A proposed miniature, electrically tunable, band-pass optical filter would have a Fabry-Perot configuration, but would be designed to trade the high spectral resolution (narrow-band pass) and small tuning range of a traditional Fabry-Perot filter for low spectral resolution (wide-band pass) and a wide tuning range. Filters like this are candidates to supplant two other types of optical filters used in remote-sensing spectrometers: namely, acousto-optical tunable filters (which are heavy and power-hungry) and liquid-crystal tunable filters (which exhibit low efficiency and work only in narrow temperature ranges).
A Fabry-Perot filter can be characterized as an interference filter and as a resonant optical cavity. It comprises a cavity bounded by partially reflective, low-absorption mirror coats on two flat, transparent plates. A traditional Fabry-Perot filter is a high-spectral-resolution (narrow-band-pass), narrow-tuning-range device constructed with low-absorption mirror coats and a cavity that is many wavelengths long. In the proposed filters, the traditional low-absorption mirror coats would be replaced by lossy metal coats only a few tens of nanometers thick, making the optical properties partly dependent on the choice of metal. In addition, the cavities would be shortened to less than one wavelength, increasing the tuning ranges for given small displacements; for example, as described below, if the distance between lossy mirror coats were made variable from 150 to 300 nm, then the tuning range would span the spectrum of visible light.
A typical proposed filter (see Figure 1) would include two glass plates with silver mirror coats 40 nm thick, separated by an airgap about half a wavelength thick. The gap thickness (cavity length) would be established by piezoelectric spacers. Thus, the filter could be tuned by applying a suitable voltage to the spacers. Figure 2 shows the calculated transmission spectra for various cavity lengths; the wavelength of peak transmission would range from 410 nm at a cavity length of 150 nm to 700 nm at a cavity length of 300 nm.
In another example, the mirror coats would be made of potassium 100 nm thick, making it possible to obtain an infrared pass band. The calculated transmission spectra for this example are also shown in Figure 2. In this case, the wavelength of peak transmission would range from 800 nm at a cavity length of 300 nm to 1,400 nm at a cavity length of 600 nm.
This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to