Multilayer thin film bandpass filter

• US 5,719,989 A
• Filed: 09/18/1995
• Issued: 02/17/1998
• Est. Priority Date: 06/28/1995
• Status: Expired due to Term

• Current Assignee: Lumentum Operations, LLC (Lumentum Holdings Inc.)
• Original Assignee: JDS Fitel, Inc. (Lumentum Holdings Inc.)
• Inventors: Cushing, David Henry
• Primary Examiner(s): Dzierzynski, Paul M.
• Assistant Examiner(s): CHANG, AUDREY Y
• Application Number:
  US08/529,449
• Time in Patent Office: 883 Days
• Field of Search: 359/580, 359/584, 359/585, 359/586, 359/588, 359/589, 359/587, 359/885, 333/212
• US Class Current: 359/589
• CPC Class Codes: G02B 5/28 Interference filters
• CPC Class Codes: G02B 5/288 comprising at least one thin film resonant cavity, e.g. in bandpass filters

Abstract

A novel design for producing bandpass filters with essentially square shapes with little or no ripple in the passband zone. Filters are of the all-dielectric type that consist of multiple cavities of bandpasses with the first and last cavities consisting of four less layers than the inner cavities. The inner cavities all have the same number of layers. All of the cavities are separated from each other by quarter wave thick layers of low index material. Layers of low index material may be added between the filter and the entrance and exit mediums. Multiple half-waves are added to the outer cavity quarter-wave stacks in particular and also may be added to some of the inner cavity stacks, These sharpen the transition from low transmission to high transmission without unduly adding ripple to the high transmission zone. The thickness of the first few layers and last few layers may be altered to enhance the transmission of the filter by matching the index structure of the passband to the adjacent mediums.

Claims

1. A bandpass filter comprising:

an array of cavities including two outer cavities and one or more inner core cavities, each cavity having a plurality of quarter-wave reflecting stacks some of the quarter-wave reflecting stacks including at least a three-quarter wave layer, the reflecting stacks comprising layers of material of alternating high and low refractive index, said plurality of stacks beings separated by a half wave layer of dielectric material, each cavity being separated by quarter-wave layer of low refractive index material, wherein the number of layers in the at least one or more inner core cavities is an odd number greater or equal to 7, and wherein the first and last cavities in said array of cavities each have two less layers in each quarter-wave reflecting stack than in other quarter-wave reflecting stacks of the one or more inner core cavities in said array, wherein the at least one three-quarter-wave layer in either of the outer cavities are for altering the slope of the passband, and wherein the at least one three-quarter-wave layer disposed in the one or more inner core cavities are for lessening ripple induced by any three-quarter-wave layers present in any of the outer cavities.

2. A bandpass filter comprising:

an array of cavities including two outer cavities and at least one or more inner cavities, each cavity having a plurality of quarter-wave reflecting stacks comprising layers of material of alternating high and low refractive index, the plurality of quarter-wave reflecting stacks being separated by a half-wave layer of dielectric material, the plurality of quarter-wave reflecting stacks of said outer cavities being separated by a multiple half-wave layer of dielectric material for broadening the bandwidth of a passband of the filter and for reducing residual reflections, each cavity being separated by quarter-wave layer of low refractive index material, wherein the number of layers in the at least one or more inner cavities is an odd number greater or equal to 7, and wherein the two outer cavities in said array of cavities each have two less layers in each quarter-wave reflecting stack than in other quarter-wave reflecting stacks of the at least one or more inner cavities in said array, wherein at least one of the quarter-wave reflecting stacks of the outer cavities have selected layers in the outer cavities being a thickness of a three-quarter wave to increase the slope of the passband, and wherein selected layers of the quarter-wave stacks of the one or more inner cavities are a thickness of a three-quarter-wave to lessen ripple induced by a three-quarter-wave layer present in one or both of the two outer cavities.

3. A bandpass filter comprising:

an array of cavities including two outer cavities and at least one or more inner cavities, each cavity having a plurality of quarter-wave reflecting stacks comprising layers of material of alternating high and low refractive index, the quarter-wave reflecting stacks being separated by a multiple half-wave layer of dielectric material, and the plurality of stacks of said two outer cavities being separated by a multiple half-wave layer of dielectric material for broadening the bandwidth of a passband of the filter and for reducing residual reflections, each cavity being separated by quarter-wave layer of low refractive index material, wherein the number of layers in the at least one or more inner cavities is an odd number greater or equal to 7, and wherein the two outer cavities in said array of cavities each have two less layers in each quarter-wave reflecting stack than in other quarter-wave reflecting stacks of the at least one or more other cavities in said array, wherein some of the quarter-wave reflecting stacks of the two outer cavities have selected layers being a thickness of three-quarter wave to alter the slope of the passband, and wherein selected layers of the quarter-wave stacks of the at least one or more inner cavities are a thickness of a three-quarter-wave to lessen ripple induced by a three-quarter-wave layer present in one or both of the outer cavities.

4. A bandpass filter as defined in claim 3, wherein the multiple half-wave layers of dielectric material of the at least one or more other cavities adjacent to the two outer cavities comprises fewer multiple half-waves than the multiple half-wave layer of the two outer cavities.