An optical filter is formed by exposing a photosensitive material to direct nd retroreflected coherent optical radiation. These radiations set up standing waves in the material such that, when the material is fixed, variations in parallel, equally spaced layers of some optical property of the material remain. The optical property may be reflectance, index of refraction, or optical density. The layers are spaced apart one-half wavelength of the coherent radiation and act as an interference filter. The material may be capable of being erased and a new filter formed, if desired.

BACKGROUND OF THE INVENTION 
This invention is in the field of optical interference filters, and is 
particularly concerned with a filter that is formable by incident coherent 
radiation and which may be selectively erased. In general, optical 
interference filters consist of layers of optical materials in which the 
layers differ in some optical property such as reflectance, refractive 
index, or density. A typical filter uses alternate layers of relatively 
high and low indices of refraction, each layer being one-quarter 
wavelength of the radiation with which one wishes to interfere. Another 
type of interference filter is the Lippmann filter, in which are contained 
partially reflecting layers spaced apart one-half wavelength of the 
radiation to be reflected. The Lippmann filter layers are formed 
photographically as follows: a medium with photosensitive particles 
therein (a photographic film) is exposed on one side to coherent radiation 
and a mirror is placed on the other side to reflect the radiation and set 
up standing waves in the medium. When the medium is developed, parallel 
and partially reflective layers form, spaced apart one-half wavelength of 
the incident radiation. The main problem with these filters is that they 
are permanent; once formed, they cannot be erased and reformed. Moreover, 
they are not formable in real time, but must be prepared in advance. The 
instant invention is able to overcome these disadvantages; it provides a 
filter which is formable in real time, which may be erased and reformed, 
and which is formed by the radiation which it is desired to block.

SUMMARY OF THE INVENTION 
The invention is a device for separating coherent optical radiation from 
noncoherent optical radiation, and includes a real-time formable bandstop 
interference filter and the method of making the filter. The filter is 
made by exposing a photosensitive material on one side to incident 
coherent radiation, and reflecting the radiation passing through the 
material back therethrough, whereby standing waves are set up in the 
material. These waves induce variations in some optical property of the 
medium, such as reflectance, index of refraction, or optical density; the 
variations act as an interference filter to the incident radiation. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention may be best understood when this description is taken in 
conjunction with the drawings. The embodiment of FIG. 1 shows lens 11 used 
to collimate and direct incoming radiation 12 onto filter 13. On the 
opposite side of 13 is a partially reflecting mirror 14, lens 15, and 
detector 16; the radiation passing through 13 and 14 is focused by 15 onto 
16. Filter 13 has electrodes 13a and 13b on opposite ends thereof, 
connected by electrical conductors 17 to switch 18. This switch controls 
the power from power supply 19 for erasing or fixing filter 13. The 
particular voltages and polarities provided by supply 19 and controlled by 
switch 18 depend upon the particular material used for 13. An example of a 
material for 13 is PLZT (lead lanthenum zirconate titanate), as described 
in the article entitled "Ion-Implanted PLZT Ceramics: A New 
High-Sensitivity Image Storage Medium" in {IEEE Transactions On Electron 
Devices}, Vol. ED-28, No. 6, June 1981, pages 756-762. 
FIG. 2 shows another embodiment of the invention similar to FIG. 1 except 
for the filter. In FIG. 2, reference numerals 21, 22 and 25-29 correspond 
to reference numerals 11, 12 and 15-19 of FIG. 1. The filter of FIG. 2, 
generally designated 23, includes photosensitive material 23a, partially 
reflecting mirror 23b, and fully reflecting mirror 24. Mirrors 23b and 24 
together act as a corner reflector and set up standing waves in 23a for 
coherent incident radiation; these standing waves cause a square honeycomb 
variation in the optical property of 23a. 
The method by which filter 13 or 23 is formed is straightforward: 13 or 23 
is sensitized with the proper voltage and polarity, coherent radiation is 
directed through (and retroreflected back through) 13 or 23 to establish 
standing waves therein, and the correct voltage and polarity are applied 
to optically fix these waves. Obviously, more than one wavelength of 
coherent radiation may be used to make the filter; the filter will act as 
an interference filter to each such wavelength of radiation. If desired, 
the filter may be erased when desired by applying the proper voltage and 
may be reformed. 
Since an interference filter is formed, this filter strongly interferes 
with or attenuates coherent radiation of the wavelength which formed the 
filter, but attenuates noncoherent radiation much less. The filter may 
thus be used to separate noncoherent and low level signal information from 
intense coherent radiation (as may come from a laser).