Reconfigurable 0ptical interconnect using dynamic hologram

A beam of coherent light is coupled into an array of light detectors using a beam splitter for dividing the beam into a probe beam and a pump beam and a spatial light modulator for varying the intensity of the probe beam in a first direction perpendicular to the direction of propagation of the beam. A photorefractive element is positioned to receive the modulated probe beam and the pump beam, the beans being oriented with respect to the photorefractive element and with respect to each other such that photorefractive two-wave mixing within the photorefractive element nonreciprocally transfers energy from the pump beam to the modulated probe beam, which is then detected by the detector array. A first lens system may be positioned in the path of each probe beam between the beam splitter and the spatial light modulator for expanding each probe beam in the first direction, while a second lens system may be positioned in the path of each pump beam between the beam splitter and the photorefractive element for expanding each pump beam in the first direction.

BACKGROUND OF THE INVENTION 
This invention is concerned with techniques for processing information 
which is transmitted optically, such as in an optical communications 
system or in an optical computer. 
The inherent parallelism of optics (i.e., a beam of light can carry 
different information on different portions of the light beam without 
interference) and the wide bandwidth which optical systems offer for 
communication are ideal for real-time image processing, optical 
interconnection schemes, and associative processing. As a result, optics 
is emerging as an area of increasing importance in high-speed information 
processing. Reconfigurable optical interconnections, for example, play a 
key role in optical computing systems, where such interconnections are 
used to link arrays of lasers with arrays of detectors. Conceptually, such 
an interconnection can be achieved by using an optical matrix-vector 
multiplication, where the input vector represents the signals carried by 
an array of lasers, the matrix represents the interconnection pattern 
which is to be implemented, and the output vector represents the signals 
which are sensed by an array of optical detectors. 
When a transparency or a spatial light modulator is used as the 
interconnection mask in an optical interconnection, an excessively large 
fraction of the light entering the device is absorbed by the transparency 
or modulator. If the interconnect is used as a crossbar switch, for 
example, it exhibits an energy efficiency of only 1/N, where N is the 
dimension of the array. This excessive energy loss occurs in the device 
because a fractional portion (N-1)/N of the light energy from each element 
of the input vector cannot pass through the crossbar mask. Furthermore, 
this loss, which is sometimes referred to as the fanout energy loss, 
increases as the dimension N of the array increases. For a 1000.times.1000 
crossbar switch, for example, as much as 99.9% of the input signal can be 
lost due to fanout. A loss of this magnitude is not acceptable in high 
speed computing applications, where signals pass through the spatial light 
modulator more than a billion times per second. This high processing speed 
would contribute to an enormous fanout energy loss in such a conventional 
optical interconnection system. In addition to the inherent fanout energy 
loss, all spatial light modulators have a finite insertion loss due to 
imperfect transmission properties and the scattering of light. If such 
insertion losses are also accounted for, the energy efficiency of a 
crossbar switch is reduced to t/N, where t is the transmittance (t&lt;1) of 
each of the optical channels through which information is transmitted in 
the switch. 
This efficiency problem has been addressed in the prior art. It is known, 
for example, to employ a holographic optical element in order to achieve a 
free space optical interconnection. In this scheme, light from each laser 
source within the input array is Bragg scattered and redirected to one or 
more detectors in the output array. Several specific requirements, 
however, such as alignment, diffraction efficiency, etc., must be met for 
a holographic optical element to be used for the interconnection of VLSI 
(Very Large Scale Integration) circuits. In addition, a new hologram must 
be provided for each new interconnection pattern. Consequently, a need 
exists for a new optical interconnection scheme which can be easily 
reconfigured while achieving a high level of efficiency. 
SUMMARY OF THE INVENTION 
This invention reconfigurable interconnect utilizes the nonreciprocal 
energy transfer of the two-wave mixing process to achieve a heretofore 
unachieveable efficiency in a reconfigurable optical interconnect. A beam 
of coherent light is coupled into an array of light detectors using a beam 
splitter for dividing the beam into a probe beam and a pump beam and a 
spatial light modulator for varying the intensity of the probe beam in a 
first direction perpendicular to the direction of propagation of the beam. 
A photorefractive element is positioned to receive the modulated probe 
beam and the pump beam, the beams being oriented with respect to the 
photorefractive element and with respect to each other such that 
photorefractive two-wave mixing within the photorefractive element 
nonreciprocally transfers energy from the pump beam to the modulated probe 
beam, which is then detected by the detector array. 
In a more particular embodiment, the photorefractive element further 
comprises a photorefractive crystal, such as, for example, AgGaS.sub.2, 
AgGaSe.sub.2, .beta.-BaB.sub.2 O.sub.4, BaTiO.sub.3, Bi.sub.12 SiO.sub.20, 
BGO, GaAs, KTN, KTaO.sub.3, LiNbO.sub.3, LiTaO.sub.3, or SrBaNb.sub.2 
O.sub.6. 
Moreover, a first lens system may be positioned in the path of each probe 
beam between the beam splitter and the spatial light modulator for 
expanding each probe beam in the first direction, while a second lens 
system may be positioned in the path of each pump beam between the beam 
splitter and the photorefractive element for expanding each pump beam in 
the first direction. 
In an embodiment designed to couple an array of coplanar, parallel beams of 
coherent light into an array of light detectors, a beam splitter divides 
the array into an array of probe beams and an array of pump beams. A two 
dimensional spatial light modulator varies the intensity of the probe beam 
array in a first direction perpendicular to the direction of propagation 
of the probe beam array and in a second direction perpendicular to the 
first direction and perpendicular to the direction of propagation of the 
probe beam array. The photorefractive element is positioned to receive the 
modulated probe beams and the pump beams, the beams being oriented with 
respect to the photorefractive element and with respect to each other such 
that photorefractive two-wave mixing within the photorefractive element 
nonreciprocally transfers energy from each pump beam to the corresponding 
probe beam. 
In addition to enhancements similar to those mentioned above, this 
embodiment may further include a third lens system positioned in the path 
of each probe beam between the photorefractive element and the detector 
array to compress the modulated and amplified probe beam in the second 
direction. 
A method of reconfigurably coupling at least one beam of coherent light 
into an array of light detectors, involves the steps of dividing each 
beams into a probe beam and a pump beam and varying the intensity of each 
probe beam in a first direction perpendicular to the direction of 
propagation of the probe beam. Each probe beam and each pump beam is then 
directed into a photorefractive element, with the beams oriented with 
respect to the photorefractive element and with respect to each other such 
that photorefractive two-wave mixing within the photorefractive element 
nonreciprocally transfers energy from each pump beam to the corresponding 
modulated probe beam. Finally, each modulated probe beam is directed onto 
the detector array.

DESCRIPTION OF THE INVENTION 
It is an outstanding feature of this invention to employ a dynamic 
holographic medium, such as a photorefractive crystal, as a reconfigurable 
optical interconnection device. This reconfigurable interconnect utilizes 
the nonreciprocal energy transfer which occurs in the two-wave mixing 
process to achieve an extremely high diffraction efficiency. FIG. 1 is a 
schematic diagram illustrating a reconfigurable optical interconnect 
constructed according to this invention. FIG. 1 depicts an interconnection 
in only one dimension in order to more clearly present the operative 
concept of this invention. A probe beam 102 containing a small fraction of 
the intensity of an input laser beam 110 is coupled out of the beam by a 
beam splitter 120, while the remainder of the beam propagates through the 
beam splitter to serve as the pump beam 122. The pump beam 12 is expanded 
by a first lens system, which includes the cylindrical lenses 130 and 132, 
and is then directed into a photorefractive crystal 134, which has its c 
axis directed as indicated by the arrow 136. 
The beam 102 is directed through a second lens system, which includes the 
cylindrical lenses 138 and 140, by a mirror 142. The lenses 138 and 140 
are provided to expand the probe beam so that separate portions of the 
beam can be directed through a pattern imposed by a spatial light 
modulator 144. In the particular configuration illustrated in FIG. 1, the 
beam 110 is to be routed to two optical detectors 148 and 152, which are 
part of an array of detectors 146, 148, 150, and 152. The connection to 
the detectors 148 and 152 is accomplished by means of a pattern of 
transparent and opaque regions which is established in the modulator 144, 
which also masks the remaining portions of the beam so that the detectors 
146 and 150 do not recieve a signal. 
After propagating through the modulator, the modulated probe beam is 
recombined with the expanded pump beam 122 inside the photorefractive 
crystal 134. The probe and pump beams are oriented with respect to the 
crystal 134 and with respect to each other such that photorefractive 
two-wave mixing within the crystal nonreciprocally transfers energy from 
the pump beam to the modulated beam. As a result, almost all of the energy 
in the pump beam is transferred to the probe beam, which is carrying the 
desired interconnection pattern. The modulated and amplified pump beam is 
then detected by the detectors 146-152. Those skilled in the art will 
appreciate that the angle between the probe beam 110 and the pump beam 122 
should be arranged to optimize the two-wave mixing process within the 
particular crystal which is utilized. In addition, each portion of the 
probe beam must intersect the corresponding portion of the pump beam 
within the crystal 134 to achieve the proper transfer of energy from the 
pump beam to the modulated probe beam. If the crystal to be used is 
smaller than the cross sectional area of the beams, it is also possible to 
focus the pump and probe beams within the crystal to achieve the proper 
conditions for two-wave mixing within a crystal of reduced size. 
The two-wave mixing process utilized in this device may be viewed as a 
real-time holographic phenomenon in which the recording and readout 
functions occur simultaneously inside the photorefractive crystal. In 
conjunction with the crystal, the beam splitter and the spatial light 
modulator cooperate to record a volume hologram which represents the 
interconnection pattern prescribed by the modulator. The energy coupling 
mechanism involved in the two-wave mixing process ensures that the 
diffraction efficiency during the readout of the crystal by the pump beam 
is almost 100%. Photorefractive crystals, such as AgGaS.sub.2, 
AgGaSe.sub.2, .beta.-BaB.sub.2 O.sub.4, BaTiO.sub.3, Bi.sub.12 SiO.sub.20 
BGO, GaAs, KTN, KTaO.sub.3, LiNbO.sub.3, LiTaO.sub.3, and SrBaNb.sub.2 
O.sub.6, are by far the most efficient of the various types of holographic 
media which are available. As those skilled in the art will appreciate, 
achieving such high efficiency levels requires that the crystal be 
properly oriented so that the energy of the readout beam is greatly 
depleted. The high level of energy efficiency occurs because most of the 
energy is carried by the readout beam, which does not pass through the 
modulator but is diffracted into the interconnection pattern by the 
hologram. 
In this manner, only a small fraction of the energy of the original beam 
110 is subjected to the attenuation introduced by the modulator 144. 
Consequently, the energy loss introduced by the spatial light modulator is 
limited to no more than the reflectivity of the beam splitter which is 
used in the device. Moreover, the beam splitter can be designed so that 
this reflectivity is small (5% or less, for example) so that this energy 
loss is minimized. Because of the nonreciprocal energy transfer which is 
accomplished in the photorefractive crystal, the optical transfer of 
information from the original beam to the detectors is thus achieved with 
a high degree of energy efficiency. 
The energy efficiency of such an interconnection pattern can be estimated 
for a crossbar switch as follows. Let R be the reflectance of the beam 
splitter. It may be assumed that the beam splitter is practically lossless 
and that the face of the photorefractive crystal is antireflection-coated, 
so that the Fresnel reflection loss can be neglected. Under these 
conditions, the two beams that arrive at the photorefractive crystal have 
energies 1-R and Rt/N, respectively. Inside the crystal, these two beams 
undergo photorefractive coupling. As a result, most of the energy (1-R) of 
the pump beam is transferred to the probe beam, with energy Rt/N, which 
carries the interconnection pattern. The energy efficiency can be readily 
derived and is given by: 
##EQU1## 
where m is the beam intensity ratio: 
##EQU2## 
and L is the interaction length, .gamma. is the coupling constant, and 
.alpha. is the bulk absorption coefficient. For photorefractive crystals 
such as AgGaS.sub.2, AgGaSe.sub.2, .beta.-BaB.sub.2 O.sub.4, BaTiO.sub.3, 
Bi.sub.12 SiO.sub.20, BGO, GaAs, KTN, KTaO.sub.3, LiNbO.sub.3, 
LiTaO.sub.3, and SrBaNb.sub.2 O.sub.6, the coupling constant is very large 
(i.e., .gamma.L&gt;&gt;1 for L=1 cm). The efficiency can be written 
approximately as: 
##EQU3## 
Note that for large N, the ultimate energy efficiency is exp(-L), obtained 
by using a beam splitter with a very small reflectance R (i.e., 
R.about.0). 
In experimental work to investigate energy efficiency and the capability of 
high data rate transmission, a large beam from an argon ion laser 
operating at 514.2 nm was used. The laser beam was collimated into a beam 
of 2 mm in diameter by using a lens of focal length f=2 m. A beam splitter 
with a reflectance of R=0.05 was used to redirect 5% of the energy through 
the spatial light modulator and to form the probe beam. The remainder of 
the energy transmits through the beam splitter and constitutes the pump 
beam. The spatial light modulator was replaced with a neutral density 
filter with a variable optical density to simulate the fanout energy loss. 
The two beams intersected inside a barium titanate crystal with an 
interaction length of approximately 5 mm. The pump beam entered the 
crystal at near normal incidence with an intensity of 9.1W/cm.sup.2. The 
probe beam, which had an intensity of 0.5 W/cm.sup.2, was incident at an 
angle of 40 degrees. The crystal was oriented such that the probe beam was 
amplified at the expense of the pump beam. After passing through the 
crystal, the pump beam was virtually depleted, whereas the intensity of 
the probe beam was elevated to 1.9 W/cm.sup.2. The same experiment was 
repeated by using neutral density filters with transmittances of 0.1 and 
0.001. The corresponding probe beam densities were 0.05 and 0.005 
W/cm.sup.2, respectively. The intensity of the amplified probe beam 
remained at 1.9 W/cm.sup.2. These measurements reflect an energy 
efficiency of about 20%. In other words, if the fanout loss is 99% (e.g., 
a 100.times.100 crossbar), the energy efficiency of this interconnect can 
be at least 20 times better than the direct approach. It should be noted 
that in these experiments the crystal was not antireflection coated and 
about 18% of the energy was lost at the front surface of the crystal. Bulk 
absorption in the crystal which was used accounted for approximately 60% 
of the energy loss. 
The interconnection pattern in this device can readily be reconfigured by 
simply substituting a different pattern in the spatial light modulator. In 
such an interconnection, the output of each input beam can be delivered to 
any one or all of the detectors. The time for the interconnect to 
reconfigure when a new interconnect pattern is introduced is governed by 
the holographic formation time within the photorefractive crystal, which 
can be very fast, i.e., on the order of 1 msec using light energy of 
approximately 1W/cm.sup.2. Once an interconnection pattern is formed 
inside the crystal as a hologram, this device can accomplish the 
interconnection function with sufficient speed to operate on high data 
rate transmission systems. Even though the reconfiguration time is limited 
by the photorefractive response time and is consequently on the order of 
milliseconds at modest intensities, the photorefractive interconnection 
system can accept very high data rate signals. To demonstrate this 
ability, temporal modulation was impressed on an Argon laser beam with a 
wavelength of 514.5 nm, using an acoustooptic device to stimulate a signal 
to be interconnected with some output. The signal used was a pulse train 
of frequency f.sub.0 =0.833 MHz, with each pulse being approximately 0.2 
microseconds wide. This rate is clearly much higher than the reciprocal of 
the photorefractive response time. The modulated laser beam was then split 
into two beams and mixed in the crystal as described before and the 
amplified probe beam was monitored with a photodetector. The results 
showed a steady-state response in which the temporally modulated pump and 
probe beams interacted simply by diffracting off the stationary index 
grating that was created in the crystal after the photorefractive response 
time. 
FIG. 2 illustrates another embodiment of the optical interconnect of this 
invention which is similar to the embodiment of FIG. 1, but effects a two 
dimensional optical interconnection. In FIG. 2, small fractions 202, 204, 
206, and 208 of the intensities of the laser beams 210, 212, 214, and 216 
in the laser beam array 218 are coupled out of each beam by a beam 
splitter 220, leaving the remaining portions of the beams, the pump beams 
222, 224, 226, and 228, propagating through the beam splitter. The pump 
beams are directed through a first lens system, including the cylindrical 
lenses 230 and 232, in order to expand the pump beams in the vertical 
direction before they propagate into a photorefractive crystal 234, which 
is oriented with its c axis directed as indicated by the arrow 236. The 
probe beams 202-208 are directed through a second lens system, including 
the cylindrical lenses 238 and 240, by a mirror 242. 
The lenses 238 and 240 expand the probe beams vertically, after which the 
probe beams are directed through a two dimensional spatial light modulator 
244. In the 4.times.4 interconnection shown, it is assumed for 
illustrative purposes that it is desired to connect the beams 210-216 to 
an array of detectors 246, 248, 250, and 252. More specifically, in the 
example depicted by FIG. 2 the beam 210 is to be connected to detectors 
246 and 250, beam 212 to detectors 250 and 252, beam 214 to detectors 246 
and 252, and beam 216 to detectors 248 and 250. In terms of a 
matrix-vector multiplication, this interconnection pattern can be written 
as: 
##EQU4## 
where v.sub.1, v.sub.2, v.sub.3, and v.sub.4 are the signals carried by 
the beams 210-216, respectively. The modulator 244 is patterned with the 
proper opaque and transparent regions to achieve the appropriate routing 
of each signal. The modulated probe beams, after traversing the modulator, 
are then recombined with the expanded pump beams 222-228 inside the 
crystal 234. 
The probe and pump beams are oriented with respect to the crystal 234 and 
with respect to each other such that photorefractive two-wave mixing 
within the crystal nonreciprocally transfers energy from each pump beam to 
its corresponding modulated probe beam. As a result, almost all of the 
energy in the pump beams is transferred to the probe beams, which are 
carrying the desired interconnection pattern. The modulated and amplified 
pump beams are then focussed from a two-dimensional array of beams into a 
vector (a one-dimensional array) by a third lens system 254. Finally, the 
beams are detected by the detectors 246-252. An advantage of this device 
is that the beams within the array 218 need not be in phase with one 
another, since each beam interacts within the photorefractive crystal only 
with a portion split off from that same beam. 
The preferred embodiments of this invention have been illustrated and 
described above. Modifications and additional embodiments, however, will 
undoubtedly be apparent to those skilled in the art. Optical phase 
conjugation, for example, could be used in conjunction with the two-wave 
mixing process to correct for any phase aberration that might be caused by 
imperfections in the photorefractive elements, as described in Chiou and 
Yeh, Optics Letters, Volume 11, Page 461 (1986). In addition, the concept 
of this invention can readily be expanded and applied to an interconnect 
with a larger number of input beams and/or a larger number of detectors. 
Furthermore, equivalent elements may be substituted for those illustrated 
and described herein, parts or connections might be reversed or otherwise 
interchanged, and certain features of the invention may be utilized 
independently of other features. Consequently, the exemplary embodiments 
should be considered illustrative, rather than inclusive, while the 
appended claims are more indicative of the full scope of the invention.