Interference filter for optical telecommunication
View Patent ↗A micro-optic Mach-Zehnder interferometer with a differential delay of n bit periods, where n is an integer number, and thermal bias control is useful as a spectral filter in front of a differential detection system to accomplish a balanced receiver. The interferometer may also be arranged as a Michelson interferometer or a dual-plate interferometer.
1. An interference filter for introducing a relative delay between components of an input optical signal, the filter comprising:
an input port for launching the input signal,
an optical splitter,
an optical combiner,
a first optical path connecting an output of the optical splitter to an input of the optical combiner, a second optical path connecting a second output of the optical splitter to a second input of the optical combiner,
a first photodetector for receiving light from a first output port of the optical combiner, a second photodetector for receiving light from a second output port of the optical combiner, wherein
a) the optical propagation delay for light travelling from the first output port of the optical splitter to the first input port of the optical combiner differs from the optical propagation delay for light travelling from the second output port of the optical splitter to the second input port of the optical combiner by an integral number of bit periods of the input optical signal,
b) the response of the first photodetector is combined with the response of the second photodetector in a negative linear combination, and
c) at least one of the optical paths is unguided.
2. An interference filter as in claim 1 in which both light paths are unguided.
3. An interference filter as in claim 1 in which the photodetectors are connected in series between a negatively biased power supply and a positively biased power supply.
4. An interference filter as in claim 1 further comprising a thermal control means to control the relative delay in the two optical paths.
5. An interference filter according to claim 4 wherein the thermal control means is a heater disposed to control the relative delay in the two optical paths.
6. The filter according to claim 4 wherein the thermal control means is a cooler disposed to control the relative delay in the two optical paths.
7. An interference filter according to claim 1 comprising
a non-polarizing beam splitter coupled with the input port for dividing the input signal into first and second sub-beams and for directing the first sub-beam along a first path and the second sub-beam along a second path;
a non-polarizing beam combiner coupled with the beam splitter for recombining the first and second sub-beam,
first reflecting means in the first path for redirecting the first sub-beam towards the beam combiner,
thermal control means in at least one of the paths for controlling relative delay between the first and second sub-beams.
8. The filter of claim 7 wherein the beam splitter and the beam combiner are in direct side-by-side contact.
9. The filter of claim 7 wherein the beam splitter and the beam combiner are spaced and separated by a spacer means.
10. The filter of claim 9 wherein the spacer means comprises a first light-transmissive spacer for transmitting the first sub-beam and a second light transmissive spacer for transmitting the second sub-beam, the first and second spacer having different optical delay characteristics.
11. The filter of claim 7 wherein the reflecting means is a flat mirror.
12. The filter of claim 7 wherein the reflecting means is a 90° corner mirror.
13. The filter of claim 9 wherein the spacer means is air.
14. The filter of claim 7 wherein the reflecting means is a means for redirecting one of the sub-beams by 180° towards the beam combining means.
15. The filter of claim 14 wherein the beam splitting means and the beam combining means are the same element.
16. The filter of claim 7 comprising two directly adjacent plates of a light transmissive material, the plates defining an interface therebetween, the interface disposed to split the input signal into the first and second sub-beam and to combine the first and second sub-beam.
17. The filter of claim 16 further comprising a first and second photodetector, wherein the plates are arranged to direct a portion of the combined first and second sub-beam to the first photodetector and another portion of the combined sub-beams to the second photodetector.