New Slitless Optical Fiber Laser-Raman Spectrometer

A new slitless optical fiber laser-Raman spectrometer has been developed in which long, thin, low loss optical fibers, either liquid filled or solid core, act simultaneously as the sample and entrance aperature. The fiber end is placed at the focus of the collimator lens, and the resultant parallel radiation is dispersed by three large triangular Steinheil prisms in series. The dispersed radiation is then focussed by the camera lens to a series of points in the focal plane. The Raman points are detected photoelectrically by scanning with either an exit pinhole or slit, and a photomultiplier whose photocathode surface can be as small as the fiber end. Because of the long fiber lengths employed and of the high transmission efficiency of the spectrometer, very large Raman signals result. For example, the extremely weak Raman intensity maximum from fused silica at Δ v̄bar; ≈ 2165 cm−1 is readily detected visually in the focal plane using a fiber length of 85 m and a few hundred milliwatts of 476.5 nm excitation. The new spectrometer will be of considerable use in Raman studies of pure and doped glasses in fiber form, and of a wide range of liquids and mixtures where small sample amounts are involved. It may also be possible eventually to extend this slitless method to studies of water, aqueous solutions, and gases by employing straight dielectric waveguide techniques.