600 W short-pulse hybrid-integrated 48-emitter DBR laser module for 3D flash LiDAR
Diode lasers generating optical pulses with high peak power and pulse lengths in the pico- and nanosecond ranges are key components for compact LiDAR (light detection and ranging) systems. These are used to scan the local environment and measure the distance of objects by, e.g., the time-of-flight (ToF) method. Flash LiDAR systems (multibeam LiDARs) are implemented without scanning mirrors. They typically comprise a wide-angle source, a detector array, and wide-angle optics to focus all the light reflected from one exposure area onto the detector.
For use in automotive LiDAR, optical pulses with peak optical power up to several hundreds of watts, a low temperature-dependent wavelength shift, and good beam quality are required. Short optical pulses (100 ps - 10 ns) enable both, a high spatial resolution and eye safety. An emission wavelength of around 900 nm allows for strongly reduced water vapor absorption in real atmospheric conditions.
For automotive 3D flash LiDAR, FBH developed distributed Bragg reflector (DBR) board area laser bars comprising 48 single emitters on one chip (filling factor ¼). The epitaxial layer structure features an InGaAs single quantum well embedded in an AlGaAs-based asymmetric waveguide and cladding. The spectral requirement can be fulfilled by means of integrating a wavelength-selective element, e.g. a Bragg grating, into the laser in a cost-effective single step epitaxial growth process. Thus, a 7th order DBR grating is implemented into the surface of the layer structure. The laser bar is 4 mm long, including the 1 mm long DBR grating. The laser bar is mounted p-side down on a CuW submount and sandwiched between an in-house developed electronic driver and a mount to minimize inductances, see Fig. 1. Thus, all 48 emitters are driven in parallel. The electronic driver features GaN-based transistors in the final stages due to their advantageous properties for high-speed high-current switching. Using four parallel final driver stages, the electronic driver provides 2 - 10 ns long, nearly rectangular shaped current pulses with peak currents up to 1000 A.
The dependence of the peak optical power on the pulse current of a 48 emitter DBR laser module is shown in Fig. 2 for a pulse width of 8 ns, a repetition rate of 10 kHz, and a temperature of 25°C and 85°C. A maximum peak optical power of about 640 W and still more than 400 W is achieved at a peak current of around 900 A at 25°C and around 860 A at 85°C, respectively. For both temperatures and for the respective maximum peak current, the corresponding optical spectra are depicted in the inset of Fig. 2. Both optical spectra feature a peak wavelength of around 900 nm and a spectral width of about 0.6 nm (95 % power content). A temperature-dependent wavelength shift of only about 60 pm/K (Dl = 3.8 nm for T = 25°C to 85°C) can be determined. The horizontal power distribution (far field) of a hybrid-integrated 48-emitter DBR laser module (see Fig. 1) is shown in Fig. 3. In the vertical direction, a divergence angle of about 24 deg is provided by a spreader lens. The residual horizontal divergence angle of the homogeneous horizontal line is about 0.1 deg (full width at 1/e² intensity). Thus, this hybrid integrated laser module provides a horizontal beam diameter of only 17 cm in a distance of 100 m.
This project was supported by the German Federal Ministry for Education and Research (BMBF) under grant no. 13N14023,26 within the funding initiative EffiLAS/PLUS.
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