JPS61190992A - Semiconductor laser with quantum well type optical modulator - Google Patents

Abstract

PURPOSE:To obtain single axial mode beams stably and simply by modulating the transmission characteristics of beams by utilizing the longitudinal electric- field effect of a quantum well layer. CONSTITUTION:A multiple quantum well active layer 3 in thickness of 0.9mum is grown on an N-type InP substrate 1, to which Sn is doped, by alternately superposing undoped InGaAs layers 75Angstrom 10, which lattice-match with InP, and undoped InAlAs layers 75Angstrom 11 at sixty periods in total, and a P-InP layer 5 to which Be is doped as a P-type impurity is grown on the layer 3, thus forming the so-called PIN structure. A Be-doped InGaAs layer 8 is grown on the layer 5 in succession, and a hole having a diameter of 0.5mm is bored to the layer 8 and a P side electrode Au:Zn:Ni 21. Accordingly, a field effect extraordinarily larger than the normal bulk structure is acquired.

Description

[Detailed description of the invention] [Field to which the invention pertains] The present invention relates to a semiconductor laser.

Specifically, it has a simple structure and is easy to manufacture.

This invention relates to a semiconductor laser that oscillates in a single-axis mode at high speed and stably.

[Conventional technology]

As the loss of optical fibers has been reduced, it has become possible to construct optical fiber transmission systems over long distances exceeding 100 km. Such an optical transmission system is affected by the wavelength dispersion of the optical fiber, so a laser that oscillates in a single axis mode is required as a light source.

A normal semiconductor laser oscillates in a single axial mode in a steady state, but when high-speed direct modulation is applied and the magnitude of the injected current is changed, several to more than 10 axial modes are generated. The modulation speed could only be increased to about 400Mh/S at most. Recently, in order to overcome these drawbacks, a so-called distributed transition type (distributed transition type) in which a diffraction grating is provided in the active layer or a waveguide layer provided adjacent to the active layer has been developed.
d Fast Each (abbreviated as DFE) laser has been proposed and is partially used. but.

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As reported by Iwashita et al. on p. 37 (1983), when modulation is applied at high speed, the spectrum broadens, and due to the influence of the wavelength dispersion of the fiber, the initial stage of light emission may be at a long wavelength or a short wavelength (on the center wavelength). Depending on the fiber propagation, the spectrum becomes narrower or wider depending on the fiber propagation, and it is currently not possible to control this.

In addition, two semiconductor lasers with different cavity lengths are arranged in series so that the cavity surfaces are parallel, and current is applied to each independently so that one of the axial modes oscillating at different wavelength intervals from each is aligned. A method of setting the current value to create a single-axis mode has been proposed by Mr. Tzang et al. of Bell Laboratories in the United States (complex optical cavity laser) (Applied P
h1yziaz Letters Volume 42, Page 650 (1
983) :I has a narrow current range in which single mode oscillation occurs and this range differs from element to element, so it is necessary to investigate this range in advance, which poses a problem that makes it difficult to use in practice.

[Purpose of the invention]

In order to eliminate these problems, the present invention uses a quantum well type semiconductor laser, and forms separate electrodes on top of each region so that current can be injected or voltage applied independently to the semiconductor laser substrate, thereby achieving luminescent recombination. A control region and a control electrode are arranged in series with the region to be controlled, and one of them is operated as a light modulator to perform stable single-axis mode oscillation.

[Principle of the invention]

It has been reported that when the thickness of the active layer is reduced to produce a quantum well effect, properties not seen in conventional semiconductors appear, mainly in GaAs/GaAλAs materials (
Journal of Applied Physics, October 1983, pages 843-851
Page), one of which is the low internal absorption loss.

We have found that the absorption loss of InGaAz/In-Az materials is 10 to 70% lower than that of conventional semiconductor lasers. Therefore, when a part of the quantum well type active layer performs oscillation operation, the active layer continuous with the two light emitting regions can function as a low-loss waveguide. or,
In a quantum well structure, when an electric field is applied in the vertical direction (perpendicular to the semiconductor layer forming the well), the absorption edge shifts to the long wavelength side at a relatively low electric field.
z system (Journal of the American Society of Applied Physics AppI1)
．． ied Physics Letters Volume 44,
(1984, pp. 16-18), but it was experimentally found that this effect similarly exists in the long wavelength band 1nGaAr/IrLAIIAs system. Figure 1 is a structural diagram of the sample used in this experiment. F), 5rL (tin)
On a 2×1018 doped n-type 1nP substrate 1, an undoped InGaAz layer 75710 lattice-matched to InP and an undoped IAlA11Az layer 75, 41 are formed.
A multi-quantum well active layer 3 with a thickness of 0.9 μm is grown by molecular beam epitaxy (MEE) for a total of 60 periods in total, and Bt (beryllium) is added as a P-type impurity thereon in a 5 x 1 Q1 layer. crt-” doped P-InP
A layer 5 is grown to form a so-called PIN structure.

On top of that, a Ba-doped InGaAz layer 8 (thickness 0.2 μm
, carrier concentration 5 x 1Q"cm-'), and this layer and the P-side electrode Aμ:Zn:
A hole with a diameter of 0.5 WLm was made in the Ni21. When the light transmission spectrum of this sample was measured using a spectrophotometer, a change appeared in the light transmission spectrum as shown in FIG. 2 due to a vertical electric field in the opposite direction. That is, at a wavelength of 1.5 μm, a decrease in the amount of transmitted light by a factor of 3.8 was observed in an electric field of 5.6XlO'. This electric field effect is unique to quantum wells, compared to ordinary bulk structures.

It's an order of magnitude bigger. This amount of reduction increases in proportion to the square of the magnitude of the applied electric field, so if a higher electric field is applied, the amount of transmitted light decreases and, therefore, the amount of light absorption increases.

The above describes the basic principle of the present invention, and the case where light is incident perpendicularly to the quantum well layer has been described, but the same effect can be expected even if the light is incident parallel to the quantum well layer. Examples will be described below.

<Embodiment 1> FIG. 3(a) is a perspective view and a sectional view showing an embodiment of the present invention.

A diffraction grating 100 is partially formed on the n-1tLP substrate 1. One period of this diffraction grating is (1Av〃) x (
m·'/2) (where -ff is the effective refractive index, chicken is a positive integer, and λ is the oscillation wavelength).

The period of the diffraction grating 100 is 467 when using the second-order period.
07 depth 1soo, 2. (2555 in the first cycle
2, depth 7001) are repeatedly formed in the <110> direction.

The diffraction grating 100 is 4250,2 of a He-Cd gas laser.
It was formed by a two-beam interference exposure method using oscillated light. In order to partially form the diffraction grating 100, 9 diffraction gratings 1
The portion where 00 is not formed may be covered with a 5iOz film. It was then used as a mask. After etching the Ot film No. 5, a normal buried semiconductor laser is manufactured.

However, the active layer has a quantum well structure and υInGaA
Z layer 1oai1o, rruJ, (JF layer 30111 are grown in 6 layers and 5 layers respectively.When forming the P-side electrode 21.22, a 20 μm separation groove 30 is formed on the diffraction grating 100 and other parts. An injection electrode 21 and a control electrode 22 were formed by sandwiching them between them.

The light emitted from the active layer 3 under the injection electrode 21 is guided to the active layer 3 under the control electrode 22 . Of the light emitted from the active layer, only the light having a wavelength corresponding to the period 4670) of the nine diffraction gratings 100 is diffracted and reflected, resulting in strong selectivity in the oscillation spectrum, resulting in single mode oscillation. This is similar to the oscillation mechanism of a normal DFB laser.

If a reverse bias is applied by the control electrode, the absorbed value of the light transmitted through the active layer under the control electrode changes depending on the bias value, so that the transmitted light intensity is modulated. If the current injected into the light emitting region is kept constant, its spectrum will not change; on the other hand, since the control electrode modulates it independently of the injected current, the overall spectrum of light emitted to the outside of the semiconductor laser is stable. And the axis mode is single.

<Embodiment 2> FIGS. 4(α) and 4(b) are a perspective view and a sectional view showing a second embodiment of the present invention. The difference from the first embodiment is that a diffraction grating 100 is formed on the optical waveguide layer 4. This structure oscillated in a single-axis mode, and its wavelength was 1.55 μm at 25° C. and an optical output of 5 nJ′. The oscillation threshold current is as small as 50rILA.

Further, the differential quantum efficiency was as high as 20 cm, which was almost the same characteristic as in Example 1.

<Embodiment 3> FIGS. 5(α) and 5(b) are cross-sectional views showing a third embodiment of the present invention. An nW InP gradient layer (thickness 3cm, Sn doping, carrier concentration 8x1) is formed on an n-type 1nP substrate 1.
Q"tM-').

On top of that, five InGaAz well layers 10 (layer thickness 10 mm,
2. A four-layer l5A2As barrier layer 11 (layer thickness 30, non-doped) is formed between them, and a P-type 1nP first cladding layer 9 (layer thickness 0.2 μm, Z
After growing n-doped, carrier concentration 1×1018cTrr5).

P-type 1nP first cladding layer 9 is partially coated with HCf! , peeled off by selective etching of the system, and only this part was etched with a period of 4670i.
A diffraction grating 100 is formed. On top of this, a P-type 1nP cladding layer 4 (layer thickness 1 μIn, Zn doped, carrier concentration 1
x I Q"z-' ). The other steps thereafter are the same as in the first embodiment.

<Embodiment 4> FIG. 6 is a perspective view showing a fourth embodiment of the present invention. In this method, the intensity of transmitted light is changed by changing the voltage at the portions of the semiconductor laser having the first and second resonator lengths different from each other, thereby performing single intensity modulation. The method of fabricating the laser is similar to conventional methods, in addition to electrically separating the first and second lasers and providing a small spacing between their optical resonators. The active layer is always a quantum well layer in the third laser, but in the first. A quantum well may not be used in the second laser.

In the above examples, the emission wavelength of the active layer was 1.55 μm, but it is not limited to this wavelength. or.

The case where the period of the diffraction grating is second-order has been explained.

It can also be applied to cases where the period is first-order. Furthermore, although the description has been made regarding the buried type laser structure, the invention also applies to lasers with other structures.

[Explanation of effects]

As explained above, the present invention can be applied to all semiconductor lasers that oscillate in a single axis mode in a steady state, since the vertical electric field effect of the quantum well layer can be used to modulate the light transmission characteristics. Single-axis mode light can be obtained stably and easily.

That is, it is possible to make a normal semiconductor laser, which inevitably causes destabilization of the axial mode and broadening of the line width due to the change in injection current due to direct modulation, to stable single-axis mode oscillation. Specifically, by using a quantum well-type active layer in an optical waveguide region in which a diffraction grating is formed, or by using a composite optical resonator, a quantum well layer can be added to a semiconductor laser that oscillates in a single-axis mode. It applies a vertical electric field to that part to change the absorption edge and change the intensity of transmitted light, thereby applying modulation.

[Brief explanation of drawings]

FIG. 1 is a structural diagram of an element for demonstrating the basic principle of the present invention. FIG. 2 is a diagram showing the results, and FIGS. 3(α) and (6) are a perspective view and a sectional view showing the first embodiment of the present invention. FIG. 6 is a perspective view showing a second embodiment of the present invention; FIG. 6 is a perspective view showing a fourth embodiment of the present invention. In the figure. 1 is an n-type IrLP substrate 2 is an n-type 1nP cladding layer 3 is IrLGaAz/In, 41. Az ff well active layer (InGaAz; 10, InAIIAz
11) 4 is InGaAzP optical waveguide layer 5 is P type 1nP cladding layer 6 is P type 1nP buried layer 7 is n type 1nP current confinement layer 8 is P type 1nGαAt cap layer 21.23 is injection electrode 22 is control electrode 30 In the separation groove 40, the n-side electrode 100 represents a diffraction grating.

Claims (1)

[Claims] 1. 1n_1_-_x_1_ which is lattice matched with InP
−_y_1Ga_x_1Al_y_1As (however, 0≦
y1) and In_1_-_x_2-_y_2Ga_x_2Al_y which is lattice-matched to InP.
an active layer having a structure in which barrier layers having a wider forbidden band width than the quantum well layer and made of _2As (y1<y2, 0≦x2) are sequentially and alternately stacked; and one of the active layers. a first cladding layer made of an InP layer having a first conductivity type formed on the surface of the active layer; and a second conductivity type opposite to the first conductivity type formed on the other surface of the active layer. In a quantum well semiconductor laser characterized in that a second cladding layer is formed of an InP layer having A semiconductor laser characterized in that a control region and a control electrode are arranged and formed in series with the active layer that emits light and recombines. 2. The period formed in a partial region of the active layer or a partial region of the waveguide layer provided adjacent to the active layer is (
1/n_e_f_f)×(m・λ/2) [n_e_f_
f: effective refractive index, m: positive integer, λ: oscillation wavelength] A semiconductor laser according to claim 1, characterized in that it has a multilayer laminated structure including a diffraction grating with repeated concavities and convexities given by f: effective refractive index, m: positive integer, and λ: oscillation wavelength. .