This PDF file contains the front matter associated with SPIE Proceedings Volume 7913, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Unique properties of unstable ring resonators are sometimes useful. A collimated beam in the gain medium may be desirable. Spatial hole burning is eliminated. Beam rotation may be helpful. 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. We have performed a geometrical and a wave-optics numerical simulation of a type of ring resonator that allows counter-propagating 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 wave-optics simulation treats a UR90, for which the output beam is unobscured.

Simulations have to accurately model thermal lensing in order to help improving resonator design of diode pumped solid state lasers. To this end, a precise description of the pump light absorption is an important prerequisite. In this paper, we discuss the frequency dependency of the pump light absorption in the laser crystal and its influence on the simulated laser performance. The results show that the pump light absorption has to include the spectral overlap of the emitting pump source and the absorbing laser material. This information can either be used for a fully frequency dependent absorption model or, at least in the shown examples, to compute an effective value for an exponential Beer-Lambert law of absorption. This is particularly significant at pump wavelengths coinciding with a peak of absorption. Consequences for laser stability and performance are analyzed for different pump wavelengths in a Nd:YAG laser.

An experimental approach in generating Petal-like transverse modes, which are similar to what is seen in porro-prism resonators, has been successfully demonstrated. We hypothesize that the petal-like structures are generated from a coherent superposition of Laguerre-Gaussian modes of zero radial order and opposite azimuthal order. To verify this hypothesis, visually based comparisons such as petal peak to peak diameter and the angle between adjacent petals are drawn between experimental data and simulated data. The beam quality factor of the Petal-like transverse modes and an inner product interaction is also experimentally compared to numerical results.

The results of laser beam refraction studies in a continuous optical discharge (COD) stabilized in a focused (f4.4) laser beam and coaxial gas flow are reported. A plasma ball formed in COD acts as a defocusing plasma lens where lensing effect occurs mainly due to distributed free electrons. The properties of the plasma lens depending on the electron density distribution and controlled through a gas flow velocity have being studied. Intensity profiles of sustaining laser beam (M2 = 6.6) transmitted through the plasma together with intensity patterns of visible plasma images were simultaneously detected and analyzed. It was found that the dependency of average refraction angle of the sustaining beam in COD plasma on a gas flow velocity drops sharply as the velocity increased 1.5 m/s from initial value of 0.06 radians to milliradians, so that at higher gas flow velocities the beam refraction does not affect plasma properties. A shape of the curve of the refraction on gas flow velocity reveals the peculiarities of lensing properties of the plasma ball. The possibility of controlling the lensing effect in plasma by means of the gas flow was also demonstrated.

A laser beam analysis system, with all passive optical components, has been developed that permits the real time measurement of a high power laser beam in the tens of kilowatts which can provide the laser's spatial profile, circularity, centroid, astigmatism and M-squared values using all the optics of a process application, including the focus lens and cover glass. At the heart of the technique is a Fabry-Perot resonator used with a focusing lens that provide a means to both attenuate and provide a multiplicity of focused laser spots each representing a spatial slice of the focused beam waist of interest onto a single CCD or CMOS camera. This arrangement provides real time data on the laser system's beam properties and is the basis upon which this work it done. The coatings of the Fabry-Perot resonator provide a high degree of attenuation of the input beam so that thermal lensing is not a factor in the measurement. By adjusting incident angle and spacing between the mirrors of the Fabry-Perot resonator, a large number of spatial cross sections can be seen on the detector. This permits then the possibility of evaluating any focusing objective whether long or short in focal length.

Due to recent advances in X-ray microscopy, we are now able to image objects with nanometer resolution thanks to Synchrotron beam lines or Free Electron Lasers (FEL). The PCI (Phase Contrast Imaging) is a robust technique that can recover the wavefront from measurements of only few intensity pictures in the Fresnel diffraction region. With our fast straightforward calculus methods, we manage to provide the phase induced by a microscopic specimen in few seconds. We can therefore obtain high contrasted images from transparent materials at very small scales. To reach atomic resolution imaging and thus make a transition from the near to the far field, the Coherent Diffraction Imaging (CDI) technique finds its roots in the analysis of diffraction patterns to obtain the phase of the altered complex wave. Theoretical results about existence and uniqueness of this retrieved piece of information by both iterative and direct algorithms have already been released. However, performances of algorithms remain limited by the coherence of the X-ray beam, presence of random noise and the saturation threshold of the detector. We will present reconstructions of samples using an enhanced version of HIO algorithm improving the speed of convergence and its repeatability. As a first step toward a practical X-Ray CDI system, initial images for reconstructions are acquired with the laser-based CDI system working in the visible spectrum.

Shack-Hartman wavefront sensors are widely used in scientific investigations of wavefronts and also, as a component of the closed-loop adaptive optical system, intended to correct for laser beam aberrations. This paper presents successful application of such type sensors in the laser systems investigations to obtain high quality laser radiation. Results of investigations of wavefronts of modern high power solid-state lasers are given in this paper.

Diffractive optical elements (DOE) play an important role for laser beam shaping in industry, for example in lithography or parallel laser material processing. Typically such applications require high damage threshold and low background illumination (high contrast and efficiency). Usual DOE with binary phase (step-like) profiles are made microlithographically and suffer from substantial scattering on profile derivative discontinuities. That gives also tendency to lower damage threshold as compared to intrinsic material values. The LIMO approach is based contrarily on a proprietary, non etching material processing and is suitable for manufacturing of high-precision free programmable continuous surface profiles in optical glasses and crystals. We report on linear symmetric diffractive beam splitter 1:11 with high homogeneity and efficiency > 95% and discuss also other DOE designs. The design data, simulations with measured surface profiles and experimental intensity distributions are in very good agreement. Furthermore we report on a new type of optical attenuator composed from two DOE gratings. Its dynamic transmission range is 0.3% to 98%. The required lateral DOE shift is only 5 - 10 μm in the present design, so that the device can be very fast and applicable for dynamic intensity stabilization.

Athermalization of focusing objectives is a common technique for optimizing imaging systems in the infrared where thermal effects are a major concern. The athermalization is generally done within the spectrum of interest and not generally applied to a single wavelength. The predominate glass used with high power infrared lasers in the near infrared of one micron, such as Nd:YAG and fiber lasers, is fused silica which has excellent thermal properties. All glasses, however, have a temperature coefficient of index of refraction (dn/dT) where as the glass heats up its index of refraction changes. Most glasses, fused silica included, have a positive dn/dT. A positive dn/dT will cause the focal length of the lens to decrease with a temperature rise. Many of the fluoride glasses, like CaF₂, BaF₂, LiF₂, etc. have a negative dn/dT. By applying athermalization techniques of glass selection and optical design, the thermal lensing in a laser objective of a high power laser system can be substantially mitigated. We describe a passive method for minimizing thermal lensing of high power laser optics.

The typical Gaussian intensity distribution generated at focus of a laser machining workstation is not always ideal for the application; instead other shapes such as ellipses, flat-tops (circular or square), or doughnuts can in some cases give better results. Also, other more complex beam profiles might be beneficial for surface micro structuring. In order to realise, and rapidly change between such beam shapes, we are investigating an adaptive optics approach based on using an iterative simulated annealing algorithm to control the actuators of a deformable mirror. A 37-element piezoelectric deformable mirror and a 37-element bimorph mirror were applied in an extracavity arrangement. Beam shaping results with these systems are presented and example laser machining is demonstrated in this paper. The results enabled by the deformable mirrors are compared to previous results using a spatial light modulator (SLM) based on a liquid crystal microdisplay. The SLM has a much higher resolution and enables complex beam shapes to be generated, however is much slower in response. Having an active beam shaping element incorporated in a laser machining workstation adds increased flexibility and improves process control.

A new type of low-voltage planar electro-optical device for fast beam deflection is reported. It contains two EO modulators, both working as multimode waveguides. The geometry of the waveguides (ratio height to length) enables an efficient self-imaging of the entrance Gaussian mode. The EO modules are from LiNbO3:MgO with the thickness of 32 μm, length 9.75 mm, and width of 26 mm. The second stage works as an active phased array with 16 channels. The design provides a flat wavefront at the exit of the system despite the discrete phase shifts in the array channels. This makes a high steering resolution and optical efficiency possible. The full angle deflection range is of ±32•(1.27λ/D) by using of very low control voltages of 10 - 15 V. The voltages can be further reduced down to 5 V through constructive improvement of the EO-modules. The deflection range can be increased 16 times implementing a 3rd EO stage with a 16- channel EO-array. The deflector provides random access to the available angle states. The access time is limited generally by the capacity of the EO modules. It is of only about 0.1 nF in the reported design. We estimate that thanks to the low control voltage and electrical capacity of EO-modules a switching frequency of about 100 MHz may be possible with an advanced electronics. A relatively large face cross-section of about 1 mm2 will allow using the system with high power lasers and short pulse duration.

There are a wide range of laser beam delivery systems in use for various purposes; including industrial and medical applications. Virtually all such beam delivery systems for practical purposes employ optical systems comprised of mirrors and lenses to shape, focus and guide the laser beam down to the material being processed. The goal of the laser beam delivery is to set the optimum parameters and to "fold" the beam path to reduce the mechanical length of the optical system, thereby allowing a physically compact system. In many cases, even a compact system can incorporate upwards of six mirrors and a comparable number of lenses all needing alignment so they are collinear. One of the major requirements for use of such systems in industry is a method of safe alignment. The alignment process requires that the aligner determine where the beam strikes each element. The aligner should also preferably be able to determine the shape or pattern of the laser beam at that point and its relative power. These alignments are further compounded in that the laser beams generated are not visible to the unaided human eye. Such beams are also often of relatively high power levels, and are thereby a significant hazard to the eyes of the aligner. Obvious an invisible beam makes it nearly impossible to align laser system without some form of optical assistance. The predominant method of visually aligning the laser beam delivery is the use of thermal paper, paper cards or fluorescing card material. The use of paper products which have limited power handling capability or coated plastics can produce significant debris and contaminants within the beam line that ultimately damage the optics. The use of the cards can also create significant laser light scatter jeopardizing the safety of the person aligning the system. This paper covers a new safety mirror design for use with at various UV and Near IR wavelengths (193 nm to 1064 nm) within laser beam delivery systems and how its use can provide benefits covering eye safety, precise alignment and beam diagnostics.

An off-axis configuration of the negative-branch confocal unstable resonator is examined numerically and experimentally for a gain medium with rectangular cross-section. Due to less diffraction effects such a configuration yields lower beam divergences than the standard on-axis resonator. The output coupling and the adaptation to the geometry of the gain medium are attained by a scraper. Two different scraper profiles are examined. One profile resembles to a rectangular bracket "[" and the other profile resembles to the letter "L". The experiments are performed with a 10 kW class chemical oxygen iodine laser (COIL), which has a medium of low gain. Both scraper profiles are applied to a resonator of the same magnification. Measurements of the intensity distributions in the near field and in the far field are presented. The setup using the [-shaped scraper yields a higher output coupling and therefore a lower output power and a lower beam divergence, whereas the setup using the L-shaped scraper makes use of the complete gain medium. Furthermore, the L-shaped scraper is reusable for different resonator magnifications.

In this paper, we report on current developments aimed at improving the focusability of the Texas Petawatt Laser. Two major campaigns have been commissioned that address the issue of focusability. First, we implemented a closed loop, 32 actuator bi-moprh deformable mirror (DFM) to compensate for aberrations in the optical train and second, a color corrector lens assembly was installed that compensates for chromatic errors accumulated in broadband (>15 nm), large aperture (>20 cm) laser systems. We will present in detail, pre and post correction results with the DFM and describe challenges faced when one activates a single shot, high energy closed loop system. Secondly, we will provide modeling and experimental results of our color correction system. This is a novel approach to a problem only seen in high energy, broadband, large aperture laser pulses. By using color correction optics we have demonstrated a 6X increase in focal intensity. With the installation of the DFM, the rms wavefront error in the system was reduced from 2.4 waves to .131 waves, further increasing intensities seen at focus by 1 order of magnitude.

This paper discusses the novel adaptive optical closed loop system with water-cooled bimorph mirror as a wavefront corrector to compensate for the aberrations of high-power CW laser beam. Shack-Hartmann wavefront sensor is used as an element for feedback control. Comparison of phase conjugation and modified hill-climbing technique is shown.

Creating of non-circular laser spots, for example of linear, elliptical or rectangle shape, with uniform intensity profile is important in various laser techniques in industry, scientific and medical applications. This task can be successfully solved with applying of refractive beam shaping optics of field mapping type in combination with some additional optical components. Due to their unique features, such as: low output divergence, high transmittance and flatness of output beam profile as well as extended depth of field, the refractive field mappers provide a freedom in further manipulation with intensity profile and shape of a laser beam. Typically design of refractive field mapping beam shapers has circular symmetry; therefore creating of non-circular spot shapes requires applying anamorphic optical components (cylinder lenses, prism pairs, etc.) ahead of or after a beam shaper. As result it becomes possible to provide various combinations of spot shape and intensity profiles, for example: roof-like spot with uniform intensity in one direction and Gaussian or triangle profile in another direction, linear spots with aspect ratio up to 1:1000, elliptical spots of uniform intensity, etc. Applications include flow cytometry instrumentation, particle image velocimetry, particle size analyzing, hardening, cladding, annealing, and others. This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts for creating laser spots of non-circular symmetry. Examples of real implementations will be presented as well.

Passive and tunable optical filters as well as optical modulators, directly fabricated on the end-faces of optical fibers can provide a fast and low cost production. A hybrid layer system can be built up to a passive Fabry-Pérot microcavity, where alternating dielectric high and low refractive materials are used as mirrors and a highly transparent polymer as the spacer material. The mirror design and the spacer thickness define the center operation wavelength and the filter bandwidth. Bandwidths of less than 1 nm (FWHM) at a wavelength of 1560 nm could be achieved for such microcavities on the end-faces of optical fibers. Enhancing the hybrid layer system by transparent conductive electrodes and by adding electro-optically active chromophores to the polymeric spacer material, the filters become tunable. The material used for the electrodes is indium tin oxide (ITO). The oxidic electrodes have to be merged with the dielectric mirrors and the polymeric spacer. Applying a voltage to the electro-optically active polymeric spacer utilizing such electrodes, the refractive index of the spacer can be changed and therefore the resonance criteria of the microcavity.

Highly prolate-shaped whispering-gallery-mode "bottle microresonators" have recently attracted considerable attention due to their advantageous properties. We experimentally show that such resonators offer ultra-high quality factors, microscopic mode volumes, and near lossless in- and out-coupling of light using ultra-thin optical fibers. Additionally, bottle microresonators have a simple and customizable mode structure. This enables full tunability using mechanical strain and simultaneous coupling of two ultra-thin coupling fibers in an add-drop configuration. We present two applications based on these characteristics: In a cavity quantum electrodynamics experiment, we actively stabilize the frequency of the bottle microresonator to an atomic transition and operate it in an ultra-high vacuum environment in order to couple single laser-cooled atoms to the resonator mode. In a second experiment, we show that the bottle microresonator can be used as a low-loss, narrow-band add-drop filter. Using the Kerr effect of the silica resonator material, we furthermore demonstrate that this device can be used for single-wavelength all-optical signal processing.

Optical nanocavities enable a strong interaction between single photons and single emitters. An appealing application is the construction of a quantum interface for photonic and solid state qubits. Since the material of the solid state qubit is often dierent from the nanocavity, there has been considerable interest in combining the two in a hybrid architecture. We describe our recent development of such a hybrid interface based a Gallium Phosphide photonic crystal nanocavity that is scanned and deterministically coupled to single emitters on a surface. The technique is used to couple the cavity to the nitrogen vacancy center in diamond, an emitter system with optically accessible electron spins and the ability to transfer electronic spin states to nuclear spins.

We investigate several possibilities of designing spectrum of a whispering gallery mode resonators to create groups of optical modes with desired free spectral range as well as group velocity dispersion. This will enable efficient resonant nonlinear frequency conversion processes such as hyper-parametric and parametric oscillations, frequency doubling, and electro-optical frequency shifting and modulation. We show that the spectral design can be achieved via a proper modification of the shape of the resonator and via the change of the distribution of the refractive index of the resonator host material.

Optical frequency combs find applications in various areas of science and technology, such as time-frequency metrology, molecular spectroscopy, and ultra-low phase noise microwave and terahertz generation. A new method has recently been demonstrated for the generation of these combs, and it is based on the excitation of the whispering gallery mode of a high Q monolithic resonator through the Kerr effect. In this paper, we will discuss some of the key challenges in achieving octave spanning combs useful for optical metrology applications.

Multiple-port directional emission microlasers are potential light sources and optical signal processing units in photonic integrated circuits. Connecting bus waveguides to a microresonator is a simple method to realize directional emission microlasers. In this paper, we investigate square and circular resonator microlasers connected with multiple bus waveguides. The mode characteristics of the microresonators connected with multiple bus waveguides are simulated by finite-difference time-domain technique, and the numerical results of mode Q factors and output coupling efficiencies show that high efficiency directional emission microresonator lasers can be realized. Furthermore, the microcylinder laser connected with a bus waveguide fabricated by planar technology processes is reported, and the lasing spectra of square microlasers with four vertices connected to bus waveguides are analyzed.

We report on lasing in conical microcavities, which are made out of the low-loss polymer poly (methyl methacrylate) (PMMA) doped with the dye rhodamine 6G, and directly fabricated on silicon. Including a thermal reflow step during fabrication enables a significantly reduced surface roughness, resulting in low scattering losses of the whispering gallery modes (WGMs). The high cavity quality factors (above 2·10⁶ in passive cavities) in combination with the large oscillator strength gain material enable lasing threshold energies as low as 3 nJ, achieved by free-space excitation in the quasistationary pumping regime. Lasing wavelengths are detected in the visible wavelength region around 600 nm. Finite element simulations indicate that lasing occurs in fundamental TE/TM cavity modes, as these modes have - in comparison to higher order cavity modes - the smallest mode volume and the largest overlap with the gain material. In addition, we investigate the effect of dye concentration on lasing wavelength and threshold by comparing samples with four different concentrations of rhodamine 6G. Observations are explained by modifying the standard dye laser model.

Leaky-waveguide laser amplifiers can sometimes have significant advantages for single-transverse-mode propagation in high-power waveguide laser systems. Most recent studies of such systems have either not included gain saturation, or they have assumed homogeneously broadened gain media. However, some of the most promising media for such amplifiers are inhomogeneously broadened. Results reported here include detailed numerical mode solutions for saturated inhomogeneous broadening, as well as useful analytical methods and approximations for both homogeneous and inhomogeneous broadening.

Ultra-sensitive and label-free chemical and biological sensing devices are of great importance to biomedical research, clinical diagnostics, environmental monitoring, and homeland security applications. Optical sensors based on ultra-highquality Whispering-Gallery-Mode (WGM) micro-resonators, in which light-matter interactions are significantly enhanced, have shown great promise in achieving compact sensors with high sensitivity and reliability. However, traditional sensing mechanisms based on monitoring the frequency shift of a single resonance faces challenges since the resonant frequency is sensitive not only to the sensing targets but also to many types of disturbances in the environment, such as temperature variation and mechanical instability of the system. The analysis of the signals is also affected by the positions of sensing targets on the resonator. Thus, it is difficult to distinguish signals coming from different sources, which introduces 'false positive' detection. We report a novel self-reference sensing mechanism based on mode splitting, a phenomenon in which a high-quality optical mode in a WGM resonator splits into two modes due to intra-cavity Rayleigh scattering. In particular, we demonstrated that the two split modes that can be induced by a single nanoparticle reside in the same resonator and serve as a reference to each other. As a result, a self-reference sensing scheme is formed. This allows us to develop a position-independent sensing scheme to accurately estimate the sizes of nanoparticles. So far we have achieved position-independent detecting and sizing of single nanoparticles down to 20 nm in radius with a single-shot measurement using an on-chip high-quality WGM microtoroid resonator.

Silicon photonics using microdisk and microring resonators are finding technologically important applications from telecommunications and on-chip optical interconnects to optofluidics and biosensing. Silicon-based microresonators that partially confine light by total internal reflection are versatile device structures which are highly wavelength-selective, reconfigurable via various refractive index tuning mechanisms, micrometer-scale footprint, and readily in/out-coupled with integrated waveguides. In this paper, we will highlight our latest progress in silicon photonics using microdisk and microring resonators for on-chip optical interconnects, optofluidics and biosensing applications including the experimental demonstrations of: (i) optical time delay and advance using silicon microring resonators integrated with pi- n diodes; (ii) photocurrent spectroscopy of microdisk resonators using two-photon-absorption induced photocarriers; (iii) optical trapping and transporting of microparticles using a water-clad silicon nitride microring resonator; and (iv) coupled microdisk resonator optical waveguide-based refractive index sensors.

We investigate high-Q microsphere resonators with whispering gallery modes using a tapered optical microfiber immersed in a liquid inside a microfluidic platform. The strength of the coupling between the cavity and the microfiber taper is shown to depend on the contact position of the microsphere along the taper and on the refractive index contrast between the microsphere and the liquid environment. We demonstrate that barium titanate glass beads with index around 1.9 are promising candidates for developing sensor and optomechanical applications of such resonator systems.

An ultrahigh-Q optical microcavity coupled with a tapered fiber is an ideal system for the cavity quantum electrodynamics (CQED). In particular realizing this system at cryogenic temperature is vitally important and has been recently explored for various CQED applications including solid-state atom-photon strong coupling, vibrational mode cooling, and photonic quantum gates. These cryogenic fiber-coupled microcavity systems, however, suffer from mechanical vibrations due to cooling systems and distortions caused by large temperature change. These factors may cause the degradation in polarization of probe light field in the system. Here we report the analysis of the polarization state in a tapered-fiber-coupled microsphere cavity at cryogenic temperatures. By scanning the wavelength of the probe light at around 637 nm, which can be used for the diamond nitrogen vacancy centers, the spectral analysis of the polarization state was performed at 8-30 K. We have found that the degree of polarization (DOP, classical analogue of purity) at cryogenic temperatures does not show significant change compared to that measured at room temperature. This fact indicates that the system can conserve the polarization at low temperature to the extent comparable to that at room temperature, which is enough for the evaluation of the quantum phase gate.

Optical interconnect and optical packet switching systems could take advantage of small footprint, low power lasers and optical logic elements. Microdisk lasers, with a diameter below 10μm and fabricated in InP membranes with a high index contrast, offer this possibility at the telecom wavelengths. The lasers are fabricated using heterogeneous integration of InP membranes on silicon-on-insulator (SOI) passive waveguide circuits, which allows to combine the active elements with compact, high-index contrast passive elements. The lasing mode in such microdisk lasers is a whispering gallery mode, which can be either in the clockwise (CW) or counter clockwise direction (CCW) or in both. The coupling to the SOI wire waveguides is through evanescent coupling. Predefined, unidirectional operation can be achieved by terminating the SOI wires at one end with Bragg gratings. For all-optical flip-flops, the laser operation must be switchable between CW and CCW, using short optical pulses. Unidirectional operation in either direction is only possible if the coupling between CW and CCW direction is very small, requiring small sidewall surface roughness, and if the gain suppression is sufficiently large, requiring large internal power levels. All-optical flip-flops based on microdisk lasers with diameter of 7.5μm have been demonstrated. They operate with a CW power consumption of a few mW and switch in 60ps with switching energies as low as 1.8fJ. Operation as all-optical gate has also been demonstrated. The surface roughness is limited through optimized etching of the disks and the large internal power is obtained through good heat sink.

The effects of periodical focusing of light were studied in chains of sapphire microspheres with 300 μm diameters assembled either on a substrate or inside capillary tubing. Dye-doped fluorescent microspheres were used as multimodal sources of light in experimental studies. Significant reduction of the focused spot sizes was observed for chains of spheres compared to a single sphere case. Numerical ray tracing simulations were performed for similar chains assembled inside hollow waveguides to be used as an optical delivery system with mid-infrared lasers for ultra-precise surgery. The device designs were optimized for contact conditions during laser surgery involving short optical penetration depths of light in tissue. It is shown that chains of spheres with n around 1.65-1.75 provide a two-fold improvement of the spatial resolution over single spheres. Potential applications of these microprobes include ultraprecise laser procedures in the eye and brain or piercing a cell, and coupling of multimodal beams into photonic microstructures.

The effect of external magnetic field on the whispering gallery optical modes (WGM) of Magnetorheological Polydimethylsiloxane (MR-PDMS) spheres is studied. Magnetically polarizable particles are mixed in with the PDMS when it is in liquid form and cured to solidify into spheres with diameters of several hundreds of microns. The spheres are then coated with a layer of pure PDMS with thickness of several microns. Light from a tunable laser is tangentially coupled into this outer layer using a tapered single mode optical fiber in order to excite the optical modes of the sphere. These composite micro-sphere resonators exhibit high Q-factors (~ 10⁶). An analysis is carried out to estimate the WGM shifts induced by the applied magnetic field. An experiment is also carried out to demonstrate the magnetic-field induced WGM shifts in a 400 μm diameter MR-PDMS resonator. The results indicate that MR-PDMS micro-spheres may be used as magnetic field sensors.

We have demonstrated a hyper coherent spectral linewidth evaluation for a frequency stabilized 405nm GaN violet laser diode (LD) based on the delayed self-heterodyne beat. The laser light source was stabilized to a reference confocal Fabry-Perot (CFP) cavity by negative electrical feedback to the injection current of the LD under the Pound-Drever-Hall technique. In addition, by introducing optical feedback from another tilted CFP cavity, the spectral linewidth has been efficiently narrowed. In this scheme, we have achieved 1.65×10^-11 estimated with Allan variance for the feedback error signal from CFP cavity. We also measured the linewidth directly by a separate FP interferometer, which resulted in close coincidence with above Allan variance estimations. In this work, we have tried to measure our narrowed linewidth by the delayed self heterodyne technique which consists of optical fiber of about 1km~3km length to have delay time the light of our stabilized violet laser. In conclusion, we have achieved a practical and inexpensive linewidth control for a high power violet LD to attain hyper coherent conditions of the semiconductor lasers.

When a photon beam is in impact with a metal, the peripheric electrons which belong to the bombarded material are made jumps, and in the same time, new photons are absorbed by electrons which had not time to come back to the fundamental levels. At a high level concentration of the radiant energy, a peripheral electron, could sequentially absorb more photons and could realize energetic jumps in succesive phase, equivalent with some photons of high energy which have wave-lenght smaller than the incidental photons. After some succesive photon absorbtion of the same electron, in the interval in which it is not activated by new photons, the electron comes back to the fundamental level and delivers the accumulated energy, in photons of higher energy, which have a lower energy than the incident beam. Comming back to the fundamental level, the electrons disturb the electronic cloud of the atom or ion they belong. After a huge number of such phenomenon the electronic cloud which is succesivelly disturbed, produces an oscillation which risez the temperature of the nucleus. The authors have studied the conditions which generated the rise of temperature and multiple radiations at the place where the photons bombard the metal.

A method of decomposing a dual-directional laser beam into a forward propagating field and a backward propagating field for an apertured plano-concave cavity is presented. An intra-cavity aperture is a simple method of laser beam shaping as higher-order transverse modes are discriminated. Two fundamental resonator theories, namely, Fox-Li and Laguerre-Gaussian decomposition are used in the determination of the respective beam profiles at a specific plane. A preliminary set-up is characterized for Gaussian propagation in an attempt to verify that the cavity is viable. A comparison of experimental data with the theories is presented.

We report on the realization of a compact-package (44x27x14mm) narrow linewidth laser based on self-injection locking of a distributed feedback semiconductor diode laser to a high-Q whispering gallery mode resonator fabricated with electro-optic material. The packaged device operates at 1,550 nm and offers instantaneous spectral linewidth performance smaller than 1.8 kHz for 3 mW of output power. We are able to tune the laser frequency by applying voltage to the resonator. This suggest that the technology enables fabrication of ultra narrow linewidth semiconductor lasers in a broad wavelength range of 390 nm to 2,900 nm. The laser source in a compact footprint enables a multitude of sensing, monitoring, and metrology applications where high resolution and precision and absolute accuracy are required.