Etalon optical logic gate apparatus and method

An optical logic apparatus includes a plurality of logic elements on a single passive, nonlinear Fabry-Perot etalon characterized by a shift in transmissivity in response to increased incident light, a source producing a probe beam having a first wavelength which does not significantly modify the index or refraction of the nonlinear medium, and a plurality of sources of input beams each representing a respective logic signal. The input beams all have a wavelength which significantly modifier the index of refraction of the nonlinear medium. The probe beams illuminate the surface of the etalon. The input beams are mapped by a lens onto the etalon, which produces a plurality of output beams each representing various logical functions of the input beams, which logical functions are determined by the frequency of the various probe beams. The output beams become input beams which are optically mapped, in combination with additional probe beams, onto a second etalon. The outputs of the second etalon can be similarly mapped onto further etalons or into the first etalon.

BACKGROUND OF THE INVENTION 
The invention relates to nonlinear Fabry-Perot etalons, and more 
specifially to use of etalons as logic elements. 
Use of nonlinear Fabry-Perot cavities or etalons is known. Typically, an 
etalon is biased by means of a bias beam having a wavelength which is 
nearly an integral multiple of the optical thickness of the etalon. 
However, prior optical switches absorb a lot of light energy, due to the 
fact that the bias beam is partially absorbed to modify the index of 
refraction to hold the switch in an on state, and also due to the fact 
that the switch is an on state, and also due to the fact that the switch 
is held in its on state for an entire cycle. This results in heating of 
the device, which can cause the switch to work improperly. Much research 
on optical bi-stable devices has been published, for example, in "Optical 
Modualtion by Optical Tuning of a Cavity" by H. M. Gibbs et al., Applied 
Physics Letters, Volume 34, No. 8, Apr. 15, 1979, page 511, "Optical 
Bi-Stable Devices: The Basic Components of All-Optical Systems?" by H. H. 
Gibbs et al., Optical Engineering, Volume 19, No. 4, July/August 1980, 
page 463, and "Switching of a GaAs Bi-Stable Etalon: External Switching On 
and Off, Regenerative Pulsations, Transverse Effects, and Lasing", by H. 
M. Gibbs et al. Other optical logic elements have also been demonstrated 
as in "Optical Logic Gates Using a Hughes Liquid Crystal Light Valve" by 
S. A. Collins, Jr., et al., SPIE, Volume 232, 1980 International Optical 
Computing Conference, page 168, and see C. P. Seaton et al., Applied 
Physics Letters, Volume 42, No. 2, Jan. 15, 1983, page 131. See also 
"Physics of Optical Switching", by R. L. Fork, Physical Review A, Volume 
26, No. 4, October 1982, page 2049. However, none of the prior optical 
logic gates has the capability of driving numerous successive similar 
logic elements, due to attenuation of light intensity or lack of a 
convenient means of regeneration of light signal strength. 
Therefore, it is a primary object of the invention to provide an improved 
optical logic element which is simple to operate, capable of reliably 
performing logical operations using a single Fabry-Perot etalon at 
substantially higher speed, and with lower power dissipation than any 
prior high speed optical logic elements. 
It is another object of the invention to provide an optical logic element 
which is capable of producing output beams representing logic combinations 
of light encoded logic signals with sufficient intensity to drive 
subsequent optical logic element stages without use of any gain medium. 
It is another object of the invention to provide an optical logic element 
which is capable of producing output beams representing logic combinations 
of light encoded logic signals with reduced intensity and yet having the 
capability of driving subsequent optical logic element stages without the 
use of any gain medium. 
It is another object of the invention to provide an optical logical element 
apparatus and method for simultaneous multiple optical logic operations in 
successive stages at very low power dissipation and high data rates. 
It is another object of the invention to provide an optical logic element 
producing high speed, low power inverting logic functions with high 
contrast output beams. 
SUMMARY OF THE INVENTION 
Briefly described and in accordance with one embodiment thereof, the 
invention provides an optical logic apparatus including an etalon having a 
nonlinear medium which is characterized by a change in its index of 
refraction and/or absorption coefficient in response to an increase in 
incident light, a source for producing a probe beam having a wavelength 
which does not significantly modify the transmissivity of the etalon, 
sources for producing a plurality of input beams each representing a 
respective data signal and each having a wavelength which does 
significantly modify the transmissivity of the etalon means for guiding 
the probe beam and input beams into the etalon so that the etalon 
undergoes an increase or decrease in transmissivity in response to 
presence or absence of light in the various input beams to produce a 
predetermined logic operation on the input data signals. In the described 
embodiment of the invention, the probe beam and the input beams are pulses 
which are short in duration compared to the relaxation time constant of 
the nonlinear medium. The probe pulses occur immediately after the input 
pulses occur. In the described embodiment of the invention, the intensity 
(or energy) of each of the input beams results in an FWHM (full width at 
half maximum) shift in the transmissivity of the etalon from its 
transmissivity level with only the corresponding probe beam incident, 
i.e., if that input beam is at a logical "1" level. For a first probe beam 
having a wavelength equal to the initial resonant wavelength of the 
etalon, the transmissivity characteristic of the etalon causes a logical 
NOR operation to be formed on the input data signals. For a second probe 
beam having a wavelength which is initially "detuned" one half of an FWHM, 
the transmissivity characteristic of the etalon produces a logical NAND 
operation on the input data signals. If another probe beam having a 
wavelength detuned in an amount equal to a full FWHM, the transmissivity 
characteristic of the etalon produces an exclusive OR function in the 
input data signals. If the probe beam wavelength is detuned by one and 
one-half FWHM, a logical OR function is performed on the input beams. If 
the probe beam wavelength is detuned by two FWHMs, a logical AND function 
is performed on the input beams. In the described embodiment of the 
invention, the output beams produced by the optical logic operations 
performed by the first etalon become input beams which, in conjunction 
with additional input and/or probe beams, are mapped into a second similar 
optical logic element, the output beams of which are mapped into 
successive etalons or into the first mentioned etalon, or into an array of 
photodetector devices which convert the output beams into corresponding 
electrical logic signals. 
In another embodiment of the invention, a continuous wave probe beam of 
relatively high intensity is directed to an etalon, the continuous wave 
probe beam having a wavelength equal to the wavelength at the base of the 
approximately linear portion of the transmissivity peak of the etalon for 
zero intensity of an input beam. The continuous wave probe beam has a 
wavelength which does not significantly modify the transmissivity of the 
etalon. The input beam has a wavelength which does significantly modify 
the transmissivity of the etalon. The input beam is modulated at a rate 
that is slow compared to the relaxation time of the non-linear medium of 
the etalon, approximately linearly modulating the index of refraction of 
the nonlinear medium and thereby producing a relatively high amplified 
modulation on the output beam produced by the etalon. 
To avoid distortion at high signal strength, the maximum input intensity 
should be such that at the maximum intensity of the input beam, the probe 
wavelength is approximately at the shoulder of approximately linear 
portion of the transmissivity peak of the etalon.

DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, optical logic apparatus 1 includes a nonlinear 
Fabry-Perot etalon 16, hereinafter referred to simply as etalon 16. In the 
experiments that lead up to this invention, dye-filled etalons were used, 
although gallium arsenide etalons would be much more practical as logic 
elements because dye-filled etalons have much slower responses. Gallium 
arsenide etalons are known to those described in the art and some of their 
properties are reported in "Optical Modulation by Optical Tuning of a 
Cavity", by H. M. Gibbs et al., Applied Physics Letter, Volume 34, No. 8, 
page 511, Apr. 15, 1979, incorporated herein by reference and in "Optical 
Bistable Devices: The Basic Components of All-Optical Systems?" by H. M. 
Gibbs et al., Optical Engineering, July/August 1980, Volume 19, No. 4, 
page 463, also incorporated herein by reference. 
Fabry-Perot etalons are well known to those skilled in the art. 
Furthermore, those skilled in the art of optical bistability know what a 
nonlinear Fabry-Perot etalon is, wherein a medium that is nonlinear with 
respect to light is positioned between the two mirror surfaces of the 
etalon. 
Referring to FIG. 1, reference numerals 16A and 16C represent spaced mirror 
surfaces which are partially transmissive to light, and a nonlinear medium 
16B disposed between mirror surfaces 16A and 16C. The absorptivity (i.e., 
absorption coefficient) and/or index of refraction of the nonlinear medium 
16B, in conjunction with the reflectivity of the mirror surfaces 16A and 
16C, determine how much of the light incident on a surface of etalon 16 is 
reflected, absorbed, and/or transmitted. 
Further details on properties of dye-filled etalons are reported in 
"Observation of Thermal Optical Bistability, Crosstalk, Regenerative 
Pulsations, and External Switch-Off in a Simple Dye-Filled Etalon" by M. 
C. Rushford et al., which was presented at the Topical Meeting on Optical 
Bistability held in Rochester, N.Y. on June 17 through June 20, 1983. This 
paper will be published in a book entitled Optical Bistability II to be 
published by Plenum Corporation. A copy of the foregoing article and is 
incorporated herein by reference. 
In FIG. 1, a polarizing cube 15 is used to combine two input beams 8 and 9 
produced by an electrical-to-optical converter device 21. Polarizing cube 
15 totally reflects these two beams from inclined surface 15A onto surface 
16C of etalon 16. Reference numerals 8-1 and 9-1 designate the reflected 
portions of beams 8 and 9, respectively. 
A beam splitter, such as a sheet of glass, represented by reference numeral 
5 transmits nearly all of the light of beams 8 and 9 directly to lens 14. 
In the experimental set up represented by FIG. 1, lens 14 is positioned as 
shown merely to focus beams 8 and 9 onto surface 15A of polarizing cube 
15. In that experimental set up, beams 8 and 9 were too far apart for them 
both to impinge on surface 15A without being focused somewhat. 
However, small portions of beams 8 and 9 designated by numerals 8-2 and 
9-2, respectively, are reflected from beam splitter 5 into two 
photo-multiplier tubes 6 and 7. The outputs of photo-multiplier tubes 6 
and 7 are connected to inputs of an oscilloscope (not shown) to allow 
monitoring of the two input beams 8 and 9. 
Polarizing cubes such as 15 have the characteristic that the surface 15A 
will totally transmit an incoming beam which is polarized in one 
direction, for example, horizontally, and totally reflect an incoming beam 
which is polarized in another direction, for example, vertically. Such 
polarizing cubes are readily commercially available from such companies as 
Karl Lambrecht Corporation. 
In FIG. 1, reference numeral 20 designates a probe beam which is polarized 
so as to be transmitted by polarizing cube 15 onto surface 16C of etalon 
16. Lens 21 focuses the probe beam 20 onto the left surface of polarizing 
cube 15. In practice, the two lenses 14 and 21 could be replaced by a 
single lens disposed between the right side of polarizing cube 15 and 
etalon 16. 
In my experiments using dye-filled etalons, the probe beam source indicated 
by block 22 in FIG. 1 was a continuous wave laser. In the preferred 
embodiments of the invention the probe light beam 20 would not be a 
continuous wave beam, but would be pulsed in synchronization with pulses 
of the input light beams 8 and 9 in the manner indicated by the upper 
graph in FIG. 3, wherein reference numeral 124 designates a single pulse 
of either one of input beams 8 and 9, reference numeral 125 indicates an 
immediately designates the simultaneous occurrence of pulses of input 
beams 8 and 9, and reference numeral 127 designates an immediately 
following pulse of probe beam 20. 
Initial probe beam pulse 130, with no input beam pulse present, is shown in 
FIG. 3, analogous to probe beam pulses 125 and 127, to show the 
transmissivity of etalon 16 with no input pulses present. The pulses 124 
and 126 can be thought of as representing energy, so input beam pulse 126 
is twice the height of input beam pulse 124. In practice, the energy or 
intensity of the probe beam pulses 125 and 127 would be much greater than 
that of both of the input pulses 124 and 126. 
For experimental purposes, it was better to use a continuous wave probe 
beam in order to show the entire time profile of etalon transmissivity of 
the probe beam. 
For a gallium arsenide etalon, the wavelength of probe beam 20 would 
probably be in the vicinity of 850 nanometers. Typically, the absorption 
edge of a gallium arsenide etalon might vary from 820 to 860 nanometers. 
Typically, the wavelength of the probe beam might be 40 or 50 nanometers 
greater than the wavelength of the absorption edge of the nonlinear 
medium. Suitable lasers for use in block are widely available, from 
companies such as Spectra-Physics. Photo-multiplier tubes such as 6, 7 and 
19 in FIG. 1 also are widely available. 
Referring again to FIG. 1, interference filter 17 is used to make sure that 
only the probe beam 20 reaches the photo-multiplier tube 19. All other 
frequencies are reflected by interference filter 17, including the 
frequency of the input beams 8 and 9. Thus, photo-multiplier tube 19 only 
sees the transmitted (etalon 16) probe beam, designated by reference 
numeral 20-1. The portions of the input beams 8 and 9 ultimately 
transmitted by etalon 16 are designated by reference numerals 8-3 and 9-3 
in FIG. 1, and are stopped by interference filter 17. 
In the experimental set-up using dye etalons, the waveforms of FIG. 4 were 
traced from displays on an oscilloscope driven by the output of 
photo-multiplier tube 19. These waveforms will be described subsequently. 
In FIG. 1, reference numeral 3 represents a flat rotating plate, or disc, 
driven about an axis 2 by an electrically controlled motor (not shown) in 
the direction of arrow 4. Small holes are provided in rotating disc 3 at 
points corresponding to points 3A and 3B to cause pulses of beams 8 and 9, 
respectively to be formed. The two beams 8 and 9 originate from block 21, 
which contains an argon laser and a beam splitter which aims the two 
"unpulsed" versions of beams 8 and 9 at the underside of rotating disc 3. 
The width of the holes 3A and 3B and the angular velocity of rotating disc 
determine the widths of the pulses of light of beams 8 and 9, 
respectively, that are transmitted to etalon 16. Note that reference 
numerals 8 and 9 are also used to designate these two beams after they 
have been pulsed by the action of rotating disc 3. 
The wavelength of the laser in block 21 was selected because it was known 
to cause a shift in the transmissivity or index of refraction of the 
dye-filled etalon being used in the experiment. 
The operation of optical logic apparatus 1 will be explained with reference 
to FIG. 2. FIG. 2 is a graph in which the vertical axis 51 represents the 
normalized transmissivity T of etalon 16, i.e., the intensity of a light 
beam transmitted by etalon 16 divided by the intensity of the incident 
portion of that beam. The horizontal axis 52 represents 
.delta..lambda./.DELTA..lambda.g the "detuning" of the wavelength of the 
probe beam 20, normalized with respect to the full width, half maximum 
(FWHM) of the transmissivity characteristic of etalon 16. 
The three transmissivity peaks 53, 54 and 55 in FIG. 2 represent the 
transmissivity of etalon 16 to probe beam 20 when zero, one or two of 
input beams 8 and 9 are incident on etalon 16. More specifically, 
transmissivity peak 53 is the transmissivity characteristic of etalon 16 
when there are no pulses of either input beam 8 or input beam 9 incident 
on etalon 16. Transmissivity peak 54 is the transmissivity characteristic 
of etalon 16 when a pulse from either one, but not both, of input beams 8 
or 9 is incident on etalon 16. Transmissivity peak 55 is the 
transmissivity characteristic of etalon 16 when pulses of both of input 
beams 8 and 9 are incident on etalon 16. In FIG. 2, transmissivity peak 54 
has the same shape as peak 53 and is one full width half maximum (FWHM) 
distance to the right of transmissivity peak 53. In other words, the 
incidence of one input pulse of either input beam 8 or input beam 9 on 
etalon 16 causes the transmissivity characteristic of etalon 16 to shift 
to the right by one FWHM. Similarly, simultaneous incidence of pulses of 
both input beam 8 and input beam 9 on etalon 16 shifts the transmissivity 
characteristic two FWHM widths to the right, to produce transmissivity 
curve 55. 
The amount of shift caused by the incidence of an input pulse is dependent 
upon the energy of that pulse, assuming the pulse has a wavelength that is 
absorbed by the nonlinear medium of etalon 16. The one FWHM width shift is 
chosen as a matter of convenience, and is not critical to operation of 
etalon 16 as an optical logic gate, as will become apparent. 
In order to understand how the transmissivity curves in FIG. 2 lead to 
operation of etalon 16 in the apparatus of FIG. 1 as a logic gate, it is 
helpful to understand how the truth table of Table 1 below is obtained. 
TABLE 1 
__________________________________________________________________________ 
COLUMN NO. 
1 
Detuning 
of Probe 
Pulse 
2 3 4 5 6 
(FWHM 
No Input 
One Input 
Two Input 
Transmitted 
Reflected 
Row No. 
Widths) 
Pulses Pulse Pulses Beam Beam 
__________________________________________________________________________ 
1 0 (A) 
1.fwdarw."1" 
(B) 
.2.fwdarw."0" 
(C) 
0.fwdarw."0" 
NOR OR 
2 .5 (D) 
.5.fwdarw."1" 
(E) 
.5.fwdarw."1" 
(F) 
.1.fwdarw."0" 
NAND AND 
3 1 (G) 
.2.fwdarw."0" 
(H) 
1.fwdarw."1" 
(I) 
.2.fwdarw."0" 
XOR 
##STR1## 
4 1.5 (J) 
.1.fwdarw."0" 
(K) 
.5.fwdarw."1" 
(L) 
.5.fwdarw."1" 
OR NOR 
5 2 (M) 
0.fwdarw."0" 
(N) 
.2.fwdarw."0" 
(P) 
1.fwdarw."1" 
AND NAND 
__________________________________________________________________________ 
What Table 1 shows is the normalized transmissivity of etalon 16 for no 
input pulses incident, one input pulse incident, and two input pulses 
incident in Columns 2, 3 and 4, respectively, for five different probe 
beam wavelengths. As will become clear subsequently, the logic function 
performed by etalon 16 upon the data signals represented by the input beam 
pulses is different for different wavelengths of the probe beam 20. Column 
1 in Table 1 indicates the relative wavelengths of the five different 
probe beams. Columns 5 and 6 indicate the logical function performed by 
etalon 16 on the transmitted probe beam and the reflected probe beam, 
respectively. Those skilled in the art will recognize that the logical 
function performed on the transmitted probe beam is the logical complement 
of that performed on the reflected probe beam. 
In Columns 2, 3 and 4 in Table 1, the capital letters in parenthesis 
indicate points which are correspondingly labeled on the graph of FIG. 2 
to make it easy to see how the data in Table 1 was obtained. 
First, with respect to Row 1 in Table 1, assume that probe beam 20 has a 
wavelength equal, for example, to the resonant wavelength of etalon 16. 
Its "detuning" is said to be zero. If no input pulses are incident, no 
shift in the transmissivity of etalon 16 occurs. The intersection of 
vertical dotted line 23 with transmissivity curve 53 occurs at point A, 
indicating that the transmissivity is 1.0. If one input pulse is present, 
the transmissivity characteristic shifts by one FWHM width, as indicated 
by 54. The intersection of vertical dotted line 23 (corresponding to the 
same probe beam wavelength) with curve 54 occurs at point B, as indicated 
by horizontal dotted line 58, whereat the transmissivity is roughly 0.2. 
Next, if input pulses of both input beams 8 and 9 are incident on etalon 
16, then its transmissivity is indicated by curve 55, and the intersection 
of vertical dotted line 23 with curve 55 is approximately 0, as indicated 
by point C. 
Thus, it can be seen that if transmissivities from 0.5 to 1.0 represent 
logical "1"s, and transmissivities of 0.2 or less represent logical "0"s, 
then Row 1 of Table 1 defines the truth table of a logical NOR function 
for the transmitted beam, the intensity of which is proportional to the 
transmissivity T, or defines the truth table of a logical OR for the 
reflected beam, the intensity of which is inversely proportional to 
transmissivity T. In other words, the transmissivity of etalon 16, for a 
probe beam having a "detuning" of zero, is a logical "1" if no input 
pulses are present, and is a logical 37 0" if either one or two input 
pulses is present. This is the definition of a logical NOR gate, as can be 
seen from Table 2, which shows the normal truth table of the logical 
functions referred to in Table 1. 
TABLE 2 
______________________________________ 
BEAM 8 BEAM 9 OR NOR AND NAND XOR 
______________________________________ 
0 0 0 1 0 1 0 
0 1 1 0 0 1 1 
1 0 1 0 0 1 1 
1 1 1 0 1 0 0 
______________________________________ 
In Table 2, beams 8 and 9 are assumed to be at a logical "0", if no pulse 
thereof is incident on etalon 16, and are assumed to be a logical "1" if a 
pulse thereof is incident on etalon 16 (by virtue of one of the holes 3A 
and 3B of rotating disc 3 being aligned with the laser source 21 so that 
the beam is not blocked by disc 3 and continues through lens 14 and is 
reflected by polarizing cube 15 onto the surface of etalon 16. 
A procedure analogous to the foregoing procedure can be used to obtain the 
other values in Table 1 for different input probe beam wavelengths. The 
values in Row 2 are determined by determining the intersection of dotted 
line 26 with curves 53, 54 and 55. The values in Row 3 are obtained by 
determining the intersections of dotted line 24 with transmissivity curves 
53, 54, and 55 etc. Row 2 corresponds to a probe beam pulse detuned to 
have a wavelength which is one-half of a FWHM width longer than is the 
case for Row 1 of Table 1 and it can be seen that in this case, the 
transmissivity of etalon 16 represents a logical "1" except when both 
input pulses are present. As can be seen from Table 2, this is the 
definition of a logical NAND gate. 
One skilled in the art can readily verify that the other logical functions 
shown in Table 1 are correct for other probe means "detuned" by one, one 
and one-half, and two FWHM widths. 
An important characteristic of the device described above with reference to 
FIGS. 1 and 2 is that it is "self-resetting". What this means is that, for 
example, when the logical operation of etalon 16 is such that the output 
beam produced in response to a particular configuration of input beam 
levels is a logical "1", i.e., the output beam has a high intensity, then 
if the logical levels associated with the input beams are modified so that 
the output of the etalon should be a logical "0", then the output beam 
produced by etalon 16 with automatically undergo a transition toward a 
logical "0" state, i.e., toward a relatively low intensity level. Stated 
differently, the transmissivity of etalon 16 automatically returns to its 
original configuration when the input beam pulses are absent. This is in 
contrast to the logic devices disclosed in the above-described Seaton 
article, in which the described element is bistable. 
FIG. 3 shows a plurality of "waveforms" or curves illustrating computer 
simulated results obtained from the arrangement shown in FIG. 1. The 
computer simulated results make no reference to any particular type of 
nonlinear medium of etalon 16. It has been found that it is possible to 
saturate the nonlinear medium material and produce the above-described 
shifts or changes in transmissivity in less than two tenths of a 
picosecond. These changes in transmissivity of nonlinear optical medium 
materials therefore can be accomplished almost instantaneously. 
In FIG. 3, curve 65 indicates the refractive index n of the nonlinear 
medium of etalon 16. Input energies computed to be enough to shift the 
transmissivity peak of the nonlinear medium by one FWHM width were 
obtained. The change in the refractive index is indicated by 65-1 in FIG. 
3. The refractive index n then exponentially recovers to its initial 
value, as indicated by 65-2. Then, at the time indicated by "4" on the 
horizontal scale, it is assumed that there is one input pulse incident to 
the etalon 16. At the time indicated by "8" on the horizontal axis, there 
are two such input pulses incident to the etalon. Reference numeral 126 
designates the presence of two input beam pulses incident on etalon 16, 
and numeral 65-3 designates the effect of two simultaneous input pulses 
incident on etalon 16 on the refractive index of the nonlinear medium. 
Numeral 65-4 designates the expotential recovery to the initial value of 
the refractive index n. The probe beam 20 is synchronized to be incident 
to the etalon 16 immediately after the occurrence of the input beams at 
times 4 and 8. These are the times during which the nonlinear medium 
material is most excited and the maximum change in refractive index or 
transmissivity has occurred, and during which the most effective logical 
function is performed on the input probe beam, because the refractive 
index has had practically no time to relax back to its original value. 
The probe beam pulses typically would have much higher intensity than the 
input beams, since the probe beam pulses which are transmitted by the 
etalon frequently need to drive two or three subsequent etalons as inputs 
thereto. However, the intensity of the probe beam is not relevant to the 
operation of a particular etalon upon which the probe beam is incident. 
A fundamental requirement for any computer is that the output of a logic 
gate be of sufficient strength to drive other logic gates. For an optical 
logic gate held in a particular state by a signal of a particular 
wavelength only, this poses certain problems. The transmissivity of that 
optical logic gate is limited, because there must be absorption of energy 
of the incident beam or beams to "drive the nonlinearity" of the nonlinear 
medium of that optical logic gate. Thus, it appears that a "single 
wavelength" optical logic gate would require that gain be somehow 
provided, or else that optical logic gate would have to undergo 
undesirable energy absorption to have any hope of adequately driving other 
optical logic gates. Optical switches having "gain" have been analyzed 
theoretically, and lasing in ultra-short etalons is not difficult to 
achieve. However, optical logic gates having gain ("active gain" which 
involves stimulated emission, such as that which occurs in a laser, rather 
than "signal gain" in which the increase of signal strength of the output 
signal occurs by any other means relative to the input signal) would be 
more complex and require more power than the optical logic gates described 
herein. Furthermore, "negative logic" is more straightforwardly obtained 
from an optical logic device in which the probe beam does not affect its 
own transmission through the optical logic element. Those skilled in the 
art know that "signal gain" is a requirement for cascadability in logic 
gates. Another requirement for cascadability of logic gates is that the 
output of one logic gate be of an appropriate nature to act as an input to 
another logic gate. 
In accordance with the present invention, the above fundamental problems 
can be overcome in a passive optical logic gate if the probe beam 
character, e.g., wavelength, differs from that of the input beams such 
that the probe beams do not significantly modify the transmissivity of the 
etalon upon which they are incident and the input beams do signficantly 
modify the transmissivity of the etalon upon which they are incident and 
wherein the intensity of the probe beam is much greater than that of any 
of the input beams, regardless of their logic levels. Before tackling the 
problem of signal propagation through many optical logic gates, first 
consider the advantages of a two-wavelength optical logic gate. Efficiency 
of the input beam pulses can be maximized by tuning them for maximum 
absorption by the nonlinear medium of the etalons. Further in accordance 
with the present invention, the probe beam can be tuned for low absorption 
by the nonlinear medium, allowing high transmissivity and finesse thereof. 
The increased finesse means that only a very small change of refractive 
index produces the desired change in the amplitude of the output beam 
produced by the etalon. In the low absorption region of some 
nonlinearities, i.e., saturation of an absortion peak, the linear 
absorption falls off inversely as the square of the detuning of the beam 
from the resonance frequency of the absorption, while the change of the 
refractive index for a given degree of saturation decreases inversely to 
the detuning of the beam frequency from the resonant frequency of the 
absorption. For nearly 100% reflecting mirrors, wherein the saturable 
absorption coefficient is much greater than the background absorption 
coefficient and the product of the saturable absorption coefficient and 
the cavity length is much greater than the scattering coefficient from 
each mirror, the required input energy is approximately inversely 
proportional to the detuning of the beam from the resonant frequency of 
the etalon, and best performance should be obtained when the cavity length 
is less than the sum of the scattering coefficient and the background 
absorption coefficient. Lowering power requirements by increasing finesse 
should not help thermal stability, since the device is correspondingly 
more sensitive to temperature change. The probe beam can have many times 
the energy of an input, in accordance with the present invention, while 
still not significantly affecting the optical logic gate's operation, and 
thus, the transmitted probe beam should be able to drive many other 
similar optical logic gates. This results in a large signal gain which is 
"total gain", rather than the differential gain observed in prior bistable 
optical switches. 
In FIG. 3, curve 66 shows the transmissivity T of etalon 16 to the probe 
beam 20 as a function of time when etalon 16 is configured by the probe 
beam wavelength to perform a logical NOR function on the input beams 8 and 
9. Reference numeral 66-1 shows a sudden decrease in transmission as a 
result of the shift in transmissivity of the etalon 16 caused by a single 
input beam pulse. See curve 54 of FIG. 2. Reference numeral 66-2 shows the 
recovery of the probe beam back to its initial value. Reference numeral 
66-3 shows the drop in transmissivity T due to the incidence of two input 
beam pulses. The exponential recovery of the transmissivity T, indicated 
by 66-4, to a normal value is somewhat longer than the exponential 
recovery indicated by 66-2. In every instance, in which it is necessary to 
determine the cycle time of an etalon logic gate, it is necessary to 
consider the longest such exponential recovery time, i.e., "relaxtion 
time". 
The curves 67, 68, 69 and 70 of FIG. 3 are entirely analogous to curve 66, 
except that differently "detuned" probe beam wavelengths are used, 
resulting in different logic functions being performed on the input beams 
8 and 9 by etalon 16. 
The appearance of some of the curves in FIG. 3 can be somewhat confusing 
because in some instances, such as the NAND curve 67, the response appears 
to be quite similar for either one or two beam inputs. But it must be kept 
in mind that the only time that the nonlinear medium response is 
meaningful is during the probe beam times indicated by 125 and 127. Dotted 
lines 127 and 128 help align these times with the various curves shown in 
FIG. 3 For example, in the NAND curve 67, for one input beam incident, the 
transmission of the probe beam will be quite high, as indicated by the 
point A, while for two input beams incident, the transmission will be very 
low, as indicated by B. Similarly, in the exclusive OR curve 68, if either 
input, but not both, is present, the high transmissivity indicated by 
point 68-1 will occur, whereas if both inputs are present, the low 
transmissivity indicated by 68-2 will be present. However, if the probe 
beam time is delayed too much, the contrast between the two levels 68-1 
and 68-2 will be rapidly degraded. 
Most of the experimental curves, shown in FIG. 4, are obtained from a 
dye-filled etalon, correspond fairly well to the results obtained from the 
computer simulation for FIG. 3. In some of the experimental curves in FIG. 
4, the order of "two-input case" and the "one-input case" has been 
reversed from the order shown in FIG. 3. Thus, the edge 75-1 of the NOR 
curve 75 in FIG. 4 corresponds to the edge 66-3 of curve 66 in FIG. 3, and 
edge 75-2 of curve 75 of FIG. 4 corresponds to edge 66-1 of curve 66 of 
FIG. 3. In FIG. 4, the darkened blocks directly beneath each horizontal 
axis have a digit "1" or "2" directly beneath them indicating whether one 
or two of the input beams 8 and 9 were incident on etalon 16. 
The experimental NOR curve 75 showed excellent agreement with the computer 
simulated curve 66. The experimental NAND curve 76 showed fairly good 
correspondence to the transmissivity results indicated in 
computer-simulated curve 67 of FIG. 3 at "probe beam times" immediately 
after the occurence of the input beams. However, the recovery from the 
incidence of one input beam for curve 76 did not correspond closely to the 
recovery indicated in curve 67 of FIG. 3. 
The experimental exclusive OR curve 77 in FIG. 4 shows some characteristics 
that I don't understand during the presence of the two incident pulses, 
but immediately afterwards, during the probe beam, the transmissivity of 
the experimental exclusive OR curve 77 (FIG. 4) are close to those of the 
corresponding simulated exclusive OR curve 68 of FIG. 3. 
The experimental OR curve 78 of FIG. 4, however, does not correspond at all 
to the computed OR curve 69 of FIG. 3. At the present time, I have no 
explanation for these unexpected results. The experimental AND curve 79 in 
FIG. 4 agrees very closely to the computed AND curve shown in FIG. 3. 
In reviewing Tables 1 and 2, it may be helpful to note that each entry in 
Column 3 of Table 1 actually corresponds to two of the four combinations 
(namely 0,1 and 1,0) of Table 2, since the entry in Column 3 corresponds 
to the transmissivity of etalon 16 produced in response to the presence of 
a pulse on either one of, but not both of input beams 8 and 9. 
Hence, it can be seen that in accordance with the present invention, etalon 
16 performs various common logic operations on the input beams. Although 
only two input beams have been shown, the results above can be extended to 
any number of input beams, as long as the incidence of each additional one 
of them produces a satisfactory corresponding shift in the transmissivity 
of the etalon. 
It can be seen that the amount of shift caused by the presence of each 
additional input beam pulse incident on etalon 16 establishes "threshold" 
points which determine when the transmissivity represents a logical "0" or 
a logical "1" and thereby determine the logical function performed by 
etalon 16 upon the signals of the input beams 8 and 9. By way of example, 
for a NOR gate, it is desirable for one input beam pulse to shift the 
transmissivity characteristic to the right more than one FWHM width in 
order to obtain best "contrast" i.e., as long as the ratio of the 
transmissivity representing a logical "1" divided by the transmissivity 
representing a logical "0" is sufficiently high to avoid confusion. 
For example, in the case of NOR gate, shifting transmissivity curve 54 more 
than one FWHM to the right in response to one input beam pulse would 
result in a transmissivity of less than 0.2 as the highest possible 
logical "0" which would be perhaps desirable. 
Actually, transitions between a logical "1" and a logical "1" as high as 
the midway points corresponding to line 57 in FIG. 2 are not particularly 
desirable, although good logic circuit operation did result in the 
experiments. 
Incidentally, those skilled in the art will recognize that the logic 
operation is performed by a modification of the etalon's transmissivity of 
reflectivity. This modification is accomplished by the input beams causing 
a change in the nonlinear medium's absorptivity and/or index of 
refraction. 
Etalon 16 can also function as an optical logic element in a different 
manner than explained above. Before explaining this, it is noted that the 
mechanism of modifying the transmissivity of etalon 16 to the probe beam 
20 (FIG. 1) as described above involves "lateral shifting" of the 
transmissivity characteristic of the etalon in response to energy received 
from the various input beams. This lateral shifting is caused by 
modification of the index of refraction of the nonlinear medium of the 
etalon. Note that the "amplitude" of the transmissivity characteristics in 
FIG. 2 does not change, however. 
In contrast, in FIG. 2A, the mechanism for modifying the transmissivity of 
etalon 16 to probe beam 20 is different because the amplitude of the 
transmissivity characteristic 53 in FIG. 2A does vary in such a way as to 
make it possible to perform common logical functions on a plurality of 
input beams incident on the etalon. This change in the amplitude of the 
transmissivity characteristic 53 in FIG. 2A is caused by modification of 
the coefficient of absorption of the nonlinear medium by the incident 
input beams. In this case, there is no change in the index of refraction 
of etalon 16 in response to incidence of an input beam. Therefore, the 
transmissivity peak does not shift laterally. In FIG. 2A, the dotted line 
29 represents the transmissivity of etalon 16 with no input beams incident 
thereon. Typically, applying one input beam to etalon 16 will not 
significantly increase the amplitude of the dotted line curve 29. However, 
if enough input beams are made incident to etalon 16, the transmissivity 
of etalon 16 at the probe beam wavelength will increase to point 53A at 
the probe beam wavelength designated by reference numeral 31. For this 
condition of many input beams incident on the etalon, the probe will be 
transmitted by the etalon. This condition complies with the definition of 
a logical AND gate. I.e., this operation occurs only if there are enough 
input beams incident with sufficient energy to saturate the absorption 
property of the nonlinear medium of the etalon. However, if the input 
beams have sufficient energy or intensity that any one of them alone is 
enough to saturate the absorption property of the etalon, then the 
operation on the probe beam will be that of a logical OR gate. 
The foregoing modes of operation of the etalon result in noninverting logic 
functions, in transmission of the probe beam and inverting logic functions 
in reflection of the probe beam whereas the technique of shifting the 
transmissivity characteristics of an etalon in response to presence of 
input beams incident on the etalon to produce a transition from a logical 
"1" to a logical "0" can produce inverting logic functions, such as the 
NOR, and NAND functions in transmission of the probe beam. 
Generally, this is an advantage, because generally, inverting and logic 
functions are more desirable to logical system designers. Although 
inverting logic can be obtained by detecting the reflected, rather than 
the transmitted component of the probe beam, generally, the "contrast" of 
the reflected beam is lower. This can present a serious problem in 
reliably detecting optical beams that represent the results of logical 
operations on signals represented by input beams, especially when the 
reflected probe beams are then utilized as inputs to a successive etalon 
also being used as a logic element. The combination of two or more such 
low contrast reflected probe beams is likely to result in erroneous or low 
reliability logical operation by a subsequent etalon that functions as a 
logic element. 
Referring to FIG. 1A, a logic "system" is disclosed using a number of 
arrays of the nonlinear Fabry-Perot etalons functioning as logic elements 
in the manner described above. Ordinarily, nonlinear media such as gallium 
arsenide having extremely fast relaxation times would be utilized in a 
practical optical logic system. In FIG. 1A, optical logic system 82 
includes a probe beam 83 which is focused by a lens 86 onto a particular 
etalon 87-1 of an array 87 of identical etalons. The input for the system 
can be provided by means of a spatial light modulator 113, which can be a 
liquid crystal array, a CCD (charge coupled device) element. A light beam 
114 illuminates the spatial light modulator 113. Modulation in the form of 
electrical signals applied to terminals 115 is accomplished. Another input 
to optical logic system 82 is provided in the form of feedback of light 
beams 102 from an output circuit, subsequently described, fed back by 
means of mirrors 109, 110, and beam splitter 85 to provide the composite 
input beams indicated by reference numeral 111 which are focused by lens 
86 onto a particular etalon element, such as 87-1 in array 87. The 
Throughput of or transmission of beams 111 by etalon 87-1 is collected by 
lens 89 and is collimated so that it goes through the beam splitter 90 and 
then is focused by lens 98 onto logic element (etalon) 99-2 of array 99. 
Reference numeral 97 indicates the subject beam after it passes through 
lens 98. 
In a system of two-wavelength optical logic gates, the optical logic gates 
cannot be identical, of course because the output from the first optical 
logic gate could not function as an input for another identical optical 
logic gate. What is possible, in accordance with the present invention, is 
a system using two similar nonlinear media or materials in two different 
successive optical logic gates, where the probe beam wavelength for the 
first optical logic gate is on the absorption peak for the nonlinear media 
of the second optical logic gate to which the output of the first optical 
logic gate is applied as an input. The nonlinear medium of the first 
optical logic gate should also have high absorption at the wavelength of 
the probe beam being supplied to the second optical logic gate. Thus, the 
wavelengths of input beams and probe beams are reversed for alternate 
cascaded optical logic gates, and propagation through an even number of 
such optical logic gates produces an output beam having the same 
wavelength as the input beams applied to the odd numbered optical logic 
gates in the group. Furthermore, operation in a closed loop manner (as 
seen in FIG. 1A) also is possible using any two optical logic gates (or 
two arrays of optical logic gates) and two wavelengths. An isolated 
saturable line absorption feature which is shifted in one material 
relative to the other should provide the desired characteristics. In 
semiconductors such as GaAs or CdS, the absorption might be an exiton 
feature well resolved from the band edge. In gallium arsenide, the peak 
can be shifted by adding a small amount of aluminum bulk material or by 
varying the well thickness in a multiple quantum well structure. 
The output of array 87 thereby becomes an input to array 99. 
Two probe beams designated by reference numerals 93 and 94, respectively, 
are produced in response to a device which can be a spatial light 
modulator similar to 113. The two probe beams 93 and 94 are reflected from 
beam splitter 91 through lens 98 onto etalons 99-1 and 99-2 of array 99. 
By placing a prism face such as 120 on cube 90, the beam emanating from 
section 113-1 of spacial light modulator 113 can be focused as an input 
not only to etalon 87-1 of array 87, as indicated by arrows 118, but also 
on other adjacent etalons of array 87, although the resulting beams for 
convenience are not shown. Probe beams 93 and 94 emanate from elements 
92-2 and 92-1 of array 92. These two probe beams, along with the outputs 
of array 87, (which will become inputs to array 99) are focused onto 
etalons 99-1 and 99-2. The transmitted beams are collected by lens 100, 
which are followed by a beam splitter 101. Beam splitter 101 takes part of 
the output signal from array 99 to produce feedback beam 102 which, as 
explained above, may be fed back and utilized as an input to one or more 
etalons elsewhere in a typical system. Some of the output beam signals 
from array 99 also are directly transmitted by beam splitter 101 to 
another lens 125 which focuses the output beams or maps them onto 
appropriate light detecting elements of a detector array 105, which in 
turn, produces electrical output signals on terminals, such as 108, 
thereby producing thereon electrical signals representing the logical 
output of optical logic system 82 in electrical form. More specifically, 
the output beams produced by the etalons 99-1 and 99-2 of array 99 are 
mapped onto elements 106 and 107, respectively, of detector array 105, 
which might, for example, be a CCD device. 
The alignment of the various arrays, lens, beam splitters, lenses and 
mirrors can be easily accomplished at the present state of the art and 
presents no problem to one skilled in the art. 
No difficult technological problems are presented by the basic system 
arrangement shown in FIG. 1A. 
The diagram in FIG. 6 illustrates the fact that there can be many cavities 
on a single device, rather than a single cavity as shown in the previously 
mentioned Fabry-Perot etalons. There can be n+1 reflecting surfaces such 
as 121. These bound n regions such as 122-1, 122-3, etc., may or may not 
contain a nonlinear medium. This kind of device can have similar 
transmissivity characteristics to a Fabry-Perot etalon in that one can 
make the medium material layers having optical thicknesses that are 
integral multiples of quarter wavelengths of the probe beam. As an example 
in which four reflecting surfaces are used, the coating characteristics 
can be optimized by providing a linear medium between the first and second 
reflecting surfaces and the third and fourth reflecting surfaces and 
providing a nonlinear medium between the second and third reflecting 
surfaces. This type of device would in essence be a nonlinear Fabry-Perot 
etalon. With proper design, such a device could be made to be highly 
transmitting to the input beams at the input beam wavelengths so that 
maximum efficiency of the input pulses would be obtained and yet the 
resulting etalon would be highly reflecting at the probe beam wavelength. 
This type of device would have the finesse that would be desired. As those 
skilled in the art know, finesse is defined as free spectral range divided 
by the FWHM width of the transmission peak in frequency where the free 
spectral range is the frequency difference between two adjacent 
transmission peaks, or the speed of light divided by twice the optical 
thickness of the etalon. 
If all of the regions between the reflecting surfaces of the device shown 
in FIG. 6 are filled with a nonlinear medium, my computer calculations 
have shown that the operation is similar to that of a nonlinear 
Fabry-Perot etalon. 
A special case of the NOR gate logic function of etalon 16 occurs if there 
is only one input beam utilized, in which case etalon 16 functions as an 
inverter. 
When there are more than two input beams present, other functions are 
possible. For example, suppose there are a number of inputs greater than 
two, each of which has an energy or intensity level which causes it to 
shift the transmissivity characteristic of FIG. 2 to the right by one FWHM 
width. Then, if the probe beam is detuned by N FWHM widths, (N being an 
integer) then etalon 16 functions as a number selector which will produce 
a high beam output level if there are exactly N input pulses impinging on 
etalon 16. However, if there is any other number of input beams impinging 
on etalon 16, it will produce a low beam output level. 
If the input beam pulses produce shifts of the transmissivity peak of the 
nonlinear medium by less than a FWHM width, then etalon 16 can function as 
a "range selector" wherein it produces a high output beam level when the 
number of input beams is in a certain range and the width of that range is 
approximately 1 divided by the number of full FWHM widths that each input 
beam shifts the peak. 
If a continuous wave probe beam of relatively high intensity (i.e., much 
higher than the input beam intensity) is directed to etalon 16, and has a 
wavelength equal to the wavelength at point A of FIG. 5 near the base of 
the approximately linear portion of the transmissivity peak of etalon 16, 
as shown, and if there is only one input beam, such as input beam 8 in 
FIG. 1, which is modulated at a rate that is slow compared to the 
relaxation time of the nonlinear medium of etalon 16, then the 
transmissivity curve shown in FIG. 5 will shift in accordance with the 
slow modulation of that input beam. This will cause the transmissivity of 
the etalon to vary along the approximately linear portion of the 
transmissivity curve shown in FIG. 5. To avoid distortion, the maximum 
modulation amplitude of the input beams should be such that point B of the 
transmissivity curve of FIG. 5 is at the wavelength of the probe beam, so 
as to avoid the highly nonlinear peak portion of the transmissivity curve. 
While the above-described shifting of the transmissivity curve of FIG. 5 in 
response to the slow modulation of the single input beam can be 
accomplished with only that input beam to provide a high degree of 
amplification of the modulation signal, an entirely similar shifting of 
the transmissivity peak of FIG. 5 can be obtained in response to many 
lower amplitude input beams instead of only on. They will achieve a form 
of digital-to-analog conversion, if all of the input beams have the same 
intensity. However, if the multiple input beams have different 
intensities, the etalon will function as a signal summing device. 
If both input beams and the probe beam are pulses of durations that are 
short compared to the relaxation time of the nonlinear medium, it is not 
necessary that the input beams and the probe beam be simultaneously 
incident on the etalon. If the nonlinear medium is such that absorption of 
one input pulse changes the refractive index of the nonlinear medium at 
the probe wavelength enough to shift the Fabry-Perot transmission peak by 
about 1 FWHM, the peak will, of course, return to its original wavelength 
within a few relaxation time constants. Thus, if the probe beam pulses is 
incident on the etalon within a time short compared to the relaxation time 
constant after the input beams have terminated, only this instantaneous 
transmission (of the probe beam through the etalon) determines the output. 
While the invention has been described with reference to several particular 
embodiments thereof, those skilled in the art will be able to make various 
modifications thereto without departing from the true spirit and scope of 
the invention. It is intended that methods and apparatus which perform 
substantially the same function in substantially the same way to achieve 
substantially the same result be encompassed by the invention. 
For example, the techniques described with reference to FIGS. 2 and 2A can 
be combined in a single device in which the transmissivity curve is both 
laterally shifted and its amplitude is simultaneously modified in response 
to the input beams. The multiple cavity etalon devices shown in FIG. 6 are 
intended to be encompassed by the word "etalon", in addition to the single 
cavity devices such as etalon 16 shown in FIG. 1.