JPS63312688A - Semiconductor laser and application thereof - Google Patents

Abstract

PURPOSE:To obtain laser beams having different wavelengths with a semiconductor laser by composing active layers of the semiconductor laser with a plurality of quantum well layers having different energy levels and by providing the control part of light absorption in a resonator and further, by mounting electrodes for applying currents so as to control active layer parts and the control part of light absorption independently, thereby controlling oscillation wavelengths through controlling light absorption. CONSTITUTION:This device is equipped with; active layers 4a and 4b which are composed of a plurality of quantum well active layers where their discrete energy levels are different one another; a control part B of light absorption which is mounted in a resonator; and electrodes for applying currents 12a and 12b which are independently prepared at the above control part B and at the active layer A other than the control part B. Since the device is able to emit laser beams having specific wavelengths by controlling respective electrodes independently, the laser lights oscillate at a number of wavelengths which have short intervals of light-emitting and are satisfactorily controlled. Then, multi-wavelength oscillation type semiconductor laser beams not only make it possible to perform switching but also are applicable to wide fields as light amplifiers, sensors, and the like.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser whose oscillation wavelength can be controlled.

[Conventional technology]

Figure 9 is, for example, published in Applied Physics Letters, Vol. 39, pp. 134-137 (Appl, Phys.
.

Lett, Vol. 39. pp134-137
) is a cross-sectional view showing the conventional quantum well semiconductor laser shown in FIG.
Substrate, 2 is n-Af2Gaz-, As kraft layer, 3 is n-, 6, f2Qaz-+As parabolic gradient index layer (2
gradually changes to y), 4 is p Alx Gat-x
As'! Koido oil well active layer p-AN, Gat-y A
s parabolic gradient index layer (y gradually changes to 2), 7 is p
-A12G a +-z A S cladding layer, 8 is p”
-GaAs cap layer.

Next, the operation will be explained.

When a thin semiconductor layer is sandwiched between semiconductor barrier layers with a large bandgap as shown in Figure 7, this thin semiconductor layer forms a potential well layer, and the electrons (or holes) confined in this well are Its specific energy En(
(when measured from the bottom of the conduction band) is Schrodinger
From the equation, n=1.2, 3+ (1), and discrete energy levels are formed. Here m
e'' is the effective mass of the electron; i is the blank constant divided by 2π; Lz is the thickness of the quantum well layer.

In this way, the electron has quantized energy En,
As shown in FIG. 8, the electron state density ρ(F,) is parabolic in the bulk crystal as shown by the broken line, but becomes step-like in the quantum well as shown in the solid line.

Therefore, if a quantum well layer is used as an active layer and a p-type semiconductor layer and an n-type semiconductor layer with a large band gap are used on both sides, carriers (electrons and holes) and light can be confined, creating a quantum well semiconductor laser. be able to.

If the semiconductor laser made in this way is made of the same material as the active layer bandgap (forbidden band width) compared to a semiconductor laser constructed with a normal double heterojunction, it will be comparable to a quantum well semiconductor laser. In this case, since the energy level of n-1 is higher than the bottom of the conduction band (solid line in Figure 8),
Compared to a normal double heterojunction semiconductor laser, which oscillates due to the energy difference between the bottom of the conduction band and the ceiling of the valence band, the laser oscillates at a shorter wavelength because the energy difference is larger. Furthermore, since the energy levels of the quantum well semiconductor laser are discrete, the spectral linewidth is narrow and a laser beam with good monochromaticity can be obtained.

The conventional example shown in FIG. 6 will be explained.

First, an n-
A1. Gaz-1As 2 is grown, followed by a parabolic index gradient layer n-Ajtz Gag-I As (z gradually changes to y) 3, a quantum well type active layer p-AI, G
at-XAs 4, parabolic graded index layer p-A'y c
a, -, As (y gradually changes to 2) 6, p-A
lz Gat-z As craft layer 7, p゛-GaAs
The cap layer 8 is grown (z>y>x.) The energy band structure diagram of the cladding layer, graded index layer, and active layer of the semiconductor laser thus produced is shown in FIG. 6(b). Shown below.

In this conventional device, there are regions on both sides of the active layer where the composition ratio of A1 is gradually increased as the distance from the active layer increases, so the refractive index as well as the band structure has a parabolic shape on the outside of the active layer 4. The refractive index decreases, and the light emitted from the active layer 4 is confined by this parabolic index gradient layer.If the end face perpendicular to the active layer 4 is formed as a reflective surface and lateral confinement is performed, a laser beam is generated. It becomes possible to oscillate. This type of laser uses a parabolic graded index waveguide and a structure that separates carrier and light confinement.
-1ndex waveguide and 5ep
arate carrier and opt-ica
l confine+5ents) laser.

In this conventional device as described above, when carriers (electrons, holes) are injected by biasing it in the forward direction, the carriers are confined in the quantum well active layer 4, and electrons and holes are exchanged between discrete energy levels. The holes recombine and emit light. At this time, a sharp emission wavelength peak corresponding to one unit of energy is obtained. Then, at -i, the gain at nwl becomes larger than the resonator loss, and oscillation occurs at the quantum level of n=1, but
As the injection current is greatly increased, carriers will combine not only between the quantum energy level n=1 of the conduction band 9 but also between the n=2 level and the corresponding level of the valence band 10. This makes it possible to generate n=2 oscillations. This n=2
Since the oscillation wavelength of is higher in energy than that of n-1, it has a shorter wavelength than the oscillation wavelength of n=1.

[Problem that the invention seeks to solve]

Since the conventional quantum well type semiconductor laser is constructed as described above, the oscillation wavelength difference corresponding to the energy difference ΔE+z between the quantum energy levels n=1 and n-'l is, for example, when the quantum well active layer has a thickness of 100 mm. In the case of human GaAs layer, ΔE,
There were problems in that t was about 110 meV and the wavelength difference was 570, which was large, and it was necessary to make a large injection current difference in switching from n=1 oscillation to n=2 oscillation.

This invention was made to solve the above-mentioned problems, and it is possible to reduce the interval between multiple wavelengths of laser oscillation, and to change the oscillation wavelength with a small difference in injection current, resulting in high controllability. The purpose is to obtain a semiconductor laser.

[Means for solving problems]

A semiconductor laser according to the present invention is a semiconductor laser having an active layer with a quantum well structure, in which a plurality of quantum wells are used as the active layer, and the discrete energy levels of the respective quantum wells are different from each other, and light is transmitted within resonance. A portion for controlling the amount of absorption is provided, and an independent current injection electrode is provided for each of the active layer portion and the control portion.

Further, the method of using the semiconductor laser according to the present invention includes an active layer formed by placing a plurality of quantum well layers having different quantized energy level structures in close proximity to each other via a barrier layer, and a resonator inside which causes laser oscillation. Use of a semiconductor laser comprising a light absorption amount control section including a part of the active layer provided in the above, the remaining active layer section, and a current injection electrode provided separately in the light absorption amount control section. The method comprises injecting a current below a threshold value at which laser oscillation occurs in at least one of the plurality of quantum well layers of the active layer, and coupling light from the outside through one end face of the resonator. It emits laser light of a specific wavelength.

[Effect]

In this invention, an active layer composed of a plurality of quantum well active layers having different discrete energy levels, a light absorption amount control section provided in a resonator, and a light absorption amount control section and other components are provided. Each of the active layers is equipped with a current injection electrode provided independently, and each of the electrodes is independently controlled to emit a laser beam of a specific wavelength, so the controllability is good and the emission wavelength interval is small. It oscillates at multiple wavelengths.

Furthermore, in the method of using the semiconductor laser of the present invention,
A current below the threshold value at which laser oscillation occurs is injected into at least one of the plurality of quantum wells of the semiconductor laser configured as described above, and light is incident and coupled from the outside through one end face of the resonator to absorb light. Since laser light of a specific wavelength controlled by the amount control section is emitted, amplification and switching can be performed using external light.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

In Fig. 1, 1 is an n''-GaAs substrate, 2 is an n-A
t1□Gaz-, As cladding layer, 3 is n-Alz G
a2-I As parabolic refractive index distribution N (z gradually changes to y), 4a is the first Aj! , lGa+-x AS active layer, 4b is the second AIXGal-XAs active layer, 5 is AI, Ga, -, As barrier layer, 6 is p-Al,
Ga, -, As parabolic graded refractive index layer (y gradually changes to 2), 7 is p-Alz Ga1-2 As cladding layer,
8a and 8b are p''-GaAs key 1-7 layers, 11 is an n-side electrode, and 12a and 12b are p-side electrodes.

Next, manufacturing of this example will be explained. First, n”-G
On the aAs substrate 1, cladding Nn-A 1zGa,-
2As layer 2, parabolic index gradient layer n-AIZGaI-7
A3 (z gradually changes to y) 3゜first activity]'
iA IX G a r-* A sll child well N 4
a, barrier layer AJ, Ga, -, As 5, second active layer A11XGa, -xAs quantum well Ji14b, parabolic graded refractive index layer p-A lyG a +-y A 3 (
y gradually changes to 2) 6, cladding Np Alz
A Ga1-z AS layer 7 and a p"-GaAs camp layer 8 are sequentially grown. Furthermore, an n-side electrode 11 is grown on the n'"-GaAs substrate l side, and a p"-GaAs layer 8 is grown on the cap layer side.
By forming the type electrodes 12a and 12b and forming a laser resonator mirror by cleaving or the like perpendicular to the active layers 4a and 4b, a quantum well type semiconductor laser having a plurality of quantum well active layers is completed.

FIG. 2 shows two quantum well active layers 4a with a barrier layer 5 in between.
, 4b, which shows the band structure when arranging the active layers 4a and 4b, have a thickness different from that of L 2 + + L 2 t. Here, according to the Tl1 equation, when Lz becomes smaller, the energy measured from the bottom of the conduction band becomes larger at the discrete energy level. Therefore, if the Lz, , LzgO values are well designed, the n=1 energy level of the active layer 4a can be set between the n=1 and nx2 energy levels of the active layer 4a (Lz, >Lzt).

Next, the operation will be explained. First, when the part A of the semiconductor laser having the active layer as described above is biased in the forward direction and carriers (electrons, holes) are injected, firstly, between the n=l level of the conduction band 9 of the active N4a and the valence band. Coupling of electrons and holes occurs at a wavelength of λ1, and when carriers are further injected, coupling occurs between the n=1 level of the conduction band 9 of active FJ4b and the valence band, and at a wavelength of λ. Emits light. Next, coupling occurs between the n=2 level of the conduction band 9 of the active layer 4a and the valence band, and light is emitted at a wavelength of λ. In this way, when the injection current is increased, from λ to λ2. It is possible to obtain a laser whose wavelength is gradually shorter than λ3.

Next, a case will be described in which the oscillation wavelength is controlled by applying an electric field using the electrodes 11 and 12b to the portion B where the amount of light absorption is controlled.

Figure 6(a) shows the gain at each quantum level when a certain value of current is applied to the quantum well active layer part A, n-
1, n'=1. n=2 is a peak corresponding to the transition between the 43 lowest quantum units of electrons and holes, the lowest quantum level of 4b, and the higher quantum level of 4a, respectively.

On the other hand, the exciton peaks in the optical absorption spectrum when no reverse bias is applied are on the slightly higher energy side (i.e., shorter wavelength side) than the corresponding laser oscillation wavelengths, as shown in Figure 3 (C). .

When part B is not reverse biased, the gain at the n=1 level exceeds the resonator loss including optical absorption, and laser oscillation occurs at this level. FIG. 3(b) shows this situation.

When an electric field is applied to a semiconductor, Franz-Keldysh
A phenomenon called "effect" occurs in which the absorption spectrum of a semiconductor shifts toward longer wavelengths. In this case, the amount of shift of the exciton peak on the higher energy side is smaller than the amount of shift of the exciton peak on the lower energy side.

Therefore, by applying a reverse bias to part B, the optical absorption spectrum shifts to the longer wavelength side as shown by the broken line in Figure 3 (C). is smaller than the light absorption at the n-1 energy level.The increase in loss due to the increase in light absorption controls the oscillation at the n-1 level.This causes stimulated emission. Population consumption by
The carriers come to occupy the n'=1 level, and as shown in FIG. 3(C), the light absorption loss at n'=1 is relatively small, so as shown in FIG. 3(d), the n' Laser oscillation at a wavelength corresponding to the level between '=1 becomes possible.

By applying a reverse bias to portion B in this manner, the laser wavelength can be switched without changing the amount of current applied to portion A.

As described above, in this example, the active layer is composed of a plurality of quantum well layers with different energy levels, and light absorption amount control is provided, so that a laser beam with a plurality of wavelengths can be generated without greatly changing the current applied to the portion A. Not only can it oscillate, but
Since the configuration is such that the oscillation wavelength can be switched by applying a reverse bias to portion B, a multi-wavelength oscillation laser with high controllability (and small wavelength difference) can be obtained at low cost.

In addition, in the above example, the thickness of the quantum wells was changed in order to change the energy level between the quantum wells, but
As shown in Figure 4, without changing the thickness of the quantum well (L2+
= LZz ), the same effect can be obtained by changing the material composition (Xz >x,).

In addition, in the above embodiment, GRIN is used to confine the laser light.
Although the case using the -SCH structure has been described, other confinement methods can also be applied.

Further, in the above embodiment, a case was described in which a GaAs-based material was used, but it may also be an InP-based material or another material, and the same effects as in the above embodiment can be expected.

Further, in the above embodiment, the case where the number of quantum well layers in the active layer is two is explained, but it may be three or more (also, the number of quantum wells having the lowest energy level is The number of quantum wells with a higher lowest energy level may be increased to facilitate oscillation at a lower energy level, and quantum wells with different thicknesses and material compositions may be used. Alternatively, barrier layers having different energy levels may be combined.

Furthermore, in the above embodiment, a case was described in which the energy levels between multiple quantum well layers do not overlap, but as shown in FIG. 5, the thickness d of the barrier layer is made thin so that the energy levels overlap with each other. Good too.

Further, as a special method of using the semiconductor laser of the present invention, a plurality of quantum well active layers 4a, 4 having different emission wavelengths may be used.
If at least one of b is forward biased to the vicinity of the threshold value that causes laser oscillation, and light with a mixture of multiple wavelengths is incident and coupled from the outside through the cavity end face, the part that is the light absorption amount control section can be It is possible to selectively amplify only the light of a specific wavelength controlled in B and take it out as a laser beam, and by using it in this way, switching by light, direct amplification, etc. can be performed.

〔Effect of the invention〕

As described above, according to the present invention, the active layer of a semiconductor laser is composed of a plurality of quantum well layers having different energy levels,
In addition, a light absorption amount control section is provided in the resonator, and a current injection electrode is provided to independently control each of the active layer section and the light absorption amount control section to control the light absorption amount and control the oscillation wavelength. Because of this configuration, laser beams with different wavelengths can be obtained with a single semiconductor laser with a simple structure, and it is possible to obtain a product with high precision at low cost.

In addition, as described above, the method of using the semiconductor laser of the present invention is to generate laser oscillation with an active layer formed by placing a plurality of quantum well layers having different quantized energy level structures in close proximity to each other via a barrier layer. a light absorption amount control section including a part of the active layer provided in the resonator, the remaining active layer section, and a current injection electrode provided separately in the light absorption amount control section. A current below the threshold value at which at least one laser oscillation occurs is injected between the plurality of quantum wells in the active layer of the semiconductor laser, and light is incident and coupled from the outside through one end surface of the resonator to generate a specific Since it emits laser light of different wavelengths, it not only enables light-based switching of multi-wavelength oscillation type semiconductor lasers, but also has the effect of being able to be applied to a wide range of applications such as optical amplifiers and sensors.

[Brief explanation of drawings]

FIG. 1 is a structural diagram of a quantum well type semiconductor laser according to an embodiment of the present invention, FIG. 2 is a quantum well energy band structure diagram for explaining the operation of the embodiment of FIG. 4 is an energy band structure diagram showing another embodiment of the present invention. FIG. 5 is a diagram for explaining the operation of the embodiment shown in FIG.
The figure is an energy band structure diagram showing still another embodiment of the present invention, FIG. 6 is a semiconductor laser structure diagram showing a conventional embodiment, and FIG. 7 is a quantum well structure diagram to explain the operation of this conventional example. , FIG. 8 is a diagram showing the density of states and energy levels of a quantum well. In the figure, l is the substrate, 4 a, 4 b, 20
a. 20b is a quantum well active layer, 5.25 is a barrier layer, 11 is an n-electrode, and 12a and 12b are p-electrodes. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (8)

[Claims] (1) In a semiconductor laser having a quantum well structure, there is an active layer made up of a plurality of quantum well layers having different quantized energy level structures placed in close proximity to each other via a barrier layer, and a cavity inside which causes laser oscillation. and a current injection electrode provided separately in the remaining active layer portion and the light absorption amount control portion. A semiconductor laser featuring: (2) The semiconductor laser according to claim 1, wherein the plurality of quantum well layers are made of materials having the same composition and have different layer thicknesses. (3) The semiconductor laser according to claim 1, wherein the plurality of quantum well layers have the same thickness and are made of materials with different compositions. (4) The plurality of quantum well layers have different materials and layer thicknesses, or are constructed by combining barrier layers with different energy heights. The semiconductor laser described. (5) The semiconductor laser according to any one of claims 1 to 4, wherein the thickness of the barrier is so thin that the energy levels of the plurality of quantum well layers overlap each other. (6) Claim 1 characterized in that the number of quantum wells having the lowest lowest energy level is greater than the number of quantum wells having a higher lowest energy level. 6. The semiconductor laser according to any one of items 5 to 5. (7) The part that controls the amount of light absorption is formed of an optical waveguide layer having a forbidden band width larger than that of the quantum well layer of the active layer, and by applying a reverse electric field, the light of the optical waveguide layer is 7. The semiconductor laser according to claim 1, wherein the amount of absorption is controlled. (8) A light absorption amount control section including a part of the active layer formed by adjoining a plurality of quantum well layers having different quantized energy level structures via a barrier, and the remaining active layer section and the light absorption amount control section. In a method of using a semiconductor laser including a current injection electrode provided separately in an absorption amount control section, a current below a threshold value at which laser oscillation occurs is applied to at least one of the plurality of quantum well layers of the active layer. 1. A method of using a semiconductor laser, which comprises injecting light into the resonator, coupling light from the outside through one end face of a resonator, and emitting laser light of a specific wavelength.