<title>Gradient index microlenses for laser diode applications</title>

We demonstrate an efficient method for fabricating high-quality cylindrical microlens arrays (CMLAs) on the surface of fused silica, fully based on spatially shaping of a femtosecond laser beam from Gaussian to Bessel distribution. As the envelope of shaped spatial intensity distribution matches the profile of cylindrical microlens perfectly, a CMLA with more than 50 uniform microlenses is fabricated by simple line scanning. The radius and height of these microlens units can be finely controlled by adjusting the power of laser pulses. Excellent optical imaging and high-speed fabrication performances are also demonstrated by our fabricated CLMA.

The cylindrical microlens array (CMLA) is an important micro-optical component, it has very significant application effects in the fields of naked eye three-dimensional display, laser line generation, and deformed beam shaping. However, the high efficiency and low-cost manufacturing of large-area CMLA is still challenging for industry and academia. Based on the electric-field-driven (EFD) jet printing technique, a new method for manufacturing a large-area CMLA with high efficiency and low cost has been proposed. We use the theoretical analysis and numerical simulation to investigate the basic principle of EFD jet printing. The influence of process parameters on the formation of CMLA was revealed by experiments. Using optimized process parameters, the fabrication of a CMLA with a patterned area of 50 × 50 mm2, a line width of 70 μm, and a period of 100 μm was achieved. The geometric and optical properties of the fabricated CMLA were characterized, showing that the products have the good geometric appearance and focusing effect. The results show that the proposed fabrication method provides a new manufacturing process for large-area manufacturing of CMLA with high efficiency and low cost.

In order to optimize the preparation process of microlens arrays, improve preparation efficiency, and reduce preparation costs, 248 nm KrF excimer laser direct writing is combined with a motion mask to prepare microlens arrays on PMMA substrates. Firstly, a specific exposure mask based on the contour characteristics of the microlens unit was designed, and the preparation principle was analyzed. Using COMSOL Multiphysics 6.3 simulation software, a microlens preparation model was built to intuitively describe the process of preparing microlenses by the motion mask method. Secondly, a preparation system was built, and the laser processing technology was optimized. Finally, microlens arrays were prepared based on the optimized process, and an optical microscope and white-light interferometer were used to observe their morphology. The experimental results show that this method can effectively prepare cylindrical and circular microlens arrays. The width of the cylindrical microlens array unit exceeded 90 μm, the height was 7.08 μm, and the roughness was 0.09 μm. The diameter of the circular microlens array unit was φ100 μm, the height was 4 μm, and the curvature radius was 230 μm. The geometric dimensions of the mask can be adjusted to obtain microlens units of the desired size, achieving personalized preparation of microlens arrays. The excimer laser motion mask method can prepare various types of microlens arrays, and the array units have a high consistency and high surface quality, which helps to improve the efficiency, flexibility, stability, and specificity of microlens array preparation.

A novel concept of a diode laser system developed by LIMO GmbH and called ring-laser (patent pending) enables a very flexible, uncomplex, simultaneous welding of plastic parts with different shapes and with different sizes without any additional changes of the system. Another diode laser system, so called line generator (patent pending), which utilises LIMO-homogenisers can be used for contour or simultaneous welding of plastics. This system is characterized by its nearly top-hat beam profile with homogeneity better than +/-6 %. Additionally, its line length, line width and its operating distance can be adapted to the geometry of a process by the use of an appropriate type of homogenisers and lens systems. Due to its outstanding features this diode laser can be also used e.g. for simultaneous soldering, thermal surface treatment, transversal pumping of solid state lasers and also in the printing industry (computer-to-plate technology).A novel concept of a diode laser system developed by LIMO GmbH and called ring-laser (patent pending) enables a very flexible, uncomplex, simultaneous welding of plastic parts with different shapes and with different sizes without any additional changes of the system. Another diode laser system, so called line generator (patent pending), which utilises LIMO-homogenisers can be used for contour or simultaneous welding of plastics. This system is characterized by its nearly top-hat beam profile with homogeneity better than +/-6 %. Additionally, its line length, line width and its operating distance can be adapted to the geometry of a process by the use of an appropriate type of homogenisers and lens systems. Due to its outstanding features this diode laser can be also used e.g. for simultaneous soldering, thermal surface treatment, transversal pumping of solid state lasers and also in the printing industry (computer-to-plate technology).

In the last few years, fiber-coupled diode lasers have shown massive applications in many fields of communication and scientific research. Particularly, the pumping of solid-state lasers is a key application for more powerful diode lasers enabling good solutions in various laser micro methods like metal cutting, sintering, structuring as well as drilling. In this work, a simple beam shaping method is demonstrated for coupling a high-power semiconductor laser diode into multi-mode fiber optic using optical lenses. The optical lenses as beam transformation components are utilized to reshape the asymmetrical irradiation of the diode laser bar and to circularize the laser beam. Using this simple method, compact, high-output-power, and high-brightness fiber-coupled laser diode systems are achieved. The obtained results show that the entire optical coupling efficiency of the system is more than 65%.

This paper presents a performance study done on a semiconductor laser diode in a moisture condensing environment. Devices with laser diodes are used in a wide variety of electronic applications and in the various climatic conditions. The motivation behind this study is a common environmental exposure, where a device using a laser diode is brought into a relatively humid building from a dry, cold, outside environments. Under such conditions, condensation occurs on various components of the device, including the diode, which could affect the laser output power. Device performance could be affected since the laser diode and the lens are susceptible to degradation due to such repetitive condensation conditions. The test vehicle chosen for this study was an optoelectronic package using a 980 nm laser diode. These are used in products for a broad range of markets, including data communications, aerospace, material processing, scientific, and defense industries [Pliska et al., "Wavelength Stabilized 980nm Uncooled Pump Laser Modules for Erbium-Doped Fiber Amplifiers," Opt. Lasers Eng., 43, pp. 271–289; Righetti, 1996, “Amplifiers Pumped at 980 nm in Submarine Applications,” European Conference on Optical Communication, Vol. 3, pp. 75–80; Pfeiffer et al., 2002, "Reliability of 980 nm Pump Lasers for Submarine, Long-haul Terrestrial, and Low Cost Metro Applications," Optical Fiber Communication Conference and Exhibit, pp. 483–484]. These products may be used in environmental conditions that could result in condensation within the product. A hermetic package could address this concern, but it is an expensive option. Nonhermetic packaging for the laser component could help to lower the cost of these devices; however, these packages have important failure mechanisms that are a potential concern. Prior research reported performance studies conducted on similar packages at elevated temperature, humidity, and power conditions using accelerated tests [Pfeiffer et al., 2002, "Reliability of 980 nm Pump Lasers for Submarine, Long-haul Terrestrial, and Low Cost Metro Applications," Optical Fiber Communication Conference and Exhibit, pp. 483–484; Park and Shin, 2004, “Package Induced Catastrophic Mirror Damage of 980nm GaAs High Power Laser,” Mater. Chem. Phys., 88(2-3), pp. 410–416; Fukuda et al., 1992, “Reliability and Degradation of 980nm InGaAs/GaAs Strained Quantum Well Lasers,” Qual. Reliab. Eng., 8, pp. 283–286]. However, studies conducted that specifically addressed condensation measurements have not been previously reported. Hence, an attempt was made to study package performance with condensation, to address the identified concern for the current package. A test method based on a military standard specification was used for this purpose. Elevated temperature and humidity (without condensation) were found to affect the laser power. These were characterized to isolate the effect of condensation alone. The package was subjected to repetitive condensing cycles and laser output power was recorded as a function of time, temperature and humidity. The variation in laser output power due to condensation was observed and quantified. Results showed a temporary power degradation of approximately 5% with condensation. This was a repeatable effect throughout the test time. Visible water droplets were found in various areas of the package after the test cycle. This could lead to potential failure mechanisms during the device life time.

: The operation regimes and dynamic properties of semiconductor laser diodes are significantly affected by external feedback. Investigation of the dynamics of Self-Sustained Pulsation (SSP) in the semiconductor laser diodes with optoelectronic feedback is presented. The Laser Diode (LD) is assumed to be a noiseless two section Fabry-Perot laser diode with linear gain in both sections. The feedback is assumed to consist of a photodetector and wide band and band pass microwave amplifier. For a certain range of the laser diode and feedback parameters the system undergoes high frequency (about 1 GHz) oscillations resulting from structural instability in system dynamics. Frequency, amplitude and other parameters of Self-Sustained Pulsation are electrically tunable. The system dynamics are investigated from the standpoint of stability analysis as well as direct numerical simulation. The stability analysis performed shows that for certain laser diode and feedback operating parameters, the feedback laser diode system exhibits an instability which results in Self-Sustained Pulsation.

A simple scanning semiconductor diode laser source and its application in wavelength modulation spectroscopy around 825 nm

In this paper we present an experimental analysis of the effect of RF modulation injection current in DFB and FP lasers nearthe first period doubling bifurcation (PDB). By increasing the amplitude of the modulation frequency at the onset of PDB thefirst results of a virtual Hopf phenomenon like behavior has been observed in both types of lasers. We characterized thisobservation by increasing the amplitude of the current modulation where two peaks appear in the spectrum, approaching oneto the other until fmally merging the onset of period doubling bifurcation as predicted by theory. The same virtual Hopfbehavior was observed when we set the amplitude modulation at a fixed value and we sweep the frequency modulation at theonset of the PDB point.Keywords: Laser Diode, Nonlinear Dynamics, Period Doubling Bifurcations. 1. INTRODUCTION Nonlinear dynamics in diode lasers has been the subject of many theoretical and experimental investigations in the lastdecade as they show a rich variety of irregular behavior in their route to chaos1. A single frequency diode laser is commonlystudied by a rate equation model which describes the time evolution of the photon and carrier densities. In these equations,the inclusion of an additional degree of freedom as modulation of the injection current, external light injection or opticalfeedback is needed for nonlinear dynamics to appear2.To our knowledge, the nonlinear behavior in semiconductor diode lasers has been extensively studied by many authors usingdelayed optical feedback and strong current modulation. In the former, the observed route to chaos was through successivesubharmonic bifurcations or by quasi periodic route3'4. In the strong current modulation case the period sequence wastruncated by period tripling before the onset of chaos5'6.On the other hand, the onset of a period doubling bifurcation point in many physical systems brings to nonlinear phenomenasuch as: 1) small signal amplification, 2) shift and suppression of the bifurcation point and 3) the emergence of a closelyspaced set of peaks in the frequency response7.In semiconductor lasers the parametric amplification was observed by Harth and co-workers8'9 which was analyzed byWiesenfeld few years after'°. In the study developed by Wiesenfeld in ref. 10, semiconductor lasers were proposed as goodcandidate to study the rich variety of nonlinear phenomena near period doubling bifurcations. A recent study has shown thatthe effect of Langevin noise in period doubling semiconductor lasers has a virtual Hopf ur12 previously predicted byWiesenfeld'34 as a virtual Hopf phenomenon in systems undergoing a period doubling bifurcations in the presence ofexternal noise.In the present study we have experimentally investigated the nonlinear dynamics of diode lasers in the period doublingbifurcation point. We study the influence of the amplitude modulation in Distributed Feedback (DFB) lasers near perioddoubling bifurcations. In Fabry-Perot (FP) lasers we performed as well the same study as DFB lasers and the analysis of thefrequency modulation near period doubling bifurcation. So in section II we described the experimental procedure for theonset of the first period doubling bifurcation performed in commercial DFB and FP InGaAsP diode lasers at 1,3 tm emission

High performance AlGaAs-based laser diodes: Fabrication, characterization and applications

A preliminary demonstration of free-space electric power transmission has been conducted using non-coherent laser diode arrays as the transmitter and standard silicon photovoltaic cell arrays as the receiver. The transmitter assembly used a high-power-density array of infrared laser diode bars, water cooled via integrated microchannel heat sinks and focused by cylindrical microlenses. The diode array composite beam was refocused by a parabolic mirror over a 10 meter path, and received on a {approximately}15 x 25 cm panel of thinned single crystal high efficiency silicon solar cells. The maximum cell output obtained was several watts, and the cell output was used to drive a small motor. Due to operating constraints and unexpected effects, particularly the high nonuniformity of the output beam, both the distance and total received power in this demonstration were modest. However, the existing transmitter is capable of supplying several hundred watts of light output, with a projected received electric power in excess of 200 watts. The source radiance is approximately 5 x 10{sup 9} W/m{sup 2}-steradian. With the existing 20 cm aperture, useful power transmission over ranges to {approximately}100 meters should be achievable with a DC to DC efficiency of greater than 10%. Non-coherent sources of this type are readily scalable to powers of tens of kilowatts, and with larger apertures can be used directly for power transmission up to several kilometers. Future non-coherent diode laser sources may be suitable for power transmission over hundreds of kilometers. Also, the experience gained with non-coherent arrays will be directly applicable to power beaming systems using coherent diode arrays or other array-type laser sources.

Three-dimensional volumetric display in rubidium vaporIsaac I. Kim, Eric Korevaar, and Harel HakakhaAstroTerra Corporation1 1526 Sorrento Valley Rd, Suite V, San Diego, CA 92121ABSTRACTThe successful demonstration of a novel three-dimensional volumetric display based on the intersection of twolow power diode laser beams in an atomic vapor is presented. A 780 nm laser and a 630 nm laser are directed via mirrorsand x-y scanners towards an enclosure containing rubidium vapor, where they intersect at 90 degrees. Rubidium atomswithin the small intersection volume undergo 1/2 to 5P3/2 excitation from the 780 nm laser, and then 5P3/2 to 6d512excitation from the 630 nm laser, resulting in red omnidirectional fluorescence from the intersection point. Tuning of thelasers to the exact excitation wavelengths resulted in an extended red spot with maximum brightness. By tuning the lasersslightly off the transition wavelengths, a very localized red spot with slightly less brightness was produced. A series ofintersection points were scanned in a time less than the eye's I 5Hzrefresh rate to create true three-dimensional volumetricimages such as a floating cube and rotating globe, which were viewable from many angles. The maximum speed of themechanical scanners limited the complexity of the three dimensional images. By incorporating higher power lasers andfaster acousto-optical scanners, this technique could allow the three-dimensional viewing of real time air traffic control,medical images, or theater battlefield management.Keywords: 3-d display, volumetric display, voxel, rubidium, diode laser, scanning, two-step excitation, fluorescence1. INTRODUCTIONThis report describes the building and performance of a novel three-dimensional volumetric display based on theintersection of two laser beams in an atomic vapor, and subsequent omnidirectional florescence from the intersection point.This 3-d display concept was patented (US Patent 4,881,068 -Nov 14, 1989) by Eric Korevaar while working for hisformer employer, ThermoTrex Corporation (TTC). TTC has granted a license to AstroTerra to manufacture productsbased on the patent. Recently, the concept has become practical because semiconductor diode lasers can be used instead oflarge, inefficient, and expensive dye lasers. Figure 1 shows the display concept. Two lasers are directed via mirrors and x-y scanners towards an enclosure containing an appropriate gaseous species (in this case, rubidium vapor), where theyintersect at 90 degrees. Either laser by itself causes no visible fluorescence. However, where both lasers are incident onthe same gas atoms, two step excitation results in florescence at the intersecting point. By scanning the intersection point intime, a three-dimensional image can be drawn in the vapor. The eye can not see changes faster than about 1 5Hz (moviesfor example are shot at 24 frames per second), so that ifthe image to be displayed is repeatedly drawn faster than this rate,the image will appear to be steady, even though light may be originating from any one point in the volume for only a smallfraction ofthe time.Government applications for the three-dimensional display include battle management (keeping track of airborneand groundborne mobile platforms and seeing how they are relatively oriented) and theater missile defense visualization(plotting missiles and interceptors). Also, air traffic control is a large potential market, showing the three dimensionalinformation to air traffic controllers. Commercial applications include displays of three dimensional CAD drawings,displays of medical images from Magnetic Resonance Imaging, visualization of complicated dynamic processes such asturbulent flow, novel advertising of products, entertainment at amusement parks, special effects in movies, and ultimatelyhome-based three-dimensional television.2. TECHNICAL OBJECTIVESFor the demonstration, two diode lasers with wavelengths of 780 nm and 630 nm were intersected in rubidiumvapor. Rubidium was chosen because the required wavelengths corresponded to currently available diode lasers. The

Coating removal during new ship construction or ship maintenance has traditionally posed environmental and production issues. Coating removal results in a large amount of waste, and other trades cannot work in or near areas where coating removal operations are going on. Hydroblasting, an environmentally friendly method, has significantly decreased in recent years; it cannot be used on interior compartments. The dilemma has been to find a surface-preparation method that is effective yet environmentally acceptable and one that can work on the outer hull as well as on the inside compartments. Further, a method is needed to remove coatings from the new "soft" substrates. Laser devolved over the years from expensive CO2 and Nd:YAG lasers to the recent innovation of using affordable semiconductor diode lasers for coatings removal. Semiconductor diode laser decoating is a nonintrusive, nonkinetic energy process that has successfully removed coatings from a variety of substrates, such as composites, fiberglass, metal, and plastics, in a controlled manner, layer by layer, without damage, leaving the surface ready for immediate repainting. The process is very rapid (up to 400 sq ft per hour), 6 to 10 times faster on average than the present manual hand-sanding methods, and involves a minimal increase in temperature, because the energy is mostly absorbed and utilized in coating decomposition (i.e., instant pyrolization and evaporation). The only waste associated with this process is the ablated coating that is vacuumed from the surface and captured in particle filters, approximately one-third of the amount of coating originally on the surface. Diode lasers are compact, durable for shipbuilding and repair industry use, and present low operating costs. Due to delays in receiving test panels from Navy and commercial ship sources, this paper will highlight data from tests of removal of coatings similar to military epoxy/polyurethane coatings from thin-skin metallic, composite, and fiberglass structures, radome material, and for removing powder coatings. Pulsed high-energy diode laser decoating technology is now commercially and economically available for ship corrosion control and maintenance, can reduce the overall environmental impact of decoating, and can increase productivity of all the ship trades.

An approach for representing and evaluating surface plasmonic lightening through cylindrical liquid crystal microlens arrays (CLCMAs) of 128×128, is proposed. The CLCMAs are typical sandwiched structures, in which the LC materials with a thickness of ~20μm is fully filled into a preshaped microcavity with a pair of parallel electrodes fabricated by silica wafers coated by an indium-tin-oxide (ITO) film. The top electrode is patterned using an arrayed micro-rectangle-hole with a size of 200×60μm 2 and a minimum spacing of 50μm. The surface plasmonic radiation is excited and further participates the focusing of incident beams in the visible range. The output light fields involving the plasmonic radiation are investigated. Rising the voltage signal from ~1.4 to ~5.5V RMS , the excited plasmonic radiation will sequentially present typical states including the beam converging state, focusing together with partial incident beams, and lightening mainly along the edge of individual ITO micro-rectangle-hole.

High Power Laser Diodes (HPLD) are increasingly used in different fields of applications such as Scientific, Industrial, Medical and Defense. Our significant performances improvements (especially in peak power, divergence and efficiency) and a reproducible manufacturing process have led to reliable, highly robust components. For industrial applications these devices are used predominantly for pumping of solid state lasers. Due to the drastically falling price per watt they are more and more replacing flash lamps as pump sources. Their compact size and high reliability make diode laser arrays ideally suited to this demanding market. For Scientific applications, the brightness as well as the cost are the two main drivers. In order to respond to this demand, QLD has focused its development on very high peak power bars associated with very high E/O efficiency. The level of energy QLD is achieving now has also pushed us to develop a power supply driver able to operate such diodes at up to 1000A under 100V.