VIS-SNIR multi-conjugate liquid crystal tunable filter

• US 8,736,777 B2
• Filed: 01/19/2012
• Issued: 05/27/2014
• Est. Priority Date: 01/19/2011
• Status: Active Grant

• Current Assignee: Chemimage Corporation
• Original Assignee: Chemimage Corporation
• Inventors: Voigt, Thomas, Shi, Lei, Venturis, George
• Primary Examiner(s): Glick, Edward
• Assistant Examiner(s): CHUNG, DAVID Y
• Application Number:
  US13/374,867
• Publication Number:
  US 20120300143A1
• Time in Patent Office: 859 Days
• Field of Search: 349/18, 359/245, 359/246, 359/256
• US Class Current: 349/18
• CPC Class Codes: G02B 27/288 Filters employing polarising elements, e.g. Lyot or Solc filters G02B5/3016 takes precedence
• CPC Class Codes: G02B 5/208 for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
• CPC Class Codes: G02B 5/3083 Birefringent or phase retarding elements G02B5/3008, G02B5/3016 take precedence; systems for polarisation control G02B27/286; manufacturing phase modulating patterns by lithographic processes G03F7/001
• CPC Class Codes: G02F 1/13363 Birefringent elements, e.g. for optical compensation
• CPC Class Codes: G02F 1/133633 using mesogenic materials
• CPC Class Codes: G02F 1/13471 in which all the liquid crystal cells or layers remain transparent, e.g. FLC, ECB, DAP, HAN, TN, STN, SBE-LC cells G02F1/13475 takes precedence
• CPC Class Codes: G02F 1/13473 for wavelength filtering or for colour display without the use of colour mosaic filters
• CPC Class Codes: G02F 1/1393 the birefringence of the liquid crystal being electrically controlled, e.g. ECB-, DAP-, HAN-, PI-LC cells G02F1/1396, G02F1/141 take precedence

Abstract

A VIS-NIR hyperspectral imaging filter has serial stages along an optical signal path with angularly distributed birefringent retarders and polarizers. The retarders can include active retarders such as tunable liquid crystal birefringent elements, passive retarders such as fixed retarders, and/or combinations thereof. Distinctly different periodic transmission spectra are provided by different filter stages, each having multiple retarders, in particular with some stages having broad bandpass peaks at wide spectral spacing and other stages have very narrow closely spaced peaks. The respective spectra include at least one tunably selectable band at which the transmission spectra of the filter stages coincide, whereby the salutary narrow bandpass and wide spectral spacing ranges of different stages apply together, resulting in a high finesse wavelength filter suitable for spectral imaging. The filter may be configured to provide faster switching speed and increased angle of acceptance and may operate in the rage of approximately 400-1100 nm.

Claims

1. A spectral imaging filter, comprising:

at least two spectral filter stages coupled along an optical signal path, wherein each of the at least two filter stages has a periodic transmission characteristic with bandpass peaks separated by free spectral bandpass gaps;

wherein each of the filter stages comprises a set of birefringent retarders and at least one polarizer, and the birefringent retarders in the set of at least one of said stages is characterized by retardation that is different from a retardation of the birefringent retarders in at least one other of the stages, such that said one of the at least two filter stages has a greater free spectral range than the other of the two filter stages, between the bandpass peaks of said one of the filter stages, and said other of the two filter stages has narrower bandpass peaks than said one of the two filter stages;

wherein each of the filter stages comprises plural rotationally distributed retarders leading into an output polarizer, whereby the output polarizer for leading stages functions as an input polarizer for following stages, wherein a number of said stages, and a number and respective thickness of retarders within the stages, are chosen to provide a free spectral range (FSR) from about 400 to 1100 nm; and

wherein the bandpass peaks of said at least two filter stages overlap in an operative state of the filter, whereby a transmission characteristic of the spectral imaging filter as a whole is characterized by said greater free spectral range and said narrower bandpass peaks.

2. The spectral imaging filter of claim 1, wherein at least one of said at least two filter stages is tunable to said operative state wherein the bandpass peaks of said at least two filter stages overlap.

3. The spectral imaging filter of claim 2, wherein a plurality of said at least two filter stages is tunable to said operative state and wherein the bandpass peaks of said plurality of filter stages overlap.

4. The spectral imaging filter of claim 1, wherein the at least two filter stages comprise filter configurations comprising a set of birefringent retarders disposed between and distributed in rotational alignment between two polarizers the retarders within each of the stages providing equal retardation with others of the retarders in the same one of the stages.

5. The spectral imaging filter of claim 4, wherein the retarders in different ones of the stages produce said retardation that is different from the retardation of the birefringent retarders in at least one other of the stages because the birefringent retarders in said one of the stages and said other of the stages have distinctly different thicknesses.

6. The spectral imaging filter of claim 5, wherein the retarders in at least two of the different stages differ in retardation by at least a factor of two.

7. The spectral imaging filter of claim 4, wherein the retarders in said stages provide said different retardation by one of a difference in materials, a difference in thickness along the optical signal path, and a tunable difference.

8. The spectral imaging filter of claim 7, wherein the retarders in at least one of the stages comprise a liquid crystal tunable birefringence element.

9. The spectral imaging filter of claim 7, wherein each of the retarders in at least one of the stages consists of a liquid crystal element.

10. The spectral imaging filter of claim 7, wherein the retarders in at least one of the stages comprise fixed retarders coupled to liquid crystal tunable birefringence elements.

11. The spectral imaging filter of claim 10, wherein the fixed retarders are index matched to at least one portion of the liquid crystal tunable birefringence elements.

12. The spectral imaging filter of claim 1, wherein the fixed retarders comprise quartz.

13. The spectral imaging filter of claim 1, wherein a number of said spectral filter stages is limited such that a transmission ratio of a selected wavelength through the spectral filter is at least about 40%.

14. The spectral imaging filter of claim 1, comprising at least three filter stages each having at least four retarders.

15. The spectral imaging filter of claim 1, comprising six filter stages, each having at least four retarders.

16. The spectral imaging filter of claim 1, wherein the retarders in at least one of the stages comprise liquid crystals coupled to a common tuning control for varying a birefringence equally for the retarders in said one of the stages.

17. The spectral imaging filter of claim 16, wherein the retarders in at least two of the stages comprise liquid crystals coupled to a tuning control that for independently varying the birefringence for the retarders in respective ones of the at least two stages.

18. The spectral imaging filter of claim 17, wherein at least a subset of the retarders coupled to said tuning control comprise fixed retarders coupled to the liquid crystals.

19. The spectral imaging filter of claim 17, wherein at least some of the stages comprise tunable retarders having a plurality of abutting rotationally aligned liquid crystals that are tunable in unison.

20. The spectral imaging filter of claim 1, comprising at least three said spectral filter stages coupled along the optical signal path, each of the stages leading into an output polarizer, whereby the output polarizer for leading stages functions as an input polarizer for following stages and a number of said polarizers is limited to a number of stages plus one.