7 W pico- and nanosecond ridge-waveguide laser source at 670 nm
Fig.1: High current ns laser driver with integrated ridge waveguide laser diode
Fig. 2: Streak camera measurement of the nearfield intensity at a driver voltage of 30
Fig. 3: Optical pulse power (red), optical pulse width (green), pulse energy (blue) vs. driver voltage
Lasers generating short optical pulses from 0.5 ns to 5 ns with peak powers from several watts to tens of watts experience an increasing demand in a broad range of applications. These include LiDAR (Light Detection and Ranging) systems, e.g., for automotive systems (autonomous driving), 3-D object detection, laser scanning (airborne, satellite and terrestrial) as well as fluorescence spectroscopy and micromachining systems.
Gain switching of laser diodes, i.e., modulating the gain of the laser by turning the current injected into the diode on and off offers a simple, cost-effective and power-efficient possibility to generate optical pulses with widths down to at least 0.5 ns. However, pulse durations below 1 ns are typically hard to achieve because of the formation of a relaxation peak at the beginning of the pulse. On the other hand, to reach optical pulse powers in the watt range electrical pulses with current amplitudes beyond 10 A must be generated.
At FBH, a new laser driver with a GaN transistor in the final stage has been developed. It generates current pulses up to an amplitude of 30 A with widths down to 0.5 ns due to the minimization of the mutual source inductance. Fig. 1 shows the developed driver with integrated ridge waveguide (RW) laser diode. The laser driver consists of two parallel circuits which can be triggered either separately (each 1 ns and 15 A) or together. In case of synchronous triggering a 1 ns current pulse of about 30 A at driver voltage amplitude of 30 V can be reached.
The used RW laser has an emission wavelength of about 670 nm. It was grown on n-type GaAs via low pressure metal-organic vapor phase epitaxy (MOVPE). The active region consists of a GaInP double quantum well (DQW). The vertical far field angle is about 30° full width at half maximum (FWHM). The laser with 3 mm cavity length and a ridge width of 15 µm was soldered p-side down on an AlN submount which has been mounted on an Al plate. To minimize the inductance for pulse applications a large number of bond wires were used.
Fig. 2 shows the temporal behavior of the nearfield intensity of an optical pulse (driver voltage 30 V, current pulse amplitude of about 30 A) measured with a streak camera. The length of the optical pulse is around 1 ns and the rise and fall times (10% - 90%) are about 0.1 and 0.3 ns, respectively. The pulse shows a nearly rectangular shape without any intensity fluctuations in the nearfield.
Fig. 3 shows the optical pulse power (red curve), the measured optical pulse width (green curve) and pulse energy (blue) in dependence on the driver voltage. As can be seen a maximum pulse power of 7.2 W was reached. Measurements with a joulemeter revealed a maximum pulse energy of about 7.7 nJ, comparable with the power measurements. The optical pulse width increased from 670 ps to approximately 1 ns with increasing driver voltage.
In summary, this laser source features outstanding properties since it allows generating short pulses with ps and ns pulse width, high output power, and a homogenous intensity profile.
Publications
A. Klehr, T. Prziwarka, A. Liero, T. Hoffmann, H.-J. Wünsche, H. Wenzel, and G. Erbert "Nanosecond high-current pulsed operation of ridge-waveguide lasers", Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. (CLEO/Europe-EQEC 2015), Jun. 21-25, Munich, Germany, ISBN: 978-1-4673-7475-0, cb-p.19-mon (2015).
A. Klehr, T. Prziwarka, A. Liero, Th. Hoffmann, T. N. Vu, O. Brox, J. Fricke, F. Bugge, P. Ressel, A. Ginolas, H. Wenzel, H.-J. Wünsche, S. Schwertfeger, G. Erbert, and G. Tränkle, "High power picosecond and nanosecond diode laser sources in the wavelength range 650 nm to 1100 nm", Proc. of 2015 High Power Diode Lasers and Systems Conference (HPD) and Photonex 2015, Coventry, UK, Oct.14-15, ISBN 978-1-4673-9177-1, pp. 3-4 (2015).
A. Klehr, T. Prziwarka, A. Liero, Th. Hoffmann, J. Pohl, J. Fricke, H.-J. Wünsche, H. Wenzel, W. Heinrich, and G. Erbert, "Generation of 7 W nanosecond pulses with 670 nm ridge-waveguide lasers", Proc. SPIE 9767, Photonics West, San Francisco, USA, Feb. 13-18, 976705 (2016).
A. Liero, A. Klehr, T. Hoffmann, T. Prziwarka, and W. Heinrich, "GaN Laser Driver Switching 30 A in the Sub- Nanosecond Range", EuMC London to be presented in Oct. 2016.