The utilization of diode lasers for Raman spectroscopy

The principle of operation and characteristics of diode lasers are briefly reviewed and critically compared to those of other lasers in regard to use as excitation light sources for laser Raman spectroscopy. Relative to other lasers frequently used for Raman spectroscopy, diode lasers have the advantages of smaller size, less interference from most sample fluorescence (compared to green, blue and UV lasers), greater efficiency, lower heat generation, less need for support equipment, potentially longer operating lifetime, virtually continuous tuning capability over approximately 30 nm, and lower cost. The disadvantages include generally lower power, smaller Raman cross section (because of restriction to near-infrared and infrared wavelengths), poor detection for large Stokes Raman shifts, decreased wavelength stability (''mode hopping''), greater beam divergence, decreased human-eye visibility, and (in some cases) decreased coherence. To date, diode lasers have only been used in conjunction with a relatively few Raman studies involving surface-enhanced Raman spectroscopy and remote optical fiber probes. As they become more powerful, stable, and coherent, diode lasers are expected to become more prominent in Raman spectroscopic studies, especially those requiring portable remote systems for process monitoring and selected environmental applications. The ease of use and handling of diode lasers will lend themselves to integration into turn-key Raman system packages which will be used as a routine analytical tool by general chemists.