SPIE Conference on Astronomical Telescopes and Instrumentation 2006 Measurement of spatialfiElering capabilities of single mode infraredfibers

Spatial filtering is necessary to achieve deep nulls in optical interferometer and single mode infrared fibers can serve as spatial filters. The filtering function is based on the ability of these devices to perform the mode-cleaning function: only the component of the input field that is coupled to the single bound (fundamental) mode of the device propagates to the output without substantial loss. In practical fiber devices, there are leakage channels that cause light not coupled into the fundamental mode to propagate to the output. These include propagation through the fiber cladding and by means of a leaky mode. We propose a technique for measuring the magnitude of this leakage and apply it to infrared fibers made at the Naval Research Laboratory and at Tel Aviv University. In froduction Direct observation of Earth-like extrasolar planets with regular telescopes is impeded by the bright star, whose light overwhelms the faint image of a planet orbiting around it. The use of interferometers for planet finding relies on suppressing the star light by combining beams from two telescopes out of phase. One of the impediments to this technique is the existence of wavefront errors in the telescopes - the wavefronts originating from the two telescope mirrors may not have the exact same shape, so the difference between the wavefronts will cause residual signal at the null. Spatial filters can help achieve deep nulls by equalizing the wavefronts. The use of single mode optical fibers for spatial filtering has been discussed previously1. A stepindex fiber consists of a high index cylindrical core and low index cylindrical cladding that encloses the core. Single mode fibers have only a single bound mode, therefore only the component of the input field that couples into this fundamental mode of the fiber will propagate to the output without loss. An ideal modal filter would have only this component of the input field reaching the output. However, in practical devices, other components of the input field may also appear at the output. (1) Some components of the input field can couple into leaky modes which carry the energy to the output with little loss. Leaky modes were considered in detail by Snyder and ~ove~ and Sammut and snyder3. (2) The energy can also be carried through the cladding of the fiber. The cladding itself acts as a light pipe that very effectively captures the light not coupled into the bound or leaky modes, as well as the light radiated from the core by leaky modes. Typically, the outside of the cladding is covered with light-absorbing material to reduce light propagation through the cladding. Previously, the following techniques have been used to ascertain the single mode operation of the fiber. One method is to construct an interferometer and measure its null depth with and without the mode-filtering fiber. This provides confirmation of single