Diffraction coherence filter

• US 4,958,892 A
• Filed: 10/18/1988
• Issued: 09/25/1990
• Est. Priority Date: 10/18/1988
• Status: Expired due to Term

• Current Assignee: Sanwa Bank California (BNP Paribas S.A.)
• Original Assignee: Physical Optics Corporation
• Inventors: Jannson, Tomasz P., Jannson, Joanna L.
• Primary Examiner(s): Arnold, Bruce Y.
• Assistant Examiner(s): Nguyen, Thong
• Application Number:
  US07/259,304
• Time in Patent Office: 707 Days
• Field of Search: 350/3.6, 350/3.66, 350/3.7, 350/3.75, 350/3.77, 350/3.78, 350/162.11, 350/162.17-162.22, 350/162.19, 350/3.85, 350/320, 350/321, 356/354-356
• US Class Current: 359/15
• CPC Class Codes: G02B 5/288 comprising at least one thin film resonant cavity, e.g. in bandpass filters
• CPC Class Codes: G02B 5/32 Holograms used as optical elements
• CPC Class Codes: Y10S 359/90 Methods

Abstract

A diffraction coherence filter based on Bragg interference principles comprises a plurality of spaced-apart lower-hierarchy optical elements, containing a series of interference structures, which form a higher-hierarchy compound optical structure. The spatial distribution of the lower-hierarchy optical element and the interference structures is governed by mathematical relationships dependent upon the coherence radius and temporal coherence length of incoming radiation. All of the lower-hierarchy elements can be coherently coupled for coherent light and randomly coupled for incoherent light, whereby the filter may differentiate between coherent and incoherent radiation.

Claims

1. A filter apparatus capable of distinguishing between highly-coherent and incoherent radiation reaching the filter apparatus at angle of incidence θ

_i, the highly-coherent radiation characterized by spatial coherence radius r_c and temporal coherence length l_c and the incoherent radiation characterized by spatial coherence radius r_i and temporal coherence length l_i, the apparatus comprising;

a plurality of diffraction optical elements separated from each other by a distance L such that

space="preserve" listing-type="equation">l.sub.i <

[L/|Cosφ

|]·

[|1+|/Cos (θ

.sub.i +φ

)|]<

l.sub.c

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">2r.sub.i <

d<

2r.sub.c

where d is the distance between successive diffractions of incident radiation wavefronts from each of said diffraction optical elements and ψ

is the angle at which incident radiation wavefronts are diffracted from each of said diffraction optical elements

said plurality of diffraction optical elements further being arranged to comprise a higher-hierarchy structure of total thickness T such that

l_c >

[T/|Cosφ

|]·

[|1+l/cos(θ

.sub.i +ψ

)|]

space="preserve" listing-type="equation">and

space="preserve" listing-type="equation">r.sub.c >

D/2

where D is the total distance across all rays diffracted from all of the optical elements.

2. A method for constructing a diffraction coherence filter from holographic media, said method comprising the steps of:

forming a first layer from the holographic medium;

transmitting electromagnetic energy through said first layer;

reflecting from a surface adjacent said first layer the electromagnetic energy transmitted through said first layer back into the holographic medium of said first layer to form a standing wave pattern therein such that an interference pattern is established in the holographic medium;

forming a second layer comprised of a holographic medium and positioning said second layer at a predetermined distance from said first layer;

transmitting electromagnetic energy through said second layer;

reflecting from a surface adjacent said first layer said electromagnetic energy transmitted through said second layer from said first layer back into the holographic medium of said second layer to create said interference pattern in the holographic medium of said second layer; and

repeating said steps of forming layers comprised of holographic medium and transmitting electromagnetic energy through said layers so formed for reflection from the preceding layer of holographic medium back into said layer so formed to create said interference patterns until a predetermined number of layers of holographic medium with interference patterns established therein have been formed.

3. A method for fabricating a diffraction coherence filter from a holographic medium, said diffraction coherence filter functioning to distinguish between coherent and incoherent radiation, said method comprising the step of:

exposing the holographic medium to electromagnetic energy through a first grating structure to create a first standing wave pattern throughout the holographic medium such that holographic interference patterns would otherwise be established throughout the holographic medium in accordance with said first standing wave pattern; and

exposing the holographic medium to electromagnetic energy through a second grating structure to create a second standing wave pattern throughout the holographic medium, said second grating structure characteristically exhibiting a large spatial period relative to the spatial period of said first grating structure, which large spatial period causes the maximum intensities of said second standing wave pattern to occur in portions of the holographic medium exhibiting strong non-linear material characteristics, effectively erasing said holographic interference patterns established by said first standing wave pattern in said portions of said holographic medium such that a series of holographic layers alternating with non-holographic spacer layers is formed in the holographic medium.

4. A method for fabricating a diffraction coherence filter from holographic media, said diffraction coherence filter functioning to distinguish between coherent and incoherent radiation, said method comprising the steps of:

building up a filter structure to a predetermined thickness by alternating layers of holographic media with distance plates to provide separation between each of said layers of holographic media, each of said layers of holographic media comprising a holographic substance which does not require any chemical processing after holographic recording;

transmitting electromagnetic energy through said filter structure and reflecting from a surface adjacent said first layer said electomagnetic energy so transmitted back into said filter structure to form a standing wave pattern in each of said layers of holographic media such that holographic intereference patterns are simultaneously established in each of said layers of holographic media.