This PDF file contains the front matter associated with SPIE Proceedings Volume 10513, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Nowadays, the development of modern laser systems tends to be more and more integrated and compact. A huge variety of components with specific purposes and in largely different dimension scales can be seen even in a single laser system. As a result, the difficulty in modeling and designing such a system increases dramatically. To model such systems, instead of attempting to employ multiple disciplines in combination, we adhere to physical optics, which is governed by Maxwell’s equations. Especially, by fast physical optics, we emphasize on highly efficient solutions to the Maxwell’s equations, based on 1) the paradigm shift of switching the main modeling domain from space to spatial frequency domain; 2) innovative Fourier transform concepts that minimize the field sampling parameters. With these techniques, components ranging from sub-wavelength gratings, to prisms, and from structured waveguides to laser crystals, can all be included. In addition to optics itself, it is possible and flexible to take other physical aspects and their impacts on the optical performance into consideration in a flexible manner, such as mechanical and thermal effects in laser packaging processes. We will present simulations of several typical modern laser systems based on fast physical optics modeling.

We report on an ultra-low loss Hollow-Core Photonic Crystal Fiber (HC-PCF) beam delivery system (GLO-GreenBDS) for high power ultra-short pulse lasers operating in the green spectral range (including 515 nm and 532 nm). The GLOBDS- Green combines ease-of-use, high laser-coupling efficiency, robustness and industrial compatible cabling. It comprises a pre-aligned laser-injection head, a sheath-cable protected HC-PCF and a modular fiber-output head. It enables fiber-core gas loading and evacuation in a hermetic fashion. A 5 m long GLO-BDS were demonstrated for a green short pulse laser with a transmission coefficient larger than 80%, and a laser output profile close to single-mode (M² <1.3).

We report on the design and fabrication of inhibited-coupling guiding hollow-core photonic crystal fiber with a transmission band optimized for low loss guidance around 2 μm. Two fibers design based on a Kagome-lattice cladding have been studied to demonstrate a minimum loss figure of 25 dB/km at 2 μm associated to an ultra-broad transmission band spanning from the visible to our detection limit of 3.4 μm. Such fibers could be an excellent tool to deliver and compress ultra-short pulse laser systems, especially for the emerging 2-3 μm spectral region.

Synthetic CVD diamond has exceptional properties, including broad spectral transmission, physical and chemical robustness, and the highest thermal conductivity of any known material, making diamond an attractive material for medium to high power optical and laser applications, minimizing the detrimental effects of thermal lensing and radiation damage. Example applications include ATR prisms, Raman laser crystals, extra- and intra-cavity laser cooling. In each case the demands on the fundamental material properties and fabrication routes are slightly different. In recent years, there has been good progress in the development of low-loss, single crystal diamond, suitable for higher power densities, higher pulse rates and more demanding intra- and extra-cavity thermal management. The adoption of single crystal diamond in this area has however, been hindered by the availability of large area, low birefringence plates.

To address this, we report a combination of CVD growth and processing methods that have enabled the manufacture of large, low defect substrates. A final homoepitaxial, low absorption synthesis stage has produced plates with large area (up to 16 mm edge length), low absorption (α<0.005 cm-1 at 1064 nm), and low birefringence (∆n <10^-5), suitable for double-sided intra-cavity cooling. We demonstrate the practical advances in synthesis, including increasing the size while reducing in-use losses compared to previous generations of single crystal material, and practical developments in processing and implementation of the single crystal diamond parts, optimizing them for use in a state-of-the-art femto-second pulsed Ti:Sa thin disk gain module, all made in collaboration with the wider European FP7 funded Ti:Sa TD consortium.

A great deal of research in optics concentrates on developing quantum methods ranging from precision measurement to optical telecommunication, and from quantum information processing to fundamental tests of nature. To this aim, engineering and characterization of quantum states of light becomes extremally important. Nonclassical sources are based on various underlying physical phenomena, from parametric downconversion and four-wave mixing to solid state and atomic single emitters to ensembles of atoms or ions. I will discuss recent efforts in engineering and characterization of nonclassical light states applicable to a broad range of sources, predominantly based on tunable narrow-band filtering.

NASA’s Goddard Space Flight Center has been developing lidar to remotely measure CO₂ and CH₄ in the Earth’s atmosphere. The ultimate goal is to make space-based satellite measurements with global coverage. We are working on maturing the technology readiness of a fiber-based, 1.57-micron wavelength laser transmitter designed for use in atmospheric CO₂ remote-sensing. To this end, we are building a ruggedized prototype to demonstrate the required power and performance and survive the required environment.

We are building a fiber-based master oscillator power amplifier (MOPA) laser transmitter architecture. The laser is a wavelength-locked, single frequency, externally modulated DBR operating at 1.57-micron followed by erbium-doped fiber amplifiers. The last amplifier stage is a polarization-maintaining, very-large-mode-area fiber with ~1000 μm2 effective area pumped by a Raman fiber laser. The optical output is single-frequency, one microsecond pulses with >450 μJ pulse energy, 7.5 KHz repetition rate, single spatial mode, and < 20 dB polarization extinction.

We present a novel compact laser device based on a semiconductor master-oscillator power-amplifier (MOPA) emitting at 772 nm, suitable for quantum optic and spectroscopy. The optical performance of the laser device is characterized. For miniaturized lasers the thermal management is challenging, we therefore perform thermal simulations and measurements.

The first demonstrator is emitting more than 3 W optical power with a linewidth below 2lMHz. Using this MOPA design also compact devices for quantum optics (e.g. rubidium atomic clock) and seed lasers for frequency conversion can be realized [1].

Challenges and resolution approaches of fiber coupling to and from optical devices will be discussed. Particular emphasis will be paid to hermetic and ruggedized assemblies.

Laser systems face massive economic challenges for cost effective, but yet ultraprecise assembly processes. Assembly costs are mainly driven by the demanding requirements of laser systems leading to challenging final assembly positions. Most challenging is the robust process control of the UV-curing adhesive bonding process. As a highly multifactorial process adhesive bonding processes are subject to a target conflict of rapid curing times within several seconds, robust mechanical properties required for reliable laser systems and before all a high reproducibility in terms of precision. The repeatability of adhesive bonding processes is limited by the shrinkage effects fast-curing adhesives inherit. The work presented aims for a significant reduction of the impact of shrinkage effects during curing and a resulting increase in assembly precision. Three key approaches are the enablers for robust bonding processes: Firstly, sophisticated curing systems tailored to the needs of the bonding task keep the variance of UV-radiation to a minimum. Therefore, the authors equip their self-developed micromanipulator with an UV-curing system. Secondly, the ultraprecise volumetric dispensing in Pico liter range with online-characterization of adhesive volume minimized variation in volume and dispensing position. The authors present a calibration station for contactless jet dispensing systems. Both volume and position are measured and adjusted automatically. Lastly, an automated characterization station for the measurement of adhesive shrinkage is presented. These technologies allow for highly reproducible adhesive bonding processes in automated production environments. They are deployed in Fraunhofer IPT’s prototyping projects and feasibility studies and Aixemtec’s assembly services and their automated assembly cells.

In present days lasers are used in many different applications like material processing, medical science, automotive, measurement and sensing applications, etc.. All of these applications require optics to make the emitted light usable. Moreover, there is a requirement for a certain beam profile to achieve the necessary characteristic. The applications are addressed by different laser types, but the optical solutions are influence by the major trends of miniaturization, improved imagine quality and cost reduction. The compactness of resent applications requires micro-optical solutions. The design, manufacturing and the evaluation of these solutions are still demanding. The presentation will look at different applications and derive the specifications for the micro-optics. The available manufacturing technologies will influence the achievable accuracy of the parts. Therefore, the talk will evaluate the effect of deviations on the performance of selected applications. The second part will focus on the selection of the appropriate manufacturing technology and the evaluation of the micro-optics. It can be demonstrated that the optimization of the process parameters during the manufacturing process improves the performance of the products.

Mastering the process of dispensing is one of the key enablers for high-precision optics assembly. Industrial dispensing mainly needs to solve the problems of droplet volume control as well as droplet positioning. Both aspects are of high importance in order to achieve a highly repeatable assembly processes. For that purpose, AIXEMTEC has developed a machine-integrated calibration module. AIXEMTEC is provider of a modular platform for automated optics assembly consisting of integration platforms, tools for manipulation, dispensing and machine vision as well as modules for tray handling, special-purpose metrology and others. After a brief overview of the jet-dispensing technology and the dispensing tool used within the machine, this paper introduces a calibration module for contact-less dispensing systems. The calibration module implements an image-based approach synchronizing image acquisition with droplet release. This calibration module allows for machine-integrated closed loop calibration of the volume in sub-nanoliter-range as well as the horizontal position of droplets. Both control variables have been benchmarked and results will be presented in the paper. The calibration process will be outlined. The paper closes with the description of a typical application that benefit from machine-integrated droplet calibration.

Improving of industrial holographic and interferometry techniques is of great importance in interference lithography, computer-generated holography, holographic data storage, interferometry recording of Bragg gratings as well as gratings of various types in semiconductor industry. Performance of mentioned techniques is essentially enhanced by providing a light beam with flat phase front and flat-top irradiance distribution. Therefore, transformation of Gaussian distribution of a TEM₀₀ laser to flat-top (top hat, uniform) distribution is an important optical task. There are different refractive and diffractive beam shaping approaches used in laser industrial and scientific applications, but only few of them are capable to fulfil the optimum conditions for beam quality demanding holography and interferometry. As a solution it is suggested to apply refractive field mapping beam shaping optics πShaper, which operational principle presumes almost lossless transformation of Gaussian to flat-top beam with flatness of output wavefront, conserving of beam consistency, providing collimated low divergent output beam, high transmittance, extended depth of field, negligible wave aberration, and achromatic design provides capability to work with several lasers with different wavelengths simultaneously. High optical quality of resulting flat-top beam allows applying additional optical components to build various imaging optical systems for variation of beam size and shape to fulfil requirements of a particular application. This paper will describe design basics of refractive beam shapers and optical layouts of their applying in holography and laser interference lithography. Examples of real implementations and experimental results will be presented as well.

New developments in industrial applications and applications under rough environmental conditions within the field of spectroscopy and quantum technology in the 935 nm wavelength regime demand new compact, stable and robust laser systems. Beside a stable laser source the integration of a compact optical isolator is necessary to reduce size and power consumption for the whole laser system. The integration of a suitable optical isolator suppresses back reflections from the following optical system efficiently. However, the miniaturization of the optics inside the package leads to high optical power density levels that make a more detailed analysis of the components and their laser damage threshold necessary. We present test results on compact stable DFB laser sources (butterfly style packages) with newly integrated optical isolators operating around 935 nm. The presented data includes performance and lifetime tests for the laser diodes as well as package components. Overall performance data of the packaged laser diodes will be shown as well.

High power diode lasers (HPDLs) offer the highest wall-plug efficiency, highest specific power (power-to-weight ratio), arguably the lowest cost and highest reliability among all laser types. However, the poor beam quality of commercially HPDLs is the main bottleneck limiting their direct applications requiring high brightness at least in one dimension. In order to expand the applications of HPDLs, beam shaping and optical design are essential. In this work, we report the recent progresses on maximizing applications of HPDLs by synergizing diode laser light source and beam shaping micro-optics. Successful examples of matching of diode laser light sources and beam shaping micro-optics driving new applications are presented.

A novel 3-dimensional arrangement of mirrors is used to re-arrange beams from 1-D and 2-D high power diode laser arrays. The approach allows for a variety of stacking geometries, depending on individual requirements. While basic building blocks, including collimating optics, always remain the same, most adaptations can be realized by simple rearrangement of a few optical components. Due to fully automated alignment processes, the required changes can be realized in software by changing coordinates, rather than requiring customized mechanical components.

This approach minimizes development costs due to its flexibility, while reducing overall product cost by using similar building blocks for a variety of products and utilizing a high grade of automation. The modules can be operated with industrial grade water, lowering overall system and maintenance cost.

Stackable macro coolers are used as the smallest building block of the system. Each cooler can hold up to five diode laser bars. Micro optical components, collimating the beam, are mounted directly to the cooler. All optical assembly steps are fully automated.

Initially, the beams from all laser bars propagate in the same direction. Key to the concept is an arrangement of deflectors, which re-arrange the beams into a 2-D array of the desired shape and high fill factor. Standard multiplexing techniques like polarization- or wavelengths-multiplexing have been implemented as well.

A variety of fiber coupled modules ranging from a few hundred watts of optical output power to multiple kilowatts of power, as well as customized laser spot geometries like uniform line sources, have been realized.

The technology of volume Bragg gratings (VBGs) recorded in photo-thermo-refractive glass allows considerable narrowing of the linewidth of laser diodes from a couple of nanometers to tens of picometeres. Such narrowband systems with GHz-wide emission spectra found broad applications for Diode Pumped Alkali vapor Lasers, optically pumped rare gas metastable lasers, Spin Exchange Optical Pumping, atom cooling, wide area Raman spectroscopy, etc. In addition to high lasing power, these applications require a very precise emission wavelength with extremely narrow linewidth, and the wavelength tunability to match an absorption line width of a particular alkali metal. For example, for Rb (794.7 nm) the linewidth dependence on pressure caused by interaction with 3He and N2 gases in the cell is on the order of 20 GHz/atm. Thus, the requirements for the pump laser systems become stricter since it requires not only narrow line but also the ability to tune the linewidth to match an absorption linewidth. In this paper, we present the results of development of a new generation of narrowband laser systems. The compact design, capability of simultaneous emission of several narrowband emission lines (e.g. for Rb/Cs or Rb/K gas systems), wavelength and linewidth tunability, high signal to noise ratio (>30 dB) makes the new system suitable for SEOP and other ultra-narrow linewidth demanding applications.

A scalable pumping method for a diode-pumped alkali laser (DPAL) is proposed. In the past, pump beams were collinearly or transversely arranged to the optical axis of the optical resonator. In our proposal, pump sources are circularly arranged around the optical axis, and the pump beams irradiate the gain cell at a small (0.1 radians) angle from the optical axis. Tens of pump beams can be coupled to the gain medium with high circular symmetry and high coupling efficiency. The idea was verified by numerical simulation, and the results were validated by experimental investigation.

Developments in diode pumped alkali laser (DPAL) systems have been impeded because of the catastrophic failure of laser windows. The window’s failure is caused by localized laser-induced heating of window material. This heating is believed to occur due to increases in absorption on or near the surface of the window. This increase is believed to be caused by either adsorption of carbon-based soot from the collisional gas or by the diffusion of rubidium into the bulk material. The work presented here will focus on the diffusion of Rb into the bulk window materials and will strive to identify a superior material to use as windows. The results of this research indicate that aluminum oxynitride (ALON), sapphire, MgAl₂O₄ (spinel), and ZrO₂ are resistant to alkali-induced changes in optical properties.

Continuous wave (CW) and quasi-continuous wave (QCW) operated diode laser bars and arrays have found a wide range of industrial, medical, scientific, military and space applications with a broad variety in wavelength, pulse energy, pulse duration and beam quality. Recent applications require even higher power, duty cycles and power density. The heat loss will be dissipated by conductive cooling or liquid cooling close to the bars.

We present the latest performance and reliability data of two novel high-brightness CW and QCW arrays of customized and mass-production modules, in compact and robust industry design for operation with high power and high duty cycles. All designs are based on single diode packages consisting of 10mm laser bars, soft or hard soldered between expansion matched submounts.

The modular components cover a wide span of designs which differ basically in water/conduction (active/passive) cooled, single, linear (horizontal and vertical) arranged designs, as well as housed and unhoused modules. The different assembling technologies of active and passive cooled base plates affect the heat dissipation and therefore the reachable power at different QCW operating conditions, as well as the lifetime.

As an example, a package consisting of 8 laser diodes, connected to a 28.8*13.5*7.0mm³ DCB (direct copper bonded) submount, passively or actively cooled is considered. This design is of particular interest for mobile applications seamless module to module building system, with an infinite number of laser bars at 1.7mm pitch. Using 940nm bars we can reach an optical output power per bar of 450W at 25°C base plate temperature with 10Hz, 1.2% duty cycle and 1.2ms pulse duration.

As an additional example, micro channel coolers can be vertically stacked up to 50 diodes with a 1,15mm pitch. This design is suitable for all applications, demanding also compactness and light weight and high power density. Using near infrared bars and others, we can reach an optical output power of 250W per bar at 25°C coolant temperature at CW operation.

Existing thermal management technologies for diode laser pumps place a significant load on the size, weight and power consumption of High Power Solid State and Fiber Laser systems, thus making current laser systems very large, heavy, and inefficient in many important practical applications. To mitigate this thermal management burden, it is desirable for diode pumps to operate efficiently at high heat sink temperatures. In this work, we have developed a scalable cooling architecture, based on jet-impingement technology with industrial coolant, for efficient cooling of diode laser bars. We have demonstrated 60% electrical-to-optical efficiency from a 9xx nm two-bar laser stack operating with propylene-glycolwater coolant, at 50 °C coolant temperature. To our knowledge, this is the highest efficiency achieved from a diode stack using 50 °C industrial fluid coolant. The output power is greater than 100 W per bar. Stacks with additional laser bars are currently in development, as this cooler architecture is scalable to a 1 kW system. This work will enable compact and robust fiber-coupled diode pump modules for high energy laser applications.

As power densities of laser diodes continuously increase, the effects of absorption losses in fast axis collimation lenses become exceedingly important. We report our analysis of two drivers of these absorption losses, coating absorption and glass bulk absorption, and how these absorption losses cause a thermal impact and have an influence on the performance of the laser beam quality emitted from a laser diode equipped with a fast axis collimation lens on a bottom tab. The presented results are derived from finite element method (FEM) simulations and the FEM model used is based on material data from data sheets and a heat transfer coefficient derived from cooling curves of components observed by a thermal infrared camera.

An insulation micro-channel cooling (IMCC) has been developed for packaging high power bar-based vertical stack and horizontal array diode lasers, which eliminates many issues caused in its congener packaged by commercial copper formed micro-channel cooler(MCC), such as coefficient of thermal expansion (CTE) mismatch between cooler and diode laser bar, high coolant quality requirement (DI water) and channel corrosion and electro-corrosion induced by DI water if the DI-water quality is not well maintained The IMCC cooler separates water flow route and electrical route, which allows tap-water as coolant without electro-corrosion and therefore prolongs cooler lifetime dramatically and escalated the reliability of these diode lasers. The thickness of ceramic and copper in an IMCC cooler is well designed to minimize the CTE mismatch between laser bar and cooler, consequently, a very low "SMILE" of the laser bar can be achieved for small fast axis divergence after collimation. In additional, gold-tin hard solder bonding technology was also developed to minimize the risk of solder electromigration at high current density and thermal fatigue under hard-pulse operation mode. Testing results of IMCC packaged diode lasers are presented in this report.

Tight tolerances for the final position and orientation of optical components are best controlled in automated and high volume production with statistical process control. Semi-automated and low-volume scenarios on the other hand are in need for a suitable approach, capable to react resiliently on remaining uncertainties of the bonding process. Active alignment has proven to lower the tolerances for finding the optimal position regarding the overall performance. We will present a novel shrinkage-compensation strategy, which extends the active control loop to integrate the curing process. We will discuss the adhesive properties necessary for the realization of our strategy and our measurement equipment used for the characterization of such properties. Besides a predicable shrinkage curve, the critical cross-linking level, until no further manipulation is possible, is a key factor. Furthermore, the machine concept, the curing capabilities and the active evaluation needs to follow special requirements. Since the shrinkage-behavior is highly sensitive to the amount of UV, the effective power on the adhesive needs to be controlled by optimizing the orientation of the light source. We integrated the UV-light in our micromanipulator in order to always ensure an optimal illumination In order to apply regression analysis for a multidimensional shrinkage model, misalignments in the selected degrees of freedom must be observable with sufficient precision. As validation of our strategy, we examine the collimation of a diode laser bar.

Next-generation industrial fiber lasers enable challenging applications that cannot be addressed with legacy fiber lasers. Key features of next-generation fiber lasers include robust back-reflection protection, high power stability, wide power tunability, high-speed modulation and waveform generation, and facile field serviceability. These capabilities are enabled by high-performance components, particularly pump diodes and optical fibers, and by advanced fiber laser designs. We summarize the performance and reliability of nLIGHT diodes, fibers, and next-generation industrial fiber lasers at power levels of 500 W – 8 kW. We show back-reflection studies with up to 1 kW of back-reflected power, power-stability measurements in cw and modulated operation exhibiting sub-1% stability over a 5 – 100% power range, and high-speed modulation (100 kHz) and waveform generation with a bandwidth 20x higher than standard fiber lasers. We show results from representative applications, including cutting and welding of highly reflective metals (Cu and Al) for production of Li-ion battery modules and processing of carbon fiber reinforced polymers.

For water-cooled fiber lasers, humidity and the resulting water-condensation has always been the biggest threat for laser reliability or power degradation, especially when used in harsh industrial environment. Here we present an innovative fiber laser packaging method featuring cast aluminum frame and an almost screw-free exterior packaging. A CW fiber laser with 1.5KW laser output power in such a compact and light-weight package has been demonstrated with an excellent beam quality and power stability for industry applications.

The amount of cladding light is important to ensure longevity of high power fiber components. However, it is usually measured either by adding a cladding light stripper (and thus permanently modifying the fiber) or by using a pinhole to only transmit the core light (ignoring that there may be cladding mode content in the core area). We present a novel noninvasive method to measure the cladding light content in double-clad fibers based on extrapolation from a cladding region of constant average intensity. The method can be extended to general multi-layer radially symmetric fibers, e.g. to evaluate light content in refractive index pedestal structures.

To effectively remove cladding light in high power systems, cladding light strippers are used. We show that the stripping efficiency can be significantly improved by bending the fiber in such a device and present respective experimental data. Measurements were performed with respect to the numerical aperture as well, showing the dependency of the CLS efficiency on the NA of the cladding light and implying that efficiency data cannot reliably be given for a certain fiber in general without regard to the properties of the guided light.

In this work we demonstrate a high transmission pump combiner that can operate uncooled at a maximum power of 400W for a continuous duration of 100hrs. The 7x1 pump combiner has seven 105/125 μm diameter 0.22 NA input pump fibers and a 247 μm diameter and 0.22 NA glass clad output fiber. The combiner has a high, 99%, pump transmission efficiency. These devices withstand without failure, a series of environmental stress tests, namely Thermal Cycling (-40 to 85ºC) and Damp Humidity (85ºC/85RH). These tests are conducted to uncover any latent defects in the device structure. The combiner’s also survive an elevated temperature of 75ºC at a power of 365W for duration of 5hrs, without any noticeable change in pump transmission.

Optical limiters transmit low intensity input light while blocking input light with the intensity exceeding certain limiting threshold. Conventional passive limiters utilize nonlinear optical materials, which are transparent at low light intensity and turn absorptive at high intensity. Strong nonlinear absorption, though, can result in over- heating and destruction of the limiter. Another problem is that the limiting threshold provided by the available optical material with nonlinear absorption is too high for many applications. To address the above problems, the nonlinear material can be incorporated in a photonic structure with engineered dispersion. At low intensity, the photonic structure can display resonant transmission via localized mode(s), while at high intensity the resonant transmission can disappear, and the entire stack can become highly re ective (not absorptive) within a broad frequency range. In the proposed design, the transition from the resonant transmission at low intensity to nearly total re ectivity at high intensity does not rely on nonlinear absorption; instead, it requires only a modest change in the refractive index of the nonlinear material. The latter implies a dramatic increase in the dynamic range of the limiter. The main idea is to eliminate the high-intensity resonant transmission by decoupling the localized (resonant) modes from the input light, rather than suppressing those modes using nonlinear absorption. Similar approach can be used for light modulation and switching.

We report on the cause and corrective actions of three amplifier crystal fractures in the space-qualified laser systems used in NASA Goddard Space Flight Center’s (GSFC) Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). The ICESat-2 lasers each contain three end-pumped Nd:YVOO4 amplifier stages. The crystals are clamped between two gold plated copper heat spreaders with an indium foil thermal interface material, and the crystal fractures occurred after multiple years of storage and over a year of operational run-time. The primary contributors are high compressive loading of the NdYVO₄ crystals at the beginning of life, a time dependent crystal stress caused by an intermetallic reaction of the gold plating and indium, and slow crack growth resulting in a reduction in crystal strength over time. An updated crystal mounting scheme was designed, analyzed, fabricated and tested. Thee fracture slab failure analysis, finite-element modeling and corrective actions are presented.

In recent past years, solid state laser have been attracted attentions in industry and scientific researches to achieve both high average power and energies with good beam quality. As an example, high energy laser with average powers over the kW have been demonstrated [1]. In these systems, predicting and managing heat generated during optical pumping is critical, as it can result in unwanted thermo-optical and thermo-mechanical effects such as thermal lensing or thermal stress fracture. The determination of temperature distribution inside the laser gain medium is essential for optimizing the laser operation. The first step in thermal analysis is to estimate the thermal volume flow deposited within the gain medium over optical pumping, but this very first point is extremely difficult to accurately measure. In this work, we propose a novel method, based on a both an experimental thermal imaging set-up and a new theoretical model, that we use to accurately measure the deposited thermal flux in a laser crystal during pumping. We use a high frequency thermal camera and a pulsed laser diode ( Pump LD 969 nm or 940 nm) to submit a photo-thermal excitation to the front surface of a non-contacted Yb:YAG laser crystal. Based on singular value decomposition and inverse methods, thermal diffusivity and the heat losses were estimated using the thermal evolution of the crystal over the relaxation step. These results and the thermal evolution of the crystal over the solicitation allowed estimating the thermal volume flow deposited within the crystal over optical pumping.

Photo-thermo-refractive (PTR) glass) is a photosensitive silicate glass doped with Ce³⁺ where a permanent refractive index decrement is produced by UV exposure followed by thermal development. This material provides high efficiency and low losses combined with high thermal, ionizing and laser tolerance of holographic optical elements (HOEs). This is why PTR glass is widely used for holographic recording of volume Bragg gratings (trivial holograms produced by interference of two collimated beams) and phase plates operating in near UV, visible, and near IR spectral regions. It would be very beneficial though to record also complex HOEs (lenses and curved mirrors) for those spectral regions. However, PTR is not sensitive to visible or IR radiation and therefore does not allow the recording of nonplanar holograms for these regions. The present paper describes a technique for recording complex HOEs using visible radiation in Ce³⁺ doped PTR glass. This two-step technique includes a blank exposure to UV radiation followed by structured exposure to a visible beam. It was found that the second exposure decreases the refractive index decrement induced in the UV exposed glass after thermal development. This means that areas, which underwent double exposure, have refractive index lower than in unexposed areas but higher than in just UV exposed ones. Thus, this technique provides refractive index increment after visible irradiation of UV exposed PTR glass. Using this approach, complex holograms (curved mirrors and lenses) operating in the visible region, were recorded in PTR glass.

High power diode laser stack is widely used in pumping solid-state laser for years. Normally an integrated temperature control module is required for stabilizing the output power of solid-state laser, as the output power of the solid-state laser highly depends on the emission wavelength and the wavelength shift of diode lasers according to the temperature changes. However the temperature control module is inconvenient for this application, due to its large dimension, high electric power consumption and extra adding a complicated controlling system. Furthermore, it takes dozens of seconds to stabilize the output power when the laser system is turned on. In this work, a compact hard soldered high power conduction cooled diode laser stack with multiple wavelengths is developed for stabilizing the output power of solid-state laser in a certain temperature range. The stack consists of 5 laser bars with the pitch of 0.43mm. The peak output power of each bar in the diode laser stack reaches as much as 557W and the combined lasing wavelength spectrum profile spans 15nm. The solidstate laser, structured with multiple wavelength diode laser stacks, allows the ambient temperature change of 65°C without suddenly degrading the optical performance.

Laser-based additive manufacturing (AM) from metal powders is emerging as the new industrial revolution, although current fabrication approaches still require long mechanical post-processing to improve the final surface quality and meet the design tolerances. To overcome this limitation, the next generation machines are expected to complement laser AM with laser ablation (LA) to implement surface finishing and micro texturing already during the device growth process. With this aim, a new beam combiner to allow the real-time interchange of additive and subtractive processes using the same scanner head has been designed. Extensive tests have been carried out using a 6 kW continuous-wave laser similar to that used for the metal powder fusion and a nanosecond 100W pulsed source similar to that used for laser ablation.

Laser-Induced Breakdown Spectroscopy (LIBS) has proven to be a useful research tool for material analysis for decades. However, because of the amount of energy required in a few nanosecond pulse to generate a stable and reliable LIBS signal, the lasers are often large and inefficient, relegating their implementation to research facilities, factory floors, and assembly lines. Small portable LIBS systems are now possible without having to compromise on energy needs by leveraging off of advances in high-density packaging of electronics, opto-mechanics, and highly efficient laser resonator architecture. This paper explores the integration of these techniques to achieve a mJ class eye-safe LIBS laser source, while retaining a small, light-weight package suitable for handheld systems.

Phase masks for mode conversion and other laser beam transformation are usually produced by surface profiling using spatially selective etching or deposition. Such fabrication techniques for making complex phase masks take significant time, effort, and expense. Surface damages and contaminations restrain the wide application of such phase elements as well. We propose an alternative, where volume phase masks were produced by spatially selective refractive index change in the bulk of plane-parallel plate of photo-thermo-refractive (PTR) glass. Those phase masks show high tolerance to harsh environments and high-power laser radiation. The approach uses a light amplitude modulating element called digital micromirror device (DMD) which has millions of micro-mirrors that tilt to ‘on’ or ‘off’ position based on the voltage applied to them. Selecting different time intervals for the mirrors to be ‘on’ or ‘off’, allows for grey level images to be generated and projected by the DMD device. Using a broadband UV light source, the desired amplitude image was projected onto a PTR glass plate for a designated period of time. The variable light amplitude exposure of the glass is converted to a refractive index change after thermal treatment step. The phase masks are recorded in the volume and are practically unsusceptible to surface damage, changes in the environment and other factors from which surface created phase masks suffer. The simplicity in fabricating grey level phase masks, the flexibility to computer design, and the robustness are the main advantages of the approach when compared to the standard phase mask fabricating techniques.

Antireflective surface structures (ARSS) are nano-patterned structures etched into the optical surface that simulate behavior like that of a moth’s eye, which has cone-like structures that reduce reflections in the visible. We have previously reported record high laser induced damage thresholds (LIDT) up to 100 J/cm2 at 1 µm (10 nsec pulsewidth) for silica glass windows with random ARSS. However, these windows with ARSS are hydrophilic, leading to practical problems for their use in fielded laser systems. Recently, we developed and reported a new process for treating the surface of silica windows with ARSS such that it becomes hydrophobic. In this paper we will report results for the LIDT for samples with these antireflective, hydrophobic surfaces, and compare to results for samples that are untreated but have ARSS, samples with hydrophobic treatment but no ARSS, and finally those having neither ARSS or hydrophobic treatment. The morphology of the laser damage observed and potential mechanisms will be discussed.

High power high energy lasers have recently emerged as potential solution for several applications such as secondary rays generation, nuclear fusion and medical therapy. One major limitation of such systems for high energy extraction is the laser-induced damage threshold (LIDT) of laser components. In this context, many studies have been devoted to the determination of the LIDT of laser materials under certain operational conditions and the identification of the limiting factors at the nanoseconds and femto/sub-picoseconds regime 1–3.However, these measurements do not consider that in most high intensity laser chains such as chirped pulsed amplification (CPA), pulses are stretched to larger duration such as hundreds of picoseconds. Thus, measuring the LIDT of laser materials under stretched pulses irradiation becomes critical. In this work, we report a study of the influence of the coating treatment and fatigue in the LIDT of Yb:YAG crystals under stretched pulses by means of 1-on-1, Rasterscan and S-on-1 tests. We use a 1mJ, 1kHz laser (S-Pulse model from Amplitude Systèmes) modified to support 150 ps-pulse duration. We show a lower LIDT of the AR-coating compared to that of the HR-coating (7J/cm2) and preliminary outcomes point out a non-deterministic effect of the fatigue at this regime of pulse duration. These results show the importance of testing critical components at hundred-picoseconds regime for high power and high energy lasers. 1. Sozet, M. et al. Laser damage density measurement of optical components in the sub-picosecond regime. 40, 2–5 (2015). 2. Smith, A. V & Do, B. T. Bulk and surface laser damage of silica by picosecond and nanosecond pulses at 1064 nm. (2008). 3. Jensen, L. O. et al. Investigations on SiO 2 / HfO 2 mixtures for nanosecond and femtosecond pulses a Laser. 7842, 1–10 (2017).

Conventional laser resonators yield multimodal output, especially at high powers and short cavity lengths. Since highorder modes exhibit large divergence, it is desirable to suppress them to improve laser quality. Traditionally, such modal discriminations can be achieved by simple apertures that provide absorptive loss for large diameter modes, while allowing the lower orders, such as the fundamental Gaussian, to pass through. However, modal discrimination may not be sufficient for short-cavity lasers, resulting in multimodal operation as well as power loss and overheating in the absorptive part of the aperture.

In research to improve laser mode control with minimal energy loss, systematic experiments have been executed using phase-only elements. These were composed of an intra-cavity step function and a diffractive out-coupler made of a computer-generated hologram. The platform was a 15-cm long solid-state laser that employs a neodymium-doped yttrium orthovanadate crystal rod, producing 1064 nm multimodal laser output. The intra-cavity phase elements (PEs) were shown to be highly effective in obtaining beams with reduced M-squared values and increased output powers, yielding improved values of radiance. The utilization of more sophisticated diffractive elements is promising for more difficult laser systems.

Diode lasers pump sources for future high-energy-class laser systems based on Yb-doped solid state amplifiers must deliver high optical intensities, high conversion efficiency (η_E = > 50%) at high repetition rates (f = 100 Hz) and long pulse widths (τ = 0.5…2 ms). Over the last decade, a series of pump modules has been developed at the Ferdinand-BraunInstitut to address these needs. The latest modules use novel wide-aperture single emitter diode lasers in passively side cooled stacks, operate at τ = 1 ms, f = 100…200 Hz and deliver 5…6 kW optical output power from a fiber with 1.9 mm core diameter and NA of 0.22, for spatial brightness B_Ω > 1 MW/cm² sr. The performance to date and latest developments in these high brightness modules are summarized here with recent work focusing on extending operation to other pumping conditions, as needed for alternative solid state laser designs. Specifically, the electro-optic, spectral and beam propagation characteristics of the module and its components are studied as a function of τ for a fixed duty cycle DC = 10% for τ = 1...100 ms, and first data is shown for continuous wave operation. Clear potential is seen to fulfill more demanding specifications without design changes. For example, high power long-pulse operation is demonstrated, with a power of > 5 kW at τ = 100 ms. Higher brightness operation is also confirmed at DC = 10% and τ = 1 ms, with > 5 kW delivered in a beam with B_Ω > 4 MW/cm₂ sr.

We present a frequency selective optical setup based on a Gires-Tournois interferometer suitable to enforce single-frequency operation of high power lasers. It is based on a birefringent Gires-Tournois interferometer combined with a λ/4 plate and a polarizer. The high-reflective part of the Gires-Tournois interferometer can be contacted to a heat sink to obtain efficient cooling (similar cooling principle as for the active medium in thin-disk lasers) enabling power scaling up to output powers in the kW range.

High power combiners are of great interest for high power fiber lasers and fiber amplifiers. With the advent of CO₂ laser splicing system, power combiners are made possible with low manufacturing cost, low loss, high reliability and high performance. Traditionally fiber optical components are fabricated with flame torch, electrode arc discharge or filament heater. However, these methods can easily leave contamination on the fiber, resulting inconsistent performance or even catching fire in high power operations. The electrodes or filaments also degrade rapidly during the combiner manufacturing process. The rapid degradation will lead to extensive maintenance, making it unpractical or uneconomic for volume production. By contrast, CO₂ laser is the cleanest heating source which provides reliable and repeatable process for fabricating fiber optic components including high power combiners.

In this paper we present an all fiber end pumped 7x1 pump combiner fabricated by CO₂ laser splicing system. The input pump fibers are 105/125 (core/clad diameters in μm) fibers with a core NA of 0.22. The output fiber is a 300/320 fiber with a core NA of 0.22. The average efficiency is 99.4% with all 7 ports more than 99%. The process is contamination-free and highly repeatable. To our best knowledge, this is the first report in the literature on power combiners fabricated by CO₂ laser splicing system. It also has the highest reported efficiency of its kind.

The end users’ biggest concern for any industrial equipment is the reliability and the service down-time. This is especially true for industrial lasers as they are typically used in fully or semi- automated processes. Here we demonstrate how to use the integrated Internet of Things (IoT) technology in industrial lasers to address the reliability and the service down-time so to improve end users’ experience.

When producing fiber lasers and amplifiers, silica glass compositions consisting of three to six different materials are needed. Due to the varying needs of different applications, substantial number of different glass compositions are used in the active fiber structures. Often it is not possible to find material parameters for theoretical models to estimate thermal and mechanical properties of those glass compositions. This makes it challenging to predict accurately fiber core refractive index values, even if the preform index profile is measured. Usually the desired fiber refractive index value is achieved experimentally, which is expensive. To overcome this problem, we analyzed statistically the changes between the measured preform and fiber index values. We searched for correlations that would help to predict the Δn-value change from preform to fiber in a situation where we don’t know the values of the glass material parameters that define the change. Our index change models were built using the data collected from preforms and fibers made by the Direct Nanoparticle Deposition (DND) technology.

In the past decade, ZBLAN (ZrF4-BaF2-LaF3-NaF) fibers have drawn increasing interest for laser operations at wavelengths where Fused Silica-based (SiO₂) fibers do not perform well. One limitation to the expansion of ZBLAN fiber lasers today is the difficulty to efficiently inject and extract light in/from the guiding medium using SiO₂ fibers. Although free space and butt coupling have provided acceptable results, consistent and long lasting physical joints between SiO₂ and ZBLAN fibers will allow smaller, cheaper, and more robust component manufacturing.

While low loss splices have been reported using a traditional splicing approach, the very low mechanical strength of the joint makes it difficult to scale. Difficulties in achieving a strong bond are mainly due to the large difference of transition temperature between ZBLAN and SiO₂ fibers (∼260°C vs ∼1175°C).

This paper presents results obtained by using the high thermal expansion coefficient of the ZBLAN fiber to encapsulate a smaller SiO₂ fiber. A CO₂ laser glass processing system was used to control the expansion and contraction of the ZBLAN material during the splicing process for optimum reliability.

This method produced splices between 125μm ZBLAN to 80μm SiO2 fibers with average transmission loss of 0.225dB (measured at 1550nm) and average ultimate tension strength of 121.4gf. The Resulting splices can be durably packaged without excessive care. Other combinations using 125μm SiO₂ fibers tapered to 80μm are also discussed.

Due to the intrinsic absorption edge in silica near 2.4 μm, more exotic materials are required to transmit laser power in the IR such as fluoride or chalcogenide glasses (ChGs). In particular, ChG fibers offer broad IR transmission with low losses < 1 dB/m. Here, we report on the performance of in-house drawn multi-material chalcogenide fibers at four different infrared wavelengths: 2053 nm, 2520 nm and 4550 nm. Polymer clad ChG fibers were drawn with 12.3 μm and 25 μm core diameters. Testing at 2053 nm was accomplished using a > 15 W, CW Tm:fiber laser. Power handling up to 10.2 W with single mode beam quality has been demonstrated, limited only by the available Tm:fiber output power. Anti-reflective coatings were successfully deposited on the ChG fiber facets, allowing up to 90.6% transmission with 12.2 MW/cm² intensity on the facet. Single mode guidance at 4550 nm was also demonstrated using a quantum cascade laser (QCL). A custom optical system was constructed to efficiently couple the 0.8 NA QCL radiation into the 0.2 NA ChG fiber, allowing for a maximum of 78% overlap between the QCL radiation and fundamental mode of the fiber. With an AR-coated, 25 μm core diameter fiber, >50 mW transmission was demonstrated with > 87% transmission. Finally, we present results on fiber coupling from a free space Cr:ZnSe resonator at 2520 nm.

The realization of the Digital Optical Twin (DOT), which is in short the digital representation of the physical state of an optical system, is particularly useful in the context of an automated assembly process of optical systems. During the assembly process, the physical system status of the optical system is continuously measured and compared with the digital model. In case of deviations between physical state and the digital model, the latter one is adapted to match the physical state.

To reach the goal described above, in a first step measurement/characterization technologies concerning their suitability to generate a precise digital twin of an existing optical system have to be identified and evaluated. This paper gives an overview of possible characterization methods and, finally, shows first results of evaluated, compared methods (e.g. spot-radius, MTF, Zernike-polynomials), to create a DOT. The focus initially lies on the unequivocalness of the optimization results as well as on the computational time required for the optimization to reach the characterized system state. Possible sources of error are the measurement accuracy (to characterize the system) , execution time of the measurement, time needed to map the digital to the physical world (optimization step) as well as interface possibilities to integrate the measurement tool into an assembly cell. Moreover, it is to be discussed whether the used measurement methods are suitable for a ‘seamless’ integration into an assembly cell.

Optimal suppression of higher-order modes (HOMs) in hollow-core antiresonant fibers comprising a single ring of thin-walled capillaries was previously studied, and can be achieved when the condition on the capillary-tocore diameter ratio is satisfied (d/D ≈ 0.68). Here we report on the conditions for maximizing the leakage losses of HOMs in hollow-core nested antiresonant node-less fibers, while preserving low confinement loss for the fundamental mode. Using an analytical model based on coupled capillary waveguides, as well as full-vector finite element modeling, we show that optimal d/D value leading to high leakage losses of HOMs, is strongly correlated to the size of nested capillaries. We also show that extremely high value of degree of HOM suppression (∼1200) at the resonant coupling is almost unchanged on a wide range of nested capillary diameter dN ested values. These results thus suggest the possibility of designing antiresonant fibers with nested elements, which show optimal guiding performances in terms of the HOM loss compared to that of the fundamental mode, for clearly defined paired values of the ratios dN ested/d and d/D. These can also tend towards a single-mode behavior only when the dimensionless parameter dN ested/d is less than 0.30, with identical wall thicknesses for all of the capillaries.

Single Crystal Diamond has been recognized as a prime material for optical components in high power applications due to low absorption and high thermal conductivity. However, diamond microstructuring remains challenging. Here, we report on the fabrication and characterization of optical diffraction gratings exhibiting a symmetric trapezoidal profile etched into a single crystal diamond substrate. The optimized grating geometry diffracts the transmitted optical power into precisely defined proportions, performing as an effective beam splitter.

We fabricate our gratings in commercially available single crystal CVD diamond plates (2.6mm x 2.6mm x 0.3mm). Using a sputter deposited hard mask and patterning by contact lithography, the diamond is etched in an inductively coupled oxygen plasma with zero platen power. The etch process effectively reveals the characteristic {111} diamond crystal planes, creating a precisely defined angled (54.7°) profile. SEM and AFM measurements of the fabricated gratings evidence the trapezoidal shape with a pitch of 3.82μm, depth of 170 nm and duty cycle of 35.5%. Optical characterization is performed in transmission using a 650nm laser source perpendicular to the sample. The recorded transmitted optical power as function of detector rotation angle shows a distribution of 21.1% in the 0th order and 23.6% in each ±1st order (16.1% reflected, 16.6% in higher orders).

To our knowledge, this is the first demonstration of diffraction gratings with trapezoidal profile in single crystal diamond. The fabrication process will enable beam splitter gratings of custom defined optical power distribution profiles, while antireflection coatings can increase the efficiency.

An optimized design of a birefringent filter with an off-plane optical axis is presented to enable multi-wavelength operation of diode-pumped Yb-ion solid-state lasers. The simulation results indicate that such birefringent filters can be used for the development of powerful tunable multi-wavelength Yb-ion laser sources. A comparison with performance of a standard birefringent filter that has its optical axis lying in plane of a plate is also given.

The 7×1 end-pumped pump combiners employing 105/125 μm multimode fibers as pump fibers are investigated. Based on the results of our theoretical analysis, sufficient taper length (TL) and low refractive index (RI) of the capillary have been adopted to fabricate high transmission efficiency combiners. A 7×1 end-pumped pump combiner with an average transmission efficiency of 98.9% and a total return loss of 1.1‰ is fabricated in experiments, which could find its application in high-power fiber laser systems.

We present our current results on the fabrication of arbitrary shaped fiber tapers on our tapering rig using a CO2-laser as heat source. Single mode excitation of multimode fibers as well as changing the fiber geometry in an LPG-like fashion is presented. It is shown that this setup allows for reproducible fabrication of single-mode excitation tapers to extract the fundamental mode (M² < 1.1) from a 30 μm core having an NA of 0.09.

A simple and powerful method using continuous wave supercontinuum lasers is demonstrated to perform spectrally resolved, broadband frequency response characterization of photodetectors in the NIR Band. In contrast to existing techniques, this method allows for a simple system to achieve the goal, requiring just a standard continuous wave(CW) high-power fiber laser source and an RF spectrum analyzer. From our recent work, we summarize methods to easily convert any high-power fiber laser into a CW supercontinuum. These sources in the time domain exhibit interesting properties all the way down to the femtosecond time scale. This enables measurement of broadband frequency response of photodetectors while the wide optical spectrum of the supercontinuum can be spectrally filtered to obtain this information in a spectrally resolved fashion. The method involves looking at the RF spectrum of the output of a photodetector under test when incident with the supercontinuum. By using prior knowledge of the RF spectrum of the source, the frequency response can be calculated. We utilize two techniques for calibration of the source spectrum, one using a prior measurement and the other relying on a fitted model. Here, we characterize multiple photodetectors from 150MHz bandwidth to >20GHz bandwidth at multiple bands in the NIR region. We utilize a supercontinuum source spanning over 700nm bandwidth from 1300nm to 2000nm. For spectrally resolved measurement, we utilize multiple wavelength bands such as around 1400nm and 1600nm. Interesting behavior was observed in the frequency response of the photodetectors when comparing broadband spectral excitation versus narrower band excitation.

We report on novel fabrication schemes of versatile cladding light strippers and end-caps via CO₂ laser radiation. We integrated cladding light strippers in SMA-like connectors for reliable and stable fiber-coupling of high-power laser diodes. Moreover, the application of cladding light strippers in typical fiber geometries for high-power fiber lasers was evaluated. In addition, we also developed processes to fuse end-caps to fiber end faces via CO₂ laser radiation and inscribe the fibers with cladding light strippers near the end-cap. Corresponding results indicate the great potential of such devices as a monolithic and low-cost alternative to SMA connectors.

Cladding mode strippers are primarily used at the end of a fiber laser cavity to remove high-power excess cladding light without inducing core loss and beam quality degradation. Conventional manufacturing methods of cladding mode strippers include acid etching, abrasive blasting or laser ablation. Manufacturing of cladding mode strippers using laser ablation consist of removing parts of the cladding by fused silica ablation with a controlled penetration and shape.

We present and characterize an optimized cladding mode stripper design that increases the cladding light loss with a minimal device length and manufacturing time. This design reduces the localized heat generation by improving the heat distribution along the device. We demonstrate a cladding mode stripper written on a 400um fiber with cladding light loss of 20dB, with less than 0.02dB loss in the core and minimal heating of the fiber and coating. The manufacturing process of the designed component is fully automated and takes less than 3 minutes with a very high throughput yield.

High power laser diodes have been found a wide range of industrial, space, medical applications, characterized by high conversion efficiency, small size, light weight and a long lifetime. However, due to thermal induced stress, each emitter in a semiconductor laser bar or array is displaced along p-n junction, resulting of each emitter is not in a line, called Near-field Non-linearity. Near-field Non-linearity along laser bar (also known as “SMILE”) determines the outcome of optical coupling and beam shaping [1]. The SMILE of a laser array is the main obstacle to obtain good optical coupling efficiency and beam shaping from a laser array. Larger SMILE value causes a larger divergence angle and a wider line after collimation and focusing, respectively. In this letter, we simulate two different package structures based on MCC (Micro Channel Cooler) with Indium and AuSn solders, including the distribution of normal stress and the SMILE value. According to the theoretical results, we found the distribution of normal stress on laser bar shows the largest in the middle and drops rapidly near both ends. At last, we did another experiment to prove that the SMILE value of a laser bar was mainly affected by the die bonding process, rather than the operating condition.