Tunable holographic drop filter with quasi phase-conjugate fiber coupling
The present invention is a method and apparatus for a rotating, tunable, holographic drop filter connected to a fiber optic source. The filter uses a quasi phase-conjugate optical system for a drop-channel fiber coupling and WDM channels which are introduced to the system. The light from these channels is collimated and passed through a volume phase holographic material so that only one WDM channel is diffracted and the rest pass through the holographic material unaffected. A quasi phase-conjugate diffracted beam is generated by the optical system to reflect the diffracted channel back towards the holographic material. The reflected light is Bragg matched to the holographic material so that it is re-diffracted along a path identical to the original incident light beam. A free-space circulator may be used to direct the diffracted beam to a fiber optic collimator, which is different from the fiber optic collimator of the incident light beam.
1. A filter system comprising:
a first fiber optic collimator;
a free space circulator coupled to said first fiber optic collimator;
a first mirror coupled to said free space circulator;
a holographic drop filter coupled to said first mirror to diffract at least one of a plurality of WDM channels to an optical system and pass through a remainder of said plurality of WDM channels;
said optical system further comprising a lens and a second mirror coupled to each other;
said optical system coupled to said holographic drop filter redirects said at least one of said plurality of WDM channels back into said holographic drop filter;
a second fiber optic collimator coupled to said holographic drop filter that collects said remainder of said plurality of WDM channels that pass through said holographic drop filter; and
a third fiber optic collimator coupled to said free space circulator that collects said one of plurality of WDM channels redirected into said holographic drop filter by said optical system.
2. The filter system of claim 1 wherein said holographic drop filter is tunable.
3. The filter system of claim 2 wherein said filter system is tuned by rotating said holographic drop filter such that its effective period length is altered.
4. The filter system of claim 1 wherein diffraction of said at least one of said plurality of WDM channels to said optical system depends on said holographic drop filter's orientation.
5. The filter system of claim 4 wherein said second mirror is positioned at a focal length of said lens such that said quasi phase-conjugate diffracted channel is in a direction opposite to said one of plurality of WDM channels diffracted to said optical system.
6. The filter system of claim 4 wherein said holographic drop filter has a refractive index that can be altered by an external electric field.
7. The filter system of claim 6 wherein said refractive index is between 1.35 and 1.45.
8. The filter system of claim 4 wherein said holographic drop filter is made from a photorefractive crystal.
9. A method to tune a filter comprising the steps of:
collimating a plurality of WDM channels by a first fiber optic collimator;
coupling a free space circulator to said first fiber optic collimator;
coupling a first mirror to said free space circulator;
diffracting to an optical system coupled to said holographic drop filter one of said plurality of WDM channels by a holographic drop filter coupled to said first mirror; passing through said holographic drop filter rest of said plurality of WDM channels; redirecting one of plurality of WDM channels back into said holographic drop filter by said optical system;
coupling a second fiber optic collimator to said holographic drop filter to collect rest of said plurality of WDM channels that pass through said holographic drop filter; and coupling a third fiber optic collimator to said free space circulator to collect said one of plurality of WDM channels redirected into said holographic drop filter by said optical system.
10. The method of claim 9 further comprising tuning of said holographic drop filter.
11. The method of claim 9 wherein tuning of said filter system is done by rotating said holographic drop filter such that its effective period length is altered.
12. The method of claim 9 wherein diffracting to said optical system one of said plurality of WDM channels depends on said holographic drop filter's orientation.