Non-polarizing thin film edge filter

• US 4,373,782 A
• Filed: 06/03/1980
• Issued: 02/15/1983
• Est. Priority Date: 06/03/1980
• Status: Expired due to Term

• Current Assignee: Optical Coating Laboratory, Inc. (Lumentum Holdings Inc.)
• Original Assignee: Optical Coating Laboratory, Inc. (Lumentum Holdings Inc.)
• Inventors: Thelen, Alfred J.
• Primary Examiner(s): ARNOLD, BRUCE
• Application Number:
  US06/156,260
• Time in Patent Office: 987 Days
• Field of Search: 350/166, 350/164, 350/172, 350/173
• US Class Current: 398/79
• CPC Class Codes: G02B 5/288 comprising at least one thin film resonant cavity, e.g. in bandpass filters
• CPC Class Codes: H04B 10/25891 Transmission components H04B10/40 takes precedence

Abstract

A thin film optical filter having a bandpass reflectance characteristic with one of the transition wavelength edges thereof being non-polarizing for radiation incident at a preselected non-normal angle. The filter comprises a transparent substrate and a multilayer thin film optical coating provided on one surface thereof. The optical coating includes a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material arranged as a sequence of dielectric multilayer stacks with intervening spacer layers formed of one of the dielectric materials. The optical thickness of the spacer layers is detuned from half-wave optical thickness to shift either the low wavelength transition edges or the high wavelength transition edges for the separate bandpass response characteristics for the two principal polarization planes substantially into alignment with each other. Various other dielectric stack designs utilizing detuned layers to accomplish the same alignment of wavelength transition edges are described. Simplex and duplex fiber optic communications systems utilizing non-polarizing edge filters of the thin film type are also disclosed.

Claims

1. A thin film optical filter having a bandpass reflectance characteristic with one of the transition wavelength edges thereof being non-polarizing for radiation incident at a preselected non-normal angle, said filter comprising:

a transparent substrate having a surface; and

a multilayer thin film optical coating provided on said surface, said optical coating including a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material arranged as a sequence of dielectric multilayer stacks with intervening spacer layers formed of one of said dielectric materials, said layers of first and second material in said multilayer stacks having optical thicknesses preselected to produce, for radiation incident at said preselected angle, a spectral response characteristic which has separate bandpass regions with separated low and high wavelength transition edges for the two principal polarization planes if said spacer layers were to have tuned optical thickness, said spacer layers having detuned optical thickness prearranged in accordance with said preselected angle to shift either said low wavelength transition edges or said high wavelength transition edges substantially into alignment.

2. The article of claim 1, wherein said multilayer thin film optical coating further includes a thin film understructure on said surface for optical matching of said coating to said substrate, and a thin film overstructure for optical matching of said coating to the surrounding medium;

and wherein each of said dielectric stacks is a symmetrical period of said thin film layers, and wherein detuned spacer layers are interposed between the first of said dielectric stacks and said thin film understructure and the last of said dielectric stacks and said overstructure.

3. A non-polarizing optical filter adapted to be disposed at a preselected non-normal angle to incident, transmitted, and reflected optical signal beams for producing wavelength division multiplexing or demultiplexing of said optical signal beams using at least a first lower signal wavelength λ

_a and a second higher signal wavelength λ

_b, said filter comprising;

a transparent substrate having a surface; and

a multilayer thin film optical coating formed on said surface and having an edge filter optical response characterized by a high reflectance region including one of said signal wavelengths λ

_a or λ

_b, a high transmittance region including the other of said signal wavelengths λ

_a or λ

_b and a non-polarizing transition edge at an intermediate wavelength λ

_c, said multilayer thin film filter including a plurality of symmetric dielectric periods, each comprising a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material having respective optical thicknesses preselected to produce a spectral response characteristic for signal beams incident at said preselected angle having separate bandpass regions with separated low and high wavelength transition edges for the two principal polarization planes if the first and last layers of said periods have tuned optical thickness, said first and last layers being formed to a detuned optical thickness preselected in accordance with said preselected angle of incidence to shift either said low wavelength transition edges or said high wavelength transition edges substantially into alignment.

4. The article of claim 3, wherein said multilayer thin film optical coating further includes a thin film understructure on said surface for optical matching to said substrate and a thin film overstructure for optical matching to the surrounding medium;

each of said understructure and said overstructure comprising a mutilayer of said first and second dielectric materials and including a layer of detuned optical thickness matching that of the detuned layers in the symmetric multilayer stack.

5. A thin film optical filter having a bandpass reflectance characteristic with one of the transition wavelength edges thereof being non-polarizing for radiation incident at a preselected non-normal angle, said filter comprising:

a transparent substrate having a surface; and

a multilayer thin film optical coating provided on said surface, said optical coating including a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material arranged in the following design configuration;

##EQU9## where A and B generally designate quarter-wave layers of said dielectric materials, X designates a preselected number of layers in each reflecting stack, ν

designates a preselected number of reflective stacks in said coating, the subscripts on the letters A and B designate the layer number within each stack and the subscripts on the parenthetical expressions designate the stack number, and ρ

_s designates a detuning factor having a value either less than or greater than one and prearranged in accordance with said preselected angle such that either the low or high wavelength transition edges of the spectral response of said filter for the two principal polarization planes are substantially aligned.

6. The filter of claim 5, wherein said matching layers adjacent to the substrate and medium are designed to reduce secondary reflectance bands in a bandpass region of interest and wherein the optical thicknesses of selected ones of said A and B layers are refined to reduce secondary reflectance bands in a bandpass region of interest.

7. A thin film optical filter having a bandpass reflectance characteristic with one of the transition wavelength edges thereof being non-polarizing for radiation incident at a preselected non-normal angle, said filter comprising:

a transparent substrate having a surface; and

a multilayer thin film optical coating provided on said surface, said optical coating including a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material arranged in the following design configuration;

##EQU10## where A and B generally designate quarter-wave layers of said dielectric materials, X designates a preselected number of layers in each reflecting stack, ν

designates a preselected number of reflective stacks in said coating, and ρ

_s designates a detuning factor having a value either less than or greater than one and prearranged in accordance with said preselected angle such that either the low or high wavelength transition edges of the spectral response of said filter for the two principal polarization planes are substantially aligned.

8. The filter of claim 7, wherein said matching layers adjacent to the substrate and medium are designed to reduce secondary reflectance bands in a bandpass region of interest and wherein the optical thicknesses of selected ones of said A and B layers are refined to reduce secondary reflectance bands in a bandpass region of interest.

9. A thin film optical filter having a bandpass reflectance characteristic with one of the transition wavelength edges thereof being non-polarizing for radiation incident at a preselected non-normal angle, said filter comprising:

a transparent substrate having a surface; and

a multilayer thin film optical coating provided on said surface, said optical coating including a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material arranged in the following design configuration;

##EQU11## where A and B generally designate quarter-wave layer thicknesses of said dielectric materials, a and b designate layer thickness design factors consisting either of a=2/3 and b=4/3 or a=4/3 and b=2/3, x designates a preselected number of layers in each reflecting stack, ν

designates a preselected number of reflective stacks in said coating, and ρ

_s designates a detuning factor having a value either less than or greater than one and prearranged in accordance with said preselected angle such that either the low or high wavelength transition edges of the spectral response of said filter for the two principal polarization planes are substantially aligned.

10. The filter of claim 9, wherein said matching layers adjacent to the substrate and medium are designed to reduce secondary reflectance bands in a bandpass region of interest and wherein the optical thicknesses of selected ones of said A and B layers are refined to reduce secondary reflectance bands in a bandpass region of interest.

11. In a fiber optic communication system employing wavelength division multiplexing,

at least a pair of optical signal transmitters for producing optical signal beams having at least two different wavelengths λ

_a and λ

_b ;

a wavelength multiplexer receiving said optical signal beams for combining said beams into a single wavelength division multiplexed beam having at least two signal components at wavelengths λ

_a and λ

_b ;

a fiber optic transmission link for transmitting said wavelength division multiplexed beam;

a wavelength demultiplexer receiving said wavelength division multiplexed beam for splitting said beam into demultiplexed optical signal beams having at least two different wavelengths λ

_a and λ

_b ;

at least a pair of optical signal detectors for separately receiving said demultiplexed optical signal beams;

said wavelength multiplexer including at least one thin film optical filter disposed at a preselected angle to each of said optical signal beams and adapted to transmit one of said signal beams having wavelength λ

_a to a multiplexed signal beam path and to reflect the other of said signal beams having wavelength λ

_b to said multiplexed signal beam path;

said wavelength demultiplexer including at least one thin film optical filter disposed at a preselected angle to said wavelength division multiplexed beam and adapted to transmit the λ

_a wavelength component thereof to a first demultiplexed signal beam path and to reflect the λ

_b wavelength component thereof to a second demultiplexed signal beam path;

each of said thin film optical filters comprising a transparent substrate having a surface; and

a multilayer thin film optical coating formed on said surface and having an edge filter optical response characterized by a high reflectance region including wavelength λ

_b and a high transmittance region including the wavelength λ

_a and a non-polarizing transition edge at an intermediate wavelength λ

_c, said multilayer thin film coating including a plurality of symmetric dielectric periods, each comprising a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material having respective optical thicknesses preselected to produce a spectral response characteristic for signal beams incident at said preselected angle having separate bandpass regions with separated low and high wavelength transition edges for the two principal polarization planes if the first and last layers of said periods were to have tuned optical thickness, said first and last layers being formed to detuned optical thicknesses preselected in accordance with said preselected angle of incidence to shift either said low wavelength transition edges or said high wavelength transition edges substantially into alignment.

12. In a fiber optic communication system employing a duplex signal transmission path using wavelength division multiplexing,

a first and a second station each having at least one optical signal transmitter and at least one optical signal detector, said transmitters at said stations producing output optical signal beams at two different wavelengths λ

_a and λ

_b ;

a fiber optic transmission link for transmitting a duplex wavelength division multiplexed beam between said stations;

a duplex beam splitter at each of said stations for separating transmitted and received optical signals, said beam splitter including at least one thin film optical filter disposed at a preselected non-normal angle to said optical signal beams and adapted to transmit one of said signal beams having wavelength λ

_a and to reflect the other of said signal beams having wavelength λ

_b ;

said thin film optical filter comprising a transparent substrate having a surface, and a multilayer thin film optical coating formed on said surface and having an edge filter optical response characterized by a high reflectance region including wavelength λ

_b and a high transmittance region including the wavelength λ

_a and a non-polarizing transition edge at an intermediate wavelength λ

_c, said multilayer thin film coating including a plurality of symmetric dielectric periods, each comprising a preselected number of alternate layers of a first higher refractive index dielectric material and a second lower refractive index dielectric material having respective optical thicknesses preselected to produce a spectral response characteristic for signal beams incident at said preselected angle having separate bandpass regions with separated low and high wavelength transition edges for the two principal polarization planes if the first and last layers of said periods were to have tuned optical thicknesses, said first and last layers being formed to detuned optical thicknesses preselected in accordance with said preselected angle of incidence to shift either said low wavelength transition edges or said high wavelength transition edges substantially into alignment.