JPH07263790A - Light bistable semiconductor laser and its manufacture - Google Patents

Abstract

PURPOSE:To reduce lowering of efficiency caused by reflection and scattering of light in a discontinuous part by providing a quantum well layer whose film thickness continuously changes from a gain region to a saturable region along a proceeding direction of light and which is formed thicker in a saturable region than in a gain region. CONSTITUTION:A quantum well layer 10 and a light confinement clad layer 20 are laminated in a surface of an n-type InP substrate 1, and a thickness of the quantum well layer 10 is continuously changed along a proceeding direction of light gradually. The quantum well layer 10 is formed so that a film thickness thereof becomes thicker gradually from one end part of light input/ output to a gain region 12 in a first tapered waveguide region 11. In the gain region 12, the film thickness is formed uniform. In a saturable absorption region 13, it is formed a slightly thicker than in a gain region 12 and is formed uniform. Furthermore, in a second tapered waveguide region 14, it is continuously formed to be thinner gually as it is far from the saturable absorption region 13 to an end part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical bistable semiconductor laser and a method for manufacturing the same.

[0002]

2. Description of the Related Art Optical fiber communication is widely used for trunk lines of public telephone lines. Recently, high-definition TV
Higher capacity communication services such as broadband ISDN that enables image transmission are planned, and optical fibers will be used not only for conventional trunk communication networks but also for subscriber communication networks including general households. The day it will be introduced is not far away. In such a system, an optical bistable semiconductor laser is required as an optical memory for temporarily holding light information so that light can be exchanged as it is.

The most widely used optical bistable semiconductor laser is a tandem type as shown in FIG.
That is, the active layer 102 is formed in the gain region 110 on the substrate 101, the saturable absorber layer 103 is formed in the saturable region 120, and the active layer 102 and the saturable absorber layer 103 are formed on the saturable region 120. The cladding layer 104 is formed. Also, the clad layer 10
The p-electrodes 105 and 106 divided along the light traveling direction are formed in the gain region 110 and the saturable region 120 above the substrate 4, and the n-electrode 107 is formed on the lower surface of the substrate 101.
In the saturable absorption region 120, carrier current injection is small.

In such an optical bistable semiconductor laser, in order to realize stable operation even when the element temperature changes, the differential gain and carrier life in the gain region 110 for laser oscillation are set to the saturable absorption region. It is essential to be as small as possible compared to 120. In order to optimize the structure in each of these regions 110 and 120,
The gain region 110 and the saturable absorption region 120 must be formed of different semiconductor layers, and the band gap (transition energy) of the saturable absorption region 120 must be slightly smaller than that of the gain region 110. For that purpose, the gain region 12
And, a method of forming the saturable absorption region 13 with different semiconductor layers is required. In the conventional technique, the gain region 12 and the saturable absorption region 13 are grown by separate thin film growth steps,
As a result, different semiconductor materials or different thickness semiconductor layers are formed in the same plane.

[0005]

However, according to the above-described thin film growth process, a discontinuous portion of the material is generated between the semiconductor layers in the two regions, and light is reflected or scattered at the discontinuous portion. There is a possibility that efficiency may drop and operation may become unstable. Further, the bistable laser is often used as a memory for optical signals in optical switching, and in that case, an optical fiber is connected to one side or both sides of the laser to input and output optical pulses. However, in the conventional optical bistable laser, the size of the input / output light beam is about 1 μm, which is greatly different from the core diameter of the fiber of about 10 μm. There is a problem that a lens needs to be interposed, a loss at the time of coupling is large, and it is not easy to manufacture a module with a fiber.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a bistable laser which is easy, highly efficient, stable, and highly reliable, and a manufacturing method thereof. Further, when forming two semiconductor regions having different compositions or layer thicknesses on the same substrate, a continuous semiconductor region is formed so as not to cause scattering at a discontinuous portion, and a high gain bistable laser and a method for manufacturing the same are provided. The purpose is to provide. It is another object of the present invention to provide a bistable laser having a small loss upon coupling with an optical system and a manufacturing method thereof.

[0007]

As illustrated in FIG. 2, the above-mentioned problem is caused by the fact that the saturable region 13 is thicker than the gain region 12 in the light traveling direction, and the saturable region 13 is thicker than the gain region 12. This is solved by an optical bistable semiconductor laser characterized by having a quantum well layer 3 which continuously changes from a region 12 to the saturable region 13.

Alternatively, the quantum well layer 3 has a film thickness changing in a taper shape from the gain region 12 to one end, and a film thickness changing in a taper shape from the saturable region 13 to the other end. It is solved by an optical bistable semiconductor laser characterized by. Alternatively, the dielectric film 7 having the opening 7a wider in the saturable region 13 than the gain region 12 is formed on the main surface of the semiconductor layer 1 and exposed from the opening by the metal organic chemical vapor deposition method. An optical bistable semiconductor including a step of selectively growing a semiconductor layer on the main surface of the semiconductor layer 1 to form a quantum well layer whose layer thickness or composition gradually changes from the gain region 12 to the saturable region 13. This is solved by a laser manufacturing method.

[0009]

[Operation] According to the present invention, since the thickness of the quantum well layer in the saturable absorption region and the gain region is continuously changed, there is little reduction in efficiency due to reflection or scattering of light in the discontinuous portion. And, it operates stably. Moreover, since the thickness and composition between the regions continuously change, reflection and scattering between the regions can be sufficiently reduced, and the operation is further stabilized.

As a method of continuously changing the film thickness of such a quantum well layer, a dielectric film having an opening is used as a mask when the quantum well layer is epitaxially grown by a metal organic chemical vapor deposition method. In addition, a method is adopted in which the width of the opening is narrower in the gain region than in the saturable absorption region. Further, by such a method, the quantum well layer is gradually thinned from the saturable region and the gain region to the end in the light traveling direction. As a result, the optical confinement effect by the quantum well layer is weakened, the beam diameter of the laser can be made closer to the beam diameter of the optical fiber, and the coupling with the optical fiber is improved.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. This quantum well optical bistable laser has a quantum well layer 10 and a cladding layer 20 for optical confinement laminated on the surface of an n-type InP substrate 1 as shown in the schematic view of FIG. The quantum well layer 10 gradually along the direction
Has a structure in which the thickness of is continuously changed.

In the quantum well layer 10, in the first tapered waveguide region 11, the gain region 12 extends from one end of the optical input / output.
The thickness of the gain region 12 is gradually increased until the temperature reaches the upper limit, the layer thickness is uniform in the gain region 12 along the light traveling direction, and the saturable absorption region 13 is formed in the gain region 1.
The thickness of the second tapered waveguide region 14 is slightly thicker than that of the second tapered waveguide region 14, and is continuously formed so that the film thickness is gradually reduced as the distance from the saturable absorption region 13 to the end is increased. .

On the other hand, the first and second gain regions 11 and saturable absorption regions 12 are formed on the cladding layer 20, respectively.
P electrodes 15 and 16 are separately formed in the light traveling direction,
Further, an n electrode 17 is formed on the back surface side of the n type InP substrate 1. The specific layer structure of the semiconductor layers forming the quantum well layer 10 and the cladding layer 20 is as shown in FIG.

That is, on the n-type InP substrate 1, an n-type GaInAsP layer 2 having a composition of about 1.1 μm and an n-type impurity concentration of 5 × 10 ¹⁷ / cm ³ , a non-doped GaInAsP layer 3 and a composition. About 1.1 μm and p-type impurity concentration 5 × 10 ¹⁷ / cm
³ p-type GaInAsP layer 4 and p-type impurity concentration 5 × 10 ¹⁷ /
a p-type InP layer 5 cm ^3, a p ⁺ -type GaInAsP layer 6 of a pair Closing 1.3 .mu.m p-type impurity concentration of 5 × 10 ¹⁹ / cm ³ are sequentially stacked.

The undoped GaInAsP layer 3 has a composition of about 1.
35 μm GaInAsP layer Well layer and GaInAsP composition of about 1.1 μm
It has a multiple quantum well structure in which AsP barrier layers are alternately repeated for 5 cycles, and corresponds to the quantum well layer 10 in FIG. The n-type GaInAsP layer 2 and the p-type GaInAsP layer 4 are SCH.
(Separate confinement heterostructure) layer,
p-type InP layer 5 is clad layer 20 shown in FIG. 1, p ⁺
The GaInAsP layer 6 of the type becomes an electrode contact layer. In addition, p-In
An n-type InP clad layer (not shown) is formed between the P substrate 1 and the n-type GaInAsP layer 2.

Next, a method of forming these semiconductor layers will be described. First, a dielectric film 7 made of silicon oxide having a thickness of 500 nm is formed on the main surface of the n-type InP substrate 1 by a thermal CVD method.
Is formed and is patterned by photolithography as shown in FIG. 3, thereby forming one opening 7a having a changed width along the light traveling direction. The opening 7a has the narrowest width in the saturable region 13, the wider width in the gain region 12, and further, in the two tapered waveguide regions 12 and 14, the gain region 12 and the saturable region. It has a substantially fan-shaped planar shape that gradually spreads away from 13.

The pattern width of the opening 7a of the dielectric film 7 controls the film thickness of the quantum well layer 10 formed in the upper layer of the InP substrate 1. Next, the n-type GaInAsP layer 2 and the GaInAsP layer 2 are formed on the n-type InP substrate 1 exposed from the opening 7a of the dielectric film 7 by the vapor phase growth method using an organic metal.
Quantum well layer 3, p-type GaInAsP layer 4, p-type InP layer 5 and p
^{A +} type GaInAsP layer 6 is sequentially grown. At this time, crystal growth is difficult on the dielectric thin film 7, so that the concentration of the material gas locally rises on the surface of the semiconductor substrate in the vicinity thereof, and the growth rate increases.

As a result, the growth rate of the compound semiconductor becomes faster as the area of the opening 7a of the dielectric film 7 becomes smaller, and the thickness of each layer becomes thicker in the saturable region 13 than in the gain region 12, and The layers formed in the tapered waveguide regions 11 and 14 gradually become thinner as the distance from the gain region 12 and the saturable absorption region 13 increases, and the cross section thereof becomes tapered as shown in FIG.

In this way, the desired film thickness can be freely obtained and can be continuously changed. In the gain region 12, the n-type GaInAsP layer 2 has a resistance of 0.
GaInAsP that constitutes the GaInAsP (quantum well) layer 3 of 1 μm
Well layer 6nm, GaInAsP barrier layer 10nm, p-type GaInAsP
Layer 4 is 0.1 μm, p-type InP layer 5 is 2.5 μm, p ⁺ type
The GaInAsP layer 6 has a thickness of 0.5 μm, the GaInAsP (quantum well) layer 3 has a GaInAsP well layer of 10 nm in the saturable absorption region 13, and the GaInAsP well layer has a thickness of 2 nm at the laser emission end face. The opening 7a is designed so that

After that, the dielectric film 17 is removed, and the planar shape of each of the layers described above is shaped into a desired pattern by lithography, and then the embedding which is usually performed on both sides of the quantum well layer 10 and the cladding layer 20 described above. A structure (not shown) or the like is formed. Next, a Ti layer, a Pt layer, and an Au layer are sequentially laminated on the p ⁺ -type GaInAsP layer 6, and these are patterned to form tandem p-electrodes 15 and 16 in the gain region 12 and the saturable region 13. . Further, an AuGe layer and an Au layer are sequentially laminated on the lower surface of the InP substrate 1 and used as the n electrode 17.

By the way, as a general property of the semiconductor material, when comparing the way of increasing the laser gain when the injection current is increased, that is, the differential gain for each wavelength, the short wavelength of the gain peak at a certain injection current is compared. There is a phenomenon that the change on the side (high energy side) is larger than that on the opposite side. This is because, as shown in FIG. 5, the peak wavelength of the laser gain shifts to the short wavelength side as the injection current increases.

Therefore, in the above-mentioned quantum well structure, the transition energy (corresponding to the band gap energy of the bulk semiconductor) changes depending on the difference in thickness, so that the saturable absorption region 13 of the GaInAsP layer 3 forming the quantum well structure is changed. Since the bandgap energy is set slightly lower than the bandgap energy of the gain region 12 (long wavelength), the differential gain of the saturable absorption region 13 is significantly larger than that of the gain region 12, An extremely stable bistable operation can be realized as compared with the conventional one.

Further, since the thickness and composition between the regions continuously change, reflection and scattering between the regions can be made sufficiently small, and the operation can be stabilized. . In addition, since the tapered waveguide region is formed before and after the gain region and the saturable absorption region that constitute the laser, the thickness of which is gently thin along the traveling direction of the light, the beam diameter of the laser is The diameter can be close to the diameter, and the bondability with the optical fiber can be improved.

As described above, according to this embodiment, a highly efficient and highly reliable quantum well bistable laser can be obtained. The composition and material of each semiconductor layer can be appropriately changed in consideration of the lattice matching with the base and the emission wavelength. The impurity concentration and the thickness of each semiconductor layer are not limited to the examples and may be changed as necessary by changing the pattern of the openings of the dielectric film.

Although GaInAsP is used as the quantum well structure in the above example, it is also possible to employ InGaAs, other compound semiconductors, or strained quantum wells. Depending on the material to be grown, decomposition of the material gas may be used. Since there is a difference in efficiency, it is possible to slightly change the composition. Further, the substrate is not limited to InP, but binary such as GaAs, ternary such as InGaAs, and other mixed crystals may be used.

[0026]

As described above, according to the present invention, since the thickness of the quantum well layer in the saturable absorption region and the gain region is continuously changed, the reflection or scattering of light at the discontinuous portion is caused. It is possible to reduce the decrease in efficiency due to and to operate stably. Further, since the thickness and composition between the regions continuously change, reflection and scattering between the regions can be made sufficiently small, and the operation can be further stabilized.

Further, the quantum well layer in the waveguide region extending from the saturable region and the gain region to the end in the light traveling direction is gradually thinned. As a result, the light confinement effect of the quantum well layer is weakened, the beam diameter of the laser can be made closer to the beam diameter of the optical fiber, and the coupling with the optical fiber can be improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a quantum well bistable laser according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a quantum well bistable laser of the present invention.

FIG. 3 is a substrate plane explanatory view of the quantum well bistable laser.

FIG. 4 is an explanatory cross-sectional view of a main part of the quantum well bistable laser.

FIG. 5 is a diagram showing a relationship between an injection current, a gain, and a wavelength.

FIG. 6 is an explanatory diagram of a conventional quantum well bistable laser.

[Explanation of symbols]

1 n-type InP substrate 2 n-type GaInAsP layer 3 GaInAsP layer (quantum well layer) 4 p-type GaInAsP layer 5 p-type InP layer 6 p ⁺ -type GaInAsP layer 7 dielectric thin film 10 quantum well layer 11 first tapered waveguide region 12 Gain Region 13 Saturable Absorption Region 14 Second Tapered Waveguide Region 15 First p-electrode 16 Second p-electrode 17 n-electrode 20 Cladding Layer

Claims (3)

[Claims] 1. The saturable region (13) is thicker than the gain region (12) along the light traveling direction, and the film thickness extends from the gain region (12) to the saturable region (13). An optical bistable semiconductor laser characterized in that it has a quantum well layer (3) that is continuously changing. 2. The quantum well layer (3) has a taper film thickness changing from the gain region (12) to one end, and has a taper film thickness from the saturable region (13) to the other end. 2. The optical bistable semiconductor laser according to claim 1, wherein 3. A saturable region (1) rather than a gain region (12).
3) a step of forming a dielectric film (7) having a wide opening (7a) on the main surface of the semiconductor layer (1), and the semiconductor layer exposed from the opening by a metal organic chemical vapor deposition method. Selectively grow a semiconductor layer on the main surface of (1),
A method of manufacturing an optical bistable semiconductor laser, comprising the step of forming a quantum well layer having a layer thickness or a composition that changes gradually from the gain region (12) to the saturable region (13).