Light frequency converter having laser device connected in series with photodetector

A structure providing a monolithic integration of an optical detector with an optical transmitter working in the region of the visible or the near IR spectrum. This source type makes it possible to transpose a perceived image belonging to a wavelength region to another wavelength region with a good output (photons/electrons coupling). Such a device may find particular application in imagery and signal processing systems.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention is directed to a light frequency converter and more 
particularly to an optical-electric-optical converter transposing a 
detected wave to a wave located in another spectral region. 
Discussion of the Background 
With the development of compounds with a quantum well base, by which it is 
possible to control the existence of absorption bands, a new class of 
detector emerges. It is based on a mastery of quantum engineering in the 
class III-V semiconducting materials. 
Concurrently, new materials emerge in the visible spectral region. The 
compounds used can be deposited on substrates identical with those used to 
achieve the function of absorption and detection at various wavelengths. 
This compatibility thus produces natural detection/emission coupling. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a novel 
light frequency converter. 
The light frequency converter according to the present invention then 
comprises: 
a photodetector with quantum multiwells comprising a group of layers 
constituting the quantum multiwells comprising a first and a second face, 
a light radiation to be converted being received on the first face and 
detected by the photodetector; 
a laser device placed on the second face of the quantum multiwell 
photodetector to be connected electrically in series with the 
photodetector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof, which shows an embodiment of a light 
frequency converter according to the present invention. 
As shown in FIG. 1, substrate 1 of semi-insulating GaAs material comprises 
a face 10. A network 2 is provided which makes it possible to optimize the 
coupling of the incident radiation wave with the dipole of the intraband 
transition between energy levels in the conduction band. This network is 
made from standard operations of mechanical-chemical polishing, resin 
coating, masking and selective etching or ion beam etching. This network 
was produced when the growth and the technological operations were 
achieved. 
On face 20 of the substrate opposite to face 10 and oriented according to 
crystallographic orientation (100) of the substrate 1, a layer 1' of GaAs 
of 50 nm is deposited, then another layer 3 of n+ doped GaAs (0.5 micron) 
is deposited on layer 1'. A layer 4 of AlAs (n+), which makes it possible 
with the preceding layer to make and to take a common electrical contact, 
is deposited on this layer 3. 
Then, a group of quantum multiwells 5 is deposited. The thickness of the 
well materials as well as the composition of the barrier materials are 
selected to obtain a resonant structure in the spectral region to be 
detected. It is possible to use a quantum multiwell or supernetwork group. 
A part of the materials constituting the wells is doped to occupy the 
first level able to be occupied by the electrons in the conduction band. 
Further, each elementary quantum well of group 5 can be designed to 
exhibit an intraband absorption at a given wavelength. 
For example: 
the elementary structure or period of the quantum multiwell can be defined 
so that it exhibits an intraband absorption in the vicinity of 10.6 
microns, for example. In this case, the composition and the thicknesses of 
the layers can be as follows: 
width of the well: 3 nm 
width of the barrier: 14 nm 
composition of the well: GaAs 
composition of the barrier: Ga.sub..75 Al.sub..25 As 
delta type doping of the well by silicon. 
The number of multiwells determines the absorption of the system at the 
wavelength to be detected. 
A layer 6 of n+ doped GaAs is then deposited on this structure 5. It has as 
its function to make possible the application of a resonant structure 
field 5 in the wavelength region to be detected. The thickness of this 
layer 6 is, for example, 1 micron. 
A Bragg mirror 7 is then produced on layer 6. This mirror consists of an 
alternation of layers of different compositions providing different 
indices of refraction. The compositions are selected so that the maximum 
reflectivity is centered on the emission wavelength of the laser active 
material. Moreover, the materials are selected so that they exhibit a low 
absorption at the laser emission wavelength. 
Bragg mirror 7 consists, for example, of about 20 Ga.sub.1-x Al.sub.x As 
layers alternated with about 20 AlAs layers. The value of x is, for 
example, 0.8. 
A group of layers 9 constituting quantum multiwells is then produced on the 
Bragg mirror 7. For example, there are 3 to 10 quantum multiwells (width 
of the wells=10 nm), (width of the barriers=10 nm) located in a GaAlAs 
material. Each ternary composition layer 8 and 11 located on both sides of 
quantum multiwells 9 is n+ and p+ doped, respectively. Then, a p+ doped 
Bragg mirror 12 is produced. 
The number of alternations of the layers of Bragg mirror 12 is lower than 
that constituting Bragg mirror 7 to optimize the escape on output mirror 
12 (optimization of the excess voltage coefficient of the cavity). 
FIG. 2 represents a detailed embodiment of the present invention according 
to which Bragg mirrors 7 and 12 and laser emission part 9 have been 
engraved to constitute a contact on quantum multiwell detector 5. 
The process of production of the device will consist in producing on 
substrate 1 the sequence of various layers described in relation to FIG. 
1. Then, the two-dimensional space distribution of the detectors is 
achieved by selective etching of all the layers. A group of MESA 
structures is thus obtained. Layer 4 of AlAs (n+) close to the substrate 
represents a buffer or stop layer relative to this etching. A second 
etching operation makes it possible to define the pattern of the laser 
diode (lateral confinement). The production of these pixels (delimitation 
of zones) can also be considered by using an ion beam etching. Then, by 
using techniques for masking and insulation by deposition of silica or 
alumina, ohmic contacts with definition of the electrode pattern are 
produced. 
For the operation of such a device, a first voltage generator 30 is 
connected to ohmic contacts 31 and 32 located respectively on layer 4 and 
on layer 6. Further, a second voltage generator 40 is connected to 
chemical contact 32 and to ohmic contact 41 located on Bragg mirror 12. 
The electrical diagram of the device is as represented in FIG. 3, in which 
detector (D.sub.d) and emission laser (D.sub.L) are connected in series 
and in opposition. 
The control of photodetector part D.sub.d is therefore achieved by applying 
a voltage to the nin zone of the structure. N+ doped layer 4 located in 
the vicinity of the substrate represents common electric part (-). Second 
n+ doped layer 6 deposited after structure 5 makes it possible to apply a 
voltage to the detection structure. 
The detection part acts as a variable resistance but also as a current 
generator. This resistance and the photocurrent generated depend on the 
incident optical intensity. This photodetector can become weaker than 
resistance Rs connected in series with the laser diode. 
The control of the emissive part requires that laser diode part D.sub.L be 
prepolarized by applying a voltage and by applying a current through 
junction pn of the active part. In the absence of detected radiation, 
diode D.sub.d does not conduct, i.e., does not generate a current. 
By acting on the values of resistance connected in series, and according to 
the structure of diodes being considered (head to foot or series), it is 
possible to achieve functions of threshold triggering or bistability in 
the laser emission as a function of the illumination. 
This linking of two functions by growth of semiconducting materials makes 
it possible to produce a multifunction component in plate form where one 
of the faces is subjected to the illumination to be detected and the other 
emits a radiation in the specific spectral region in the active material 
employed in junction pn of the laser diode. 
The type of integration described above is not specific to the use of 
quantum multiwell detectors. The same type of integration of the two 
functions can be considered by using, for example, a heteroepitaxy of 
II-VI or else III-V materials. 
For example, the deposition of HgCdTe achieved from the method of molecular 
jet epitaxy on GaAs makes it possible also to integrate the detection 
function on one of the faces of the substrate and the emission function on 
the other face. 
A monolithic device making it possible to transpose one radiation located 
in a spectral region (invisible radiation, for example) to another has 
been described. The selection of materials deposited on the resonant 
quantum multiwell structure for a wavelength will make it possible to 
generate an optical wave in a desired spectral region. 
One face of the device receives the radiation to be detected, the other 
face emits a laser wave located in the near IR or in the visible spectrum. 
Thus, it is possible to design brilliance amplifier type systems able to 
be inserted in imager assemblies. In this case, the use of III-V materials 
(in detection and in emission) has an advantage of being able to produce 
quick imagers (absence of remanence). 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.