JPS58218188A - Optical transmitter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to optical transmission devices, and more particularly to devices that operate in the optical frequency range and can be fabricated on small semiconductor chips.

In order to achieve the maximum possible bandwidth and thus the maximum possible transmission rate in the transmission of large amounts of information, it is advantageous to use the highest possible carrier frequency. Therefore, it is best to use carrier frequencies in the optical range for high-speed data transmission. Important applications of optical transmission devices include computer-to-computer connections, either in the same location or over long lines, or connections between computers and display terminals that interact with them, and for multiplex television communications. Loop information links interconnecting interacting terminals with sufficient capabilities, real-time production line monitoring at the same location or between locations, video conferencing, information management of integrated aeronautical circuit equipment and ship communication loops. etc. Pictorial information, such as X-ray and thermistoscope images, real-time television, etc., can also be easily handled. Also, the requirements for improved communication technology associated with urban planning or "new towns" where as many as 30 equivalent television channels are planned connecting each home to each other can also be achieved with a single optical communication link.

The objects and advantages of the present invention include (1) simplicity in both operational and conceptual design and fabrication;
2) be compatible with integrated electronic circuits; (3) be economical in terms of price compared to lower carrier frequency systems;
4) Easy time division multiplexing, (5) Frequency division multiplexing possible, (6) Light weight, (7
) that only materials with less restrictive requirements are used; (
8) It can alleviate RFI (radio frequency interference), crosstalk and grounding problems, and (9) It has a wide bandwidth.

Many people today recognize the need for miniaturized optical transmitters that couple with optical fiber transmission lines. Recently, optical fiber transmission lines with losses as low as the required: 20 dB/Km have been developed. In the present invention, an integrated optical circuit (IO
C) The transmitter is described. What you need is (1
) a source of light pulses of different wavelengths with a predetermined repetition rate; (2) an optical waveguide transmission line; (3) a pulse code modulator for a train of light pulses; (4) a single optical guide for several trains of light pulses. and (5) a coupling technique from the surface waveguide on the chip to the optical fiber transmission line. Apparatus for accomplishing each of these is provided in the present invention.

Miniaturizing coherent radiation sources requires the use of surface lasers whose output is then easily coupled into optical waveguides. If such a surface laser is not used,
It is necessary to couple energy into the waveguide from an external source.

This has been accomplished in the prior art using prisms and grating couplers. In the present invention, these techniques are used to couple YAG-Nd or other pulsed laser radiation to optically pump a surface laser. The surface laser of the invention provides several radiation sources with different wavelengths, thereby allowing frequency division multiplexing.

Several prior art surface lasers have been developed.

Examples are PrCl3 and various dye lasers. However, none of them allows for easy generation of multiple frequencies at desired wavelengths. Although dye lasers can be easily tuned to different wavelengths, they do not operate in the infrared, the desired wavelength region, and do not have long lifetimes. The surface laser of the present invention is an optically and/or electrically pumped semiconductor laser, for example made of GaAs, other III-V compounds and ternary mixed III-V material systems. The bandgap between the valence band and the conduction band is adjusted by the material composition and appropriate doping in order to obtain radiation in the wavelength range from 0.6 to 1.3 μm. The carrier lifetime is short enough for the laser to respond to pumping from a mode-locked or cavity-damped YAG Nd laser to produce superradial laser emission with minimal feedback. Epitaxially grown regions of a semiconductor laser medium are created on a substrate by selective epitaxial techniques, different regions being made of materials of different composition and doping to provide a variety of surface lasers emitting radiation at several different wavelengths. ing.

The light beam attaches, grows, and
Alternatively, it can be introduced into an implanted transparent dielectric structure. The waveguide region consists of a strip of material of a slightly higher refractive index embedded in a medium of a certain refractive index. It is desirable that the light waves propagate in the lowest fundamental mode of the waveguide.

Considering the requirements for fundamental mode propagation, the losses caused by waveguide bending or radiation losses and mode coupling, and the possibility of fabrication on nickel-sized chips by photolithography, the desired wavelength range is 0. 6-1.3μm
It is. This requires the width of the waveguide to be ~10 μm, the thickness to be ~0.3 μm, and the refractive index of the material to be ~4% greater than the refractive index of the material embedding it. Then, the minimum allowable coupling radius is ~4 cm and the edge regularity is ~0.1 μm. According to the invention, these optical waveguides can be made of chalcogenide glass.

Fabricating these structures is within the scope of current photolithography technology. The use of chalcogenide glasses is compatible with semiconductor surface lasers operating at 0.6-1.3 μm as described above. Optical analogs can be fabricated for all techniques of making couplers, filters and other passive microwave devices. Additionally, waveguide structures with gain may be fabricated as described below. A substrate overlying epitaxial semiconductor region with a suitable bund gap is formed beneath the optical waveguide. The oily material is then optically pumped by an external energy source, causing a distribution inversion in the material adjacent the optical waveguide. Optically guided waves are amplified by a decaying field extending from the optical waveguide into this material. Attenuators can also be fabricated using similar techniques.

Modulators compatible with this fabrication technique are created using acousto-optic effects. A piezoelectric material, such as ZnO, is deposited in selected areas on the material in which the optical waveguide structure is embedded, and a finger-like metallization pattern suitable for transmitting sound waves into the piezoelectric material is placed thereon.

The layers are made in such a way that they are compared to the wavelength of the sound waves, so the light waves are diffracted by the surface sound waves. For this purpose, the width of the optical waveguide is made several times larger than that required for single-mode propagation, and it is possible to have several exits at suitable angles so that the diffracted light waves intersect. is provided.

These devices, fabricated on nickel-sized chips, provide optical transmitters.

The above and other objects, features and advantages of the present invention will become apparent from the following embodiments, which are described by way of example in conjunction with the accompanying drawings.

1 to 8b show embodiments of an optical transmission device according to the invention.

The future of optical communications depends on the rapid processing of large amounts of information by integrated optical circuits (IOCs) that transmit information through optical fibers. Achieving this requires the development of techniques to create reliable and cost effective fiber and integrated optical circuit transmitters and receivers.

Recently, optical fibers with very low loss (~2 dB/km) transmission have been reported, and sufficient photodetectors that couple well with the fibers are commercially available. Additionally, non-coherent detection is also sufficient for integrated optical circuits. Therefore, basic optical fiber and photodetector technology is already possible.

The situation is different for transmitters. Light emitting diodes do not couple conveniently to fibers, and their response times are too long to allow high speed (>100 MHz) data transmission. Single-chip designs include the necessary information processing functions, such as multiplexing, modulation, and switching, on a single chip, although separate semiconductor lasers can be directly modulated faster and more effectively coupled into optical fibers. It is not possible. Thus, the most important technical problems exist in the field of transmitter development. Monolithic manufacturing techniques must be developed to develop reliable and cost effective integrated optical circuit transmitters that meet all circuit functionality requirements.

FIG. 1 shows an integrated optical circuit 30 having the basic functionality required for a high speed data transmitter. injection type surface laser 32
, 34 are three-dimensional waveguides 33, 35, 36, 37, 40
sends out laser radiation. Radiation modulation and switching occur within the waveguide structure and are controlled separately. The entire transmitter is a single chip functional block 31 containing device components compatible with low cost integrated circuit manufacturing.

The material system chosen for the development of such a transmitter is II
IV group semiconductor alloys (Ga, In)As and (Ga,
Al)As. These semiconductors have the properties to perform the basic functions involved in the device of FIG. 1, namely, coherent light emission, passive optical coupling, optical waveguiding, electro-optic modulation, and acousto-optic modulation. . The emission wavelength of the surface laser can be tailored to the low optical loss region of the fiber by changing the alloy composition.

A particular object of the invention is to develop an electrically injected semiconductor laser localized on the surface of the chip. This laser generates a "mirror surface" by plane-facing a mesa crystal grown in vapor phase on an epitaxial substrate through an oxide mask.
A pn junction device that performs reflection may be used.

An integrated optical transmitter has three main components: (1)
It has an injection laser source, (2) a three-dimensional waveguide, and (3) a modulator/switch. The surface laser source is the most important of the three components. A suitable three-dimensional waveguide is one to which the output of the laser is coupled. Since surface-injected lasers can be directly modulated, the development of waveguide modulators is less important.

The optically pumped (Ga,In)As-GaAs surface laser is a vapor-grown mesa structure that combines a waveguiding composite layer and a chalcogenide glass waveguide deposited on a separate substrate for the laser output. It has a phase grating coupled to the tube. The basic concept of semiconductor mesa laser structures is extended from optically pumped devices that require an external laser pump to electrically pumped p-n junction devices that operate with their own internal injection current. This injection laser is designed such that its output is coupled into an epitaxial waveguide on the same substrate.

In this way, a truly integrated single coherent light source is obtained which is coupled to an integrated three-dimensional waveguide occupying the substrate.

INTEGRATED LIGHT SOURCE We now describe in detail a new architecture for the integration of injection semiconductor lasers. The device structure is shown schematically in FIG. 2 and operates as follows. Grown (GaI
n) The pn junction formed in the As alloy mesa 52 is lased by injecting minority carriers (electrons) through the junction. Optical feedback is provided by parallel facing faces of the mesa. The laser output is generated by coupling with a gradually disappearing field.
coupled to an As waveguide. GaAs is (Ga, In)A
Since the energy bandgap is larger than that of the s-mesa, the waveguide can be said to be nearly transparent to mesa laser emission (loss is ~1 dB/cm). Three-dimensional confinement of the radiation in the waveguide is achieved through the overlying structure described below.

The device of FIG. 2 is optically characterized as follows.

GaAs epitaxial film 57 away from the mesa
(n~1016 cm-3) was heavily toped (n~1
018cm-3) Form a waveguide layer on top of the substrate 56. Optical confinement occurs because the refractive index of epitaxial layer 57 is ˜10 −3 greater than the refractive index of substrate 56 . As shown in the TE mode dispersion curve of FIG. 3, confinement occurs when the waveguide thickness d is 1.6λ. λ~
1μm, so single TE0 mode propagation is 1.6μm ■
There are two optical waveguides (one optically active junction region of the mesa) coupled by a lower index buffer layer as seen in d■. The decaying field coupling between these two waveguides (high index regions) is caused by field leakage through the buffer layer (n~1017 cm-3) at the bottom of the mesa. Calculations of the degree of coupling for a similar structure containing an integrated optical circuit injection laser with a step discontinuity in refractive index of ~10-2 indicate that fairly effective coupling occurs for a buffer layer of ~1 μm thickness. It turns out that For the simple pn junction laser of FIG. 2, the refractive index profile near the junction is not an ideal step function and the optical radiation is not very well confined. More effective bonding was actually obtained with thicker (several μm) buffer layers.

Figure 2 and the device can be fabricated using vapor phase epitaxial growth, conductivity type reversal by diffusion and/or ion implantation, and photolithographic masking techniques. First, a uniform GaAs epitaxial film 57 (
A waveguide (waveguide) is grown on a heavily doped GaAs substrate 56. Next, this semiconductor thin piece 51 is coated with a SiO2 film 53.

An opening for mesa growth is provided in this SiO2 film 53 by etching. An n-type mesa 52 is grown through the opening in the mask 53. A SixNy diffusion mask 60 is then deposited over the grown mesa. Zn is diffused or implanted through the etched opening at the top of the mesa to convert top layer 55 to p-type and form a p-n junction. Following formation of the pn junction, metal contacts 61, 62 are connected to the top of mesa 52 and substrate 5.
6. Attach the alloy to the top. The device is completed by soldering the device header and bonding the lead wires.

A new dual source reactor was constructed. Such a device is shown in FIG. In this apparatus, separate gallium and indium metal sources 72 and 73 are exposed to a HCl/H2 gas mixture from conduits 74 and 75. In and Ga
The volatile chloride formed in the reaction between HCl and HCl is a separate input gas, A resulting from the decomposition of AsH3 from conduit 78.
s mixed with steam. The volatile chloride and As vapor then pass over the GaAs substrate 76. Flow conditions and temperature are adjusted to deposit an epitaxial film on the GaAs substrate. Such an open-tube device produces a sudden change in heterogeneous epitaxial (Ga, In)A with an InAs content of about 20%.
The alloy layer used to grow the s-alloy is of n-type with a carrier concentration of about 1Q16 cm-3. Reference numeral 70 is a heating furnace, and portions 70a, 70b, and 70c are heated so that the temperatures of the gallium source 72, indium source 73, and GaAs substrate 76 are set to appropriate values (for example, 750° C. and 850° C., respectively).
℃, 750℃). 80 is reaction tube 7
1 gas outlet.

Two additional features can be added to this reactor. First, supplying excess HCl/H2 through conduit 77 can prevent deposition from occurring at temperatures above the substrate temperature. This feature allows for best control of the components during deposition. Second, In/Ga in the gas phase
It is possible to control the ratio independently. by this,
It becomes possible to gradually change the composition of the film to minimize lattice mismatch effects between the GaAs substrate and the (Ga,In)As alloy. Additionally, using a separate Group III material source simplifies compositional variations in heterogeneous epitaxial growth.

However, even more important than the above features are the
It would be easy to control the gas phase composition to control the growth of faceted mesas of GaAs and (Ga,In)As. By controlling the Group III material/Group V material ratio in the gas phase, the growth rates of the (111) planes of opposite polarity can be made equal. This effect is illustrated in FIG. 5 for epitaxial growth of GaAs. This effect, combined with proper substrate orientation, results in the growth of embossed parallel sides of the mesa. The exact relationship of the (111) planes on the (110) substrate is shown in the plan projection view of FIG.

This projection is surrounded by a diamond shape whose sides are (111) planes. This diamond structure, or a variant thereof, is produced in GaAs and (Ga,In)As after etching into SiO2 on a GaAs (110) substrate.
provides the basis for mesa growth. Changing one of the two factors in the composition disrupts the two-fold axis of rotation of the grown mesa. This effect is also seen in (Ga,In)As alloys, although it is not as drastic as in the case of GaAs mesa growth.

The main feature of the invention is the development of an injection type semiconductor mesa laser. Therefore, Zn diffusion and/or implantation is performed on the GaAs mesa.

This includes masking and photolithographic techniques to define the diffusion area at the top of the mesa. Silicon nitride (deposited with RF plasma) is a good mask and is impermeable to zinc in diffusion using a closed ampoule.

Three-dimensional waveguide integrated optical circuits must incorporate three-dimensional optical waveguides that can be arranged in three-dimensional patterns for modulation, switching, and multiplexing. However, many past studies of optical waveguides have dealt with two-dimensional thin film waveguides. Two-dimensional waveguides do not provide the radiation channeling required for various circuit functions. There are three types of three-dimensional waveguides as circuit configurations in semiconductor epitaxial layers, as shown in FIGS. 7a-7c. Figure 7a shows a buried channel formed by proton bombardment, ion implantation or diffusion; Figure 7b shows a raised or raised channel waveguide formed by chemical etching or ion milling; Figure 7c shows a dielectric. It is a waveguide with a

Buried and raised channel waveguides in III-V epitaxial layers require smooth, well-defined edges for low loss propagation. For waveguides with a dielectric on top, propagation and confinement occur in a two-dimensional membrane, and the requirements on edge regularity are relaxed. Moreover, the demands on the optical properties of the dielectric are not stringent.

As a result of the inventor's studies, the following was established.

(1) High quality single and composite epitaxial GaAs
and (Ga,In)As layer with low loss (~1dB/c
m) waveguiding occurs. (2) Low loss (■
2 dB/cm) three-dimensional waveguiding occurs. Evidence of optical confinement occurring at 1.15 μm in a 7 μm thick vapor grown (Ga,In)As epitaxial layer on a GaAs substrate is shown in Figure 8a.

An abrupt sharpening of the transmitted intensity profile is observed when focusing the incident laser beam 92 onto the epitaxial layer 91 on the GaAS substrate 90 (see FIG. 8a). The waveguide scanning device used for these measurements is No. 8b.
As shown in the figure.

A vibrating mirror 99 repeatedly sweeps the beam magnified by a waveguide output lens system 93 across a narrow slit 94 to produce an intensity profile display on an oscilloscope screen 95 through a phototube 96. The waveguide output is also reflected by the half mirror 100 and is monitored by the °C television monitor 98 through the infrared vidicon 97.

Intensity measurements show that the attenuation of the three-dimensional waveguide is not significantly greater than that of the two-dimensional membrane.

Two basic components of integrated optical circuit transmitters over modulators are discussed: injection laser sources and three-dimensional optical waveguides. The third component needed in the transmitter is a modulator. Although the laser source can be directly modulated by varying the injection current, the device will require additional modulation capabilities (eg, multiplexing) for maximum bandwidth and versatility.

Electro-optic and acousto-optic effects or three-dimensional optics I
Useful for modulating optical beams in II-V waveguides.

Although the invention has been described with respect to specific preferred embodiments, many changes and modifications will become apparent to those skilled in the art. It is, therefore, to be understood that the claims are intended to include all such changes and modifications.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a monolithic integrated optical circuit transmitter;
FIG. 2 is an injection mesa laser/waveguide structure according to an embodiment of the present invention, FIG. 3 is a dispersion curve for the TE mode of the nn waveguide in FIG. 2, and FIG. 3a is an injection mesa laser/waveguide structure in FIG. 2. The refractive index profile of the mesa laser/waveguide structure, Figure 4 is (G
a, In) Gas phase reactor for growing As epitaxial alloys, Figure 5 shows A and B {111}GaAs
Effect of composition on the relative growth rate of the surface, Figure 6 is a {110} stereographic projection of the diamond mesa geometry;
Figures 7a to 7c are possible geometries of three-dimensional optical waveguides, with Figure 7a being a buried channel;
Figure 7b shows a raised channel, Figure 7c shows a waveguide with a dielectric on top, and Figure 8a shows a 7μ thick layer on a GaAs substrate.
1.15 m in (Ga,In)As epitaxial layer
Near field scanning showing optical waveguiding in μm, Figure 8b shows the apparatus used to obtain such a scanning graph. Agent Akira Asamura

Claims (1)

[Claims] A substrate of a semiconductor material selected from a group of materials consisting of III-V semiconductor compounds and ternary mixed III-V semiconductor compositions; an epitaxial waveguide of a semiconductor material of a first conductivity type provided on the substrate; and a mesa of a semiconductor material selected from the group of materials consisting of III-V semiconductor compounds and ternary mixed III-V semiconductor compositions disposed on the epitaxial waveguide layer. a first region of semiconductor material of the first conductivity type in contact with the wave tube layer; and a second region of semiconductor material of the second conductivity type disposed on the first region; ni pn
a laser having a mesa with parallel opposed surfaces defining a junction and both regions being planarized to provide optical feedback; a top of the second region of the mesa and the epitaxial surface of the substrate; electrical contacts each provided on a surface opposite the waveguide side, and connecting said electrical contacts on top of said second region of said mesa to a power source, thereby causing a small number of crosses across said p-n junction by operation of said power source. means for causing laser radiation power to be coupled to the epitaxial waveguide layer by injection of carriers.