Narrowband GaN-based diode laser system for the generation of DUV laser light for Raman spectroscopy
Fig. 1: Schematic view of the ECDL system in Littrow configuration
Fig. 2: a) Emission spectra at an injection current 0.49 A and 20°C: Fabry-Perot laser diode (grey), ECDL system (blue). b) power-current characteristics: FP diode (dashed), ECDL (blue). c) emission spectrum of ECDL system with 44 dB side-mode suppression ratio
Deep ultraviolet (DUV) lasers are of great interest for Raman spectroscopy. In this spectral range, the signal increases with the fourth power of the inverse wavelength and the typically disturbing fluorescence background is significantly lower. Up to now, realizing suitable light sources was rather complex and difficult.
In a novel approach, the FBH only recently started to use commercially available diode lasers in the blue spectral range with more than one Watt optical output power. The institute, at the same time, works on efficient frequency doubling of this emission by utilizing nonlinear crystals. At the FBH, a high-power external cavity diode laser (ECDL) system making use of such diode lasers has been realized for the first time. It delivers an optical output power of 400 mW and narrowband emission at 445 nm.
The system is based on a GaN laser diode (OSRAM Opto Semiconductors, 15 µm aperture) that emits with a spectral width of 1 nm without external feedback. An optimized cavity in Littrow configuration enables to reduce the spectral width to 20 pm, the side-mode suppression ratio is 44 dB. At 400 mW output power, the beam quality shows M2 values of around 5.
Due to the high optical output power, the small spectral width, and the sufficient beam quality, the ECDL system is suitable as pump source for nonlinear frequency conversion into the deep ultraviolet wavelength region. In first experiments with this ECDL system, UV radiation with an emission wavelength around 223 nm has already been generated by simple single-pass frequency doubling within a BBO crystal. The hereby achieved output powers in the lower µ-Watt range are already sufficient for Raman spectroscopy applications.
Publication:
N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, and G. Tränkle, "400 mW external cavity diode laser with narrowband emission at 445 nm", Opt. Lett., vol. 39, no. 13, pp. 3794-3797 (2014).
FBH research: 10.11.2014