JP3468612B2 - Semiconductor laser 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 structure of a semiconductor laser, and more particularly, it has a structure in which a stripe width is changed in a resonator direction, such as a high power semiconductor laser and a low cost semiconductor laser in an optical communication system and an optical information system. Regarding semiconductor lasers.

[0002]

2. Description of the Related Art Currently, there is a demand for a high-power and high-reliability semiconductor laser for writing on optical disks and magneto-optical disks. Further, a high output and high reliability 0.98 μm band semiconductor laser has been actively studied as a light source for exciting a rare earth-doped optical fiber amplifier used in a repeater or a receiver in an optical transmission system. Further, a semiconductor laser capable of passive alignment is being researched for the purpose of cost reduction. These semiconductor lasers are required to operate in a stable fundamental mode and have a wide light spot size at the light emitting end face. In a semiconductor laser, lateral mode control is performed by providing stripe-shaped regions having a high refractive index in the cavity direction. The width of the region called the stripe is generally constant along the cavity direction. By the way, in order to obtain stable transverse mode characteristics, it is necessary to narrow the stripe width. On the other hand, widening the stripe width is an effective method for realizing a wide light spot. In order to satisfy these two contradictory requirements, a structure in which the stripe width is changed in the resonator direction has been reported. For example, SPIE vol. There is a report by Kenji IKEDA et al. Described on pages 893 and 79.

[0003]

[Problems to be Solved by the Invention] FIG.
3 shows the stripe structure reported by KEDA et al. The stripe width increases linearly in the resonator direction. However, if the stripe width is changed in the resonator direction by such a method, the transverse mode conversion due to the change of the stripe width cannot be smoothly performed. For this reason, mode loss occurs during conversion, and lateral modes such as side peaks in the far-field image in the horizontal direction of the active layer due to defective mode conversion become unstable, which is not suitable for practical use.

It is an object of the present invention to provide a semiconductor laser having a structure in which the stripe width is changed in the cavity direction, in which no mode loss occurs due to mode conversion and the transverse mode is stable.

[0005]

The above object is to provide an active layer for generating light on a semiconductor substrate, a semiconductor clad layer for confining the light, a resonator structure for obtaining laser light from the generated light, and a resonator structure along the resonator direction. In a semiconductor laser device having a region having a higher effective refractive index than other portions in a stripe shape, the width of the region having a higher effective refractive index changes along the cavity direction, and the width of the region is 1 in the cavity direction. This is achieved in a semiconductor laser device in which the second derivative is continuous in the cavity direction and changes without an inflection point. Further, the above object is achieved by changing the width of a region having a high effective refractive index along the resonator direction according to an exponential function. Further, it is achieved when the width of the region having a high effective refractive index changes along the resonator direction according to an exponential function, and the width of the region having a high effective refractive index is constant in the vicinity of the end face. In particular, this is achieved when the portion where the width of the region having a high effective refractive index is constant is 100 μm or less. Furthermore, the above-mentioned object is significantly effective in a semiconductor laser module using these semiconductor laser devices. Further, the above-mentioned purpose is to set the oscillation wavelength to 0.
In a semiconductor laser device of 9 μm to 1.1 μm, the width of the region having a high effective refractive index is constant inside the resonator and the width of the region having a high effective refractive index at the end face is 1 μm or more wider than inside the resonator, and It is achieved when the length of the width of the region having a high effective refractive index changes from 60 μm to 200 μm.

[0006]

The function of the present invention will be described below.

First, the change in stripe width in the resonator direction will be described. First, the realization of a high power and high reliability laser will be described. The main factor limiting high power operation is the occurrence of kinks. The kink is generated by the beam shift or the start of oscillation in the higher order mode. In any case, this is a practical problem. These kinks occur because the carrier density in the central portion of the stripe, where the optical density is high, decreases as compared with the peripheral portion, and the gain distribution inside the stripe becomes large in the peripheral portion. Therefore, in order to increase the kink generation light output, the waveguide may be formed so that the light distribution becomes smaller with respect to the carrier diffusion length. That is, it is effective to reduce the stripe width. On the other hand, the cause of element deterioration during high-power operation is melting of crystals in the high light density portion of the end face portion. This is COD (Catastrophic)
This is called Optical Damage) deterioration. This CO
In order to suppress the D deterioration, the operating light density at the end face may be reduced. That is, the light spot size may be increased. For this purpose, it is effective to widen the stripe width. Thus, in order to realize a high-power and high-reliability laser, the contradictory conditions described above must be satisfied. Therefore, a wide stripe is formed on the front surface where high light density is a problem to widen the light spot to reduce the light density, and a narrow stripe is formed on the rear end surface where the light density is relatively small to stabilize the transverse mode. The structure in which the stripe width changes in the cavity direction is an effective method for realizing a high power and high reliability laser. Also,
Another effective method is to narrow the stripe width inside the element that determines the transverse mode and widen the stripe width at the end face where high light density is a problem. At this time, a sufficient effect can be obtained by making the stripe width at the end face portion 1 μm or more wider than that inside the element. Next, a semiconductor laser capable of passive alignment for the purpose of cost reduction will be described. At present, when a module is manufactured using a semiconductor laser, an axis deviation tolerance is as small as about 1 μm when performing alignment with an optical fiber, so that the optical axis is adjusted while operating the semiconductor laser. By omitting this step, it is possible to significantly reduce the cost. That is,
Cost reduction can be realized by performing alignment with the optical fiber without operating the semiconductor laser. Therefore, it is necessary to increase the axis deviation tolerance to about 10 μm. For a semiconductor laser, it is required that the spot size on the light emitting end face be about 10 μm.
As an approach for this, there is a method in which the stripe width at the end face of a laser having a ridge waveguide structure is extremely narrowed to 1.5 μm or less. According to this method, since the stripe width becomes narrow and the optical field distribution is large and exudes to the outside of the stripe, the area of the low refractive index outside the ridge effectively pushes the optical field distribution largely to the substrate side. Can be expanded. In this method, since the confinement coefficient of light into the active layer is small in the portion where the stripe width is small, if the same narrow stripe structure is used in all the resonators, the threshold current is increased and the device characteristics are deteriorated. Therefore, it is desirable that the stripe width is narrow at the light emitting end face of the element and wide at the other portions. As mentioned above,
It can be seen that the structure in which the stripe width changes in the cavity direction is extremely effective in realizing a high-power high-reliability laser and a semiconductor laser capable of passive alignment for the purpose of cost reduction.

Next, a method of changing the stripe width in the resonator direction with high efficiency and without mode conversion loss will be described. Optical field distribution E (x,
y, z) follows the following formula.

E (x, y, z) = X (x, z) Y (y)
exp (i (ωt−βz)) where ω is the angular frequency and β is the propagation constant.
Further, x, y, and z respectively indicate the horizontal direction, the vertical direction, and the resonator direction in the active layer. As shown in this equation, it is understood that the optical electric field distribution is distributed in the cavity direction by an exponential function. The change in the optical electric field distribution due to the change in stripe width is represented in X (x, z). Since the z-direction dependence of X (x, z) mainly depends on the change in stripe width, the mode can be smoothly changed in the resonator direction by using the change in stripe width as an exponential function.

[0010]

Embodiments of the present invention will be described below with reference to FIGS.

[Embodiment 1] A first embodiment of the present invention is shown in FIG.
The description will be made using .about.2. In this embodiment, the present invention is applied to a 0.98 μm band high power semiconductor laser for pumping a rare earth-doped optical fiber amplifier used in a repeater or a receiver in an optical transmission system. Figure 1 shows a striped structure
FIG. 2A shows a sectional structure, and FIG. 2B shows an enlarged view of the active layer. Next, a method of manufacturing the device will be described. GaAs buffer layer 2, G on n-GaAs substrate 1
n-InGaP cladding layer 3 lattice-matched to an aAs substrate, In (1-x) Ga (x) As (y) P (1-y)
Barrier layer (x = 0.82, y = 0.63, barrier layer thickness 35n
m) 13 and 15 and a strained quantum well active layer 4 composed of In (z) Ga (1-z) As strained quantum well layers (z = 0.16, well layer thickness 7 nm) 14, lattice-matched to a GaAs substrate. The p-InGaP cladding layer 5, the p-GaAs optical waveguide layer 6, the p-InGaP cladding layer 7 lattice-matched to GaAs, and the p-GaAs cap layer 8 are sequentially formed by the MOVPE method, the gas source MBE method, or the CBE method. To do. Next, using the oxide film as a mask, a ridge as shown in FIG. 2A is formed by a photoetching process. Etching at this time may be performed by any method such as wet etching, RIE, RIBE, and ion milling. The oxide film that serves as a mask has the shape shown in FIG. Etching is p-
The GaAs optical waveguide layer 6 is completely removed, and the p-InGaP clad layer 5 is formed so as not to reach the strained quantum well active layer 4.
Stop halfway through. Next, the oxide film used as the etching mask is used as a mask for selective growth, as shown in FIG.
As shown in (a), the n-InGaP current confinement layer 9 is
Selective growth is performed by the VPE method. After that, the wafer is taken out from the growth furnace, and the oxide film used as the selective growth mask is removed by etching. Then, the p-GaAs contact layer 10 is formed by MOVPE method or MBE method. After forming the p-side electrode 11 and the n-side electrode 12, a laser device having a cavity length of about 900 μm was obtained by the cleavage method. After that, a low reflection film made of Al2O3 having a thickness of λ / 4 (λ: oscillation wavelength) is formed on the front surface (ws (L) side) of the element, and SiO2 and a-Si are formed on the rear surface (ws (0) side) of the element. A high-reflectivity film consisting of a four-layer film was formed. Then, the device was bonded on a heat sink with the bonding surface facing down. The prototype device continuously oscillated at room temperature with a threshold current of about 10 mA, and its oscillation wavelength was about 0.98 μm. The element is 6
Transverse single mode oscillation was stably performed up to 00 mW. No side peak was observed in the far-field image at this time. Even if the light output is increased, the end face deterioration does not occur, and the maximum light output is 8
00 mW was limited by thermal saturation. Further, when 30 elements were continuously driven at a constant light output of 200 mW under an environment temperature of 80 ° C., the initial drive current was about 250 mA, and all the elements stably operated for 100,000 hours or more.

[Embodiment 2] A second embodiment of the present invention is shown in FIG.
And 5 will be described. The present embodiment is used for writing on an optical disk or a magneto-optical disk, 0.8 μm.
It is applied to a high power semiconductor laser. Figure 4
FIG. 4A shows a sectional structure, FIG. 4B shows an enlarged view of the active layer, and FIG. 5 shows a stripe structure. Here, in the case of the present embodiment, the length (Lc) of the region where the stripe width is constant
Is 50 μm. Next, a device manufacturing method will be described. GaAs buffer layer 1 on n-GaAs substrate 16
7, n-Al (x) Ga (1-x) As cladding layer (x
= 0.5) 18, Al (y) Ga (1-y) As barrier layer (y = 0.3, barrier layer thickness 5 nm) 28 and Al (z) Ga
(1-z) As quantum well layer (z = 0.1, well layer thickness 7n
m) 29, and Al (s) Ga (1-s) As SCH
(Separate Confinement Het
erostructure) layer (s = 0.35, layer thickness 1
0 nm) 30 and the quantum well active layer 19, p
-Al (t) Ga (1-t) As cladding layer (t = 0.
5) 20, p-Al (u) Ga (1-u) As beam expanding layer (u = 0.3) 21, p-Al (v) Ga (1-)
v) As clad layer (v = 0.55) 22, p-GaA
The s cap layer 23 is sequentially formed by MOVPE method or MBE method. Next, a ridge as shown in FIG. 4A is formed by a photoetching process using the oxide film as a mask. Etching at this time is wet, RIE, RIB
Any method such as E or ion milling may be used. The oxide film that serves as a mask has the shape shown in FIG. The etching completely removes the p-Al (u) Ga (1-u) As beam expanding layer (u = 0.3) 21 and p-Al (t) Ga so as not to reach the quantum well active layer 19. (1-t)
The As clad layer (t = 0.5) 20 is stopped halfway. Next, using the oxide film used as the etching mask as a mask for selective growth, as shown in FIG.
The GaAs current confinement layer 24 is selectively grown by the MOVPE method. After that, the wafer is taken out from the growth furnace, and the oxide film used as the selective growth mask is removed by etching. After that, the p-GaAs contact layer 25 is formed by MOVP.
It is formed by the E method or the MBE method. p-side electrode 26, n
After forming the side electrode 27, the resonator length is about 60 by the cleavage method.
A laser device of 0 μm was obtained. After this, the front surface of the device (ws
SiO2 with a thickness of λ / 4 (λ: oscillation wavelength) on the (L) side
On the rear surface (ws (0) side) of the element,
A high-reflection film composed of a four-layer film composed of O2 and a-Si was formed. Then, the device was bonded on a heat sink with the bonding surface facing upward. The prototype device continuously oscillates at room temperature with a threshold current of about 14 mA and its oscillation wavelength is about 0.78.
was μm. The device stably oscillated in the transverse single mode up to 300 mW. Also, the maximum light output is 500 mW
The above optical output was obtained. Further, when 30 elements were continuously driven at a constant light output of 150 mW under an environmental temperature of 80 ° C., the initial drive current was about 200 mA, and all the elements stably operated for 50,000 hours or more.

[Embodiment 3] A third embodiment of the present invention will be described with reference to FIGS. This embodiment is applied to a 1.3 μm band semiconductor laser capable of passive alignment for the purpose of cost reduction. FIG. 6 shows a sectional structure, and FIG. 1 shows a stripe structure. Next, a device manufacturing method will be described. n- (100) InP substrate 31
N-InGaAsP layer (composition wavelength: 1.10 μm, layer thickness: 0.05 μm) and n-InP layer (layer thickness: 0.05 μm)
10-period superlattice structure auxiliary waveguide layer 32, n-InP
The spacer layer 33, the InGaAsP well layer (composition wavelength: 1.
37 μm, well layer thickness 6 nm) and InGaAsP barrier layer (composition wavelength 1.10 μm, barrier layer thickness 8 nm) 7
The periodic multiple quantum well active layer 34, the InGaAsP upper optical guide layer (composition wavelength 1.10 μm) 35, the p-InP cladding layer 36, and the p-InGaAs cap layer 37 are MO.
The layers are sequentially formed by the VPE method. Next, using the oxide film as a mask, a ridge as shown in FIG. 6 is formed by a photoetching process. The oxide film serving as a mask has the shape shown in FIG. The stripe direction is the [011] direction. At this time, the p-InP clad layer 36 is etched by wet etching using a mixed solution of hydrobromic acid and phosphoric acid to form an inverted mesa-shaped ridge waveguide having a (111) A plane as a side wall as shown in the figure. Can be formed.
Next, a silicon oxide film 38 is formed on the entire surface of the substrate, and then a polyimide resin 39 is formed on the entire surface of the substrate. Further, a silicon oxide film window is formed on the upper surface of the ridge by the etch back method. Finally, after forming the p-side electrode 40 and the n-side electrode 41, a laser element having a cavity length of about 400 μm was obtained by the cleavage method. After that, SiO2 is formed on the rear surface (ws (L) side) of the device.
And a highly reflective film composed of a four-layer film of a-Si was formed.
Then, the device was bonded on a heat sink with the bonding surface facing upward. The prototype device has the element front end face (ws
(0) stripe width 0.5 μm, device rear end face (ws
A device having a stripe width of 2.5 μm (on the (L) side) continuously oscillates at room temperature with a threshold current of about 10 mA and has a luminous efficiency of 0.
It was 53 W / A. The spot diameter of the laser beam emitted from the front facet of the element is 10 μm and the ridge width is 2.5 μm.
4 compared to the beam spot diameter of about 2.5 μm at the rear end face
It was doubled. From these, it is understood that the mode conversion is efficiently performed without loss. When this laser and a single mode fiber with a core diameter of 10 μm were coupled using an optical lens, a coupling loss of 2 dB or less was realized with horizontal and vertical positioning accuracy of ± 3 μm. Furthermore, when a module was produced using this laser and a single mode fiber having a core diameter of 10 μm, a module with a yield of 90% and a coupling loss of 2 dB or less could be obtained without driving the laser during alignment of the laser and the fiber. . Also, the ambient temperature for 30 elements is 90 ° C
When the long-term reliability was evaluated under the above condition, all the devices operated stably for 100,000 hours or more.

[Fourth Embodiment] A fourth embodiment of the present invention will be described with reference to FIGS. In this embodiment, the present invention is applied to a 0.98 μm band high power semiconductor laser for pumping a rare earth-doped optical fiber amplifier used in a repeater or a receiver in an optical transmission system. 7 and 8 show a stripe structure, FIG. 2A shows a sectional structure, and FIG. 2B shows an enlarged view of the active layer. Next, a method of manufacturing the device will be described. A GaAs buffer layer 2 on the n-GaAs substrate 1, an n-InGaP clad layer 3 lattice-matched to the GaAs substrate, In (1-x) Ga (x) As (y) P (1
-Y) barrier layers (x = 0.82, y = 0.63, barrier layer thickness 35 nm) 13 and 15 and In (z) Ga (1-z) A
s strained quantum well layer (z = 0.16, well layer thickness 7 nm) 14
Strained quantum well active layer 4, p-InGaP clad layer 5 lattice-matched to a GaAs substrate, p-GaAs
Optical waveguide layer 6, p-InGaP lattice-matched to GaAs
The cladding layer 7 and the p-GaAs cap layer 8 are MOVPE
Method, gas source MBE method, or CBE method. Next, using the oxide film as a mask, a ridge as shown in FIG. 2A is formed by a photoetching process. Etching at this time is wet, RIE, RIB
Any method such as E or ion milling may be used. The oxide film that serves as a mask has the shape shown in FIG. 7 or 8.
The etching completely removes the p-GaAs optical waveguide layer 6 and p-In so as not to reach the strained quantum well active layer 4.
The GaP clad layer 5 is made to stop halfway. next,
Using the oxide film used as the etching mask as a selective growth mask, the n-InGaP current confinement layer 9 is selectively grown by the MOVPE method as shown in FIG. After that, the wafer is taken out from the growth furnace, and the oxide film used as the selective growth mask is removed by etching. Then p-
The GaAs contact layer 10 is formed by MOVPE or MBE.
It is formed by the method. After forming the p-side electrode 11 and the n-side electrode 12, a laser device having a cavity length of about 900 μm was obtained by the cleavage method. At this time, the length (Lt) of the region where the stripe width changes was 100 μm. In the stripe-shaped element shown in FIG. 8, the region (Lf) having a constant stripe width near the end face was set to 5 to 20 μm. In any case, the stripe width (wi) inside the element is 2.5.
and the stripe width (wf) at the end face was 4.5 μm. After that, a low reflection film of Al2O3 having a thickness of λ / 4 (λ: oscillation wavelength) is formed on the front surface of the element, and SiO 2 is formed on the rear surface of the element.
A high reflection film was formed by a 6-layer film composed of 2 and a-Si. Then, the device was bonded on a heat sink with the bonding surface facing down. The prototype device produced continuous oscillation at room temperature at a threshold current of about 10 to 14 mA and an oscillation wavelength of about 0.98 μm in any stripe shape. The device stably oscillated in transverse single mode up to 580 mW. No side peak was observed in the far-field image at this time. Further, even if the light output was increased, the end face deterioration did not occur, and the maximum light output 800 mW was limited by heat saturation. Further, when 30 elements were continuously driven at a constant light output of 200 mW under an environmental temperature of 80 ° C., the initial drive current was about 240 mA, and all the elements stably operated for 100,000 hours or more.

The GRIN-SCH (Gra) in which the composition of the SCH layer is changed stepwise in the active layer of the above-mentioned embodiment.
ded Index-Separate Confin
element Heterostructure) As an active layer. Further, since the present invention does not depend on the waveguide structure, for example, in addition to the above-described embodiment, a BH (Buried Heterostructure) is used as the waveguide structure.
e) A structure may be used. Further, since the present invention does not depend on the material system, InGaAsP on the InP substrate described above is used.
System, InGaAsP system on GaAs substrate, AlGaAs system on GaAs substrate, as well as AlI on InP substrate
It can also be applied to nGaAsP type, InAlGaP type on a GaAs substrate, and the like. Further, as the oscillation wavelength, the above-mentioned 0.
98 μm band, 0.8 μm band, 1.3 μm band, and 0.6
It goes without saying that the present invention can be applied to all wavelength ranges that can be realized by a semiconductor laser, such as the μm band and the 1.55 μm band.

[0016]

According to the present invention, in a semiconductor laser having a structure in which the stripe width is changed in the cavity direction, a semiconductor laser in which a mode loss due to mode conversion does not occur and a transverse mode is stable is realized. Therefore, high reliability is realized in the high power semiconductor laser. Furthermore, we have realized a semiconductor laser that can be passively aligned by an easy method to improve yield and reduce cost.

[0017]

[Brief description of drawings]

FIG. 1 is a diagram showing a stripe structure according to the present invention.

FIG. 2 is a diagram showing a first embodiment according to the present invention.

FIG. 3 is a diagram showing a stripe structure according to a conventional structure.

FIG. 4 is a diagram showing a second embodiment according to the present invention.

FIG. 5 is a diagram showing another stripe structure according to the present invention.

FIG. 6 is a diagram showing a third embodiment according to the present invention.

FIG. 7 is a diagram showing another stripe structure according to the present invention.

FIG. 8 is a diagram showing another stripe structure according to the present invention.

[Explanation of symbols]

1 n-GaAs substrate, 3 n-InGaP clad layer, 4 strained quantum well active layer, 5 p-InGaP clad layer, 6 p-GaAs optical waveguide layer, 7 p-InGaP clad layer, 16 n-GaAs substrate, 18 n-AlGaAs cladding layer, 19 quantum well active layer, 20 p-AlGaAs cladding layer, 21 p-AlGaAs beam expanding layer, 22 p-AlGaAs cladding layer, 31 n-InP substrate, 32 auxiliary waveguide layer, 34 quantum well active layer, 36 p-cladding layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazunori Shinoda 1-280 Higashi Koigokubo, Kokubunji City, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor Kazuhisa Uomi 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi Ltd. (72) Inventor Masahiro Aoki 1-280, Higashi Koigokubo, Kokubunji, Tokyo Hitachi, Ltd. Central Research Laboratory (72) Inventor, Hiroshi Sato 1-280, Higashi Koikeku, Kokubunji, Tokyo Hitachi Central Lab. 56) References JP 5-67845 (JP, A) JP 6-174982 (JP, A) JP 9-23036 (JP, A) JP 7-106686 (JP, A) JP 59-144193 (JP, A) JP-A-1-140787 (JP, A) IEEE Journal of Quantu Electronics, 1992 years, 28 [2], p. 447-458 Electronics Letters, 1988, 24 [18], p. 1182-1183 (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (2)

(57) [Claims] 1. A resonator structure for obtaining a laser beam from the generated light, an active layer for generating light on a semiconductor substrate, a semiconductor clad layer for confining the light, and a stripe structure along the resonator direction from other portions. Also in a semiconductor laser device having a region with a high effective refractive index, the width of the region with a high effective refractive index changes along the cavity direction according to an exponential function , and the first derivative of the width in the cavity direction is the cavity. It is continuous in the direction and changes without an inflection point , and
The width at the light emitting end face from the laser is the light emitting end face
A semiconductor laser device characterized in that it is larger than the end face on the side opposite to . 2. A resonator structure for obtaining a laser beam from the generated light, an active layer for generating light, a semiconductor clad layer for confining the light, and a stripe structure along the resonator direction from other portions. Also in a semiconductor laser device having a region with a high effective refractive index, the width of the region with a high effective refractive index changes along the cavity direction according to an exponential function, and the width of the region with a high effective refractive index is near the end face. A semiconductor laser device, wherein the width is constant at the light emitting end face from the laser and is larger than the end face on the side opposite to the light emitting end face.