Optical Contact
Optical contact is defined as glass to glass physical contact that results in the zero or dark fringe of the well known Newton's Rings experiment. Surfaces of the same refractive index that are in contact are not reflecting and are therefore no longer visible. Optically, the surfaces effectively disappear.
In an evanescent wave coupler, the fibers, supported in substrates, have been polished to form plane facets in the cladding just above the core. The facets are optically contacted resulting in a coupling region where the two cores appear to be contained in a continuous cladding. This physical contact resists motion due to glass to glass static friction. The assembly is a glass only structure and therefore thermally stable as there are no interleaved layers of materials with highly variable refractive indices. Fiber facets are created slightly above the substrate surface so that only the facets themselves are in optical contact. There is a small gap of about 0.1 micron between the substrates.
The substrates are fixed, with a small elastic contact pressure, such that variations in the assembly due to thermal or mechanical stress are not passed onto the fibers. This maintains the birefringent state of polarization maintaining fiber and low cross-coupling.
Coupling ratio is essentially flat over a large thermal range.
Another advantage of optical contact is that the cores of the fibers in the coupling region are not distorted. There is therefore very little loss of guidance. Polishing the facets to equal depths results in similar refractive index profiles and the contact is non reflective and non scattering. Low loss (excess loss) and low backreflection (return loss) are therefore typical of polished couplers. Well matched coupler halves also show similar propagation constants allowing 100% coupling. Resonant rings with very high finesse are possible.
Very deeply polished fibers and long interaction lengths allow over-coupling cycles where the light beats from core to core along the direction of propagation. Coupling to the first 50% beat gives the lowest wavelength dependence. This is typically 2.5% change in ratio for 1% change in wavelength. Wavelength dependence of couplers at the first 100% beat position is reduced by an order of magnitude.
