Method and apparatus to effectively eliminate optical interference structure in detector response

• US 5,554,845 A
• Filed: 11/14/1994
• Issued: 09/10/1996
• Est. Priority Date: 11/14/1994
• Status: Expired due to Term

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Santa Barbara Research Center
• Inventors: Russell, Edgar E.
• Primary Examiner(s): Westin, Edward P.
• Assistant Examiner(s): CALOGERO, STEPH
• Application Number:
  US08/337,720
• Time in Patent Office: 666 Days
• Field of Search: 250/208.1, 250/208.2, 250/216, 250/226, 257/432, 257/437, 359/581, 359/582, 359/589
• US Class Current: 250/226
• CPC Class Codes: H01L 31/02162 for filtering or shielding light, e.g. multicolour filters for photodetectors

Abstract

A method and apparatus are provided to eliminate spectral interference variation contained in optical signals transmitted to an optical detector or array of optical detectors, the spectral variation deriving from the presence of a layer deposited on the detector having a different index of refraction from the detector. The practice of the invention involves optically coupling a plate to the layer using an adhesive, with the proviso that the plate, layer and adhesive have substantially the same index of refraction so that a composite thickness with the refractive index of the layer is formed. Given the inverse relationship between thickness of the layer and period of the interference in wavenumber, the effective increase in thickness of the layer yields a corresponding decrease in the period. Hence, the thickness of the plate may be selected such that the composite thickness of the layer, the plate, and the adhesive is sufficient to decrease the period so that it is masked by the normal shift of the interference structure over the range of incident angle of the optical input onto the detector or array of detectors, thereby effectively eliminating the spectral interference variation.

Claims

1. A method of fabricating a device in which spectral variation in optical signals transmitted to an optical detector or array of optical detectors is reduced, said spectral variation resulting from a layer over the active area of the optical detector or array of optical detectors, said method comprising:

(a) providing an optical detector having an active area, at least a portion of said active area being covered by a layer, said layer and said optical detector having different indices of refraction such that said layer introduces spectral variation into optical signals incident to said active area, said spectral variation having a period of interference defined by the formula

space="preserve" listing-type="equation">1/(2nt cos θ

'"'"'),

where n represents the index of refraction of said layer, t represents the thickness of said layer, and θ

'"'"' represents the angle of incidence of said optical signals within said layer;

(b) providing a plate having an index of refraction substantially equal to the index of refraction exhibited by said layer, said plate having an areal extent at least equivalent to that of said at least a portion of said active area covered by said layer; and

(c) bonding said plate and said layer with an adhesive such that said plate is parallel to said layer across said at least a portion of said active area covered by said layer, said adhesive having an index of refraction substantially equal to the index of refraction exhibited by said layer and said plate,

whereby said bonding forms a structure having a composite thickness comprising the thicknesses of said plate, said adhesive, and said layer, said structure having an index of refraction substantially equal to said index of refraction n of said layer, such that the variable t of said formula is effectively increased and said period of interference is reduced according to the inverse relationship between t and period defined in said formula.

2. The method of claim 1 wherein said indices of refraction of said layer, said plate, and said adhesive are within ±

5% of each other.

3. The method of claim 1 wherein said layer comprises an oxide layer.

4. The method of claim 1 wherein said plate comprises an optically transparent material.

5. The method of claim 4 wherein said optically transparent material comprises glass.

6. The method of claim 1 wherein said adhesive comprises an optically transparent material.

7. The method of claim 1 further comprising coating said plate with an anti-reflection coating, said anti-reflection coating covering at least a portion of said active area.

8. An apparatus in which spectral variation is substantially reduced in optical signals transmitted to an optical detector or array of optical detectors, where the spectral variation results from a layer over the active area of the optical detector or array of optical detectors, comprising:

(a) an optical detector having an active area, at least a portion of said active area being covered by a layer, said layer and said optical detector having different indices of refraction such that said layer introduces spectral variation into optical signals incident to said active area, said spectral variation having a period of interference defined by the formula

space="preserve" listing-type="equation">1/(2nt cos θ

'"'"'),

where n represents the index of refraction of said layer, t represents the thickness of said layer, and θ

'"'"' represents the angle of incidence within said layer of said optical signal; and

(b) a plate having an areal extent at least equivalent to that of said at least a portion of said active area covered by said layer, said plate bonded to said layer with an adhesive such that said plate is parallel to said layer across said at least a portion of said active area covered by said layer, said adhesive and said plate having indices of refraction substantially equal to the index of refraction exhibited by said layer,

wherein said layer, said plate, and said adhesive comprise a structure having a composite thickness and an index of refraction substantially equal to said index of refraction n of said layer, such that the variable t of said formula is effectively increased from said thickness of said layer to said composite thickness, such that said period of interference is reduced according to the inverse relationship between t and period of interference defined in said formula.

9. The apparatus of claim 8 wherein said indices of refraction of said layer, said plate, and said adhesive are within ±

5% of each other.

10. The apparatus of claim 8 wherein said layer comprises an oxide layer.

11. The apparatus of claim 8 wherein said plate comprises an optically transparent material.

12. The apparatus of claim 11 wherein said optically transparent material comprises glass.

13. The apparatus of claim 8 wherein said adhesive comprises an optically transparent material.

14. The apparatus of claim 8 further comprising coating said plate with an anti-reflection coating, said anti-reflection coating covering at least a portion of said active area.