JPH065970A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a distributed feedback type semiconductor laser in which a kink of current/light output characteristics is scarcely generated as compared with prior art and a light outputting efficiency is improved. CONSTITUTION:An n-type InGaAsP waveguide layer 13 having lambdag of 1.10mum and a thickness of 0.13-0.15mum, an n-type InP spacer layer 21 having a thickness of 50-60nm, an undoped InGaAsP active layer 15 having lambdag of 1.55mum and a thickness of 0.10-0.13mum, a p-type InGaAsP buffer layer 17 having lambdag of 1.35mum and a thickness of 40-50nm and a p-type InP clad layer 19 having a thickness of 0.5-0.7mum are sequentially provided in this order on an n-type InP substrate 11 having diffraction gratings 11a each having a height of its crest of a predetermined value of 20nm or less at a pitch of 241nm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser.

[0002]

2. Description of the Related Art Conventionally, a semiconductor laser of this type has been disclosed in, for example, a document ("Optical integrated circuit", Japan Society of Applied Physics, Optical Society, Asakura Shoten, p.59). FIG. 4 is a sectional view schematically showing this semiconductor laser cut along a direction parallel to the axis of the laser resonator. However, illustration of electrodes and the like is omitted.

This semiconductor laser has a diffraction grating 11 on the surface.
an n-type InP substrate 11 having a and an n-type InGa layered on the diffraction grating formation surface of this substrate in order from the substrate 11 side.
AsP waveguide layer 13, undoped InGaAsP active layer 15, p-type InGaAsP buffer layer 17, and p-type In
It is mainly composed of the P clad layer 19. In this configuration, the diffraction grating 11a and the n-type InGaAsP waveguide layer 13 form a resonator.

In this semiconductor laser, the light generated in the active layer 15 propagates through the n-type InGaAsP waveguide layer 13 and is reflected by the diffraction grating 11a. for that reason,
The light output from the semiconductor laser to the outside has a wavelength of a wavelength selected by the pitch of the diffraction grating 11a and the effective refractive index obtained by collecting the refractive indexes of the waveguide layer 13, the active layer 15 and the buffer layer 17 together. That is, light, that is, laser light in a single longitudinal mode.

[0005]

However, in the conventional semiconductor laser, the coupling coefficient K (kappa) determined by the pitch of the diffraction grating 11a and the effective refractive index of each of the layers 13, 15 and 17 and this semiconductor. The value of the product "KL" with the laser cavity length L (see Fig. 4) is large (KL
When ≧ 2), the coupling of the light propagating (trapping) through the waveguide layer 13 becomes too strong. Therefore, the light intensity distribution in the semiconductor laser has a maximum at the central portion in the longitudinal direction of the resonator, so that the efficiency of extracting the laser light to the outside (hereinafter,
It is called "light extraction efficiency". ) Was reduced. Further, since the distributed feedback mode is likely to change as the injection current increases, the light output characteristic (hereinafter referred to as “IL characteristic”) of the device with respect to the injection current has a bending portion, so-called kink. There was also a problem that it would end up.

In consideration of the practicality of the semiconductor laser, the cavity length L cannot be shortened so much. Therefore, in order to solve the above-mentioned problems, the height of the peak of the diffraction grating 11a has been lowered or the layer of the waveguide layer 13 has been conventionally used. It has been performed to increase the thickness to reduce the coupling coefficient K. However, since the diffraction grating is formed by selectively etching the substrate, there is a limit to reducing the height of the peak. Further, if the waveguide layer is too thick, the beam spread of the laser light becomes large and the operation mode of the semiconductor laser includes high-order modes, which is not preferable.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a distributed feedback type semiconductor laser having a structure capable of reducing the coupling coefficient K.

[0008]

In order to achieve this object, according to the distributed feedback semiconductor laser of the present invention, a refractive index lower than the refractive index of these layers is provided between the active layer and the waveguide layer. It is characterized by comprising a spacer layer composed of the material shown.

Here, the material of the spacer layer satisfies the conditions necessary for constructing a semiconductor laser other than satisfying the above-mentioned refractive index conditions, for example, other conditions such as lattice matching with the active layer or the waveguide layer. Selected from the ones that satisfy. As a specific example, in the case of an InP-based distributed feedback type, the spacer layer material is, for example, InP or InGaAsP, and InG having a composition having a lower refractive index than the waveguide layer is used.
aAsP can be mentioned. In the case of a GaAs type distributed feedback type, the spacer layer material may be, for example, GaAs or AlGaAs having a composition having a refractive index lower than that of the waveguide layer. For the purpose of the present invention, InP is used for InP and GaAs is used for GaAs.
Is particularly preferable as the spacer layer material.

If the spacer layer is too thick, the light of the active layer does not seep into the waveguide layer and the action of distributed feedback cannot be obtained in the first place.

[0011]

According to the present invention, since the spacer layer having a predetermined refractive index is provided, the light generated in the active layer is easily trapped in the active layer and is less likely to reach the waveguide layer. In other words, this means that the effective refractive index of the double heterojunction structure portion of the semiconductor laser becomes small. Therefore, the coupling coefficient K determined by this effective refractive index and the pitch of the diffraction grating is provided with the spacer layer. It will be smaller than it would be without it.

[0012]

EXAMPLE An example will be described below with reference to an example in which the present invention is applied to the distributed feedback semiconductor laser described with reference to FIG. This description will be made with reference to several figures. However, these drawings merely show the dimensions, shapes, and positional relationships of the respective constituents to such an extent that the present invention can be understood.

FIG. 1 is a sectional view schematically showing the semiconductor laser of the embodiment cut along a direction parallel to the axis of the laser resonator. However, illustration of electrodes and the like is omitted.

The semiconductor laser of this embodiment has an n-type InP substrate 11 having a diffraction grating 11a on its surface, and an n-type IP layered on the diffraction grating formation surface of this substrate in order from the substrate 11 side.
nGaAsP waveguide layer 13 and n-type InP spacer layer 2
1, undoped InGaAsP active layer 15, p-type InG
aAsP buffer layer 17 and p-type InP clad layer 19
It is mainly composed of and. Although illustration of the current confinement structure portion is omitted, for example, an internal current confinement structure having a so-called BH structure can be used.

Here, the diffraction grating 11a has a waveform formed on the surface of the substrate 11 at a pitch and a depth corresponding to a desired oscillation wavelength. When the oscillation wavelength is about 1.5 μm, a diffraction grating having a predetermined value with a pitch of 241 nm and a peak height of 20 nm or less may be used. Such a diffraction grating 11a can be formed by, for example, a two-beam interference exposure method and an etching technique.

In addition, the waveguide layer 13 to the upper clad layer 19
In this case, the composition and film thickness of each layer are as follows.

The n-type InGaAsP waveguide layer 13 has λg
Of 1.10 μm and a thickness of 0.13 to 0.
The thickness is 15 μm. n-type InP spacer layer 21
Has a thickness of 50 to 60 nm. The undoped InGaAsP active layer 15 has a composition of λg of 1.55 μm and a thickness of 0.10 to 0.13 μm. The p-type InGaAsP buffer layer 17 has λg
Has a composition of 1.35 μm and a thickness of 40 to 50 nm
It is as The p-type InP clad layer 19 has a thickness of 0.5 to 0.7 μm. Each of these layers can be formed by a suitable crystal growth method.

In the semiconductor laser of this embodiment, the averaged refractive index of the laminated portion of the InP spacer layer 21 and the InGaAsP waveguide layer 13 is lower than that of InGaAsP only because of the presence of the InP spacer layer 21 (described later). Value of n in FIG. 2). Therefore, the light generated in the active layer 15 is less likely to seep out to the vicinity of the diffraction grating 11a as compared with the case where the spacer layer 21 is not provided. An intensity distribution is obtained. The refractive index distribution I and the light intensity distribution II are shown in FIG. However, in FIG. 2, the vertical axis represents the position in the thickness direction (thickness direction of the substrate 11) of the semiconductor laser, and the horizontal axis represents the refractive index and the light intensity (arbitrary unit). Further, in FIG. 2, numbers corresponding to the respective constituent components in FIG. 1 are attached to the vertical axis to show the correspondence between the substrates and the respective semiconductor layers and their refractive indexes. Further, for comparison, FIG. 3 shows the refractive index distribution III and the light intensity distribution IV accompanying it in the conventional semiconductor laser described with reference to FIG. 4, that is, in the case where the spacer layer 21 is not provided, as shown in FIG. The same notation is used.

As described above, in the semiconductor laser of the present invention,
Since the light intensity distribution in the vicinity of the diffraction grating 11a is smaller than the conventional one, the coupling coefficient K is smaller than the conventional one, so that K · L is smaller than the conventional one when the resonator length L is the same. Therefore, it is possible to suppress the deterioration of the IL characteristics and the decrease of the light extraction efficiency due to the large coupling coefficient K.

The light intensity distribution as shown in FIG. 2 can be obtained by increasing the layer thickness of the waveguide layer 13 without providing the spacer layer 21, but the thickness of the waveguide layer in such a case. Is the spacer layer 21 and the waveguide layer 13 in this embodiment.
And the total thickness of 0.18 to 0.21 μm (0.25 μm or more). When the thickness of the waveguide layer is increased, the beam spread of the laser light is increased and the operation mode of the semiconductor laser tends to include a higher order mode. On the other hand, the structure of the present invention is advantageous in that K · L can be made smaller than in the conventional case and high-order mode oscillation can be suppressed.

Although the embodiments of the distributed feedback semiconductor laser of the present invention have been described above, the present invention is not limited to the above embodiments.

For example, the above-described embodiment is an example in which the present invention is applied to a distributed feedback type semiconductor laser having a waveguide layer between the substrate and the active layer. Of course, it can be applied to a structure having a waveguide layer between the layers.

Further, in the above-mentioned embodiment, the constitutional example in which the buffer layer is provided on the active layer is shown, but this buffer layer is not essential. Further, in the above-mentioned embodiment, the example of the structure in which the diffraction grating 11a is provided on the surface of the substrate 11 and the waveguide layer 13 is provided on this substrate has been described, but if necessary, I can be provided on the substrate.
An nP buffer layer may be provided and a diffraction grating may be provided on this surface.

In the above-mentioned embodiment, the spacer layer is made of In.
Although the spacer layer is composed of the P layer, the spacer layer may be composed of an InGaAsP layer having a composition lower than that of the waveguide layer, if necessary.

Further, although the n-type InP substrate is used in the above-mentioned embodiment, the same effect as that of the embodiment can be obtained when the substrate is of p-type and the conductivity type of each semiconductor layer is opposite to that of the embodiment. You can Further, the refractive index and the film thickness of each layer shown in the above embodiments are merely examples within the scope of the present invention, and can be changed according to the oscillation wavelength and the material used. The present invention can also be applied to a distributed feedback semiconductor laser using a material other than InP. For example, in the case of a GaAs system, the same effect as that of the embodiment can be obtained by using a GaAs layer as the spacer layer or an AlGaAs layer having a lower refractive index than the waveguide layer.

[0026]

As is apparent from the above description, according to the distributed feedback semiconductor laser of the present invention, since the appropriate spacer layer is provided between the active layer and the waveguide layer, the waveguide layer The coupling coefficient K (kappa) can be made smaller than before without increasing the thickness. Therefore, it is possible to suppress a kink in the IL characteristic and a decrease in light extraction efficiency that occur due to a large coupling coefficient. Therefore, for example, a semiconductor laser suitable for a light source for long-distance optical communication can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of essential parts for explaining a semiconductor laser according to an embodiment.

FIG. 2 is a diagram for explaining an example, and is a diagram showing a refractive index distribution and a light intensity distribution in a semiconductor laser of the example.

FIG. 3 is a diagram for explaining a comparative example, and is a diagram showing a refractive index distribution and a light intensity distribution in a semiconductor laser of the comparative example.

FIG. 4 is a cross-sectional view of essential parts for explaining a conventional semiconductor laser.

[Explanation of symbols]

 11: n-type InP substrate 11a: diffraction grating 13: n-type InGaAsP waveguide layer 15: undoped InGaAsP active layer 17: p-type InGaAsP buffer layer 19: p-type InP clad layer 21: InP spacer layer

Claims (1)

[Claims] 1. A distributed feedback semiconductor laser comprising a spacer layer formed between the active layer and the waveguide layer, the spacer layer being made of a material having a refractive index lower than the refractive index of these layers. Type semiconductor laser.