Sebastian Sauer

Model-based adhesive shrinkage compensation for increased bonding repeatability

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.

Strategies for precision adhesive bonding of micro-optical systems

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.

Automated assembly of lens barrels with active wavefront sensor guiding

Miniaturized optics are main-components in many different areas ranging from smart devices over medical products to the area of automotive and mobility. Thus several millions if not billions of small lenses are merged into objectives. One characteristic type of objective holder is the lens barrel. The successful assembly of lenses with diameters of just a couple of millimeters into a lens barrel is an error-prone task antagonized with mass production and an optical inspection at the end of the assembly. Obviously, this process is neither time- nor cost-effective. Furthermore, the increasing imaging qualities demand for highly accurate aligned lens systems. The demand for high-quality optics in large quantities together with the small dimensions of the lenses make assembling a complex process. The Fraunhofer IPT investigates a much more elegant way inspecting the optical system during the fully automated assembly. In the assembly cell, our six-axis micromanipulator aligns the lens camera-led in the lens barrel. Next, the wavefront sensor analyses the imaging function of the lens and compares the actual status with the data from the optic model. This feedback loop between wavefront sensor and micromanipulator continues until the best position is found. We save this information as a digital twin and continue with the next lenses until the optics is completed. The observation of the optical function during the assembly process leads to high quality objectives produced in short cycle times. Moreover, our assembly cell is modular and this allows us to adopt the setup for new lens barrels easily.

Characterization of individualized assembly for BFL-compensated FAC on bottom tab modules (Conference Presentation)

Uncertain glue gaps lead to challenging assembly tasks in respect to shrinkage control. With decreasing back-focal lengths (BFL) in collimators, the tolerance window for correct alignments decreases as well and forces manufactures to find novel approaches to realize the bonding process. We present performance characteristics of an automated assembly cell for individualized FAC on bottom tab modules. BFL-compensated collimators allow minimizing the critical adhesive gap between substrate and diode laser. This provides optimal control over shrinkage, as well as thermal aspects of the bonding properties. We will focus on the active alignment, which provides the individual focus distance, as well as the relative image processing necessary to assemble both components with ±1 µm precision. Our machine concept and measurement equipment is suitable as stand-alone process for optic manufacturers, as well as integrated part in the final application assembly. In last year’s publication (SPIE 10086), we presented the general concept and can now support our approach with more details from our operating data. With minimized adhesive gaps, the robustness of the proposed concept and a precise characterization of its process window is key, as minimal variations lead to rejects and cause high costs during the final application assembly. Besides classic properties, many more characteristics, e.g. smile behavior of the optic module, are potential optimization factors to increase beam quality. Characterization data from both optic and laser allow applying tolerance matching, where alignment is physically constraint. Performance wise, we will discuss the repeatability, achievable precision and the implications on process time.

Individualized FAC on bottom tab subassemblies to minimize adhesive gap between emitter and optics II (Conference Presentation)

The quality of High Power Diode Laser (HPDL) systems highly depends on the assembly precision. Nowadays, neither the precision of the manipulation tools (step resolution < 10 nm) nor the measurement systems utilized in active alignment algorithms (alignment precision of ~50 nm) are the quality limiting factors but the bonding process is. This is due to the volumetric shrinkage of fast curing UV-adhesives in the curing process. The objective of this work is to minimize the absolute volumetric shrinkage of the UV curing adhesives between edge emitter and bottom tab so no significant misalignment while curing is expected. The approach was first described in the paper [SPIE 10086-28] and aims for minimizing the glue gap and therefore the amount of adhesive through combining active alignment of fast axis collimators (FAC) to edge emitter with a tolerance compensated individualized FAC on bottom tab subassembly in a fully automated production process. With less adhesive the absolute volumetric shrinkage is reduced. The expected benefits are the reduction of the misalignment through volumetric shrinkage and a 100% quality assessment without additional costs. Lens quality data such as smile, residual divergence and optical surface imperfections can be characterized. A permanent data collection provides feedback for all previous and following production systems and allows the improvement of the quality for the whole HPDL production chain. This paper presents the results gathered by implementing the individualized FAC on bottom tab process in an industrial production environment and compares it to the expected benefits to conventional HPDL production.

Curing-in-the-loop strategy for multidimensional-shrinkage compensation in active alignment FAC assembly (Conference Presentation)

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.

Active alignment of DOE based structured light application in consumer electronics

New applications with 3D sensing technologies are entering the market every day. Based on diffractive optical elements, structured light is a key factor to realize computer vision based sensing solutions. Active alignment can compensate uncertain tolerances during early stages of the product development, which eases prototyping and ramp up of production. Built upon a customizable micromanipulator design, a specialized part handling system with integrated measurement and UV curing capabilities has been designed. We will discuss our production solution and alignment algorithm and present how it can be scaled for high volume production.

Individualized FAC on bottom tab subassemblies to minimize adhesive gap between emitter and optics

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.

Machine platform and software environment for rapid optics assembly process development

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.

Simultaneous power and beam-shape optimization of an OPSL resonator

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.