Tunable optical filters are versatile devices with a place in a host of photonic applications. They are essential in wavelength-flexible wavelength division multiplexing systems, and they also can play a key role in wavelength-tunable lasers for such systems. But tunable filters that use bulk (i.e., free-space) components introduce loss when radiation is coupled into and out of the fiber, and they generally are large and expensive. As a result, many researchers are examining approaches to all-waveguide tunable filters, and a group from NTT Corp. in Kanagawa, Japan, has demonstrated a filter fabricated in a monolithic, InGaAsP/InP planar waveguide.Figure 1. A ladderlike system of waveguides and multimode interference couplers forms the basis of a novel tunable optical filter.The new filter is a ladderlike structure, each section of which resembles a Mach-Zehnder interferometer (Figure 1). The output waveguide across the top and the input waveguide across the bottom are joined by regularly spaced linking waveguides, each longer than the previous one by ΔS in the figure. For constructive interference to occur at each coupler in the output waveguide, ΔS must be equal to an integral number of wavelengths of the incoming radiation. However, just as a Mach-Zehnder can be tuned by adjusting the refractive index of one or both legs, the filter can be tuned by adjusting the refractive index of the legs (L1 and L2 in the figure) with an injected current.The NTT researchers fabricated an experimental device with 15 linking waveguides such that the ΔS for each waveguide was 20 of the C-band (~1.5 µm) wavelengths (Figure 2). Physical asymmetries in the cross section of the waveguides introduced an effective birefringence, so the scientists evaluated the interferometer using polarized light. The birefringence could be eliminated, they believe, by eliminating the asymmetries.Figure 2. A photograph of the experimental filter reveals the electrodes on each leg of the structure that change their refractive indices.Illuminating the filter with ampli-fied spontaneous emission from a fiber amplifier, they observed a single peak transmitted through the device at 1539.5 nm. They also observed side lobes approximately 7 nm from the peak and suppressed by 12.5 dB. The 3-dB bandwidth of the peak was measured at 4.5 nm. With no current injected through either leg, adjacent transmissions were separated by 74.4 nm, the interferometer's free spectral range. Injecting a current of 30 mA into one leg tuned the peak upward by 29 nm, and injecting a current of 30 mA into the other leg tuned it downward by 29 nm, for a total tuning range of 58 nm. The switching speed of the filter was less than 10 ns, limited by the time constant of the capacitance and resistance of the device.Other researchers have suggested a conceptually similar approach that employs electrodes on a conventional arrayed waveguide grating. The drawback of that approach, compared with the current scheme, is that the grating's electrodes necessarily occupy a large physical space on the device, which results in a limited tuning range and greater power consumption.