Symmetric interferometers unbalanced by using light exposure to tailor the spectral behavior

Abstract

Unbalanced Mach-Zehnder interferometers (MZI) are useful for a number of applications including wavelength filters, gain flattening or gain equalization elements, and band splitters or combiners. A MZI is comprised of two couplers surrounding a phase shifting region, which consists of two arms with differential propagation constants. We disclose a means of using light exposure to unbalance a symmetric MZI consisting of substantially the same lengths of substantially the same fiber. In particular, the index of refraction of a fiber can be increased by exposure to ultraviolet light, and the magnitude of the change can be increased by using higher germanium doping or by hydrogen loading the fiber. The magnitude of the phase shift can be controlled accurately by varying the fiber length exposed, the light exposure intensity, the light exposure time, and the hydrogen loading or fiber composition. In addition, exposing the other arm,of the MZI to trim back the phase shift can compensate any over-exposure of the fiber. By starting with a symmetric MZI made from substantially the same fiber, low polarization sensitivity or polarization dependent loss can be achieved, and the device can operate over a wider overall wavelength range. Also, by using a MZI with substantially the same lengths of fibers and by placing those fibers parallel to one another, low environmental sensitivity can be achieved. Moreover, the use of light exposure to unbalance the MZI has the advantage of enabling precise tailoring of the spectral response with ease of manufacturing and high yields. The MZI devices can be cascaded to achieve more complicated filtering functions in lattice devices such as Lyot-Ohman filters or Solc filters.