Two-mode fibers that support two spatial modes are used for several useful applications for which single mode fibers can not be used. Mode couplers, which exchange the energy between the two modes, are fundamental components for the use of two-mode fibers.
In this work, mode couplers were fabricated by applying periodic microbends to fibers. The periodic bends were formed by pressing the fiber between two plates having grooves with a phase-matched period. The mode coupling mechanism was analyzed theoretically by a simple model and coupling ratio was calculated by coupled mode equation derived from the Maxwell's equations.
Mode couplers were fabricated in two types. One is a tunable coupler which enables us to tune its coupling strength and coupling wavelength. The other is a static coupler having a micro-bending structure which was fixed by arc and therefore maintained without grooved plates. This type is more adequate when stable coupling is needed. Mode coupling over 99% and insertion loss less than 0.22 dB at 1.064 ㎛ were measured.
Three examples of application of the mode couplers were demonstrated. A two-mode interferometer using a pair of mode couplers showed sinusoidal output in response to the stain applied to a fiber. A mode coupler with 28 bending points could be used as wavelength notch filter with 100 nm bandwidth. A sinusoidal amplitude modulation of a few MHz was produced by a mode coupler followed by a frequency shifter.
