Proc. SPIE. 10086, High-Power Diode Laser Technology XV
KEYWORDS: Packaging, Lasers, High power lasers, Collimators, Collimation, Micro optics, Optical alignment, High power diode lasers, Heatsinks, Adhesives, Tolerancing, Assembly tolerances, Laser systems engineering
High Power Diode Laser (HPDL) systems with short focal length fast-axis collimators (FAC) require submicron assembly precision. Conventional FAC-Lens assembly processes require adhesive gaps of 50 microns or more in order to compensate for component tolerances (e.g. deviation of back focal length) and previous assembly steps.
In order to control volumetric shrinkage of fast-curing UV-adhesives shrinkage compensation is mandatory. The novel approach described in this paper aims to minimize the impact of volumetric shrinkage due to the adhesive gap between HPDL edge emitters and FAC-Lens. Firstly, the FAC is actively aligned to the edge emitter without adhesives or bottom tab. The relative position and orientation of FAC to emitter are measured and stored.
Consecutively, an individual subassembly of FAC and bottom tab is assembled on Fraunhofer IPT’s mounting station with a precision of ±1 micron.
Translational and lateral offsets can be compensated, so that a narrow and uniform glue gap for the consecutive bonding process of bottom tab to heatsink applies (Figure 4). Accordingly, FAC and bottom tab are mounted to the heatsink without major shrinkage compensation.
Fraunhofer IPT’s department assembly of optical systems and automation has made several publications regarding active alignment of FAC lenses [SPIE LASE 8241-12], volumetric shrinkage compensation [SPIE LASE 9730-28] and FAC on bottom tab assembly [SPIE LASE 9727-31] in automated production environments. The approach described in this paper combines these and is the logical continuation of that work towards higher quality of HPDLs.
The assembly process of optical components consists of two phases – the alignment and the bonding phase. Precision - or better process repeatability - is limited by the latter one. The limitation of the alignment precision is given by the measurement equipment and the manipulation technology applied. Today’s micromanipulators in combination with beam imaging setups allow for an alignment in the range of far below 100nm. However, once precisely aligned optics need to be fixed in their position. State o f the art in optics bonding for laser systems is adhesive bonding with UV-curing adhesives. Adhesive bonding is a multi-factorial process and thus subject to statistical process deviations. As a matter of fact, UV-curing adhesives inherit shrinkage effects during their curing process, making offsets for shrinkage compensation mandatory. Enhancing the process control of the adhesive bonding process is the major goal of the activities described in this paper. To improve the precision of shrinkage compensation a dynamic shrinkage prediction is envisioned by Fraunhofer IPT. Intense research activities are being practiced to gather a deeper understanding of the parameters influencing adhesive shrinkage behavior. These effects are of different nature – obviously being the raw adhesive material itself as well as its condition, the bonding geometry, environmental parameters like surrounding temperature and of course process parameters such as curing properties. Understanding the major parameters and linking them in a model-based shrinkage-prediction environment is the basis for improved process control. Results are being deployed by Fraunhofer in prototyping, as well as volume production solutions for laser systems.
Depending on the application, high-power diode lasers (HPDL) have individual requirements on their beam-shaping as well as their mechanical fixation. In order to reduce assembly efforts, laser system manufacturers request pre-assembled beam-shaping systems consisting of a support structure for adhesive bonding as well as one, two or more lenses. Therefore, manufacturers of micro-optics for HPDL need flexible solutions for assembling beam-shaping subassemblies. This paper discusses current solutions for mounting optical subassemblies for beam-shaping of high-power diode lasers and their drawbacks regarding quality and scalability. Subsequently, the paper presents a device which can be used for the sensor-guided assembly of beam-shaping systems based on bottomtab support structures. Results from test productions of several hundred modules are presented showing that repeatability in the range of 1 μm is feasible on an industrial level.
The assembly of optical components for laser systems is proprietary knowledge and typically done by well-trained personnel in clean room environment as it has major impact on the overall laser performance. Rising numbers of laser systems drives laser production to industrial-level automation solutions allowing for high volumes by simultaneously ensuring stable quality, lots of variants and low cost. Therefore, an easy programmable, expandable and reconfigurable machine with intuitive and flexible software environment for process configuration is required. With Fraunhofer IPT’s expertise on optical assembly processes, the next step towards industrializing the production of optical systems is made.
Today’s piezo-based micromanipulator technology allows for highly precise manipulation of optical
components. A crucial question for the quality of optical assemblies is the misalignment after curing. The
challenge of statistical deviations in the curing process requires a sophisticated knowledge on the relevant
process parameters. An approach to meet these requirements is the empirical analysis such as characterization of
shrinkage. Gaining sophisticated knowledge about the statistical process of adhesive bonding advances the
quality of related production steps like beam-shaping optics, mounting of turning mirrors for fiber coupling or
building resonators evaluating power, mode characteristics and beam shape. Maximizing the precision of these
single assembly steps fosters the scope of improving the overall efficiency of the entire laser system. At
Fraunhofer IPT research activities on the identification of relevant parameters for improved adhesive bonding
precision have been undertaken and are ongoing. The influence of the volumetric repeatability of different
automatic and manual dispensing methods play an important role. Also, the evaluation of UV-light sources and
the relating illumination properties have a significant influence on the bonding result. Furthermore, common
UV-curing adhesives are being examined on their performance and reliability for both highest precision
prototyping, as well as their application as robust bonding medium in automated optics assembly cells. This
paper sums up the parameters of most influence. Overall goal of these activities is the development of a
prediction model for optimized shrinkage compensation and thus improved assembly quality.
In many applications for high-power diode lasers, the production of beam-shaping and homogenizing optical systems
experience rising volumes and dynamical market demands. The automation of assembly processes on flexible and
reconfigurable machines can contribute to a more responsive and scalable production. The paper presents a flexible
mounting device designed for the challenging assembly of side-tab based optical systems. It provides design elements for
precisely referencing and fixating two optical elements in a well-defined geometric relation. Side tabs are presented to
the machine allowing the application of glue and a rotating mechanism allows the attachment to the optical elements.
The device can be adjusted to fit different form factors and it can be used in high-volume assembly machines. The paper
shows the utilization of the device for a collimation module consisting of a fast-axis and a slow-axis collimation lens.
Results regarding the repeatability and process capability of bonding side tab assemblies as well as estimates from 3D
simulation for overall performance indicators achieved such as cycle time and throughput will be discussed.
In this paper, we present hybrid assembly technology to maximize coupling efficiency for spatially combined laser systems. High quality components, such as center-turned focusing units, as well as suitable assembly strategies are necessary to obtain highest possible output ratios. Alignment strategies are challenging tasks due to their complexity and sensitivity. Especially in low-volume production fully automated systems are economically at a disadvantage, as operator experience is often expensive. However reproducibility and quality of automatically assembled systems can be superior. Therefore automated and manual assembly techniques are combined to obtain high coupling efficiency while preserving maximum flexibility. The paper will describe necessary equipment and software to enable hybrid assembly processes. Micromanipulator technology with high step-resolution and six degrees of freedom provide a large number of possible evaluation points. Automated algorithms are necess ary to speed-up data gathering and alignment to efficiently utilize available granularity for manual assembly processes. Furthermore, an engineering environment is presented to enable rapid prototyping of automation tasks with simultaneous data ev aluation. Integration with simulation environments, e.g. Zemax, allows the verification of assembly strategies in advance. Data driven decision making ensures constant high quality, documents the assembly process and is a basis for further improvement. The hybrid assembly technology has been applied on several applications for efficiencies above 80% and will be discussed in this paper. High level coupling efficiency has been achieved with minimized assembly as a result of semi-automated alignment. This paper will focus on hybrid automation for optimizing and attaching turning mirrors and collimation lenses.
In the assembly of optical resonators of optically pumped semiconductor lasers (OPSL), the highly reflective resonator mirror is the most crucial component. In previous cooperation, Coherent and Fraunhofer IPT have developed a robust active alignment strategy to optimize the output power of the OPSL resonator using search strategies for finding the laser threshold as well as hill-climbing algorithms for maximizing the output power. Beam-shape as well as the laser mode have major influence on the quality and the duration of subsequent beam-shaping and fiber-coupling steps. Therefore, the alignment algorithm optimizing the output power has been extended recently by simultaneous image processing for ensuring a Gaussian beam as the result of alignment. The paper describes the enhanced approach of automated alignment by additionally scanning along the optical resonator and subsequently evaluating and optimizing the roundness of the beam as well as minimizing the beam radius through twisting and tilting of the mirror. A quality metric combining these measures is defined substituting an M² measurement. The paper also describes the approach for automated assembly including the measuring setup, micromanipulation and dispensing devices.
In science and industry, the alignment of beam-shaping optics is usually a manual procedure. Many industrial applications utilizing beam-shaping optical systems require more scalable production solutions and therefore effort has been invested in research regarding the automation of optics assembly. In previous works, the authors and other researchers have proven the feasibility of automated alignment of beam-shaping optics such as collimation lenses or homogenization optics. Nevertheless, the planning efforts as well as additional knowledge from the fields of automation and control required for such alignment processes are immense. This paper presents a novel approach of planning active alignment processes of beam-shaping optics with the focus of minimizing the planning efforts for active alignment. The approach utilizes optical simulation and the genetic programming paradigm from computer science for automatically extracting features from a simulated data basis with a high correlation coefficient regarding the individual degrees of freedom of alignment. The strategy is capable of finding active alignment strategies that can be executed by an automated assembly system. The paper presents a tool making the algorithm available to end-users and it discusses the results of planning the active alignment of the well-known assembly of a fast-axis collimator. The paper concludes with an outlook on the transferability to other use cases such as application specific intensity distributions which will benefit from reduced planning efforts.
In this contribution, we present a novel approach to enable virtual commissioning for process developers in micro-optical assembly. Our approach aims at supporting micro-optics experts to effectively develop assisted or fully automated assembly solutions without detailed prior experience in programming while at the same time enabling them to easily implement their own libraries of expert schemes and algorithms for handling optical components. Virtual commissioning is enabled by a 3D simulation and visualization system in which the functionalities and properties of automated systems are modeled, simulated and controlled based on multi-agent systems. For process development, our approach supports event-, state- and time-based visual programming techniques for the agents and allows for their kinematic motion simulation in combination with looped-in simulation results for the optical components. First results have been achieved for simply switching the agents to command the real hardware setup after successful process implementation and validation in the virtual environment. We evaluated and adapted our system to meet the requirements set by industrial partners-- laser manufacturers as well as hardware suppliers of assembly platforms. The concept is applied to the automated assembly of optical components for optically pumped semiconductor lasers and positioning of optical components for beam-shaping
In micro-optical assembly, the mastering of the steps of passive and active alignment, bonding, and part feeding as well as their interdependencies are crucial to the success of an automation solution. Process development is therefore complex and time consuming. Separation of assembly process planning and assembly execution decouples both phases so that production and process development can take place in parallel and even in spatially separated stations. The work presented in this paper refines the concept of flexible assembly systems by separating the phases of assembly process planning and assembly execution by providing a dedicated process development platform on the one hand and by providing automatisms regarding the transfer from the development platform into industrial production on the other. For this purpose, two key concepts are being developed by the research carried out at Fraunhofer IPT. The paper introduces the overall approach and formalisms as well as a form of notation based on part lists, product features and key characteristics and it shows industrial use cases the approach has been applied to. Key characteristics are constraints on spatial relations and they are expressed in terms of optical functions or geometric constraints which need to be fulfilled. In the paper, special attention is paid to the illustration of the end-user perspective.
Diode lasers are gaining importance, making their way to higher output powers along with improved BPP. The assembly of micro-optics for diode laser systems goes along with the highest requirements regarding assembly precision. Assembly costs for micro-optics are driven by the requirements regarding alignment in a submicron and the corresponding challenges induced by adhesive bonding. For micro-optic assembly tasks a major challenge in adhesive bonding at highest precision level is the fact, that the bonding process is irreversible. Accordingly, the first bonding attempt needs to be successful. Today’s UV-curing adhesives inherit shrinkage effects crucial for submicron tolerances of e.g. FACs. The impact of the shrinkage effects can be tackled by a suitable bonding area design, such as minimal adhesive gaps and an adapted shrinkage offset value for the specific assembly parameters. Compensating shrinkage effects is difficult, as the shrinkage of UV-curing adhesives is not constant between two different lots and varies even over the storage period even under ideal circumstances as first test results indicate. An up-to-date characterization of the adhesive appears necessary for maximum precision in optics assembly to reach highest output yields, minimal tolerances and ideal beamshaping results. Therefore, a measurement setup to precisely determine the up-to-date level of shrinkage has been setup. The goal is to provide necessary information on current shrinkage to the operator or assembly cell to adjust the compensation offset on a daily basis. Impacts of this information are expected to be an improved beam shaping result and a first-time-right production.
Beam-shaping is essential for any kind of laser application. Assembly technologies for beam-shaping subassemblies are subject to intense research and development activities and their technical feasibility has been proven in recent years while economic viability requires more efficient engineering tools for process planning and production ramp up of complex assembly tasks for micro-optical systems. The work presented in this paper aims for significant reduction of process development and production ramp up times for the automated assembly of micro-optical subassemblies for beam-collimation and beam-tilting. The approach proposed bridges the gap between the product development phase and the realization of automation control through integration of established software tools such as optics simulation and CAD modeling as well as through introduction of novel software tools and methods to efficiently describe active alignment strategies. The focus of the paper is put on the methodological approach regarding the engineering of assembly processes for beam-shaping micro-optics and the formal representation of assembly objectives similar to representation in mechanical assemblies. Main topic of the paper is the engineering methodology for active alignment processes based on the classification of optical functions for beam-shaping optics and corresponding standardized measurement setups including adaptable alignment algorithms. The concepts are applied to industrial use-cases: (1) integrated collimation module for fast- and slow-axis and (2) beam-tilting subassembly consisting of a fast-axis collimator and micro-lens array. The paper concludes with an overview of current limitations as well as an outlook on the next development steps considering adhesive bonding processes.
Due to the architectural advantage of an external cavity architecture that enables the integration of additional elements into the cavity (e.g. for mode control, frequency conversion, wavelength tuning or passive mode-locking) VECSELs are a rapidly developing laser technology. Nevertheless they often have to compete with direct (edge) emitting laser diodes which can have significant cost advantages thanks to their rather simple structure and production processes. One way to compensate the economical disadvantages of VECSELs is to optimize each component in terms of quality and costs and to apply more efficient (batch) production processes. In this context, the paper presents recent process developments for the automated assembly of VECSELs using a new type of desktop assembly station with an ultra-precise micromanipulator. The core concept is to create a dedicated process development environment from which implemented processes can be transferred fluently to production equipment. By now two types of processes have been put into operation on the desktop assembly station: 1.) passive alignment of the pump optics implementing a camera-based alignment process, where the pump spot geometry and position on the semiconductor chip is analyzed and evaluated; 2.) active alignment of the end mirror based on output power measurements and optimization algorithms. In addition to the core concept and corresponding hardware and software developments, detailed results of both processes are presented explaining measurement setups as well as alignment strategies and results.
In this paper details on the analysis and optimization of a flexure-based micromanipulator for the alignment of optical
components will be presented. The developments are motivated by a concept for flexible precision assembly that will be
described in the first section. The development of the manipulator itself is based on a systematic approach to first define
suitable kinematical structure for a six-axes device. The kinematics have modeled to allow mathematical analyses of the
main geometric parameters on relevant performance characteristics. A stepwise optimization procedure has been
developed to define the smallest possible configuration of the mechanism for specific workspace requirements.
The realized design of the manipulator will be presented which is taking into account further design aspects such as the
choice of suitable actuators and the design of flexure joints. By means of interferometer measurements a motion
resolution in the nanometer range could be proved as well as a high repeatability of 0,15 μm.
Despite major progress in developing brilliant laser sources a huge potential for cost reductions can be found in simpler
setups and automated assembly processes, especially for large volume applications. In this presentation, a concept for
flexible automation in optics assembly is presented which is based on standard micro assembly systems with relatively
large workspace and modular micromanipulators to enhance the system with additional degrees of freedom and a very
high motion resolution. The core component is a compact flexure-based micromanipulator especially designed for the
alignment of micro optical components which will be described in detail. The manipulator has been applied in different
scenarios to develop and investigate automated alignment processes.
This paper focuses on the automated alignment of fast axis collimation (FAC) lenses which is a crucial step during the
production of diode lasers. The handling and positioning system, the measuring arrangement for process feedback during
active alignment as well as the alignment strategy will be described. The fine alignment of the FAC lens is performed
with the micromanipulator under concurrent analysis of the far and the near field intensity distribution. An optimization
of the image processing chains for the alignment of a FAC in front of a diode bar led to cycle times of less than
An outlook on other applications and future work regarding the development of automated assembly processes as well as
new ideas for flexible assembly systems with desktop robots will close the talk.