JP2003283046A - Semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser element suitable for the forward direction excitation light source of a Raman amplifier. <P>SOLUTION: The semiconductor laser element 100 is provided with an active layer 45 of multiplex quantum well structure and separate closing heterodyne structure (SCH) layers 42, 44, 45, 52 which are provided above and below the active layer 45. The upper surface of the SCH layer 52 positioned at the uppermost position is coated with a lower intermediate layer 15a. An upper intermediate layer 15b is attached to the upper surface of the lower intermediate layer 15a. The upper surface of the upper intermediate layer 15b is coated with an upper clad layer 16. A uniform diffraction grid 36 is provided in an interface between the upper intermediate layer 15b and the clad layer 16 along the active layer 45. The semiconductor laser element oscillates in multimode oscillation whereby the output light of the same is hardly scattered in an optical fiber. Further, the semiconductor laser element has low RIN and excellent in wave length stability also. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device that oscillates a laser, and more particularly to a semiconductor laser device suitable as a pumping light source for a Raman amplifier.

[0002]

2. Description of the Related Art An external grating resonator type semiconductor laser is known as a pumping light source for a Raman amplifier. This is a laser in which one facet of a Fabry-Perot (FP) type laser has a low reflection coating, and the other facet and a grating (diffraction grating) arranged outside the FP type laser constitute an external resonator. Recently, a fiber Bragg grating (FBG) is often used as an external grating. Such an external FBG resonator type laser has an advantage of having good wavelength selectivity. In addition, the external FBG resonator type laser has a current dependence of the oscillation wavelength compared to the FP type laser.
There is also an advantage that the temperature dependence is small.

[0003]

In order to improve the performance of the Raman amplifier, a bidirectional pumping system in which pumping light is added to the signal light from both the forward direction and the backward direction is desirable. In forward pumping, the noise characteristics of the pumping light source strongly affect the signal light.
Therefore, an excitation light source having an extremely low relative noise intensity (RIN) is required. The external FBG resonator laser has excellent wavelength stability, but it is difficult to make RIN low enough for forward pumping of the Raman amplifier. In fact, such low RIN external FBG cavity lasers have not yet been realized.

Therefore, an object of the present invention is to provide a semiconductor laser device suitable for a forward pumping light source of a Raman amplifier.

[0005]

A semiconductor laser device according to the present invention comprises: (a) a substrate made of a semiconductor of a first conductivity type;
(B) a lower clad layer provided on the upper surface side of the substrate and made of a semiconductor of the first conductivity type; (c) a lower separated confinement heterostructure layer provided on the lower clad layer; and (d) a lower separated confinement. An active layer having a multiple quantum well structure provided on the heterostructure layer, (e) an upper isolation confinement heterostructure layer provided on the active layer, and (f) an upper surface of the upper isolation confinement heterostructure layer. A deposited lower intermediate layer, (g) an upper intermediate layer deposited on the upper surface of the lower intermediate layer, and (h) a second conductive layer deposited on the upper surface of the upper intermediate layer and different from the first conductivity type. An upper clad layer made of a semiconductor of the type, (i) an upper electrode provided on the upper clad layer, and (j) a lower electrode provided on the lower surface side of the substrate.

The lower intermediate layer has a lower index of refraction than the upper isolated confinement heterostructure layer. The upper intermediate layer has a higher refractive index than the lower intermediate layer and the upper cladding layer. A diffraction grating is provided along the active layer at the interface between the upper intermediate layer and the upper cladding layer. This diffraction grating may be along the entire length of the active layer, or may be along only a part of the length of the active layer. The diffraction grating may be a uniform diffraction grating or a chirp diffraction grating. The semiconductor laser device of the present invention has an emission spectrum of two or more oscillation longitudinal modes. That is, the semiconductor laser device of the present invention performs multimode (longitudinal multimode) laser oscillation.

The semiconductor laser device of the present invention may have a normalized coupling coefficient of less than 0.8. The semiconductor laser device of the present invention may further include a front end face having a low reflection coating and a rear end face facing the front end face and having a high reflection coating. The substrate, the lower clad layer, the lower intermediate layer and the upper clad layer may be made of InP, and the lower isolated confinement heterostructure layer, the active layer, the upper isolated confinement heterostructure layer and the upper intermediate layer may be made of InGaAsP. Bottom isolation confinement heterostructure layer,
The active layer, the upper separate confinement heterostructure layer, the lower intermediate layer and the upper intermediate layer may have a mesa structure.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments of the present invention, the outline of the present invention will be described.

The distributed feedback (DFB) semiconductor laser has characteristics of high wavelength stability and low RIN. This point is suitable for the forward pumping light source of the Raman amplifier. However, when the existing DFB laser is actually used as the pumping light source for the Raman amplifier, it is impossible to maintain sufficient pumping light power. This is because the DFB laser light causes Brillouin scattering inside the optical fiber.

The Brillouin scattering of the DFB laser light is remarkable because the DFB laser oscillates in a single mode (longitudinal single mode oscillation). Therefore, the present inventor
We have devised a semiconductor laser device that has a diffraction grating similar to a laser, and yet performs multimode oscillation (longitudinal multimode oscillation). This is the semiconductor laser device of the present invention.

The present inventor has found that even a semiconductor laser device having a structure similar to that of an existing DFB laser has a low standardized coupling coefficient and multimode oscillation occurs. In particular, if the normalized coupling coefficient is less than 0.8, multimode oscillation is likely to occur. The normalized coupling coefficient is often expressed as κL. Here, κ is the coupling coefficient and L is the length of the diffraction grating. According to "semiconductor laser" (edited by the Japan Society of Applied Physics / edited by Kenichi Iga, Ohmsha), the coupling coefficient κ is the degree to which light propagating in opposite directions is diffracted by a diffraction grating and is coupled per unit length. Show.

The semiconductor laser device of the present invention has a separate confinement hetero (SCH) structure. At this time, SC
It is conceivable to form a diffraction grating at the interface between the H layer and the cladding layer. However, in this case, it is not easy to achieve sufficient optical confinement in the SCH structure and control the normalized coupling coefficient κL to a small value. This is because if the difference in the refractive index between the SCH layer and the cladding layer is small, κL of the diffraction grating provided at the interface between them will be small, but at the same time the optical confinement ratio will also be decreased.

Therefore, the present inventor has devised a structure in which κL can be easily controlled to a small value without deteriorating the optical confinement. That is, in the present invention, the lower intermediate layer and the upper intermediate layer are provided between the SCH layer and the cladding layer, and the diffraction grating is provided at the interface between the upper intermediate layer and the cladding layer. The lower intermediate layer has a lower refractive index than the SCH layer. Therefore, SC
Light confinement in the H structure is enhanced. The upper intermediate layer has a higher refractive index than the lower intermediate layer and the cladding layer. Therefore, part of the light is confined in the upper intermediate layer. However, since the optical confinement in the SCH structure is enhanced, the optical coupling to the upper intermediate layer is not strong. Therefore, if a diffraction grating is provided in the upper intermediate layer, a small κL can be obtained. Thus, the present invention can achieve both good optical confinement to the SCH structure and small κL.

In the semiconductor laser device of the present invention, the diffraction grating is provided along the active layer. For this reason, it is possible to generate laser light in a multi-mode, but with a sufficiently narrow line width (light emission spectrum width). Like the DFB laser, the semiconductor laser device of the present invention has high wavelength stability, and
Is low. Since the multi-mode oscillation occurs, the Brillouin scattering in the optical fiber can be suppressed. Therefore, the semiconductor laser device of the present invention is suitable as a pumping light source for a Raman amplifier.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. Further, for convenience of illustration, the dimensional ratios in the drawings do not necessarily match those described.

FIG. 1 shows a semiconductor laser device 1 according to this embodiment.
It is a perspective view which shows the structure of 00. Semiconductor laser device 10
Reference numeral 0 denotes a lower cladding layer 12 sequentially deposited on the upper surface of the substrate 10, a multiple quantum well-separation confinement structure (MQW-S).
The CH) layer 14, the lower intermediate layer 15a, the upper intermediate layer 15b, and the first upper cladding layer 16 are provided. They are,
Both are mesa-type semiconductor layers. Both the substrate 10 and the lower cladding layer 12 are made of n-InP. MQW-SCH layer 14 and upper intermediate layer 15b
Is composed of quaternary compound semiconductor InGaAsP. Lower intermediate layer 15a and first upper clad layer 16
Is composed of p-InP. Lower clad layer 1
2 has a thickness of 5000 Å, and the first upper cladding layer 16
Has a thickness of 1000Å. Details of the MQW-SCH layer 14 and the intermediate layers 15a and 15b will be described later.

Lower clad layer 12, MQW-SCH layer 1
4, the intermediate layers 15a and 15b and the first upper cladding layer 1
Both side surfaces of 6 are covered with a buried layer 18.
The buried layer 18 is composed of p-InP. A current blocking layer 20 is provided on the buried layer 18. The current block layer 20 is composed of n-InP. A second upper cladding layer 22 is deposited on the first upper cladding layer 16, the buried layer 18, and the current blocking layer 20. The thickness of the second upper cladding layer 22 is about 15000Å. Second upper clad layer 22
Is composed of p-InP. As described above, the n-type current blocking layer 20 serves as the p-type second upper cladding layer 2
2 and the p-type buried layer 18 are interposed.

A contact layer 24 is deposited on the upper surface of the second upper cladding layer 22. The thickness of the contact layer 24 is about 5000Å. The contact layer 24 is p ⁺
-It is composed of InGaAs. Contact layer 24
An upper electrode layer 28 is deposited on the upper surface of the. Board 1
A lower electrode layer 30 is deposited on the lower surface of the layer 0. The second upper cladding layer 22 adjacent to the contact layer 24 may have higher p-type conductivity than the first upper cladding layer 16. The substrate 10 adjacent to the lower electrode layer 30 may have n-type conductivity higher than that of the lower clad layer 12.

The first upper clad layer 16, the second upper clad layer 22 and the lower clad layer 12 are formed of MQW-SCH.
It is for confining the light generated in the layer 14 in the MQW-SCH layer 14 and the upper intermediate layer 15b. Therefore, these cladding layers have a lower refractive index than the MQW-SCH layer 14 and the upper intermediate layer 15b.

In the following, referring to FIG. 2, MQW-
SCH layer 14, lower intermediate layer 15a, and upper intermediate layer 1
The structure of 5b will be described in detail. FIG. 2 shows MQW-SCH
It is a figure which shows the distribution of the band gap wavelength of the layer 14 and the intermediate | middle layers 15a and 15b. In FIG. 2, the vertical axis (x) represents the vertical (thickness direction) distance of the semiconductor laser device 100, and the horizontal axis (λ _g ) represents the bandgap wavelength.

As shown in FIG. 2, the MQW-SCH layer 14 includes a first SCH layer 42, a second SCH layer 44, an active layer 45, a third SCH layer 50 and a third SCH layer 50 from the lower cladding layer 12 toward the lower intermediate layer 15a. It is a multilayer semiconductor in which the fourth SCH layer 52 is sequentially stacked. Each layer is composed of quaternary compound semiconductor InGaAsP.

The first SCH layer 42 and the fourth SCH layer 52 are
Paired to form a separate confinement heterostructure. The first SCH layer 42 and the fourth SCH layer 52 have the same band gap. In the present embodiment, both the first SCH layer 42 and the fourth SCH layer 52 have a bandgap wavelength of 1.05 μm.

The second SCH layer 44 and the third SCH layer 50 are also
Paired to form a separate confinement heterostructure. The second SCH layer 44 and the third SCH layer 50 have the same band gap. In the present embodiment, both the second SCH layer 44 and the third SCH layer 50 have a bandgap wavelength of 1.1 μm.

The active layer 45 comprises a multiple quantum well (M) in which a plurality of barrier layers 46 and a plurality of well layers 48 are alternately laminated.
QW) structure. Each barrier layer 46 is 1.2μ
has a bandgap wavelength of m. Each well layer 48 has a bandgap wavelength according to a desired oscillation wavelength. In this embodiment, each well layer 48 has a bandgap wavelength of 1.48 μm. The number of well layers 48 is 3 to
It is preferably 5. The well layer 48 may have compressive strain. Specifically, the well layer 48 may have a compressive strain of 1% or more and 1.5% or less.

The MQW-SCH layer 14 has a refractive index distribution similar to the bandgap wavelength distribution. That is,
Well layer 48, barrier layer 46, SCH layers 44 and 50,
The SCH layers 42 and 52 have a higher refractive index in that order. The second SCH layer 44 and the third SCH layer 50 have the same refractive index. In addition, the first SCH layer 42 and the fourth SCH layer 52
Have the same refractive index. With such a refractive index distribution,
The light generated in the active layer 45 is well confined in the MQW-SCH layer 14.

Specifically, each well layer 48 has three layers.
It has a refractive index of 506. Each barrier layer 46 is
It has a refractive index of 312. The second SCH layer 44 and the third SCH layer 50 each have a refractive index of 3.244. First SCH layer 42 and fourth SCH layer 52
Respectively have a refractive index of 3.206. The lower clad layer 12 and the first upper clad layer 16 each have a refractive index of 3.100.

The lower intermediate layer 15a is deposited on the upper surface of the fourth SCH layer 52. The lower intermediate layer 15a, like the upper cladding layer 16, is made of p-InP.
The band gap of the lower intermediate layer 15a is determined by the fourth SCH layer 5
Wider than 2 bandgap. The band gap wavelength of the lower intermediate layer 15a is 1.0 μm. Lower middle layer 1
The refractive index of 5a is lower than that of the fourth SCH layer 52. The lower intermediate layer 15a has a refractive index of 3.100.

The upper intermediate layer 15b is deposited on the upper surface of the lower intermediate layer 15a. The upper intermediate layer 15b is MQW-
Like the SCH layer 14, it is made of InGaAsP. The band gap of the upper intermediate layer 15b is narrower than the band gaps of the lower intermediate layer 15a and the first upper cladding layer 16. The upper intermediate layer 15b has a bandgap wavelength of 1.05 μm. The refractive index of the upper intermediate layer 15b is lower than that of the lower intermediate layer 15a and the first upper cladding layer 1.
It is higher than the refractive index of 6. The upper intermediate layer 15b is 3.20.
It has a refractive index of 6.

The bandgap wavelength distribution and the refractive index distribution as described above can be realized by adjusting the composition of InP and InGaAsP.

The structure of the semiconductor laser device 100 will be further described below with reference to FIGS. 3 and 4. FIG. 3 is a vertical sectional view of the semiconductor laser device 100 taken along the line 3-3 in FIG. FIG. 4 is a plan sectional view of the semiconductor laser device 100 taken along the line 4-4 in FIG.

The front end face 32 of the semiconductor laser device 100 has an AR (Anti-Reflection: AR) so that the reflectance of the light generated in the active layer 45 is smaller than that of the crystal cleavage plane.
(Low reflection) coat is applied. The reflectance (AR reflectance) of the front end face 32 is preferably 0% or more and 5% or less. In this embodiment, the AR reflectance is 0.5%.
An HR (High Reflection) coating is applied to the rear end face 34 of the semiconductor laser device 100 so that the reflectance with respect to the light generated in the active layer is higher than that of the crystal cleavage plane. The reflectance (HR reflectance) of the rear end face 34 is 8
It is preferably 0% or more and 95% or less. In this embodiment, the HR reflectance is 90%. Semiconductor laser device 1
The laser light generated at 00 is emitted from the front end face 32 as indicated by an arrow 60.

Upper intermediate layer 15b and first upper cladding layer 1
A diffraction grating 36 is provided at the interface with 6. The diffraction grating 36 is a periodic structure provided on the upper surface of the intermediate layer 15b along the vertical direction (z direction in FIG. 1) of the semiconductor laser device 1. In this embodiment, the diffraction grating 36 has a sinusoidal shape. As shown in FIG. 4, the shape of the diffraction grating 36 is uniform along the horizontal direction (y direction in FIG. 1).

The diffraction grating 36 is formed along the entire length of the active layer 45. The diffraction grating 36 is a uniform diffraction grating having a uniform period (reference numeral Λ in FIG. 3) and a uniform depth (reference numeral a in FIG. 3). That is, the diffraction grating 36 has no phase shift. Depth a of the diffraction grating 36 (diffraction grating 3
The distance between the uppermost part and the lowermost part of 6) is 300Å.

The diffraction grating 36 constitutes a laser resonator.
Therefore, the length of the diffraction grating 36 is equal to the cavity length.
The length of the diffraction grating 36 is 900 μm. In the composition of the semiconductor laser device 100, it is preferable that the cavity length exceeds 600 μm, and particularly the cavity length is 900 to 1800 μm.
It is preferably m.

Referring again to FIG. 2, the MQW-SCH layer 14
Constituting each layer, the lower intermediate layer 15a and the upper intermediate layer 1
The dimension of 5b will be described. First to fourth SCH layers 4
The thicknesses of 2, 44, 50 and 52 are 150Å, respectively. The thickness of each barrier layer 46 is 100Å. The thickness of each well layer 48 is 50Å.

The thickness of the lower intermediate layer 15a is 3000Å. The basic thickness of the upper intermediate layer 15b is 150Å.
As described above, the diffraction grating 36 (depth 300Å) having a periodic structure is provided on the upper surface of the upper intermediate layer 15b. Therefore, the thickness of the upper intermediate layer 15b changes periodically along the vertical direction (z direction). In the present specification, the “basic thickness” means the diffraction grating 36 of the upper intermediate layer 15b.
It means the thickness of the part excluding. This is the upper intermediate layer 15b
Corresponds to the distance from the lower surface of the to the lowermost portion (the deepest portion) of the diffraction grating 36. The basic thickness is constant regardless of the vertical position. When the basic thickness is d and the depth of the diffraction grating 36 is a, the thickness of the upper intermediate layer 15b periodically fluctuates between d and d + a along the vertical direction.

By adjusting the width of the active layer 45, the transverse mode can be made single. For single mode in the lateral direction,
The width of the active layer 45 is preferably about 1 to about 5 μm. In this embodiment, the width of the active layer 45 is about 4 μm. When the transverse mode is single, the far field pattern (FFP) is stable, so the semiconductor laser device 100
And easy to couple with optical fiber. This is useful for using the semiconductor laser device 100 as a pumping light source for a Raman amplifier.

The standardized coupling coefficient of the semiconductor laser device 100 of this embodiment is less than 0.8. The normalized coupling coefficient is theoretically represented by κL. Here, κ is the coupling coefficient, and L is the length of the diffraction grating 36. Since the diffraction grating 36 constitutes a resonator as described above, L is equal to the resonator length.

The normalized coupling coefficient κL is obtained by measuring the spectrum of the output light of the semiconductor laser device 100. Specifically, if the width of the stop band (also called “forbidden band”) is obtained from the spectrum and the stop band width and the oscillation condition based on the mode coupling equation are used, the value of κL can be obtained. The formula expressing the oscillation condition is the same as that of the DFB laser. Note that the standardized coupling coefficient and oscillation conditions are "Basics of Semiconductor Lasers" (Toshiaki Suhara, Kyoritsu Shuppan) and "Semiconductor Lasers" (Japan Society of Applied Physics).
Edited by Kenichi Iga, Ohmsha).

When a forward voltage is applied to the semiconductor laser element 100 via the upper electrode layer 28 and the lower electrode layer 30, the semiconductor laser element 100 oscillates a laser. The principle of laser oscillation is the same as that of the DFB laser. That is, the diffraction grating 36 functions as a laser resonator to promote stimulated emission and laser oscillation.

FIG. 5 shows the spectrum of the output laser light from the semiconductor laser device 100. As shown in FIG. 5, the semiconductor laser device 100 performs multimode oscillation (longitudinal multimode oscillation). This is because κL is as small as less than 0.8.

The number of longitudinal modes contained in the emission spectrum of the semiconductor laser device of the present invention is arbitrary and it is not necessary to set an upper limit. The emission spectrum width is preferably about 10 nm or less. here,
"Emission spectrum width" refers to the width of the envelope of a plurality of longitudinal modes included in the emission spectrum. In the semiconductor laser device of the present invention, the emission spectrum width broadens as the normalized coupling coefficient κL decreases.

The reason why a small κL is obtained will be described below. By providing the lower intermediate layer 15a having a lower refractive index than the fourth SCH layer 52 on the fourth SCH layer 52, light is strongly confined in the MQW-SCH layer 14. The upper intermediate layer 15b includes the lower intermediate layer 15a and the upper clad layer 1.
Since it has a refractive index higher than 6, it functions as a second waveguide. However, the light confinement ratio of the upper intermediate layer 15b is low. This is because the optical confinement in the MQW-SCH layer 14 (first waveguide) is strong, and the upper intermediate layer 15
This is because b is relatively far from the active layer 45. When the light confinement ratio of the upper intermediate layer 15b is low, κL of the diffraction grating 36 provided in the upper intermediate layer 15b also becomes small.

The distance between the active layer 45 and the diffraction grating 36 in the vertical direction (x direction) is equal to the distance between the lower and upper intermediate layers 15.
It changes according to the thickness of a and 15b. κL is determined according to this distance. Therefore, the lower and upper intermediate layers 1
By adjusting the thickness of 5a and 15b, κL can be controlled accurately.

The semiconductor laser device 100 can be suitably used as a forward pumping light source for a Raman amplifier. This includes
There are four reasons. First, the laser oscillation light of the semiconductor laser device 100 is less likely to be scattered in the optical fiber. this is,
This is because the semiconductor laser device 100 oscillates in multimode. Secondly, the semiconductor laser device 100 has a smaller relative noise intensity (RIN) than the external fiber Bragg grating resonator type laser. This is because the semiconductor laser device 100 is an internal resonator type, and thus disturbance is unlikely to enter the resonator. Thirdly, the semiconductor laser device 100 has excellent wavelength stability. The laser oscillation wavelength of the semiconductor laser device 100 is substantially determined by the period of the diffraction grating 36. Therefore, like the existing DFB laser, the current dependence and temperature dependence of the wavelength are small. Fourthly, the semiconductor laser device 100 has a line width sufficiently narrow as a forward pumping light source of a Raman amplifier. This is because the oscillation wavelength is limited by the diffraction action of the diffraction grating 36.

The semiconductor laser device 100 also has the advantage of being easy to manufacture. This is because it is not necessary to form a shallow diffraction grating to achieve low κL. Although κL can be reduced by making the diffraction grating 36 shallow (thin), it is difficult to uniformly form the shallow diffraction grating over a wide area. In addition, it is difficult to control κL and the yield is low. In the present embodiment, the lower and upper intermediate layers 15
Low κL can be achieved by adjusting the thicknesses of a and b. Therefore, the diffraction grating 36 can be provided with a sufficient depth so as not to be difficult to manufacture. Generally, since the film thickness can be controlled with high accuracy, it is easier to adjust the thickness of the intermediate layers 15a and 15b than to control the depth of the shallow diffraction grating 36. Therefore, the semiconductor laser device 100 of the present embodiment can be manufactured while accurately controlling the standardized coupling coefficient κL to a small value.

The semiconductor laser device 100 also has an advantage that its electric resistance and thermal resistance can be suppressed low. The electric resistance and the thermal resistance increase as the distance between the active layer 45 and the diffraction grating 36 increases. In this embodiment, even if the distance between the active layer 45 and the diffraction grating 36 is relatively short, sufficiently low κL and multimode oscillation can be achieved. Thereby, electric resistance and thermal resistance can be suppressed. In the present embodiment, the lower intermediate layer 15a having a lower refractive index than the MQW-SCH layer 14 is provided on the MQW-SCH layer 14. Thereby, the optical confinement in the MQW-SCH layer 14 becomes strong. Accordingly, the optical coupling to the diffraction grating 36 becomes weak. Therefore, even if the distance between the active layer 45 and the diffraction grating 36 is relatively short, κL becomes sufficiently low, and multimode oscillation can be achieved. As a result, the distance between the active layer 45 and the diffraction grating 36 can be shortened, so that the electric resistance and thermal resistance of the semiconductor laser device 1 can be suppressed.

In contrast, it is possible to provide the diffraction grating 36 on the upper surface of the fourth SCH layer 52 without providing the intermediate layers 15a and 15b. In this case, to obtain low κL,
By increasing the distance between the active layer 45 and the diffraction grating 36,
Optical coupling to the diffraction grating 36 must be weakened. For that purpose, it is necessary to make at least one of the third and fourth SCH layers 50 and 52 thick. However, as the distance between the active layer 45 and the diffraction grating 36 increases, the electric resistance and thermal resistance of the semiconductor laser element also increase. This is not desirable.

The present invention has been described above in detail based on the embodiments thereof. However, the present invention is not limited to the above embodiment. The present invention can be variously modified without departing from the gist thereof.

In the above embodiment, the diffraction grating 36 is provided along the entire length of the active layer 45. However, the diffraction grating 36 may be provided along only a part of the length of the active layer 45. In the above embodiment, the diffraction grating 3
6 has a uniform period. However, the diffraction grating 36
May be a chirped diffraction grating having a period that continuously changes along the longitudinal direction.

[0051]

Since the semiconductor laser device of the present invention oscillates in multimode, it emits laser light that is difficult to scatter in the optical fiber. Further, in the semiconductor laser device of the present invention, since the laser resonator is constituted by the diffraction grating provided inside the device, the RIN is low and the wavelength stability is excellent. Therefore, the semiconductor laser device of the present invention can be suitably used as a forward pumping light source for a Raman amplifier. Further, by providing the lower intermediate layer on the SCH layer, light confinement in the SCH structure is enhanced. Therefore, the semiconductor laser device of the present invention is easy to manufacture, and the electric resistance and thermal resistance are easily suppressed.

[Brief description of drawings]

FIG. 1 is a perspective view showing a structure of a semiconductor laser device 100 according to an embodiment.

FIG. 2 is an MQW-SCH layer 14 and intermediate layers 15a, b.
It is a figure which shows the distribution of the band gap wavelength of.

FIG. 3 is a vertical sectional view of a semiconductor laser device 100.

FIG. 4 is a plan sectional view of a semiconductor laser device 100.

5 is a diagram showing an emission spectrum of the semiconductor laser device 100. FIG.

[Explanation of symbols]

10 ... Substrate, 12 ... Lower clad layer, 14 ... MQW-S
CH layer, 15 ... Intermediate layer, 16 ... First upper cladding layer, 1
8 ... Buried layer, 20 ... Current blocking layer, 22 ... Second upper cladding layer, 24 ... Contact layer, 28 ... Upper electrode layer, 30 ... Lower electrode layer, 42 ... First SCH layer, 44 ... Second SCH layer, 45 ... Active layer, 46 ... Barrier layer, 48 ... Well layer, 50 ... Third SCH layer, 52 ... Fourth SCH layer, 10
0 ... Semiconductor laser device.

Claims (4)

[Claims] 1. A substrate made of a first conductive type semiconductor, a lower clad layer provided on the upper surface side of the substrate and made of a first conductive type semiconductor, and provided on the lower clad layer. A lower isolation confinement heterostructure layer, an active layer provided on the lower isolation confinement heterostructure layer and having a multiple quantum well structure, an upper isolation confinement heterostructure layer provided on the active layer, the upper portion A lower intermediate layer deposited on the upper surface of the separate confinement heterostructure layer, an upper intermediate layer deposited on the upper surface of the lower intermediate layer, an upper intermediate layer deposited on the upper surface of the upper intermediate layer, and the first conductivity type. An upper clad layer made of a semiconductor of a different second conductivity type; an upper electrode provided on the upper clad layer; and a lower electrode provided on a lower surface side of the substrate, wherein the lower intermediate layer is , Above Has a lower refractive index than the isolated confinement heterostructure layer, the upper intermediate layer has a higher refractive index than the lower intermediate layer and the upper clad layer, and an interface between the upper intermediate layer and the upper clad layer. In the semiconductor laser device, a diffraction grating is provided along the active layer, and the oscillation longitudinal mode has an emission spectrum of two or more lines. 2. The semiconductor laser device according to claim 1, wherein the normalized coupling coefficient is less than 0.8. 3. The semiconductor laser device according to claim 1, further comprising a front end face having a low reflection coating and a rear end face facing the front end face and having a high reflection coating. 4. The substrate, the lower clad layer, the lower intermediate layer and the upper clad layer are composed of InP, and the lower isolated confinement heterostructure layer, the active layer, the upper isolated confinement heterostructure layer and the upper intermediate layer are InGa.
The semiconductor laser device according to claim 1, which is composed of AsP.