Tech Briefs

Compact f/2.4 design efficiently handles large numbers of optical fibers at the entrance aperture.

Ray trace images (see Figure 2) represent the re-imaged back focal plane spot size of a 50-μm diameter fiber positioned at the top, middle, and bottom of a 3-mm tall entrance aperture in a Raman Explorer spectrometer module designed for 532-nm laser excitation; a diffraction-limited image of the fiber core is provided for a reference. In this case, the re-imaged spot energy is well contained at near the diffraction-limited space, allowing the Raman Explorer spectrometer to accurately reproduce tall spatial images at the back focal plane for each input aperture. As demonstrated by these ray trace diagrams, this optical design is well suited to process large stacks of linearly aligned fiber arrays. The system’s spatial-imaging performance also can process several discreet fiber bundles within each entrance aperture.

Headwall Photonics has improved the system’s optical efficiency by combining an optimized holographic grating design with an f/2.4 platform. The f/2.4 optical speed compliments the NA 0.22 of optical fiber, thereby utilizing the majority of the input signal. This optical speed provides many more times the efficiency of typical f/4-f/6 systems. The holographic grating design used for Raman Explorer is highly optimized to place the entire dispersed signal within one diffracted order. Combined with holographic gratings, these spectrometers provide superior stray light rejection of less than 10-5. Raman Explorer is relatively compact at 7 × 7 × 10 in., and weighs 12 lbs. The ability to spectrally image large apertures is of particular interest to those involved in multiple-site industrial process monitoring, and optical fiber imaging endoscopy.

This article was written by Jay Zakrzewski, Business Development Manager, at Headwall Photonics, Inc. For more information, call 978- 353-4036.

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