Diode laser based deep ultraviolet Raman excitation
Fig. 1: Emission spectrum of the ECDL system (blue) at PECDL = 400 mW and T = 20°C. Spectrum of the FP diode laser (black) for comparison.
Fig. 2: Experimental setup for simple single-pass second harmonic generation based on a high-power GaN ECDL system.
Fig. 3 left: Spectrum of the generated second harmonic at λ= 222.5 nm and the ECDL emission at λ= 445 nm. Right: Generated SHG power versus fundamental pump power.
One of the biggest challenges in Raman spectroscopy is to avoid or minimize the typically strong fluorescence background that can, in some cases, even mask the relatively weak Raman signal completely. The major advantage of Raman spectroscopy with excitation wavelengths in the deep UV below 260 nm over visible or NIR excited Raman spectroscopy is that the Raman signal can be spectrally separated from this fluorescence background. Due to the ω4 dependence of the Raman scattering cross section the Raman signal also increases significantly for lower excitation wavelengths. Additionally - depending on the analyzed substance - deep UV excitation leads to resonant effects that can enhance the Raman signal by a few orders of magnitude.
Up to now, the realization of suitable light sources in this spectral range was rather complex and difficult and established DUV laser systems suffer from high power consumption and a large footprint. To utilize the benefits of DUV Raman spectroscopy in fields of application outside the laboratory environment the demand for a compact and reliable DUV laser light source is steadily increasing.
The Ferdinand Braun Institute follows a new approach using commercially available diode lasers in the blue spectral range with more than one Watt optical output power and works on their efficient frequency doubling in a nonlinear crystal. By using such diode lasers a high-power external cavity diode laser (ECDL)-system with an optical output power of 400 mW and narrowband emission at 445 nm has been realized for the first time. The system is based on a commercially available GaN laser diode (OSRAM Opto Semiconductors, 15 µm emitting aperture) that emits with a spectral width of 1 nm without external feedback. With an optimized cavity in Littrow configuration it was made possible to reduce the spectral width to 20 pm. Figure 1 shows the emission spectra of the FP diode and the ECDL system at T = 20°C. The side-mode suppression ratio of the ECDL system is 44 dB. At 400 mW output power, the beam quality shows M2 values around 5. Thanks 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, UV radiation with an emission wavelength around 223 nm has been generated with this ECDL system by simple single-pass second harmonic generation (SHG) in a BBO crystal. The experimental setup is shown in figure 2. Longitudinal mode selection in the ECDL system is realized by a holographic surface diffraction grating with 3600 lines/mm. The beam from the ECDL system is focused into a BBO crystal where the UV light is generated. The second harmonic and the fundamental beam are spatially separated by a dispersive CaF2 prism.
Figure 3a shows a spectrum of the generated second harmonic light at 222.5 nm and the fundamental light at 445 nm at a pump power from the ECDL of 400 mW. Figure 3b shows the generated UV power at 222.5 nm versus the pump power from the ECDL. For a pump power of 680 mW a UV power of 16 μW is generated.
With the achieved output power in the lower μW-range this concept enables compact and efficient diode laser based light sources emitting in the deep UV spectral region potentially suitable for Raman spectroscopy.
Publications:
N. Ruhnke, A. Müller, B. Eppich, R. Güther, M. Maiwald, B. Sumpf, G. Erbert, G. Tränkle, "Single-pass UV generation at 222.5 nm based on high-power GaN external cavity diode laser", Opt. Letters (2015).
N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, G. Tränkle, "400 mW external cavity diode laser with narrowband emission at 445 nm", Opt. Lett. 39, 3794-3797 (2014).