Evaluation of external-cavity diode lasers for Raman spectroscopy

We have evaluated the suitability of two commercial external-cavity, tunable diode lasers (tuning range approximately 770-800 nn) for use as Raman excitation sources. The tests consisted of relatively long-term (up to 14 h) measurements of wavelength and power stability as well as measurements of reproducibility of emission wavelength during repeated on/off cycling. We find that, for both tested lasers, the external cavity configuration either eliminates or dampens ''mode-hopping'' and wavelength drift to a level suitable for obtaining good-quality Raman spectra in most routine experiments. Over the course of several hours, wavelength change from both mode-hopping and drift is less than approximately 0.05 nm for the New-Focus laser and 0.07 nm for the SDL laser. Reproducibility over several on/off cycles is of the same order of magnitude. In all cases, resolution measurements were limited by the spectrographs.