Raman detection of nitrogen gas is very difficult without a multi-pass arrangement and high laser power. Hollow-core
photonic bandgap fibers (HC-PBF) provide an excellent means of concentrating light energy in a very small volume and
long interaction path between gas and laser. One particular commercial fiber with a core diameter of 4.9 microns offers
losses of about 1dB/m for wavelengths between 510 and 610 nm. If 514nm laser is used for excitation, the entire Raman
spectrum up to above 3000 cm-1 will be contained within the transmission band of the fiber. A standard Raman
microscope launches mW level 514nm laser light into the PBF and collects backscattered Raman signal exiting the fiber.
The resulting spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about
several thousand over what is attainable with the objective in air and no fiber. The design and fabrication of a flow-through
cell to hold and align the fiber end allowed the instrument calibration for varying concentrations of nitrogen.
The enhancement was also found to be a function of fiber length. Due to the high achieved Raman signal, rotational
spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge.
Photon counting detectors are used in many diverse applications and are well-suited to situations in which a weak signal
is present in a relatively benign background. Examples of successful system applications of photon-counting detectors
include ladar, bio-aerosol detection, communication, and low-light imaging. A variety of practical photon-counting
detectors have been developed employing materials and technologies that cover the waveband from deep ultraviolet
(UV) to the near-infrared. However, until recently, photoemissive detectors (photomultiplier tubes (PMTs) and their
variants) were the only viable technology for photon-counting in the deep UV region of the spectrum. While PMTs
exhibit extremely low dark count rates and large active area, they have other characteristics which make them
unsuitable for certain applications. The characteristics and performance limitations of PMTs that prevent their use in
some applications include bandwidth limitations, high bias voltages, sensitivity to magnetic fields, low quantum
efficiency, large volume and high cost.
Recently, DARPA has initiated a program called Deep UV Avalanche Photodiode (DUVAP) to develop semiconductor
alternatives to PMTs for use in the deep UV. The higher quantum efficiency of Geiger-mode avalanche photodiode
(GM-APD) detectors and the ability to fabricate arrays of individually-addressable detectors will open up new
applications in the deep UV. In this paper, we discuss the system design trades that must be considered in order to
successfully replace low-dark count, large-area PMTs with high-dark count, small-area GM-APD detectors. We also
discuss applications that will be enabled by the successful development of deep UV GM-APD arrays, and we present
preliminary performance data for recently fabricated silicon carbide GM-APD arrays.