Although the principal motivation for developing inexpensive tunable optical filters is the potentially massive market for such devices in telecommunications, these filters will find important, albeit less remunerative, applications in spectroscopy, interrogation of optical sensors and elsewhere. A research group at Virginia Polytechnic Institute & State University in Blacksburg recently proposed and demonstrated a straightforward design for a tunable optical filter that would be extremely inexpensive to manufacture yet would meet the requirements of many of these applications.Figure 1. The Fabry-Perot filter is formed by fusion welding two single-mode fibers inside a borosilicate tube. When heated, the tube expands more than the fiber, increasing the spacing between the Fabry-Perot's mirrors. ©2004 IEEE. The filter is an extrinsic (glass-air-glass) fiber Fabry-Perot interferometer mounted inside a borosilicate tubing (Figure 1). The operating principle is simple: The mirrors on the ends of the two fibers form a Fabry-Perot interferometer, whose transmission wavelength is a function of the separation between the mirrors. When the device is heated, the borosilicate tubing, whose thermal expansion coefficient is roughly six times that of the glass fiber, expands more than the fiber, which increases the spacing between the mirrors. The fusion between the borosilicate tube and the fiber is performed with a carbon-dioxide laser. Because the outer diameter of the fiber is an extremely close fit to the inner diameter of the tubing, mirror alignment is straightforward, eliminating one of the most expensive steps in a manufacturing process. In an experimental device, the reflectivity of the mirrors varied between 87 and 87.5 percent from 1530 to 1600 nm, with the maximum at roughly 1565 nm.Figure 2. The filter's resolution -- that is, the width of the peaks in this constant-temperature scan -- could be improved by increasing the mirror reflectivity or by pulling the mirrors farther apart. (Note that the vertical scale is calibrated in decibels, so the full width half maximum is measured 3 dB beneath the peak.) ©2004 IEEE.The researchers mounted the filter on a 14-pin butterfly package with a thermoelectric cooling module. The stability of the cooler -- better than 0.003 °C over 24 hours -- ensured the stability of the filter's pass wavelength. The constant-temperature spectrum of the filter showed a free-spectral range of 118.8 nm and a 3-dB bandwidth of 7.4 nm (Figure 2). For applications requiring greater resolution, the width of the passband could be decreased either by increasing mirror reflectivity to enhance the interferometer's finesse or by increasing the initial separation between the mirrors, which would compress the spectrum.The scientists adjusted the temperature of the thermoelectric cooler from 13.5 to 69.3 °C, and over that range, they observed the filter pass-wavelength tune from ~1516 to ~1606 nm. The relationship between the pass wavelength and temperature was very linear (Figure 3).Figure 3. The filter's 90.8-nm tuning range was very linear with the temperature of the thermoelectric cooler. ©2004 IEEE.Insertion loss of the filter was 2.7 dB, a figure that the scientists believe could be reduced to ~1 dB by further tightening the fit between the fiber and the borosilicate tubing and by applying a higher-quality coating to the ends of the fibers. The thermal tuning time over the spectral range of Figure 3 was less than 2 s, but even faster times could be obtained when necessary, they say, by reducing the package size (thereby reducing thermal mass) or by running an electrical current through a resistive coating applied to the borosilicate tubing.