Fiber-optic probes can efficiently collect Raman spectra in process monitoring and art/archeological investigations, for in vivo tissue analysis, and for any other applications in which free-space access to the sample is difficult. While traditional fiber-optic Raman probes have one fiber for delivering source power to the object (typically with a 105 μm core diameter) and another for coupling the scattered light back into a spectrometer (typically with a 200 μm core diameter), these designs have higher coupling losses during excitation and collection than most free-space Raman systems.
To combat this drawback, Innovative Photonic Solutions (IPS; Monmouth Junction, NJ) developed a fiber-optic Raman probe with throughput comparable to typical free-space Raman setups. By incorporating a volumetric Bragg grating (VBG)-based wavelength-stabilized diode laser into the probe, coupling losses associated with the excitation fiber are eliminated by shaping the laser output into a collimated elliptical beam with an elliptical spot at the sample to maintain the étendue of the laser since the active area of the diode has an elliptical cross-section. The sample then emits Raman-scattered photons from the elliptical spot that, when recollimated by the collection optics, again forms an elliptical beam and is coupled into a custom fiber-optic cable with an elongated core that is rotationally aligned to the entrance slit of the spectrometer. The étendue is maintained from the active area of the diode to the sample and back through the collection optics to the entrance slit of the spectrometer. This elliptical-core design results in a throughput enhancement of 3 to 5X compared to traditional Raman probe designs. By eliminating fiber-coupling losses in the excitation source, more power at the sample allows operation of the diode at a lower current for increased useful lifetime. The integrated fiber-optic Raman probe includes an OEM laser driver and thermoelectric cooler. Reference: R. Chimenti,The SCIentific eXchange (SciX) 2015 – the FACSS Annual Meeting, poster board #39, Providence, RI (Sept. 29, 2015).
