Improved Beam Quality Research Articles

Beam-shaping technique for improving the beam quality of a high-power laser-diode stack

We report a beam-shaping technique that reconfigures the beams to improve the beam quality and enhance the power density for a ten-array high-power laser-diode stack by using two optical rectangular cubes and two stripe-mirror plates. The reshaped beam has threefold improvement in beam quality, and its power density is effectively enhanced. On the basis of this technique, we focus the beam of the high-power laser-diode stack to effectively end pump a high-power fiber laser.

Beam quality improvement at Raman conversion of multimode conical beam

The spatial structure of Raman-converted radiation was investigated numerically and experimentally for the case when the pump is a conical beam produced by an axicone from multimode laser radiation. Both conical and axial Stokes beam generation was found to be possible. The improvement of the laser beam quality was demonstrated numerically and supported by experiment on Raman conversion in barium nitrate crystal. The laser beam with the beam quality factor M2=11 was converted into the axial Stokes beam with M2=3.5 at the conversion efficiency of 20%. A two-times increase in the Stokes beam brightness was obtained.

Improvement of electron beam quality in optical injection schemes using negative plasma density gradients

Enhanced electron trapping using plasma density down-ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector is examined in detail using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over distances greater than the plasma wavelength leads to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength--e.g., approximately 5 fs), high charge ( > 20 pC at modest injection laser intensity approximately 10(17) W/cm(2)), with a relative energy spread of a few percent at a mean energy of approximately 25 MeV, and a normalized root-mean-square emittance of the order of 0.5 mm mrad.

Self-compensating amplifier design for cw and Q-switched high-power Nd:YAG lasers

We experimentally demonstrate a self-adaptive compensation of the pump power dependent thermal lens in an Nd:YAG laser through a thin layer of a medium with a negative temperature dependence of the refractive index. The layer is thermally coupled to the laser rod and leads to a strikingly improved beam quality over a large stability range. The scheme allows for a scaling to high powers as well as pulsed-mode operation.

High-power Yb-doped multicore ribbon fiber laser

A highly elongated double-clad ribbon fiber that comprises a pure-silica inner cladding with transverse dimensions of approximately 1.4 mm by 0.23 mm with a linear array of ten ytterbium-doped cores has been fabricated and operated in a simple laser configuration pumped by two diode stacks. The fiber laser yielded 320 W of output power at a center wavelength of 1045 nm in a combined beam with beam propagation factors of approximately 2 (perpendicular to the array) and approximately 150 (parallel to the array) for 576 W of launched pump power. The slope efficiency with respect to absorbed pump power was 62%. The prospects for further power scaling and improved beam quality and efficiency are discussed.

Control of spatial polarization by use of a liquid crystal with an optically treated alignment layer and its application to beam apodization

We have investigated the alignment of a liquid crystal whose orientation is controlled by photoisomerization reaction for use in developing optical devices to improve beam quality. A glass window of a liquid-crystal cell that is coated with poly(vinyl alcohol) doped with azo dye was illuminated with a Hg lamp. We confirmed the dependence of the spatially controlled alignment direction of a liquid crystal on the irradiation time of this ultraviolet light. The new azo dye used in this study substantially reduced the illumination energy density required for aligning liquid-crystal molecules. We have demonstrated the control of polarization and successfully fabricated a serrated apodizing aperture and a soft aperture.

Efficient improvement of laser beam quality by coherent combining in an improved Michelson cavity

The efficient improvement of laser beam quality is demonstrated by coherent combining of two Nd:YVO4 lasers in an improved Michelson cavity. When the two lasers are separately operated with beam qualities of M(x)2 = 1.74 and M(y)2 = 1.34 for laser 1 and M(x)2 = 1.53 and M(y)2 = 1.39 for laser 2, by coherent combining of the two lasers, a single beam output exceeding 2 W with a nearly perfect TEM00 mode (M(x)2 = 1.08, M(y)2 = 1.04) is obtained. Moreover, it can withstand environmental perturbations with no change in beam quality or output power and has the potential for scaling to much higher output power with high beam quality by coherent addition of multiple lasers in this configuration.

Modeling of patterned contacts in tapered lasers

We study the influence of a patterned contact design on the beam properties of high-brightness tapered lasers. A simple approach to simulate patterned injection has been developed and introduced into a quasi-three-dimensional tapered laser model. The method is applied to tapered lasers, whose performance is limited by two different mechanisms: self-focusing and poor modal filtering. The results show an improvement in beam quality in comparison to standard devices.

Improvement in ion beam quality for thin-film formation

Low-energy ion beam technique is useful for the study on ion–surface interaction and deposition processes. In the low-energy ion beam system, ions are accelerated and decelerated to a desired energy just before deposition. The high-energy neutrals contained in the low-energy ion beam cannot be decelerated and make damage in the deposition film. By rejecting high-energy neutrals from a low-energy ion beam, improvement of the quality of the beam might be achieved. Thus, we modified the low-energy ion beam deposition system in order to reduce them. With this modification, the amount of high-energy Ar neutrals in a 100-eV Ar + ion beam was reduced to 0.5% in this work.

High power density beam from narrow diode-laser arrays in axial symmetry.

A multiplexing method based on narrow diode-laser arrays (DLAs) in an axially symmetric configuration is described. The use of submillimeter narrow DLAs improves beam quality considerably in the slow-axis direction compared with typical 1-cm-wide DLAs. The axially symmetric geometry is advantageous for efficient spatial, wavelength, and polarization multiplexing. With narrow DLAs and an axially symmetric geometry, a small circular focus with more than 10-kW/mm2 average power density is possible. Theoretical calculations and preliminary experimental results are presented.

High-power CO2 laser design “from resonator end mirror to cutting lens focal point”

An algorithm is derived that allows calculating all optical parameters relevant to the design of a high-power CO2 laser, taking into account the practical needs of a flying-optics type of laser cutting machine. The novelty of the approach taken is that the resonator and the beam delivery optics are treated simultaneously, which allows for greater flexibility in design. To begin with, the often-used optical configuration of a stable resonator combined with a beam expander for collimation is modeled and optimized. The remaining focal shift over the cutting table, mainly due to the fact that the part of the beam close to the resonator still has appreciable wavefront curvature variation, is calculated. The resulting function is programmed into the machine CNC, which continuously adjusts the lens position depending on the actual beam length, to compensate for this effect. Next, attention is paid to the well-known problem of changing beam parameters with thermal load of the output coupler. It is found necessary to model both mechanical deformation of the ZnSe window and lensing due to the temperature dependent index of refraction. The former is derived from a finite element model, for the latter, the formula of Miyamoto et al. [Proc. SPIE 1276, 112 (1990)] is used. Finally, as an example of novel resonator designs studied using this approach, the characteristics of an intracavity reflective telescope are determined for two different goals: first, to increase the fundamental mode spot size for the given resonator (and hence to improve beam quality without sacrificing power), and second to preserve beam quality when extra resonator layers are added to increase power. The beam is modeled using the concept of the “embedded Gaussian” and the “complex radius of curvature” due to Siegman [Lasers (University Science Books, CA, 1986)].

Dynamic filamentation and beam quality of quantum-dot lasers

We present a comparative study of numerical simulations and experiments on the spatiotemporal dynamics and emission characteristics of quantum-well and quantum-dot lasers of identical structure. They show that, in the quantum-dot laser, the strong localization of carrier inversion and the small amplitude–phase coupling enable a significant improvement of beam quality compared to quantum-well lasers of identical geometry. Near-field profiles and beam quality (M2) parameters calculated on the basis of time-dependent effective Maxwell–Bloch equations into which the physical properties of the active media are included via space-dependent material parameters, effective time constants, and matrix elements are fully confirmed by experimental measurements. Together they indicate that, in the quantum-dot laser, the strong localization of carrier inversion and the small amplitude–phase coupling enable a significant improvement of beam quality compared with quantum-well lasers of identical geometry.

Iterative method for the design of a dual-phase-conjugation-mirror resonator with multiple apertures.

Apertures have been used to select the low-order transverse modes in resonators. The additional diffraction losses result in a change in the transverse-mode structure, and the presence of apertures inside a resonator generally distorts the mode shape. The optimization of a multiple-aperture resonator demands an approach that differs from the conventional method in which the mode theory is used. We demonstrate an iterative design method to find optimal phase profiles for the reflector surfaces to build a resonator with multiple apertures to produce a lowest-order mode with much smaller diffraction loss and to satisfy the phase-conjugation conditions at the mirrors. The results are compared with conventional stable resonators, and we show that substantial improvement in round-trip loss and beam quality can also be obtained.

Spatial Coherence and Thermal Lensing in Broad-Area Semiconductor Lasers

Injection profiled broad-area edge-emitting semiconductor lasers demonstrate single transverse mode operation and near-diffraction-limited beam output when driven by pulsed pump current. Thermal effects arising from CW operation induce filamentary dynamics, thus degrading the beam. Transition from the stable nonthermal to the unstable CW regime is analyzed experimentally and numerically, and techniques to improve beam quality in the thermal regime, based on feedback or thermal profiling, are proposed.

Highly sensitive beam quality measurements on large-mode-area fiber amplifiers

The beam quality of large-mode-area fiber amplifiers was investigated at the 10 W power level using a tunable ring cavity, that is also used in the laboratory system of GEO600 as a pre-mode-cleaner for mode filtering. More than 98% of the overall output power were contained within the polarized (200:1) TEM00 mode with an appropriate choice of coiling diameter. With the high sensitivity ring cavity analysis, the beam quality improvement caused by decreasing the coiling diameter was verified, while this could not be seen within conventional M2-measurements. The results are compared with the properties of single-mode fibers.

Multifunctional gratings for surface-emitting lasers: design and implementation.

We experimentally demonstrate the use of two different multifunctional grating couplers in surface-emitting lasers for improved beam quality and advanced beam profiles. The lasers used for the demonstration are grating-based unstable resonator lasers, each with a grating coupler for surface emission and beam shaping. The new design method, described in detail, allows for simultaneous optimization of arbitrary feedback and outcoupling characteristics of the grating coupler. The first coupler is designed to reduce feedback to the resonator that would otherwise disturb the operation of the laser and lower the beam quality and to produce an output beam focused to four spots. The second coupler is designed to provide the feedback needed to support the unstable resonator, eliminating one feedback grating, and simultaneously focus the output beam to a single spot. As far as we know, this is the first time such multifunctional couplers are used in grating-coupled surface-emitting lasers. The couplers provide near-diffraction-limited spots that are a considerable improvement compared with previous lasers with no feedback control in the couplers.

Increasing the power density and quality of laser radiation

The results of studies on the improvement of the laser beam quality initiated by N.G. Basov and performed at the Laboratory of Quantum Radiophyisics (FIAN) are briefly discussed.

Comparison of adaptive optics and phase-conjugate mirrors for correction of aberrations in double-pass amplifiers.

Correction of birefringence-induced effects (depolarization and bipolar focusing) were achieved in double-pass amplifiers by use of a Faraday rotator between the laser rod and the retroreflecting optic. A necessary condition was ray retrace. Retrace was limited by imperfect conjugate-beam fidelity and by nonreciprocal refractive indices. We compared various retroreflectors: stimulated-Brillouin-scatter phase-conjugate mirrors (PCMs), PCMs with rod-to-PCM relay imaging (IPCM), IPCMs with astigmatism-correcting adaptive optics, and all-adaptive-optic imaging variable-radius mirrors. Results with flash-lamp-pumped, Nd:Cr:GSGG double-pass amplifiers showed the superiority of adaptive optics over nonlinear optic retroreflectors in terms of maximum average power, improved beam quality, and broader oscillator pulse duration/bandwidth operating range. Hybrid PCM-adaptive optics retroreflectors yielded intermediate power/beam-quality results.

Improvement of the spatial beam quality of laser sources with an intracavity Bragg grating.

We demonstrate a novel method for improvement of spatial beam quality by use of the angular selectivity of a thick phase grating inserted into various laser cavities. Preliminary experiments with an intentionally degraded beam from a Nd:YVO4 laser have enhanced beam propagation factor M2 by a factor of 4.5. Operation with a broad-area laser diode in an external cavity with a thick grating was achieved. We obtained 100 mW of cw power in a beam with an M2 of 1.3. Moreover, a reduction of the output spectrum by as much as a factor of 3.8 was obtained.

A second-generation ion beam buncher and cooler

A radiofrequency quadrupole (RFQ) ion accumulator and buncher has been designed for the low-energy beam and ion-trap (LEBIT) facility which is being set up at the NSCL/MSU. The LEBIT buncher will be a cryogenic system. Compared to room-temperature systems an improved beam quality and overall efficiency are expected. It will feature a novel electrode structure with a drastically reduced number of electrodes for simplified operation. Its design is presented and Monte-Carlo type ion-trajectory calculations are discussed which predict excellent beam quality and high performance.