Two-fiber spectroscopic probe with improved scattered light rejection

Our angled two-fiber probe design for in situ spectroscopic measurements (e.g., fluorescence, phosphorescence, or Raman) through a sapphire window has been modified to provide improved rejection of scattered excitation light while maintaining good efficiency of collection of the desired signal. The improvement is achieved by changing the probe axis to an off-normal configuration to minimize back reflection from window surfaces while maximizing the overlap of the light cones of the excitation and collection fibers at the outer window surface where the sample is in contact with the window. The results of laser-induced fluorescence measurements on phenol solution and sand samples, demonstrating the improved performance of the new probe, are reported. Up to 90-fold improvement in the fluorescence/scattered light ratio has been observed with the new probe.