JPH03240285A - Bistable semiconductor laser - Google Patents

Abstract

PURPOSE:To control hysteresis characteristic in current vs. optical output charac teristic and optical input/output characteristic of a laser by introducing a superlattice structure to a gain region and a saturable absorption region, and forming a resonator perpendicularly to the growing surface of a crystal. CONSTITUTION:A distributed reflection type reflecting mirror 2 made of a multilayer film of AlyGa1-yAs/AlzGa1-zAs (0<y<z), a saturable absorption region 3 having an AlxGa1-xAs/GaAs superlattice structure, an N-type AluGa1-uAs (u>=w) clad layer 4, a gain region 5 having an AlwGa1-wAs/GaAs superlattice structure, a P-type AluGa1-uAs clad layer 6, and a P<+> type AlyGa1-yAs cap layer 7 are formed on a GaAs substrate 1. Here, the relationship between the (w) and the (x) is that the exciton absorption peak wavelength of the region 5 is equal to or longer than that of the region 3. The (r) of the layer 7 is so set that the wavelength of the band gap of the cap layer is shorter than an oscillation wavelength. Electrodes 8, 9 on the region 3 are for injection a current, and an electrode 10 on the mirror 2 is for applying an electric field to the region 3.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention provides optical logic and optical switching operations that can be used in optical exchanges, optical repeaters, etc., which are expected to be components of optical communication and optical information systems. The present invention relates to bistable semiconductor lasers.

(Prior art) Bistable semiconductor lasers, which have hysteresis phenomena in current vs. optical output characteristics and optical input/output characteristics, are used in functional devices that constitute optical communication and optical information systems (e.g., high-speed optical switches, optical logic operations, optical memories, etc.) It is expected that

FIG. 5 shows a cross-sectional view of a conventional bistable semiconductor laser in a direction perpendicular to the crystal plane. The structure shown in FIG. 5 consists of a P-side electrode of a semiconductor laser that emits light in a direction parallel to the semiconductor growth layer;
Of the n-side electrodes, the electrode located on the epitaxial growth surface side is divided into two or more parts, which are electrically separated from each other with a high-resistance m region 14 in between. In FIG. 5, the case of two electrodes is shown as an example. In FIG. 5, one region 12 is a gain region that generates and amplifies light, and the other region 13 is a saturable region that absorbs light generated in the gain region 12 and guided through the laser resonator. This is an absorption area. When the injected current or external input light is small, the light inside the resonator is absorbed by the saturable absorption region 13, so the resonator loss is large and oscillation does not occur.

When the injection current or input light is increased, the light absorption coefficient of the saturable absorption region 13 depends on the light intensity within the resonator, and the electrons excited to the conduction band by the light accumulate at the bottom of the energy band, resulting in absorption. The coefficient decreases. Therefore, since the resonator loss is reduced, oscillation occurs after a certain excitation intensity is reached.

This situation is shown in FIG.

Kawaguchi (H , Kawaguchi)
and “°Iwane” (Magazine Electronics Letters 1981, 17).
Volumes 167-168 (true). The gain region 12 and the saturable absorption region 13 are formed by multiple quantum wells (multiple quantum wells).
An attempt was made to create an edge-emitting bistable semiconductor laser with a tum well structure that emits light in the same manner as described above.
About aAs/GaAs system
S, Tarucha) and “°Okamo)” (H, Oka
moto) (Magazine Applied Physics Letters, 1986, Vol. 49, 543-545)
page).

As shown in Figure 7, a surface-emitting bistable semiconductor laser has features that are suitable for two-dimensional parallel processing and monolithic formation by using a bulk structure and emitting light in a direction perpendicular to the crystal growth plane. For more information, click here.
(J, Nttta) and Iga (K, Iga) (IEICE Transactions Cl987 J7
0-C, pp. 517-524).

(Problem to be solved by the invention) As proposed by Kawaguchi et al. and Futatsuta et al., a bistable semiconductor laser using a valve structure is capable of absorbing light until saturation of absorption in the saturable absorption region occurs. Since the strength is large, the power consumption during operation is large.Also, the amount of change in the absorption coefficient of the valve structure due to the applied electric field, known as the Franz-Kjelditssch effect, is small, making it difficult to control the hysteresis characteristics by applying an external electric field. Scarce. “Kawaguchi”.

In a typical edge-emitting bistable semiconductor laser, as proposed by et al. and "Talcha" et al., the waveguide region for guiding light is about 5 μm wide and 0.1 μm thick, making it difficult for external input to occur. It is difficult to efficiently couple light into the waveguide region, and it is not suitable for two-dimensional parallel processing, which is thought to be the mainstay of the next generation.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a bistable semiconductor laser which eliminates the above-mentioned drawbacks, has excellent controllability of hysteresis characteristics, and is highly suitable for two-dimensional parallel processing.

(Means for Solving the Problems) Means provided by the present invention to solve the above-mentioned problems are as follows:
In a bistable semiconductor laser, the gain region a has a gain region a that generates and amplifies light, and a saturable absorption region that exists in the resonator and absorbs light and generates bistable operation by saturating its own absorption coefficient. By forming a and a saturable absorption region in the growth direction of the semiconductor crystal, light is emitted perpendicularly to the semiconductor crystal plane, and the gain region a and the saturable absorption region are formed into a two-dimensional superlattice structure. By applying the properties of excitons, that is, the property that the wavelength dependence of the absorption coefficient of a saturable absorption region changes depending on the electric field applied to the saturable absorption region, the so-called quantum confined Stark effect, The electric field applied to the absorption region controls the hysteresis characteristics in the optical output characteristics and optical input/output characteristics of the bistable semiconductor laser.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a cross-sectional view in a direction perpendicular to the crystal growth plane of an embodiment of the present invention.

In this embodiment, AlyGa, AlyGa, and
.

Distributed reflection composed of a multilayer film of As/AIJa+-uAs (0<y<z)
Bragg reflector.

DBR) type reflector 2, a saturable absorption region 3 made of AlGaAs, n-A1. Ga+-uAs
(u≧W) cladding layer 4, A1wGa+-wAs/Ga
Gain region 5 of As superlattice structure (The relationship between w and X is such that the exciton absorption peak wavelength of gain region 5 is equal to or longer than the exciton absorption peak wavelength of saturable absorption region 3. setting), p-A1uGa+-uAs cladding layer 6, p'' AIrGa+-rAs key 117
A cap layer 7 (r is set such that the wavelength of the band gap of the cap layer is shorter than the oscillation wavelength) is grown. DBR
The thickness of the two layers of the anti-shaped reflecting mirror is a value obtained by further dividing one quarter of the oscillation wavelength by the refractive index of each layer, and the thickness of each layer of the saturable absorption region 3 of the superlattice structure is set to a value of 100 Å or less. Also, the thickness of each layer in the gain region 5 of the superlattice structure is 1
Set to 00 Å or less. To form the electrode, first DB
After a mesa is formed by etching up to the R-shaped counter-reflecting mirror 2, a mesa having an area smaller than the saturable absorption region 3 is formed by etching to the depth of the saturable absorption region 3.

The cap layer 7, the electrode 8.9 on top of the saturable absorption region 3,
The electrode IO of the DBR type counter-reflector 2 is used for current injection, and the electrode IO of the DBR type counter-reflector 2 is used for applying an electric field to the saturable absorption region 3. The reflective mirror 11, which requires p-type growth, is a dielectric multilayer film such as TiO□/sio, or a metal vapor deposited film such as Au.

Next, the operation of the bistable semiconductor laser of this embodiment will be explained. FIG. 3 shows the wavelength dependence of the absorption coefficient of the superlattice structure. The oscillation wavelength of the bistable semiconductor laser in this example is
As shown by the arrow in FIG. 3, due to the band gap reduction effect, the wavelength becomes longer than the exciton absorption peak of the saturable absorption region 3, so when the voltage applied to the saturable absorption region 3 is Ov. has low absorption. However, as the reverse bias applied electric field to the saturable absorption region 3 is increased, the exciton absorption peak shifts to longer wavelengths due to the quantum confined Stark effect, and the resonance loss increases, so the oscillation darkness value increases. As the temperature increases, the bistable property becomes more pronounced. 2
The optical intensity of absorption saturation by dimensional excitons is smaller than that of the bulk, so it operates with low power, and the absorption coefficient is large, making it possible to miniaturize the device. Furthermore, since the differential gain coefficient is large due to the quantum effect, it is effective for speeding up and reducing dark values. When the hysteresis is controlled by applying a reverse bias to the saturable absorption region 3, carriers can be sucked out by the reverse bias, so that the switch-off time can be shortened. Furthermore, because of the vertical cavity structure in which the cavity is formed perpendicularly to the crystal growth plane, a two-dimensional arrangement as shown in Figure 4 is easy, making it suitable for two-dimensional parallel processing and capable of monolithic fabrication. It is. In FIG. 4, 34 is a bistable semiconductor laser of the present invention.

The results of numerical analysis of the characteristics of the present invention are shown in FIG.

Figure 2(a) shows the current vs. output characteristics, Figure 2(b) shows the optical input/output characteristics, and Figure 2(C) shows the absorption coefficient at the oscillation wavelength by applying an electric field to the saturable absorption region 3. It shows the current vs. light output characteristics when changed. In the analysis, Nitsuta' (J, N1tta) and "Iga" (K, Iga)
IEICE Journal of Electronics, Information and Communication Engineers, Vol. 5, J70-C, 1987
The procedure described on pages 17-524 was used. Second
In figure (a), the solid line shows the analysis results assuming a superlattice structure, and the dashed line shows the analysis results assuming a bulk structure. Both show hysteresis, but the oscillation dark value current is lower in the superlattice structure. ing. In the analysis, the resonator length was 7 μm.
, the total thickness of the CaAs layer with small band gap energy in the gain region 5 shown in FIG. 1 and the thickness of the gain region 25 shown in FIG. Among them, the total thickness of the CaAs layer with small band gap energy and the thickness of the saturable absorption region 24 shown in FIG. 7 are l grm , and the reflectance is 0.97.
, the wavelength was set to 0.88 μm. In FIG. 2(b), the solid line indicates the analysis result when a superlattice structure is used, and the broken line indicates the analysis result when a bulk structure is assumed, and the same values of the structural parameters as in FIG. 2(a) are used. However, the differential gain coefficients of the superlattice structure and the bulk structure in the gain region are respectively 4X10"'c1.2.57xlO-"c-,
The differential gain coefficient of the superlattice structure and bulk structure in the saturable absorption region was set to 1.5 times the value in the gain region. In addition, the bias current value in the case of a superlattice structure is 15.4 kA/c
n+2, the bias current value in the case of bulk structure is 23 to 4
7 cm". Hysteresis is also obtained in the optical input/output characteristics. Figure 2 (C) shows the analysis results of the electric field dependence of the current vs. optical output characteristics on the electric field applied to the saturable absorption region 3. As the electric field applied to 3 becomes smaller (in the case of reverse bias, the absolute value becomes larger), that is, as the absorption coefficient becomes larger, both the rising threshold and the falling threshold tend to become larger, and the hysteresis width becomes wider. However, in the analysis, the value of the differential gain coefficient of saturable absorption region 3 is 6 Xl0-"cm" (A)
Calculations are made based on 2 times (B) and 1/3 (C).

The above description has been about the AlGaAs/GaAs system, but the InGaAsP/InP system, InGaAs/Ga
Similar actions and effects can be obtained also in the As strained superlattice system.

(Effects of the Invention) As explained above, the present invention introduces a superlattice structure into the gain region 5 and the saturable absorption region 3 in a bistable semiconductor laser, and aligns the resonator in a direction perpendicular to the crystal growth plane. By forming a vertical cavity structure, (1) the wavelength dependence of the absorption coefficient of two-dimensional excitons changes with the applied electric field, making use of the quantum-confined Stark effect, which allows for greater controllability of hysteresis characteristics; (2) Since the light intensity at which absorption saturation occurs is low, operation at low power is possible. (3) Downsizing is possible by taking advantage of the large absorption coefficient due to two-dimensional excitons. (4) Because the differential gain coefficient increases due to quantum effects,
(5) When hysteresis is controlled by applying a reverse bias to the saturable absorption region 3, the switch-off time can be shortened; (6) Vertical resonator structure Therefore, two-dimensional parallel processing and monolithic formation are possible.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view in the direction perpendicular to the crystal growth plane of an embodiment of the present invention, FIG. FIG. 2(1)) is a diagram showing the analysis results of the optical input/output characteristics of an embodiment of the present invention, FIG. 2(C) is a diagram showing the dependence of the current vs. optical output characteristics on the applied electric field according to the present invention, Fig. 3 is a diagram showing the wavelength dependence of the absorption coefficient of the superlattice structure, Fig. 4 is a perspective view of the two-dimensional array configuration of the present invention, and Fig. 5 is a diagram showing the crystal growth plane of a conventional bistable semiconductor laser. Vertical cross-sectional view, Figure 6 (a) is a current vs. optical output characteristic diagram, Figure 6 (b) is an optical input/output characteristic diagram, and Figure 7 is a surface proposed by “Futatsuta'° and Iga”. It is a sectional view of a light-emitting type bistable semiconductor laser. 1...GaAs substrate 2...^!, Ga, -, As/A1.Ga, -, As
DBR thick reflector 3...^1xGa I-XAs/GaAs superlattice structure saturable absorption region 4-n-AluGaI-uAs cladding layer 5...
A1wGa+-wAs/GaAs superlattice structure gain region 6-P-^1uGaI-aAs cladding layer 7''・p
” A1rGa+-rAsAs character 8... P-side electrode of gain region 9... N-side electrode of gain region 10... Electrode for applying electric field to saturable absorption region 11...
・P detection reflector 12...Gain region 13...Saturable absorption region 14...Isolation region 15...N type substrate 16...N type rad layer 17...Narrow gap semiconductor 18 ...p-shaped rad layer 19...n-side electrode 20
P-side electrode 21 of gain region P-side electrode 22 of saturable absorption region N-type TE plate 23 N-shaped rad layer 24 Saturable absorption region (n-type) 25... Gain region (p type) 26... P-shaped rad layer 27... Cap layer 28... N-side electrode 29
...p-side electrode 30...n-side reflecting mirror 31.
... P detection reflector 32... Optical resonance 33... Optical output @1 Fig. Ice East R Fig. 2 ζC) 41 to bow day - Nu Z cheap foot - Gi j season ≧ A pull L -
Figure 6 a) (1)? First input Figure 5 n Figure 7

Claims (1)

[Claims] 1. In a bistable semiconductor laser that has a gain region a that generates and amplifies light and a saturable absorption region that exists in the resonator and absorbs light and generates bistable operation by saturating its own absorption coefficient, the gain By forming the region a and the saturable absorption region b in the growth direction of the semiconductor crystal, light is emitted perpendicularly to the semiconductor crystal plane, and the gain region a and the saturable absorption region b are formed into a two-dimensional superlattice structure. Due to the properties of excitons, the wavelength dependence of the absorption coefficient of saturable absorption region b can be changed by the electric field applied to saturable absorption region b (quantum confined Stark effect), and the saturable absorption region can be made bistable by the electric field applied to it. A bistable semiconductor laser characterized by controlling hysteresis characteristics in the current versus optical output characteristics and optical input/output characteristics of the semiconductor laser.