Monolithic active waveguide optical crossbar switch

A semiconductor active waveguide optical crossbar switch to allow optical signals to be routed to any number of ports with no net attenuation of signal strength. The optical crossbar switch is comprised of a network of optical waveguides wherein both lateral and transverse carrier confinement is provided. Redirection of the optical waveguide is accomplished by means of total internal reflectance turning mirrors. Furthermore, the optical waveguides are formed such that electrical current may be applied to the semiconductor structure to amplify the optical signal travelling therein. Electro-absorption switches formed from distinct metallic contacts overlying portions of the optical waveguides amplify the optical signal travelling through the waveguide underneath the switch if sufficient electrical stimulation is applied to the switch. Additionally, if no stimulation is applied to the electro-absorption switch, the optical signal travelling in the portion of the optical waveguide beneath the electro-absorption switches is attenuated to a level below that of the noise in the optical system such that no signal will be received by the output. Thus, by appropriately patterning the optical waveguide network in conjunction with turning mirrors and metallization contacts to provide attenuation or amplification, an input optical signal applied to this semiconductor active waveguide optical crossbar switch may be directed to any number of output ports. By utilizing optical signals throughout the design with no conversion to electrical signals, the transmission bandwidth of the switch is very large and the switching network is impervious to EMI.

BACKGROUND OF THE INVENTION 
This invention relates generally to optical crossbar switches and more 
particularly to reconfigurable monolithic optical crossbar switches 
capable of amplifying or attenuating the switched signals. 
In numerous applications, such as high computation rate parallel or 
distributed computing architectures with numerous processors which 
transmit information to each other or share common resources, 
communication switching systems as in telephone switching centers, and 
aircraft fiber optic buses that require reconfigurability to allow for 
redundancy for fault tolerance as well as the ability to share several 
sensors with each of several processors it is desirable to utilize a 
reconfigurable interconnection network. One such type of reconfigurable 
interconnection network is a reconfigurable crossbar switch which is in 
effect an N.times.M array of switches for connecting each of N inputs for 
any or all of M outputs. 
Initially, electrical crossbar switches were developed and utilized, 
however, the use of such electrical switches severely limits the bandwidth 
of the signals transmitted through the switch as well as being quite 
sensitive to electromagnetic interference (EMI). Additionally, electrical 
crossbar switches generate EMI themselves. With the advances of speed and 
volume in data communications, the limitations imposed by electrical 
crossbar switches have proven to be too constraining and reconfigurable 
interconnection networks including optical switches or hybrid 
electrooptical switches have been developed. 
Optical or hybrid electrooptical crossbar switches have significantly 
improved the transmission bandwidth in comparison with electrical crossbar 
switches as well as reducing the effects of EMI on the switch. Several 
methods have been utilized in attempts to design optical interconnection 
networks. One such design involves the electrical detection of an optical 
signal followed by relaying the transduced electrical signal to the 
appropriate channel and regeneration of the optical signal at the 
appropriate output optical channel. Such a process, however, is prone to 
the introduction of errors during conversion as well as during the 
relaying processes. Additionally, bandwidth is still limited to the 
bandwidth of the device electronics and transmission via an electrical 
signal path can take place in only one direction as opposed to 
bidirectional optical transmission. 
An alternative optical interconnection network is a passive optical 
transmission path crossbar switches in which an input optical signal is 
made available to N output ports with the particular output port 
subsequently selected. Such an optical interconnection network, however, 
suffers from several deficiencies. In addition to the typically massive 
size of such systems, the signal in such a device is reduced in strength 
by a factor of N due to the splitting of the signal in order to make it 
available to each of the N output ports. 
Yet another approach to developing an optical interconnection network is 
illustrated by the networks disclosed in U.S. Pat. No. 5,037,173 issued on 
Aug. 6, 1991. In the optical interconnection network discussed in the 
5,037,173 patent bifurcated optical fibers with a common end for both 
emitting light and receiving light are positioned such that light may be 
emitted from the fibers toward a spatial light modulator which reflects 
the input light to the desired fiber through which it is to be output. 
Additionally, the patent discloses the use of a deformable mirror device 
for reflecting the input light to the desired output fiber. While the 
optical interconnection network disclosed by the 5,037,173 patent may 
allow transmission of signals having a wide bandwidth while not being 
impeded by EMI, such an optical interconnection network must be aligned 
precisely in order to operate properly and may prove difficult to 
fabricate. 
It would be desirable therefore, for an optical interconnection network to 
be designed that can provide a high level of signal splitting without 
signal corruption or loss in bandwidth. Furthermore, it would be desirable 
for such an optical interconnection network to be bi-directional and to be 
capable of being fabricated by conventional integrated circuit fabrication 
techniques with a high level of crossbar function integration on a single 
monolithic chip. It would also be desirable for the optical 
interconnection network to be easily reconfigurable to assure system 
architectural flexibility and fault tolerance. 
SUMMARY OF INVENTION 
There is provided by this invention a semiconductor active waveguide 
optical crossbar switch to allow optical signals to be routed to any 
number of ports with no net attenuation of signal strength. The optical 
crossbar switch is comprised of a network of optical waveguides. The 
optical waveguides are typically formed from a plurality of semiconductor 
layers having therein a single quantum well between a pair of graded index 
confinement layers. The optical waveguides require lateral confinement as 
well, typically implemented by a rib waveguide construction such that the 
optical signals propagate within the quantum well region located beneath 
the rib. Redirection of the optical signals along the optical waveguide is 
accomplished by means of total internal reflectance turning mirrors in 
which a portion of the waveguide along with the surrounding semiconductor 
layers are etched to a point beneath the single quantum well and 
confinement layers such that optical signals arriving at this etched facet 
are redirected. Furthermore, the optical waveguides are formed such that 
metallization layers may be applied to the opposing major faces of the 
semiconductor structure such that electrical current may be applied to the 
semiconductor structure to amplify the optical signal travelling therein. 
The metallization layer may be patterned such that separate contact may be 
made with each optical waveguide. Electro-absorption switches formed from 
distinct metallic contacts overlying portions of the optical waveguides 
amplify the optical signal travelling through the waveguide underneath the 
switch if sufficient electrical stimulation is applied to the switch. 
Additionally, if no stimulation or a negative bias is applied to the 
electro-absorption switch, the optical signal travelling in the portion of 
the optical waveguide beneath the electro-absorption switches is 
attenuated. If the length of the electro-absorption switch is sufficiently 
long, the attenuation reduces the signal strength to a level below that of 
the noise in the optical system such that no signal will be received by 
the output. Alternatively, the signal may be reduced in strength below the 
noise level with a shorter electro-absorption switch if the switch is 
electrically reverse biased. Thus, by appropriately patterning the optical 
waveguide network in conjunction with turning mirrors and metallization 
contacts to provide attenuation or amplification, an input optical signal 
applied to this semiconductor active waveguide optical crossbar switch may 
be directed to any number of output ports. Additionally, with the use of a 
sensor and a controller, the incoming signals may dictate the 
configuration of the network and their ultimate destruction. By utilizing 
optical signals throughout the design with no conversion to electrical 
signals, the transmission bandwidth of the switch is very large and the 
switching network is impervious to EMI. While such an optical crossbar 
switch may be made to any N.times.M size, typical sizes of such crossbar 
switching networks are 2.times.2, 4.times.4, 8.times.8, and 16.times.16 
networks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown an exemplary plan view of a 
semiconductor active waveguide optical crossbar switch 10 allowing signals 
to be routed between various input and output ports which illustrates the 
potential optical paths and switches. The semiconductor active waveguide 
optical crossbar switch 10 is comprised of a plurality of components each 
of which will be subsequently described. There is shown in FIG. 2, a 
sectional side view of a semiconductor structure 20 which typically forms 
the optical waveguides and amplifiers of the semiconductor active 
waveguide optical crossbar switch 10. 
While the semiconductor structure may be comprised of any of a variety of 
materials and the physical dimensions of the semiconductor structure also 
varied, as is well known to those skilled in the art, a gallium arsenide 
(GaAs) semiconductor structure will be described in more detail as an 
example. Additionally, while the semiconductor structure may be comprised 
of any of a variety of layers such that vertical, or transverse, 
confinement and lateral confinement of the propagating light is achieved, 
it is preferable if the semiconductor structure is designed such that the 
energy band gap is a minimum and the index of refraction is a maximum in 
the active region. The minimization of the energy band gap in the active 
region is preferable so as to optimize hole-electron recombination while 
the maximization of the index of refraction in the active region is 
desirable to provide vertical optical confinement. As an exemplary 
structure, a graded index single quantum well structure is described in 
detail although multiple quantum well structures or structures having a 
broad active region could be used as well. Also, the graded index layers 
are only one manner of achieving vertical confinement and others may be 
utilized as is well known to those skilled in the art. The exemplary GaAs 
semiconductor waveguide structure 20 shown in FIG. 2 is fabricated on an 
N+ doped GaAs substrate 22 upon which an N-doped AlGaAs cladding layer 24 
is deposited. Upon the N-doped AlGaAs cladding layer 24 is deposited a 
pair of graded index confinement layers 26 surrounding a single quantum 
well 28 forming a graded index separate confinement heterostructure 
(GRIN-SCH). In this GRIN-SCH region, the graded index confinement layers 
26 have varying percentages of aluminum and gallium comprising their 
composition of Al.sub.x Ga.sub.1-x As. The confinement layers 26 have a 
percentage of aluminum that decreases as the layer approaches the 
interface of the confinement layer and the quantum well. Thus, at the 
interface of the cladding layer, and the confinement layer the percentage 
of aluminum in the confinement layer is equal to that in the cladding 
layer. The percentage of aluminium then decreases in a direction towards 
the quantum well region. 
The quantum well 28 is formed with GaAs such that it has a smaller energy 
gap and a higher refractive index than the graded index confinement layers 
26 bounding it so as to confine the carriers within the quantum well 28 to 
increase the waveguide's efficiency. Upon the quantum well 28 and second 
graded index confinement layer 26b is deposited a P-doped AlGaAs cladding 
layer 30. A P+ doped GaAs layer 32 is subsequently deposited upon the 
P-doped AlGaAs cladding layer 30. Through conventional photolithographic 
etching, a rib or ridge of P+ GaAs and P-doped AlGaAs layers is defined 
upon which a dielectric layer 34 is deposited. The dielectric layer 34 is 
subsequently etched such that the P+ GaAs layer 32 is exposed upon the top 
of the ridge. Subsequently, metallization layers 36 are applied to both 
major surfaces of the semiconductor structure such that the electrical 
contact can be maintained. 
While the semiconductor structure 20 depicted in FIG. 2, was described in 
terms of construction utilizing GaAs and AlGaAs, similar structures can be 
formed from a variety of optically active III-V, II-V, II-VI and IV-VI 
semiconductor materials as well known to those skilled in the art. 
Examples of alternative materials systems for use in various wavelength 
spectra include the use of alloys of Indium Gallium Arsenide (InGaAs) for 
propagating light having a wavelength from 900 nm to 1100 nm; alloys of 
Indium Gallium Arsenide Phosphide (InGa As P), typically deposited or 
Indium Phosphide (InP) substrates, for propagating light having wavelength 
from 1300 nm to 1600 nm; alloys of Indium Arsenide Antimonide (InAsSb) for 
propagating light having a wavelength from 2000 nm to 5000 nm; and alloys 
of II-VI materials for propagating visible light having a wavelength of 
450 nm to 650 nm. The semiconductor structure, however, must operate as a 
single, lowest order mode optical waveguide so that it is not bandwidth 
limited by multi-mode signal distortion. Additionally, as previously 
discussed, a semiconductor structure regardless of particular materials 
utilized and the sequence of layers deposited thereupon, should be grown 
to form a structure in the vertical direction that: 1) has a P-N junction 
in the vertical direction, 2) confines the optically amplified signal to 
the P-N junction by means of grading the refractive index to maximum at 
the junction, and 3) attracts and confines both electrons and holes to the 
junction by means of achieving a minimum energy band gap at the junction. 
Lateral optical confinement may also be accomplished by any number of 
structural configurations in addition to the rib structure described in 
detail herein. While the rib structure provides lateral optical 
confinement due to the presence of a slightly higher refractive index in 
the area underneath the rib than that on either side of the rib, such 
lateral confinement may also be obtained with other semiconductor 
waveguide configurations such as etched grooves, typically V-shaped 
between adjuacent waveguides, the introduction of materials having lower 
indices of refraction between adjacent waveguides, or by the introduction 
of impurity induced disordering in the portions of the active layer 
laterally adjacent to the desired optical waveguide, as is well known to 
those skilled in the art. The structure 20 shown in FIG. 2 and described 
heretofore accomplishes each of these objectives with vertical confinement 
accomplished by means of the graded index structure 26 surrounding the 
single quantum well 28 and lateral optical confinement provided by means 
of the rib structure 12. 
The waveguide structure 20 illustrated in FIG. 2 has metallization layers 
36 on both major surfaces of the structure 20 such that current may be 
introduced between the metallization layers 36 as is well known to those 
skilled in the art. With no bias or a reversed bias applied to the rib 
structure, the semiconductor waveguide attenuates a travelling optical 
signal. However, with an appropriate amount of current supplied between 
the metallization layers 36, the optical path becomes transparent, i.e. 
having a unity gain. With increased levels of current supplied, the 
optical waveguide becomes an optical amplifier such that gain may be 
applied to the optical signal travelling therethrough. Thus, the 
semiconductor structure 20 shown in FIG. 2 may serve to extinguish, 
attenuate, pass or amplify an optical signal. The optical 
waveguide/amplifier structure 12 shown in FIG. 2 is utilized in the 
semiconductor active waveguide optical crossbar switch 10 shown in FIG. 1 
to transmit, amplify or attenuate the optical signal. 
As shown in FIG. 1, redirection of a guided optical signal through the 
semiconductor active waveguide optical crossbar switch 10 is required in 
order to route the optical signal as needed. Such redirection may be 
accomplished by means of a total internal reflection (TIR) turning mirror 
40 as shown in FIG. 3. To form such a turning mirror 40, the optical 
waveguides 12 required for the signal to travel are initially formed by 
deposition and etching upon the semiconductor substrate 22. Thereafter the 
mirror surface 42 may be formed by selectively etching a portion of the 
epitaxial layers along with a portion of the ribbed waveguide structure. 
The portion of the ribbed waveguide structure to be etched is that portion 
at which the reflection is to occur. The epitaxial layers and the ribbed 
waveguide structures are both etched to a point below the graded index 
structure 26 of the optical waveguide. The cavity 44 which has been etched 
from the semiconductor structure may be filled with a dielectric layer, 
such as SiO.sub.2, or may be left empty. Such etching and selective 
patterning can be achieved by microelectronic photolithographic techniques 
followed by etching with a dry or wet etching process. The optical signal 
propagating through the optical waveguide is totally reflected from the 
etched facet 42, as shown in FIG. 3, due to the high index of refraction 
in the semiconductor device material in comparison to the material filling 
the cavity 44. 
Redirection of a waveguide signal may also be achieved by means of 
gradually bending the waveguide while increasing the depth of the etch 
which forms the rib 12 and establishes the rib's height. Such bending is 
effective, however, only so long as the radiation mode losses which occur 
as the guided optical wave negotiates the bend in the waveguide do not 
excessively attenuate the guided signal. 
Two typical configurations used to divide signals are a Y-branch structure 
50 as shown in FIG. 4 or a T-branch structure 60 as shown in FIG. 5. In a 
Y-branch structure 50 as illustrated in FIG. 4, a guided wave is gradually 
widened and split into two or more signals. The splitting of the signals 
may be aided by process induced stress at the edge of the waveguide 12 
which increases the refractive index at the edge 52 of the guide and thus, 
the mode strength along either edge of the waveguide 12. 
As shown in FIG. 5, a T-branch 60 provides for gradual widening of a guided 
optical signal. The widened optical signal is thereafter redirected in two 
different directions as the widened signal strikes a double faceted 
splitting mirror 62 similar to those previously described and illustrated 
in FIG. 3. The optical signals may thereafter be redirected in any desired 
direction consistent with achieving total internal reflectance within the 
waveguide by means of two additional turning mirrors 42. 
By propagating through N-stages of Y-and/or T-branches, a single optical 
signal may be split into as many as 2.sup.N signal paths. Similarly, 
N-stages of Y-and/or T-branches may be utilized to channel N signal paths 
into one path by introducing the optical signals from the opposite end of 
the semiconductor structure since each of these structures function 
bi-directionally. 
While redirection and splitting of optical signals is required for the 
functioning of an optical crossbar switch, such optical signal splitting 
results in resultant signals which are reduced in magnitude. For example, 
if an input optical signal is split into two portions of equal strength, 
each of these resultant portions is reduced by a factor of two relative to 
the original signal's power. Such an attenuation in signal strength, 
however, may be compensated for by the active waveguide portion of the 
semiconductor structure in which current may be introduced between the 
metallization layer 36 such that the signal is amplified to its original 
signal strength or to a greater level of signal strength if desired. 
The operation of such a semiconductor active waveguide optical crossbar 
switch 10 may be illustrated by reference to FIG. 1. For example, an 
optical signal at port 14a may be connected with either or both of ports 
14b and 14c, by means of the electro-absorption switches 16. Each 
electro-absorption switch 16 is a separately contacted portion of the 
semiconductor waveguide structure such that current introduced by the 
electro-absorption switch 16 electrically pumps the modulator section of 
the waveguide directly beneath the electro-absorption switch 16 into an 
"on" state so as to pass an optical signal propagating in the portion of 
the waveguide beneath the switch. If no current or a negative bias is 
applied to an electro-absorption switch 16, the portion of the waveguide 
beneath the switch operates to attenuate the optical signal. Thus, with a 
sufficiently long section of optical waveguide beneath an 
electro-absorption switch 16, a signal may become attenuated to a level 
substantially below the spontaneous emission noise background of the 
device and thus become obscured such that no signal is discernable after 
passing through the electro-absorption switch 16 in such an "off" state. 
By selecting which electro-absorption switches 16 are biased in an "on" or 
"off" state, the input optical signals may be routed to any one or a 
number of selected output ports. Additionally, when biased to an "on" 
state, the electro-absorption switch 16 may also provide gain to the 
optical signal should a sufficiently high level of current be applied to 
the switch 16. While electrical current has heretofore been discussed as 
supplying the required excitation for passing or amplifying an actively 
waveguided signal, the semiconductor optical amplifier may also obtain 
gain from optical pumping such that it may be controlled through optical 
stimulation by coupling light through the amplifying structure in a 
direction perpendicular to the waveguide. 
The pattern of the metallization layers are such that each 
electro-absorption switches 16 is electrically distinct from every other 
electro-absorption switch 16 as well as from every metallization layer 36 
which overlying the optical waveguides 12. Additionally, the metallization 
layers overlying each optical waveguide is typically fabricated so as to 
be electrically distinct from the metallization layers overlying every 
other optical waveguide on the monolithic structure as well as each 
electro-absorption switch 16. This metallization pattern is preferred 
since the gain or attenuation of each waveguide can then be individually 
controlled; however, if such control is unnecessary for the particular 
application, a single metallization layer over every waveguide may be used 
to simplify the fabrication process as long as the electro-absorption 
switches 16 are electrically distinct from the metallization layer 36 and 
every other switch 16. 
Thus, as illustrated in FIG. 1, an input supplied to any one of the four 
ports 14 may be directed to either or both ports on the opposing side of 
the crossbar switch 10. Additionally, each optical path is bidirectional 
so signals may travel in either direction through the crossbar switch 10. 
Since such an optical crossbar switch 10 may be fabricated monolithically 
on a semiconductor structure 20, high levels of integration may be 
achieved by means of utilizing a larger network of active waveguides 12, 
turning mirrors 42, splitters 62, and electro-absorption switches 16 such 
as 4.times.4, 8.times.8, and 16.times.16 arrays of optical crossbar 
switches 10 in addition to the illustrative 2.times.2 crossbar switch 
shown in FIG. 1. 
The optical crossbar switching network 10 as shown in FIG. 1 may also be 
reconfigured quickly by means of applying a different pattern of 
stimulation to the electro-absorption switches 16 so as to redirect or 
reroute any combination of input optical signals to any combination of 
output ports. The only delay in such reconfigurability is due to the 
inherent resistance and capacitance characteristics of the circuitry 
associated with the optical crossbar switch 10. Thus, reconfiguration 
times in nanoseconds are readily achievable. 
An alternative use of such a switching network 10 is to utilize it in 
conjunction with a sensor 70 and a controller 72. The optical sensor 70 
and the controller 72 are well known to those skilled in the art and are 
shown in FIG. 6. The sensor can measure the current or voltage of the 
incoming signals to a port 14. The controller 72 may then use the measured 
signal and based upon a predetermined set of instructions reconfigure the 
network to route the signals appropriately. Thus, the incoming signals may 
themselves control their destination. 
While FIG. 1 illustrates the layout of the optical waveguides 12 for a 
monolithic crossbar switching network 10, optical signals may be coupled 
to or form the input/output ports 14 of the crossbar switch by means of 
optical fibers or microoptics as is well known to those skilled in the 
art. 
Although there has been illustrated and described in specific detail 
instruction of operations, it is clearly understood that the same were for 
purposes of illustration and that changes and modifications may be made 
readily therein by those skilled in the art without departing from the 
spirit and scope of this invention.