High output power and reliability due to improved laser facet stability
Fig. 1. High annular dark field STEM images of passivated facets in the quantum well region fabricated by hydrogen cleaning (a) and cleaving in vacuum (b).
Fig. 2. FBH’s novel UHV cleaving and passivation system.
Fig. 3. Lifetime tests for two types of RWLs investigated (a) 980 nm RWLs, (b) 1064 nm RWLs.
GaAs-based laser diodes are among the most energy efficient light sources currently available. Therefore, they have a wide application spectrum, ranging from material processing to sensing applications. Future space-based applications require ridge-waveguide lasers (RWLs) and amplifiers with a very high output power and a simultaneously high reliability. These requirements cannot be met using the long-proven laser facet passivation process employed at FBH [1], [2]. This patented method relies on removing the native oxide from the laser facet by means of atomic hydrogen cleaning and subsequent passivation of the laser facet by ZnSe. However, it is not possible to remove the complete surface oxide from the (Al,Ga)As layers of the laser diode using this method. The presence of this oxide layer can be clearly seen in Fig. 1(a) as a dark line in cross-sectional scanning transmission electron microscopy (STEM) images of the laser facet of a passivated RWL. It is likely that this oxide limits the reliability of the laser diodes. Facets without the native oxide are therefore desirable.
Cleaving laser bars in ultra-high vacuum (UHV) prevents laser facets from oxidation and permits entirely oxide-free ZnSe passivated facets. To improve the reliability of edge-emitting lasers, FBH has acquired a system capable of UHV cleaving and ZnSe passivation in 2020, see Fig. 2. The STEM image of a laser facet passivated with this system is shown in Fig. 1(b). It is easy to observe that no oxide is present between (Al,Ga)As and ZnSe, because no dark line shows. This illustrates that facet passivation with this new approach works as expected.
We investigated the potential of this new passivation method on device reliability using 980 nm and 1064 nm RWLs, see Fig. 3. The output power of the RWLs was increased step by step up to the maximum output power (limited by thermal rollover). For the 980 nm RWLs, we observed excellent results for both types of passivation methods. For the RWLs fabricated by UHV cleaving, the aging was stopped after 2,000 hours due to time limitations. Within this timeframe, a single failure unrelated to the facet occurred. By contrast, the 1064 nm RWLs obtained by hydrogen cleaning have only limited reliability, failing at an output power of 0.4 W. The RWLs fabricated by UHV cleaving, on the other hand, didn’t show any sign of facet degradation (again a single RWL died from an internal defect) and reached output powers of 1.4 W [3].
These results demonstrate that a significant higher stability of laser facets can be realized using UHV cleaving compared to hydrogen cleaning. This improvement is a key enabler to qualify lasers and amplifiers for future space-based applications. Building on these results, similar improvements are expected for lasers operating at other wavelengths as well as for broad area lasers.
Publications
[1] P. Ressel, G. Ebert, U. Zeimer, K. Häusler, G. Beister, B. Sumpf, A. Klehr and G. Tränkle, “Novel passivation process for the mirror facets of Al-free active-region high-power semiconductor diode lasers,” IEEE Photonics Technol. Lett., vol. 17, no. 5, pp. 962–964, May 2005, doi: 10.1109/LPT.2005.846750.
[2] P. Ressel and G. Erbert, “Method for the passivation of the mirror-faces surfaces of optical semi-conductor elements,” US7338821B2 (2008).
[3] J. E. Boschker, U. Spengler, P. Ressel, M. Schmidbauer, A. Mogilatenko and A. Knigge, "Stability of ZnSe-Passivated Laser Facets Cleaved in Air and in Ultra-High Vacuum", IEEE Photonics Journal, vol. 14, no. 3, pp. 1-6, June 2022, Art no. 1531606, doi: 10.1109/JPHOT.2022.3176675.