This PDF file contains the front matter associated with SPIE Proceedings Volume 8236, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Modal properties of two architectures for coherent beam combining are theoretically analyzed and experimentally verified. The supermodes of a two-laser spatially filtered cavity exhibit two distinctly different types of behavior depending on the path length error. When the error is small, the two modes present different cavity loss values and can be differentiated by gain. However, cavities containing path length errors greater than a critical value produce modes with identical losses and different resonant frequencies. A diode-bar side-pumped plano-concave Nd:YAG laser cavity is built for experimental verification of the theory. Experiments have shown two distinct regions as predicted by theory. In the small path length error region, the cavity runs in one single mode; however, when the path length error goes beyond a critical value, the cavity lases in two modes simultaneously or alternates between two modal states. Detailed loss versus phase error curves are presented and compared to theory. The modal behavior in this spatial filtering architecture is quite different from that found in a superposition architecture for coherent beam combining where the fundamental mode always has smaller loss per round trip than the second mode. The modal curves for a superposition architecture are provided for comparison with those from spatial filtering.

A dual seeded (pump at 1069 nm and second Stokes at 1178 nm) Raman laser system involving amplification of the second Stokes in a Raman resonator having high reflector Bragg gratings tuned to the first Stokes at 1121 nm is proposed. This system has the potential of generating 50 W of output power at 1178 nm. Because the laser is seeded with the desired output wavelength (second Stokes), outputs having narrow or broad linewidths can be achieved. Highly efficient Raman conversion is achieved in a short Raman fiber (< 20 m) due to high 1121 nm circulating power levels in the Raman resonator. Stimulated Brillouin Scattering, the 1121 nm circulating power level, and the effects of modal instability are constraints on the design of a narrow linewidth system. Length of the resonator cavity, power level of the 1069 nm pump, and power level of the 1178 nm seed are parameters that can be adjusted to optimize the design. Usage of germanosilicate fiber improves the performance of the system. Finally, because of the low power level of available 1178 nm seeds, a two stage system (low and high power stages) is necessary to achieve 50 W of 1178 nm output power.

The influence of a variety of parameters, such as the gas composition, pressure and lateral variation of Joule heat release of the discharge at employed frequency, are taken into account to derive a distributed thermal lensing expression and to numerically calculate the focal length as a function of position within the inter-electrode gap. The process of widening of the beam inside the resonator is investigated by means of complex ray methodology. The findings are incorporated into the optimization process of the optical resonator in the stable direction and the impact on the beam quality and power stability is verified by experimental results.

The application of standard unstable resonators does not allow for an independent adjustment of the resonator magnification and the output coupling. Either you get a high magnification together with a high output coupling, or vice versa. Certain laser types, like e.g. thin-disc lasers or chemical oxygen iodine lasers, permit only quite low optimum output couplings. The corresponding low resonator magnification is equal to a poor beam quality. In order to apply unstable resonators with a high magnification also to low gain media an additional mirror surface retroreflects a part of the out coupled radiation back into the cavity. The output coupling is reduced efficiently, whereas the resonator magnification stays high. Accordingly low gain media can be operated with high power extraction in combination with a good beam quality. Numerical and experimental investigations are shown. The experiments are performed with a chemical oxygen iodine laser operating at a wavelength of 1.315 μm and demonstrate the feasibility of this resonator design.

In this work we study the effect of the ring width on the performance of a ring generated Bessel beam. Experimental results and simulation model for ring generated Bessel beams are investigated and compared. The simulation model is based on Fourier optics. The effect of varying the ring radius and the ring width on the Bessel beam parameters like the axial intensity and the detected output power transported by the beam passing through the ring is studied. A good agreement is found between the simulation model and the measurements. Larger ring width led to higher efficiency (output power) but to less beam quality.

Diffractive optical elements (DOEs) are of rising importance for many industrial laser applications, especially for laser beam shaping and laser beam splitting. Typically, such applications require high damage threshold of the diffractive optical elements as well as high diffraction efficiency. Usually DOEs with multilevel (step-like) phase profiles are made microlithographically and suffer from "quantisation" errors and scattering on profile derivative discontinuities. The steplike structure lowers the DOE damage threshold compared to the intrinsic material values. LIMO's microoptical technology is suitable for the production of high-precision free programmable continuous surface profiles in optical glasses, crystals and metals. It can be applied for manufacturing of microlens and micro-mirror arrays as well as for manufacturing of diffractive optics with continuous reliefs. Both the arrays and DOEs with continuous relief are suitable for high efficiency laser beam splitting. However, the design approaches to obtain a desirable solution for the corresponding continuous phase profiles are different. The results of the wave-optical simulations made by LIMO's own program and by VirtualLab software, and experimental studies for a 1 to 11 beam splitter with a continuous profile for the wavelength of 532 nm are presented. Continuous phase profiles for the DOEs were designed by a procedure based on the theory of beam splitting by a phase grating. Comparative theoretical and experimental studies were also done for splitting with a double-sided microlens array. For both types of beam splitting the efficiency can be very high (> 98%). The DOEs show especially high homogeneities of the resulting intensity distribution, however, they are much more sensitive to wavelength variations. The microlens arrays demonstrate even weaker ghost orders as the DOE splitters and their surface profiles are simpler. However, the efficiency and homogeneity suffer on interlens gaps.

Direct laser patterning of various materials is today widely used in several micro-system production lines like inkjet printing, solar cell technology, flat-panel display production, LEDs, OLEDs, semiconductors and medicine. Typically single-mode solid state lasers and their higher harmonics (e. g. 266, 355, 532 and 1064 nm) are used especially for machining of holes and grooves. The striking advantages of flat top intensity distributions compared to Gaussian beam profiles with respect to the efficiency and quality of these processes were already demonstrated. Here we will give an overview of parameters, methods and applications of Gaussian-to-top-hat beam shaping. The top hat field size can start from about 30 μm with no upper size limitation in the far field of the optics. Beam shaping for various wavelengths were realized with field geometries of squares, rectangles and circles. With LIMO's compact Gaussian-to-top-hat converter an inhomogeneity better than 5% contrast was reached. Special focus is put on the integration of Gaussian-to-top-hat beam shapers in fast scanning systems employing Galvo mirrors and a specially developed f-Theta lens to avoid destruction of the top hat profile within the scan field. Results with a 50x50μm² top hat size (inhomogeneity down to <10%) in a scan area of 156x156mm² are presented. The minimal distortions of the top hat observed within the scan area make LIMO's compact Gaussian-to-top-hat converter excellently suited for industrial scanning applications, e.g. for the processing of solar panels.

We propose a new method to determine the wavefront of a laser beam, based on modal decomposition using computer-generated holograms (CGHs). Thereby the beam under test illuminates the CGH with a specific, inscribed transmission function that enables the measurement of modal amplitudes and phases by evaluating the first diffraction order of the hologram. Since we use an angular multiplexing technique, our method is innately capable of real-time measurements of amplitude and phase, yielding the complete information about the optical field. A measurement of the Stokes parameters, respectively of the polarization state, provides the possibility to calculate the Poynting vector. Two wavefront reconstruction possibilities are outlined: reconstruction from the phase for scalar beams and reconstruction from the Poynting vector for inhomogeneously polarized beams. To quantify single aberrations, the reconstructed wavefront is decomposed into Zernike polynomials. Our technique is applied to beams emerging from different kinds of multimode optical fibers, such as step-index, photonic crystal and multicore fibers, whereas in this work results are exemplarily shown for a step-index fiber and compared to a Shack-Hartmann measurement that serves as a reference.

Modern laser scientific techniques and industrial technologies require not only simple homogenizing of a beam but also more freedom in manipulation of intensity profile and generating such profiles like super-Gaussian, inverse-Gaussian, skewed flattop and others. In many cases the task of variable beam shaping can be solved by refractive beam shaping optics of field mapping type which operational principle presumes saving of beam consistency, providing collimated output beam of low divergence, high transmittance and flatness of output beam profile, extended depth of field; another important feature is negligible residual wave aberration. Typically the fields mapping refractive beam shapers, like πShaper, are designed to generate flattop intensity profile for a beam of pre-determined size and input intensity profile. Varying of the input beam diameter lets it possible to realize either super-Gaussian (smaller input) or inverse-Gaussian (bigger input) intensity profiles of output beam that are important in pumping of solid-state lasers, hardening, cladding and other techniques. By lateral shift of a beam with respect to a πShaper the output flattop profile gets a skew in direction of that shift, the skew angle corresponds to the shift value. The skewed profile is important, for example, in some acousto-optical techniques where compensation of acoustic wave attenuation is required. All variety of profiles can be provided by the same beam shaper unit. This paper will describe some design basics of refractive beam shapers of the field mapping type and techniques to vary the output intensity profile, experimental results will be presented as well.

We outline a theory for the calculation of the laser beam quality factor of an aberrated laser beam. We provide closed form equations which show that the beam quality factor of an aberrated Gaussian beam depends on all primary aberrations except tilt, defocus and x-astigmatism. The model is verified experimentally by implementing aberrations as digital holograms in the laboratory. We extend this concept to defining the mean focal length of an aberrated lens, and show how this definition may be of use in controlling thermal aberrations in laser resonators. Finally, we look at aberration correction and control using a combination of spatial light modulators and adaptive mirrors.

We present a novel laser beam measurement setup which allows the determination of the beam diameter for each single pulse of a pulsed laser beam at repetition rates of up to 200 kHz. This is useful for online process-parameter control e.g. in micromachining or for laser source characterization. Basically, the developed instrument combines spatial transmission filters specially designed for instantaneous optical determination of the second order moments of the lateral intensity distribution of the light beam and photodiodes coupled to customized electronics. The acquisition is computer-based, enabling real-time operation for online monitoring or control. It also allows data storage for a later analysis and visualization of the measurement results. The single-pulse resolved beam diameter can be measured and recorded without any interruption for an unlimited number of pulses. It is only limited by the capacity of the data storage means. In our setup a standard PC and hard-disk provided 2 hours uninterrupted operation and recording of varying beam diameters at 200 kHz. This is about three orders of magnitude faster than other systems. To calibrate our device we performed experiments in cw and pulsed regimes and the obtained results were compared to those obtained with a commercial camera based system. Only minor deviations of the beam diameter values between the two instruments were observed, proving the reliability of our approach.

The key objective of this article is to underscore that as engineers, we need to pay close attention in repeatedly validating and re-validating the underlying physical processes behind a working theory that models a phenomenon we are using to create tools and technologies. We use the test case, the prevailing mode-lock theory, to illustrate our views by identifying existing contradictions and showing approach towards their resolution by identifying the relevant physical processes. The current theory tells us that the Fourier summation of all the allowed cavity modes directly produces the train of pulses. It effectively assumes that electromagnetic (EM) waves are capable of re-organizing their spatial and temporal energy distribution to generate a train of temporal pulses while preserving the spatial mode energy distribution. The implication is that EM waves interact with each other by themselves. Even though the theory is working, we have three logical problems. First, in the real world, in the linear domain, waves never interact with each other. On careful analysis of all types of interference experiments, we will recognize that only in the presence of some interacting material medium can we observe the physical superposition EFFECT. In other words, detectors carryout the superposition effect we call interference phenomenon, through the summation of their multiple simultaneous linear stimulations and then absorbing energy proportional to the square modulus of the sum total stimulation. Second, a Fourier monochromatic wave, existing in all space and time, is a non-causal hypothesis. Just because our theories are working does not mean that we have understood the real physical interaction processes in nature. We need to build our theories based upon space and time finite EM wave packet containing a finite amount of energy, which is a causal approach. Third, in spite of staggering successes of Quantum Mechanics, we do not yet have a self consistent model for space and time finite model of a photon. QM only predicts that EM energy emission (spontaneous and stimulated) takes place only in a discrete amount at a time from atoms and molecules. It does not give us recipe about how to visualize a propagating photon as it expands diffractively. However, Huygens-Fresnel's classical diffraction integral gives us a rigorous model, which is the cornerstone of modeling evolution of laser cavity modes, CW or pulsed. In this paper, we highlight the contradictions that arise out of the prevailing mode-lock theory and resolve them by using causal models, already underscored above. For example, there are now a wide range of very successful technological applications of the frequency comb extracted out of fs lasers. If the Fourier summation were the correct physical process, then all the cavity modes would have been summed (converted) into a single mean frequency around the gain line center for perfectly mode-locked systems. Further, sending such fs pulses through an optical spectrometer would have always displayed a transform limited fringe, centering on the mean Fourier frequency, rather than generating the comb frequencies, albeit instrumentally broadened. Output pulse train from a phase locked laser is functionally produced due to the oscillatory time-gating behavior of the intra-cavity phase-locking devices. So, we need to pay more attention to the fast temporal behavior of the materials we use for achieving very fast time-gating, since this material imposes phase locking on the cavity modes to enhance its own high-contrast time-gating behavior.

A laser beam waist analyzer system has been developed that permits the real time focal spot measurement of a high power fiber laser in excess of ten of kilowatts and the ability to monitor thermal lensing of a laser and its optical system. The analyzer can provide a laser's spatial profile, circularity, centroid, astigmatism and M-squared values inclusive of the optics used in the process application. The system is very compact, measures in real time with no moving parts by incorporating an all passive optical design to obtain spot diameter, M-squared, Rayleigh length, beam waist position at power levels in excess of 20 kW in less than one second from laser off to laser on.

Higher oder modes in multi mode fibers determine fundamental beam properties. Hence, the knowledge about the modal content becomes essential to understand the underlying physical effects. Therefore, different modal decomposition techniques have been developed. In this paper we present a comparison of two methods. At first the modal decomposition by a correlation filter method (CFM) and secondly using a numerical phase retrieval algorithm. Limitations of both techniques are demonstrated and theirs overcoming by combining both is presented.

With the development of the thin-disk laser, thermal lensing issues have been drastically reduced. However, fundamental mode operation at high output powers is still limited by severe efficiency reductions due to diffraction losses introduced by the temperature gradient at the boundary of the pump spot. To countervail these aspherical wavefront deformations, high-power laser mirrors featuring deformable surfaces have been developed. The surface of these mirrors resembles the shape of the thermally induced change of optical path length in the pumped laser crystal. The magnitude of the surface deformation can be actively controlled to provide optimal compensation at different power levels.

When ultra high intensity lasers facilities were in their early development, the only concern was getting laser pulses with the right energy and pulse duration. As facilities are orienting toward the end users, they are now required to deliver a laser beam with additional qualities like a focal spot with constant quality. That is why Adaptive Optics is now a standard feature for the current ultra high intensity lasers facilities to correct for the aberrations of the beam exiting the laser chain. However, the very last optical components, like the off axis parabola to focus the beam induce aberrations that cannot be directly corrected as they are located after the wavefront sensing. We present a new technology of deformable mirror and a new correction strategy to get optimal focal spot in the experiment chamber as well as measurement of the actual beam quality in the chamber while the beam is used for experiments. These deformable mirrors were designed taking into account needs of ultra intense laser applications. They provide exceptional stability, optical quality and innovative features like scalability and maintenance of the reflective surface. The method of correction proposed uses usual adaptive optics loop to correct for all the aberration from the laser chain, as well as additional steps to get an optimal focal spot in the experiment chamber on a non amplified beam, and to correct and measure the actual beam quality on the amplified beam while it is used for experiments.

This paper presents the two types of Hartmann wavefront sensors to measure the aberrations of CO₂ laser radiation. One sensor is baser on commercially available IR camera while the second one - on so-called thing film sensors. We discuss both positive and negative attributes of these sensors and the possibility to use them in the real laser systems.

Unidirectional-emission microlasers are greatly demanded for photonic integrated circuits and optical interconnection. In this paper, the mode characteristics of circular and coupled-circular microresonators with a bus waveguide are numerically simulated by finite-difference time-domain technique. For a circular microresonator connected with a bus waveguide, coupled-mode between two whispering-gallery modes can have high mode Q factor for realizing unidirectional-emission lasing. In addition, symmetry and antisymmetry coupled modes are analyzed for the coupledcircular microresonator with a middle bus waveguide. Furthermore, the output characteristics of a coupled-circular microlaser, which is fabricated by standard photolithography and inductively-coupled-plasma (ICP) etching techniques, are reported. Single mode operation is realized for the coupled-circular microlaser with a radius of 20 μm and a 2-μmwidth bus waveguide.

Crystalline whispering gallery mode (WGM) resonators are small and structurally monolithic, yet capable of ultra-high quality factors and dramatically enhanced optical nonlinearities. These properties can be exploited in developing miniature optical clocks. Several studies have explored using WGM resonators as frequency reference cavities, and there also exists great research interest in using WGMs for frequency comb generation. In this paper, we will describe our efforts in pursuing laser stabilization using WGM reference cavities with both passive and active temperature stabilization schemes. We will also present our latest analysis and experimental results in frequency comb generation in WGM resonators.

Whispering gallery mode (WGM) resonators are attracting increasing interest as possible frequency reference cavities. Unlike commonly used Fabry-Perot cavities, however, WGM resonators are dielectric resonators lled with a bulk medium whose properties have a signicant impact on the stability of its resonance frequencies. Thermal and other refraction index uctuations of the medium induce additional frequency instability that has to be reduced to a minimum. On the other hand, a small monolithic resonator provides opportunity for integration and immunity against vibration and acceleration disturbances. This feature is essential when the cavity operates in a non-laboratory environment. In this paper, we report a case study of vibration sensitivity reduction for a crystalline resonator and discuss a pathway towards the inhibition of vibration- and acceleration- induced frequency uctuations.

Stability and footprint of optical parametric oscillators (OPOs) strongly depend on the cavity used. Monolithic OPOs tend to be most stable and compact since they do not require external mirrors that have to be aligned. The most straightforward way to get rid of the mirrors is to coat the end faces of the nonlinear crystal. Whispering gallery resonators (WGRs) are a more advanced solution since they provide ultra-high reflectivity over a wide spectral range without any coating. Furthermore, they can be fabricated out of nonlinear-optical materials like lithium niobate. Thus, they are ideally suited to serve as a monolithic OPO cavity. We present the experimental realization of optical parametric oscillators based on whispering gallery resonators. Pumped at 1 μm wavelength, they generate signal and idler fields tunable between 1.8 and 2.5 μm wavelength. We explore different schemes, how to phase match the nonlinear interaction in a WGR. In particular, we show improvements in the fabrication of quasi-phase-matching structures. They enable great flexibility for the tuning and for the choice of the pump laser.

The quest for low power and high frequency electro-optical modulator has been one of the important endeavors in microwave photonics. The advent of microdisk electro-optic modulator created a new domain in optical modulator and photonic microwave receiver design by exploiting the unique properties of high quality (high-Q) Whispering-Gallery Mode (WGM) optical cavities. High-Q crystalline WG cavities were the first devices used as compact and low power resonant electro-optical modulators and gradually semiconductor and polymer based microdisk and microring modulators emerged from this core technology. Due to its small size, high sensitivity and limited bandwidth, originally microdisk modulator was developed with the objective of replacing the conventional microwave wireless receiver frontend with a sensitive photonic front-end. Later it was shown that the electro-optic microdisk modulator could also function as a microwave frequency mixer in optical domain. Starting from fundamentals of resonant electro-optic modulation in high-Q WGM cavities, in this paper we review the development of high sensitivity microdisk modulators and the recent progress toward more efficient modulation at higher frequencies. Next related topics such as singlesideband modulation, all-dielectric photonic receiver, and semiconductor microring modulators are briefly discussed. Finally, photonic microwave receiver configurations that employ high-Q optical resonance for modulation, filtering and mixing are presented. We will show that high-Q optical resonance is one of the promising routes toward the general idea of an all-optical microwave receiver free of high frequency electronic transistors, mixers and filters.

In order to fully exploit the unique properties of micro-optical resonators with whispering gallery modes (WGMs), both for fundamental investigations as well as for practical applications, a critical point is an efficient, controllable, and robust coupling of the light to the cavity WGMs. We present the results of our studies on phase-matched evanescent field couplers, with particular reference to the coupling to high-index crystalline resonators like lithium niobate disks. We focus on couplers based on different types of waveguide configurations and include demonstration of optical coupling to high-Q lithium niobate resonators from integrated planar waveguides as well as from angle polished waveguides. These systems are all in guided optics architectures. We also briefly present our recent achievements in the development of microbubble resonators fabricated from silica capillaries. We show that, as high-Q hollow 3-D cavities with intrinsic microfluidics, these resonators represent a promising biochemical sensing platform.

Passive Q-switched lasers are constructed using saturable absorbers (SA). One characteristic of these lasers is that they are built with small dimensions. There are difficulties in designing lasers with a given pulse repetition rate or pulse energy using saturable absorbers. Numerical simulation of Q-switches facilitates the design and production of such lasers and helps to reduce development time and cost. This paper presents a new simulation method which calculates beam quality, maximal output power, pulse-width and pulse energy.

During the past few years the Monoblock laser has become the laser-of-choice for Army laser range-finders. It is eyesafe with emission at 1570 nm, high pulse energy, simple construction, and high efficiency when pumped by a laserdiode stack, providing advantages that are not available with other laser types. Although the divergence of the Monoblock output beam is relatively large, it can be reduced to <1 mR using a telescope with a large magnification. This solution, however, is not acceptable for applications where the laser and telescope size must be kept to a minimum. In this paper we present a simple and compact technique for achieving significant reduction in the Monoblock beam divergence using a partial reflector that is placed a short distance from the optical parametric oscillator (OPO). Using an ultra-compact 38 mm Monoblock with a 10 mm long KTP OPO, we achieved a beam divergence of <4 mR, corresponding to a >2.5 X reduction from the unmodified laser. Performance using this technique with various feedback and etalon spacings will be presented. Laser diode array and VCSEL pumping were both investigated with similar results.

The Gouy phase is an important phase, however rather poorly understood. It has been shown recently that it can be used to control the frequency of a ring laser and can help in achieving a control over mode-locking problem of ring laser gyro. It is demonstrated now that with the help of the Gouy phase one discovers further characterization of a laser involving cascaded optics in terms of what may be called longitudinal Gouy phase branches. On such a Gouy phase branch one can tune the laser in a manner described elsewhere. It is noted that the longitudinal Gouy-phase-branches play significant role in cascaded optical systems as well as have potential characterization feature in several ring configurations that get naturally selected in a random lasing medium. The concept is further clarified with Collins chart diagrams of several cascaded optics lasers.

High fabrication costs of complex photonic nano-devices make it imperative to have access to high-performance computational tools, which can greatly accelerate the device design process by reducing the design-fabricationtesting cycle. To this end, in this article we briefly review a numerical method based on the multiple scattering algorithm, which we used to describe the second-harmonic generation in plasmonic nanostructures. Then, by using specific examples, we illustrate how this method can be employed to characterize the nonlinear optical modes of microcavities made of plasmonic nanowires. In particular, we show that such plasmonic microcavities have three distinct types of modes, namely, plasmonic cavity modes, multipole plasmon modes generated via the hybridization of modes of single nanowires, and whispering gallery modes. We also show that due to the sensitivity of the nonlinear plasmonic cavity modes to the changes in the environment, they are ideal candidates for nano-scale sensing devices, e. g., plasmonic sensors.

Due to its closed-loop structure, the silicon microring resonator can have a very high quality factor value and its output intensity is very sensitive to different wavelengths. These characters, plus the potential in CMOS compatible fabrication process, are making the silicon microring resonator a key building block of photonic integrated circuits for the applications of filtering, switching, modulation, and wavelength conversion. In parallel, the silicon microring resonator has also been proposed as a compact and highly sensitive optical sensor. In recent years, microring sensing related research and development has been growing consistently and rapidly for the purposes of environment monitoring, health diagnosis, and drug development. This article will firstly review the recent development in silicon microring sensors, which are mainly concentrated in two areas: to improve the sensitivity and to enhance selectivity. In order to have better understanding of this class of sensors, the basic structure of the microring sensors, its working principle, and its unique properties will be described. Then, our own works, in particular, the methods to improve dynamic range, sensitivity, and stability of the silicon microring sensors, will be presented. By studying the fundamental optical properties of the coupled microring resonators, novel silicon microring sensors with better and more reliable sensing performance are constructed. Finally, through studying the interactions between chemistry and optics, acceleration and optics, the specific applications in glucose sensing and seismic sensing are discussed.

Whispering-Gallery-Mode (WGM) microresonators have shown great promise for ultra-sensitive and label-free chemical and biological sensing. The linewidth of a resonant mode determines the smallest resolvable changes in the WGM spectrum, which, in turn, affects the detection limit. The fundamental limit is set by the linewidth of the resonant mode due to material absorption induced photon loss. We report a real-time detection method with single nanoparticle resolution that surpasses the detection limit of most passive micro/nano photonic resonant devices. This is achieved by using an on-chip WGM microcavity laser as the sensing element, whose linewidth is much narrower than its passive counterpart due to optical gain in the resonant lasing mode. In this microlaser based sensing platform, the first binding nanoparticle induces splitting of the lasing line, and the subsequent particles alter the amount of splitting, which can be monitored by measuring the beat frequency of the split modes. We demonstrate detection of polystyrene and gold nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions. The built-in self-heterodyne interferometric method achieved in the monolithic microlaser provides a self-referencing scheme with extraordinary sensitivity, and paves the way for detection and spectroscopy of nano-scale objects using micro/nano lasers.

Optical detection of ultrasound is an emerging technique based on the interaction of strain field and optical field, modulating the optical properties of the resonance cavity for sensitive detection. Such a detection scheme can have several unique advantages, such as broadband response and size-independent sensitivity, compared with conventional piezoelectric transducers. Detector's high sensitivity is essential for deep penetration depth, especially for highresolution imaging because of the strong attenuation of high-frequency ultrasound. Besides, small element size has the advantage of realizing wide acceptance angle of ultrasound detection. Previously we have demonstrated optical-based ultrasound detectors using polymer microring resonators, showing flat spectral response from dc to high frequency, over 90 MHz at -3-dB. By improving the fabrication process, we are able to achieve a new generation device with higher sensitivity and smaller element size. An ultralow noise-equivalent pressure of 21.4 Pa over 1-75 MHz range has been achieved using a fabricated detector of 60 μm diameter. We will mainly demonstrate its applications to photoacoustic imaging, including photoacoustic computed tomography and photoacoustic microscopy. The new generation device can also be applied to other ultrasound-related imaging and detection.

Over the past several years, fiber-optic resonators have been successfully used as mechanical probes by virtue of their intrinsic sensitivity to length changes. In a recent work, we devised a diode-laser system for strain interrogation of a high-finesse fiber Bragg-grating cavity, achieving a 10^-13 resolution in the infrasonic and acoustic frequency ranges, thanks to the exceptional stability of an optical frequency comb (OFC). A Pound-Drever-Hall (PDH) frequency locking technique is implemented for low-noise and wide dynamic range readout of the sensor keeping the interrogating laser always linked to the OFC reference. In this way, the low-frequency strain noisefloor is free from laser noise and possibly limited by thermodynamic phase fluctuations or other thermal effects in the fiber.

Optical microresonators have been proven effective for developing sensitive chemical and biological sensors by monitoring the changes in refractive index or mass near the resonator surface. The rotationally symmetric structures support high quality (Q) whispering gallery modes (WGMs) that interact with the local environment through the evanescent field. The long photon lifetime of the high-Q resonator (thus the long light-material interaction path) is the key reason that a microresonator can achieve very high sensitivity in detection. In this paper, we present our recent research on using porous wall hollow glass microsphere (PW-HGM) as an optical microresonator for chemical vapor detection. The diameter of the PW-HGM ranges from 10μm to 100μm. The wall thickness is about 2μm and the pore size is about 20nm. The Q-factors and free spectrum ranges (FSR) of PW-HGMs were measured by coupling light into the PW-HGM using a single mode fiber taper. Various types of chemical vapors were used to characterize the PW-HGM resonator. The resonant wavelength shift was measured as a function of vapor concentration. Comparisons between a PW-HGM and a solid glass microsphere indicated that a PW-HGM can effectively adsorb vapor molecules into its nanosized pores, providing a direct and long light-material interaction path for significant sensitivity enhancement for chemical vapor detection.

A quantum cascade laser at IR wavelengths with an open external cavity presents an opportunity for spectral sensing of molecular compounds that have low vapor pressure. The sensitivity of such a system is potentially very high due to extraordinarily long effective optical paths that can be achieved in an active cavity. We demonstrate here an external cavity mid-IR QCL molecular absorption sensor using a fixed Fabry-Perot etalon as the spectrum analyzer. The system is sensitive to the water vapor present in the laboratory air with an absorption coefficient of just 9.6 x 10^-8 cm^-1. The system is sensitive enough to detect the absorption coefficient of TNT vapor at room temperature.

In principle, a microspherical resonator pendulum can be trapped using the optical forces produced by two simultaneously excited whispering gallery modes of a photonic molecule, comprising two microspherical cavities. The cavity-enhanced optical force generated in the photonic molecule creates an optomechanical potential that can trap the pendulum at an equilibrium position by carefully choosing the excitation laser frequencies. In this paper, we study the mechanical characteristics of microsphere pendulums by evanescently detecting the pendulum motion. Nonlinear effects like electromagnetically induced transparency and absorption, and Fano resonances have also been observed using a microsphere-fiber taper system and the results are presented here.

We investigate opto-mechanical oscillation (OMO) and subsequent generation of acoustic wave frequency combs in monolithic crystalline whispering gallery mode (WGM) resonators. The OMO is observed in resonators made of electro-optic (lithium tantalate), non-electro-optic birefringent (magnesium fluoride), and non-birefringent (calcium fluoride) materials. The phenomenon manifests itself as generation of optical harmonics separated by the eigenfrequency of a surface acoustic wave (SAW) mechanical mode of the same WGM resonator. We show that the light escaping the resonator and demodulated on a fast photodiode produces a spectrally pure radio frequency (RF) signal. For instance, we demonstrate generation of 200 MHz signals with instantaneous linewidth of 0.2 Hz.

Using ab initio approach to the theory of electromagnetic interaction of a small particle and spherical whispering gallery mode resonator, we derive the optical forces experienced by the particle and it's resulting motion. The form of the forces differs from that expected by the traditional gradient/scattering approach due to the modification of the field by the particle. The main effect of the confinement of the field in a cavity consists in making the component of the optical force usually interpreted as gradient, manifestly non-conservative, and hence not presentable in the gradient form. It is shown how the standard gradient/scattering formalism can be modified for the cavity confined optical field.

We study the propulsion of polystyrene microspheres along water immersed silica tapered fibers. We observed a nearly linear increase of the propulsion velocity with the sphere diameter increasing from 3 to 20 μm. By measuring the fiber transmission spectra we demonstrate efficient evanescent coupling of light to whispering gallery modes (WGMs) in large (>10 μm) polystyrene spheres. For 20 μm spheres we observed the depth of resonant dips ~ 3.5 dB in combination with the Q-factors ~ 10³. Due to small losses in the fiber ~1-2 dB we are able to determine the power in the tapered region and to characterize quantitatively the optical propelling forces. The maximum value of the propelling velocity was 260 μm/s and was observed for 15 μm spheres with guided power of only 43 mW. Such velocities are nearly an order of magnitude higher than those observed for similar powers on waveguide structures. Using simple physical arguments we show that for spheres with diameters larger than 10 μm the experimentally observed velocities of propelling are too high to be explained by the conventional nonresonant scattering forces. We propose that these high velocities indicate that the optical forces are enhanced in such cases due to resonant coupling effects.

In the 1550-nm wavelength range, silicon waveguides exhibit a plethora of non-linear phenomena arising when the optical power is sufficiently intense. Inside optical resonators the efficiency of wave mixing effects is largely enhanced and can be effectively exploited for on-chip all-optical signal processing. By cascading silicon microring resonators, we demonstrated the concept of travelling-wave resonant four-wave mixing (FWM) and we realized a 630-μm-long wavelength converter with a conversion gain 28-dB higher than a bare waveguide of the same physical length. Both the conversion efficiency and the bandwidth of the device are enhanced with respect to a single silicon resonator. However, one of the major impairments associated with nonlinearity in silicon is the TPA-induced local heating of the waveguides. In coupled resonator architectures, this produces not simply a rigid red-shift of the spectral response, but a resonance spread disrupting the frequency response of the devices. Active thermal control of the individual resonators is proposed as a viable strategy to adaptively compensate for intensity-dependent distortions and cross-talk increase in silicon coupled resonator filters, demonstrating that device performance can be preserved regardless of the aggregate power transmitted through the waveguide.

Cryogenic sapphire resonators operating in Whispering Gallery Modes have very high Q-factors (> 10⁹) at microwave frequencies . Such a property makes them useful for a host of applications, which are only possible due to the additional inclusion of residual paramagnetic impurities that annul the frequency-temperature dependence of sapphire. More recently, residual Fe³⁺ impurities with parts-per-billion concentration within the lattice have been shown to create a three level system corresponding to the spin states of the ion. By pumping at 31.3 GHz, a stable 12.04 GHz maser signal (stability of parts in 10¹4) has been created without any stabilization circuitry. In addition, we have observed the fundamental thermal noise limit near 4 K by operating such masers in a bimodal conguration. Annealing one resonator in air has led to conversion of Fe²⁺ ions in the lattice to Fe³⁺, leading to an orders of magnitude increase in active ion concentration. At the post-annealing Fe³⁺ concentration of 150 ppb , we observe nonlinear eects such as a degenerate four-wave mixing due to a _χ⁽³⁾ magnetic nonlinearity as well as stable frequency comb generation.

Ab initio approach to description of nonlinear dynamics of individual and coupled microdisk whispering-gallerymode resonators is developed for the nonlinearities of Kerr type. Distinction between scattering resonances and eigenfrequencies, which is important for resonators with moderate quality factors is discussed. Properties of various parameters important for nonlinear dynamics, such as coupling to external radiation, linear inter-disk coupling parameter, nonlinear mode overlap coefficients are discussed from the point of view of the ab initio formulation.

Electronic Kerr effect in a polyfluorene derivative is used to reversibly switch near infrared probe beam resonantly coupled to a hybrid polymer-silica microspherical resonator. NIR pumping at 780 nm in pulsed laser regime is used for non-linear switching of the WGM resonances that shift as much as 2 GHz for 50 mW of average pump power, compared to a shift of 250 MHz for the same average pump power at CW regime. The absence of temporal drift and the magnitude of this shift confirm the Kerr nature of the switching, ruling out thermooptical effects.

Coating of spherical microresonators is a very promising technique for optimizing their optical properties. Optical coatings are constituted by glasses, polymer, and glass ceramics, passive or activated by luminescent species, Glass ceramic activated by rare earth ions are nanocomposite systems that exhibit specific morphologic, structural and spectroscopic properties allowing to develop interesting new physical concepts, for instance the mechanism related to the transparency, as well as novel photonic devices based on the enhancement of the luminescence. At the state of art the fabrication techniques based on bottom-up and top-down approaches appear to be viable although a specific effort is required to achieve the necessary reliability and reproducibility of the preparation protocols. In particular, the dependence of the final product on the specific parent glass and on the employed synthesis still remain an important task of the research in material science. Looking to application, the enhanced spectroscopic properties typical of glass ceramic in respect to those of the amorphous structures constitute an important point for the development of integrated optics devices, including coating of spherical microresonators. Here we present a review regarding spherical microresonators coated by glass and glass-ceramic film activated by Er³⁺ ions. Er³⁺ ions appear to be embedded in a crystalline or amorphous environment and the lifetime dynamic is influenced by the geometry and by the morphology of the system. Photoluminescence results and morphologic properties are discussed for both amorphous and glass ceramic films.

Polymer-based, multi-layered dielectric microspheres are investigated for high-resolution electric field sensing. The external electric field induces changes in the morphology of the spheres, leading to shifts in the whispering gallery modes (WGMs). Light from a distributed feedback (DFB) laser is sidecoupled into the microspheres using a tapered section of a single mode optical fiber to interrogate the optical modes. The base material of these multi-layered spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are investigated: (1) cores comprised of a 60:1 volumetric ratio of PDMS-to-curing agent mixture that are mixed with varying amounts of barium titanate (BaTiO3) nano particles, (2) cores comprised of 60:1 PDMS that are coated with a thin layer of 60:1 PDMS that is mixed with varying amounts of barium titanate and (3) a composite Carbon Black-BaTiO3 prototype. The outermost layer for all sphere geometries is a thin coat of 60:1 PDMS which serves as the shell waveguide. Light from the tapered laser is coupled into this outermost shell that provides high optical quality factor WGM (Q ~ 10⁶). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field. Preliminary results show that electric fields of the order of 100 V/m can be detected using these composite micro-resonators.

Silicon photonic resonators, implemented using silicon-on-insulator substrates, are promising for numerous applications. The most commonly studied resonators are ring/racetrack resonators. We have fabricated these and other resonators including disk resonators, waveguide-grating resonators, ring resonator reflectors, contra-directional grating-coupler ring resonators, and racetrack-based multiplexer/demultiplexers. While numerous resonators have been demonstrated for sensing purposes, it remains unclear as to which structures provide the highest sensitivity and best limit of detection; for example, disc resonators and slot-waveguide-based ring resonators have been conjectured to provide an improved limit of detection. Here, we compare various resonators in terms of sensor metrics for label-free bio-sensing in a micro-fluidic environment. We have integrated resonator arrays with PDMS micro-fluidics for real-time detection of biomolecules in experiments such as antigen-antibody binding reaction experiments using Human Factor IX proteins. Numerous resonators are fabricated on the same wafer and experimentally compared. We identify that, while evanescent-field sensors all operate on the principle that the analyte's refractive index shifts the resonant frequency, there are important differences between implementations that lie in the relationship between the optical field overlap with the analyte and the relative contributions of the various loss mechanisms. The chips were fabricated in the context of the CMC-UBC Silicon Nanophotonics Fabrication course and workshop. This yearlong, design-based, graduate training program is offered to students from across Canada and, over the last four years, has attracted participants from nearly every Canadian university involved in photonics research. The course takes students through a full design cycle of a photonic circuit, including theory, modelling, design, and experimentation.

A Fabry-Perot etalon, consisting of two π phase shifted reflecting volume Bragg gratings, is presented. These gratings are obtained as a moiré pattern resulting from sequential recording of interference patterns with different periods in photo-thermo-refractive glass and called moiré volume Bragg gratings (MVBGs). A detailed investigation of the fundamental operating principles and measurement techniques for phase shifted gratings is shown. Experimental results demonstrating a MVBG with a 15 pm bandwidth and 90% transmission at resonance are presented. The use of the MVBG for longitudinal mode selection in a laser resonator is shown.

By using polystyrene microspheres with index n=1.59 as a model system we study light focusing and transport properties of chains formed by spheres with diameters varying from 2 to 30 μm. We used techniques of imaging based on light scattering perpendicular to the axis of the chain to visualize and study periodically focused beams in such structures. The results demonstrate good agreement with geometrical optics modeling for sufficiently large spheres, D>>10λ, where D is the sphere diameter and λ is the wavelength of light. For mesoscale structures with 4<D/>><10 we observed two effects which cannot be explained by geometrical optics. One is a "beam tapering" effect which is stronger in normalized units than it is theoretically possible in the limit of geometrical optics in short chains. Another effect is reduced power attenuation in sufficiently long chains which is found to be smaller than it is possible in the limit of geometrical optics. Both effects are ascribed to the increased role of physical optics properties. We can also suggest some role of the microjoints developing between polystyrene microspheres in self-assembled structures. The results are important for developing applications requiring focusing of multimodal beams such as mid-infrared laser surgery.

In several applications of optical whispering-gallery resonators, in particular for Kerr frequency comb generation, a possibility to calculate accurately the eigenfrequencies and geometry dependent dispersion is an important prerequisite for device optimization. While these calculations with significant time consumption may be performed numerically, analytical approximation are still quite desirable for theoretical considerations. Spheroidal geometry can fit the form of most types of whispering gallery resonators that are in use today. We propose and check numerically new accurate approximations for the frequencies and geometrical dispersion of fundamental and transversal modes of oblate and prolate spheroids using both the eikonal approximation and WKB analysis. We also suggest approximations for the distribution of optical fields in a spheroidal resonator.

A generalized model for beam-path variation analyzed with vector method in square ring resonators is established. The model can be applied to analyze beam-path variation in various ring resonators induced by all the possible perturbation sources. The generalized model is useful for the cavity design, cavity improvement, alignment of planar ring resonators and research on backscattering coupling effect. Backscattering coupling effect in square ring resonator has been chosen as examples to show its application. Backscattering coupling coefficient r is obtained as a function of mirror's axial displacements. Some novel results of backscattering coupling effect have been acquired. The results indicate that r can not be reduced to zero because of the initial machining errors of surfaces of plane mirrors. However, r can be reduced to zero almost when stabilizing frequency of laser gyro by take the suitable values of axial displacements of plane mirrors. These results are important for high precision laser gyro.

The uniform irradiation performance of the small target spots for indirect-drive inertial confinement fusion is studied. A lens array and the technique of smoothing by spectral dispersion are included in the frequency-tripled high-power laser driver. The intensity distributions of the target spots are simulated and their spatial power spectra are analyzed. The results show that spots of clear envelopes can be obtained with a lens array and smoothing by spectral dispersion has obvious effects in eliminating high-contrast intensity modulation inside the spots, even in the cases they are very small. With the two methods of beam smoothing, performance of indirect-drive experiments is expected to be evidently improved.

The energetic relief depends on the nature of the considered element which has its own internal energy, cohesion, structure and other characteristic parameters. Former experiences has shown that we could appreciate how and when a metal part was cut, with the aid of an electron counter system - in a determined period of time. To join or aggregate metal parts - or particles like powder - we must reduce or eliminate the influence and effect of the energetic relief between the considered parts or particles. Different types of fields or energy may be applied in order to do that. Authors experienced some kind of energies and studied the resulting plasma with the magnetohidrodynamic theory. They found that the variation of plasma density and the intensity of the magnetic field appears in a time interval which is under - or comparable - with the wave lenght at this was the start point to develope further research. The experiences were made with the aid of an original device. Of course, after measuring and studying the main obtained data, authors have designed installation and devices, for industrial use. One of the - economical - conclusion, was that is possible to influence the energetic relief in order to diminish costs of all type of aggregation processes, like sintering, welding a.s.o. In the same time, the processing time, may be reduced.