JPH07221403A - Variable wave-length semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a variable wave-length semiconductor laser for a wide frequency range, which has a flat gain independent of oscillating frequency. CONSTITUTION:A variable wave-length semiconductor laser includes an active layer that has a periodical arrangement composed of a plurality of semiconductor layers each having wells 4, 5 and 6 with sequentially different thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable semiconductor laser, and more particularly to its structure.

[0002]

2. Description of the Related Art FIG. 2 (a) is a cross-sectional view in the cavity direction showing the structure of a conventional wavelength tunable laser. In the figure, 10 is an active layer, 11 is a diffraction grating, 12 is an active region, and 13 is a DB.
This is an R (Distributed Bragg Reflector) region.

2 (b) and 2 (c) are band diagrams of the quantum well structure of the above tunable laser,
It is a figure which shows the wavelength dependence of and gain. In the figure, 1 is a p-InP clad layer, 2 is an n-InP clad layer, 3 is In0.71Ga0.29As0.64P with a layer thickness of 10 nm.
0.36 barrier layer, 4 is In0.53 Ga0 with a well layer thickness of 5 nm.
47As well layer, 5 is In0.53 with a well layer thickness of 5.5 nm
Ga0.47As well layer, 6 is an In0.
53 Ga 0.47 As well layer. Reference numeral 7 is a curve showing the wavelength dependence of the gain of a well layer having a well layer thickness of 5 nm, and 8
Is a curve showing the wavelength dependence of the gain of the well layer having a well layer thickness of 5.5 nm, and 9 is a curve showing the wavelength dependence of the gain of the well layer having a well layer thickness of 6 nm.

The semiconductor laser shown in FIG. 2 (a) has a tunable wavelength D
This laser is called a BR laser, and this laser has a D region in which an active region 12 and a diffraction grating 11 are formed in the optical resonator direction.
The BR area 13 is divided into two areas. The reflectance of the DBR becomes maximum at the Bragg wavelength λB given by the following equation.

Λ B = 2 (neff Λ) (1) where Λ is the period of the diffraction grating and neff is the equivalent refractive index of the waveguide in the DBR region.

As can be seen from this equation, the reflectance of the DBR becomes maximum near the Bragg wavelength. On the other hand, in order to oscillate the semiconductor laser, it is necessary to satisfy the phase matching condition. The oscillation wavelength of the DBR laser is a wavelength that satisfies the phase matching condition in the vicinity of the Bragg wavelength having the highest reflectance. If the equivalent refractive index neff of the DBR region is changed,
The Bragg wavelength given by equation (1) changes and the oscillation wavelength changes. When a current (hereinafter also referred to as a DBR region current) Id is injected into the DBR region 13, the equivalent refractive index neff changes due to the plasma effect, and the Bragg wavelength also changes accordingly. In this case, the operating current required for laser oscillation is maintained by the current Ia injected into the active region 12. Figure 2
As shown in (d), the laser oscillation wavelength is also changed by changing the DBR region current Id. In this example, when the DBR region current Id is changed from 0 mA to 100 mA, the laser oscillation wavelength is from 1.558 μm to 1.552.
6 nm change to μm.

The active layer 10 of the wavelength tunable DFB laser is usually constructed by a quantum well structure. This is because the quantum well structure reduces the threshold current and improves the modulation characteristics.
This is because the laser characteristics are remarkably improved. The quantum well structure is composed of a well layer and a barrier layer, and the layer thickness of the well layer is 20 nm or less. In used for optical communication
In a semiconductor laser made of a GaAsP-based material, a plurality of well layers are formed in order to increase the gain, that is, the gain. In the wavelength tunable semiconductor laser, as shown in the band diagram of FIG. 2 (b), the number of well layers 4, 5 and 6 of the same number is 5 nm, 5.5 nm and 6 nm, respectively.
It is slightly changed from nm. This is to flatten the wavelength dependence of the gain. By changing the thickness of each well layer, as shown in Fig. 2 (c), the wavelength dependence of the gain is better than in the case of a single well layer. Because it can be alleviated. That is, the wavelength dependence of the gain is strongly influenced by the well layer thickness. When the active layer has such a quantum well structure, the wavelength variable region can be greatly expanded as compared with a semiconductor laser having a single well layer.

[0008]

Although the conventional wavelength tunable semiconductor laser is constructed as described above, the well structure in the conventional wavelength tunable semiconductor laser has two problems. The first is that the gain peak changes with the change of the well layer, and in the example of FIG.
The gain peak of the well layer 4 of nm is the maximum, and the peak value becomes smaller as the well layer becomes thicker. However, the gain peak decreases to the right as shown in the figure and is not completely flat. It will be. That is,
The gain on the long wavelength side is lower than that on the short wavelength side, which makes oscillation difficult.

Another problem is that in the case of a quantum well structure having a plurality of well layers, the holes have a large mass, and the barrier effect of the barrier layer 3 on the holes is high, so that the p-InP cladding layer 1 side. The holes injected into the quantum well structure from are recombined with electrons in the well layer close to the p-InP clad layer 1 and do not reach the vicinity of the n-InP clad layer 2. Therefore, in the well layer on the n-InP side, The problem is that laser oscillation becomes difficult. That is, since the electrons are light, they can enter the quantum well structure uniformly, but the holes are heavy, so they cannot enter the quantum well structure uniformly, so that uniform oscillation does not occur. Therefore, there is a problem that the gain becomes smaller than the theoretical value.

The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to obtain a wavelength tunable semiconductor laser having a flat gain wavelength dependency and a wide wavelength tunable width. .

[0011]

A tunable semiconductor laser according to the present invention has an active layer of a quantum well structure including a plurality of types of well layers having different layer thicknesses, and the plurality of types of well layers described above. Is arranged such that the wavelength dependence of the gain in the active layer is reduced.

Further, the wavelength tunable semiconductor laser according to the present invention has an active layer of a quantum well structure including a plurality of types of well layers having different layer thicknesses, and the plurality of types of well layers are arranged in the order. , In the arrangement order such that the wavelength dependence of the gain in the active layer is reduced,
An active region including the active layer of the quantum well structure, and a distributed Bragg reflector region continuously arranged at the end of the active region in the cavity length direction and capable of changing its equivalent refractive index. Be prepared.

The tunable semiconductor laser according to the present invention has a plurality of types of well layers having different layer thicknesses, and a plurality of well layers having different layer thicknesses are arranged in this order. The active layer has a quantum well structure in which a plurality of semiconductor laminated structures are periodically arranged.

Further, the wavelength tunable semiconductor laser according to the present invention has a plurality of types of well layers having different layer thicknesses, and a plurality of sets of semiconductor laminated layers each including a well layer having a different layer thickness for each set. The quantum well structure in which the structures are arranged in the order of the layer thickness of the well layer is provided in the active layer, and the number of the well layers is increased as the semiconductor laminated structure of the group including the well layer having the high gain is increased. The number of well layers is increased as the number of sets of semiconductor laminated structures is reduced and the well layer having a lower gain and including well layers is included.

Further, the wavelength tunable semiconductor laser according to the present invention has a plurality of types of well layers having different layer thicknesses, and a plurality of sets of semiconductor laminated structures each including a well layer having a different layer thickness for each set. The quantum well structure formed by arranging the above well layers in the order of the layer thickness is arranged in the active layer, and the number of well layers is reduced in a semiconductor laminated structure of a group including well layers having a high gain. However, in the case where the number of well layers is increased as the number of well layers increases in a group including well layers having a low gain, the number of well layers having i-th (i = 1 to n) well layers is the same. The number of wells in the structure, Ni,
Optical confinement coefficient Γi of well layer with i-th well layer thickness
And the hole injection efficiency η of the well layer having the i-th well layer thickness
The product of i and the gain gi of the well layer of the i-th well layer is N1.GAMMA.1 .eta.1 .g1 = ... = Ni .GAMMA.i .eta.i .gi = ... = Nn .GAMMA.n .eta.n. gn.

[0016]

In the tunable semiconductor laser according to the present invention, the tunable semiconductor laser has an active layer of a quantum well structure including a plurality of types of well layers having different layer thicknesses, and the arrangement order of the plurality of types of well layers is Since the arrangement order is such that the wavelength dependence of the gain in the active layer is reduced, a semiconductor laser having a wide wavelength variable width can be obtained.

Further, the wavelength tunable semiconductor laser according to the present invention has an active layer having a quantum well structure including a plurality of types of well layers having different layer thicknesses, and the arrangement order of the plurality of types of well layers. In an arrangement order such that the wavelength dependence of the gain in the active layer is reduced, and an active region including the active layer of the quantum well structure,
Since the active region is provided with a distributed Bragg reflector region which is continuously arranged at the end portion in the cavity length direction and whose equivalent refractive index can be changed, a wavelength having a wide wavelength tunable width is provided. A tunable DBR laser is obtained.

In the tunable semiconductor laser according to the present invention, a plurality of types of well layers having different layer thicknesses are provided, and a plurality of well layers having different layer thicknesses are arranged in order of layer thickness. Since the active layer has a quantum well structure formed by periodically arranging a set of semiconductor laminated structures, the wavelength dependence of gain is flat, and a semiconductor laser having a wide wavelength tunable width can be obtained.

In addition, the wavelength tunable semiconductor laser according to the present invention has a plurality of types of well layers having different layer thicknesses, and each set includes a plurality of semiconductor laminated layers including well layers having different layer thicknesses. The active layer has a quantum well structure in which the structure is arranged in the order of the thickness of the well layer,
Further, the number of well layers is reduced as the group of semiconductor stacked structures including the well layer having a high gain layer thickness is increased, and the number of well layers is increased as that of the group including a well layer having a low gain layer thickness. , The gain wavelength dependency becomes flat, and a semiconductor laser having a wide wavelength tunable range can be obtained.

In addition, the wavelength tunable semiconductor laser according to the present invention has a plurality of types of well layers each having a different layer thickness, and each set includes a plurality of sets of semiconductor layers including different well layers having different layer thicknesses. The number of well layers is higher in a group of semiconductor laminated structures having a quantum well structure in which the laminated structure is arranged in the order of the layer thicknesses of the above well layers in the active layer and including well layers having a high gain layer thickness. And the number of well layers is increased as the number of well layers increases in a group including well layers with a low gain layer thickness, a group including a well layer having an i (i = 1 to n) th well layer thickness The number of wells Ni of the semiconductor laminated structure, the optical confinement coefficient Γi of the well layer of the i-th well layer, the hole injection efficiency ηi of the well layer of the i-th well layer, and the well layer of the i-th well layer thickness. The product of the gain of 1 · Γ1 · η1 · g1 = ・ ・ ・ = Ni · Γi · ηi · gi = ・ ・ ・ = Nn · Γn · ηn · gn Since the wavelength dependence of the gain is flat and the wavelength tunable A wide semiconductor laser can be obtained.

[0021]

EXAMPLES Example 1 Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram for explaining a wavelength tunable semiconductor laser according to a first embodiment of the present invention. FIG. 1 (a) is a band diagram of a quantum well structure of the semiconductor laser, and FIG.
Shows the wavelength dependence of the gain of each well layer constituting the quantum well structure. In the figure, 4 is a well layer thickness of 5n
m is an In0.53Ga0.47As well layer, 5 is an In0.53Ga0.47As well layer having a well layer thickness of 5.5 nm, and 6 is an In0.53Ga0.47As well layer having a well layer thickness of 6 nm.
Further, 7 is a curve showing the wavelength dependence of the gain of the well layer having a well layer thickness of 5 nm, 8 is a curve showing the wavelength dependence of the gain of the well layer having a well layer thickness of 5.5 nm, and 9 is a well layer thickness 6
Curves showing the wavelength dependence of the gain of the well layer in nm are respectively shown.

Since the structure of the semiconductor laser of this embodiment is the same as that of the conventional example, detailed description thereof will be omitted. In the wavelength tunable DBR laser of the first embodiment, the quantum well structure has an energy band diagram as shown in FIG. 1 (a). In FIG. 1 (a), the energy is higher on the left side. In this structure, a plurality of well layers 4, 5 and 6 having different well layer thicknesses are alternately and periodically laminated, and the total number of the well layers 4, 5 and 6 is , Are equal.
With this configuration, even if holes injected from the p-InP clad layer 1 do not reach the n-InP clad layer 2 and are non-uniformly distributed in the quantum well structure, these holes are not uniform. Since it does not exist in the well layer with an uneven thickness, the well layers are arranged in order according to the thickness of the well layer as shown in FIG. 1 (b). However, the wavelength dependence of the gain will be greatly relaxed. Therefore, the tunable width of the tunable semiconductor laser is also greatly increased.

In the wavelength tunable semiconductor laser of the first embodiment, the active layer of the quantum well structure has a plurality of types of well layers having different layer thicknesses, and the plurality of well layers having different layer thicknesses are provided. Since a plurality of sets of semiconductor laminated structures, which are arranged in the order of the layer thickness, are arranged periodically,
The characteristic of the gain with respect to wavelength of the quantum well structure is high on the low wavelength side, for example. Therefore, in this configuration, holes injected from the p-InP cladding layer 1 reach the n-InP cladding layer 2. Even if they are distributed unevenly in the quantum well structure without being distributed, the well layers do not deviate to a well layer having a specific layer thickness, and the well layers are arranged in order by the well layer thickness over the entire active layer as in the conventional example. The wavelength dependence of the gain is greatly relaxed more than the above, and there is an effect that a wavelength tunable semiconductor laser capable of increasing the wavelength tunable width can be obtained.

Example 2 In the second embodiment of the present invention, in order to eliminate the influence that the magnitude of the gain changes depending on the well layer thickness, the number of well layers having the highest gain is set to a constant value, and the well layer thickness having the smallest gain is set. The number of wells is gradually increased so that a flat gain as a whole, that is, a gain with little wavelength dependence can be obtained.

That is, FIG. 3 shows a semiconductor laser device according to a second embodiment of the present invention. As shown in FIG. 3A, the well layer 4 having the highest gain, in this example 5 nm, is formed. The thickness of the well layer with a small gain of 5.5n
If the number of m, 6 nm wells is increased in order, for example, to 4 layers and 5 layers, it is possible to obtain a flat gain as a whole, that is, a gain with little wavelength dependence. If this is expressed as a general formula, N1.GAMMA.1 .eta.1 .g1 = ... = Ni .GAMMA.i .eta.i .gi = ... = Nn .GAMMA.n .eta.n .gn. Where Ni is the number of wells in the i-th (i = 1 to n) -th well layer, Γi is the optical confinement coefficient of the i-th well-layer, and ηi is each i-th The hole injection efficiency of the well layer having the th well layer thickness, gi is
The gain of the well layer having the i-th well layer thickness is shown. If the number of wells is selected so as to satisfy the above equation, a flat wavelength-dependent gain can be obtained, and as a result, a semiconductor laser having a wide wavelength variable width can be obtained.

In the wavelength tunable semiconductor laser according to the second embodiment, the active layer has a quantum well structure in which a plurality of well layers having different layer thicknesses are arranged in the order in which the layer thickness changes, In the quantum well structure, the number of well layers is decreased as the well layer having a higher gain is increased and the number of well layers is increased as the layer thickness is decreased as a lower gain. The opened hole is n-I
Even if the nP clad layer 2 does not reach the nP clad layer 2 and becomes unevenly distributed in the quantum well structure, it is necessary to increase the number of well layers even in the well layer portion having a low gain. As a result, the wavelength dependence of the gain is greatly alleviated, and the wavelength tunable range can be increased accordingly.

In each of the above-mentioned embodiments, the case where the invention is applied to the wavelength tunable DBR laser in which the DBR region is provided and the oscillation wavelength is controlled by changing the equivalent refractive index of the DBR region has been described. Can be applied to a wavelength tunable laser in which the oscillation wavelength is controlled by a structure other than the DBR, and the same effect as that of the above embodiment can be obtained.

Further, it is needless to say that the semiconductor material forming the laser is not limited to those shown in the above-mentioned embodiment, but can be applied to the laser formed by using other material system such as GaAs system.

[0029]

As described above, the wavelength tunable semiconductor laser according to the present invention has an active layer having a quantum well structure including a plurality of types of well layers having different layer thicknesses, and the plurality of types described above. Since the arrangement order of the well layers is such that the wavelength dependence of the gain in the active layer is reduced, there is an effect that a wavelength tunable semiconductor laser having a wide wavelength tunable width can be obtained.

Also, according to the tunable semiconductor laser of the present invention, the active layer has a quantum well structure including a plurality of types of well layers having different layer thicknesses, and the plurality of types of well layers are arranged. In the order in which the wavelength dependence of gain in the active layer is reduced, an active region including the active layer of the quantum well structure and an end portion of the active region in the cavity length direction are arranged. Arranged consecutively,
Since the distributed Bragg reflector region capable of changing its equivalent refractive index is provided, the wavelength tunable DBR laser having a wide wavelength tunable width can be obtained.

Further, according to the wavelength tunable semiconductor laser of the present invention, a plurality of types of well layers having different layer thicknesses are provided, and a plurality of well layers having different layer thicknesses are arranged in order of layer thickness. Since the active layer has a quantum well structure formed by periodically arranging a plurality of sets of semiconductor laminated structures, the wavelength dependence of the gain is flat, and the wavelength tunable semiconductor has a wide wavelength tunable width. There is an effect that a laser can be obtained.

Further, according to the tunable semiconductor laser of the present invention, a plurality of sets of semiconductor layers each having a plurality of types of well layers having different layer thicknesses, each group including a well layer having a different layer thickness, are provided. The number of well layers is higher in a group of semiconductor laminated structures having a quantum well structure in which the laminated structure is arranged in the order of the layer thicknesses of the above well layers in the active layer and including well layers having a high gain layer thickness. Since the number of well layers is increased in a semiconductor laminated structure of a group including well layers having a low gain and a low gain, the wavelength dependence of gain becomes flat, and a wavelength tunable semiconductor laser having a wide wavelength tunable width is obtained. There is an effect that can be.

According to the tunable semiconductor laser of the present invention, the number of wells in each well layer Ni, the optical confinement coefficient Γi of each well layer, the hole injection efficiency ηi of each well layer, and the well injection efficiency ηi of each well layer. Since the product of the gain gi is N1 · Γ1 · η1 · g1 = N2 · Γ2 · η2 · g2 = ..., the wavelength dependence of the gain is further flattened and the wavelength with a wide wavelength tunable range is widened. There is an effect that a variable semiconductor laser can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a band diagram of a quantum well structure of a wavelength tunable semiconductor laser according to a first embodiment of the present invention (FIG. 1 (a)), and a diagram showing wavelength dependence of gain (FIG. 1).
(b)).

2 is a sectional view of a conventional tunable semiconductor laser device (FIG. 2 (a)), a band diagram of its quantum well structure (FIG. 2 (b)), and a graph showing wavelength dependence of its gain ( 2 (c)), and FIG. 2 (d) showing the relationship between the oscillation wavelength and the injection current into the DBR region.

FIG. 3 is a diagram showing a band diagram of a quantum well structure of a wavelength tunable semiconductor laser according to a second embodiment of the present invention (FIG. 3 (a)), and a diagram showing wavelength dependence of its gain (FIG. 3).
(b)).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 p-InP clad layer 2 n-InP clad layer 3 InGaAsP barrier layer 4 Well layer thickness 5 nm InGaAs well layer 5 Well layer thickness 5.5 nm InGaAs well layer 6 Well layer thickness 6 nm InGaAs well layer 7 Well layer thickness 5 nm Curve showing the wavelength dependence of the gain of a well layer thickness of 5.5 nm Curve showing the wavelength dependence of the gain of 5.5 nm 9 Curve showing the wavelength dependence of the gain of a well layer thickness of 6 nm 10 Active layer 11 Diffraction grating 12 Active region 13 DBR region

Claims (5)

[Claims] 1. A tunable semiconductor laser having an active layer capable of generating laser light of a plurality of wavelengths, the active layer having a quantum well structure including a plurality of types of well layers having different layer thicknesses, A tunable semiconductor laser, wherein a plurality of types of well layers are arranged in such an order that the wavelength dependence of gain in the active layer is reduced. 2. The wavelength tunable semiconductor laser according to claim 1, wherein an active region including an active layer of the quantum well structure and an equivalent region of the active region which is continuously arranged at an end portion in the cavity length direction of the active region. A tunable semiconductor laser comprising a distributed Bragg reflector region capable of changing a refractive index. 3. The wavelength tunable semiconductor laser according to claim 1, further comprising a plurality of types of well layers having different layer thicknesses, and a plurality of well layers having different layer thicknesses are arranged in order of layer thickness. A tunable semiconductor laser having a quantum well structure in which a plurality of sets of semiconductor laminated structures are periodically arranged in its active layer. 4. The tunable semiconductor laser according to claim 1, further comprising a plurality of types of well layers having different layer thicknesses, each set including a plurality of semiconductor layers each including a well layer having a different layer thickness. The active layer has a quantum well structure in which the well layers are arranged in the order of the thickness of the well layers, and the number of well layers is reduced in a semiconductor laminated structure of a group including well layers having a high gain. A tunable semiconductor laser, wherein the number of well layers is increased in a semiconductor laminated structure of a set including well layers having a low gain layer thickness. 5. The tunable semiconductor laser according to claim 4, wherein the number of wells Ni of a semiconductor laminated structure of a set including a well layer having an i (i = 1 to n) well layer thickness and an i th well layer. The product of the optical confinement coefficient Γi of the thick well layer, the hole injection efficiency ηi of the well layer of the i-th well layer, and the gain gi of the well layer of the i-th well layer is N1 · Γ1 · η1 · A wavelength tunable semiconductor laser characterized in that g1 = ... = Ni..GAMMA.i..eta.i.gi = ... = Nn..GAMMA.n.eta.n.gn.