Method and means for detecting optically transmitted signals and establishing optical interference pattern between electrodes

A photodetector for detecting signal pulses transmitted in an optical carrier signal relies on the generation of electron-hole pairs and the diffusion of the generated electrons and holes to the electrodes on the surface of the semiconductor detector body for generating photovoltaic pulses. The detector utilizes the interference of optical waves for generating an electron-hole grating within the semiconductor body, and, by establishing an electron-hole pair maximum at one electrode and a minimum at the other electrode, a detectable voltaic pulse is generated across the electrode.

BACKGROUND OF THE INVENTION 
This invention relates generally to optical signal transmission, and more 
particularly the invention relates to an improved photodetector and method 
of detecting optically transmitted signals. 
The transmission of data optically through optical fibers permits the rapid 
transfer of large quantities of information. While a television channel, 
for example, transmits information in a bandwidth of 6 MHz, light, in 
theory, has a bandwidth that is many million times that of a TV channel. 
Potentially, using optical transmission, 10 trillion bits of information 
could be sent in a second. 
However, in order to use light to send large amounts of data at high speed, 
one needs very short light pulses and therefore very fast photodetectors. 
Conventional photodetectors are either very small or very inefficient, and 
are therefore limited in their ability to measure the intensity of light 
with fine time resolution. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is an improved method of detecting 
optically transmitted signals. 
Another object of the invention is an improved high-speed photodetector. 
Still another object of the invention is an improved method and means for 
optically transmitting data. 
Briefly, a photodetector in accordance with the invention includes a 
semiconductor body with electrodes positioned on at least one surface. 
Means is provided for transmitting optically encoded signals into the 
semiconductor body whereby photovoltaic pulses are produced. The impulses 
result from the diffusion of electrons to the electrodes in response to a 
grating of photo-excited electron-hole pairs. The grating is established 
whereby a maximum of the carrier density occurs at one electrode and a 
minimum occurs at the other electrode. 
In accordance with one embodiment of the invention, the two electrodes are 
optically transparent. The optical carrier signal is split, and the two 
signal components are transmitted through the electrodes and interfere 
inside of the semiconductor body to establish the grating. 
In accordance with another embodiment of the invention, one electrode is 
transparent and one electrode is reflective. The optical carrier signal is 
transmitted through the transparent electrode into the semiconductor body 
and is reflected off the other electrode. The incoming signal and the 
reflected signal interfere to establishing the grating. 
In accordance with another embodiment of the invention, the optical carrier 
signal is split into components such as by a Ronchi ruling, and the 
components are then imaged onto the semiconductor body between the 
electrodes. The components interfere within the semiconductor body to 
establish the grating. While physical thickness of the semiconductor body 
and electrodes are critical in the first two embodiments, the dimensions 
are not critical in the third embodiment. 
A measure of the photovoltaic pulses is obtained by suitable means such as 
a high-speed sampling oscilloscope or by electro-optic sampling. 
The invention and objects and features thereof will be more readily 
apparent from the following detailed description and appended claims when 
taken with the drawing.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
FIGS. 1-3 are schematic representations of three embodiments of the 
invention. In each of the embodiments, the photodetector relies on 
diffusion of electrons and holes to produce very short (i.e., less than 
one picosecond with counterpropagating optical beams) photovoltaic impulse 
responses. This is to be contrasted with the conventional photodetectors 
which rely upon large electric fields and small geometries to allow all of 
the photocarriers to be swept into the contacts in a short time. The 
detector in accordance with the present invention does not require an 
electrical bias or recombination as in the prior-art high speed devices. 
Referring now to FIG. 1, a photodetector in accordance with one embodiment 
of the invention comprises an optically thin slab of semiconductor 
material 10, single crystal silicon or gallium arsenide for example, with 
optically transparent electrodes 12 and 14 provided on opposing surfaces 
thereof. Suitable electrode material can be a very thin coating of gold on 
silicon material or aluminum gallium arsenide on gallium arsenide 
material. The electrode spacing as determined by the slab thickness must 
be an odd integer times one-fourth the carrier signal wavelength in the 
slab. It will be appreciated that the two beams may be offset by an angle 
of 2.theta. whence the appropriate thickness is as above divided by cos 
.theta.. In this embodiment a coherent lightbeam from laser 16 is encoded 
by modulator 18 and then split by beam-splitter 20. The two beams from 
beam-splitter 20 are then directed along equal length paths and directed 
by mirrors 22 and 24 to the semiconductor body 10 through the optically 
transparent electrodes 12 and 14. The two beams interfere within the 
semiconductor body and establish an electron-hole pair grating therein. By 
positioning the semiconductor body whereby an intensity maximum occurs at 
one electrode and a minimum at the other electrode, as illustrated 
schematically at 26 and 26, a current is generated by the diffusion of 
electrons and holes with the generated current being proportional to the 
difference in the electron-hole pair densities at the two electrodes. The 
sinusoidal distribution of the electron-hole pairs, and thus the current, 
decays rapidly by diffusion. The photovoltaic pulses generated by the 
diffusion of electrons to the electrodes can then be measured by means of 
a high-speed sampling oscilloscope 30, for example. For common materials, 
such as silicon and gallium arsenide, and visible wavelengths, the 
response time can be as short as a hundred femtoseconds. This is much 
shorter than the impulse responses of conventional fast photodetectors. 
FIG. 2 is a schematic diagram of another embodiment of a photodetector of 
the present invention. In this embodiment, the active detector portion is 
fabricated on a supporting substrate 32 of gallium arsenide, for example. 
The substrate 32 is etched using conventional semiconductor processing 
techniques to form a window 34 therein. At the base of window 34 is an 
optically transparent electrode 36 which can be N++ gallium arsenide. The 
contact 36 may also function as an etch stop in the fabrication of the 
window 34. On the surface of the contact 36 is an active layer 38 of 
gallium aluminum arsenide, and on the opposing surface of layer 38 is a 
second electrode 40 of N++ gallium arsenide. Finally, over the surface of 
electrode 40 is a reflective metal layer 42 of aluminum, for example. The 
active layer 38 and the electrode 40 have approximately the same 
dimensions as the window 34. 
In this embodiment, a coherent light beam from laser 44 is modulated at 46 
and then directed by mirror 48 through the window 34 and transparent 
contact 36 to the active layer 38. The light reflects off of mirror 
surface 42, and the reflected light interferes with the impinging light 
directed by mirror 48 to establish an electron-hole grating between 
electrodes 36 and 40. 
In the embodiments of FIGS. 1 and 2, the thickness of the semiconductor 
slab must be an odd integer times a quarter of the wavelength of the 
optical carrier signal in the material. Thus, fabrication of the 
semiconductor body can be critical. However, FIG. 3 is another embodiment 
of a photodetector in accordance with the invention in which the thickness 
of the semiconductor material is not critical because the imaging system 
can be adjusted to compensate for thickness variations. In this 
embodiment, a beam from laser 50, after passing through modulator 52, is 
divided into a number of components by suitable means such as a Ronchi 
ruling 54. The components 56 of the laser beam are then imaged by means of 
a lens 58 on a semiconductor body 60. Electrodes 62 and 64 are provided on 
opposing surfaces of the semiconductor body 60, and a signal detector 66 
is connected to the electrodes to measure the photovoltaic pulses 
generated thereon. In this embodiment the components 56 created by the 
Ronchi ruling 54 generate the electron-hole grating within the 
semiconductor body 60. 
Experimental results using a photodetector in accordance with the invention 
have proved to be consistent with theoretical results. FIG. 4 is a plot of 
time constant vs. grating k vector with the solid line 70 represented the 
theoretical results and the experimental results being denoted by squares. 
These measurements were made on a silicon sample of the type shown in FIG. 
3. In the experimentally accessible region for our apparatus the impulse 
response is given by: 
##EQU1## 
where D.sub.a is the material's ambipolar diffusion constant, .tau. is the 
carrier lifetime, and K is 2.pi. divided by the period of the photocarrier 
grating. The various K's were produced by using Ronchi rulings of various 
periods and imaging systems of various magnifications. The measurements go 
from the regime where the response time is dominated by recombination (260 
nanoseconds response at K=314 cm.sup.-1) to that where the diffusion time 
dominates the problem (the remaining points). The fastest time (650 ps at 
K=15700 cm.sup.-1) is almost a factor of two faster than the saturation 
velocity induced transit time limit (0.011 cm/(10.sup.7 cm/s)=1.1 ns). One 
of the important aspects of the photodetector is its ability to operate 
more quickly than the transmit time limit, while still providing high 
quantum efficiency. 
An optical communications system using a photodetector in accordance with 
the presence invention should be capable of producing detectable light 
pulses that last one picosecond, or a millionth of a millionth of a 
second, thus, more fully utilizing the bandwidth of a light carrier 
signal. While the invention has been described with reference to specific 
embodiments, the description is illustrative of the invention and is not 
to be construed as limiting the invention. For example in the embodiment 
of FIG. 1, two lasers may be applied and mixed down by the moving 
interference pattern within the semiconductor body. Further, while in the 
preferred embodiments the two electrodes are on opposing surface, the two 
electrodes can be in spaced alignment on a single surface, Thus, various 
modifications and applications will occur to those skilled in the art 
without departing from the true spirit and scope of the invention as 
defined by the appended claims.