A selective review of the developments of laser resonators relevant to practical applications is given, with a main concern to recent advances connected with diode-pumped solid state laser revolution. An attempt to foresee some possible future developments is made.

The results of the use of two types of adaptive mirrors to generate quasi Q-switch pulses of the CO₂ laser radiation are presented. The excess of the output power in peak above the average level in CW regime was more than 2.5 times. The power of the lasers that were used in the experiments was in the range of 1 - 3 kW CW. Also, the design of the different types of the active correctors used in the experiments is discussed.

Aspherical (graded-phase) mirrors are potentially useful in stable resonators to obtain a desired intensity distribution at the output of a low-loss or moderate-loss resonator. The mode volume for single-mode operation can also be increased over that which can be obtained using a stable resonator. Much of the work on this concept to date has concentrated on resonators that have a super-gaussian mode at the output mirror. Here we explore some concepts involving other amplitude distributions that may be used to further enlarge the mode volume, although the output distribution may not be as desirable as the super gaussian.

A diffusion cooled lineselective CO₂ laser with a variable reflectivity grating is presented in this paper. The line-selective CO₂ laser acts as a pump source for far infrared lasers. We use an rf-excited slab laser geometry to achieve a compact laser design due to the area power scaling in contrast to the conventional longitudinal DC excited lasers with length power scaling. The huge Fresnel number of a slab system normally leads to a higher order mode operation of the laser and uncontrolled transversal mode-hops. In order to reduce this effect, so called Variable Reflectivity Mirrors (VRM) were investigated for stable and unstable resonators in the past. Instead of a VRM we use a modified binary Littrow grating to achieve the same effect in a lineselective resonator setup. The reflectivity of a binary Littrow grating with constant depth and grating period depends on the width of the grating bars. The grating splits the incident power into the -1st and 0th diffraction order. The splitting radio depends on the duty cycle of the grating. The -1st diffraction order is reflected back into the resonator whereas the 0th diffraction order is used to couple out the laser beam. Therefore the grating acts as lineselective, outcoupling and, due to the nature of VRM resonators, as a modeselective element as well. The gratings are realized by a microgalvanic process on copper substrates. Results of different resonator concepts (stable and unstable) with planar and convex gratings are presented.

The features of fundamental modes in large-aperture unstable resonators with helical mirrors are for the first time to our knowledge investigated via computer simulation. It is shown that a large-aperture laser can oscillate at a single transverse mode with axially symmetric intensity and nonzero topological charge determined by the charge of the helical coupling mirror. Energy efficiency of such a laser does not differ from that for conventional unstable cavity.

Detailed laser beam characterization is essential for the proper choice of lasers source according to the respective application as well as for the optimization of optical systems. Since most lasers generate partially coherent beams, intensity and phase distributions are not enough to describe them. In addition the knowledge of the coherence distribution is necessary. So far different setups have been used to measure phase and coherence distribution with limited accuracy. Here we demonstrate a new measurement procedure which is based on the retrieval of the Wigner distribution, from which all relevant information can be derived. The setup is very simple and the results seems to be fairly accurate.

The multi-Gaussian beam shape is proposed as a model for aperture functions and laser beam profiles which have a nearly flat top, but whose sides decrease continuously. Beams and apertures of this type represent a simple, elegant, and intuitive alternative to super-Gaussian beams. The design of lasers that have these beams as its eigenmodes is discussed. Such laser resonators would have increased energy extraction from the laser amplifier.

Excimer lasers are the preferred source of illumination for photolithographic systems. Lenslet array optical integrators are very frequently used to achieve good uniformity of illumination at the plane of the mask--the target to be recorded by the photolithographic system. Lenslet arrays break the incident beam up into subapertures and the resulting beams are overlapped at the plane of the target with variable incident angles. The overlapping of the tilted beams segments results in coherent interference effects leading to fine-scale intensity modulation called orange peel. We show that the ideal method of characterization of excimer lasers for this application is the Young's double hole test with hole spacing set to the lenslet element separation. This method to be superior to other common measures of laser quality such as angular divergence and M².

The numerical and experimental investigation of a self- starting Nd:YAG laser oscillator with a cavity completed by population gratings induced in the laser crystal by generating beams themselves are reported. The spatio- temporal characteristics of the laser comprising three Nd:YAG amplifiers and a saturable absorber were investigated. The generation of single mode beam with an average power of as large as 100 W and high quality was achieved.

We have described resonator designs for stable and high- brightness beam generation by intracavity frequency doubling. The combination of a novel pump configuration and the bifocusing compensation resonator design provides efficient and stable harmonized beam generations with good beam quality.

Light possesses both spin and orbital angular momentum. Whereas the spin component arises from the spin of individual photons and is associated with circular polarization, the orbital component arises from the azimuthal component of the Poynting vector and is associated with helical wavefronts. This paper considers the generation of beams with orbital angular momentum, their interaction with mater, behavior in nonlinear media and frequency shifts arising from their rotation. In each case the properties of beams containing orbital and/or spin angular momentum is compared and explained.

Several papers contain calculated optical losses of CO₂ waveguide resonators considering attenuation and coupling losses due to interference between modes within the space between the end of the waveguide and the mirror. A strong dependence of these losses on guide dimensions, especially those dimensions affecting the Fresnel number of the resonator, was found. To measure these losses we used a method based on a scanning Fabry-Perot interferometer. By determining the bandwidth total losses are calculated.

As an important step towards integration of microspheres in compact functional photonics devices, we demonstrate direct efficient coupling of light in and out of high-Q whispering- gallery (WG) modes in silica microspheres using angle- polished single mode fibers. Based on this principle, we present a 1-inch fiber-pigtailed microsphere module that can be used for fiber-optic applications, and a fiber-coupled erbium-doped microsphere laser at 1.55 micrometers . In addition, we report preliminary data on the intensity modulation based on high-Q WG modes in a lithium niobate sphere. We also demonstrate a novel geometry WG-mode optical microcavity that combines Q approximately 10⁷, typical for microspheres, with few-nanometer mode spacing either available in lower quality factor Q approximately 10⁴ microfabricated planar rings.

2-micrometers thick GaN microdisks with the following shapes: circular; square; and quadrupolar with various deformation amplitudes have been investigated by optical pumping. For circular microdisks, the minimum laser threshold was found when the pump is a ring-shaped 355 nm laser beam. An imaging technique is used to photograph (with a CCD camera) the sidewall of the microdisk at various angles in the horizontal plane. From the imaging results as a function of observation angle, it is possible to extract information about the laser output location along the sidewall and its far-field angular pattern. Image results for the quadrupoles (with optical pumping of the top face) suggest that the directionality of laser emission is associated with chaotic- orbits that emerge just outside the highest curvature edges. With focused pumping of the same quadrupole structures at the middle of the top face, only Fabry-Perot modes involving the two lowest curvature interfaces are observed.

We discuss the optical whispering gallery modes in single and coupled spheres in the small size range. We study the emission from a dye doped polystyrene spheres with diameter ranging from 2 to 5 micrometers under quasi-steady state optical excitation. By studying the spectral characteristics of the emission of the individual 5 micrometers sphere, we observe the transition from spontaneous emission to lasing. Although the dye molecule linewidth is broader than the free spectral range of whispering-gallery modes for such a sphere, more than 30% of the spontaneous emission is emitted into the lasing mode. By monitoring the frequencies of fluorescence peaks of individual spheres, we sort out two spheres with appropriate size matching and make them in contact. We observe coherent resonator coupling of optical whispering- gallery modes in fluorescence from two-sphere system (bisphere). By taking into account harmonic coupling of the whispering-gallery modes, the obtained features of the normal mode splitting are well explained by the tight binding photon picture.

The high Q of a microsphere whispering-gallery mode allows for sensitive resonant detection of atoms or molecules. The species being detected absorbs energy from the mode's evanescent field. It can be identified by knowing the resonant wavelength of the driving laser, and its concentration can be determined from the absorption signal on the light in reflection or transmission. High sensitivity results from the long effective absorption path length provided by the whispering-gallery mode's large Q. There are many possible implementations of and applications for such a sensor; several of each are described herein. In particular, for atmospheric trace-gas sensing, the microsphere has the potential to rival the performance of the multipass cell, but in a much more compact and rugged system. Our construction of a prototype system for detection of carbon monoxide, carbon dioxide, and ammonia is described.

Microring resonators are attractive for Very Large Scale Integrated Photonic Circuits. They have been shown to be capable of many optical signal processing functions, and their small dimensions could lead to integration densities of 10,000 devices per square-cm. Here, the analytic theory of higher order mutually coupled resonators is derived. Experiments involving fabricated ring resonators and resonator arrays is described.

Multicore fiber-lasers are designed to build high-power short length fiber-lasers. In our case the active single- mode cores (micro cores) are placed on a ring inside a big pump core. The micro cores are placed together very closely so that evanescent coupling between adjacent micro cores should be provided. To understand the coupling behavior in a multicore fiber in order to phase-lock all the micro cores we measured experimentally the coupling constant between the micro cores. Simultaneously we calculated the evolution of an injected field in a multicore fiber. In the experiment and in the simulations 38 micro cores with a diameter of approximately 6.9 micrometers are placed on a ring with a diameter of 115 micrometers . The distance between adjacent cores is about 2.6 micrometers . The measured coupling constant of 2c_exp equals 0.82 mm^-1 is in good agreement with the value 2c_th equals 0.83 mm^-1. Furthermore the phase evolution in each micro core was evaluated.

Multimode interference is fairly known from the 1D case of slab waveguides. We present for the first time to our knowledge the reconstruction of the 2D radial symmetric structure of a multicore fiber laser in a multimode fiber. In the concept of multicore fiber, rare earth-doped single mode waveguides (micro cores) are placed on a ring inside a big pump core. The situation of injecting radiation from N incoherent emitting sources into a multimode waveguide is described analytically. Experimental and numerical results for various multimode diameters and fiber lengths dealing with the reconstruction of the injected near-field pattern and the corresponding far-field patterns are presented. We propose that the reconstructed field could be re-injected into the multicore fiber-laser in order to introduce parallel coupling of all emitters. Additionally, using the multimode fiber as a passive element, without re-injection, the on-axis intensity of the multicore laser radiation is significantly increased by a single pass through a multimode fiber with a certain length. This effect takes place without any loss of energy.

An analytical/numerical approach, based on the Van Vleck Green function, was developed in order to analyze spectrally multiplexed beam combining of a linear array of fiber lasers, in which a blazed diffraction grating located in an external cavity plays the role of a spatially dispersive element. The focus is on the laser-external cavity coupling, which determines excess cavity loss, as affected by primary aberrations of the transform lens. Other issues are also touched upon, however, such as beam quality and bandwidth dependence on element location, and the optimization of the latter. Analytical results were supported by a more general numerical implementation. Typical values for bandwidth were found optimally as low as a few GHz, which, being substantially narrower than the free-running fiber laser line-width, maximum power limits are foreseeably determined by stimulated light scattering. A rough but encouraging degree of agreement of the resonator theory with independent lens aberration calculations and preliminary experiments performed at MIT/LL using triplet and quadruplet transform lenses was encountered so far.

Using the most complete in literature physical model of the non-steady-state stimulated Brillouin scattering (SBS), the numerical study is carried out of phase conjugation (PC) in the SBS-mirror that consists of an angular selector of Stokes radiation, an ordered raster of small lenses, a main focusing lens, and an SBS-cell. The ordered raster with controlled varying of its parameters allows to perform the effective angular filtering of non-conjugated Stokes components, to reduce the local light loads, and to avoid the competitive nonlinear effects. An optimal configuration of such SBS-mirror has been determined. It has the unique properties as compared with the current SBS-mirrors. It fixedly yields the PC quality that is near to an ideal (the PC coefficient is about of or more than 95%) at the selector transmittance 50 - 70% and any level of SBS saturation, i.e. any reflection coefficient. In the SBS-mirrors of different types the high PC quality in the focused beams takes place at the high reflection coefficients only that is difficult to realize as a rule. The first experimental data obtained at a Nd laser facility show the validity of the simulation results. The developed concept of SBS-mirror with unique properties can be applied for the improvement of wide class of industrial lasers.

A novel automatic output stabilized tunable CO₂ laser has been developed providing a new quality level of investigations in the areas of atmosphere monitoring, active media diagnostics, spectroscopy, etc. The optimally designed and arranged laser is composed of sites and systems with reliable operational resources. Mean time till failure of the laser is 1000 h. About 0.3 cm^-1 resolution of its high-Q and highly selective resonator allows extraction of most lines in sequence and hot bands without using a hot cell in the laser cavity. The laser is also distinguished among many other lasers by generating a high power (up to 50 W) at traditional lines and a great amount (near 160 totally) of sequence and hot lines (up to 10 W) in the 9 - 12 micrometers range which can be qualitatively changed in favor of sequence or hot lines by replacing some of the sealed-off active element or the whole set of them. Software and hardware developed for handling the laser operation offer the possibility to extract a needed set of lines in any succession and with a required run duration.

The thermally induced lens is a critical issue in high-power diode-pumped solid-state lasers. A self-adaptive scheme to balance the thermal lenses in laser resonators is presented. The requirements for the thermo-optical self-adaptive element and its influence on the resonator are discussed. With an appropriate compensating element and the correct resonator design, constant beam parameters are expected to be achieved over a pump range of several kilowatts.