Fast Axis Collimation Research Articles

Variable magnification beam expander system based on semiconductor laser

Due to the different divergence angles in two perpendicular directions and the presence of astigmatism in edge-emitting semiconductor lasers, the communication distance and efficiency of underwater communication are seriously affected. Therefore, based on the fact that the semiconductor laser beam belongs to Gaussian beam, satisfying Gaussian characteristics and collimating and expanding characteristics, a system composed of fast-axis collimation mirror and variable magnification beam expander is designed. Simulation is performed, and the final divergence angle is calculated. Experimental results show that the divergence angles of the fast axis and the slow axis have been compressed from 49 and 9, respectively, to 0.315 mrad and 0.180 mrad. Simulation demonstrates that the system can achieve propagation over a distance of 100m underwater and solve the alignment difficulty of APT. The designed structure is compact, easy for processing and adjustment, with high practical value, which helps to solve the problems of optical energy loss and low coupling efficiency in underwater long-distance communication.

The SMILE Effect in the Beam Propagation Direction Affects the Beam Shaping of a Semiconductor Laser Bar Array

Near-field bending of a laser diode bar (i.e., the SMILE effect) degrades the laser beam brightness, adversely affecting optical coupling and beam shaping. Previous reports mainly focused on the two-dimensional near-field bending of a laser diode bar. However, the near-field bending of a laser diode bar not only occurs in the laser bar growth direction, but also in the beam propagation direction. The present article proposes the three-dimensional near-field bending of a laser diode array, which is commonly known as the three-dimensional spatial SMILE effect. Through theoretical and simulated investigations, it has been found that a laser bar array not only deforms in the fast axis direction to cause the traditional two-dimensional SMILE effect but also experiences an additional deformation of approximately 2 μm in the laser emission direction simultaneously. Due to the SMILE effect in the beam propagation direction, not all emitters are aligned in a straight line, and some emitters experience defocusing during collimation. Consequently, there is an increase in the residual divergence angle and beam width, resulting in a degradation of the laser bar array’s beam quality. According to the theoretical calculations, ZEMAX simulations, and experimental results, for a FAC (fast axis collimation) with a focal length of 300 μm, the divergence angle of single emitter after collimating in the fast axis increases from 4.95 mrad to 6.46 mrad when the offsetting of the working distance between the incident beam waist and FAC lens increases from 0 μm to 2 μm.

Immersion laser separation: Enhancing efficiency and quality in cutting irregular lenses

Irregular lenses are commonly employed in various beam shaping, imaging, and other optical applications. They require secondary processing to meet specific design and assembly criteria. Bessel beam lasers have gained popularity for their ability to efficiently separate hard-brittle materials owing to their high processing quality and speed. However, the complex surface of irregular lenses creates challenges such as refraction and scattering, which impede the laser beam’s ability to form a laser center lobe inside the material. This paper proposes an innovative technique called immersion laser separation (ILS), where an irregular lens is immersed in a refractive index matching medium to create a compound with a uniform refractive index. This approach mitigates the impact of surface complexity on the optical path of the Bessel beam. The separation of a fast axis collimator (FAC) by ILS is used as an example to test the proposed technique. The distribution characteristics of the Bessel beam light field inside the lens material under different refractive index were investigated to find that the laser consistently forms a laser center lobe inside the material during cutting when the matching deviation is small. This results in a modified area that covers the entire surface. The maximum cutting speed can reach 50 mm/s, the cutting surface roughness is less than 700 nm, and the technique produces no chips or micro-cracks, ensuring uniformly high-quality separation. Experimental and simulation results are in close agreement, suggesting that the proposed technique is effective for the efficient, high-quality cutting of irregular lenses.

Inhomogeneous thermal analysis of microlayer mold for glass molding of fast-axis collimation lens array

Fast-axis collimation (FAC) lens arrays play a crucial role in laser systems, offering precise beam shaping and focusing for applications such as laser marking, and optical communication. Achieving high precision in these applications requires a FAC lens array with a long Rayleigh range. While precision glass molding with nickel-phosphorus (Ni-P) microlayer molds has revolutionized the mass production of FAC lens arrays, the extreme conditions during the molding process can lead to poor profile accuracy. To address this issue, we developed an optical collimation system to derive the final characteristic parameters and obtain the beam waist radius and Rayleigh range of the collimated laser. Additionally, we conducted a thorough analysis of the inhomogeneous thermal expansion process of the microlayer mold and established a mathematical solution for thermal compensation to modify the mold's shape. We validated the proposed method by fabricating an FAC lens array mold and measuring the profile error and waist radius of a molded FAC lens array. We also calculated the profile error of this method using a finite element (FE) model in ABAQUS. The results indicate that incorporating the inhomogeneous thermal expansion reduces the shape error of the FAC lens array from 0.577 μm to 0.135 μm. Moreover, using inhomogeneous thermal compensation significantly improves the Rayleigh range of the FAC lens array from 1.285 μm to 9.8 × 104 μm.

Pulsed Diode Laser Minibars for Pumping Space-Borne Solid-State Lasers

Laser diode benches (LDBs) with lasing wavelength of 808 nm were developed, manufactured and endurance-tested for utilization as pump lasers for the Nd:YAG oscillator in the climate satellite MERLIN. The LDBs combine customized TE-polarized GaAs-based 5-mm minibars with fast axis collimation lenses to produce a reliable, space qualified component at optical power of 63 W per minibar. Accelerated life tests of 20 LDBs are presented. Except for a small increase in threshold current, no wear out in operating current or sudden failures were observed, and reliable operation over the full mission time of >4 gigashots was confirmed. The electro-optical characteristics remain in the specified ranges after exposure to the total operational load of the mission.

Micro-optics assembly for fast axis collimation by means of convolutional neural network.

The manufacture of high-power diode laser systems highly depends on the quality of the collimation of semiconductor laser diodes. The collimation process is conducted by precise alignment of micro-optics, where the first and the most critical step is the placement of the fast axis collimator (FAC) lens. The sub-micron positioning of the FAC lens is conventionally conducted with an active alignment strategy by monitoring changes in the laser beam profile with an external sensitive camera. The optimization of the beam profile characteristics is controlled by a specifically programmed motorized robotic aligner. However, active alignment, a very accurate method, often results in a higher cycle time than the passive approach, where the lens is placed in a pre-measured position. Here, we developed a new active approach, without closed loop control to position the micro-optics, that relies on the use of a pretrained convolutional network (CNN). We trained and evaluated three CNNs that can predict the optimal lens position using the single camera image of the laser beam. We predict that implementation of the best performing CNN-based model would lead to a decrease in alignment time from tens of seconds to hundreds of milliseconds and will be broadly applied in a high-volume manufacturing environment.

Fiber coupling technology of high brightness blue laser diode

With the development and application of blue semiconductor lasers, it has become a research hotspot to obtain high brightness blue light source by beam combining technology. In order to obtain high brightness blue light output, 48 single tube semiconductor lasers with wavelength of 450 nm and output power of 3.5 W are focused and coupled into 105 μm/0.22 NA fiber by fast slow axis collimation and spatial beam combination. The blue light with power of 144.7 W and brightness of 11 MW/(cm2str) is obtained. The coupling efficiency is 93.78%, and the optical to optical conversion efficiency of the whole system is 86.13%.

GPU-accelerated wave-optical model for external-cavity diode lasers.

The spatial distribution of the optical feedback field in an external-cavity diode laser (ECDL) is an important parameter influencing its lifetime and filamentation. From a modeling point of view, the evaluation of the full diffraction integral is required to calculate the propagation of light through thick lenses such as fast axis collimation lenses or beam transformation systems used for high-power diode lasers. In this Letter, we illustrate an algorithm for the accelerated computation of the Rayleigh-Sommerfeld diffraction integral on a graphics processing unit. We further show a validation of the results by measurements on an ECDL.

Fast-axis collimating lens recognition algorithm based on machine vision

In order to solve the problem of low efficiency and low accuracy of manually picked fast shaft collimation lens in actual production, the method of recognition of fast shaft collimation lens based on machine vision is studied. Using industrial camera to achieve image acquisition, using HALCON software to pre-process the image of fast-axis collimated lens, edge detection and other aspects of research, through the platform verification, the results show that the processed image is clear and easy to identify. In the lens placement is not accurate, the background is dirty can still ensure a better positioning effect, This study has some reference significance for visual recognition in the actual production of fast-axis collimation lens.

Laser-diode-array side-zigzag-pumped polygon thin-disk laser

The high-average-power diode-pumped solid-state laser is one of the main research directions in the field of international laser technology, and has major applications in the fields like space exploration, precise detection, fusion research, etc. Under high-power pumping conditions, the conventional rods or slabs are not conducive to effectively removing waste heat. And the thermal effect causes the quality of the laser beam to deteriorate, which limits the further increase of the output power. In this article, the polygonal Nd: YAG thin disk is taken as a gain medium for the laser, and experiments verify that the side pumping method can obtain a higher output power of the all-solid-state pulsed laser while ensuring high beam quality. The gain medium is a regular pentagonal Nd: YAG thin disk with a side-cut-angle of 45°, the crystal thickness is 1.5 mm, and the diameter of the inscribed circle on the front face is 16 mm. Five laser diode arrays are placed symmetrically around the disk, and the pump surfaces are parallel to the sides of the disk. The pump laser propagates along the zigzag path between the upper and lower surface of the disk, thus improving the absorptive efficiency and pump uniformity. Through the optimization study of its gain characteristics and optical characteristics, the high-efficiency high-uniform pumping is achieved. Along the pump light coupling transmission path, the fast-axis collimator is used to control the beams in the fast-axis direction to be nearly parallel, and the large-area pump light is compressed through the coupling structure of cylindrical lens and light guide to match the size of the thin disk, and the pump coupling efficiency measured experimentally is 97%. When the Nd<sup>3+</sup> doping concentration in the crystal is 0.3 at.%, the gain medium absorptive efficiency is 87%, and the root mea squared pump absorptive distribution uniformity in the gain medium is 3.21%. The fluorescence distribution of the gain medium is in good agreement with the simulated data. When the pump energy is 2.2 J, a laser output with an energy of 0.85 J is obtained, and optical-to-optical efficiency and slope efficiency are 38.8% and 40.1%, respectively. The single pulse energy stability is 2.7%(RMS) at 1 Hz frequency. In the stable cavity, the beam quality <i>β</i> - factor is measured to be about 10.

Influence of positioning errors of the laser collimator on the beam shape and coupling efficiency

The positioning error of optical components is one of the crucial factors affecting the beam quality for high-power semiconductor laser diode and the fiber coupling efficiency. This paper, derives the evaluation equations for the influence of collimator positioning error of single emitter laser array on light intensity distribution, and explores the effect of positioning error on the shape and position of beam spot through simulation. Moreover, the effects of positioning error on the coupling efficiency are researched by the experiment and are supported by simulations. The results show that the most influential positioning error, a 4μm displacement error in the y-axis of the fast axis collimator will decrease the coupling efficiency by 78.5%. Besides, the sensitivity analysis of each positioning error on coupling efficiency is investigated.

Influence of positioning errors of optical shaping components for single emitter laser diode on beam shaping effects

Beam shaping is required for semiconductor lasers to achieve high optical fiber coupling efficiency in many applications. But the positioning errors on optics may reduce beam shaping effects, and then lead to low optical fiber coupling efficiency. In this work, the positioning errors models for the single emitter laser diode beam shaping system are established. Moreover, the relationships between the errors and the beam shaping effect of each shapers are analysed. Subsequently, the relationship between the errors and the optical fiber coupling efficiency is analysed. The result shows that position errors in the Z axis direction on the fast axis collimator have the greatest influence on the shaping effect, followed by the position errors in the Z axis direction on the converging lens, which should be strictly suppressed in actual operation. Besides, the position errors have a significant influence on the optical fiber coupling efficiency and need to be avoided.

Automated UV-Epoxy-Based Micro-Optic Assembly for Kilowatt-Class Laser-Diode Arrays and Modules

This paper outlines the numerous interdisciplinary activities required to implement two ultraviolet (UV) epoxy-based automated micro-optic assembly tools within a “smart factory” for high-power semiconductor laser manufacturing. After a brief overview of the motivation and approach, two examples of automated micro-optic assembly tools [e.g., fast-axis collimation (FAC) lens attach and mirror stripe alignment] for kilowattclass laser-diode arrays and modules are given that demonstrate the breadth of automation activities required within a modern factory. For each system, the complete assembly sequence is outlined, and the steps that presented fully automated challenges are highlighted. In contrast, to the previous published results that focused on the details of the automated micro-optic alignment, the largest challenges primarily involved automation and repeatability of the UV-epoxy dispense and attach processes for these high-power semiconductor devices. These were solved by precise and stable mechanical design, customized lighting, and machine vision solutions. Finally, the complete integration of the tools into a “smart-factory” system is outlined.

High-power direct diode laser output by spectral beam combining

We demonstrate a spectral beam combining scheme based on multiple mini-bar stacks, which have more diode laser combining elements, to increase the combined diode laser power and realize equal beam quality in both the fast and slow axes. A spectral beam combining diode laser output of 1130 W is achieved with an operating current of 75 A. When a 9.6X de-magnifying telescope is introduced between the output mirror and the diffraction grating, to restrain cross-talk among diode laser emitters, a 710 W spectral beam combining diode laser output is achieved at the operating current of 70 A, and the beam quality on the fast and slow axes of the combined beam is about 7.5 mm mrad and 7.3 mm mrad respectively. The power reduction is caused by the existence of a couple resonator between the rear facet of the diode laser and the fast axis collimation lens, and it should be eliminated by using diode laser chips with higher front facet transmission efficiency and a fast axis collimation lens with lower residual reflectivity.

Application of Gaussian beam ray-equivalent model and back-propagation artificial neural network in laser diode fast axis collimator assembly.

The paper presents the development of a tool based on a back-propagation artificial neural network to assist in the accurate positioning of the lenses used to collimate the beam from semiconductor laser diodes along the so-called fast axis. After training using a Gaussian beam ray-equivalent model, the network is capable of indicating the tilt, decenter, and defocus of such lenses from the measured field distribution, so the operator can determine the errors with respect to the actual lens position and optimize the diode assembly procedure. An experimental validation using a typical configuration exploited in multi-emitter diode module assembly and fast axis collimating lenses with different focal lengths and numerical apertures is reported.

Multi-parameter processing tolerance theory of fast axis collimation cylindrical lens of LD

To achieve the high efficiency and accuracy of multi-parameter processing tolerance of fast axis collimation cylindrical lens used by semiconductor laser, we have established the corresponding theoretical model for multi-parameter theoretical model processing tolerances by geometrical optics method based on the theory of collimating the fast axis beam. Using the limit deviation of all the structural parameters as the initial value of tolerance, processing precision as the boundary conditions and optimizing the tolerance according to the requirement of design, the tolerances of all the parameters are set quickly and logically. Having designed the fast axis collimation cylindrical lens for a semiconductor laser with 36divergence in fast axis of TO-MOUNT model, we developed quickly the tolerance of the fast axis collimation cylindrical lens by the model in the paper. The simulation result in ZEMAX by importing the tolerance meets the design requirement.

High power high efficiency fiber coupled diode laser module

The high coupling-efficiency fiber coupled diode laser module is obtained by beam shaping. The wavelength of the CW diode laser bar is 976 nm, and five bars are collimated by a fast axis collimation lens. Then the beams are transformed and collimated by slow axis collimation optics. Then all the beams are combined in space, and focused into a multimode fiber (400 m core diameter, the numerical aperture (NA) is 0.22). Experimental results show that the laser output from fiber could reach 327 W, and the coupling-efficiency is more than 93.6%. Experimental results demonstrate that this module lays a foundation for multi-wavelength, polarization beam combining and the high beam quality, high coupling efficiency diode laser module with CW thousands watts class output power can be developed.

Stable-wavelength and narrow-bandwidth high-power external-cavity laser diode stack

We demonstrated a simple external-cavity configuration to effectively suppress the spectral bandwidth of a high-power laser diode (LD) stack. The external cavities consisted of a set of microcylindrical lenses for fast-axis collimation and a single volume Bragg grating. On the basis of this configuration, we reduced the bandwidth from 2.34 to 0.36 nm (6.5-fold reduction) and achieved a peak power output of 335 W. The external-cavity LD stack showed very good spectrum stability in the entire current and wide temperature ranges considered.

Beam shaping of laser diode stacks for compact and efficient illumination devices at the French-German Research Institute of Saint-Louis

Abstract Laser diode stacks are applied for the realization of compact and efficient laser illumination devices for active imaging (e.g., laser gated viewing). An efficient use of illumination power and sensor arrays has to adapt the laser illumination to the sensor’s field of view (FOV) with a perfectly matched and homogeneous illumination field. In the past decades, ISL has studied different methods for beam shaping and homogenization of laser diode stacks. In this publication, the authors give a review of the development of different beam-shaping techniques for auto-stack, minibar stack in a specific configuration and standard laser diode stacks. The presented methods are based on the application of wedge-type waveguides, fast axis and slow axis collimation, and the rearrangement of the laser beams by polarization overlapping and virtual restacking. All presented methods have very high transmission efficiencies (>80%) and lead to a homogeneous illumination matched to the sensor’s field of view.

Laser diode stack beam shaping for efficient and compact long-range laser illuminator design

Abstract Laser diode stacks are interesting laser sources for active imaging illuminators. They allow the accumulation of large amounts of energy in multi-pulse mode, which is best suited for long-range image recording. Even when the laser diode stacks are equipped with fast-axis collimation (FAC) and slow-axis collimation (SAC) micro-lenses, their beam parameter products BPP are not compatible with direct use in highly efficient and compact illuminators. This is particularly true when narrow divergences are required such as for long-range applications. A solution to overcome these difficulties is to enhance the poor slow-axis BPP by virtually restacking the laser diode stack. We present a beam shaping and homogenization method that is low-cost and efficient and has low alignment sensitivity. After conducting simulations, we have realized and characterized the illuminator. A compact long-range laser illuminator has been set up with a divergence of 3.5×2.6 mrad and a global efficiency of 81%. Here, a projection lens with a clear aperture of 62 mm and a focal length of 571 mm was used.