact of microresonator-based approaches to generating frequency combs – so-called “Micro-combs or “Kerr combs” – has reached far beyond metrology applications. Here we summarise our theoretical and experimental results for solitons and Turing patterns in a system comprising a micro-resonator nested in an auxiliary fibre-gain cavity.

We demonstrate the all-precision-machining fabrication of ultrahigh-quality-factor (Q) crystalline optical microresonators. The obtained Q exceeds 100 million for both magnesium fluoride (MgF₂) and calcium fluoride (CaF₂) materials. This constitutes the highest Q factor obtained for whispering gallery mode crystalline microresonators fabricated solely by ultra-precision machining. We also achieve octave-wide optical parametric oscillation using an MgF₂ resonator fabricated with a computer-controlled machining process. This method readily enables advanced dispersion engineering for optical parametric oscillation and makes it possible to explore microcavity nonlinear optics and quantum optics without the need for skilled manual polishing techniques.

Coherent ultra violet (UV) light has broad applications in optical atomic clocks and quantum computers. Since availability of semiconductor tunable monochromatic lasers is limited at short wavelengths, optical nonlinear processes are frequently used to produce the desirable radiation wavelength. Optical crystals with quadratic nonlinearity are promising for this purpose because of their transparency and high optical damage threshold. In this paper we discuss a possibility of generation of coherent deep UV light utilizing high quality factor (-Q) monolithic optical microcavities made of crystalline optical materials characterized with cubic nonlinearities. We show that the generation is feasible in spite of the relatively small cubic nonlinearities, due to the ultra-broad- band high optical transparency of the crystals and associated high-Q achievable in the cavities created out of the materials. We found that the birefringence as well as the tensor nature of the nonlinear susceptibility of the crystalline optical materials can simplify the phase matching of the nonlinear processes.

The structuring of laser light with non-diffracting vector distributions, optical vortices, or high angular momentum are being increasing exploited. We present a system for the synthesis of these arbitrary intensity and phase profiles through a number of carrier-envelope-phase stabilized coherent frequency and spatial combs. Adjusting the differences between adjacent combs allows synthesis of a combined field which can be structured, directed, and improved in the presence of propagation noise. Additionally, we present a method for the optimization of such systems in free space with a Fourier optics based genetic algorithm that converges to <π/10 accuracy of the initial parameters.

In this invited article, we report the experimental demonstration of the simultaneous coherent locking of two independent lasers with arbitrary multi-FSR (free spectral range) frequency separation to a Kerr microcomb soliton, resulting in the creation of synthetic microcomb soliton crystals. Each of the two pumps is self-injection- locked to its neighboring microcavity mode and acts as an arbiter linking the microcomb to the cavity. We show that the beating of the two pumps creates a manifest discrete symmetry and that certain microcomb states generated in this pumping scheme spontaneously break this symmetry, thereby realizing dissipative discrete time crystals. Apart from introducing a powerful platform leveraging advanced photonics for the creation and scientific exploration of various types of dissipative time crystals and their properties, our results constitute a decisive step towards the two-point locking of Kerr microcombs with moderate bandwidths much smaller than an octave which cannot be self-referenced through standard approaches such as the f - 2f technique.

Coherently driven Kerr microresonators have attracted significant interest due to their ability to produce optical frequency combs with attractive characteristics. Of particular significance has been the realisation that such resonators can sustain pulses of light known as dissipative Kerr cavity solitons. Here we discuss recent advances related to the generation of repetition-rate-tunable soliton combs and describe novel soliton dynamics that can manifest themselves in multimode resonators. We introduce rational harmonic driving as a new pulsed pumping modality that allows access to a variety of repetition rates, and we show that cavity solitons can undergo spontaneous symmetry breaking in multimode resonators.

Self-injection locking (SIL) is an effect of the oscillator frequency stabilization by means of a passive external high-quality cavity enabling frequency filtered coherent optical feedback to the oscillator. It is widely used in various photonic applications, including compact narrow-linewidth lasers and microcomb sources. While basic properties of this effect were studied in many theoretical and experimental works, deeper insight on its physical features and parameter space analysis allows us to build a model that describes its behavior and predicts at least an order of magnitude improvement of the stabilized laser linewidth reduction as compared to the best previous results. We find out a global maximum over all parameters and obtain analytical expression for the optimal stabilization coefficient. Influence of the resonator non-linearity and transition from the SIL to single cavity regime are discussed. Quality factor of the resonator appears to be a key parameter for effective SIL and oscillator stabilisation. Crystalline microresonators demonstrated the highest Q and a prism coupling is a robust method of its excitation, broadly used in applications. We developed and verified experimentally a new method of determining the key parameters of the chosen mode of the microresonator - quality factor and vertical index - based on the measurement of the locking bandwidth as a function of the resonator to prism distance. Unlike other methods it allows for the measurement to be made right in the SIL regime and does not require narrow-line lasers or fast photodetectors. A comparison with the FWHM and ringdown methods demonstrated excellent agreement.

Lithium niobate (LN) is a well-known material extensively used for optical modulators, thanks to its large electro-optic coefficient. The advancement of thin-film LN technology has sparked tremendous interest in the field of integrated optics. The capability of co-integration ultralow loss monolithic optical waveguides with highly efficient microwave control could enable next-generation, completely novel electro-optic and nonlinear photonic devices with advanced functionalities in a small footprint. Here, we discuss the recent development of electro-optic frequency shifters and beamsplitters, acousto-optic modulators, and electro-optic frequency combs, powered by integrated LN photonics.

In many applications of whispering gallery (WG) resonators and optical nanofibers as well as their coupling systems, the freedom of polarization is not critical. Inspired by the results from near-field optics, we found the freedom of polarization may bring up interesting ideas into these systems. In this talk, I will introduce three experiments about the polarization-controlled light coupling in WG resonator-nanofiber coupling system. The first one is the realization of the full polarization control of a nanofiber; The second one is the cavity enhanced directional coupling; The third one is the polarization-controlled cavity input-output relations.

Due to their high circulating intensities, ultra-high quality factor dielectric whispering-gallery mode resonators have enabled the development of low threshold Raman microlasers. Subsequently, other Raman-related phenomena, such as cascaded stimulated Raman scattering (CSRS) and stimulated anti-Stokes Raman scattering (SARS), were observed. While low threshold frequency conversion and generation have clear applications, CSRS and SARS have been limited by the low Raman gain. In this work, the surface of a silica resonator is modified with an organic monolayer, increasing the Raman gain. Up to four orders of CSRS is observed with sub-mW input power, and the SARS efficiency is improved by three orders of magnitude compared to previous studies with hybrid resonators.

Hexagonal boron nitride (hBN)—a 2D crystalline sheet consisting of alternating boron and nitrogen atoms—is emerging as a promising candidate for integrated photonics due to its exceptional properties. It is one of the widest bandgap materials (6 eV) with a transparency window including ultraviolet and visible regime. Importantly, hBN hosts ultra-bright single photon emitters (SPEs) operating at room temperature, which have attracted intense research attention since their discovery in 2016. hBN hosts a broad range of SPEs in energies showing exceptional brightness with several million counts at the detector. In addition, these emitters benefit from weak electron-phonon coupling indicated by the high intensity of zero phonon lines and weak intensity in the phonon sideband. This, together with the high degree of inherent chemical inertness, makes hBN an attractive candidate for next-generation photonic quantum integrated circuits.

Whispering-gallery mode resonators (WGMR) have been used instead of fiber delay lines for energy storage in ultra-pure microwave generation using optoelectronic oscillators (OEOs). They can reduce the size of the OEOs by replacing the optical fiber delay line, thereby simplifying the OEO architecture. So far, there has been no mathematical framework to understand the dynamics of these miniature OEOs. We propose a theoretical model based on a field envelope approach that describes their nonlinear dynamics. We perform a stability analysis of the stationary states and characterize the critical gain leading to microwave oscillations as a function of the detuning frequency of the central mode, and as a function of the intrinsic properties of the WGMR.

The coupling between mechanical motion and optical fields can be exploited for exquisite force sensing. This optomechanical interaction is further amplified with optical resonances, leading to unprecedented displacement sensitivities beyond 10^-18 m/Hz ^1/2, as exemplified by the Laser Interferometer Gravitational Wave Observatory (LIGO). In this talk I will introduce a cavity optomechanics platform for motion sensing based on optical whispering gallery mode (WGM) resonances. I will describe the evolution of a WGM accelerometer from the laboratory to a hand-fabricated proof-of-concept prototype, and now, towards chip-scale fabrication. Our goal is to reach acceleration sensitivities below 100 ng/Hz^1/2 (where g=9.81 ms^-2) whilst ensuring low power operation, high linearity, and low drift. Through the lens of commercial feasibility, we use finite element modelling to simulate the optical, mechanical, and thermal behaviour of a range of MOEMS designs. Preliminary results from chip fabrication and chip-testing will also be presented.

This work demonstrates the feasibility of the evanescent field perturbation sensor approach for both three-dimensional and planar resonator geometries. An optical fiber-coupled silica sphere resonator and an integrated waveguide-ring resonator are used with a smaller sphere inserted in their evanescent tail. In both experiments, the motion of the perturber across the evanescent tail leads to a measurable shift in the resonators’ whispering-gallery-modes. The results show a consistent relationship between the mode shift and the position of the perturber.

Shrinking the volumetric footprint of gas sensors is desirable as it allows for nonintrusive, nonperturbing gas mixture analysis and access to tight enclosures. Micro-resonators are a perfect candidate for these sensors as their size parameter (~micron) is minimal, and the typical surface propagating whispering gallery modes can interact with an analyte without disrupting the environment. The large, quality factor (Q) of these resonant cavity modes enables long interaction lengths on the order of 100s of centimeters between the optical field and analyte. Thus, the presence of a gas different than the nominal environment will result in a shift of the resonant properties, including the resonant wavelength, amplitude, and quality factor, that can be detected in real-time. To illustrate this effect, we utilized a spherical micro resonator on the end of a piece of optical fiber, formed using standard ball lens fabrication, and excited the resonant modes using a tapered optical fiber connected to tunable Infrared laser. The resonator was fixed in contact with the tapered region of fiber, and the assembly was placed inside an in-house, optically coupled, vacuum-tight vessel for gas testing. We compared the spectral response of air, pure CO2, and pure N2 gas, observing spectral shifting and broadening of the cavity resonances. In addition, the effect of vessel temperature on resonance peak position due to the thermo-optic effect was investigated and quantified. Lastly, a feedback arm was added to the setup to reduce signal noise and automated data analysis was implemented to improve data clarity.

The proposed sensor utilizes a whispering gallery mode (WGM) resonator to measure velocity-induced non-coherent Doppler shifts in Mie-scattered light from particles moving with the air-flow. The resonator replaces the typical Fabry-Perot instrument to measure the Doppler shift. A prototype sensor was developed and experiments were carried out in an atomizing nozzle with a 25μm-diameter water-droplet-seeded air jet. Individual velocity measurements were made at the center of the nozzle, switching the flow on and off. Preliminary results show promise for the WGM-based velocity sensor concept, which would significantly reduce the size and weight of future Direct-Detection-Doppler systems for air-speed measurement, with possible applications in airborne-platform weather-monitoring and planetary studies.

Chiral light-matter interactions have emerged as a promising area in biophysics and quantum optics. Remarkable progress in enhancing chiral light-matter interactions have been investigated through passive resonators or spontaneous emission. Nevertheless, the interaction between chiral biomolecules and stimulated emission remains unexplored. Here we introduce the concept of a biological chiral laser by amplifying chiral light-matter interactions in an active resonator through stimulated emission process. Green fluorescent proteins or chiral biomolecules encapsulated in Fabry-Perot microcavity served as the gain material while excited by either left-handed or righthanded circularly polarized pump laser. Owing to the nonlinear pump energy dependence of stimulated emission, significant enhancement of chiral light-matter interactions was demonstrated. Detailed experiments and theory revealed that lasing dissymmetry factor is determined by molecular absorption dissymmetry factor at its excitation wavelength. Finally, chirality transfer was investigated under stimulated emission process through Forster resonance energy transfer. Our findings elucidate the mechanism of stimulated chiral light-matter interactions, providing new prospects for understanding light-matter interaction in biophysics, chiral sensing, and quantum photonics.

Using a specially designed all-fibre mode converter we generate and deliver so-called “petal” beam with high-power output. We observe that under partial obstruction, the petal beam can be reconstituted through propagation.

Adaptive system for wavefront correction based on 240-mm bimorph deformable mirror and Shack-Hartmann wavefront sensor is presented. The dynamic characteristics of the deformable mirror and the performance of the wavefront correction in various operating modes of the PEARL facility as well as the features of phase distortion compensation in a single-shot generation regime are studied. An improvement in the quality of focusing that led to an increase in the Strehl ratio to 0.6 is demonstrated.

The article discusses the use of stacked-actuator adaptive mirrors to improve the focusing of laser radiation. The criterion of focusing efficiency is the fraction of the energy of the laser radiation passing through the pinhole located in the focal plane of the focusing lens.

Beam-shaping of homogeneous line profiles at ultra-violet wavelengths has wide applicability in the flat-panel display industry. Besides the well-established Excimer-Laser-based setups, diode-pumped-solid-state lasers with high repetition rates and comparatively small pulse energies have proven to be capable of providing a cost-effective alternative for different process steps e.g. Laser-Lift-Off or Solid-State-Laser-Annealing. We give a short summary about challenges emerging during the design of these system and demonstrate the generation of state-of-the-art laser-lines, offering a super-Gaussian, top-hat-shaped short-axis profile..

Using fly’s eye homogenizer has proven applicability in various fields, ranging from early-stage microscopy to state-of- the-art lithography. Hereby periodic lens arrays are commonly used to form a homogenous top -hat distribution. In turn, this periodicity can limit the attainable homogeneity by interference based micro - inhomogeneities. Furthermore, the positioning of the lens arrays results in small spot sizes directly at the lens surface, compromising functionality for large pulse energies. Here we present the challenges emerging due to the usage of non-periodic lens-arrays and introduce our design concept, suppressing interference based micro-inhomogeneities while being suitable for applications with large pulse energies.

Lasers in this new decade are much more industrial than scientific. Lasers help produce many of the products consumers take for granted. This is especially the case with computers, smart phones, and cars but of course lasers touch many parts of other products we use every day. A laser is a tool no different than any other mechanical machine tool. The only difference is how the tool is measured. A mechanical tool can obviously be measured with a micrometer or calliper, but a laser needs a power or energy meter and an M-square measurement device to measure its size and performance. Power meter measurements are straight forward and well understood. M-square measurements have been less so. Most lasers are measured by the manufacturer in a "free space" condition. This is how it has been done for more than two decades. Given the large number of lasers in production, it is important to be able to measure these tools in the application for which they are being used. The time has come for M-square measurement to leave the "Scientific" realm and enter the "Industrial" realm so that the performance of production lasers can be quantified in their production space. This work covers the finite number of application specific M-square measurement techniques for industrial lasers which include fiber only; collimated, free space beam; post focus and pre-focus applications.

Achieving ever narrower linewidths in diode lasers has become of paramount importance for coherent communications. Coupling a laser chip to a fiber however introduces external optical feedback (EOF) to the cavity which is notoriously detrimental to the stability of the device, in all but very specific and difficult conditions. A better understanding of the interplay between EOF and the laser dynamics is thus crucial for designing narrow- linewidth diode lasers to be used in field-testing scenarios. The standard formalism of EOF in diode lasers relies on the assumption that feedback is present on one side of the laser cavity. However, given the currently available integration technologies, this assumption is no longer justified. In this work a revision of EOF theory is explored based on the updated assumption that feedback can be introduced from both sides of the laser cavity. This is done by obtaining the dynamic equations of the proposed system, including an expression for the power spectral density and linewidth. Additionally, the stability of the system is discussed. Results show that the proposed revised theory can yield stable laser performance while simultaneously reducing laser linewidth.

Laser applications like 3D sensing, multifocal microscopy and material processing require high uniformity of the dot patterns created by diffractive optical elements (DOEs). Using an inverse design method for such DOEs, based on gradient-optimization and rigorous coupled-wave analysis, we have investigated a few case studies. We will discuss beam splitters generating a 1D 1×15 fan-out for 1550 nm wavelength, a 1D 1×16 fan-out for 532 nm wavelength and a 2D 3×5 fan-out for 405 nm wavelength with full-pattern angles up to 54°. We obtained uniformity errors as low as 3% for the elements fabricated in fused silica.

Optical frequency combs (OFC) using different kinds of whispering-gallery-mode (WGM) microresonators have already shown different applications and especially their applications in fiber optical communication systems as replacements of laser-arrays. For this application the free spectral range (FSR) of 200 GHz or less is desirable. Besides the fabrication material for microspheres, the resonator radius can be modified to change the FSR. In this paper use of silica microspheres for OFC represents an inexpensive alternative over the other microcombs: microring, microdisk, and microtoroid. We experimentally present a microsphere fabrication process from a different kind of silica (SiO₂) fibers by use of the hydrogen-oxygen melting technique. We experimentally review the OFC generation process the main microresonator parameters as FSR, Q-factor and evaluate the resulting WGM resonator generated OFC comb light source for further applications. An OFC was excited inside a 166 μm silica microsphere WGM resonators using a 1548 nm laser light. The obtained broadband OFC spanned from 1400-1700 nm with FSR of (397 ± 10) GHz.

Bimorph and stacked actuator deformable mirrors were used to increase the efficiency of focusing of partially coherent laser radiation propagated through the scattering suspension that was equivalent to the middle-density fog layer with the length ranging from 300–500 meters up to 5 kilometers. We used Shack-Hartmann sensor to measure the averaged wavefront distortions and CCD camera to estimate the intensity distribution of the focal spot in the far-field. Numerical and experimental investigations of focusing improvement demonstrated that it is possible to increase the peak intensity of the focal spot by more than 60 %.

Stacked-array deformable mirror is one of the most popular tools for correction of wavefront aberrations. We manufactured the stacked-array deformable mirror with small diameter by using piezoceramic combs with few actuators on them. In this case the aperture of mirror will be equal to 30 mm. The stroke of such mirror would be about 5 microns. The thickness of the mirror substrate is 1 mm. Developed deformable mirrors will be suitable for fast adaptive optical systems for optical radiation transferring through turbulent atmosphere tasks.

Whispering gallery mode resonators (WGMRs) are small axial symmetrical structures from transparent material, that can exceptionally well confine light within, thus making them ideal for studying light-matter interactions and using them as sensors. Various WGMR designs can be simulated using COMSOL Multiphysics. Sometimes an extra layer is coated on the surface of the resonator for achieving desirable effects. The extra layer changes quality factor of the resonator and ads extra modes for some frequencies. Different methods and studies are used for the exploration of this topic such as changing the thickness of the coating and using random functions to describe the roughness of the surface, which in micro and nanoscale makes a difference.

The whispering gallery modes (WGM) micro resonators are based on elliptical objects, which can be made from optically transparent materials, The geometry of the object enables optical wave circulating inside the ellipse using total internal reflection. If there is a monochromatic light source with constant intensity to the ellipse, constructive interference may be observed. Poly methyl methacrylate acrylic (PMMA) WGM micro resonators are commercially available with typical optical quality factor of 10³-10⁴. These could limit problems with WGM micro resonator expensive manufacturing. Thanks to advances in high resolution image processing, read-outs using spectroscopy (single photo detector) could be replaced with image processing. Image processing (4.5μm/px) allows to split elliptical WGM micro resonator in regions and analyze separate sectors of the ellipse, which can used as a representation of surface irregularity interaction with higher order special mode groups. In the present work new type of image processing for micro-resonators were developed, to analyze intensity distribution in separate regions for the PMMA WGM micro-resonators (40-70 μm). Resonators were coupled using a tapered fiber and fixed wavelength VCSEL laser (760nm). Temperature was change from 20-80 ⁰C which affected the PMMA refractive index (α), for 760nm dn/dT = -1.32 10^-4 (⁰C^-1) and thermal expansion (β) dR/dT = 2.60 10^-4(⁰C^-1). Combining the following physical changes, total changes (α+ β), WGM PMMA micro-resonator mode mapping was obtained. The following work offers new type of intensity processing methods for measuring applications using PMMA WGM micro resonators.