Spectral filter comprising at least two coupled Fabry-Perot structures

Patent metadata

Field	Value
Publication number	US-11326947-B2
Application number	US-201917255820-A
Country	US
Kind code	B2
Filing date	Jun 25, 2019
Priority date	Jun 28, 2018
Publication date	May 10, 2022
Grant date	May 10, 2022

Abstract

Disclosed is a spectral filter including a plurality of Fabry-Perot structures which are collectively carried by a support, and coupled together so as to produce a coupling resonance that is distinct from individual resonances of each of the Fabry-Perot structures. The coupling resonance may have a higher quality factor than the quality factors of the individual resonances. Each Fabry-Perot structure includes two portions of metal surfaces which are parallel to a face of the substrate, so that electromagnetic radiation wave components propagating within each Fabry-Perot structure, and which contribute to the coupling resonance, have a direction of propagation that is parallel to the face of the substrate.

First claim

The invention claimed is:

1. A spectral filter comprising at least one group of a plurality of Fabry-Perot structures which are collectively carried by a face of a support of the filter, each Fabry-Perot structure comprising two portions of reflecting surfaces which are arranged facing one another, parallel to one another, and separated from one another inside the structure by a dielectric medium,
each of the Fabry-Perot structures of the group being sized so that a wave component of electromagnetic radiation can propagate between the two portions of reflecting surfaces of said structure, from one to the other, and so that a standing wave results from multiple reflections of the wave component which occur in alternation at the two portions of reflecting surfaces, and an individual resonance of said Fabry-Perot structure corresponding to a maximum of amplitude of said standing wave inside said structure when a wavelength of an electromagnetic radiation incident on the filter varies,
at least one of the portions of reflecting surfaces of each Fabry-Perot structure of the group being separated from at least one of the portions of reflecting surfaces of each other structure by an intermediate space parallel to the face of the support,
wherein at least one parameter which determines the individual resonance of each Fabry-Perot structure, separately from each other Fabry-Perot structure, has values which differ between at least two of the structures of the same group, so that said at least two structures of the group have respective values of individual resonance wavelength, effective for the electromagnetic radiation incident on the filter, which are different, and said at least two structures with different values of individual resonance wavelength having respective values of an individual resonance quality factor such that, on a wavelength axis of the incident radiation, the following ranges of individual resonances: [λ ri ·(1−3/Q i ); λ ri ·(1+3/Q i )], have pairwise overlaps, where i is an integer that identifies each Fabry-Perot structure within the group, and λ ri and Q i are respectively the values of the resonance wavelength and of the quality factor of the individual resonance of Fabry-Perot structure i,
and wherein a thickness h i of the dielectric medium, relative to Fabry-Perot structure i and measured perpendicularly to a direction of propagation of the wave components forming the standing wave inside said Fabry-Perot structure i, is greater than or equal to 0.125·λ ri /n eff_i , n eff_i being an effective refractive index for the dielectric medium of Fabry-Perot structure i,
and wherein a coupling between two of the Fabry-Perot structures of the same group, whose individual resonance wavelength values are different, is produced by the intermediate space existing between said two structures, possibly also by a material present in said intermediate space, a separation distance between the two Fabry-Perot structures thus coupled, determined by said intermediate space and measured parallel to the face of the support, being less than a resonance wavelength value relating to the coupling, called coupling resonance wavelength, which is effective for the electromagnetic radiation incident on the filter, and which results from an interference between at least three waves which include:
a first wave which originates from a reflection of the incident radiation on the face of the support, or from a transmission of the incident radiation through the support;
a second wave which comes out from a first of the Fabry-Perot structures of the group, and which results from a superposition of several wave components among which at least one of said wave components has completed at least one round trip inside a second of the Fabry-Perot structures of said group, coupled with said first structure; and
a third wave which comes out from said second of the structures of Fabry-Perot of the group, and which results from another superposition of several other wave components among which at least one of said other wave components has completed at least one round trip inside said first of the Fabry-Perot structures of said group,

and wherein each Fabry-Perot structure of the group comprises two portions of metal surfaces which are parallel to the face of the support, and an internal volume which is limited between said two portions of metal surfaces perpendicularly to the face of the support, and limited parallel to the face of the support between opposite edges of at least one of said two portions of metal surfaces of the structure, said two opposite edges determining positions of the portions of reflecting surfaces of the Fabry-Perot structure, so that the wave components which contribute to the coupling resonance propagate parallel to the face of the support inside each of the coupled Fabry-Perot structures of the group.

2. The spectral filter of claim 1 , wherein the parameter which determines the individual resonance of each Fabry-Perot structure of the group, separately from each other Fabry-Perot structure of said group, and which has values which differ between two structures which are coupled with each other, is one of the following parameters or a combination thereof:
a width of the internal volume of each structure, measured parallel to the face of the support between the portions of reflecting surfaces of said structure,
a refractive index of the dielectric medium in the internal volume of each structure,
a fill ratio and/or a composition of the dielectric medium in the internal volume of each structure, and
a coefficient of electromagnetic radiation absorption by each structure.

3. The spectral filter of claim 2 , wherein each Fabry-Perot structure of the group is designed so that the value of the individual resonance quality factor of said structure is less than 20,
and wherein the group of Fabry-Perot structures is designed so that a quality factor associated with the coupling resonance is greater than 20.

4. The spectral filter of claim 2 , wherein, for each Fabry-Perot structure of the group, the dielectric medium in the internal volume of said structure consists of a respective portion of a layer having parallel faces and uniform thickness, of a solid dielectric material, said layer being identical for all structures of the group.

5. The spectral filter of claim 1 , wherein each Fabry-Perot structure of the group is designed so that the value of the individual resonance quality factor of said structure is less than 20,
and wherein the group of Fabry-Perot structures is designed so that a quality factor associated with the coupling resonance is greater than 20.

6. The spectral filter of claim 5 , wherein each Fabry-Perot structure of the group that is designed so that the value of the individual resonance quality factor of said structure is less than 20, comprises a thickness of the internal volume between the two portions of metal surfaces of said structure, measured perpendicularly to the face of the support, and
wherein the group of Fabry-Perot structures that is designed so that a quality factor associated with the coupling resonance is greater than 20 comprises the intermediate space between two of the structures of said group which are coupled.

7. The spectral filter of claim 1 , wherein, for each Fabry-Perot structure of the group, the dielectric medium in the internal volume of said structure consists of a respective portion of a layer having parallel faces and uniform thickness, of a solid dielectric material, said layer being identical for all structures of the group.

8. The spectral filter of claim 1 , wherein a pattern which comprises the Fabry-Perot structures of the group, as separated by each intermediate space between two of said structur

Assignees

• Office National Detudes Rech Aerospatiales

Inventors

• Fix Baptiste
• Jaeck Julien
• Bouchon Patrick
• Haidar Riad

Classifications

• G02B5/288 Primary

  comprising at least one thin film resonant cavity, e.g. in bandpass filters · CPC title

• G01J2003/2826

  Multispectral imaging, e.g. filter imaging · CPC title

• G01J3/26 Primary

  using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters · CPC title

• G01J3/2823

  Imaging spectrometer · CPC title

• G02B26/001 Primary

  based on interference in an adjustable optical cavity (interference filters G02B5/28; devices or arrangements using multiple reflections in spectrometry or monochromators G01J3/26) · CPC title