The usage of a quality management, in combination with a standard certification, is nearly inevitable for today's industrial manufacturing. In laser materials processing, a periodical beam diagnosis is to be executed as a quality-maintaining measure with any change of the workpiece geometry to guarantee an unambiguous allocation of the beam quality factors. Otherwise changes in the beam quality, caused by pollution, aging or defect of the optical components, remain unidentified for a long time, leading to impairments of the treatment quality or even costly down-times. As a solution a diagnosis system is integrated into a laser system. Data sources like measuring instruments, sensors and laser control transmit the diagnosis data to a diagnosis PC. A user-friendly software, based on Fuzzy algorithms, enables the operator to retrace changes in the beam quality to failures of the laser system. All diagnosis data are getting archived in a databank. The access to the archived data through the World Wide Web allows remote diagnoses. With the help of the beam diagnosis system failures can be discovered in advance, and losses of production can be avoided. The gained transparency of the beam characteristic values facilitates the integration of the laser system in the quality management. A prototype installation has been realized and latest results will be demonstrated.

Industrial Lasers must have high reliability and repeatability so that the process for which they are to be used is profitable and successful. Current methodology for measuring the performance of high power industrial lasers is incapable of real time data. We describe two new instruments; a new on-line, in-line laser beam performance monitoring system that not only calculates M² and divergence angle, but also provides real time beam profiling, and an at-line beam profiling system that can calculate beam widths and many other critical parameters in real time. The information obtained from these measurements allows the operator to monitor beam performance in real time, speed tuning and adjustment. more accurately predict maintenance intervals and aid in troubleshooting malfunctions. Actual examples of different lasers will show the value of these systems.

The knowledge of such beam parameters as beam diameter, divergence angle, beam propagation factor M² is very useful for many laser applications. Thus it is necessary to know these parameters of a laser beam while doing some laser experiments or for characterization a laser beam of industrial lasers. We developed and constructed a low-cost M² -sensor allowing to measure the main parameters of a laser beam. The algorithm of the measurements is based on double and multiple beam diameter measurements. Beam diameter is defined as the second moment of the intensity distribution function of the beam at some cross-section. The software of M² -sensor includes options for evaluation of short-term and long-term laser power stability. It is also possible to do Gaussian and flat-top fit to the beam intensity that is used to evaluate how close the beam is to TEM₀₀ mode or to uniform beam.

We will report experimental results on the coherent laser operation of a bar of broad area lasers. The array is 1 cm wide and consists of 25 lasers of 200 micrometer width each. Under normal conditions the lasers possess no mutual coupling and, therefore, emit incoherently. The resulting beam quality is correspondingly very low and typically more than 2000-fold diffraction-limited. To coherently couple the emitters we operate them in an external cavity. Inside the cavity a multiplexing and mutual coupling is achieved by means of a 16 level diffractive optic (DO) designed as a 1:25 beam splitter. We show that a stable phase-locked laser mode can build up inside the resonator. Its pronounced central angular emission lobe possesses a beam quality of approximately 60 times diffraction limit. In a passive setup, in which the internal dynamics of the broad area lasers can be suppressed by use of a seeding laser of good beam quality (M²<2), the central emission lobe is better than five times diffraction-limited.

RF CO₂ slab lasers with waveguide-unstable resonators demonstrated last years to be an effective tool for many applications. Increase of the total power using technique of phase locking of lasers can be achieved without deterioration of beam quality. Till now, no attempt was made to develop such technique for slab-laser arrays. Authors propose to arrange optical coupling between individual resonators using beams diffracting over the edge of convex mirror in asymmetric unstable resonator. 3-dimensional diffraction code was applied taking into account refractive index gradients and gain saturation effects in combination with diffraction and waveguiding effects in a whole optical tract including the coupling channel. In-phase and out-of-phase mode competition was studied in dependence on parameters of resonators and active medium, assuming the lasers were identical. Key parameters for phase locking are defined: optical coupling channel length and position of the resonator optical axis relative to the nearest edge of the convex mirror. The numerical simulations evaluate tolerances for the length of optical coupling channel. A range of the key parameters providing in-phase mode stable operation at high above threshold conditions is found for a particular laser construction. An option to generalize optical coupling method to N-slab-laser array is discussed.

Defocusing, angular, and linear alignment tolerances, and the effects of transform lens aberrations are calculated for spectral beam combining of fiber lasers, first for a cavity in which no microlens array is used in the collimating optics. Two design criteria are derived and used to compare four lenses: a simple bi-spherical lens, a compound quadruplet, a plano-aspheric lens, and a plano-parabolic lens. The results point to superior performance of simple aspheric lenses over compound lenses with spherical surfaces. A kind of equivalence of efficiency and beam quality is demonstrated. Partial results on the inclusion of a microlens array for improved beam collimation predicts marked increase of the maximum array size and overall efficiency.

Diode laser arrays present challenges to delivering maximum brightness laser energy to remote sites. Coupling with optical fibers is key to achieving this goal. Fiber core diameters are chosen to capture all the energy from the slow axis, with fiber placement and/or lensing to assure capture output from the fast axis. Multiple fibers are then bundled as tightly as possible and their output focused into a single output fiber. The optimum brightness is achieved by using as small as possible bundle dimensions before reduction to the output fiber's dimensions. A major aim is thus to minimize jacketing and cladding thickness. Data and analysis of the effects of cladding thickness on the spectral transmission of optical fibers having core diameters between about 100 micrometers to about 300 micrometers are presented. Particularly below a 200 micrometers core, cladding thickness can significantly alter the transmission of laser energy in the visible and near infrared spectral regions, especially between 600nm and 1700 nm. Data primarily deals with low-OH, 'water-free' fibers having cladding thicknesses between 5 to 20micrometers . Especially for fibers having cladding/core ratios below 1.2, care must be taken to either use core sizes approaching 200 micrometers or work in the UV or lower visible wavelength region. Further guidelines are given below.

We report a monolithic low threshold 482nm Tm:ZBLAN upconversion fiber laser. The laser cavity consists of a directly coated single-mode fluoride fiber. The vapor deposit coatings significantly reduce the coupling losses and are suitable to be pumped by laser diodes. The laser operation and threshold characteristics have been investigated. The output stability and beam quality was tested.

Coherent combination of free-space beams from continuous-wave, 1547 nm, narrow-band, single-mode fiber-lasers was experimentally investigated. Beams from fiber lasers (in a master oscillator/ power amplifier arrangement) were collimated. The beams were then overlapped on a video camera to form an interference pattern. The interference pattern drifted slowly under lab conditions. The drift was observed to mainly be due to: a) wavelength drift of the master oscillator combined with an optical path difference and b) small thermal fluctuations in the optical fibers that, in turn, cause wavefront phase changes. Images from the video camera were acquired by computer and analyzed in real-time. A phase control signal was fed back to a fiber stretcher to achieve a stable interference pattern. The interference pattern peak could also be steered within the beam overlap region to a desired location.

A lenslike diverging medium is potentially useful in semiconductor lasers. A laser of this type was previously fabricated and tested. The performance was substantially better than a comparable laser without a lenslike region, although refinements of the design were needed to obtain an output beam that was essentially diffraction limited. For this type of laser, the structure of the wafer is designed so that the effective refractive index depends on x, the lateral distance from the center of the wafer. The shape of the wavefronts of the mode depends on the functional form of the index of refraction. In principle, the index can be tailored so that the wavefronts will be cylindrical. In practice, this profile will not be achieved perfectly, and it is important to be able to calculate the aberrations that are introduced. It is also useful to calculate the index profile that would result in ideal cylindrical wavefronts. Equations are derived for the mode wavefronts given the index profile and for the index profile that will result in a given wavefront, and examples are discussed.

Industrial high precision micro-material processing with solid-state lasers needs the reliable and efficient generation of high brightness laser beams. The key problems for this goal are the control of the thermal effects (lensing) in the active material and the overlap efficiency between the resonator mode and the pumped active material volume. Modern solid state lasers with low thermal effects such as zig-zag slab lasers have non circular geometries difficult to adapt for high efficiency and brightness simultaneously. Resonators comprising a cylindrical telescope, resulting in an elliptical beam section in the active material of rectangular geometry but nevertheless a circularly symmetric output beam can increase the efficiency and beam quality and also compensate for eventual astigmatic effects of the active medium. These lasers yield therefore TEM00 output beams (pulsed, free running) with a beam quality of M²<1.7, pulse powers up to several kW and intensities up to 500MW/cm² for a spot diameter of 10 to 15micrometers . Such lasers are ideally suited for industrial high precision cutting and drilling, but also for quasi-cw harmonic generation, where the beam quality influences directly the conversion efficiency via the limited angular phase matching acceptance angles. Laser cutting with fundamental mode Nd-YAG lasers at 1.06micrometers and at 532nm (SHG efficiencies up to 17%) yields a minimal kerf width down to 15 micrometers and heat affected zones of less than 2micrometers .

To change the intensity distribution of the fundamental mode in a Nd:YAG laser resonator to a top-hat profile we developed and used a dielectric graded-phase mirror. A super-Gaussian mode of the sixth order was generated by means of a graded-phase mirror with a simple ring-shaped phase step on a spherical reflector. The depth of the ring was 90 nm. Measurements with continuous-wave and repetitively pulsed diode-laser pumping were performed and compared. The results are in excellent agreement with the theory. To allow for real-time modification of the modes, adaptive optics such as deformable mirrors can be used. Experiments with an adaptive mirror featuring eight actuators on a glass substrate were performed. It was not yet possible to generate super-Gaussian modes with a deformable mirror but the beam quality in the multi-mode regime was improved significantly without closed-loop electronics. A reduction of the M² value by 39 % was achieved.

A new design of the laser resonator using dual phase conjugation mirrors is discussed. It is shown that using the modified Gerchberg-Saxton algorithm, phase conjugation criteria at the both mirrors with finite apertures can be met for the given shape of desired mode. The flexibility of the design method allows the method can be applied to any other resonator design using either transmissive or reflective element(s) inside the cavity to satisfy the given amplitude/phase requirement of desired mode at any location. Unlike previous studies on the phase conjugation resonators, the new dual phase conjugation resonator is shown to provide extremely low loss for the desired mode while providing higher discrimination against the higher order modes. This approach to designing a resonator has been applied to the design of a large Fresnel number cavity for a ideal diode pumped laser with the objective achieving a large filling factor with a single mode and a design example will be presented.

In order to compensate for the thermally induced lenses in high-power laser rods we investigate self-adaptive techniques based on thermo-optical processes. Recently we have demonstrated that the influence of the thermal lense in high-power lasers can be reduced significantly by means of a thin liquid layer located within the resonator. Here we report on the investigations of different liquids and gels for the generation of the adaptive lens and discus an improved implementation of the technique, with the compensating layer placed directly in contact with the laser rod.

The rod geometry for high power solid state lasers has been proven to be both, reliable and reasonably inexpensive. On the other hand, setting up rod lasers with excellent beam quality implies a number of problems that have to be tackled. One of the most serious problems for isotropic crystals like Nd:YAG might be the effect of depolarization and bifocusing due to thermally induced birefringence (tib). The effect of tib can be compensated by a 90 degree(s) polarization rotation between two identical rods. However, because of the finite length of the laser rods in an optimized compensation scheme, matching the two paths in the two laser rods becomes necessary. By doing this, one wide stability region is yielded. The laser behaves like a single rod laser, and the beam path for the radial and tangential eigenmode becomes the same. Thereby, excellent beam qualities at high output powers can be achieved. Our Nd:YAG double rod system provides a maximum average output power of 180 W with a beam propagation factor of M²< 1.2. It is quasi cw diode pumped with a repetition rate of around 1 kHz and a pump pulse length of 250 microsecond(s) . The laser heads contain 3 star like arranged sets of diode bars being set up in a complementary position to each other. The maximum average pump power per laser head is 800 W.

In this report, a Nd:YLF laser with pulse duration of 2 ns, synchronized with a femtosecond laser is discussed. The pulse duration and synchronization are provided by two Pockels cells, in which voltage pulse is synchronized with the femtosecond laser by means of a special electronic unit. One Pockels cell ensures Q-switching, whereas the other cuts a shorter pulse out of a Q-switched pulse with 15ns duration. In such an approach, the requirements for jitter between a Q-switched pulse and a Q-switched pulse are not so strict and may be easily fulfilled. Experimental results presented herein show that the suggested two-step scheme of synchronization of a Q-switched laser and a cw laser with passive mode-locking may simply and reliably synchronize these lasers with jitter better than 100 ps.

We present electro-optic modulator based on toroidal lithium niobate cavity with whispering-gallery modes, superimposed with stripline resonator. With microwave resonance (quality-factor Q ~ 10²) tuned to the free spectral range of optical modes (Q ~ 5x10⁶), controlling power ú10mW is achieved in 9GHz prototype, and preliminary results with 33GHz prototype are obtained. Further efficiency improvement will enable various applications in microwave photonics.

Er:ZBLALiP microsphere laser has been studied under pumping by a fiber taper at 1480 nm. Whispering Gallery Mode laser spectra were analyzed for different sphere diameters and Erbium concentrations (from 0.01 % to 0.2% by mole). The gain spectrum is calculated for the transition ⁴I_13/2->⁴I_15/2 around 1550 nm. Red-shift effect on the frequencies of both fluorescence and laser spectra is experimentally observed when the pump power is increased, originating from thermal effects. Temperature inside the microsphere cavity and thermal expansion coefficient were determined by employing the intensity ratio of two green up- conversion emission centered at 526 and 550 nm respectively which quantitatively explain this red-shift.

Measurements of the optical spectra of semiconductor injection lasers with deformed cylinder resonators show direct and unique signatures of laser action on scar-modes and of the Kolmogorov-Arnold-Moser (KAM) transition from integrability to chaos. The diode lasers studied in the present work operate with transverse electric (TE) polarization resulting in laser action on (partially) chaotic whispering gallery modes for all deformations. This observation is in contrast to the stable and quasi-stable, bouncing-ball type bow-tie modes of unipolar semiconductor lasers having the same resonator geometry but emitting with transverse magnetic (TM) polarization.

Optical sensing of biomolecules on microfabricated glass surfaces requires surface coatings that minimize nonspecific binding while preserving the optical properties of the sensor. Microspheres with whispering-gallery (WG) modes can achieve quality factor (Q) levels many orders of magnitude greater than those of other WG-based microsensors: greater than 10¹⁰ in air, and greater than 10⁹ in a variety of solvents, including methanol, H₂O and phosphate buffered saline (PBS). The presence of dyes that absorb in the wavelength of the WG excitation in the evanescent zone can cause this Q value to drop by almost 3 orders of magnitude. Silanization of the surface with mercapto-terminal silanes is compatible with high Q (>10⁹), but chemical cross-linking of streptavidin reduces the Q to 10⁵-10⁶ due to build-up of a thick, irregular layer of protein. However, linkage of biotin to the silane terminus preserves the Q at a ~2x10⁷ and yields a reactive surface sensitive to avidin-containing ligands in a concentration-dependent manner. Improvements in the reliability of the surface chemistry show promise for construction of an ultrasensitive biosensor.

Rather powerful output-stabilized sealed-off cw Co-lasers tunable over lines in a wide spectral range find ever expanding applications to solving numerous problem sets in ecology monitoring, medicine, spectroscopy, metrology, nonlinear optics, communication systems, laser chemistry, diagnostics of active media of high-power CO lasers and other areas. We have created an automatic output-stabilized CO-laser model tunable within a broad spectral range (~300sm^-1) and based on a sealed-off (i.e. without need for mixture flow and re-filling) active element. The main model characteristics are: total of lines to be selected for single-wave generation - about 90; tuning order - regular or irregular - to be set manually or under the program guidance and operated by computer; cw power in single strong spectral lin - up to 2,000 mW; possibility to operate both at negative and room temperatures; possibility to operate on isotopically replaced molecule CO; long- lasting output power instability - better than 1%; radiation source structure - with enhanced anti-acoustic resistivity; service life - several thousands of hours (about 3,000- 5,000).

We demonstrated generation of square-shaped ultraviolet pulses and that they reduces the emittance of electron beams from laser-photocathode RF-gun. Some frequency components of femtosecond pulses from a Ti:sapphire oscillator are modulated with a pulse shaper that consists of a lens pair, gratings, and a liquid crystal spatial light modulator (LC-SLM) to generate square-shaped pulses. The shaped pulses were amplified up to 3 mJ through a regenerative amplifier, and were converted to ultraviolet region with two different nonlinear crystals. Energy of the shaped pulses was about 100 (mu) J which is sufficient to generate electron charge of 1 nC. In our experiment, emittance of electron beam was reduced to as low as almost the half of that with non-shaped pulses.

We report on the measurement of small stress fluctuations in loaded fused silica fibers using high-Q optical microcavity sensor with whispering-gallery modes. Axial force applied to the spherical whispering-gallery mode microcavity by the fiber attached in the polar area displaced the frequencies of whispering-gallery mode resonances, and enabled high-sensitivity measurement of the stress force variations. With quality factor of integrated microcavity sensor of (2-7)x10⁷, the achieved spectral sensitivity of force measurement was <1x10^-8 N/(root)Hz in the 10²-10³ Hz frequency range and about 2x10^-10N/(root)Hz above 10⁴ Hz.

The use of semiconductor saturable absorbers has emerged as an enabling technology in modern passively modelocked laser systems. Their application to high power picosecond lasers, most notably Nd-doped lasers, has produced systems with average power levels of a few tens of watts. In this paper, the development of these laser systems to the 100W level and above will be outlined.

We describe a novel scheme consisting of two deformable mirrors that can free ultrashort laser pulses from simultaneously present severe wavefront distortions and strong intensity-profile modulations. This scheme is applied to the MPQ 10-TW TiS laser facility ATLAS. We demonstrate that thereby the focusability of the ATLAS pulses can be improved from 10¹⁸ to 2x10¹⁹ W/cm² without any penalty in recompression fidelity by using adaptive optical system.

Planar photonic crystals are constructed by combining two-dimensional periodic structures with high refractive index contrast slabs. By suppressing the loss in these structures due to imperfect confinement in the third dimension, one can fully take advantage of their relatively simple fabrication, and achieve the functionality of three-dimensional photonic crystals. One of the greatest challenges in photonic crystal research is a construction of optical nanocavities with small mode volumes and large quality factors, for efficient localization of light. Beside standard applications of these structures (such as lasers or filters), they can potentially be used for cavity QED experiments, or as building blocks for quantum networks. This paper will address our theoretical and experimental results on optical nanocavities based on planar photonic crystals, with mode volumes as small as one half of cubic wavelength of light in material, and with Q factors even larger than 1x10⁴.

The artificial resonance of optical ring resonators can be used to enhance the nonlinear phase shift resulting from a third-order material response. This enhancement results form the interaction of two effects: the internal intensity build up within the resonator which introduces a single pass nonlinear phase shift, and the resulting detuning of the resonance which causes a change in phase at the outputs. Devices consisting of multiple resonators in series can provide additional improvements in terms of allowing a large phase shift of the output. As compared to same material in bulk, devices consisting of single and multiple ring resonators constructed from an absorbing material can provide a much greater nonlinear phase shift, in some case by many orders of magnitude. In addition, these phase shifts exceed the maximum value allowed by absorption in bulk materials.

This report discusses the design and installation of a static wavefront correction optic on the HELEN laser at AWE. The element is designed to compensate for static phase errors and prompt thermally induced aberrations on the backlighter beam of the laser. Partial compensation of cooling effects is also included in the design. A phase element has been fabricated using a recently developed novel wet etch figuring tool at LLNL. Performance evaluation through comparison of the focal spot pre- and post-installation is provided. The element has been tested on the laser to produce a 2x reduction in focal spot size.

We investigate the influence of thermally induced spherical aberrations on the fundamental mode of a rod solid-state laser. Results concerning the additional losses and beam quality degradation are presented and point out that for a large volume fundamental mode resonator, a spherical aberration greater than 0.5(lambda) dramatically deteriorates the laser performances. To control the resonator performances, we present a new phase control technique with an intracavity optically addressed liquid-crystal spatial light modulator. The presented Nd:YAG resonator is able to deliver beams with various spatial profiles, as flattop super-Gaussian or square-shaped beams, and is thus potentially able to compensate for the thermally induced aberrations of the laser medium.