We present an optical design concept to create a two-dimensional tweezer array for neutral atom quantum computing. The first part of the setup splits the four input laser beams into 20 individual beam lines that correspond to the columns of the tweezer array. A combination of an acousto-optic deflector (AOD) and a suitable imaging lens for each beam line creates a magnified image of the one-dimensional columns in the tweezer array with a spacing of less than 200 μm. The spacing between the 20 columns is reduced to this value as well to achieve a spot array with an equidistant spacing in both axes. This is done successively in four steps by using a combination of one discrete mirror cascade and three mirror cascades integrated into monolithic mirror arrays. The final tweezer array with a spacing of 3.5 μm is then created by a two-stage telecentric demagnifying lens group. Unlike other approaches to create a two-dimensional tweezer array (e.g. crossed AODs), the use of a separate AOD for each column allows for an independent adjustment of the spacings within each column and thus a flexible adjustment of the local interaction of neighboring atoms. A raytracing analysis demonstrates that the whole setup produces a telecentric and diffraction-limited image of 20 × 100 spots in the following vacuum chamber.
In this work, we compare four different design concepts for external-cavity laser diodes (ECDL) with respect to the maximum achievable output power before the onset of catastrophic optical damage (COD). A multiphysics model of the ECDL with a self-consistent description of the electrical, optical and thermal properties of the device is used to evaluate the COD level. The feedback-induced failure is provoked by shifting the fast axis collimation (FAC) lens along the fast axis (smile error) resulting in an absorption of the feedback radiation within the highly p-doped and contact metal layers. The investigated design concepts include three local modifications at the front facet of the laser diode chip itself which are supposed to suppress injected current, optical absorption and leakage current from the quantum well. Within the considered parameter space these approaches lead to an increase of the COD level by 8%, 27% and 27% respectively, however at the cost of drawbacks like slightly reduced efficiency or beam quality along the fast axis. By combining all three approaches the output power can be increased by 37%. The fourth approach uses an additional lens within the external resonator to make it bi-telecentric and allows for a feedback field without image reversal. This approach completely removes the sensitivity of the setup regarding a vertical misalignment of the FAC lens. The drawback in this case is the increase of the resonator size by approximately a factor of 20.
Pursuit of an academic career is often co associated with a PhD which also is a qualification for all types of jobs in industry. However, in Germany most PhD programs focus on university-based, basic, and applied research and aim to demonstrate concepts while the transfer to industry and real products is subordinate. This is where Fraunhofer comes in: our natural science and engineering PhD students participate in solving real-world problems for our industrial customers with innovative and scientific approaches while they simultaneously pursue basic research questions with an application relevance for their PhD thesis. As an example, in this paper we present a multiphysics laser diode simulation software (SEMSIS) which was developed within two industrially funded PhD projects at the Fraunhofer Institute for Laser technology ILT. In the fusion research, a vast number of high-power laser diodes are used as pump sources for the high-energy pulsed lasers in inertial confinement fusion. Improving their electro-optical efficiency and making them more robust against external optical feedback represents a crucial step towards their use in economically competitive fusion power plants. In the presented simulation software tool SEMSIS, the complex interaction of electrical, optical, thermal as well as mechanical properties and their impact on efficiency, filamentation and reliability of high-power diode lasers can be analyzed to address the previously mentioned requirements in fusion research.
In this work, we use a multiphysics model of an external-cavity laser diode to study the influence of misaligned external optical feedback on the COD level of the device. The model solves the drift-diffusion equations for the electrical transport in the vertical-longitudinal plane self-consistently with a wave-optical model (including semiconductor chip and external resonator) and a 3D thermal model of chip and submount. A vertical misalignment of the FAC lens in an external resonator configuration consisting only of the FAC lens and feedback mirror leads to strongly reduced COD levels within the simulation if the feedback radiation hits the metal layers on the p-side of the device. The absorbed feedback radiation is the initial driver for the COD, whereas vertical leakage currents lead to ever increasing temperatures during thermal runaway. Experimental data of pulsed COD tests confirm the simulation results qualitatively. The minimum absorbed optical feedback power leading to COD depends on the operating point of the device. It increases with increasing external reflectivity due to the onset of COD at lower currents and corresponding lower internal optical power densities. For a low external reflectivity the output power is limited by thermal rollover instead of COD. The surface recombination velocity as the parameter quantifying the facet passivation quality has only a minor influence on the COD level in the simulation as for a low surface recombination velocity (high facet quality) the carriers can still recombine nonradiatively in the bulk layers due to the vertical leakage currents.
KEYWORDS: 3D modeling, Thermal modeling, Optical damage, High power lasers, Diodes, Quantum wells, Waveguides, Thermography, Temperature metrology, Semiconductor lasers
The process of catastrophic optical damage (COD) in 9xx-nm laser diodes is typically divided into three phases. In this work we model the first phase of COD by placing a localized additional heat source near the front facet corresponding to accumulated defects or misaligned optical feedback. We then compare two different multiphysical models to investigate thermal runaway, the second phase of COD. The first model considers only the carrier density within the quantum well coupled to a lateral-longitudinal optical model and a 3D thermal model. For this model, the temperature distribution converges within a few iteration steps without indication of thermal runaway and irrespective of the power of the additional heat source. The second model self-consistently computes the electrical and optical properties in the vertical-longitudinal plane and the 3D temperature distribution of the device. A critical power of the additional heat source is found above which the temperature distribution does not converge anymore and the maximum temperature increases to values above 1000 K. This strong temperature increase is accompanied by a thermally induced current crowding near the front facet and excessive carrier leakage from the quantum well. An analysis of the contributions of different heat sources shows that the nonradiative recombination in the waveguide and cladding layers exhibits the strongest changes during thermal runaway. The results of the two models indicate that the frequently proposed explanation of the feedback loop for thermal runaway consisting of a thermally induced bandgap shrinkage and increasing nonradiative recombination needs to be supplemented by thermally induced current crowding.
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