Monolithic apparatus comprising optically interconnected quantum well devices

Apparatus comprising a monolithic structure having an array of substructures, e.g., mesas, each including first and second photodetectors electrically connected as components of different self electro-optic effect devices. The devices are optically interconnected due to the positioning of the component photodetectors within a single mesa.

TECHNICAL FIELD 
This invention relates to nonlinear optical devices. 
BACKGROUND AND PROBLEM 
A nonlinear, bistable optical device, referred to as a self electro-optic 
effect device (SEED) and described in U.S. Pat. No. 4,546,244 issued to D. 
A. B. Miller on Oct. 8, 1985, has a semiconductor quantum well region that 
is electrically controlled to change the state of the device via a 
photodetector. Many SEED devices must be interconnected in typical signal 
processing applications such as optical computing or photonic switching. 
Two approaches have been proposed for interconnecting the devices. In one 
approach, the devices are constructed in a manner where optical signals 
travel through the devices in optical waveguides. Waveguides are also used 
to interconnect several devices on a chip in a manner similar to that used 
for titanium-diffused lithium niobate directional couplers. Devices on 
different chips are then interconnected with optical fibers. A 
disadvantage of this approach is that not many devices can be integrated 
on a single substrate because of the amount of room required for the 
waveguide interconnections. 
A second approach is to build the devices in two-dimensional arrays, where 
the optical signals travel normal to the plane of the devices. The devices 
are optically interconnected using bulk optical elements such as lenses 
and beam splitters. However, the bulk element arrangements required to 
perform even relatively simple optical interconnections may be quite 
complex. 
In view of the foregoing, a need exists in the art for a simple optical 
interconnection mechanism for interconnecting a large number of optical 
devices. 
SOLUTION 
The aforementioned need is met and a technical advance is achieved in 
accordance with the principles of the invention in an exemplary monolithic 
structure having an array of substructures, e.g., mesas, each including 
first and second photodetectors electrically connected as components of 
different self electro-optic effect devices. The devices are optically 
interconnected simply due to the positioning of the component 
photodetectors within a single substructure. Because of the monolithic 
construction, the number of interconnectable devices is restricted only by 
the size of the structure and the size and spacing of the individual 
substructures. 
The apparatus of the invention comprises a monolithic structure with a 
first group of photodetector layers including a quantum well region and a 
second group of photodetector layers. Light transmitted through the 
quantum well region of the first group of photodetector layers is incident 
on the second group of photodetector layers. The structure has a number of 
individual substructures formed therein each having a first photodetector 
from the first group of photodetector layers and a second photodetector 
from the second group of photodetector layers. Electrical connections are 
provided to connect ones of the first and second photodetectors of ones of 
the substructures for operation in a predefined circuit comprising 
self-electro-optic effect devices that are optically interconnected within 
individual ones of the substructures. 
In one embodiment, the second photodetector of a first substructure is 
electrically connected in parallel with the first photodetector of a 
second substructure for operation together with a load as a first self 
electro-optic effect device. The second photodetector of the second 
substructure is electrically connected in parallel with the first 
photodetector of a third substructure for operation together with a load 
as a second self electro-optic effect device optically interconnected with 
the first device. 
In the other embodiments, the interconnected devices are symmetric self 
electro-optic effect devices comprising series-connected photodetector 
arrangements. In one such embodiment comprising a linear optical shift 
register, individual second photodetectors of a first pair of 
substructures are electrically connected in parallel with respective first 
photodetectors of a second pair of substructures for operation as a first 
symmetric self electro-optic effect device. Individual second 
photodetectors of the second pair of substructures are electrically 
connected in parallel with respective first photodetector of a third pair 
of substructures for operation as a second symmetric self electro-optic 
effect device optically interconnected with the first device. The second 
group of photodetector layers also includes a quantum well region such 
that light transmitted through the quantum well region of the second group 
of photodetector layers is incident on the first group of photodetector 
layers. The substructures are arranged as a planar array having two 
dimensions, with the individual substructures of a pair aligned in a first 
dimension and the three pairs of substructures aligned in the second 
dimension. The second self electro-optic effect device is optically 
interconnected with the first device for shifting optical data in either 
direction of the second dimension. Other embodiments include a symmetric 
self electro-optic effect device with a fan-out of two operable as a 
1.times.2 optical switch, a two-dimensional shift register capable of 
shifting optical data in either direction of both dimensions, and an 
arrangement of two symmetric self electro-optic effect devices each 
optically interconnected with a third symmetric self electro-optic effect 
device for operation as an optical logic circuit or as a 2.times.1 optical 
switch.

DETAILED DESCRIPTION 
The optical devices used in the exemplary embodiments described herein are 
the above-referenced SEED devices. The first embodiment described involves 
such SEED devices generally. The remaining embodiments concern a specific 
type of SEED, referred to as a symmetric SEED (S-SEED), described in U.S. 
Pat. No. 4,754,132 issued to H. S. Hinton et al. on Jun. 28, 1988. The 
S-SEED is a device that acts as an optically bistable set-reset latch or 
flip-flop. 
In the operation of a SEED there are typically several input beams. The 
input beams comprise signal beams and bias or clock beams. The signal 
beams determine the logic state of the device and the bias or clock beams 
provide the gain needed to cascade devices and to provide the output 
signal beam. In the basic SEED described in the above-referenced U.S. Pat. 
No. 4,546,244, the signal and bias beams are applied concurrently. In the 
S-SEED described in the above-referenced U.S. Pat. No. 4,754,132, the 
signal and clock beams are applied in a time sequential manner. The signal 
and bias or clock beams are typically applied to the same quantum well 
p-i-n diode photodetector. For the SEED devices used in the monolithic 
structure of the present invention, the signal beams are applied to a 
photodetector that is electrically connected in parallel with the quantum 
well photodetector that has the bias or clock beams incident thereon. 
The exemplary monolithic structure of the present invention is fabricated 
as follows. First a wafer is grown using standard epitaxial growth 
techniques, such as molecular beam epitaxy. First and second groups of 
photodetector layers are separated by an insulating layer. The signal 
beams are incident on photodetectors of the first group of layers which 
may or may not include quantum wells. The bias or clock beams are incident 
on photodetectors of the second group of layers which must have quantum 
wells such that the logic states of individual SEED devices can be read 
out. Individual mesas are formed by etching through portions of the 
material and photodetectors are electrically interconnected by making 
ohmic contacts and using gold metalization over an insulating material 
such as polyimide or silicon nitride. 
FIG. 1 is a diagram of an illustrative optical apparatus comprising three 
substructures, e.g., mesas, in a monolithic structure--a first mesa 
comprising photodetectors 402 and 495, a second mesa comprising 
photodetectors 406 and 404, and a third mesa comprising photodetectors 502 
and 408. Photodetectors 402 and 404, connected in parallel via conductors 
425, 426, 418 and 417, together with series-connected load 496 operate as 
a SEED. Similarly, photodetectors 406 and 408, connected in parallel via 
conductors 585, 586, 587, and 588, together with series-connected load 589 
operate as a second SEED. Loads 496 and 589 may be resistors, 
phototransistors, photodiodes, transistors, or any elements that convert a 
current change to a voltage change. (In the S-SEED, the load is a 
photodetector comprising a p-i-n diode with a quantum well region.) 
Electrical power is provided by a DC voltage source 461. The two SEEDs are 
cascaded in the device of FIG. 1 because of the use of two photodetectors 
electrically connected in parallel to operate together with a load as a 
single SEED and because of the positioning of the second photodetector of 
the first SEED and the first photodetector of the second SEED within a 
single mesa. Because of the positioning within the monolithic structure, 
light transmitted by the second photodetector of the first SEED is 
incident on the first photodetector of the second SEED. Signal beam 456, 
which is shown in FIG. 1 as being transmitted from photodetector 495 but 
which is also receivable from an external array of devices, is incident on 
photodetector 402. In response, photodetector 402 generates photocurrent 
which flows through load 496. Similarly, photodetector 404 responds to 
incident bias beam 434 by generating photocurrent that also flows through 
load 496. The voltage developed across load 496 in response to 
photocurrent effectively controls the optical absorption of quantum well 
region 414 of photodetector 404 thereby controlling the fraction of bias 
beam 434 that is transmitted from photodetector 404 through insulating 
layer 493 as the signal beam 436 to photodetector 406 of the second SEED. 
In response, photodetector 406 generates photocurrent which flows through 
load 589. Photodetector 408 responds to incident bias beam 440 by 
generating photocurrent that also flows through load 589. The voltage 
developed across load 589 in response to photocurrent effectively controls 
the optical absorption of quantum well region 416 of photodetector 408. 
The remainder of the description involves the interconnection of S-SEEDs. 
Recall that the basic form of the S-SEED comprises two series-connected 
photodetectors each having quantum wells in the intrinsic region. Optical 
data is represented by two beams rather than one as in the basic form of 
the SEED. If the power in a first beam is greater than the power in a 
second beam, the S-SEED is in one logical state; otherwise, the S-SEED is 
in the other logical state. Since the S-SEED device is operated in time 
sequential fashion, there are two signal beams used to determine the state 
of the device and two clock beams that subsequently read the device state 
and provide optical gain necessary for cascading devices. In typical 
operation, one signal beam and one clock beam are incident on one 
photodetector of the S-SEED and a second signal beam and a second clock 
beam are incident on the other photodetector. However, in order to 
transfer data from one S-SEED to another in a planar structure, each 
photodetector is replaced by two photodetectors, one on which the signal 
beam is incident and a second on which the clock beam is incident. Only 
the second photodetector need have a quantum well region in order to 
obtain a cascaded structure that shifts optical data in one direction. 
FIG. 2 is a schematic diagram of the components of an S-SEED flip-flop 470. 
A first pair of photodetectors 401 and 403 are connected in parallel via 
conductors 423 and 424; a second pair of photodetectors 402 and 404 are 
connected in parallel via conductors 425 and 426. The first and second 
pairs of photodetectors are connected in series via conductor 418. 
Electrical power is provided by DC voltage source 461. Signal beams 455 
and 456 determine the absorption in the quantum well regions 413 and 414 
to set the state of S-SEED flip-flop 470. Substantially equal clock beams 
433 and 434 read out the state of S-SEED flip-flop 470. The output beams 
435 and 436 have intensities dependent on the absorption in the quantum 
well regions 413 and 414, and are shown in FIG. 2 being transmitted to 
subsequent photodetectors 405 and 406. To operate in time-sequential 
fashion, it is necessary that clock beams 433 and 434 are of low intensity 
when the signal beams 455 and 456 are applied, and that the beams 455 and 
456 are of low intensity when the high intensity clock beams 433 and 434 
are subsequently applied to read the state. (The timing required for 
S-SEED operation is described in detail in the above-referenced U.S. Pat. 
No. 4,754,132.) The physical positioning of the components of S-SEED 
flip-flop 470 in a monolithic structure comprising cascaded S-SEEDs is 
shown in FIG. 5. Output beams 435 and 436 pass through insulating layers 
with little or no attenuation and are incident on photodetectors 405 and 
406 that are part of a second S-SEED. 
FIG. 3 is a schematic diagram of a master-slave arrangement comprising a 
master flip-flop 470 cascaded with a slave flip-flop 471. Such 
master-slave arrangement is the basic unit of a linear shift register made 
by cascading several of the basic units. Master flip-flop 470 has been 
described previously with respect to FIG. 2. Slave flip-flop 471 is 
substantially identical and comprises a first pair of photodetectors 405 
and 407 connected in parallel via conductors 427 and 428 and a second pair 
of photodetectors 406 and 408 connected in parallel via conductors 429 and 
430. The first and second pairs of photodetectors are connected in series 
via conductor 421. Beams 435 and 436, transmitted from photodetectors 403 
and 404 respectively, determine the absorption in the quantum well regions 
415 and 416 to set the state of slave flip-flop 471. Substantially equal 
clock beams 439 and 440 read out the state of slave flip-flop 471. The 
output beams 451 and 452 have intensities dependent on the absorption in 
the quantum well regions 415 and 416. The master-slave arrangement is 
bidirectional in that data can be shifted from left to right or from right 
to left. FIG. 3 illustrates the light beams required for right shifting. 
FIG. 4 is a diagram showing the same master-slave arrangement of FIG. 3 
illustrating the light beams required for left shifting. The physical 
arrangement of a monolithic structure implementing the cascaded S-SEEDS of 
FIGS. 3 and 4 is shown in FIG. 5. For clarity of presentation, the 
electrical connections between photodetectors shown in FIGS. 3 and 4 are 
omitted from FIG. 5. Note that in order for the structure of FIG. 5 to be 
capable of either right shifting or left shifting, all of the 
photodetectors must have quantum well regions because the roles of the 
photodetectors are reversed for left shifting. 
There are three sets of input beams incident on the master-slave 
arrangement of FIG. 5. A first set of input beams 455 and 456 is used for 
right shifting operation and emanates from the complementary outputs of a 
previous master-slave arrangement to the left in FIG. 5 having output 
photodetectors 307 and 308 with quantum well regions 315 and 316 
respectively. A second set of input beams 431 and 432 is receivable from 
an external source. A third set of input beams 453 and 454 is used for 
left shifting operation and emanates from the complementary outputs of a 
previous master-slave arrangement to the right in FIG. 5 having output 
photodetectors 501 and 502 with quantum well regions 509 and 510 
respectively. The direction of shifting is determined by using only three 
of a possible five sets of clock beams and only one of a possible three 
sets of input beams. 
To perform right shifting as shown in FIGS. 3 and 5, the first set of clock 
beams 433 and 434 reads out the state of master flip-flop 470 providing 
output beams 435 and 436 which become the input signal beams for slave 
flip-flop 471. The second set of clock beams 439 and 440 reads out the 
state of slave flip-flop 471 to provide output beams 451 and 452 as the 
input signal beams to a subsequent master-slave arrangement. A third set 
of clock beams 437 and 438, referred to herein as monitor beams, is shown 
only in FIG. 5. Beams 437 and 438 are incident on photodetectors 405 and 
406 respectively to provide additional output beams 441 and 442. Output 
beams 441 and 442 are identical to output beams 451 and 452 respectively, 
and are usable to drive external devices. Clock beams 433 and 434 are 
complementary to clock beams 439 and 440; clock beams 433 and 434 are also 
complementary to monitor beams 437 and 438. This assures that signal beams 
and clock beams do not coincide in time. 
To perform left shifting as shown in FIGS. 4 and 5, the fourth set of clock 
beams 443 and 444 and the fifth set of clock beams 445 and 446 are used 
rather than the first and second sets of clock beams. Beams 443 and 444 
are incident on photodetectors 405 and 406 respectively, and beams 445 and 
446 are incident on photodetectors 401 and 402 respectively, to effect 
left shifting. The monitor beams are usable for either direction of 
shifting. For left shifting, clock beams 445 and 446 are complementary to 
clock beams 443 and 444; clock beams 445 and 446 are also complementary to 
monitor beams 437 and 438. 
FIG. 6 is a schematic diagram of an S-SEED 473 similar to that of FIG. 2 
but having two pairs of complementary output beams rather than one. The 
arrangement of FIG. 2 has a fan-out of one; the arrangement of FIG. 6 has 
a fan-out of two. In FIG. 6, an additional photodetector 465 having 
quantum well region 467 is connected in parallel with photodetector 403 
via conductors 469 and 480. An additional photodetector 466 having quantum 
well region 468 is connected in parallel with photodetector 404 via 
conductors 481 and 482. An additional set of clock beams 483 and 484 
provides an additional set of output beams 485 and 486. Output beams 485 
and 486 become the input signal beams incident on a third S-SEED including 
receiving photodetectors 487 and 488. In effect, S-SEED 473 is a 1.times.2 
switch. The output of S-SEED 473 can be directed to one S-SEED including 
photodetectors 405 and 406 and/or a second S-SEED including photodetectors 
487 and 488 by turning on the set of clock beams 433 and 434 and/or the 
set of clock beams 483 and 484. 
FIG. 7 is a schematic diagram of a master-slave arrangement that is the 
basic unit of a two-dimensional shift register that can shift data in any 
one of four planar directions (left, right, up, down). The accompanying 
physical structure is shown in FIG. 9. The layer structure is identical to 
the structure shown in FIGS. 1 and 5; the three layers that are shown in 
FIG. 9 correspond to the bottom photodetector, insulating layer, and upper 
photodetector of each mesa in FIG. 5. The master-slave arrangement has 
nine sets of clock beams and five sets of input beams. Each set of clock 
beams comprises two beams, one for each half of a S-SEED. The clock beams 
comprise right shifting clock beams, right shifting complementary clock 
beams, left shifting clock beams, left shifting complementary clock beams, 
down shifting clock beams, down shifting complementary clock beams, up 
shifting clock beams, and up shifting complementary clock beams. In 
addition there is a set of monitor clock beams that is synchronized with 
the complementary clock beams. The monitor clock beams are used to provide 
additional output light beams corresponding to the state of the 
master-slave arrangement. At most three sets of clock beams are on at a 
given time: the set of clock beams for a particular direction, the set of 
complementary clock beams for that direction, and, optionally, the monitor 
beams. The five sets of input beams comprise one set of input beams to 
provide for external input data, and sets of input beams from master-slave 
arrangements to the left, up, right, and down. There are five sets of 
output beams as well. One set of output beams results from the application 
of the monitor beams and the other four sets provide input beams to 
master-slave arrangements to the right, down, left, and up. 
In the arrangement of FIGS. 7-9, a single S-SEED flip-flop comprises six 
photodetectors. A master flip-flop comprises photodetectors 601, 603, and 
605 connected in parallel via conductors 632, 633, 636 and 637 and 
photodetectors 602, 604, and 606 connected in parallel via conductors 630, 
631, 634, and 635. The two sets of parallel photodetectors are connected 
in series via conductor 628 and connected to DC voltage source 461 via 
conductors 625 and 629. A slave flip-flop comprises photodetectors 607, 
609, and 611 connected in parallel via conductors 643, 644, 647 and 648 
and photodetectors 608, 610, and 612 connected in parallel via conductors 
641, 642, 645 and 646. The two sets of parallel photodetectors are 
connected in series via conductor 640 and connected to DC voltage source 
461 via conductors 626 and 639. In the structure of FIG. 9, up to three 
layers of metalization may be required to provide the electrical 
connections. 
For right shifting operation, the state of the master flip-flop is set 
using one of the sets of input beams incident on photodetectors 601 and 
602. These sets comprise beams 654, 655, 656, 649, 650 and 651. Then the 
data is read out using the right shifting clock beams 659 and 660. The 
output beams 671 and 672 have intensities determined by the absorption in 
quantum well regions 615 and 616. Output beams 671 and 672 each propagate 
through an insulating layer and become the signal input beams incident on 
photodetectors 607 and 608 for the slave flip-flop. Then the right 
shifting complementary clock beams, 683 and 684, read out the state of the 
slave flip-flop, providing output beams 687 and 688. The output beams 687 
and 688 have intensities determined by the absorption in quantum well 
regions 622 and 621 and become the input signal beams to the next 
master-slave flip-flop arrangement located physically to the right, 
including photodetectors 701 and 702 in FIG. 9. At the same time, monitor 
beams 679 and 680, which are synchronized with the complementary clock 
beams, provide output beams 681 and 682 that are substantially identical 
to output beams 687 and 688 and that are determined by the absorption in 
quantum well regions 619 and 620 of photodetectors 607 and 608, 
respectively. 
For down shifting operation, the state of the master flip-flop is set using 
one of the sets of input beams incident on photodetectors 601 and 602. 
These sets comprise beams 654, 655, 656, 649, 650 and 651. Then the data 
is read out using the down shifting clock beams 661 and 662. The output 
beams 673 and 674 have intensities determined by the absorption in quantum 
well regions 617 and 618. Output beams 673 and 674 each propagate through 
an insulating layer and become the signal input beams incident on 
photodetectors 607 and 608 for the downward slave flip-flop. Then the down 
shifting complementary clock beams, 685 and 686, read out the state of the 
downward slave flip-flop, providing output beams 689 and 690. The output 
beams 689 and 690 have intensities determined by the absorption in quantum 
well regions 623 and 624 and become the input signal beams to the next 
master-slave flip-flop arrangement located physically downward, including 
photodetectors 801 and 802 in FIG. 9. 
For left shifting operation, the data is read in via the input signal beams 
652 and 657 incident on photodetectors 609 and 610 respectively. The left 
shifting complementary clock beams 668 and 667, incident on photodetectors 
607 and 608 respectively, read out the state of the first flip-flop and 
provide output beams 675 and 677. The output beams 675 and 677 have 
intensities determined by the absorption in quantum well regions 620 and 
619. The output beams propagate to photodetectors 603 and 604 where they 
become input beams, setting the state of the second flip-flop. Clock beams 
663 and 664 are used to provide output beams 691 and 693 which serve as 
input beams for the next flip-flop to the left. 
Up shifting operation is similar to left shifting operation except clock 
beams 669, 670, 665 and 666 are used and signal beams 653 and 658 are 
incident on photodetectors 611 and 612. The output beams 676 and 678 from 
the first flip-flop propagate to the second flip-flop and are incident on 
photodetectors 606 and 605 respectively. The output beams 692 and 694 
become the input signal beams for the next flip-flop upward. 
For all four directions of shifting, the monitor beams 679 and 680, which 
are synchronized with the complementary clock beams, provide output beams 
that can be used to drive external devices. In addition, there is 
provision for a set of external input beams. 
FIG. 10 is a diagram of an illustrative 2.times.1 optical switch that may 
be constructed as a monolithic device in accordance with the invention. 
Photodetectors 901 and 902, connected in series via conductor 910, 
comprise a first S-SEED having input signal beams 923 and 925 set the 
state and clock beams 911 and 913 passing through quantum well regions 905 
and 907 to provide output beams 912 and 914. Output beams 912 and 914 pass 
through an insulating layer of the monolithic structure (not shown) and 
are incident as input signal beams on a second S-SEED comprising 
photodetectors 801 and 802 connected in series via conductor 818. 
Photodetectors 801 and 802 have quantum well regions 809 and 810. 
Photodetectors 903 and 904, connected in series via conductor 909, 
comprise a third S-SEED having input signal beams 924 and 926 set the 
state and clock beams 915 and 917 passing through quantum well regions 906 
and 908 to provide output beams 916 and 918. Electrical power is provided 
by DC voltage source 461. Output beams 916 and 918 pass through an 
insulating layer (not shown) and are incident as input signal beams on 
photodetectors 801 and 802. If clock beams 911 and 913 are turned off, the 
data on input signal beams 924 and 926 appears at output beams 921 and 922 
after the state of the second S-SEED is read using complementary clock 
beams 919 and 920. Alternatively, if clock beams 915 and 917 are instead 
turned off, the data on input signal beams 923 and 925 appears at output 
beams 921 and 922 after the state of the second S-SEED is read using 
complementary clock beams 919 and 920. 
The arrangement of FIG. 10 is usable as a 2.times.1 switch as described 
above where the clock beams are turned on or off to determine which input 
beams are selected. However, since there is an input beam pair incident on 
each of the two photodetectors of an S-SEED, each S-SEED is also usable to 
perform logic functions such as AND, OR, NAND, and NOR, by presenting both 
input beam pairs at the same time. Logic operation using S-SEEDS is 
described in the paper "Photonic Ring Counter and Differential Logic Gate 
Using Symmetric Self Electro-Optic Effect Devices", A. L. Lentine, D. A. 
B. Miller, J. E. Cunningham, and J. E. Henry, Conference on Lasers and 
Electro-Optics, Optical Society of America (1987). The arrangement of FIG. 
10 is not monolithically cascadable with other such arrangements. To make 
the arrangement cascadable, photodetectors are added in parallel in a 
manner analogous so that shown herein for the two-dimensional shift 
register. Further, since the S-SEED is usable to perform logic functions, 
it is therefore possible to obtain monolithic structures that perform a 
variety of complex logic functions by interconnecting S-SEEDs, for 
example, a structure comprising a full adder or a programmable logic 
array. 
It is to be understood that the above-described arrangements are merely 
illustrative of the principles of the invention and that many variations 
may be devised by those skilled in the art without departing from the 
spirit and scope of the invention. It is therefore intended that such 
variations be included within the scope of the claims.