JPH03235390A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To make possible an oscillation in a stable single mode in a simple periodic structure by a method wherein an active layer is formed into a quantum well structure. CONSTITUTION:An n-type buffer layer 2, an active layer 9 of a quantum well structure and a p-type clad layer 7 are provided in order on an n-type substrate 1 and thereafter, n-type regions 8 are formed periodically on the layer 7 and moreover, a p-type clad layer 5 and a p<+> cap layer 6 are formed. By this constitution, when a forward current is made to flow, an injection current flows periodically. Since the layer 9 has the quantum well structure, the gain coefficient of the layer 9 is high. Moreover, an oscillation in a stable single mode becomes possible by setting a Bragg wavelength of a periodic structure on the side of a wavelength longer than an absorption edge wavelength of the layer 9.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser, and particularly to a semiconductor laser device that oscillates in a single mode.

[Prior Art] Conventionally, as this type of semiconductor laser element structure, a distributed feedback semiconductor laser (DFB-LD) shown in FIG. 2 is known. This DFB-LD includes an n-type buffer layer 2. an undoped active layer 3; a guide layer 4 having a structure in which the thickness changes periodically; It has a structure in which a p-type cladding layer 5 and a p3-type cap layer 6 for making ohmic contact with an electrode are sequentially formed. The arrows in the figure indicate the emitted light. DBF- with such a structure
The LD is good (as is known, the oscillation threshold gain decreases equally at the frequencies at both ends of the stop band determined by the periodic structure of the guide layer 6, so oscillation is unlikely to be uniform. Avoid this. To make the periodic structure uneven,
There is a method to create a single mode by shifting the phase by λ/4 midway, but it is difficult to create such a periodic structure, and the oscillation mode may change due to external reflected light injection. There was a drawback as mentioned.

In order to overcome this drawback, a semiconductor laser having the structure shown in FIG. 3 has been considered. The structure of this laser is different from that of the B-LD shown in FIG. 2, in addition to the guide layer 4 whose thickness changes periodically, an n-type block region 8 is formed on a flat p-type cladding layer 7. These points are provided periodically.

Now, when a power supply is connected so that a forward current is injected in the structure shown in FIG.
The junction becomes reverse biased, and the lower part of the n-type region 8 becomes a region where forward current is blocked and is not injected. Therefore, if there is periodicity in the injected carriers, periodicity will occur in the gain,
As a result, a gain distribution type B-LD structure is obtained. It has been pointed out by Kogelnik et al. that the gain distribution type DFB-LD is different from the normal refractive index distribution type DFB-LD, and the oscillation threshold gain is minimized at the center frequency of the Bragg wavelength (References, H, Kogelnik and C,V
, 5hank, “Coupled-wavetheo
ry of distributed feedback
k1asers, J.

Appl, Phys,, vol. 43, No.
5. pp, 2327-2325°1972, ).

Therefore, although the semiconductor laser shown in FIG. 3 theoretically has a single oscillation mode, it is difficult to oscillate it in reality for the reasons described below.

FIG. 4 shows the change in absorption coefficient of the active layer with respect to the amount of minority carriers injected. Curve A represents the change in the active layer of the structure of FIG. 2, and curve B represents the change of the absorption coefficient in the active layer of the structure of FIG. In the structure shown in FIG. 3, carriers are not injected into the half region of the active layer, that is, the region below the n-type block region 8, so the injection amount in FIG. It will be a big loss. As a result, as shown in curve B, even if the amount of injected carriers was increased, a gain state could not be achieved, making it difficult to cause oscillation.

1 Problems to be Solved by the Invention 1 As mentioned above, the conventional graded index B laser has the disadvantage that it is difficult to fabricate a periodic structure, the oscillation mode fluctuates in response to external reflected light injection, and the gain Distributed DFB
Laser oscillation itself is difficult.

An object of the present invention is to provide a semiconductor laser device that causes single mode oscillation with a simple periodic structure and is strong against reflected light.

[Means for Solving the Problems 1] In order to achieve such an object, the semiconductor laser of the present invention has a semiconductor laser having a junction of first and second conductivity type regions different from each other along the propagation direction of light. A semiconductor laser having a junction of periodically arranged second and first conductivity type regions is characterized in that the active layer has a quantum well structure.

1 Effect 1 In the semiconductor laser according to the present invention, the forward current injection region in the active layer has periodicity, and the active layer has a quantum well structure, so the gain coefficient is high, and the Bragg wavelength of the periodic structure is low. By setting the wavelength on the longer wavelength side than the absorption edge wavelength of the active layer, stable single mode oscillation is possible.

[Example 1 The present invention will be explained below by way of examples.

FIG. 1 is a sectional view illustrating the structure of an embodiment of the present invention. In this example, n-type InGa is formed on an n-type InP substrate 1.
Buffer layer of AsP (ng=1.3 um) 2°InG
The active layer 9 has a quantum well structure consisting of a laminated structure of aAsP and InP and has an absorption edge wavelength of 1.55 μm, and a p-type!
After successively forming a cladding layer 7 of nGaAsP (λg=1.3 μm), n-type InP regions 8 are periodically formed on the cladding layer 7, and further p-type InP cladding layer 5 and p″″-type InGaAsP are formed. A cap layer 6 of /InP (band gap wavelength λg=1.55 μm) is formed.

FIG. 5 compares the gain and absorption wavelength characteristics when carriers are injected into the active layer and when carriers are not injected into the active layer, when the active layer has a quantum well structure, and when the active layer has a normal bulk structure. Curves E1 and F2 represent the characteristics when carriers are injected and not injected into the active layer of the quantum well structure, respectively, and curves F1 and F2 represent the characteristics when carriers are injected and not injected into the active layer of the bulk structure, respectively. show. The curve E1 corresponds to the characteristics of the portion of the quantum well active layer other than the lower part of the n-type region 8 when a forward current is applied to the embodiment shown in FIG.
This corresponds to the characteristics of the lower part of the mold region 8. Similarly, the curve F1 corresponds to the characteristics of the area other than the lower part of the n-type region 8 of the bulk active layer of the conventional example in FIG. 3, and the curve F2 corresponds to the characteristics of the lower part of the n-type region 8.
When a forward current is caused to flow in the structure of the embodiment shown in FIG. 1, the injected current has periodicity as explained with reference to FIG.

By the way, the gain-wavelength characteristic when carriers are injected into the active layer of a quantum well structure is as shown by curve E in FIG. The wavelength may be longer than the absorption edge wavelength rounding when there is no carrier injection. On the other hand, in the case of the conventional bulk structure, as shown by curves F and F2, at the peak wavelength of gain, the absorption coefficient in the lower part of the n-type region 8 becomes thick (and thus shows a large loss). On the other hand, in this example, the absorption at the peak wavelength of the gain in the curve E1 (shown in the curve E2) is small.

Therefore, if the Bragg wavelength of the periodic structure is the absorption edge wavelength λ1
If a longer wavelength is set, the oscillation wavelength is uniquely determined by this Bragg wavelength. Furthermore, at this wavelength, absorption loss is extremely small (and can be ignored).

For comparison with the conventional example, the change in absorption coefficient for injected carriers in the example of the present invention is shown as a curve C in FIG. 4. In this example, the oscillation threshold gain is reached at the carrier injection amount Nc, so oscillation is possible, and the difference from the conventional example shown by curve B is remarkable. Moreover, since the active layer has a quantum well structure, the gain coefficient itself becomes large, and there is a large difference in the oscillation threshold current compared to the oscillation threshold current NA of the conventional example shown in curve A, which has a bulk structure and has no periodicity in gain. There is no.

Although an example using an n-type substrate has been described above, the characteristics are exactly the same even if a p-type substrate is used and the structure is of the opposite conductivity type to the embodiment shown in FIG.
It is also clear that the present invention can be applied to lasers using compound semiconductors other than P and InGaAsP.

[Effects of the Invention] As explained above, since the DFB-LD of the present invention has small loss in the non-injected region, (1) it easily oscillates in a single mode. (2) The oscillation threshold current is small. (3) The periodic structure is simple and manufacturing is easy. (4) Strong against external disturbance light; (5) Can be easily integrated in two-dimensional form.

There is an effect.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the structure of an embodiment of the device of the present invention, FIGS. 2 and 3 are cross-sectional views showing the structure of a conventional DFB-LD, and FIG. 4 is absorption of the amount of minority carriers injected into the active layer. FIG. 5 is a characteristic diagram showing changes in coefficients. FIG. 5 is a characteristic diagram showing wavelength characteristics of gain and loss. DESCRIPTION OF SYMBOLS 1... N-type substrate, 2... N-type buffer layer, 3... Active layer, 4... P-type guide layer with periodic structure, 5... P-type cladding layer, 6...・p type cap layer, 7... p type cladding layer, 8... n type periodic block region, 9... active layer having quantum well structure. This invention is exempt 0 Tachibana 1 Rin also Takemen Tadashi Figure 3
Figure 4

Claims (1)

[Claims] 1) Junctions of second and first conductivity type regions periodically arranged along the propagation direction of light of a semiconductor laser having junctions of mutually different first and second conductivity type regions 1. A semiconductor laser having a quantum well structure in which an active layer has a quantum well structure. 2) The semiconductor laser according to claim 1, wherein the Bragg wavelength of the period of the junction between the second and first conductivity type regions is longer than the absorption edge of the active layer.