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Machined Fibers Improve Midinfrared Spectrometry

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Richard Gaughan

Optical fibers have improved the versatility of near-infrared spectrometers, but instruments operating in the mid-IR have yet to see the development of efficient fibers or coupling. Bernhard Lendl, leader of the chemical analysis and vibrational spectroscopy group at Vienna University of Technology, hopes to change that by using flexible and relatively soft polycrystalline silver halide fibers, which are transparent from the visible spectrum down to 600 cm21.

But when these plane-polished fibers are coupled with a cryogenically cooled mercury cadmium telluride detector, their distribution of light is so wide that they require a relatively large detector. Because the noise in the system goes up as the area of the detector increases, a larger detector implies a lower-than-optimum signal-to-noise ratio. On the other hand, a smaller detector will not collect all of the signal, again implying a lower than optimum ratio.

A figured exit face of a silver halide fiber allows coupling of mid-IR radiation to a small detector. Improved efficiency will improve the flexibility, robustness and signal-to-noise ratio of spectroscopic instruments. Courtesy of the Vienna University of Technology.

Lendl's solution is to machine the output end of the fiber to concentrate the light in a smaller area. This allows use of smaller detectors to obtain the same signal at a reduced noise level.

The researchers used a computer-controlled turning lathe to produce spherical, paraboloid and hyperboloid fiber exit faces with different radii. Because the silver halide degrades when in contact with metal, a Teflon sleeve holds the fiber, which is then machined with a glass knife.

After characterizing the repeatability of the machining and alignment processes, Lendl measured the output distribution of the fibers with a 100-µm2 HgCdTe detector translated with 10-µm precision. He found that all the figured output faces had more concentrated light distribution, but that the best concentration was in fibers where the surface radius matched the 1-mm fiber diameter. There is little difference between the different surface figures, although that may be partially due to the finite detector size.

A proof-of-concept experiment, even with a nonoptimum flow cell size, displayed excellent sensitivity for varying concentrations of sucrose in water.

Lendl's group is developing a compact unit consisting of a quantum cascade laser, an infrared optical fiber, an improved flow cell and a fiber detector coupling. The instrument will be a portable unit for applications in process analytical chemistry, as well as a detection unit for chromatography and flow-injection analysis.

Photonics Spectra
Mar 2001
Basic ScienceResearch & TechnologySensors & DetectorsspectroscopyTech Pulse

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