Rugged Fiber-Optic Raman Probe for Process Monitoring Applications

We report on the development of a simple, rugged fiber-optic probe for process Raman measurements, in which laser line rejection is based on an absorptive longpass filter made from a direct bandgap CdTe semiconductor. The probe can be used with a fixed wavelength laser at 830 nm, and Raman spectra can be recorded down to 200 cm−1 from the laser line. The filter thickness can be adjusted for final turning of the filter edge, as the edge slope is almost independent of thickness in the range 0.1 to 1 mm. Other properties of the probe, such as its signal-to-noise ratio and signal-to-background ratio, are shown to compare well with those of a state-of-the-art probe based on holographic notch filter techniques.

Dielectric Filter for Highly Sensitive Raman Spectroscopy

A filter spectrometer using dielectric bandpass filters eliminates Rayleigh scattering down to 10-5 while passing more than 90% of Raman signal from 50 to 2000 cm−1. It improves the optical throughput about 8–9 times in comparison to a double-stage subtractive monochromator, and about 20% in comparison to a holographic notch filter. Moreover, transmittance of the filter spectrometer does not depend on polarization of the incident light at the optimum angle. Raman spectra from powder sample of bismuth oxide and copper phthalocyanine and liquid sample of carbon tetrachloride in a glass capillary were easily observed down to 50 cm−1 with the use of the filter spectrometer combined with a single-stage monochromator.

Performance of a Holographic Supernotch Filter

Results are presented of a comparative evaluation of a holographic supernotch filter (HSNF) and a holographic notch filter (HNF) as a Rayleigh line rejection filter for Raman spectroscopy. The filter permits acquisition of both Stokes and anti-Stokes spectra down to ±200 cm−1 shift from excitation simultaneously, without filter angle adjustment. With slight angle adjustment, spectra can be recorded as close as 41 cm−1 from the excitation line. Performance of the HSNF is evaluated by measuring the low-frequency Raman spectra of Tb2(MoO4)3, water, and naphthalene.

Construction and Operation of a dispersive Laser Raman Spectrograph using interference filter

A dispersive laser Raman system was designed and constructed using a helium-neon (He-Ne) laser as an excitation source, and an interference filter in the reflection mode for Raleigh filtering instead of the more common holographic notch filter. A commercially available spectrograph equipped with a cooled CCD camera was used to acquire the Raman spectra. The constructed laser Raman spectrograph was found to have excellent performance and sensitivity. Stokes Raman spectra of some common chemicals were acquired by the system, and the wavelengths of spectral lines agreed well with the literature values, within experimental error. The useful spectral range of the system is about 200-4000 cm-1. It was also possible to acquire anti-Stokes Raman spectra of one chemical (CCl4) without much difficulty. We hope to use the system for chemical identification of molecules as well as quantitative chemical analysis. To our knowledge, this is the first laser Raman system constructed in Bangladesh. doi: 10.3329/jbas.v32i1.2451 Journal of Bangladesh Academy of Sciences, Vol. 32, No. 1, 121-129, 2008