This PDF file contains the front matter associated with SPIE Proceedings Volume 8960, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Various aspects of the nonlinear dynamics of Kerr frequency comb generation in optical microresonators are considered. It is shown that the comb generation process can, for the case of a single continuous wave pump, be given a simple interpretation in terms of modulational instability and that the essential dynamics can be captured using a three wave mode truncation for the pump mode and the dominant sideband pair. This idea is also extended using a four wave model to analyze an alternative dual pump configuration, for which comb generation may occur without a pump intensity threshold in both the normal and the anomalous dispersion regime.

Kerr frequency comb generation from a nonlinear high-Q resonator becomes an interdisciplinary research topic emerging from nonlinear optics, integrated photonics, and ultrafast optics. We show that ultrashort cavity solitons can be generated from a mode-locked Kerr frequency comb in a dispersion-engineered nonlinear microresonator. The spectral flatness of the comb is greatly improved by making the cavity soliton as short as two optical cycles, with a comb line power variation below 20 dB over an octave-spanning bandwidth from near infrared to mid infrared, while excellent spectral coherence is achieved by soliton-based mode locking. It is shown by simulation that the two-cycle solitons are robust to the wideband soliton perturbation effects such as all-order dispersion, frequency-dependent Q-factor, dispersive wave generation, Kerr self-steepening, and stimulated Raman scattering. The pump power used to generate an octave-spanning combs can be significantly reduced when a dispersion profile with four zero-dispersion frequencies, which paves the way to achieve a fully integrated frequency comb generator on a chip.

The theoretical understanding of Kerr combs has been the object of extensive efforts worldwide in the last ten years. Several insights have been provided since then into this problem and have enabled significant progress for the optimization and tailoring of these combs. Here, we investigate the formation of dissipative structures in crystalline whispering-gallery mode disk resonators that are pumped in different dispersion regimes. In the Fourier domain, these dissipative structures correspond to specific types of mode-locked Kerr optical frequency combs. Depending on the sign of the second-order chromatic dispersion and on the pumping conditions, we show that either bright or dark cavity solitons can emerge, and we show these two regimes are associated with characteristic spectral signatures that can be discriminated experimentally. We use the Lugiato-Lefever spatiotemporal formalism to investigate the temporal dynamics leading to the formation of these azimuthal solitons, as well as the emergence of Turing patterns. The theoretical results are in excellent agreement with experimental measurements that are obtained using calcium and magnesium fluoride disk resonators pumped near 1550 nm.

Whispering gallery optical parametric oscillators are millimeter-sized monolithic sources for tunable coherent light. Several experiments have revealed that during optical parametric oscillation the pump resonance strongly differs from a Lorentzian shape. We theoretically and experimentally analyze these line-shape distortions. It turns out that the line shape of the pump resonance strongly depends on the coupling strength of the pump light and on the loss ratio between generated light and pump light. The line-widths, i.e. the losses, for the light generated by the parametric process can be deduced without measuring them directly.

High quality factor whispering gallery mode microresonators are ideally suited for nonlinear optical interactions. We demonstrate x⁽³⁾-based nonlinear interactions in silica microspheres, consisting in third harmonic generation and Raman assisted TSFG in the visible. A tunable, CW multicolour emission has been quantitatively measured controlling the cavity mode dispersion by choosing suitable sized microspheres and exciting the proper modes for efficient frequency conversion.

Resonant light pressure effects provide new degrees of freedom for optical manipulation of microparticles. In particular, they can be used for optical sorting of photonic atoms with extraordinary uniform resonant properties. These atoms can be used as building blocks of structures and devices with engineered photonic dispersions. To study the spectral shape of the force peaks, we developed a method to precisely control the wavelength detuning between the tunable laser emission line and central position of the whispering gallery mode (WGM) peaks in tapered fiber-to-microsphere water-immersed couplers. Our method is achieved by integrating optical tweezers to individually manipulate microspheres and based on preliminary spectral characterization of WGM peak positions followed by setting a precise amount of laser wavelength detuning for optical propulsion experiments. We demonstrated dramatic enhancement of the optical forces exerted on 20 μm polystyrene spheres under resonant conditions. Spectral properties of the resonant force enhancement were studies with controlled laser line detuning. In addition, we observed the dynamics of radial trapping and longitudinal propelling process and analyzed their temporal properties. Our studies also demonstrated a stable radial trapping of microspheres near the surface of tapered fiber for high speed resonant optical propulsion along the fiber.

Microresonators have drawn a great deal of interest for their importance in both practical applications and fundamental physics in light-matter interaction. The optical confinement provided by a microresonator greatly enhances the interaction between optical spatial mode and the light emitting materials. Conventional fabrication of microresonators adopting semiconductor processing technology (no matter top-down or bottom-up approach) still faces some challenges. Here we report the feasibility of constructing solid state microresonators with various configurations including spheres, hemispheres and fibres from organic polymer in a flexible way. We realize optically pumped lasing from these structures after incorporating organic dye materials and/or colloidal quantum dots into the resonators. The lasing characteristics have been systematically examined in terms of size dependence and polarization. The longitudinal optical modes are well defined by whispering gallery modes. We are also able to tune the resonance modes by deforming the shape of micro-spheres, representing the facile manipulation of light-matter interaction. Finally, refractive index sensing with high sensitivity can be readily realized from these structures enabled by the existence of evanescent waves and improved by Vernier effect in coupled resonators.

Optical cavities have successfully demonstrated the ability to detect a wide range of analytes with exquisite sensitivity. However, optimizing other parameters of the system, such as collection efficiency and specificity, have remained elusive. This presentation will discuss some of the recent work in this area, including 3D COMSOL Multiphysics models including mass transfer and binding kinetics of different cavity geometries and covalent attachment methods for a wide range of biological and synthetic moieties. A few representative experimental demonstrations will also be presented.

Optical resonances of spherical microresonators are of great interest measurements with high sensitivity. Usually the quantity to be measured is determined by the shift of the resonances of a single particle. Unfortunately, for this purpose, an expensive low-bandwidth tunable laser system with high accuracy is needed. When using an array of microresonators with slightly different size, each particle has a different resonance behavior. A change of the quantity to be measured leads to a change of the intensity distribution over the entire array. Therefore, using a microresonator array it is sufficient to measure the intensity distribution over all particles at a fixed wavelength.

The optical properties and sensing capabilities of fused silica microbubbles are studied numerically through a finite element method. Mode characteristics, such as the quality (Q) factor and the effective refractive index, can be determined for different bubble diameters and shell thicknesses. For sensing with whispering gallery modes, thinner shells lead to an improved sensitivity. However, the Q- factor decreases as the shell thickness reduces and this limits the final resolution. Here, we show that high resolution can be achieved when the microbubble acts as a quasi-droplet even for a water-filled cavity at the telecommunications C-band. Different sensing scenarios can be studied such as thermal sensing, pressure sensing, and nanoparticle detection. We investigate the onset of the quasi droplet regime for different modes in the microbubble.

In this study, we carry out an analytical investigation to determine the efficacy of multi-layer dielectric micro-sphere resonators as high-resolution electric field sensors. The use of a large number of layers with different electrical and mechanical properties allows for optimum dielectric constant and elastic modulus gradients within the sphere to optimize sphere resonator’s sensitivity to external electric field. The external electric field applied to a dielectric sphere induces an elastic deformation of the resonator (electrostriction effect), leading to shifts in its whispering gallery optical modes (WGM). The non-uniform distribution of the dielectric constant leads to a gradient in the electric field within the sphere. This in turn induces non-uniform body force on the sphere when subjected to an external electric field. By also appropriately varying the elastic modulus of the layers, the electrostictive deformation of the sphere can be optimized. In this paper we present a mathematical model describing the effect of the electric field on the WGM shift of layered micro-spherical resonators.

We demonstrate a novel simplified hollow-core microstructural optical fiber (SHMOF) laser with intense output and purely radial emission. The SHMOF comprises a large hexagonal core with six surrounding crown-like air holes. The microfluidic channel is composed of a nearly cylindrical center hole of the SHMOF, which is made by selectively blocking off other holes of the SHMOF. The novel SHMOF based laser device is integrated with the silica ring surroundings and the gain medium microchannel. The fiber core formed cavity which filled with a microfluidic gain medium (Rhodamine 6G) plug was lateral pumped by a nanosecond pulse laser. For pump energy at threshold as low as 80 nJ/pulse, single mode laser oscillation was observed at about 570 nm. And when pump energy was added to a higher value above the threshold, an intense output of a unique radiating field pattern characterized by cylindrical symmetry emerged in the azimuthal direction of the fiber. The explanation of this result lies in the particularity structure of the SHMOF in our experiment. This attractive capability presents opportunities in fiber laser array composed microsystems and fluorescence signal amplification for chemical and biological analysis.

We report the observation of multiphoton excitation of organic chromophores in microbubble whispering gallery mode resonators. High-Q microbubble resonators are a formed by heating a pressurized fused silica capillary to form a hollow bubble which can be filled with liquid. In this case, the microbubble is filled with a solution of Rhodamine 6G dye. The resonator and dye are excited by evanescently coupling CW light from a 980nm laser diode using a tapered optical fiber. The two-photon fluorescence of the dye can be seen with pump powers as low as 1 mW.

In this paper, we demonstrate a hybrid plasmonic bottle microresonator (PBMR) which supports whispering gallery modes (WGMs) along with surface plasmon waves (SPWs) for high performance optical sensor applications. The BMR was fabricated through “soften-and-compress” technique with a thin gold layer deposited on top of the resonator. A polarization-resolved measurement was set-up in order to fully characterize the fabricated PBMR. Initially, the uncoated BMR with waist diameter of 181 μm, stem diameter of 125 μm and length of 400 μm was fabricated and then gold film was deposited on the surface. Due to surface curvature, the gold film covering half of the BMR had a characteristic meniscus shape and maximum thickness of 30 nm. The meniscus provides appropriately tapered edges which facilitate the adiabatic transformation of BMR WGMs to SPWs and vice versa. This results in low transition losses, which combined with partially-metal-coated resonator, can result in high hybrid-PBMR Q’s. The transmission spectra of the hybrid PBMR are dramatically different to the original uncoated BMR. Under TE(TM) excitation, the PBMR showed composite resonances with Q of ~2100(850) and almost identical ~ 3 nm FSR. We have accurately fitted the observed transmission resonances with Lorentzian-shaped curves and showed that the TE and TM excitations are actually composite resonances comprise of two and three partially overlapping resonances with Q’s in excess of 2900 and 2500, respectively. To the best of our knowledge these are the highest Qs observed in plasmonic microcavities.

Label-free biosensors that combine high sensitivity and high specificity characteristics have shown tremendous potential for applications in medical diagnostics, and have more recently been extended to the food safety and environmental monitoring arenas. A unique type of label-free, optical biosensor, based on Whispering Gallery Mode microresonators, has tremendous potential to revolutionize biodetection due to its extreme sensitivity. The primary limitation of these biosensors, however, is that they require the addition of biorecognition elements to specifically target a biological species of interest. Therefore, the ability to selectively functionalize the microresonator for a specific target molecule, without degrading device performance, is extremely important, and represents the next step in translating these devices from laboratory to field environments. Here, we demonstrate a variety of straightforward bioconjugation strategies that not only impart specificity to optical microresonators, but also allow for the creation of multi-use platforms for complex environments. Of particular interest is the ability to detect harmful bacteria, insects, and fungi in crop and water systems. The resulting surface chemistries are illustrated with XPS, SEM, and fluorescence and optical microscopy, and the device sensitivity is determined via quantitative microcavity analysis. The ability to minimize non-specific adsorption and target unique molecules in complex environments is demonstrated via ellipsometry and in situ device testing. The resulting devices can be recycled several times without loss of sensitivity. By combining these high sensitivity biosensors with appropriate biochemistries, the resulting platforms can be extended to address broader issues in environmental biosensing that directly impact agriculture.

High optical field intensities build up inside microtoroids owing to its ultra-high quality factor, making them an ideal platform for plasmonic-photonic interactions with noble metals and a suitable pump source for microlaser. In this work, a microlaser based on hybrid silica microtoroids coated with gold nanorods is theoretically modeled and experimentally demonstrated. Theoretically, we used 3-D Comsol Multiphysics and modeled the interaction between the optical mode of the microtoroids and the surface plasmonic resonance of gold nanorods, both on and off resonance. To thoroughly study the role that the polymer layer plays in the plasmonic laser system, we perform a series of finite element method simulations in which the polymer layer thickness and refractive index is varied, and its effect on the plasmonic resonance is quantified. Experimentally, we demonstrated a visible laser at 575nm from hybrid microtoroids with a 30μWthreshold and an approximately 1nm linewidth. We have also varied the gold nanorod concentration on the microtoroids surface, and studied its effect on the Quality factor and the threshold power in order to get the optimum concentration for lasing.

Due to their narrow linewidth lasing peaks, microlasers based on whispering gallery mode optical resonators are becoming increasingly useful in sensing applications. Changes in the microlaser’s environment cause the lasing wavelength to shift, enabling detection with high resolution. However, the performance of these devices is often limited by the optical spectrum analyzers used in the detection experiments, which lack the speed and sub-picometer resolution needed to measure small changes in the microlaser’s output. One promising approach to overcome this limitation is heterodyning the microlaser’s output. In the present work, we successfully heterodyne a microlaser sensor based on a neodymium-doped silica toroid platform pumped at 765nm. Combining the microlaser's 1064nm emission with a 1064nm reference laser produces an easily detectable low frequency beat signal. Monitoring the beat frequency on an electrical spectrum analyzer (ESA) enables wavelength shifts to be detected with high speed and subpicometer resolution. As a proof of concept, temperature sensing experiments are performed by tracking the beat frequency as the microlaser is heated. To directly determine the improvement in sensitivity and in signal to noise, comparison experiments are also performed by tracking the resonant wavelength and lasing wavelength. We experimentally show that heterodyning improves the microlaser's detection limit, signal to noise, and time resolution by as much as 50-fold compared to the non-heterodyned laser approaches. Narrowing the microlaser's linewidth and reducing noise could increase this enhancement even further. Therefore, heterodyning will significantly benefit both the performance of microlaser sensors and their many applications.

A six sigma laser processing system is proposed that utilizes real time measurement of ISO 11146 and ISO 13694 laser beam parameters without disrupting the process beam and with minimal loss. If key laser beam parameters can be measured during a laser process, without a disruption to the process, then a higher level of process control can be realized. The difficulty in achieving this concept to date is that most accepted beam measurement techniques are time averaged and require interruption of the laser beam and therefore have made it impractical for real time measurement which is necessary to consider six sigma process control. Utilizing an all passive optical technique to measure a laser’s beam waist and other parameters for both focused and unfocused beams, the direct measurement of the ISO laser beam parameters are realized without disruption to the process and with minimal loss. The technique is simple enough to be applied to low and high power systems well into the multi-kilowatt range. Through careful monitoring of all laser beam parameters via software control of upper and lower limits for these parameters, tighter quality control is possible for achieving a six sigma process. In this paper we describe the optical design for both low and high power laser systems and how six sigma laser processing may be realized.

We present an adaptive optics system for active wavefront correction of the first stage amplifier of a multi-slab laser system capable of generating 100 W of average output power and transverse dimensions of 20 mm x 20 mm. The results of this experiment are compared with numerical simulations.

We discuss our recent progress in iimproving the phase noise of a semiconductor laser using self-injection locking of to a mode of a high-Q whispering gallery mode resonator. Locking efficiency is analyzed for semiconductor distributed feedback (DFB) as well as Fabry-Perot (FP) lasers operating at 690 nm, 1060 nm, 1550 nm, and 2 μm. Instantaneous linewidth below 300 Hz is realized with telecom DFB lasers. Tunability of the lasers is demonstrated. Commercially available packaged ”plug-and-play” devices are manufactured.

We present studies of using high Q whispering gallery mode resonators in Mid-IR laser spectroscopy. Several crystalline materials have high transparency in Mid-IR wavelength region and can be made into high Q optical resonators. We report recent measurements of Q values of greater than 1x10⁸ in the wavelength region longer than 3 μm using one of these materials, Magnesium Fluoride (MgF₂). These resonators are being used for cavity ring-down measurements, optical frequency comb generation, and their applications in Mid-IR spectroscopy

High speed modulation characteristics are investigated for microcircular lasers connected with an output waveguide theoretically and experimentally. The injection current profile and carrier spatial hole-burning and diffusion are accounted in the rate equation model by radially dividing the microcircular resonator into two regions under the approximation of uniform carrier densities. The numerical results indicate that wide mode field pattern in radial direction has merit for high speed modulation, which is expected for coupled modes in circular microlasers connected with an output waveguide. Small signal response curves and large signal modulation responses are investigated for a 15-μmradius microlaser connected with a 2 μm wide output waveguide. The highest resonance frequencies of 7.2, 5.9 and 3.9 GHz are obtained at the temperatures of 287, 298 and 312 K from the small signal response curves, and clear eye diagrams at 12.5Gbit/s with an extinction ratio of 6.1 dB are observed for the microlaser at biasing current of 38 mA and the temperature of 287 K.

We demonstrate a free running 10 GHz microresonator-based RF photonic hyper-parametric oscillator characterized with phase noise better than -60 dBc/Hz at 10 Hz, -90 dBc/Hz at 100 Hz, and -150 dBc/Hz at 10 MHz. The device consumes less than 25 mW of optical power. A correlation between the frequency of the continuous wave laser pumping the nonlinear resonator and the generated RF frequency is confirmed. The performance of the device is compared with the performance of a standard optical fiber based coupled opto-electronic oscillator of OEwaves.

Different applications of crystalline whispering gallery mode resonators call for different properties of the resonator host material. We report on our recent study of resonators made out of sapphire, diamond, and quartz crystals and discuss possible applications of these resonators. In particular, we demonstrate Kerr frequency comb generation in sapphire microresonators.

We introduce tunable optofluidic microlasers based on active optical resonant cavities formed by optically stretched, dye-doped emulsion droplets confined in a dual-beam optical trap. To achieve tunable dye lasing, optically pumped droplets of oil dispersed in water are stretched by light in the dual-beam trap. Subsequently, resonant path lengths of whispering gallery modes (WGMs) propagating in the droplet are modified, leading to shifts in the microlaser emission wavelengths. We also report lasing in airborne, Rhodamine B-doped glycerolwater droplets which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Furthermore, biological lasing in droplets supported by a superhydrophobic surface is demonstrated using a solution of Venus variant of the yellow fluorescent protein or E. Coli bacterial cells expressing stably the Venus protein. Our results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium.

When liquid crystals are dispersed in an immiscible fluid, microdroplets of liquid crystal are spontaneously formed in a fraction of a second. They have optically anisotropic internal structure, which is determined by the ordering of liquid crystal molecules at the interface. Spherical droplets of a nematic liquid crystal can function as whispering-gallery-mode microresonators with an unprecedented width of wavelength tunability by an electric field. WGM pulsed lasing in dyedoped nematic microdroplets is sensitive to strain, temperature and presence of molecules that change molecular orientation at the interface. Omnidirectional 3D lasing was demonstrated in droplets of chiral nematic liquid crystals that form 3D Bragg-onion resonators. We present recent progress in this field, including electric tuning of 3D lasing from chiral nematic droplets and self-assembly of ferroelectric smectic-C* microdroplets with the onion-Bragg structure. We show that anisotropic fibres could be self-assembled from smectic liquid crystals.

A novel, integrated photonic device has been demonstrated, which combines a microstructured optical fiber (MOF) and a Whispering-Gallery Mode (WGM) microsphere resonator encapsulated inside one of its capillaries. Studies have been focused on the light coupling to and from the micro-resonator, as well as, the resonating properties of such a photonic system. Spectral resonation in a spectral band spanning between 500 nm – 1600 nm has been observed, with Q-factors exceeding 2x10³ for a single microsphere. Research on the functionality and performance of different fiber/microsphere combinations focuses on the material and size of the microspheres as key parameters for the optimization of the corresponding spectral patterns and Q-factors. This newly developed resonating system constitutes an optical platform with various potential applications.

The fabrication and performance of a new optical microstub resonator is presented. The technique is simple and versatile enabling fabrication of high quality resonators. Q factors in excess of 10⁺⁶ have been demonstrated. Furthermore, active Ytterbium-doped microstub resonators are fabricated and characterized which show potential applications in fiber lasers and sensors.

We report on fabrication of three-dimensional (3D) high-quality (Q) whispering-gallery-mode microcavities by femtosecond laser micromachining. The main fabrication procedures include the formation of on-chip freestanding microdisk through selective material removal by femtosecond laser pulses, followed by surface smoothing processes (CO₂ laser reflow for amorphous glass and focused ion beam (FIB) sidewall milling for crystalline materials) to improve the Q factors. Fused silica microcavities with 3D geometries are demonstrated with Q factors exceeding 106. A microcavity laser based on Nd:glass has been fabricated, showing a threshold as low as 69μW via free space continuous-wave optical excitation at the room temperature. CaF₂ crystalline microcavities with Q factor of ~4.2×10⁴ have also been demonstrated. This technique allows us to fabricate 3D high-Q microcavities in various transparent materials such as glass and crystals, which will benefit a broad spectrum of applications such as nonlinear optics, quantum optics, and bio-sensing.

Short-pulse laser systems have found entry into industrial micro material fabrication processes on a large scale during the past ten years. In the same way the demand of simple, compact and cost-efficient seed sources has grown. The physical parameters needed for short-pulse laser processing range between a few femtoseconds to some ten picoseconds at repetition rates of up to 1 MHz. Up to now these laser systems are based on high repetition rate oscillators and regenerative amplifiers. These systems are rather complex and expensive. In contrast a Q-switched microchip laser in combination with a single pass amplifier permits a much simpler approach. In the following we present a 50 μm Nd³⁺:YVO₄ microchip laser that is passively Q-switched by a semiconductor saturable absorber mirror. To overcome handling problems of the small crystal dimensions the 50 μm 3 at.-% doped Nd³⁺:YVO₄ crystal is optically bonded to an undoped YVO₄ crystal of a length of about 500 μm. The system provides pulse widths around 26 ps at a repetition rate of up to 0.9 MHz. The average output power is 15 mW at a wavelength of 1,064 nm, at an energy of 17 nJ. We will discuss the prospects and limits in terms of pulse width, repetition rate, output power, and system stability. The experimental data are compared to theoretical calculations.

A procedure is described to accurately measure the self phase-tuning in a coupled fiber laser. A fiber is designed and fabricated to eliminate the effects of self-tuning from wavelength shifting, thermal expansion, and thermally induced index change, allowing us to study the phase effects produced by Kramers-Kronig phase shifting as a function of various cavity parameters. For sufficient pump power, we observe that the Kramers-Kronig effect is capable of compensating for all path length errors introduced into the cavity, resulting in efficient lasing under all path length conditions. We have directly measured the Kramers-Kronig-induced phase shift and present experimental evidence that this additional phase compensates for the applied phase error and promotes efficient lasing.

To scale to power levels of up to tens of kW with a few fiber lasers, the best candidates are large core fibers guiding a few large-area higher order modes with the outputs of these fibers combined into a single beam. However, in many applications it is desirable to convert these higher order modes into a Gaussian profile. Here, we propose a method to accomplish this task via single volume phase element. This element accepts multiple higher order mode beams and simultaneously converts and combines them to a single Gaussian profile in the far field.

Wide aperture bimorph mirrors for laser beam correction and formation were developed and investigated. Different types of substrates and active piezoceramics materials were considered to fabricate temperature independent shape of the mirror surface and to maximize the sensitivity of the mirror. High reflectivity coatings for different wavelengths were studied.

We explore an intra-cavity beam shaping approach to generate a Gaussian distribution by the metamorphosis of a Gaussian beam into a flat-top distribution on opposing mirrors. The concept is tested external to the cavity through the use of two spatial light modulators (SLM), where the first SLM is used to transform a collimated Gaussian into a flat-top distribution and the second SLM is encoded with the conjugate phase of the flat-top for conversion back to a Gaussian. We implement this intra-cavity selection through the use of two optical elements of the refractive variant that are designed from the phase profiles addressed to the SLMs. We consider a solid-state diode side-pumped laser resonator that consists of two planar mirrors where the refractive optics are positioned at the mirrors. We out couple the Gaussian and show that the output beam size is comparable with the theoretical predictions and that we have an increase in optical brightness when compared to the cavity without any optics.

In this paper we will outline our recent advances in all-digital control of light. Importantly, we will outline how to create a so-called “digital laser”, where a conventional laser mirror is replaced with a phase-only spatial light modulator. This allows the mirror properties to be dynamically changed by altering only the image sent to the device: on-demand laser modes. We demonstrate a myriad of laser beams that can be created from the same device without any realignment or additional custom optics.

In this paper we experimentally demonstrate a simple laser cavity that produces spatial tuneable laser modes from a Gaussian beam to a Flat-top beam and a Donut-beam. The laser cavity contains an opaque ring and an adjustable circular aperture that could be varied and thus allows for tuneability of the cavity without it being realigned. A digital laser with an intra-cavity spatial light modulator is used to demonstrate and confirm the predicated properties of the resonator.

We propose a new technique for laser beam shaping namely the reshaping of the laser beam into a desirable beam profile by the use of a laser amplifier with a pump beam that has a modified intensity profile. We developed the analytical formula which describes the transformation of the seed beam into the desired beam profile in the amplifiers small signal regime. In the case were high pump power saturated the laser crystal we have shown the method of reshaping of the seed beam into desirable beam by a numerically obtained pump intensity profile. The method can also be extended for shaped seed beams with fixed profile pump.

Beam-shaping is essential for any kind of laser application. Assembly technologies for beam-shaping subassemblies are subject to intense research and development activities and their technical feasibility has been proven in recent years while economic viability requires more efficient engineering tools for process planning and production ramp up of complex assembly tasks for micro-optical systems. The work presented in this paper aims for significant reduction of process development and production ramp up times for the automated assembly of micro-optical subassemblies for beam-collimation and beam-tilting. The approach proposed bridges the gap between the product development phase and the realization of automation control through integration of established software tools such as optics simulation and CAD modeling as well as through introduction of novel software tools and methods to efficiently describe active alignment strategies. The focus of the paper is put on the methodological approach regarding the engineering of assembly processes for beam-shaping micro-optics and the formal representation of assembly objectives similar to representation in mechanical assemblies. Main topic of the paper is the engineering methodology for active alignment processes based on the classification of optical functions for beam-shaping optics and corresponding standardized measurement setups including adaptable alignment algorithms. The concepts are applied to industrial use-cases: (1) integrated collimation module for fast- and slow-axis and (2) beam-tilting subassembly consisting of a fast-axis collimator and micro-lens array. The paper concludes with an overview of current limitations as well as an outlook on the next development steps considering adhesive bonding processes.

Widening of using high power multimode lasers in industrial laser material processing is accompanied by special requirements to irradiance profiles in such technologies like metal or plastics welding, cladding, hardening, brazing, annealing, laser pumping and amplification in MOPA lasers. Typical irradiance distribution of high power multimode lasers: free space solid state, fiber-coupled solid state and diodes lasers, fiber lasers, is similar to Gaussian. Laser technologies can be essentially improved when irradiance distribution on a workpiece is uniform (flattop) or inverse-Gauss; when building high-power pulsed lasers it is possible to enhance efficiency of pumping and amplification by applying super-Gauss irradiance distribution with controlled convexity. Therefore, “freeform” beam shaping of multimode laser beams is an important task. A proved solution is refractive field mapping beam shaper like Shaper capable to control resulting irradiance profile – with the same unit it is possible to get various beam profiles and choose optimum one for a particular application. Operational principle of these devices implies transformation of laser irradiance distribution by conserving beam consistency, high transmittance, providing collimated low divergent output beam. Using additional optics makes it possible to create resulting laser spots of necessary size and round, elliptical or linear shape. Operation out of focal plane and, hence, in field of lower wavefront curvature, allows extending depth of field. The refractive beam shapers are implemented as telescopes and collimating systems, which can be connected directly to fiber-coupled lasers or fiber lasers, thus combining functions of beam collimation and irradiance transformation.

Laser beams with cone-refracted output from the plane mirror is demonstrated for the first time in lasers based on intracavity conical refraction (CR) phenomenon. Transverse profile of such lasers comprises a crescent ring of CR-like distribution, where any opposite points are of orthogonal linear polarizations. We confirm the existence of such mode of CR lasers by two observations. First, cascaded CR in reflection geometry has been demonstrated for first time and it provides experimental prove that a light beam passed along optic axis of a biaxial crystal, reflected back from a plane mirror and passed again through the crystal is restored. Second, CR cavity mode with CR-like pattern through the plane mirror is experimentally and theoretically demonstrated for the first time.

In order to provide the end user with a diffraction limited collimated beam, adaptive optics phase correction systems are now a standard feature of ultra intense laser facilities. Generally speaking, these systems are based on a deformable mirror controlled in closed loop configuration in order to correct the aberrations of the beam measured by the wavefront sensor. Such implementation corrects for most of the aberrations of the laser. However, the aberrations of the optical elements located downstream of the wavefront sensor are not measured and therefore not corrected by the adaptive optics loop while they are degrading the final focal spot. We present an improved correction strategy and results based on a combination of both usual closed loop and phase retrieval in order to reach the diffraction limit at the focal spot inside the interaction chamber. The off axis parabola alignment camera located at the focal spot is used in combination of the deformable mirror and wavefront sensor to get images of the focal spot. The residual aberrations of the focal spot are measured by a Phase Retrieval algorithm using the acquired focal spot images. Then the adaptive optics loop is run in order to precompensate for these aberrations, which leads to diffraction limited focal spot in the interaction chamber.

We experimentally investigate a technique to reduce speckle contrast in multimode fibers by using a piezoelectric vibrator. The speckle patterns in multimode fiber are attributed to the interference between the propagation modes of the fiber. If the optical fiber is externally vibrated, the output pattern is modulated due to mode couplings of the propagation modes. In this experiment, a part of fiber was fixed on the piezoelectric plate and applied high frequency oscillations. It was clearly observed that the speckle contrast of the output pattern was dramatically reduced by applying vibration. We investigated the factors which affect the speckle contrast, such as core diameter, fiber bending radius, shape of fiber core, exposure time of camera, etc. We found that non-circular shaped core fiber might be more effective to reduce speckle contrast in multimode fibers.

In this paper we present a general approach to determine the stability of a laser cavity which can include nonconventional phase transformation elements. We consider two pertinent examples of the detailed investigation of the stability of a laser cavity firstly with a lens with spherical aberration and thereafter a lens axicon doublet to illustrate the implementation of the given approach. In the particular case of the intra–cavity elements having parabolic surfaces, the approach comes to the well–known stability condition for conventional laser resonators namely 0 (1 / 1 )(1 / 2 ) 1. ≤ − z /R₁ − z/ R₂ ≤ The method can be considered as a technique for intra–cavity beam shaping.

An 1178 nm seeded and 1069 nm pumped Raman laser system where the second Stokes is amplified in a 1121 nm resonator defined by high reflector fiber Bragg gratings (FBGs) has the potential of producing high output power of narrow linewidth 1178 nm. However, 1121 nm power leakage out of the resonator cavity around the gratings was found to impact the performance of the laser and needs to be dealt with in order to obtain high 1178 nm output power levels. In order to address this problem, the causes of linewidth broadening must be understood. A fully nonlinear model has been built which involves propagation of the spectral wave shape via the nonlinear Schrödinger equation in addition to the Raman processes. It was found that increases in 1121 nm cavity power, fiber Bragg grating bandwidth, and the nonlinear index of refraction n2, as well as a decrease in group velocity dispersion β 2 leads to an increase in linewidth broadening. It is concluded that the magnitude of linewidth broadening seen experimentally can only be explained if the spectral components outside the bandwidth of the FBGs are being amplified. Experimentally, 1121 nm power leakage can be handled by using a three wavelength WDM on either side of the rare earth doped amplifier. In addition, usage of a fiber having a high value for group velocity dispersion and/or a low value for nonlinear index of refraction n2 in addition to narrower bandwidth fiber Bragg gratings may help reduce the amount of linewidth broadening.

It is experimentally shown that a large thermal blue shift of up to 100 GHz/K (0.2 nm/K at a wavelength of 775 nm) can be achieved with higher order radial modes in an ethanol-filled microbubble whispering gallery mode resonator (WGR). Q-factors for the most thermally sensitive modes are typically 10⁵, equivalent to a measurement resolution of 8.5 mK. The thermal shift rate is determined for different modes when the core of the microbubble is filled with air, water, and ethanol. The measured shifts are compared against Finite Element Model (FEM) simulations. It is also shown that, if the microbubble is in the quasi-droplet regime, the fundamental TE mode in a bubble with a 500 nm wall is estimated to experience a shift of 35 GHz/K, while the effective index is still high enough to allow efficient coupling to a tapered optical fiber. Nonetheless, at a wall thickness of 1 μm, the most sensitive modes (n = 2) observed were still strongly coupled.

In this paper we outline a simple laser cavity which produces customised on-demand digitally controlled laser modes by replacing the end-mirror of the cavity with an electrically addressed reflective phase-only spatial light modulator as a digital addressed holographic end-mirror. We show that on-demand digitally controlled laser modes are possible by changing the phase and amplitude of the computer generated hologram in a form of a grey-scale image on the holographic mirror. We demonstrate that customised digitally controlled laser modes can be generated on-demand by switching to several different spatial modes in real-time with the first the ‘digital laser’.

The standard Findlay Clay Analysis cannot be applied to diode side-pumped Nd:YAG lasers because both the pump wavelength as well as the gain change with the diode current and the cooling water temperature. We have developed a modified method which is based on the variation of the cooling water temperature to determine the lowest threshold for each output coupler. We have applied this technique to a 300 Watt class side Nd:YAG rod laser. The resulting Findlay Clay plot exhibits very good linearity and the measured gain and loss were confirmed by comparing measured output power at different cooling water temperatures with theoretical values provided by a Rigrod power model.

Ring resonators have unique properties that are sometimes desirable. Spatial hole burning is eliminated. Beam transformation, such as image rotation which may reduce the magnitude of certain aberrations, can be implemented in a traveling-wave region. There is a drawback, however. As usually constructed, a ring resonator has half as many passes through the gain medium as can be achieved with a standing-wave resonator. This may have a detrimental effect on laser efficiency. We have constructed a type of ring resonator that allows counterpropagating collinear passes through the gain medium, while there is also a section with a unidirectional beam. The resonator includes a polarizing beam splitter. The linear polarization is transformed to the orthogonal state by optical elements at the two ends of the region with counter-propagating beams. The beams passing through the gain medium in opposite directions are linearly polarized with orthogonal states.