JP2687404B2 - Distributed feedback semiconductor laser - Google Patents

Description

The present invention relates to a structure of a distributed feedback semiconductor laser having a diffraction grating.

[Conventional technology]

Since the distributed feedback semiconductor laser oscillates in a stable single longitudinal mode, it has the excellent characteristic that the oscillation wavelength does not fluctuate even if the injection current or operating temperature changes. It is getting attention. As a conventional technique, a distributed feedback semiconductor laser as shown in FIG. 2 has been proposed. This structure is made of an IuGaAsP / InP crystal, and a diffraction grating 8 having an uneven periodic structure is formed on a substrate crystal 1 made of InP, and an InGaAsP optical guide layer 2, I is formed on the diffraction grating 8.
An nGaAsP active layer 4, an InP clad layer 5, and an InGaAsP cap layer 6 are laminated. Therefore, only the wavelength determined by the periodic condition of the unevenness is amplified and laser oscillation occurs, so that a single oscillation spectrum is obtained.

[Problems to be solved by the invention]

However, the distributed feedback type semiconductor laser of this structure is designed to increase the reflection from the diffraction grating, so that the optical guide layer 2
It is necessary to have a layer structure having a composition that reduces the difference in refractive index between the active layer 4 and the active layer 4. As a result, the coupling efficiency with the diffraction grating is improved, but a new problem occurs.

Reducing the difference in the refractive index between the optical guide layer and the active layer is nothing but the approach of the forbidden band width of both. In order to confine the carriers injected into the active layer in this layer, a corresponding forbidden band width is required between the layers sandwiching the active layer. Therefore, according to this method, carriers in the active layer may easily leak to the light guide layer. This phenomenon becomes remarkable especially at high temperature operation, and the temperature characteristics of the threshold current of the laser deteriorate.

[Means for solving the problem]

An object of the present invention is to provide a distributed feedback semiconductor laser capable of stably manufacturing one having excellent characteristics.

The distributed feedback semiconductor laser of the present invention has a multilayer structure in which the active layer is removed by a clad layer having a larger forbidden band width than the active layer, and an optical guide layer having a smaller forbidden band width than the active layer is provided adjacent to one clad layer. Is laminated on a semiconductor substrate having a diffraction grating formed on the surface thereof.

Embodiment 1 Next, the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a distributed feedback semiconductor laser according to an embodiment of the present invention. In this embodiment, a crystal layer having a band gap larger than that of the active layer 4 made of InGaAsP, that is, a clad layer made of InP.
The active layer 4 is sandwiched between 3 and 5, and the optical guide layer 2 made of InGaAsP is provided on the InP clad layer side. The semiconductor multilayer film crystal including the active layer is laminated on the InP substrate crystal on which the diffraction grating is formed. There is something. Next, the manufacturing method of this embodiment will be described. First, a substrate crystal 1 made of n-InP
Apply about 500Å of photoresist film on the surface. After this,
The photoresist film is exposed in the shape of a diffraction grating using an interference diffraction exposure device using a He-Cd laser light source with a wavelength of 3250Å
Form a 2010 Å diffraction grating. Next, using this as a mask, the substrate crystal 1 is chemically etched to form an uneven diffraction grating 7. After removing the photoresist film, this n-In
The following layers are successively grown on the P substrate 1 by liquid phase epitaxial growth. First, n-InGaA with a composition of 1.4 μm
The sP light guide layer 2 is grown, and then the n-InP clad layer 3, the 1.3 μm InGaAsP active layer 4, the P-InP clad layer 5, and the cap layer 1.1 μm P-InGaAsP6 are grown and completed. The typical thickness of each layer is 0.08 μm for the light guide layer 2 and n-
The clad layer 3 is 0.05 μm, the active layer 4 is 0.1 μm, the P-clad layer 5 is 1.5 μm, and the cap layer 6 is 3 μm. Finally, the P-side electrode 8 and the n-side electrode 9 are respectively formed and completed.

Example 2 In the above example, the case where the wavelength of the active layer is 1.3 μm has been described, but the present invention can be applied even when the wavelength is shortened to 1.2 μm. Example 2 has an oscillation wavelength of 1.2μ.
The case of m will be described. In this example, the active layer composition is 1.2
μm, light guide layer composition 1.3 μm, diffraction grating period about 1
It is only 850 Å, and other than that, for example, it is not necessary to change each layer thickness, InP as a clad layer, and the like. Therefore, the structure is similar to that of the first embodiment.

〔The invention's effect〕

As described above, according to the structure of the present invention, a sufficiently large forbidden band difference is formed between the active layer and the clad layer, so that diffusion of carriers injected into the active layer is prevented,
The carrier is confined. This effect is not impaired even at high temperature operation. On the other hand, since the clad layer is thin, the laser light seeps into the light guide layer through this layer. Sufficient diffraction efficiency can be obtained by providing the diffraction grating in the light guide layer. By providing a thin cladding layer between the active layer and the light guide layer, the composition of the light guide layer can be designed with only the refractive index as a parameter without considering the phenomenon of carrier leakage to the light guide layer side. Becomes

This brings about the following advantages in the manufacturing method. By making the refractive index of the optical guide layer larger than that of the active layer, the difference in refractive index formed between the diffraction grating on the InP substrate and the optical guide layer grown on the InP substrate causes the distributed feedback semiconductor of the conventional configuration. Can be larger than that of a laser. That is, in order to form the diffraction efficiency required by the semiconductor laser, the present invention can reduce the height of the unevenness of the diffraction grating. As a result, the manufacturing method of the diffraction grating substrate becomes easy.

Furthermore, the semiconductor laser of the present invention becomes a distributed feedback semiconductor laser that is stable with respect to return light. The conventional distributed feedback semiconductor laser is unstable with respect to the return light because the waveguide region composed of the cladding layer, the active layer and the optical guide layer is transparent to the return light. Is to penetrate deep into the waveguide region and disturb the electromagnetic field distribution. As in the present invention, when the forbidden band width of the light guide layer is larger than the forbidden band width of the active layer and the light guide layer has an action as an absorber for oscillated light, the return light returns to the semiconductor laser However, it is absorbed in the vicinity of the end face of the emitted light and does not penetrate deep into the waveguide region. Therefore, the disturbance of the electromagnetic field distribution is unlikely to occur, and a distributed feedback semiconductor laser that is strong (stable) against return light can be obtained.

Although the electrode stripe structure is used in the embodiment as can be seen from FIG. 1, the stripe structure for transverse mode control may be any structure such as a buried stripe structure, a planar stripe structure, or an oxide stripe structure. The present invention can be applied.

[Brief description of the drawings]

FIG. 1 is a perspective view of a semiconductor laser according to an embodiment of the present invention, and FIG. 2 is a perspective view of a conventional semiconductor laser. 1 ... n-InP substrate crystal, 2 ... n-InGaAsP optical guide layer, 3 ... n-InP cladding layer,
4 ... Active layer made of InGaAsP, 5 ... Clad layer made of P-InP, 6 ... Cap layer made of P-InGaAsP,
7 ... Diffraction grating, 8 ... P-side electrode, 9 ... N-side electrode.

Claims (1)

(57) [Claims] 1. An optical guide layer on a semiconductor substrate, a clad layer having a band gap larger than that of the light guide layer, an active layer having a band gap smaller than that of the clad layer and a band gap larger than that of the optical guide layer, and an active layer. A distributed feedback semiconductor laser comprising a multilayer structure in which clad layers having a larger forbidden band width are sequentially stacked, and a diffraction grating is provided at a boundary between the semiconductor substrate and the optical guide layer.