JPH06105817B2 - Semiconductor laser with quantum well optical modulator - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a semiconductor laser, and more specifically, to a quantum laser which has a simple structure, is easy to manufacture, and oscillates in a single axis mode stably at high speed. The present invention relates to a semiconductor laser with a well type optical modulator.

[Conventional technology]

With the reduction of optical fiber loss, it has become possible to construct an optical fiber transmission system for long distances exceeding 100 km.
In such an optical transmission system, a laser that oscillates in a single axis mode is required as a light source because it is affected by wavelength dispersion of an optical fiber. A normal semiconductor laser oscillates in a single-axis mode in a steady state, but if a high-speed direct modulation is applied to change the magnitude of the injected current, several to 10
Several axis modes were generated, and the modulation speed could only be increased to about 400 Mb / S. Recently, in order to overcome such a drawback, a so-called distributed feedback type (Distribution Feedback Type) in which a diffraction grating is provided in an active layer or a waveguide layer provided adjacent to the active layer
Ted Feed Back (DFB) laser has been proposed and partially used. However, Electronics Letters 19: 937 (1
(983) published by Iwashita et al., The spectrum spreads when modulated at high speed, and it depends on whether the initial light emission is longer or shorter than the central wavelength due to the influence of fiber chromatic dispersion. As a result, the spectrum after fiber propagation becomes narrower or wider, and the current situation is that it cannot be controlled.

In addition, two semiconductor lasers with different cavity lengths are directly arranged so that the cavity surfaces are parallel to each other, and currents are independently applied to them.
Tsang et al. Of Bell Laboratories, Inc. of the United States have proposed a method of setting a current value so that one axial mode oscillating at a different wavelength interval from each other is set to coincide with each other (complex optical resonance Laser) [Applied Physics Letters42
Vol. 650 (1983)], it is necessary to examine this region in advance because the current region for single-mode oscillation is narrow and this region differs from device to device, and there is a problem that it becomes difficult to use in practice. .

[Object of the Invention]

In order to eliminate these problems, the present invention uses a quantum well semiconductor heterostructure formed on a substrate, and allows each region to be independently current-injected or voltage-applied to the quantum well semiconductor heterostructure. A separate electrode is formed on the upper part of the structure to form a light emitting part and a modulating part, and the light emitting part has a built-in diffraction grating, so that stable single axis mode oscillation can be performed even during high speed modulation.

[Principle of Invention]

It has been announced that properties that cannot be seen in conventional semiconductors appear when the active layer thickness is reduced to produce the quantum well effect, mainly in GsAs / GAAlAs materials (October 1983, Journal of Japan Society of Applied Physics). Pp. 843-851). One of them is the low internal absorption loss.
We have found that the absorption loss is as low as 70% in InGaAs / InAlAs based materials. Therefore, when oscillating operation is performed in a part of the quantum well type active layer, the active layer continuous to the light emitting region can function as a low-loss waveguide.
In the quantum well structure, when a vertical electric field (direction perpendicular to the semiconductor layer that forms the well) is applied, development in which the absorption edge shifts to the long wavelength side with a relatively low electric field is reported in the GaAs / GaAlAs system (US Journal of Applied Physics
Letters 44, pp. 16-18, 1984), but experimentally found that this effect also exists in the long-wavelength InGaAs / InAlAs system. Figure 1 is a structural diagram of the material used for this experiment. It is an undoped undoped InP that is lattice-matched with InP on an n-type InP substrate 1 doped with Sn (tin) at 2 × 10 ¹⁸ cm ^-3 .
InGaAs layers 75Å10 and undoped InAlAs layers 75Å11 are alternately grown for a total of 60 periods, and a large number of quantum well active layers 3 with a thickness of 0.9 μm are grown by the molecular beam epitaxy (MBE) method, and then P
A so-called PIN structure was formed by growing a P-InP layer 5 doped with Be (beryllium) as a type impurity at 5 × 10 ¹⁷ cm ^-3 , and a Be-doped InGaAs layer 8 (thickness: 0.2 μm, carrier concentration: 5 × 10 ¹⁸ cm ⁻³ ) was sequentially grown, and a 0.5 mm diameter hole was formed in this layer and the P-side electrode Au; Zn; Ni21 as a light window. When the transmission spectrum of light was measured using a spectrophotometer in this material, a change was observed in the transmission spectrum of light as shown in FIG. 2 due to the vertical electric field in the opposite direction. That is, a decrease in transmitted light amount of 3.8% was observed at an electric field of 5.6 × 10 ⁴ V / cm at a wavelength of 1.5 μm. This field effect is unique to quantum wells, and is orders of magnitude greater than that of ordinary bulk structures.
The amount of decrease increases in proportion to the square of the magnitude of the applied electric field, so that if a higher electric field is applied, the amount of transmitted light decreases and the amount of light absorption increases accordingly.

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

Example 1 FIGS. 3 (a) and 3 (b) are a perspective view and a sectional view showing an example of the present invention.

The diffraction grating 100 is partially formed on the n-InP substrate 1.
This diffraction grating has a period of (1 / n _eff ) × (m · λ / 2) (where n _eff is the effective refractive index, m is a positive integer, and λ is the oscillation wavelength).
It is a diffraction grating with repeated concavities and convexities given by.

When the second period is used as the period of the diffraction grating 100, 4670Å depth 1500Å, (2335Å in the first period, depth 700Å) <110
Repeatedly formed in the> direction. The diffraction grating 100 was formed by a two-beam interference exposure method using an oscillation light of 4250 Å of He-Cd gas laser. In order to partially form the diffraction grating 100, the portion where the diffraction grating 100 is not formed may be covered with a SoO ₂ film. Next, after etching the SiO ₂ film used as a mask, a normal embedded semiconductor laser is manufactured. However, the active layer has a quantum well structure and the InGaAs layer 100Å1
0 and InAlAs layers 30 to 11 are grown in 6 layers and 5 layers, respectively. When the P-side electrodes 21 and 22 are formed, 20 is formed on the diffraction grating 100 and other portions.
A separation groove 30 of μm is sandwiched between the injection electrode 21 and the control electrode 22 to be formed.

The light emitted from the active layer 3 below the injection electrode 21 is emitted from the control electrode 22.
Is guided to the active layer 3 in the lower region of. As for the light emitted from the active layer, only the light having a wavelength corresponding to the period 4670Å of the diffraction grating 100 is diffracted and reflected, so that the oscillation spectrum has strong selectivity and single mode oscillation occurs. This is a normal DF
It is similar to the oscillation mechanism of B laser.

If a reverse bias is applied by the control electrode, the value of the light transmitted through the active layer in the lower region of the control electrode will be changed depending on the value of the bias, and the transmitted light intensity will be modulated. If the current injected into the light emitting region is kept constant, its spectrum does not change, while the control electrode modulates the current independently of the injected current. Is stable and single in axial mode.

Example 2 FIGS. 4 (a) and 4 (b) are a perspective view and a sectional view showing a second example of the present invention. The difference from the first embodiment is that the diffraction grating 100 is formed on the optical waveguide layer 4. The structure oscillates in a single axis mode, and its wavelength is 1.55 μm at a light output of 5 mW at 25 ° C. The oscillation threshold current was as small as 30 mA and the differential quantum efficiency was as high as 20%, which was almost the same characteristic as in Example 1.

<Embodiment 3> FIGS. 5A and 5B are sectional views showing a third embodiment of the present invention. On the n-type InP substrate 1, an n-type InP graded layer (thickness 3 μm, sN-doped, carrier concentration 8 × 10 ¹⁷ cm ^-3 ), and 5 InGaAs well layers 10 (layer thickness 100 Å, non-doped) between them 4 Layer of InAlAs barrier layer 11 (layer thickness 30Å, non-doped) is sandwiched between layers, and the P-type InP first cladding layer 9 is formed thereon.
(Layer thickness 0.2 μm, Zn-doped, carrier concentration 1 × 10 ¹⁸ cm _-3 )
Then, the P-type InP first cladding layer 9 is partially peeled off by selective etching of HCl system, and the diffraction grating 100 having a period of 4670Å is formed only in this portion. P-type InP cladding layer 4 on top of this
(Layer thickness 1 μm, Zn doping, carrier concentration 1 × 10 ¹⁸ cm ⁻³ ) is grown. The other steps thereafter are the same as those in the first embodiment.

<Embodiment 4> FIG. 6 is a perspective view showing a fourth embodiment of the present invention. A laser beam (composite optical resonator laser) whose axial modes are unified by introducing currents into semiconductor lasers having different first and second cavity lengths independently is introduced into a third modulation laser chip. The intensity of the transmitted light is changed by changing the voltage of the part to perform intensity modulation. The laser manufacturing method is the same as the usual method, and in addition to that, the first and second lasers are electrically separated and a small space is provided between their optical resonators. Although the quantum well layer is always used for the active layer in the third laser, the quantum well may not be used in the first and second lasers.

In the above embodiments, the emission wavelength of the active layer was 1.55 μm, but the wavelength is not limited to this. Further, the case where the period of the diffraction grating is the second order has been described, but the case where the period is the first order is also applicable. Further, the laser structure has been described as an embedded type, but it is also applicable to lasers having other structures.

[Explanation of effects]

As described above, according to the present invention, since the transmission characteristic of light is modulated by utilizing the longitudinal electric field effect of the quantum well layer, it can be applied to all semiconductor lasers that oscillate in a single axis mode in a steady state. It is possible to stably and easily obtain the single-axis mode light. That is, it is possible to make a stable single-axis mode oscillation in a normal semiconductor laser in which the axial mode is destabilized and the line width is inevitably changed because the injection current is changed by the direct modulation. Specifically, by using a quantum well type active layer in the optical waveguide region in which the diffraction grating is formed or by using a compound optical resonator, the quantum well layer is attached to a semiconductor laser that oscillates in a single-axis mode. A vertical electric field is applied to change the absorption edge to change the intensity of transmitted light and modulate.

In the present invention, the use of InP as the cladding layer has the following advantages.

1.InP has a binary composition, so its thermal conductivity is much higher than that of a ternary composition (usually about an order of magnitude higher than that of a mixed crystal system).

Therefore, in order to perform laser operation, In
The structure sandwiched between P is advantageous and suitable for high temperature operation.

2. The cladding layer InP according to the present invention is easy to regrow with respect to the thick cladding layer (including the substrate InP), and is extremely advantageous for use as a buried layer. Furthermore, since the optical refractive index of InP is smaller than that of InAlAs of the active layer, a difference occurs in the optical waveguide characteristics, and a single transverse mode (in the thickness direction) is easy to form.

And so on.

[Brief description 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 (a) and 3 (b) are the first embodiment of the present invention. 4A and 4B are perspective views and sectional views showing a second embodiment of the present invention, and FIGS. 5A and 5B are third perspective views of the present invention. FIG. 6 is a sectional view showing an embodiment, and FIG. 6 is a perspective view showing a fourth embodiment of the present invention. In the figure, 1 is an n-type InP substrate, 2 is an n-type InP cladding layer, 3 is an InGaAs / InAlAs quantum well active layer (InGaAs; 10, InAlAs; 11), 4 is an InGaAsP optical waveguide layer, 5 is a P-type InP cladding layer, and 6 is a P-layer. The type InP buried layer 7 is the n type InP current confinement layer 8 is the P type InGaAs cap layer 21, 23 is the injection electrode 22, the control electrode 30, the separation groove 40, and the n side electrode 100 is a diffraction grating.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yuzo Yoshikuni 1839 Ono, Atsugi City, Kanagawa Pref., Atsugi Electro-Communications Research Laboratories, Nippon Telegraph and Telephone Corporation (72) Inventor, Hitoshi Asahi 1839, Ono, Atsugi City, Kanagawa Prefecture (56) Reference JP 59-165480 (JP, A) JP 59-139691 (JP, A) JP 59-181588 (JP, A) Applied Physics Letters 42 [10] ], 15 May 1983 P.M. 845 ~ 847

Claims (1)

[Claims] 1. A quantum well layer made of InGaAs lattice-matched with InP and a barrier layer having a wider band gap than the quantum well layer made of InAlAs lattice-matched with InP are alternately arranged. A first active layer having a laminated structure, a waveguide layer formed on one surface of the active layer, and an InP layer having a first conductivity type formed on the surface of the waveguide layer. 1 having a second conductivity type opposite to the first conductivity type formed on the other surface of the cladding layer and the active layer.
A quantum well semiconductor heterostructure in which a second cladding layer made of a P layer is formed, and has a period of (1 / n _eff ) × (m · λ / 2) [n _eff : effective refractive index,
m: a positive integer, λ: oscillation wavelength], and a diffraction grating having a concavo-convex pattern is formed on a part of the active layer or a part of the waveguide layer, and the first electrode includes the diffraction grating. A semiconductor laser with a quantum well type optical modulator, wherein a second electrode separated from the first electrode is formed on each of the regions except the diffraction grating.