Narrow-band filters in the form of phase-shifted Fabry-Perot Bragg gratings incorporated into optical fibers are being developed for differential-absorption lidar (DIAL) instruments used to measure concentrations of atmospheric water vapor. The basic idea is to measure
the relative amounts of pulsed laser light scattered from the atmosphere at two nearly equal wavelengths, one of which coincides with an absorption spectral peak of water molecules and the other corresponding to no water vapor absorption. As part of the DIAL measurement process, the scattered light is made to pass through a filter on the way to a photodetector. Omitting other details of DIAL for the sake of brevity, what is required of the filter is to provide a stop band that:
- Surrounds the water-vapor spectral absorption peaks at a wavelength of ≈946 nm,
- Has a spectral width of at least a couple of nanometers,
- Contains a pass band preferably no wider than necessary to accommodate the 946.0003-nm-wavelength watervapor absorption peak [which has 8.47 pm full width at half maximum (FWHM)], and
- Contains another pass band at the slightly shorter wavelength of 945.9 nm, where there is scattering of light from aerosol particles but no absorption by water molecules.
Whereas filters used heretofore in DIAL have had bandwidths of ≈300 pm, recent progress in the art of fiber-optic Bragg-grating filters has made it feasible to reduce bandwidths to ≤20 pm and thereby to reduce background noise. Another benefit of substituting fiber-optic Bragg-grating filters for those now in use would be significant reductions in the weights of DIAL instruments. Yet another advantage of fiber-optic Bragg-grating filters is that their transmission spectra can be shifted to longer wavelengths by heating or stretching: hence, it is envisioned that future DIAL instruments would contain devices for fine adjustment of transmission wavelengths through stretching or heating of fiber-optic Bragg-grating filters nominally designed and fabricated to have transmission wavelengths that, in the absence of stretching, would be slightly too short. Prototype fiber-optic Bragg-grating filters were designed so that their grating structures were chirped and each filter included π-radian phase shifts at two locations along its length. In each filter, the chirp was characterized by 200 uniform- pitch fields concatenated along a total length of about 6 cm. The chirp rate was 0.3 nm/cm, with a pitch centered at 648.9 nm. The π-radian phase shifts were located at lengthwise positions of 29 and 31 cm, respectively. The particular combination of chirping parameters and phase-shift locations was chosen to yield the desired pass bands at wavelengths of 945.9 and 946.0003 nm in a stop band 2.66 nm wide upon stretching of the fiber at a tension equivalent to the terrestrial weight of a mass of 140 mg (see figure). The filters were fabricated in a multistep process, starting with electron- beam patterning of step-chirp corrugations into a mask. Hydrogen-loaded single-mode optical fibers were irradiated through the mask by light from an ultraviolet excimer laser, then the fibers were annealed by heating.
The prototype fiber-optic Bragg-grating filters were subjected to several tests that demonstrated their potential utility for DIAL water-vapor measurements. Measurements of the transmission spectra of the filters were found to be well approximated by theoretical calculations, which were made by use of a piecewisematrix form of a coupled-mode equation. Tension tuning was also demonstrated.
This work was done by Leila B. Vann and Russell J. DeYoung of Langley Research Center and Stephen J. Mihailov, Ping Lu, Dan Grobnic, and Robert Walker of the Communications Research Centre Canada. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. LAR-17039-1