Technique to improve the efficiency of nonlinear optical processes

The invention provides structure and method for improving the efficiency of nonlinear optical processes where two light beams interact in a material to produce an output beam at a new frequency. Effective phase matching is accomplished by periodically removing the output beam energy so that it does not destructively interfere with the input. The invention can also be utilized to increase the efficiency of optical devices where partial phase matching is present without further adjusting the phase of the beams involved in the nonlinear interaction. The invention also provides effective beam phase matching where the beams passing through the nonlinear material cannot normally be phased matching by present methods.

BACKGROUND OF THE INVENTION 
The invention is directed to apparatus and method for improving the 
efficiency of nonlinear optical process where the phase matching of the 
light beams passing through the nonlinear material, cannot be achieved by 
conventional means. This invention effectivly phase matches partially 
phased or non-phased matched beams by periodically removing the output 
energy of the beams after mixing within the nonlinear material. 
A large body of literature exists on nonlinear processes and phase matching 
techniques employed to phase match light beams passing through nonlinear 
material. None of these techniques teach phase matching by removal of the 
output beam. 
A key requirement for efficient nonlinear operation in devices of this type 
is that the beams be phased matched. Phase matching involves setting up 
the operating conditions so that the propagation vector of the output beam 
is equal to the sum of the propagation vectors of the input beams, ie. 
the, output beam is equal to the sum of the propagation vectors of the 
input beams, ie. 
##EQU1## 
.lambda..sub.o' .lambda.j =vacuum wavelengths of output and input beams 
respectfully 
n.sub.o' n.sub.j =indices of refraction for output and input beams 
respectfully 
N=number of input beams 
s.sub.o' s.sub.j =unit vectors in the propagation direction 
Typically, phase matching is achieved by selecting the material, operating 
wavelengths and propagating directions correctly. The problem is well 
understood and detailed descriptions of phase matching techniques can be 
found in the technical literature e.g., in Applied Nonlinear Optics by 
Fritz Zernike and John E. Midwinter, chapter 3. 
An important concept to understand about phase matching is that the waves 
interact constructively over a length l.sub.c called the coherence length. 
After propagating a length l.sub.c the waves interfere destructively for a 
length l.sub.c and the process repeats. The distance l.sub.c is equal to: 
##EQU2## 
For a crystal of length L&gt;&gt;l.sub.c the total nonlinear output appears to 
come from a region only l.sub.c long and the material is used very 
inefficiently. 
Another important characteristic of the state of the art nonlinear 
processes is that many of the materials with the largest nonlinear 
coefficient cannot be phased matched because there is no combination of 
n.sub.o, n.sub.j, .lambda..sub.o, .lambda..sub.j, s.sub.o, and s.sub.j 
that leads to .DELTA.k=0. 
U.S. Pat. No. 3,983,406, by inventors Lax et al., describes a technique to 
improve the efficiency of a nonlinear process by using multiple internal 
reflections. The converted signal and the reference signals all travel 
through the nonlinear medium where some adverse reverse mixing occurs and 
the reference signals and the converted signal are not separated until 
they leave the nonlinear medium. 
The present invention improves the present state of the art by more 
efficiently separating the input beams and the resulting output beam from 
each other, thus preventing distructive interference mixing thereof 
without concern to the phase relationship of the input and output beams. 
SUMMARY OF THE INVENTION 
The present invention is directed to a new approach to nonlinear process 
involving two or more light beams which interact in a medium to form a new 
beam with a minimum of destructive interface of the new beam with the 
input beams without concern to related phase relationships of the two or 
more light beams. 
The invention reduces or eliminates the destructive interference by 
providing a means of removing the output beam from the nonlinear material 
after the beams have propagated through the distance d=m l.sub.c (where 
m=integer&gt;0). The input beams continue in the material for another 
interval d where the output is again removed. The process continues as 
long as practicable. By removing the output beam in this manner the 
destructive interference phenomenon is prevented and a large fraction of 
the propagation length in the material can be used efficiently. 
An object of this invention is to provide efficient nonlinear processes, 
e.g. mixing of incoming light beams into a third different frequency light 
beam, by passing the beams through a nonlinear material with a minimum of 
destructive interference phenomenon without concern to phase matching. 
An other object of this invention is to provide the mixing of at least two 
separate input frequency light beams within a nonlinear material to 
produce a third frequency light beam produced therefrom and provide means 
for removing the third frequency output light beam produced by the 
nonlinear material after the beams have propagated through a distance d=m 
l.sub.c (where m=integer&gt;0), allowing the input beams to continue in the 
nonlinear material for another interval d where the third or mixed 
frequency output is again removed and continuing the process as long as 
practicable. 
Still another object of the invention is to provide passband dielectric 
reflector coatings to the light receiving and light exiting surfaces of a 
nonlinear material whereby the input frequency light beams entering the 
nonlinear material are reflectively contained, reflect back and forth 
between the input and output surfaces and propagate within the nonlinear 
material and the third frequency light beam produced by the mixing of the 
input light beam signals pass through a dielectric reflector coatings with 
minimum resistance thereto each time the input frequency signals strike 
the coatings. 
Still another object of the invention is to remove a third frequency light 
beam at yet a different frequency produced by the mixing together of at 
least two different input frequencies light beams within a nonlinear 
material at predetermined distance intervals sums together these removed 
light beams to produce the single third frequency light beam at a combined 
elevated intensity level which exits said nonlinear material. 
The features of the present invention which are believed to be novel are 
set forth with particularly in the appended claims. The organization and 
manner of operation of the invention, together with further objects and 
advantages thereof may be best understood by reference to the following 
description taken in connection with the accompanying drawings, in the 
several figures of which like reference numerals identify like elements 
and in which:

DETAILED DESCRIPTION OF THE FIRST EMBODIMENT 
Referring now specifically to FIG. 1, this first embodiment teaches a 
resonant cavity technique of practicing the invention. A resonate cavity 
10 is constructed by high reflectance mirrors 12 formed from dielectric 
coatings positioned on each side of nonlinear material 14 forming and 
etalon. This nonlinear material may be GaAs, CdTe crystals or the like 
suitable for the purpose intended. The reflectance mirrors 12 are made 
from multiple dielectric layers and are not designed as ordinary mirrors. 
Instead they are multi-layer interference filters with a peak 
transmittance at a selected output wavelength. 
For example, if the frequencies of the two out of phase frequencies to be 
combined were 10.6.mu. and 1.06.mu. the selected transmission wave length 
would be 0.96.mu. or if the frequencies were 10.6.mu. and 0.598.mu. the 
selected transmission wave length would be 0.566.mu., ie. the coatings 
have a low reflection at the selected transmission wave length to let the 
up-converted light escape the etalon. Any selected frequencies can be 
handled in this manner so long as the low reflection characteristics of 
the etalon is selected to the frequency of the combined incoming 
frequencies. Although generally the power of one of the incoming 
frequencies is much greater than the other, for example, in a laser system 
the pump frequency (one of the incoming frequencies) will be in the range 
of -10.sup.3 -10.sup.6 watts/cm.sup.2 and the other frequency in the range 
of -10.sup.-8 -10.sup.-10 watts. In this example the etalon resonates at 
the pump frequency to amplify the intensity of the pump frequency in the 
material and it is approximately resonant to the other incoming signal. 
The thickness t of the etalon produced in this manner is made 
approximately equal to ml.sub.c (for m as small as practical) and the 
etalon is tuned to resonance with the two input beams 16 and 18. The 
resonance condition can be achieved by choosing t and the temperature 
correctly for given values of n.sub.j and .lambda..sub.j. At resonance the 
intensity of the beams 16 and 18 inside the cavities increases, causing 
the efficiency to rise. Because the mirrors 12 pass only the output wave 
length which is a summation of beams 16 and 18 the output energy leaves 
the cavity and destructive interference between the photons propagating 
back and forth between the mirrors and the output photons does not occur. 
Various means of collecting the output beam 20 are available and even the 
backward propagating output beam 22 can be used efficiently. An example of 
a collecting means is a simple focus lens 22 that focuses the beam 20 on a 
detector 24 such as, for example, a photomultiplier tube, for visible 
light, a silicon photo diode, for infrared light, or similar devices which 
passes the beam energy to signal processing electronics 26. 
FIG. 2 depicts a second embodiment of the invention which teaches a 
multiple reflection technique. In this embodiment, an etalon formed from a 
piece of nonlinear material 14 is fabricated with plane parallel faces 24 
and each face 24 is covered with passband interference reflector 12 
similar to or the same as the one used on the resonant cavity 10 above. 
The propagation length between the faces of the crystal is made equal to 
ml.sub.c again m is as small as possible. As the beams 16 and 18 bounce 
back and forth between the parallel faces the output beam escapes each 
time they travel ml.sub.c, again preventing destructive interference. All 
of the output beams are collected and added together, increasing the 
efficiency by the number of bounces. By tilting the input beams 16 and 18 
slightly the multiple bounces will carry the beams in a third dimension 
(perpendicular to the plane of the Figure) and the number of bounces can 
be increased. 
FIG. 3 depicts a third embodiment of the invention. In this embodiment the 
device is very similar to the plane parallel multiple reflection resonant 
etalon device of FIG. 1 except that the input beams 16 and 18 propagate in 
a waveguide of nonlinear material 14. The advantage of the waveguide 
approach is that a long path with no beam divergence can be made. The 
zig-zag path is constructed so that the output escapes from the guide 
every time the beams propagate a distance equal to ml.sub.c. The output 
beams are collected by the means mentioned above and added together 
increasing the efficiency by the number of separate outputs. 
Further modifications of the invention herein disclosed will occur to 
persons skilled in the art and all such modifications are deemed to be 
within the spirit and scope of the invention defined herein as defined by 
the claims: