Wavelength conversion using self electrooptic effect devices

Abstract: Information borne by an optical signal at first wavelength (λ₁) is transferred intact to another optical signal at a second and different wavelength (λ₂) and vice versa via an optoelectronic circuit employing quantum well devices (2,4) connected serially to facilitate self electrooptic effects therein. The optoelectronic circuit accepts two input signals (30,31)) and provides two output signals (32,33) wherein an input signal and its corresponding output signal are the same wavelength. Bidirectional information transfer equivalent to bidirectional wavelength conversion is permitted b the optoelectronic circuit.

Description: Technical Field This invention relates to the field of optical devices and, more particularly, to devices comprising semiconductor quantum wells utilizing the self electrooptic effect. Background of the Invention Quantum well devices have been developed recently to provide classical optical communication system functions such as modulation, detection, and optical signal generation. See, for example, U. S. Patent 4,595,687. By employing the nonlinear bistability of certain devices, it has been possible to extend the growth of quantum well devices into the areas of switching systems and optical computing. In the latter areas, quantum well devices have been fabricated as basic Boolean logic devices such as AND and OR gates, complex memory and processing devices such as S-R flip-flops and logic device arrays for parallel processing, and switching devices such as n x n switching arrays. Improvements to the latter quantum well devices have centered on the development of the self electrooptic effect device, more commonly referenced as the SEED device. See U. S. Patent 4,546,244 (corresponding to EP-A-0 155 802). In the operation of the SEED device, a photocurrent generated from the absorption of optical energy by a diode may change the voltage across the diode which, in turn, causes a change in the absorption characteristic of the diode. The diode is generally a GaAs/AlGaAs p-i-n structure wherein the intrinsic region (i) comprises one or more quantum well layers. Connection of the diode to a proper electrical load such as a resistive load and tuning the optical signal wavelength coincidently with the heavy hole exciton resonance wavelength permits switching and bistable behavior (hysteretic operation) to be achieved via a positive feedback mechanism. An integrated structure for the SEED device has been proposed in which a p-i-n diode having quantum wells in the intrinsic region is vertically integrated in series with a standard photodiode that is used as a "resistive" load. See Appl. Phys. Lett., Vol. 49, No. 13, pp. 821-3 (1986). The two diodes are responsive to different wavelengths such that the load photodiode is transparent to the infrared wavelengths used in the quantum wells of the other diode and is opaque to wavelengths shorter than infrared. When operated, a control light beam shone on the load photodiode effectively varies the resistance of the load in the circuit thereby affecting the switching characteristics (speed and power) of the integrated SEED device. In addition to the control light beam, an input infrared light beam is directed on the SEED device and a single infrared light beam is output from the device. In another related development, a symmetric SEED device has been proposed in which two serially connected p-i-n photodiodes are fabricated with substantially identical quantum well layers in the respective intrinsic regions. See Conference on Lasers and Electrooptics , Paper ThT12 (1987). This structure is called symmetric because each photodiode operates as the load for the other. It has been proposed that this structure act as a bistable optical memory element such as an optical S-R flip-flop. As an S-R flip-flop, the structure supports dual inputs and dual outputs wherein the optical wavelengths for all inputs and outputs are identical. Summary of the Invention A device according to the invention as set out in the claims is capable of receiving dual input optical signals at different wavelengths and, in response to the dual input signals, generating dual output signals at the corresponding different wavelengths wherein at least one output signal is functionally related to the input signal of the different wavelength. For example, it is possible for information borne by an optical signal at a first wavelength to be transferred intact to another optical signal at a second wavelength and vice versa. Brief Description of the Drawing A more complete understanding of the invention may be obtained by reading the following description of a specific illustrative embodiment of the invention in conjunction with the appended drawing in which: FIG. 1 is a schematic diagram of an optoelectronic circuit in accordance with the principles of the invention; FIG. 2 is a simplified cross-sectional diagram of the semiconductor layers employed by the circuit shown in FIG. 1; FIG. 3 is a graphical plot of optical intensities to illustrate the hysteresis of the circuit in FIG. 1; and FIG. 4 is a cross-sectional view of an integrated circuit embodying the principles of this invention. Detailed Description FIG. 1 shows a schematic diagram of two reverse-biased multiple quantum well diodes arranged electrically in series in accordance with the principles of this invention. Diode 2 includes a quantum well region responsive to optical signals substantially at wavelength λ₁. Diode 4 includes a quantum well region responsive to optical signals substantially at wavelength λ₂. Diodes 2 and 4 are connected in series aiding configuration via electrical lead 3. Reverse bias voltage is applied to the diodes by source 6 connected to diode 2 via electrical lead 1 and connected to diode 4 via electrical lead 5. The optoelectronic circuit shown in FIG. 1 was realized as a hybrid circuit using discrete components. Diode 2 was fabricated as an InGaAs multiple quantum well p-i-n diode wherein the quantum well region was located in the intrinsic region of the diode. Fabrication of diode 2 is explained for those skilled in the art in Appl. Phys. Lett., Vol. 50, No. 15, pp. 1010-2(1987). This diode was illuminated by a tunable NaCl color center laser at the heavy hole exciton resonance wavelength (λ₁ = 1610 nm) for the InGaAs quantum wells. Diode 4 was fabricated as a GaAs multiple quantum well p-i-n diode wherein the quantum well region was located in the intrinsic region of the diode. Fabrication of diode 4 is explained in U. S. Patent 4,525,687. This diode was illuminated by a tunable dye laser (LDS 821) at the heavy hole exciton resonance wavelength (λ₂ = 850 nm) for the GaAs quantum wells. By ramping the intensity of the color center laser up and down while maintaining a constant intensity for the dye laser, it was observed that the output intensity from diode 4 varied as a function of the input intensity to diode 2. In other words, the information carried by the modulated color center laser beam at wavelength λ₁ was transferred intact to the beam at wavelength λ₂ output from diode 4. The results of this experiment are plotted in FIG. 3. In a related experiment, by ramping the intensity of the dye laser up and down while maintaining a constant intensity for the color center laser, it was observed that the output intensity from diode 2 varied as a function of the input intensity to diode 4. In other words, the information carried by the modulated dye laser beam at wavelength λ₂ was transferred intact to the beam at wavelength λ₁ output from diode 2. The result of this experiment when taken with the result above indicates clearly that the single optoelectronic circuit in FIGs. 1 and 2 is capable of bidirectional information transfer with bidirectional wavelength conversion. FIG. 2 shows a simplified cross-sectional layer diagram for the exemplary devices included in the optoelectronic circuit of FIG. 1. It will be understood by those skilled in the art that the layers shown in the FIG. are not drawn to scale and only depict the important active layers while others such as the substrate have been omitted. As shown in FIG. 2, diode 2 comprises a p-i-n quantum well photodiode structure in which layer 21 is n-doped InP (1.0µm thickness), intrinsic region 22 includes a plurality of alternating layers (≈ 100 pairs) of InGaAs wells (10 nm thickness) and InP barriers (10 nm thickness) to form the quantum well region, and layer 23 in p-doped InP (1.0µm thickness). Electrical contact is made to layer 21 via metallic contact 25 and to layer 23 via metallic contact 24. In a similar fashion, diode 4 comprises a p-i-n quantum well photodiode structure in which layer 41 is n-doped AlGaAs(1.0µm thickness), intrinsic region 42 includes a plurality of alternating layers (≈ 100 pairs) of GaAs wells (10 nm thickness) and AlGaAs barriers (10 nm thickness) to form the quantum well region, and layer 43 in p-doped AlGaAs (1.0µm thickness). Electrical contact is made to layer 41 via metallic contact 45 and to layer 43 via metallic contact 45. Biasing potential of 20 volts is supplied to contact 24 (V-) and contact 44 (V+) to achieve a reverse bias condition. The diodes are interconnected in series-aiding configuration via lead 3 connecting contacts 25 and 44. During operation, light beams 30 and 31 are directed on diodes 2 and 4, respectively. Light beam 30 has a wavelength λ₁ = 1610nm which approximates the heavy hole exciton resonance for InGaAs/InP quantum wells. Likewise, light beam 31 has a wavelength λ₂ = 850nm which approximates the heavy hole exciton resonance for GaAs/AlGaAs quantum wells. When light beam 30 is modulated and light beam 31 is maintained at a constant intensity, light beam 33 emerges from diode 4 bearing the modulation information from light beam 30 but at the wavelength λ₂. Alternatively, when light beam 31 is modulated and light beam 30 is maintained at a constant intensity, light beam 32 emerges from diode 2 bearing the modulation information from light beam 31 but at the wavelength λ₁. While the combination of GaAs/AlGaAs and InGaAs/InP quantum well devices has been described above, it is to be understood by those skilled in the art that other combinations of quantum well types are possible. For example, it is expected that combinations of quantum wells from the Group III-V system of semiconductor compounds such as InGaAsP/InP, GaAs/AlGaAs, GaSb/GaAlSb and InGaAs/InP and the like may be substituted for the quantum well regions shown in FIG. 2 to achieve modulation conversion at other wavelengths. Actual heavy hole exciton resonance wavelength varies according to the particular mole fraction (0≦x≦1 and 0≦y≦1) used during device fabrication and the range of wavelengths existing for each compound is approximately as follows: In x Ga 1-x As y P 1-y /InP 1.1-1.6µm GaAs/Al x Ga 1-x As 0.82-0.86µm GaSb/Ga x Al 1-x Sb 1.4-1.6µm In 0.47 Ga 0.53 As/In 0.48 Al 0.52 As 1.3-1.6µm Devices incorporating these and other compounds may be discrete, hybrid or integrated. Fabrication techniques of the basic p-i-n quantum well structure for Group III-V compounds is well known from the technical literature and is not repeated herein. FIG. 3 shows an actual graphical plot of the output intensity for the optical signal from diode 4 at wavelength λ₂ as a function of the input intensity of the optical signal from diode 2 at wavelength λ₁. The experimental configuration and procedures which produced these results have been described above in reference to FIG. 1. FIG. 4 shows a cross-sectional view of an integrated semiconductor circuit embodying the principles of the present invention. By employing standard epitaxial growth techniques for creating the semiconductor heterostructure, standard photolithography, masking and etching techniques for defining the various mesas, and standard alloying techniques to form the contacts, it is possible to form the integrated circuit shown in FIG. 4. As shown in FIG. 4, the integrated circuit comprises two series-aiding multiple quantum well photodiodes grown on InP substrate 100. Substrate 100 is transparent to the operating wavelengths of interest thereby permitting optical access to the active regions of the device. Diodes 200 and 400 comprise epitaxial p-type, intrinsic and n-type layers. Doping concentrations are generally in the range 10¹⁷ to 10¹⁸cm⁻³. For diode 200, n-doped layer 201 is approximately 1µm thick InGaAsP; intrinsic region 202 comprises approximately 100 pairs of thin (10 nm ) InGaAs well layers and InGaAsP barrier layers; and p-doped layer 203 is approximately 1µm thick InGaAsP. Both the InGaAsP (λ ≈ 1.3µm) and the InGaAs (λ ≈ 1.5µm) are selected to be lattice matched to InP. Alloyed contact 210 is formed in n-doped layer 201 for connection to the power supply. Alloyed contact 211 is formed in p-doped layer 203 for interconnection from diode 200 to diode 400 via gold interconnection pad 301. Diode 200 is responsive to input light beam 300 at a wavelength of 1.5µm. Light beam 320 at 1.5µm is the output beam for diode 200. For diode 400, n-doped layer 401 is approximately 1µm thick InP; intrinsic region 402 comprises approximately 100 pairs of thin (10 nm ) InGaAsP well layers and InP barrier layers; and p-doped layer 403 is approximately 1µm thick InP. InGaAsP (λ ≈ 1.3µm) is selected to be lattice matched to InP. Alloyed contact 410 is formed in p-doped layer 403 for connection to the power supply. Alloyed contact 411 is formed in n-doped layer 401 for interconnection from diode 400 to diode 200 via gold interconnection pad 301. Diode 400 is responsive to input light beam 310 at a wavelength of 1.3µm. Light beam 330 at 1.3µm is the output beam, for diode 400. Various modifications to the above-described embodiments may be made by those skilled in the art. For example, the optoelectronic circuit may be utilized in a waveguide mode with a possible reduction of the number of layers in the intrinsic region and a possible elimination of the biasing source. It is also understood that in such a waveguide structure proper fabrication of the layer thicknesses may result in an antiresonant reflector optical waveguide with vertical coupling between the photodiodes.

Claims: An optical device comprising a first p-i-n diode (2) including a light-sensitive quantum well region (22) in the i-region thereof and a second p-i-n diode (4) including a quantum well region (42) in the i-region thereof, said first diode being connected electrically in series with said second diode for electrically controlling an optical absorption or an index of refraction in the quantum well region of said second diode, CHARACTERISED IN THAT said quantum well region of said first diode includes semiconductor material such that its electrical properties are sensitive to light of a first wavelength (λ₁), said quantum well region of said second diode includes semiconductor material such that the said controlled optical absorption or refractive index is that which applies to light of a second wavelength (λ₂) said first and second wavelengths being different from each other, and the electrical properties of said second diode are also sensitive to light at said second wavelength for electrically controlling the optical absorption or index of refraction applying to light of said first wavelength in the quantum well region of said first diode. A device as claimed in claim 1 wherein said quantum well regions comprise multiple quantum wells. A device as claimed in any of the preceding claims wherein the semiconductor materials are Group III-V compounds. A device as claimed in claim 3 wherein the semiconductor material of said quantum well region in said first diode consists of compounds from the AlGaAs/GaAs system and the semiconductor material of said quantum well region in said second diode consists of compounds from the InGaAs/InP system. A device as claimed in claim 3 wherein the semiconductor material of said quantum well region in said first diode consists of compounds from the InGaAs/InGaAsP system and the semiconductor material of said quantum well region in said second diode consists of compounds from the InGaAsP/InP system. A device as claimed in any of the preceding claims wherein light of said first wavelength is within a heavy hole exciton resonance of the quantum well region of the first diode and light of said second wavelength is within a heavy hole exciton resonance of the quantum well region of the second diode.

Inventors: BAR-JOSEPH ISRAEL, CHEMLA DANIEL SIMON, MILLER DAVID ANDREW BARCLAY, BAR-JOSEPH, ISRAEL, CHEMLA, DANIEL SIMON, MILLER, DAVID ANDREW BARCLAY
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1988
Main classification: G02F1
IPC codes: G02F2, G02F1, H01L31, G02F3, H01L27