TUNABLE OPTICAL FILTERS WITH LIQUID CRYSTAL RESONATORS
A voltage-tuned optical filter that is low cost and simple to fabricate uses cascaded etalon modules, each module comprising a liquid crystal etalon, such as a Fabry-Perot etalon, having a relatively small Free Spectral Range (FSR). At least two of the modules are provided with a voltage control to enable Vernier tuning control. For a given overall scan, the voltage-tuned optical filter may operate with reduced voltage ranges for each liquid crystal etalon.
1 . A method for tuning an optical filter having at least two voltage controlled Fabry-Perot liquid crystal etalon modules N 1 and N 2 , wherein the overall filter has a required total free spectral range (FSR) of X, and module N 1 has an FSR of X 1 which is in the range 0.008X to 0.5X, and module N 2 has an FSR of X 2 that is less than X 1 and in the range 0.008X to 0.5X, the method comprising:
simultaneously changing the voltage of the N 1 and N 2 modules over a range of V 1 to V 2 .
2 . The method of claim 1 , wherein the voltages of N 1 and N 2 are cycled through C cycles, and each cycle comprises:
simultaneously changing the voltage V N1 of the N 1 module over a range of V 1 N1 to V 2 N1 and changing the voltage V N2 of the N 2 module over a range of V 1 N2 to V 2 N2 , where the voltage difference V N2 −V N1 is fixed during any one cycle and changes from cycle to cycle.
3 . The method of claim 2 , wherein C is at least 2.
4 . The method of claim 2 , wherein three or more voltage controlled Fabry-Perot liquid crystal etalon modules are used.
5 . The method of claim 1 , wherein the voltage change occurs while an optical signal is transmitted through the optical filter.
6 . The method of claim 5 , wherein the optical signal has a center wavelength near 1.55 microns.
7 . The method of claim 1 , wherein X 2 is approximately 0.9*X 1 .
8 . An optical filter, having a required total free-spectral range (FSR) of X, comprising:
a liquid crystal Fabry-Perot etalon module N 1 , the module N 1 having a Free Spectral Range (FSR) of X 1 , which is in the range 0.008X to 0.5X;
an N 1 voltage control for controlling the voltage of the module N 1 ;
a liquid crystal Fabry-Perot etalon module N 2 spaced from and optically aligned with the module N 1 , the module N 2 having a FSR of X 2 that is less than X 1 and in the range 0.008X to 0.5X; and
an N 2 voltage control for controlling the voltage of the module N 2 .
9 . The optical filter of claim 8 , wherein the N 1 and N 2 voltage controls are controlled to simultaneously change the voltage of the N 1 and N 2 modules over a range of V 1 to V 2 .
10 . The optical filter of claim 9 , wherein V 1 is greater than V 2 .
11 . The optical filter of claim 9 , wherein V 1 is less than V 2 .
12 . The optical filter of claim 9 , wherein V 1 is greater than V 2 during one cycle, and V 1 is less than V 2 during another cycle.
13 . The optical filter of claim 9 wherein the change in the voltage of the N 1 and N 2 modules follows a sinusoidal waveform.
14 . The optical filter of claim 8 wherein the etalon modules comprise nematic liquid crystals.
15 . An optical filter having a plurality of liquid crystal etalon modules connected in a cascaded manner, wherein a Free Spectral Range (FSR) of the optical filter is X and the FSR of each of the etalon modules is equal to or less than 0.5*X.
16 . The optical filter of claim 15 , wherein at least three liquid crystal etalon modules are connected in a cascaded manner and the FSR of each of the etalon modules is equal to or less than 0.33*X.
17 . The optical filter of claim 15 , wherein each of the etalon modules include a voltage control unit to enable a Vernier tuning control of the optical filter.
18 . The optical filter of claim 17 , wherein the voltage control units are controlled to simultaneously change the voltage of the etalon modules over a range of V 1 to V 2 .
19 . The optical filter of claim 18 , wherein the FSRs of at least two of the etalon modules differ by 5% to 40%.
20 . The optical filter of claim 15 , wherein the liquid crystal etalon modules comprise Fabry-Perot etalon modules having nematic liquid crystals.