We report the experimental characterization of mechanically induced long-period fiber gratings (LPFG's) made by pressing a plate with periodic grooves against a short length of fiber. This filter, which is simple and inexpensive, exhibits transmission spectra and temperature stability similar to those of photoinduced LPFG's. It also offers the unique advantages of being tunable, erasable, and reconfigurable. Its polarization dependence also makes it useful as a polarizer, although it can also be eliminated with simple mechanical designs.

An all-fiber-ring resonator has been constructed using a single strand of single-mode optical fiber and a directional coupler. Derivation of the resonator finesse in terms of fiber and directional coupler parameters is given. A finesse of 80 has been achieved experimentally.Applications of such a fiber-ring resonator are discussed.Single-mode optical fiber can be used to make a highfinesse optical resonator by forming a short piece of fiber into a closed ring to constitute a low-loss cavity.With the recent advances in single-mode fiber directional couplers,l such a fiber ring can be closed in a low-loss manner.A schematic of such an optical-fiber resonator is shown in Fig. 1. If the directional coupler has large coupling, light trapped in the fiber ring will couple from port 2 to port 3 and will continue to circulate. Similarly, light introduced to the input port 1 will couple mostly to the output port 4.Consider the case in which the fiber-loop length is adjusted for constructive interference (addition) between coherent components entering port 3 from ports 1 and 2. The small fraction of light from port 2 to port 4 will destructively interfere with the light coupling from port 1 to port 4. The circulating field will grow until an equilibrium is reached. With an optimum value of coupling that depends on losses, the two destructively interfering components emerging from port 4 are equal in amplitude and completely cancel each other. From an energy-conservation point of view, the circulating power grows until the power dissipated by losses in the loop equals the input power at port 1.If the light frequency is now varied continuously, the power emerging from port 4 will show a series of sharp minima whenever the input optical frequency matches the resonant condition. The behavior is similar to a Fabry-Perot-type resonator whose reflected power has sharp minima at resonance. For a resonator of this type to function properly, the directional coupler must have a low insertion loss. As the optimum value of coupling depends on losses, a variable coupling coefficient is desirable. In this experiment, an evanescent-field coupler is used. A single strand of optical fiber is bonded into two slotted quartz blocks a distance L apart. Each fiber-block unit is ground and polished to within a few micrometers of the fiber core. Placing the two units in contact, oriented as in Fig. 1, produces a ring resonator of perimeter L. Coupler tuning is accomplished by sliding one block over the other to vary the core-to-core separation and optimize the coupling coefficient.A device similar to this, using a modulated incoherent source, has been demonstrated as a recirculating delay line transversal filter for high-speed signal processing. 2The directional coupler is modeled as a perfect (lossless) device with an added lumped loss that is independent of the coupling coefficient. Referring to Fig. 1, the fractional coupler intensity loss yo is given bywhere Ei is the complex field amplitude at the ith port.Typically -y will be 5-10%. The com...

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