Dynamic optical grating device

The invention relates to an apparatus including a semiconductor grating whose optical properties can be changed electrically in order to steer a diffracted laser beam with no moving parts. Lithographically defined portions, stripes or areas formed in a semiconductor quantum well region used in association with selectable voltage supply means enable control of the optical characteristics of the grating. The optical properties of the semiconductor quantum well region vary in response to variations in voltage applied to the areas which in turn change the transmissivity or reflectivity of the areas. By selectively applying voltages, the diffraction pattern obtained in the far-field from illuminating the areas is thus controlled and beam steering results. By using a two-dimensional array of areas, or alternatively using two such one dimensional arrays, beam steering in two dimensions may be accomplished. Using a Fabry-Perot cavity allows large reflectivity changes to occur from the absorption changes, and another external Fabry-Perot cavity increases the optical power steered into the diffracted modes. Electronically addressing a large number of areas can be achieved using a number of standard methods.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to optical devices, and more particularly 
to an improved optical grating device and system employing electrical 
means to control the direction or spatial patterns of optical beams. 
2. Brief Description of the Prior Art 
Devices for controlling the direction of an optical beam or the spatial 
patterns of illumination produced by a laser have been very limited in the 
past, and confined almost entirely to mechanical methods, such as galvanic 
mirrors. An alternative method and apparatus whereby this spatial control 
could be performed electronically with no moving parts would allow much 
faster and more reliable control of optical beams. 
Spatial light modulators, devices in which optical properties of the 
material are spatially controlled, have previously been very large 
compared to the wavelength of light, and have therefore been useless for 
obtaining diffraction patterns. Present semiconductor technology, however, 
allows the use of quantum well devices to make much smaller spatial light 
modulators where diffraction effects dominate. Since these devices are 
intrinsically very fast, and can be made lithographically, they are useful 
in rapidly controlling far-field patterns of illumination and obtaining 
beam steering through diffraction. These devices can work in transmission 
mode, where the light passes directly through the quantum well region, or 
in reflection mode, where external or integrated mirrors are used to 
enhance reflectivity changes and contrast. 
SUMMARY OF THE INVENTION 
A presently preferred embodiment of the invention includes a semiconductor 
optical grating device wherein the optical properties of the device 
material are controlled spatially by applying a voltage to defined areas. 
By using a semiconductor quantum well material, whose optical properties 
such as refractive index and absorption coefficient depend on the electric 
field, optical properties can be changed independently in each area. Since 
the dimensions of the defined areas are on the order of the wavelength of 
light, strong diffraction effects occur to produce changes in the 
far-field optical pattern. Consequently, electronic control of the 
far-field pattern is obtained with no moving parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a generalized block diagram showing an exemplary embodiment of 
the invention. A light source 100 generates an input light beam 102 which 
is then modified by optics 104 to impinge upon a block of grating material 
106. The output light beam 108 is again modified by output optics 110 to 
focus on a detecting device 112. The properties of material 106 are 
controlled by an electronic circuit 114 which can be partially or entirely 
integrated with the material 106, or can be completely separated. 
The optical characteristics of the material 106 must be such that the 
intensity or the phase of the output light beam changes with applied bias 
supplied by the electronic circuit 114. Material 106 must also be 
physically defined into independently controlled areas such as stripes or 
squares. The electronic circuit 114 will therefore be able to control 
independently the phase of intensity of the output light resulting from 
the illumination of each defined area. If the impinging illumination upon 
the material 106 is coherent, a diffraction pattern will be obtained in 
the far-field. This diffraction pattern can be calculated by many methods, 
a simple approximation being the Huygens-Fresnel integral: 
##EQU1## 
where the left hand side of the equation represents the complex electric 
field of the optical beam at the output coordinates x, y, z. E.sub.0 
represents the complex electric field of the output beam 108 at the 
position of the material 106. k and .lambda. represents the wave number 
and wavelength of the optical beam respectively, and the integral is 
carried out over the two transverse coordinates of the material 106, 
x.sub.0 and y.sub.0. In the limit where the object 106 is placed at 
z.sub.0 =0, and that the coordinates in the far-field are much larger than 
the spatial coordinates of the object, then this becomes a simple 
two-dimensional Fourier Transform: 
##EQU2## 
where I is the intensity at the far-field and 
##EQU3## 
represent the frequencies. 
An example of an exemplary embodiment of material 106 is shown in FIG. 2 
and comprises a number of quantum wells (QWs) 120 of indium gallium 
arsenide (InGaAs) grown upon a gallium arsenide (GaAs) substrate 122. This 
structure is generally similar to those characterized in prior art, such 
as in Applied Physics Letters vol. 49, p. 135. Here the quantum wells are 
thin enough to exhibit quantum confinement and the optical absorption 
properties of the quantum wells are dependent on he applied electric 
field. Typical optical absorption properties for various electric fields 
are shown in FIG. 3. The quantum wells are sandwiched between a top p-type 
GaAs layer 124 and a bottom n-type GaAs layer 126. A reverse bias applied 
to such a structure changes the electric field in the intrinsically doped 
quantum well region 120. 
In accordance with the present invention, the surface of the epitaxially 
grown region is lithographically defined into stripes 128, and isolation 
mesas 130 are etched. The depth of the mesa 130 is enough to electrically 
isolate each stripe. An ohmic metal contact pad 132 is lithographically 
defined and evaporated at the end of each stripe, and wires 134 connect 
each contact pad to the drive electronics 114 (FIG. 1). Since the top of 
each stripe is heavily doped and p-type, the voltage across the whole 
stripe is a constant and is controlled by the voltage applied at the 
contact pad. To increase the conductivity of the top layer for use with 
higher optical powers, a transparent contact such as indium-tin-oxide 
(ITO) can be applied across the length of the whole stripe. Metal lines 
that only partially cover the stripe can also be used, though they will 
decrease efficiency. In this embodiment, since the areas are defined as 
stripes, the far-field intensity profile can be changed in one dimension 
only, and as previously discussed, the far-field intensity pattern depends 
on the Fourier Transform of the spatial reflectivities. To obtain a 
two-dimensional Fourier Transform, we need to be able to vary the spatial 
variance of reflectivity in two dimensions. Consequently, using squares 
instead of stripes is a logical alternative. FIG. 4 shows a typical 
embodiment of this idea. In this case the quantum well material is 
identical to FIG. 3, but the wafer is patterned into squares instead of 
stripes. Contacting such an array can be difficult. However, for small 
arrays, simply depositing metal lines on an insulating layer 136 will be 
sufficient. We shall discuss the issue of more complex addressing schemes 
later in this document. 
An application of this embodiment for laser beam steering (for example to 
scan a laser beam for a laser printer) is schematically shown in FIG. 5. A 
laser 100 outputs a collimated beam 102 that impinges on the material 106. 
The transmitted beam 108 is focused by a simple lens 136 and is spatially 
cut off by an aperture 138. These two elements make up the output optics 
110 shown previously in FIG. 1. Only the first order diffracted beam is 
allowed through the aperture 138 to fall upon the screen or detector 112. 
If the spatial frequency of the stripes in the transmissive state is now 
changed by the controlling electronics 114, the angle that the beam is 
scanned will also be changed. Suppose that the material is configured 
exactly the same as the material in the previous reference with 
characteristics shown in FIG. 3, and that the apparatus is operated at 
wavelength 0.95 .mu.m. As a bias voltage is applied to each 
lithographically defined region, the absorption is reduced, and the 
intensity of the transmitted light increases, turning the area on. If a 
stripe geometry is used, as in FIG. 3 with each stripe 2 .mu.m wide and 
with a 1 .mu.m isolation mesa, then one can simply compute the deflection 
angles of the output beam. For example, an on-off-on-off-on pattern would 
have an effective grating spacing of d=6 .mu.m and would produce a 
deflected beam at 9.11 degrees (given by well-known diffraction formula 
d*sin(angle)=wavelength). Similarly, an on-on-off-off-on-on-off-off 
pattern would have an effective grating spacing of d=12 .mu.m and lead to 
deflection at 4.54 degrees. Of course, the stripes can also be in the 
intermediate states of being partially on. This additional degree of 
freedom would increase the amount of optical power in the fundamental mode 
and thereby increase the overall efficiency. Simple diffraction theory 
governs the scanning rules and in this case a continuous scanning range 
between zero and 9.11 degrees can be achieved by changing the spatial 
frequency. The function of the aperture 138 is to block all diffracted 
beams outside this angular range, since there will also be higher order 
diffraction modes. The angular width of the diffraction mode is defined as 
the resolution of the instrument and increases as the number of stripes 
increases. From well-known grating theory we know that this angular width 
of the resonance is equal to the wavelength divided by the total width of 
the grating. 
An embodiment of the invention working in the reflection mode was 
fabricated in the gallium arsenide/aluminum gallium arsenide system 
(GaAs/AlGaAs). The reflective mode is preferred for two reasons. Since in 
this material the substrate is opaque to the appropriate wavelength, 
constructing a transmissive device would require the removal of the 
substrate. Furthermore, by working in the reflection mode, one can obtain 
higher contrast and lower loss due to the interaction of the Fabry-Perot 
cavity with the quantum wells. The epitaxial growth of the material is 
explained in depth in Applied Physics Letters, vol. 57, p. 1491. In brief, 
the material was grown on an n-type GaAs substrate using molecular beam 
epitaxy (MBE). The mirrors composing the Fabry-Perot cavity consisted of 
1/4 wave stacks of aluminum arsenide (AlAs) and AlGaAs. The bottom mirror 
was n-doped and had a calculated reflectivity of 98.8%, while the top 
mirror was p-doped with a reflectivity of 50.3%. The cavity contained 19 
quantum wells that were left undoped. The total reflectivity of the device 
changed from about 10% to 76% when a 5-volt bias was applied. In this 
embodiment of the invention a wet-etching technique was used to fabricate 
15 stripes, each 10 .mu.m wide and with a 2 .mu.m wide mesa. The stripes 
were ii individually contacted with an ohmic contact around the edge of 
the wafer. FIG. 6 is a photograph of the fabricated device. 
The device was tested using a tunable Ti:Sapphire laser. FIG. 7 is a 
schematic of the experimental apparatus. A 20 cm focal length lens 140 
focused the laser beam onto the device 106. when illuminating the device, 
it is important to have a large enough spot size to illuminate as much of 
the stripes as possible without getting reflections from other parts of 
the wafer, such as the contact pads. In this case, the reflected beam was 
deflected with a small mirror 142 onto an imaging lens 144 and a silicon 
CCD camera 112. The contact pads of device 10 were wire-bonded onto a 
package (not shown), and an external bias could be applied to the stripes. 
The experimental results are displayed in FIG. 8a. In the figure a "1" 
implies a reflective state, and a "0" implies a non-reflective state. In 
the first plot, where all the elements are in a reflective state, the main 
reflected peak appears at the left and a peak corresponding to the spatial 
frequency of the isolation mesas is to the right. Since this peak is due 
to the reflectivity difference between the mesas and the stripes, it 
occurs with differing intensity in almost all plots. Had the mesas been 
smaller in size, this peak would have been correspondingly reduced. The 
main reflected peak on the left is due to the DC reflectivity value of the 
pattern, and is the main power loss mechanism. In the second plot (b) the 
pattern 0:1:0:1:0:1 . . . causes a main peak to occur in the center of the 
range. The position of this peak corresponds to the spatial frequency 
imposed on the striped and is twice the stripe period (24 .mu.m). In the 
next three plots (c,d,e), the peak moves to the left as the spatial 
frequency decreases. The final plot (f) corresponds to all the elements in 
the off state, and only the main reflective peak appears. The peak due to 
the mesas disappears in this plot since the off state reflectivity 
corresponds roughly to the reflectivity of the mesa regions. FIG. 8a 
clearly demonstrates that beam steering is possible in accordance with the 
present invention. 
There are a number of modifications that will increase the performance of 
the device. In the prototype embodiment, simple one-to-one connections 
were made to each stripe. To obtain better resolution, more stripes are 
required and would make the prototype form of addressing impractical. 
Since the semiconductor material is ideal for fabrication of drive 
electronics, it is possible to integrate drive electronics on chip. FIG. 9 
shows an implementation where D-type flip-flops in a shift register 
configuration are utilized to decode serial information for the states of 
each stripe and apply the bias to each stripe. Alternatively, one could 
fabricate a large shift register to address all the stripes, or a number 
of smaller shift registers that would each address a smaller number of 
stripes. Each n-bit shift register would reduce the number of wires to the 
chip by n. 
One could also address the stripes using a read-only memory (ROM) in a 
look-up table configuration. This idea is shown schematically in FIGS. 10a 
and 10b. The look-up table corresponding to ROM 146 gives the output 
pattern corresponding to the input code. This is a typical pattern that 
might be used, where the increasing binary code corresponds to increasing 
frequency. This is similar to the demultiplexer logic, where n lines can 
address 2.sup.n devices, in this case, the devices being the stripes. 
To perform beam steering in two dimensions, a simple modification would be 
to make a two-dimensional array of devices, employing squares instead of 
stripes, as shown schematically in FIG. 4. In large arrays of this kind 
the addressing becomes much more complicated, since attaching discrete 
wires to a large matrix is impractical. By using transparent 
interconnects, such as ITO, layers of contact stripes separated by 
transparent insulators such as silicon dioxide would enable a greater 
matrix to be addressed. The gallium arsenide chip could also be inverted 
and placed on a fabricated silicon wafer with bonding pads in the correct 
position. In this case the reflection modulator would have to work from 
the substrate side, with either the substrate transparent (for example by 
using InGaAs wells), or with the substrate removed. 
One could alternatively use optical methods to address a large number of 
regions. For example, CCD technology has a well-established capability for 
low power and high speed. This ii form of addressing would make the 
information transfer to the chip optical instead of electronic, and very 
much faster. An experimental demonstration of such a technique is 
explained in Applied Physics Letters, vol. 52, p. 1116. FIG. 11 is a 
schematic of the device (from reference). Briefly, a conventional CCD 
structure is grown over a multi-quantum well region, and the presence of 
pockets of charge in the CCD layer changes the electric field in the 
quantum wells and thereby changes the absorption. To set a particular 
region to a particular voltage, a specific amount of charge is first 
injected through an ohmic contact on the periphery, and clocked down to 
the appropriate place, moving to the adjacent contact on each clock pulse. 
Note that the CCD region is fabricate with AlGaAs, and is therefore 
transparent at the wavelength where the QW modulation occurs. 
Consequently, it does not decrease the intensity of the light to be 
modulated, nor are the pockets of charge change affected by this light. 
Optical addressing could also be performed using Self Electro-Optic Effect 
Device (SEED) technology. Integrated resistors to normally-off quantum 
well modulators can be fabricated in small sizes and exhibit optical 
bistability. An explanation of such operating principle and fabrication 
can be found in U.S. Pat. No. 4,546,244. A typical embodiment shown in 
FIG. 12 would be to fabricate a large array of SEEDs composed of many 
small squares with integrated resistors. The SEED itself is simply a 
normally-off transmission modulator with a bottom n-region 126, a top 
p-region 124, and an intrinsic quantum well region 120. Integrated with 
each modulator is a resistor 160. Only two electrical lines supplying bias 
to the device would be required, one to the substrate and a top contact 
164 that addresses all devices. This contact would have to be transparent 
to allow optical input. All the devices would also have to be laterally 
isolated from one another with an electrically insulating material 166. To 
operate, a DC electrical bias may be applied uniformly to all the devices 
by an external voltage supply 168. An optical bias illuminating all the 
devices at a power level where the device has two stable states would 
enable the device. If an image were to be focused onto the array, some 
pixels would switch to the low transmission state, and stay in that state 
even when the focused image is removed. In the far-field one would obtain 
a two-dimensional Fourier Transform of the image. Since analytical methods 
of Fourier Transforms, especially in two dimensions are very computer 
intensive, this would provide a fast optical alternative. 
There are also modifications to the optical set up that can improve the 
characteristics of the device. The main reflected lobe represents wasted 
power, since it can not be deflected electronically. By placing the device 
in a external Fabry-Perot cavity using a partially transmitting second 
mirror, the power in the main lobe can be recovered. FIG. 13 schematically 
depicts this embodiment. A partially reflecting mirror 150 is set to have 
the same reflectivity as the average reflectivity of the device 106, and 
placed an integral number of half wavelengths away to form an external 
cavity. The net result is that the beam directly reflected by the material 
106 builds up inside this external cavity until it is deflected into a 
useful side order mode 152. If the reflectivity of the partially 
reflecting mirror 106 is correct, there will be no net reflection either 
from the main lobe or from mirror 150 back towards the light source. 
Efficiency can also be increased by modulating the phase rather than by 
modulating the intensity of the output light from each area. In this case 
one would rely on refractive index changes in the quantum well region to 
modulate the phase of the output beam. One could also use other materials 
like ferroelectrics where the refractive index is a function of electric 
field to achieve the same effect. These phase modulators can also be 
integrated with intensity modulators to control both optical properties. 
In a typical example the light would first pass through quantum well 
region where the operating wavelength is close to the exciton, and 
electric fields would primarily influence the intensity, and then pass 
through a quantum well region where the operating wavelength is further 
from the exciton and the path length is therefore field dependent. 
A practical way to implement a device where the reflected beam only changes 
phase by 180 degrees and stays the same in intensity is using a properly 
optimized reflection electroabsorption modulator. In such a device the 
total reflectivity of the device depends on the front mirror reflectivity 
R.sub.f, the rear mirror reflectivity R.sub.b, the round trip cavity 
absorption e.sup.-2.alpha.L, as follows: 
##EQU4## 
where R.sub.b' =R.sub.b e.sup.-.alpha.L, an effective back mirror 
reflectivity that takes into account cavity absorption. In conventional 
intensity modulators, the cavity absorption is changed from a low value 
where R.sub.b'&gt;R.sub.f to a high value where R.sub.b' =R.sub.f. When this 
matching condition is satisfied, the total reflectivity goes to zero, thus 
yielding a high contrast device. However, if the cavity absorption is 
changed from a specific value above this matching condition to another 
specific value below this condition, then the intensity of the reflected 
beam will remain constant, while the phase will switch by 180 degrees. 
FIG. 14 is a plot of total reflectivity as a function of R.sub.b' for 
R.sub.f =0.9. As a numerical example, assume R.sub.b is almost 1 and 
R.sub.f =0.9. Suppose also that the cavity absorption can be changed by a 
factor of 10, then by changing .alpha.L by a factor of 10 from -0.033 to 
-0.333, R.sub.b' is changed from 0.9672 to 0.7164, and although the total 
reflectivity stays at 0.269, the phase of the reflected beam changes by 
180 degrees. This is shown on FIG. 14, as moving from point A to point B. 
Of course, higher efficiencies than 0.269 can be obtained by better 
quantum wells that can change .alpha.L by larger amounts. 
Optimization can also be made in the definition of the active areas. It has 
previously been mentioned that the areas can be stripes for beam 
deflection in one dimension, or squares for control in two dimensions. In 
the above-described embodiment the active regions were equally spaced 
stripes of equal width. However, better efficiency can be obtained in a 
finite grating if the stripes are made uneven, the stripe width depending 
on the position of the stripe in the grating. This is schematically shown 
in FIG. 15. The same applies to a two-dimensional grid for both horizontal 
and vertical control; the size of the individual squares could be 
dependent on the position of the square to increase efficiency. 
Two-dimensional control of the beam can also be obtained by using two 
gratings. The first grating device would enable control in one direction, 
while the other would deflect in a perpendicular direction to the first. 
This embodiment is schematically shown in FIG. 16. The incoming light beam 
102 is first deflected by a first device 170 which controls the deflection 
in the x direction. The reflected beam then falls upon a second device 172 
which controls deflection in the y direction. Consequently, with 2 inputs 
deflection can be controlled in both directions. The situation is 
analogous to two sets of galvanic mirrors used to get both up and down, 
and left and right control of a laser beam. 
The main application for this invention is steering laser beams. It can be 
used in producing graphics, such as in laser printers, or controlling a 
scanning laser beam to produce a dynamic image, such as a TV picture. For 
applications where a visible image is produced, lasers are required in the 
visible, as well as material that can modulate the intensity or the phase 
of visible light. To produce a color image for a TV, a red, green, and a 
blue laser and gratings are required. A typical set up for such a system 
is shown in FIG. 17. The red laser 190, the green laser 192 and the blue 
laser 194 are directed at their own respective beam steering devices 196, 
198, and 200 respectively. The beams then pass through focusing lenses and 
fall upon a screen 202. The intensity of the laser beams and the 
deflection angles are controlled by electronic circuits 204. In this way a 
high resolution composite color image can be produced. The invention can 
also be used in optical switching, for example directing a single incoming 
beam to a number of output detectors or fibers. This could have 
applications in digital or analog information demultiplexing, such as in a 
telephone switching or optical computer networks. FIG. 18 is a schematic 
diagram for how this invention can be used to demultiplex information from 
an optical fiber bundle and send the data to different photodetectors. The 
property that the far-field is a Fourier Transform of the spatial variance 
of the device also has a host of applications. It could be used in signal 
processing, for example in obtaining a fast frequency scan of radar 
signals. FIG. 19 is a schematic of this implementation. The incoming beam 
102 is once again deflected by the grating device and focused onto a 
detector array. Since the deflection angle depends on the spatial 
frequency of the device reflectivity, we obtain a spatial representation 
of the signal's spectrum. To translate the time varying incoming signal to 
spatial variance, a series of delay units can be used. As well as the 
one-dimensional transform used in signal processing, two-dimensional 
transforms can be performed with an array such as that sketched in FIG. 4. 
These have applications in image processing and image recognition as well 
as many other areas. 
Though reflection mode devices produce the best contrast and reflectivity 
change, transmission mode devices are easier to apply due to the simpler 
geometry. All the previous applications mentioned above can be performed 
either with transmission mode or reflection mode devices. A dual 
transmission mode device combination is shown in FIG. 20 wherein two 
grating devices are used one after another to first steer the beam 
horizontally, and then vertically This enables x-y scanning without 
complex alignment of the gratings 
Materials for making this grating device may include ternaries or 
quarternaries such as InGaAlAs, InGaAsP, InGaAs, InAlAs and other 
semiconducting materials such as InP. For example, alloys or compounds 
made of Group III-V elements and Group II-VI elements are particularly 
useful for this device. For devices working in the reflection mode, 
mirrors may also be made of the previously listed materials, or 
dielectrics such as ZrO.sub.2, Ti.sub.2 z, or metal films such as gold or 
aluminum. They can be made in single layers, or using 1/4 wave stacks as 
previously mentioned.