Hermetic and reliable packaging of single-emitter laser diode chips in TO package for operation in harsh environments
Fig. 1: Laser diodes sealed in TO packages.
Fig. 2: PUI characteristics of an uncapped and capped 633 nm laser diode for external resonator operation.
Fig. 3: Comparison of the aging results of 420 nm GaN laser diodes sealed into a TO package and mounted on a C-mount.
Laser diodes manufactured at FBH can usually be operated without being sealed under normal room atmosphere. However, for operation in harsh environments with, e.g., high humidity, fine dust or reactive gases, hermetic packaging of the laser chips is mandatory.
For some time now, we have been hermetically packing laser modules that require a considerable amount of space to integrate components such as chips and/or lenses, etc. using so-called butterfly packages or similar housings. Here, the light is coupled out via an optical window integrated into the package with an anti-reflective coating or via a fiber. After all components have been integrated into the butterfly package, the housing is filled with a buffer gas and hermetically sealed with a metal lid using the roll seam welding process.
With a focus on simplicity and compactness, a slightly different technological approach has now been established at FBH for single-emitter lasers. It can be used e.g. for economic lifetime investigations under defined buffer gas compositions. Transistor Outline (TO) packages have been successfully introduced for this purpose. The laser diode is mounted into this package on a pillar, located approximately in the middle of the header. In contrast to the butterfly package, hermetic sealing is achieved by projection welding of the caps, and the laser light is also emitted differently through an anti-reflective coated optical window in the top of the welded cap.
We have been investigating TO packages of various sizes for small laser diode chips emitting from the UV to NIR spectral range for some time. Processes for gluing and soldering the submount with the laser chip to the header were developed and qualified, e.g., via shear tests and thermal resistance measurements. For example, for p-down mounted broad area lasers based on GaAs with 130 µm x 2000 µm stripes emitting at a wavelength of 808 nm, an overall thermal resistance of about 11 K/W could be derived – regardless of whether the submount was soldered or glued. The estimated contribution of the bonding layer between submount and package is only about 3 K/W.
After mounting the laser chips on TO heads, geometrically and optically matching caps (see Fig. 1) were welded on top. The tightness of the TO housings was confirmed by gross and fine leakage tests. Reasonable leakage rates below 5 x 10-8 mbar l/s (gas-tight) were determined. Measurements to determine the performance of 633 nm lasers based on GaAs before and after capping did not show any discernible changes in the PUI characteristics. This means that optical power losses due to shadowing or back reflection at the cap can be neglected, as can be seen in Fig. 2. Furthermore, laser diode chips based on GaN emitting at around 420 nm have been mounted into TO packages as well. They seem to show a more stable operation compared to unsealed lasers (see Fig. 3). Further studies are needed to support this finding.
These and further investigations can now be easily realized, as FBH has a simple, compact and cost-effective hermetic mounting solution using TO packages. We are thus able to provide our partners from research and industry with innovative lasers in TO packages for use in harsh environments.
This work was supported by the German Space Agency DLR with funds provided by the Federal Ministry for Economic Affairs and Climate Action (BMWK) under grant number 50WM2179. Further funds came from the Federal Ministry of Education and Research (BMBF) with co-funding from the European Union Horizon 2020 program under grant numbers 01QE1903C and E112864.