JP3010817B2 - Semiconductor optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical device, and more particularly to a semiconductor optical device suitable for application to a semiconductor laser, a semiconductor optical amplifier and the like used in an optical communication system.

[0002]

2. Description of the Related Art In a conventional semiconductor laser, particularly a distributed feedback (DFB) semiconductor laser which oscillates in a single longitudinal mode even during modulation, a method of increasing the relaxation oscillation frequency which determines the upper limit of high-speed modulation is known as DFB. It is known that a minus detuning system in which the laser oscillation wavelength λB determined by the Bragg reflection of the structure is set to a shorter wavelength side than the gain peak wavelength λg of the active region medium is extremely effective.
On the other hand, a quantum well semiconductor laser that oscillates at a wavelength of 1.55 μm with the least optical fiber loss is expected as a light source for long-distance optical fiber communication due to its low chirping property reflecting the quantum size effect of the quantum well structure. I have. One example of this is IEE Photonics Technology Letters, Boroom 2, Number 4,
(1990), pp. 229-230 [IEEE Pho
tonics Technology Letters, vol.2, No.4, pp.229-23
0, (1990)].

[0003]

First, the minus detuning method in the above prior art will be described. D
The laser oscillation wavelength λB determined by the Bragg reflection of the FB structure is 1.55 μm, and the degree of minus detuning is particularly preferably −10 to −30 nm. Therefore, it is necessary to set the gain peak wavelength λg of the active region medium to about 1.57 μm. In order to satisfy this, the I shown in FIG.
From the relationship between the nGaAs quantum well layer thickness and the gain peak wavelength λPL of the active region medium (Δa / a = 0%), InGaAs
It is necessary to set the thickness of the s quantum well layer to about 90 °. However, this method has the following problems. In FIG.
The dependency of the differential gain (dg / dn) representing the degree of the quantum size effect of the quantum well structure on the thickness of the InGaAs quantum well layer is shown. To perform minus detuning, InGa
When the thickness of the As quantum well layer is increased to about 90 °, the differential gain (d
g / dn) is greatly reduced. For this reason, although the negative detuning was performed as indicated by the white circles in FIG. 4, the relaxation oscillation frequency fr was conversely reduced . Activity of quantum well structure
A semiconductor optical device having a region is disclosed in, for example, Japanese Patent Application Laid-Open No.
JP, JP-A-3-3384, and JP-A-2-1309
No. 88, which solves the above problem
No technology could be found.

It is an object of the present invention to provide a semiconductor optical device in a 1.55 μm band quantum well type DFB semiconductor laser which has both a negative detuning method and a quantum size effect of a quantum well structure.

[0005]

Means for Solving the Problems To achieve the above object, the present inventors have proposed a method of forming a quantum well layer made of a compound semiconductor material such as InGaAs in order to sufficiently exploit the quantum size effect of a quantum well structure. 40-80 °, and the gain peak wavelength λg of the medium in the active region is set to about 1.57 μm.
It discloses a configuration in which a relatively small strain is introduced into the quantum well layer in order to perform the negative detuning by setting to m. In other words, the element composition ratio (for example, InGaAs of InGaAs) of the compound semiconductor constituting the quantum well layer
(Molar ratio) is slightly different from the lattice matching composition to the semiconductor substrate.

[0006]

The effects of the present invention will be described below with reference to FIGS. The strain amount of the InGaAs layer on the InP substrate changes depending on the In molar ratio. This strain amount Δa / a is determined by InP
Let the lattice constant of the substrate be as and the lattice constant of the InGaAs layer be a
If it is w, it can be expressed by the following equation.

Δa / a = (aw−as) / as An example of the dependency of the gain peak wavelength λg of the active region medium on the thickness of the InGaAs quantum well layer when Δa / a is used as a parameter (Δa / a = ± 0) .4%) is shown in FIG. For example, when Δa / a is + 0.4%, it can be seen that a gain peak wavelength λg of 1.57 μm can be obtained by setting the thickness of the InGaAs quantum well layer to 54 °. As can be seen from FIG. 3, the thickness of the quantum well layer is a thickness that can sufficiently bring out the quantum size effect of the quantum well structure. The film thickness is 55 ° at the distortion amount + 0.4% thus manufactured.
The detuning dependence of the relaxation oscillation frequency fr of the 1.55 μm band quantum well type DFB semiconductor laser having the InGaAs quantum well layer of FIG. It can be seen that the relaxation oscillation frequency fr increases with the degree of minus detuning.

As described above, the gain peak wavelength λg can be controlled by the distortion amount Δa / a, and the degree of detuning of the DFB laser can be adjusted. At this time, if the strain amount Δa / a is too large, a defect occurs in the semiconductor crystal. Therefore, the maximum value is preferably ± 0.5%. On the other hand, if it is too small, the control width of the gain peak wavelength λg is small,
The absolute value of a / a is required to be at least ± 0.2%. Therefore, the range of the strain amount Δa / a in the present invention is:
It is desirable that 0.2% ≦ | Δa / a | ≦ 0.5% and the thickness of the quantum well layer Lw be 40 ° ≦ Lw ≦ 80 °, which is a range that sufficiently brings out the quantum size effect of the quantum well structure. The In molar ratio y of the InGaAs layer corresponding to the above strain amount Δa / a is 0.46 ≦ y ≦ 0.50 or 0.56 ≦
y ≦ 0.61.

The present invention is also effective in increasing the gain saturation output level in a semiconductor optical amplifier in the 1.55 μm band, since the quantum size effect of the quantum well structure can be sufficiently obtained. Further, the present invention is similarly effective for a high-output semiconductor laser having a wavelength of 1.48 μm for an erbium-doped fiber optical amplifier.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

[Embodiment 1] FIG. 1 shows the present invention at 1.55 μm.
This is applied to a band quantum well type DFB semiconductor laser. InGaP on n-InP substrate 1 having diffraction grating 2
AsP light guide layer 3, multiple quantum well active region 4, p-I
The nP cladding layer 5 is formed sequentially. After a mesa stripe is formed by etching until it penetrates the active region, p-InP6 and n-InP7 are formed so as to bury the active region. Thereafter, a p-side electrode 8 and an n-side electrode 9 were formed, and cleaved to a resonator length of 150 to 500 μm to form an element. Here, the multiple quantum well active region 4 has a thickness of 60
Å, InGaAs quantum well layer (In molar ratio: 0.575) 4a having a strain amount Δa / a of 0.3% and In
This is a periodic structure of the GaAsP barrier layer 4b. This cycle is
It can be 1 to 25 cycles. The oscillation wavelength of the prototype device was about 1.55 due to Bragg reflection by the diffraction grating.
0 μm, and the detuning is about −20 nm compared to about 1.570 μm of the gain peak wavelength λg of the active region medium.
Can be controlled. As a result, the relaxation oscillation frequency fr (black circle) at a pulse current of 40 mA is about 12 GHz as shown in FIG.
And fr can be doubled as compared with the conventional detuning method (open circles) for increasing the thickness of the InGaAs quantum well layer. In addition, since the quantum size effect of the quantum well structure is sufficiently taken out, 20% at 2.4 Gbit / s modulation.
The chirping amount of dB down is the conventional quantum well structure DF.
Compared to the B laser, the current can be reduced to about 0.20 nm, which is about a half of that of the B laser. Further, the threshold current can be effectively reduced, and a threshold current of 10 mA or less can be stably obtained.

[Embodiment 2] FIG. 5 shows the present invention at 1.48 μm.
This is applied to a high power semiconductor laser. n-In
On the P substrate 1, an InGaAs quantum well layer 10a having a thickness of 55 ° and a strain amount Δa / a of −0.3% and InG having a thickness of 100 ° are formed.
A multiple quantum well active region 10 having a 2 to 15 period structure of the aAsP barrier layer 10b and a p-InP cladding layer 5 are sequentially formed. Thereafter, a mesa stripe is formed by etching until it penetrates the active region in the same manner as in the first embodiment to form a stripe structure with an active region width of about 1.5 μm, and then a p-side electrode 8 and an n-side electrode 9 are formed. Then, cleavage is performed to a resonator length of 150 to 1000 μm to form an element. The prototype device has a threshold current of about 1 at a wavelength of 1.48 μm.
It oscillates at 5mA and reflects the quantum size effect and has a maximum output of 3
00mW.

Embodiment 3 FIG. 6 shows an embodiment in which the present invention is applied to a semiconductor optical amplifier. Film thickness 5 on n-InP substrate 1
A multiple quantum well active region 11 having a 2-15 periodic structure of an InGaAs quantum well layer (In molar ratio: 0.59) 11a having a thickness of 5% and a strain amount Δa / a of + 0.4% and an InGaAsP barrier layer 11b having a thickness of 100%. , P-InP cladding layer 5
Are sequentially formed, and a mesa stripe is formed by etching until it penetrates the active region in the same manner as in the first embodiment to form a stripe structure. Next, a window structure in which the semiconductor film near the end face is removed and embedded with the Fe-doped InP layer 12 is obtained. A p-side electrode 8 and an n-side electrode 9 are formed, cleaved to a resonator length of 200 to 1000 μm, a low-reflection film 13 made of a dielectric film is formed on both end surfaces, and the reflectance is further reduced. The rate can be 0.003%. The gain peak wavelength λg of the prototype device is about 1.550 μm, and the high quantum size effect of the quantum well structure can be sufficiently brought out, so that the gain saturation output level is 20 to 25 dBm, which is equivalent to that of the conventional quantum well structure semiconductor optical amplifier. It can be larger than that.

[0014]

According to the present invention, the distortion amount Δa / a or I
By controlling the n mole ratio, it is possible to provide a 1.55 μm band quantum well type DFB semiconductor laser that has both the negative detuning method and the quantum size effect of the quantum well structure. As a result, it is effective in reducing the threshold current, increasing the relaxation oscillation frequency, and reducing chirping. The present invention is also effective for increasing the gain saturation output level in a semiconductor optical amplifier in the 1.55 μm band, since the quantum size effect of the quantum well structure can be sufficiently obtained. Further, the present invention is similarly effective for a high-output semiconductor laser having a wavelength of 1.48 μm for an erbium-doped fiber optical amplifier.

[Brief description of the drawings]

FIG. 1 is a structural diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the present invention.

FIG. 3 is a diagram for explaining the operation of the present invention.

FIG. 4 is a diagram for explaining the operation of the present invention.

FIG. 5 is a diagram for explaining an embodiment of the present invention.

FIG. 6 is a diagram for explaining an embodiment of the present invention.

[Explanation of symbols]

1 ... n-InP substrate, 2 ... diffraction grating, 4, 10, 11 ...
Multiple quantum well active region, 4a, 10a, 11a... Strain amount Δ
InGaAs having an absolute value of a / a of 0.2 to 0.5%
Quantum well layers, 4b, 10b, 11b... InGaAsP barrier layers.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Makoto Takahashi 1-280 Higashi Koikebo, Kokubunji-shi, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (56) References JP-A-5-29715 (JP, A) JP-A-4 -373190 (JP, A) JP-A-4-284683 (JP, A) JP-A-2-130988 (JP, A) Electronics Letters Vol. 22, No. 23, pp. 1246-1247 (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3/18 JICST file (JOIS)

Claims (4)

(57) [Claims] A semiconductor substrate, an active region formed on the semiconductor substrate and having a quantum well layer and a barrier layer having a larger bandgap than the quantum well layer, and a cladding layer for confining light. And the lattice constant of the semiconductor substrate is a
s, the lattice constant of the quantum well layer is defined as aw, the strain amount Δa / a of the quantum well layer is defined as Δa / a = (aw−as) / as, and the film thickness of the quantum well layer is defined as Lw. 0.2% ≦ | Δa / a | ≦ 0.5% 40 ° ≦ Lw ≦ 80 ° A semiconductor substrate, an active region formed on the semiconductor substrate and having a quantum well layer and a barrier layer having a larger bandgap than the quantum well layer, and a cladding layer for confining light. Wherein the semiconductor substrate is InP;
The quantum well layer is formed of InGaAs.
When the In molar ratio of aAs is y and the thickness of the quantum well layer is Lw, 40 ° ≦ Lw ≦ 80 ° and 0.56 ≦ y ≦ 0.61 or 0.46 ≦ y ≦ 0.50. A semiconductor optical device that satisfies the relationship. 3. The semiconductor optical device according to claim 1, wherein the semiconductor optical device has a diffraction grating near the active region. 4. An active region having a semiconductor substrate, a quantum well layer formed on the semiconductor substrate, and a barrier layer having a larger bandgap than the quantum well layer, a cladding layer for confining light, A diffraction grating disposed in the vicinity of the active region, wherein the peak wavelength of light generated from the active region is λg, the Bragg wavelength of the diffraction grating is λB, the thickness of the quantum well layer is Lw, Let the lattice constant of the semiconductor substrate be a
s, when the lattice constant of the quantum well layer is aw and the strain amount Δa / a of the quantum well layer is defined as Δa / a = (aw−as) / as, 0.2% ≦ | Δa / a | ≦ 0.5%, λB−λg <0.