This PDF file contains the front matter associated with SPIE Proceedings Volume 7579, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We have developed a fast multi-wavelengths switching laser platform. The tuning mechanism is based on a micro-mirror array DLP chip from a commercial pico-projector forming the end mirror of an external cavity laser diode. We report progress on a working prototype of a single frequency laser with a wavelength that is switchable between any five wavelengths spanning 765 nm to 783 nm. Switching time between any two wavelengths is equal to the switching time of the DLP micro-mirrors (milliseconds). We show that there is a clear path to realizing a tunable laser with over 50 discrete wavelengths. In addition to the fast switching time between any wavelengths, this laser has a compact form factor (<1cm²) and the design is applicable to a broad spectral range spanning 400 nm to 3,000 nm.

In this paper we outline new approaches to old problems, namely understanding the transverse modes in Porro prism resonators, and creating methods to select Gaussian beams by phase-only intra-cavity elements. In the process we outline some of the recent research that has taken place within the Mathematical Optics research group.

We investigate numerically and analytically the effects of gain saturation on the propagation of the fundamental mode in a gain-guided index-antiguided slab waveguide. The propagating mode adapts to gain saturation by becoming less confined, while at the same time its peak intensity increases more slowly. The modal gain coefficient is shown to scale linearly with the modal confinement in the cladding. At steady state, both the mode shape and power remain constant. We show that, at large index antiguiding, the field profile inside the core remains nearly constant during propagation, which allows an analytical description of the evolution of the optical power inside the core.

A laser resonator configuration is proposed in which the fundamental oscillating mode is an odd mode with a line singularity. The resonator is based on the replacement of one reflector by a bi-lens split mirror. The beam emitted by such a laser is an ideal source for the recently introduced singular beam microscopic procedure. We numerically investigate resonators with convex and concave faces of the reflector. It is shown that resonators of the latter type of reflectors have higher effectiveness in shaping modes with linear singularity. Their selectivity is also higher, than resonators with roof-mirrors that were investigated earlier.

We discuss practical aspects of the use of the finite-difference alternating-direction implicit (ADI) algorithm for the free-space propagation of light beams. Results of calculations solving the finite-difference equations are compared with fresnel-integral solutions. Calculations are for round beams, but the field is represented in cartesian coordinates. Modes for empty unstable resonators are also obtained using the finite-difference algorithm and are compared with fresnel-integral solutions.

Because the Fox-Li diffraction integral iteration method is not suitable to calculate transverse-mode fields of a resonator with big Fresnel number, the tansfer matrix method is used for analysis of the eigen modes of an optical resonator. The process of this method is as follows: from the Collins Formula, the diffraction integral equation of a resonator is obtained and transformed to the finite-sum matrix equation. Finally, the transverse-mode distribution and loss of the resonator can be calculated by use of the eigen function and eigen value of the matrix. In this paper, the transfer matrix method is discussed, and the precision of the method is control. It is shown from the simulated results of a general confocal resonator that numberical results by use of the transfer matrix method are accord with that of the intergral iterative method, and the convergence problem of the intergral iterative method can be overcome and the analysis of laser resonators can be met by the transfer matrix mothod, as long as the calculation precision is reasonably controlled.

We present a new treatment of optical forces, revealing that the forces in virtually all optomechanically variable systems can be computed exactly and simply from only the optical phase and amplitude response of the system. This treatment, termed the response theory of optical forces (or RTOF), provides conceptual clarity to the essential physics of optomechanical systems, which computationally intensive Maxwell stress-tensor analyses leave obscured, enabling the construction simple models with which optical forces and trapping potentials can be synthesized based on the optical response of optomechanical systems. A theory of optical forces, based on the optical response of systems, is advantageous since the phase and amplitude response of virtually any optomechanical system (involving waveguides, ring resonators or photonic crystals) can be derived, with relative ease, through well-established analytical theories. In contrast, conventional Maxwell stress tensor methods require the computation of complex 3-dimensional electromagnetic field distributions; making a theory for the synthesis of optical forces exceedingly difficult. Through numerous examples, we illustrate that the optical forces generated in complex waveguide and microcavity systems can be computed exactly through use of analytical scattering-matrix methods. When compared with Maxwell stress-tensor methods of force computation, perfect agreement is found.

The integration of a higher index chalcogenide strip as a guiding layer on top of diffused lithium niobate waveguides is presented. The mode transfer to upper cladding through a 2D taper structure is discussed theoretically. A review of the fabricated waveguide structures and the results are also provided. A slight modification to current taper design is proposed to further improve the bend radius and create much smaller ring resonators. Future perspectives for devices and their potential impacts on integrated optics are also discussed.

A 2kW cylindrically polarized laser beams by using a triple-axicon optical resonator is demonstrated. The rear mirror of a commercial CO₂ laser is replaced by the optical component that is composed of a waxicon and an axicon accurately fitted together. Selection of the polarization is made by the reflectivity difference between p and s polarizations at the inclined surfaces. The reflectivity is designed to r_s>r_p so that the resonator is oscillated in azimuthally polarized mode. The output beam is converted to radially polarized beam by a converter composed of four λ/4 phase retarders. The polarization conversion efficiency is 98.5% and the power conversion efficiency is 95%.

Recently the importance of numerical simulations for the design of laser resonators has grown considerably. This applies in particular if the alignment of components within the resonator is crucial for its stability. In such cases a tolerance analysis is required that can be done most efficiently using numerical simulation tools. In this paper, we introduce a computer model for resonators based on components and their combination using absolute or relative positioning. We show that this approach is the basis for tolerancing and sensitivity analysis. Further we discuss the concepts of field tracing and unified optical modeling that allow the coupling of several propagation methods within one modeling task. For laser resonators this involves in particular free space propagation methods as the Fresnel integral, geometrical optics and split step beam propagation methods. The primary goal is to provide a fully vectorial simulation as accurate as required and as fast as possible. This approach covers in particular general eigenmode models and general geometries including micro-structured surfaces that can be used for additional beam control as it is shown in the examples.

Here we investigate closed-loop adaptive optical system to compensate for laser beam aberrations. A bimorph mirror is used as a wavefront corrector and Shack-Hartmann wavefront sensor is an element for feedback control. Comparison of phase conjugation and multi-dither technique is shown.

The sensitivity of coupled laser cavities to path length errors for coherent laser combining is analyzed, and new methods are described for reducing this sensitivity. We show that certain resonator structures are able to tolerate path length errors better than others, and present experimental measurements of this tolerance.

A unique technology for the fabrication of high-quality and robust beam delivery optics for fiber lasers is presented. CO₂ lasers are used to reshape the spherical surface of plano-convex fused-silica rod-lenses, and then fuse the optical fiber directly to these lenses. A specific fiber collimating system is presented and analyzed in terms of aberrations, insertion loss, M2, and return loss. Test results are compared to the theoretical modeling, demonstrating the accuracy and repeatability of this technology.

Laser beam transformation utilizing the effect of multimode interference in multimode (MM) optical fiber is thoroughly investigated. When a Gaussian beam is launched to an MM fiber, multiple eigenmodes of the MM fiber are excited. Due to interference of the excited modes, optical fields that vary with the MM fiber length and the signal wavelength are generated at the output facet of the MM fiber. Diffractive propagation of these confined fields can yield various desired intensity profiles in free space. Our calculations show that, an input fundamental Gaussian beam can be transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and low-divergence Bessel-like within either the Fresnel or the Fraunhofer diffraction range, or even in both ranges. Experiments on a monothic fiber beam transformers consisting of a short piece of MM fiber (~ 10 mm long) and a single-mode signal delivery fiber were carried out. The experimental results indicate the functionality and high versatility of this simple fiber device. The performance of this fiber device can be easily and widely manipulated through parameters including the ratio between the core diameters of the SM and MM fiber segments and the length of the MM fiber segment. In addition, the intensity profile of the output beam can be controlled by tuning the signal wavelength even after the fiber device is fabricated. Most importantly, this technique is highly compatible with the technology of high power fiber lasers and amplifiers and fiber delivery systems.

Different scientific and industrial laser techniques require not only intensity profile transformation but also creating various shapes of final spots like circles of different diameter, lines and others. As a solution it is suggested to apply combined optical systems consisting of a refractive beam shaper of field mapping type providing a required intensity transformation and additional optical components to vary the shape of final spots. The said beam shapers produce low divergence collimated flattop beam that makes it easy to vary the shape of the beam spot with using either ordinary relay imaging optics, including zoom one, or anamorphotic optics. And the design features of the refractive beam shapers allow controlling the intensity distribution in the final spot (most often flattop one) and providing wide range of spot sizes. This paper will describe some design examples of combined beam shaping systems to create round spots of variable diameter as well as linear spots of uniform intensity. There will be presented results of applying these systems in such applications as laser hardening and others.

Micro-lenses and micro-lens arrays are widely used for various applications. Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages to laser applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMO's unique production technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis. First application results of such microlens arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100 W laser with tilted illumination under an angle of 30-50° is shown. A homogeneity of better than 90% was achieved for a field size of 270 mm x 270 mm. In laser direct write processes a top hat profile has several advantages compared to a Gaussian beam profile, especially the throughput of the system and quality of the structures can be improved. Novel patterning results with TopHat-converted single mode lasers and a special Gaussian-to-TopHat galvo scan system are demonstrated for solar cell technology.

The coexistence of different propagating modes inside an optical multi mode fiber (MMF) determines the properties of the emerging beam. Hence, detailed knowledge of the (transversal) modal content became essential to understand the underlying physical effects. Different approaches were applied to acquire knowledge about modal content of MMFs such as interferometry or M² respectively S² measurement. In this paper we present new results concerning the access to full field information of vector beams, emerging from two step-index LMA fibers (V -parameter 3.96 and 4.72, respectively). Thereby, the direct approachable measurement data consists in modally resolved information about polarization state, intermodal power distribution and intermodal phase delay. By means of the reconstructed field, "global" characterizations of the investigated beams, e.g., in form of the widespread beam propagation ratio M², are possible.

Spatial light modulators based on liquid-crystal-on-silicon micro-displays were investigated with respect to their capability to flexibly shape complex wavefields from femtosecond pulses. Experiments were performed with a Ti:sapphire laser oscillator emitting linearly polarized radiation at pulse durations in 10 fs range. It is shown that the transfer characteristics well enable for an undistorted adaptive shaping of microoptical phase profiles which are linearly dependent on the gray values at such ultrashort pulses. In particular, beam arrays consisting of individually programmable nondiffracting Bessel-like beams, needle beams and beam slices of high aspect ratios were generated. By composing complex patterns of nondiffracting subbeams, image information was propagated nearly undistorted over certain distances ("flying images"). Cross-talk was minimized by diffractive background management. Further applications like adaptive wavefront sensing, advanced autocorrelation as well as statistical encoding are discussed.

This paper gives expressions to calculate the fraction of power, fPIB, from a given multimode gaussian laser beam that can be deposited within a bucket of radius, r_T, on a target at a range, z_T, using a focusing optic of diameter, D_f. We relate the power in the bucket, fPIB, to the M² parameter, both of which can be experimentally measured. In this paper, we have also presented relationships between these two parameters and BQ and Strehl, which have not been unambiguously defined for a multimode laser beam in the literature. We propose f_PIB and M² to be used as standard design parameters instead of BQ and Strehl for laser-target interaction tests with multimode laser beams from stable resonators.

Providing optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 μm wavelength to signal light being tunable from 1.25 to 1.85 μm and to corresponding idler light from 2.3 to 5.3 μm. We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of 1.35 THz and a power of 2 μW. Now, the straightforward next development step is to reduce the footprint of such devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity mode for 3 hours.

The electrostriction effect on the whispering gallery modes (WGM) of polymeric micro-spheres is investigated analytically and experimentally. Electrostriction is the elastic deformation (mechanical strain) of a dielectric material due the force exerted by an electric field. The elastic deformation also leads to mechanical stress which perturbs the refractive index of the sphere. Both if these effects (strain and stress) induce a shift in the WGM of the dielectric microsphere. We develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled micro-shells. The analytical results show that detection of electric fields < 1000 V/m is possible using water filled PDMS microshells. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid polydimethylsiloxane (PDMS) spheres agree well with the analytical prediction. These results are encouraging for future development of WGM-based optical switches/filters as well as electric field sensors.

Microfluidic lasers, which utilize liquid as a gain medium, are of great interest for lab-on-a-chip devices due to their small size, tunability, and cost-effectiveness. We demonstrate a soft-lithography-based opto-fluidic ring resonator (OFRR) laser which can be produced in arrays of identical rings in polydimethyl siloxane (PDMS). The PDMS structures are produced from a silicon mold fabricated using reactive ion etching (RIE) and are both robust and reusable. Using rhodamine 6G in a tetraethylene glycol (TEG) dye solvent provides enough refractive index contrast with PDMS to generate a multimode lasing signal from rings 200 to 400 microns in diameter and lasing thresholds of 2.7 μJ/mm² centered around 580 nm. These rings are coupled to liquid waveguides which conveniently direct the lasing emission to other on-chip devices. Since the rings and waveguides are not in fluidic contact, many rings may potentially be coupled into a single waveguide for multi-color emission. Separating the ring and waveguide fluidics also prevents unwanted absorption of the lasing signal by extra dye molecules.

It is described how strongly directional emission may be achieved from whispering gallery modes in optical resonators which are only very slightly deformed from spherical or circular geometry. A theoretical description is offered which uses an extension of eikonal approximation to complex ray families. Natural boundaries arise in the complexified ray families which prevents direct application of ray-theoretical methods. By treating the ray dynamics perturbatively, however, it may be possible to obtain explicit approximation of the exterior field for sufficiently weakly deformed resonators.

We introduce a novel approach to assemble fundamental nanophotonic model systems. The approach is based on the controlled manipulation of single quantum emitters (defect centers in diamond) via scanning probes. We demonstrate coupling of a single diamond nanocrystal to a planar photonic crystal double-heterostructure cavity as well as to a silica toroidal resonator. Our studies represent an important step towards well-controlled cavity-QED experiments with single defect centers in diamond.

Recent studies on optical rotation sensors employing slow-light media show great potential for the realization of compact, yet sensitive devices. In particular, the rapid progress in micro and nano fabrication methods render coupled cavity based slow-light structures as promising candidates for the realization of such devices. In slow-light structures, the impact of rotation is manifested in a completely different way than it does in conventional Gyros, thus giving rise to extremely different characteristics such as exponential sensitivity, phase-shift control and more. In this paper, I review the principles of slow-light rotation sensors with emphasis on the differences from conventional optical Gyros. The underlying physical mechanisms of the variously studied slow-light Gyros as well as the expected performances will be presented and compared.

Three-mode opto-acoustic interactions can occur in optical cavities when a mirror acoustic mode has the appropriate mode shape and frequency to scatter carrier light into a cavity high order mode that has matching mode shape and frequency. The interaction can be very strong since the strength scales as the product of two optical and one acoustic quality factor. The phenomenon enables a new class of transducer, amplifier or optical cooler. Small-scale devices are predicted to enable efficient cooling to the quantum ground state, while in the long optical cavities of gravitational wave detectors, the phenomenon can lead to acoustic instability. Experimental results are presented for both large and smallscale systems, and the design of systems for optical cooling to the quantum ground state is discussed.

Microwave optical systems for frequency generation are described in this paper. The goal is to reach high spectral purity in the microwave frequency range using ultra high Q optical resonators. The resonators investigated are of two types : resonant (passive) fiber rings and WGM tridimensional resonators. They all feature ultra high optical Q factors, in excess of 10⁸ or 10⁹ near 1550 nm. These resonators also sustain a large number of optical resonances, and the microwave signal is stabilized on two (or more) resonances of this optical comb. Different problems have to be overcome in order to reach a functional system, such as : resonator design and coupling, laser stabilization on a resonance, overall system design, noise optimization... This paper gives an overlook on these problems, and on some solutions we found to work towards a compact and efficient microwave opto-electronic oscillator (OEO). A first result is presented on a 10 GHz OEO based on a resonant fiber ring.

We review the improved performances of a narrow linewidth laser using negative electrical feedback obtained through advances on narrowband FBG filters. Noteworthy, the tolerance of the laser to vibrations is significantly improved. As an extension of this work, these narrow filters are proposed for filtering optical signals in RF photonics systems.

We have developed thin film Fabry-Perot filters directly coated on optical fibers to archive a high level of integration with a reduction of optical elements. Such band-pass filters can be used in fiber optical sensor systems, and for fiber communication, e.g. CWDM applications. The filters cavities consist of a single spacer and two dielectric mirrors. The dielectric mirrors are deposited by PVD directly on end-faces of single-mode optical fibers. Dielectric as well as polymeric materials were applied as the spacer layer. Polymeric spacer layers were deposited by dip coating. The influence of the mirror reflectivity on the transmission band of the Fabry-Perot filters was investigated. Furthermore, the optical performance of filters with first order (λ/2) as well as higher order spacers was analyzed. The experimental results are compared with numerical analysis of Fabry-Perot cavities on the end-face of cylindrical waveguides. The spectral characteristic of the filters are calculated using a software solving Maxwell´s equations by a FDTD method. The layer design of the filters and the deposition process were optimized for maximum transmission and narrow bandwidth of the transmission peak. Passive band-pass filters on fiber end-faces were designed, fabricated and characterized for transmission wavelengths of 945 nm, 1300 nm, as well as 1550 nm. Bandwidths as narrow as 1 nm could be achieved for 945 nm.

Outstanding evolutions in fiber technology made it possible to overcome restrictions due to nonlinear pulse distortions in the amplification fiber and revealed the full potential of rare-earth-doped fibers as a power-scalable solid-state laser concept in the short pulse regime. State-of-the-art femtosecond fiber lasers in our labs deliver average power as high as 1 kW and pulse energies above 1mJ in the 1 μm wavelength region. This performance, in particular the significantly higher repetition rate compared to conventional femtosecond lasers, allows for unique approaches in several application fields. Beside the fiber designs, the experimental strategies, performance and limitations of these systems we will discuss selected applications.

The augmented 5×5 ray matrix with the consideration of spherical mirror's axial displacement has been proposed in this article. By applying it to a planar square ring resonator and a monolithic triaxial ring resonator, the optical-axis perturbation induced by spherical mirror's axial displacement has been obtained. The mismatching error C of the monolithic triaxial ring resonator has been found out that it can be decreased and can be even reduced to 0. These interesting findings are important to cavity design, cavity improvement and alignment of monolithic triaxial ring resonator.

An approximate analytical formula for the cross-spectral density matrix of the electric field of diffracted electromagnetic Gaussian Schell-model (EGSM) beams is derived by using the method of the complex Gaussian function expansion. We show both analytically and by numerical examples the aperture effects of spectra, coherence and polarization of electromagnetic Gaussian Schell-model beams. It is shown that the larger the truncation parameter the spectral degree of polarization becomes non-uniform on propagation, the spectral density shape is more tightly. The spectral degree of coherence oscillates, and the larger the value of truncation parameter the spectral degree of coherence exhibits rapid oscillation.

We investigate the temperature dependence of lasing properties of GaAs powders with non-resonant feedback from T = 30 to 300 K. The lasing peak energy, emission intensity, and width of the lasing emission band are found to be strongly dependent on the temperature. The dependence of the lasing peak energy on T is well explained by a theoretical model for the calculation of the gain spectra of heavily doped n-GaAs. The temperature dependence of the lasing emission intensity is different from the spontaneous emission intensity. We also find that the width of the lasing emission band is linearly proportional to the spontaneous emission band width. The linearity corresponds to the prediction by a diffusion theory.