Various resonator concepts are discussed with respect to the improvement of beam quality, especially for high power solid state lasers. By compensating the thermally induced birefringence and using adaptive mirrors considerable improvement is possible. The values of output power and beam propagation factors M2 for various laser systems are compiled.
Possible designs of high power fiber lasers are discussed in this paper related to the future power delivery system in laser material processing factories. The performance of high power fiber lasers is analyzed using basic equations and optical parameters of laser cavities. High efficiency operation of 73% was demonstrated by clad pumping scheme using homogeneous absorption regime of Nd3+-doped rectangular double clad fibers. The possibility of solar pumping without any concentrator was evaluated using power scaling law of pumping to lasing. Fiber embedded lasing disk and tube are proposed for the high power laser which produces more than 1 kW from a single core of 50 micro-meter in diameter.
The problem of the continuity of knowledge in the field of laser resonators and the beam divergence is considered. Some interesting episodes of the laser history are expounded; one gives a grand attention to works of Russian scientists.
A Nd:YAG microchip laser array is developed and the spatial phase correlation among each the microchip laser is attained utilizing the Talbot effect in a three-mirror cavity configuration. The laterally coupled Nd:YAG microchip laser array produces a pair of spots with angular separation of (lambda) /d in its far field, where d is the distance between the adjacent sources, corresponding to an out-of-phase spatial mode coupling. The far field spot size is reduced by a factor of 4.7 compared with that obtained by incoherent adding of the individual microchip lasers. Although an external mirror positioned at the 1/4 Talbot distance from the microchip lasers source is very effective to introduce the coherence among the laser chips, the present long Talbot distance limits the output laser power due to the extremely high diffraction loss in the auxiliary cavity. Smaller beam sizes for each microchip lasers will decrease the filling factor and improve the efficiency in the Talbot cavity.
The results of intracavity beam correction and formation by active bimorph mirror are presented in this paper. Different types of lasers were used to show the efficiency of such method of laser beam control. Also, the possibility to improve beam quality parameter of YAG and copper-vapor lasers is shown.
The graded reflectivity mirror (GRM) unstable resonator has become the resonator of choice for many laser systems. It provides a large size fundamental mode while reducing detrimental diffraction effects encountered in hard-edge unstable resonators. It also allows some control over the near-field profile of the output beam through a proper choice of the reflectivity profile of the GRM. This added flexibility in design translates into an apparent increase in complexity to the potential user. This tutorial review discusses through a step-by-step approach the inner workings of such resonators, in order to identify tradeoffs which are involved in their design. Geometrical and diffractive contributions to the mode definitions are discussed, as well as the beam shaping action of amplification. Particular attention is given to super-Gaussian resonators, which are nowadays the most widely used.
A diffusion-cooled CO2-laser with an output power of 1.5 kW was developed and set up with a coaxial RF-excited discharge. The laser beam is extracted by a resonator with helical mirrors, which is stable in the radial direction and unstable in the azimuthal direction. Without any beam transformation this laser emits a general astigmatic beam with an elliptical power density distribution, which is rotating in a plane perpendicular to the beam axis while propagating. Also, the beam shows hard edge diffraction effects due to the hybrid unstable/stable design of the resonator. In order to make the beam applicable to standard materials processing applications a beam shaping device has to be used to transform the general astigmatic beam into one of more rotational symmetry. The resulting beam has a times- diffraction-limited factor M2 equals 1.5.
The diffraction analyses of formation of the given fundamental mode intensity distribution in near-field zone by intracavity adaptive mirrors are discussed. Such mirror was a bimorph flexible one with three concentric rings of controlled electrodes. The calculations were carried out for industrial CW CO2 laser.
During the last five years, INO has conducted a program to develop high performance diode laser illuminator for ATV (active TV) system. Illuminator made using INO proprietary collimating method, which uses GRIN (mu) -lenses for the fast axis and plano-convex (mu) -lenses for the slow axis, were found to provide uniform and tightly collimated beam, diffraction-limited for the fast axis. Such collimated laser diode arrays were proven to efficiently illuminate non- cooperative scene at 1 km with only 6 W of average power.
Semiconductor diode lasers have significant advantages over other types of lasers in compactness and efficiency. They also can be mass produced using techniques of the electronics industry, offering the hope for very low cost per laser. However, the power of an individual single- spatial-mode diode laser is limited to a few watts. The coherent power of diode lasers can be increased by making a coherent array. We will discuss a variety of approaches that have been employed to make coherent arrays of diode lasers. The first group of techniques are those using a master oscillator, either in a branching master-oscillator-power- amplifier configurations, or in a multiple injection locked configuration. Systems without a master oscillator can either be one large resonator with many segmented gain regions, or coupled oscillators. The coupled oscillators are divided into those coupled to adjacent elements only (series coupling), and those coupled to many other elements (parallel coupling). We also will show how nonlinear optics have been used to implement some of the above architectures. These various methods have produced scientific success in coupling arrays together, but each has drawbacks that have prevented coherent diode arrays from achieving commercial success.
Monolithic linear arrays of diode lasers, also known as diode laser bars, are the basic elements for most high-power laser applications such as solid-state laser pumping or material processing. Cylindrical microlenses used as fast- axis collimators for 10-mm diode bars require very high angles of aperture (up to 100 degree FW1/e2, equivalent to a numerical aperture of approx. 0.8) to capture most of the emitted laser power. For the efficient longitudinal pumping of laser rods, or the narrow focusing of the diode laser radiation (fiber coupling, material processing), high- quality microlenses with small lens aberrations are necessary to avoid power losses and beam quality degradation. A technique for coupling the output of high- power diode laser bars into one multimode fiber with high efficiency, easy alignment requirements and low manufacturing costs is demonstrated using a single fiber with core diameter down to 400 micrometers . This technique comprises two micro step-mirrors for beam shaping. The overall efficiency from one diode-laser bar to fiber is 71% with 20 W cm laser power through the fiber. Coupling of 12 diode laser bars and power of 200 W out of a fiber with core diameter of 0.8 mm and NA equals 0.2 is achievable with this technique.
Several unconventional laser resonators are described, and their application to various laser systems is explored. Spatial mode control of vertical cavity surface emitting lasers is enhanced by two spatial filtering methods. The first employs a GRIN lens with a patterned mirror designed to reflect power only from desired modes. The angular selectivity of a Fabry-Perot etalon is used in the second system to improve spatial modal discrimination. Diffractive optical elements are used with a solid-state laser for beam shaping and aberration correction. An analysis of the effects of fabrication errors shows that higher-order diffraction terms and phase errors can result in distorted modes. However, proper cavity design can help to reduce these effects and insure proper mode shape.
High power diode lasers based on multistripe, multimode array bars enter numerous fields of applications due to their high performance and high reliability. Beam shaping optics that transfer the light from the facet of the diode laser to the target play a key role for most of the applications. Sophisticated optics must be adapted to a single or multiple number of diode laser bars to generate high power densities and small spots at the target. The physical and practical limitations are discussed.
The application of laser welding has been introduced to industry during the last decade. Most of the work done has concentrated on the reliability of the process itself. Now, the next step is to increase the reliability of the laser source to build lasers more simple and to make them cheaper. This paper presents the results of the theoretical calculation of the coaxial laser resonator. The data of the raw and the focused beam have been investigated and first welding samples are demonstrated.
Aspherical resonators were developed for an industrial high- power CO2 laser with a rated output power of 20 kW. The task of the aspherical resonators is to perform intra-cavity beam shaping in order to generate process-adapted laser beams for laser materials processing. To evaluate the capability of aspherical resonators for lasers materials processing their performance is compared to conventional spherical and unstable resonators. The times-diffraction- limit factor M2 obtained with an aspherical resonator has a value of 6.3 and is comparable to a spherical reference resonator. Compared to the spherical and the unstable resonator the electro-optical efficiency can be improved up to 50%. In the far field a considerable enhancement of the peak intensity compared to the spherical resonator is obtained. Application of the aspherical resonator for welding demonstrates the essential importance of this resonator type for materials processing.
Investigations aimed at the production of high power beams with diffractionally limited quality are carrying out in the Institute for Problems in Mechanics since 1992. Up to 5 kW CW output beam characterized by M2 less than 1.7 was produced with Lantan-5 industrial fast-transverse-flow CO2 laser by means of the unstable resonator with variable reflectivity output coupling mirror (VRM). Both theoretical and experimental investigation of the parameters of the beam produced by VRM unstable resonator are presented. The results of beam quality measurements are in good agreement with theoretical evaluations, and processing performance of the beam corresponds to the measured value of M2. The quality of the output beam in resonators with VRM may be further improved by appropriate choice of the parameters of variable thickness reflective layer.
The beam quality of high average power solid state lasers degrades due to thermal lensing and stress birefringence, similarly, discharge inhomogeneities limit the performance of excimer lasers. To improve the beam quality, self-pumped phase conjugating mirrors based on stimulated Brillouin scattering (SBS) were applied in different oscillator and master oscillator power amplifier systems (MOPA) producing radiation from the mid infrared down to the ultraviolet range. Pulsed Nd-lasers for industrial applications with up to 500 W average output power of 1 micron wavelength and high beam quality were realized. Due to phase conjugation the beam diameter and divergence do not depend on the average output power. Also for medical applications, high beam quality is essential. Therefore we investigated a MOPA- system with phase conjugating mirror at a wavelength of 2.8 micron. In the ultraviolet range, a commercial XeCl laser from Lambda Physik was equipped with an SBS-mirror resulting in a self starting oscillator with phase conjugation at a wavelength of 308 nm. As an alternative to widely used SBS- materials like liquids and gases we developed a new kind of self-pumped SBS-mirror based on multimode quartz fibers. These exhibit lower SBS thresholds and therefore continuously pumped Q-switched laser systems with SBS-mirror can be realized.
We have implemented an Optical Parametric Oscillator (OPO) with an unstable cavity. The cavity is a telescope made of a concave mirror and a convex mirror. This latter is coated only in its center an is the pupil of the cavity. The nonlinear crystal is a 5 cm long LiNbO3. The OPO is pumped by a 20 ns Q-switch Nd:YAG laser and is operated at 1.9 micrometers for the signal and 2.4 micrometers for the idler. The OPO threshold is 7 mJ. For a pump energy of 30 mJ, the signal and idler energy produced is 9 mJ. Those results are comparable to those obtained with plane mirrors cavities. The near field of the output beam has an annular distribution imposed by the pupil. The distribution is not uniform but has 2 lobes located on opposite sides one from another. This uniformity can be improved increasing in the magnification factor from 1.2 to 1.33. The far field shows a central peak with side lobes in the vertical direction. This intensity distribution can be calculated from the diffraction of a plane wave by a uniform annular pupil similar to the near field intensity distribution. This indicates that the field is close to the diffraction limit in this direction. In the horizontal direction, there is peak which is twice as large as the previous one, and there is no side lobes. The field can be estimated to be 2 times the diffraction limit in this direction. This experiment shows that infrared OPO's with unstable cavities can produce beams close to the diffraction limit with a good energy conversion.
New principles of distributed feedback lasers with cavity formed by refractive index gratings induced in the laser crystal by generating beams themselves have been discussed. Different geometries of the cavity (linear, loop and multi- loop) with Nd:YAG crystal have been investigated experimentally. The oscillator with a nonlinear mirror has displayed self-adaptivity to strong intracavity distortions. Generation of single transverse mode beams with near- diffraction limited quality in millisecond pulse-repetitive regime with 12 W average power has been achieved in the oscillator with single Nd:YAG amplifier.
The problem of reaching diffraction divergence in annular lasers is extremely complex, and, at this time, is quite far from being solved. The following paper considers traditional attempts of solving this problem, based upon the usage of AXICON optics, as well as some of the newer ways. The three main approaches that will be looked at use resonator unstable in azimuthal direction, astigmatic transformation of annular cross section along with polarization, and the idea of coherent self-imaging in annular geometry.
Self-pumped phase-conjugate (SPPC) loop resonators using four-wave mixing (FWM) in solid-state gain media are investigated in both injected and self-starting configurations. A model, using a transient treatment of FWM with the appropriate boundary conditions imposed by the loop geometry, allows us to analyze the threshold condition for oscillation, the temporal dynamics and the energy characteristics of such resonators. The influence of the input energy in the injected configuration, and the influence of the different gain gratings and output coupler reflectivity in the self-starting configuration are also analyzed. A SPPC loop resonator using FWM in a flash-lamp- pumped Nd:YAG amplifier is experimentally investigated. In the injected case, a maximum phase-conjugate reflectivity of 42, and a maximum extraction of 47 mJ are obtained. In the self-starting case, a TEM00 mode output of approximately 130 mJ in a 13 ns single-longitudinal-mode pulse is produced up to 30 Hz. Experimental and theoretical results are in good agreement.
Transversely cooled, cylindrical Nd:YAG laser rods exhibit a strong thermally induced birefringence when they are optically pumped. The birefringence leads to different refractive indices for radially and tangentially polarized light which affects the beam-quality and the degree of polarization achieved with conventional optical resonators. In spite of this drawbacks the traditional rod-geometry for Nd:YAG lasers is still of great interest because of its technological simplicity, comparatively low costs and acceptable efficiency. In the present study the limitations for the output power of single rod Nd:YAG lasers in TEM00- operation is investigated theoretically and experimentally. Furthermore the possibility of improving the brightness of Nd:YAG rod lasers beyond the traditional limits using dual rod systems with birefringence compensation is demonstrated. Birefringence compensation is established by image-relaying between two identical Nd:YAG laser rods and an additional 90 degree(s) polarization rotation. This compensation method requires specific dynamically stable optical resonators in order to achieve optimal compensation and high efficiencies. With a dual rod Nd:YAG-system up to 80 Watt TEM00 output power is achieved with a beam quality of M2 equals 2.8. The output power is approximately 8 times higher than using a single rod-system separately.
We first present a brief review of our experimental results with a cw CO2 laser using a `custom phase-conjugate resonator' designed for a super-Gaussian output beam profile. Preliminary results with a new cavity design adapted for a peculiar fundamental model are also reported.
The National Ignition Facility (NIF) will house a 2 MJ Nd:glass laser system to be used for a broad range of inertial confinement fusion experiments. This record high energy laser output will be initiated by a single low energy, fiber-based master oscillator which will be appropriately shaped in time and frequency prior to being split into 48 beams for intermediate amplification. These 48 intermediate energy beams will feed the 192 main amplifier chains. We report on the baseline design and test results for an amplifier subsystem in the intermediate amplifiers. The subsystem is based on a diode pumped, Nd:glass regenerative amplifier. The amplifier is comprised of a linear, folded, TEM00, 4.5 m long cavity and represents the highest gain (approximately 107) component in the NIF laser system. Two fundamentally important requirements for this amplifier include output energy of 20 mJ with a square pulse distortion of less than 1.45. With a single 48 bar 4.5 kW peak power diode array and lens duct assembly, we pump a 5 mm diameter X 50 mm long Nd-doped, phosphate glass rod, and amplify the mode-matched, temporally shaped (approximately 20 ns in duration) oscillator seed pulse to 25 mJ of output energy with a very acceptable square pulse distortion of 1.44. This most recent design of the regenerative amplifier has increased the performance and reduced the cost, enabling it to become a solid baseline design for the NIF laser system.
The Whispering Gallery Modes of fused silica microspheres hold the promise for simultaneous achievement of high Q (> 109) and small mode volumes (<EQ 10-14 m3) necessary for strong coupling in cavity QED. Here we present results for high Q measurements into the NIR along with some progress towards experimental implementation in cavity QED.
A general model is presented for coupling of high-Q whispering-gallery modes in optical microsphere resonators with coupler devices possessing discrete and continuous spectrum of propagating modes. By contrast to Fabry-Perot resonators, in microspheres independent high intrinsic quality-factor and controllable parameters of coupling via evanescent field offer variety of regimes earlier available in RF devices. Latest results on realization of material- limited Q approximately 1010 in microspheres in the visible and near-infrared band and preservation of very-high Q in surface-hydration-preventing environment are presented.
A review of ways of longitudinal mode selection in solid- state Q-switched oscillators is presented. The following methods are discussed: high precision temperature stabilization of cooling liquid, active element and air; injection of cw radiation from an auxiliary diode-pumped laser into the master cavity; using feedback for compensation of cavity length changes; fast change of round trip cavity length during pumping in combination with a slight exceeding of threshold; using a ring cavity having better selective properties as compared with the linear one. The present survey of various methods for constructing oscillators with 100% probability of single-frequency lasing demonstrates the results achieved in this field as well as the ways of further development. Comparison of advantages and disadvantages of the methods allows choosing an optimal type of master oscillator for each specific application. The ways of Q-switching is also compared from the point of view of mode selection.
Industrial and scientific applications require lasers with high beam quality in Q-switched operation. In oscillators the high reflecting mirror can be substituted by a phase conjugating mirror based on stimulated Brillouin scattering (SBS). Due to the nonlinear response of such a mirror not only the beam quality can be improved also Q-switching can be obtained with the same element. Up to now toxic liquids or gases under high pressure were used as SBS-media in pulsed solid state oscillators. These media can be substituted by a simple multimode fiber. A pulsed oscillator with a multimode fiber inside was realized. Nd:YALO was used as active medium. The flashlamp duration is about 140 microsecond(s) and the threshold energy for laser operation is 5 J. The repetition rate can be varied from 1 Hz up to 25 Hz. The energy per shot is about 16 mJ. Due to the relatively low SBS threshold a burst of Q-switched pulses per flashlamp shot occurs. The pulse width can be varied between 0.5 ns and 10 ns which depends on the used resonator configuration and the fiber.
The Kerr-lens mode-locked (KLM) laser with a novel five- mirror cavity is theoretically examined by applying the ABCD-matrix formalism for a Gaussian beam. Since the optimum configuration of the five-mirror cavity is obtainable at the middle of confined cavity condition, one can easily achieve the KLM alignment and stable mode-locking operation. Influence of the self-focus appearing in the five-mirror cavity configuration upon a semiconductor saturable-absorber mirror is also analyzed.
The generation of near Gaussian beams with small beam propagation factors M2 is investigated for excimer laser applications requiring small focal spots. A considerable transverse beam quality improvement can be achieved not only with small internal apertures but also be operating the excimer laser with a phase-conjugating SBS-mirror. No additional start resonator was used. SBS-cells filled with a liquid mixture of dimethylbutane/n-pentane exhibit high optical breakdown intensities.
We discuss a new resonator design for sub-pm bandwidth (FWHM) KrF excimer laser using an etalon, a grating and prisms. This resonator uses a polarization beam splitter prism (PBSP) which has both functions of splitting polarized beam and beam expansion to reduce the resonator length. In our experiments, the output power is improved 29.4% by using PBSP in comparison with the conventional method, while the bandwidth (FWHM) is reduced from less than 0.9 pm to less than 0.6 pm at the same time. A calculation has been done to confirm the output power dependence on the resonator length and the loss of optics. Its result explains our experimental results approximately. We demonstrated that PBSP has a significant effect to improve the output power of the narrow bandwidth KrF excimer laser that would result in the extension of chamber lifetime. Furthermore, this technique will help the development of the band narrowing excimer laser for microlithography.
In this paper, simultaneous frequency, power and beam direction stabilization of transverse Zeeman Laser is reported. The axial mode of He-Ne laser is collapsed into two polarized components perpendicular to each other in a transverse imagination field. By setting up a proper thermal field, taking a frequency difference, between the two composts as the stabilization parameter, the output angle floating is less than 2 X 10-4, the relative power stability of 0.089% and the qencystability of 4 X 10-10 is obtained.
Are known powerful technological YAG:Nd-lasers with passive Q-switching on a LiF:F2- crystal. At use of the unstable resonator with a convex mirror was received single- mode pulse-periodic radiation with high spatial brightness, small divergence and large coherent length. The phase conjugation of technological Nd-lasers on the dynamic hologram in a LiF:F2- crystal has allowed to increase efficiency of laser processing. In the present work the modes of generation and technological opportunities of YAG:Nd-lasers and on Nd-glass c by adaptive resonators on a basis of Sagnac interferometer (SI) are investigated at pulse-periodic pumping. Phase conjugation and passive Q- switching were carried out simultaneously with the help of a LiF:F2- crystal, placed in SI. Crystal transparency linearly changes on length, therefore by moving of a crystal normally to an optical axis it is possible over a wide range to operate power and temporary parameters of laser radiation. High power and spatial parameters of radiation, as the opportunity of their control, directly during processing by crystal moving, have allowed effectively to carry out the calibrated drilling in various materials by a diameter 50 - 400 micrometers and depth up to 20 mm. For increase of efficiency of fragile materials destruction the combination of spatial and temporary modulation of laser radiation was used. The formation in a volume or on the material's surface of alternating areas with high gradients of thermally induced pressure has allowed to decrease the destruction's power threshold in some times. Besides by optimization of spatial parameters it is possible to improve quality of laser processing by decreasing thermally induced pressure and deformations.
Dispersion optical resonators of tunable lasers often contain one or more plane dispersive elements (PDE), e.g., refracting boundaries or diffraction gratings. The characteristic feature of such structures is that their optical length varies over resonator cross-section, this causes specific diffraction distortions of the light beam. But conventional ideas concerning processes in dispersion resonators are based on the analysis of simplified models, where each PDE conditionally is removed or replaced by certain thin optical element, so that particular nature of PDE is neglected. An attempt of systematic theoretical consideration of optical resonator with PDE is presented. On the base of general formulae for paraxial beam transformation in PDE the integral equation describing wide class of dispersion resonators is derived. It is analyzed and discussed; for some cases there are given approximate analytical and numerical solutions. They demonstrate slight asymmetry in beam profile and in dependence of resonator losses on its misalignment. Besides, it is shown that tuning of the resonator can be changed not only by angular but by certain progressive shifts of resonator elements. These effects can give new possibilities for the control of tunable laser radiation and must be allowed for in precision measurements.