Since the advent of fiber optics, the fiber optical communication infrastructures have become more diverse and sophisticated. The fiber optic applications range from low speed, local area networks to high speed, long distance telecommunication systems. In recent years, the demands for greater bandwidth and lower network costs have resulted in increasing use of dynamic, tunable components.
Tunable optical filters are of particular importance because they can be configured to perform a variety of critical network functions, including channel selection and optical power monitoring.
Prior art techniques to construct tunable optical filters include the acousto-optic tunable filter which operates by using an acoustic wave simulated by a radio-frequency power supply and transducer to induce densification and rarefaction in an optical waveguide material. In practice, acoustic-optic tunable filters usually work by changing the polarization of light at a wavelength that is matched to the acoustically induced grating which results in separation of tuned wavelength from the other wavelength components. Tuning is accomplished by changing the frequency of the applied acoustic wave. Acoustic-optic devices provide rapid tuning in the microsecond range and complete blanking of the filter, however they are not polarization independent devices and suffer from poor adjacent channel rejection and high insertion loss.
Optical nanostructures have been the object of scientific investigation for several years but advances in material science and imprint lithography have only recently resulted in their cost effective manufacturing and availability. An optical nanostructure is derived with feature sizes below the wavelength of light, so they offer uniform behavior over a broad wavelength, wide acceptance angles and unique optical properties by function of varying dimensions of the underlying grating features. Most recently, optical nanostructures have been designed to function as a resonant waveguide, which, when coupled to an active layer capable of changing its index of refraction, is a foundation for tomorrows tunable waveguide filter.
Liquid crystals are known to change their index of refraction with the application of voltage and can be dynamically controlled and configured to enable a range of optical switching and signal conditioning applications. Formed with opposing plates of sealed substrates, liquid crystal cells are considered a prospect technology and integration target capable of supplying the active layer to a nanostructure integrated therewith. Wang et. Al has recently demonstrated an experimental electrically tunable filter based on a waveguide resonant sub-wavelength nanostructure-grating filter incorporating a tuning mechanism in a thin liquid crystal. The device experiment was functional and exhibited performance of 30 nanometer tuning, however this device i) does not function in a polarization independent capacity; ii) does not offer a wide tuning range required for operation in different network bands, and; iii) does not address temperature stability issues associated with robust control of liquid crystal devices.
The advantages of liquid crystal based tunable filter over existing technologies include durability due to the absence of mechanical moving parts, no stretchable medium required as in prior art tunable filters and derivatives, no loss of optical performance in the event of mechanical failure, no fatigue resulting from mechanical failure occurring over time and the ability to provide tunable filter arrays with multiple tuning pixels.
Given the assertion that tunable devices can be achieved at low cost by way of integrating active liquid crystal with passive integrated nanostructured gratings, the present invention addresses a strong need for a low cost polarization independent tunable filter that offers a wide tuning range that operates in a reliable manner across a range of temperature and atmospheres.
The present invention tunable filter utilizes active liquid crystal in conjunction with passive optical elements to vary the index of refraction of the media. A change in index of refraction creates different waveguide conditions and affects the incident light propagation in the media. Wavelength tuning is achieved from the liquid crystal material's ability to change the index of refraction as a function of an external electrical field.