JPH08195529A - Semiconductor laser epitaxial crystalline laminate and semiconductor laser - Google Patents

Abstract

PURPOSE: To provide a semiconductor laser which can oscillate at the exciting wavelength of an optical fiber amplifier and has a narrow vertical radiation angle and high reliability of a stable lateral mode even at the time of a high optical output and a semiconductor laser epitaxial crystalline laminate used therefor. CONSTITUTION: Low refractive index layers 4 and 12 having lower refractive index than those of upper and lower clad layers 3 and 13 are provided between the layers 3 and 13 for holding active layers 7 and 9 and upper and lower guide layers 5 and 11 provided between the clad layers and the active layers. The upper and lower clad layers and the upper and lower guide layers may be vertically asymmetrical with respect to the Al composition or the thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to form a semiconductor laser which can be used as a pumping light source for an optical fiber amplifier used for optical communication or the like and an SHG (optical second harmonic generation) light source, and a semiconductor laser therefor. Of the semiconductor epitaxial crystal laminate.

[0002]

2. Description of the ^Related Art An optical fiber amplifier using a silica-based fiber doped with Er ³⁺ ions as an amplification medium operates in the 1.55 μm band where the optical propagation loss of a silica-based single mode fiber (SMF) is minimized. Since it is possible, it is attracting attention as a key device for optical communication. As the Er ³⁺ ion excitation light source wavelength used for laser oscillation or amplification, the 1.48 μm, 0.98 μm, and 0.82 μm bands have been studied. In addition, recently, a 1.3 μm band optical fiber amplifier using a fluoride fiber doped with Pr ³⁺ ions has also been proposed and tested. In this case, the optimum excitation wavelength is 1.017 μm. In particular, it has been confirmed that the amplification efficiency is high and the noise characteristic is good in the 0.98 μm band, which is a promising excitation wavelength for the 1.55 μm band optical fiber amplifier. A Ti: sapphire laser has been used as the experimental pump laser in the 0.98 μm / 1.02 μm wavelength band.

On the other hand, recently, a strained quantum well laser using an InGaAs layer having a layer structure as shown in FIG.
Since it oscillates in the above wavelength band, it has been actively studied as a compact semiconductor laser for excitation. Regarding such a semiconductor laser, a 0.98 μm / 1.02 μm laser having a low threshold and high efficiency characteristics has been reported, but the vertical light emission angle is large and the coupling efficiency with a single mode fiber is insufficient. In addition, there is a problem that the transverse mode is not stable and the light is not sufficiently coupled into the single mode fiber at the time of high light output with increased drive current.

In order to improve the optical coupling efficiency with a single-mode fiber, it is important to reduce the vertical emission angle of light and to make the circle circular and enlarge the spot size. These can be realized by making the refractive index of the clad layer closer to the refractive index of the active layer or by thinning the active layer to loosen the light confinement in the active layer. From the results of the studies so far, a vertical light emission angle of 23 °, a characteristic temperature of 100 K, and a coupling efficiency of about 50% with a single-mode fiber have been obtained. If the optical confinement is reduced to meet the above requirements, a sharp increase of the oscillation threshold, deterioration of external differential quantum efficiency, occurrence of saturable absorption, deterioration of temperature characteristics, and reliability, which are problems.

[0005]

The object of the present invention is to
Highly reliable semiconductor laser that oscillates in the 86 μm to 1.07 μm band, particularly wavelengths of 0.98 μm and 1.02 μm, has a narrow vertical emission angle of 20 ° or less, and has stable transverse mode even at high light output, and its fabrication Another object of the present invention is to provide a semiconductor laser epitaxial crystal laminated body used for.

[0006]

In order to achieve the above object, the invention according to claim 1 comprises an active layer, an upper clad layer and a lower clad layer sandwiching the active layer from above and below,
A semiconductor epitaxial crystal laminated body including an upper guide layer and a lower guide layer respectively provided between the both clad layers and the active layer, wherein between the upper clad layer and the upper guide layer and the lower clad. A low-refractive index layer having a refractive index smaller than that of a nearby clad layer is provided between the layer and the lower guide layer.

Here, in the semiconductor laser epitaxial crystal laminate according to claim 1, the active layer comprises at least one layer.

[0008]

[Outside 2]

The upper or lower cladding layer is made of Al _x Ga _1-x As,
The upper or lower guide layer may be Al _y G _1-y As (y <
x), wherein the low refractive index layer is Al _z Ga.
_It may be composed of _1-z As (z> x).

The upper and lower clad layers may have asymmetrical Al compositions, and the upper and lower guide layers and the upper and lower low refractive index layers may have asymmetrical Al compositions and asymmetric thicknesses.

Further, each layer has In of 1 × 10 ¹⁹ to 2 × 1.
The layered body, which is doped at a concentration of 0 ²⁰ cm ⁻³ and has the In concentration in the above range, has a layered structure including the above layers.
It may be provided on the aAs substrate.

The invention according to claim 8 is a semiconductor laser, which is characterized in that it includes each of the semiconductor epitaxial crystal laminates.

[0013]

In the semiconductor epitaxial crystal laminate of the present invention, a low refractive index layer having a smaller refractive index than the cladding layer is introduced between the cladding layer and the guide layer to slightly increase the light confinement in the active layer. Alternatively, the distance of leakage of light to the cladding layer can be increased while suppressing the amount to a slight extent. Therefore, the leakage of light can be controlled to the far side of the clad layer and the light emitting region can be expanded without substantially deteriorating the light confinement near the active layer in the optical waveguide.

In general, the light emitted from the crystal facet is diffracted and has a finite radiation angle (far field image).
However, if the light source is small, the diffraction will be large. Therefore, if the leakage of light is large as in the present invention, the radiation angle is expected to be small.

Here, the far-field image is approximately given by the Fourier transform of the electric field distribution (near-field image) in the near field, which coincides with the case of the diffraction image by the slit.
It can be qualitatively understood that the larger the slit, the larger the diffraction, and the smaller the slit, the smaller the diffraction.

In addition to the structure having the low refractive index layer, by introducing an asymmetrical structure of the upper and lower refractive index distributions, the point of maximum light intensity is determined from the quantum well layer where electrons and holes are recombined to the SCH. Degradation due to end face melting (COD: Catalyst
Hic Optical Damage), and the presence of the low refractive index layer,
The deterioration of optical output characteristics due to carrier overflow is also improved. Therefore, the low refractive index layer and the asymmetrical refractive index structure enable the vertical narrow emission angle and stable high light output operation.

Further, as a substrate, In-codoped n-Ga is used.
If each epitaxial layer is co-doped with In of approximately the same concentration using an As substrate and has the above-described refractive index structure, the deterioration characteristics of the laser can be dramatically improved while maintaining the above characteristics. Can be improved. This is because In pinning the propagation of crystal transition, which is a major cause of the deterioration mode.

[0018]

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

(First Embodiment) FIG. 1A is a sectional view showing a first embodiment of a semiconductor laser epitaxial crystal laminated body according to the present invention, and FIG. 1B is an Al in the structure of FIG. It is a graph which shows composition and In composition. In FIG. 1, 1 is an n ⁺ -GaAs substrate, 2 is an n ⁺ -GaAs buffer layer, 3 is an n-Al _x Ga _1-x As clad layer (lower clad layer), and 5 and 11 are Al _z Ga _1-z. As guide layer (lower guide layer and upper guide layer), 6 and 1
0 is an AlGaAs SCH layer (SCH: Separate)
e-Confinement-Heterostruc
true, AlGaAs: Al composition ratio is any value from 0 to guide layer composition ratio z), 8 is AlG
aAs barrier layer (Al composition is equal to or smaller than SCH layer), 7 and 9 are

[0020]

[Outside 3]

(Although it is a double quantum well in the figure, it can be selected from a single layer to five layers), 13 is p-Al _x G
a _1-x As clad layer (upper clad layer), 14 is p ⁺
-GaAs cap layer. In the semiconductor epitaxial crystal laminate of this example, the upper and lower clads Al _x G
a _1-x As layers 13 and 3 and upper and lower guides Al _y Ga _1-y
Al from the clad layer near the As layers 11 and 5
It is characterized by having Al _z Ga _1-z As low refractive index layers 12 and 4 (upper low refractive index layer and lower low refractive index layer) having a large composition. The guide layer has a thickness d _g of 0.1 to 0.
And the thickness d _n of the low refractive index layer is 0.1 to 5 μm.
It is set to 0.5 μm. For comparison, FIG. 2 shows a structure of a conventional example having no low refractive index layer as described above.

In order to realize the structure shown in FIG. 1A, first, an epitaxial crystal growth apparatus (MOVPE) is used.
Method: metalorganic vapor phase epitaxy or MBE: molecular beam epitaxy) to grow epitaxial layers 2 to 14. As a typical value, the Al composition x of the lower clad layer 3 and the upper clad layer 13 is 0.15 to 0.3.
5 μm, the Al composition z of the lower guide layer 5 and the upper guide layer 11 is 0.1 to 0.3 μm, and Se, Si or the like as the n-dopant is the lower cladding layer 3, the lower low refractive index layer 4 and the lower guide layer 5. To about 5 × 10 ¹⁷ cm ⁻³ . Zn, Mg, Be or the like is used as the p-dopant,
The upper guide layer 11, the upper low refractive index layer 12 and the upper cladding layer 13 are each doped with about 5 × 10 ¹⁷ cm ⁻³ . The lower guide layer 5 and the upper guide layer 11 are used by lightly doping n or p or undoped. Further, in order to enable laser oscillation at 0.98 μm / 1.02 μm,

[0023]

[Outside 4]

As a typical example, the In composition ratio α and the thickness t of α = 0.2, t = 10 nm or α = 0.
3, t = 7.5 nm. The cap layer 14 has an ohmic electrode of 5 × 10 ¹⁹ cm ⁻³ or more of Zn or the like p.
Dopant high concentration doping.

After the crystal growth, the cap layer (contact layer) 14 and the cladding layer 13 are processed to form a ridge having a width of about 1.5 to 3 μ as shown in FIG. To form this ridge, a resist is patterned by photolithography, and the contact layer 14 and the clad layer 13 are etched by wet or dry etching using the resist as a mask. The depth is determined in consideration of the transverse mode within a range in which the higher mode does not stand, and the upper low refractive index layer 12
Alternatively, the upper guide layer 11 may be etched. After forming the ridge, the mask is removed, an insulating film 15 (SiO ₂ , SiN, etc.) is formed on the entire surface by sputtering or the like, and the insulating film on the ridge is etched off. Ai or Ti / P
The p electrode 16 such as t / Au is formed and n ⁺ -GaA is formed.
An n-electrode 17 made of AuGeNi or the like is formed on the lower surface of the s substrate 1. After that, ohmic sintering is performed to obtain the semiconductor laser structure shown in FIG.

In the present embodiment, the ridge laser is shown as an example of the refractive index guiding structure, but other laser structures such as a buried laser are naturally conceivable.

FIG. 4 shows an example of the optical field distribution (near-field image) in the waveguide of the semiconductor laser including the conventional semiconductor laser epitaxial structure not including the low refractive index layer, and FIG.
An example of the optical field distribution (near-field image) in the waveguide of the semiconductor laser including the structure sandwiching the low refractive index layer of the present invention is shown in FIG. As shown in FIG. 5, in the present invention, the introduction of the low-refractive index layer slightly increases the light confinement in the active layer and increases the light leakage from the active layer to the cladding layer. 6 and 7 show calculated values of far field patterns (far field images). The vertical radiation angle full width at half maximum is 23 ° and 13 °, respectively. As shown in FIG. 7, it can be seen that in this example, the introduction of the low refractive index layer broadens the field distribution and reduces the vertical radiation angle.

FIG. 8A shows the relationship between the light confinement efficiency and the vertical light emission angle, and FIG. 8B shows the experimental result of the relationship between the oscillation threshold and the characteristic temperature. Circles in FIGS. 8A and 8B represent the results of the present invention. The vertical light emission angle of 16 ° (calculated value of 13 °) was experimentally obtained without lowering the light confinement efficiency. From this result, according to the present invention, the light confinement efficiency is not reduced, that is, It can be seen that the vertical light emission angle can be reduced without increasing the oscillation threshold Ith and deteriorating the characteristic temperature To. Further, as shown in (b), in the temperature characteristic of the threshold value, the deterioration of the characteristic temperature at 60 ° C. or higher is improved as compared with the case where the low refractive index layer is not provided, and the conventional 500 m
It was also confirmed that the light output characteristics that were thermally saturated at about W were able to achieve high output up to about 600 mW because the presence of the low-refractive layer having a high Al composition suppresses thermal overflow of carriers into the cladding.

On the other hand, the horizontal radiation angle of the ridge laser can be controlled by the ridge depth, and can be realized in the range of 6 to 13 ° almost independently of the vertical radiation angle. Therefore, in the semiconductor laser having the epitaxial structure of the present invention, the ratio of the vertical radiation angle to the horizontal radiation angle (elliptic ratio) is 1.
It can be seen that the spot can be rounded down to about 2 to 2 and that the coupling rate with the single-mode fiber can be easily obtained at 70 to 80% in combination with the reduction of the absolute value of the vertical radiation angle. It was

As described above, according to this embodiment, a high-power optical fiber pumping laser required for optical communication in the 0.98 μm / 1.02 μm band having a long life can be easily realized.

(Second Embodiment) FIG. 9 is a sectional view showing a second embodiment of the semiconductor laser epitaxial crystal laminate of the present invention. In the present embodiment, the Al composition of the lower cladding layer 3, the lower low refractive index layer 4 and the lower guide layer 5 is lower than the Al composition of the upper layers 11 to 13, and the low refractive index layers 4 and 12 and the guides. Regarding the thicknesses of the layers 5 and 11, the upper portion is thinner, and the upper and lower layers are asymmetrical with respect to the active layer.

In this embodiment, as compared with the first embodiment, the place where the light intensity is maximum in the waveguide is shifted from the active layer due to the asymmetry of the refractive index distribution introduced into the semiconductor laser epitaxial structure. Therefore, it was found that the mode stability at high light output was increased, and the narrow vertical emission angle and the low threshold were realized, while the crystallographic damage of the active layer was not easily received. It was also found that in this example, if the light leakage to the upper guide layer is selected smaller than that of the lower clad layer, the deterioration of the oscillation threshold can be suppressed without making the upper clad layer so thick. .

(Third Embodiment) FIG. 10A is a sectional view showing a third embodiment of the semiconductor laser epitaxial crystal layered body of the present invention, and FIG. 10B is a cross sectional view of the structure A of FIG.
It is a graph which shows 1 composition and In composition. In this embodiment,
On the In-codoped GaAs substrate 18, a laser epitaxial layer having the same structure as that of the first embodiment in which all layers are In-doped is laminated. In FIG. 10A, the epitaxial structure of the first embodiment is shown, but a laminated body of the epitaxial structure of the second embodiment may be laminated on the In co-doped GaAs substrate 18. The surface morphology of the obtained crystals is
It is lattice-matched to the substrate 18 co-doped with In, no cracks and the like are seen, which is comparable to that of a normal GaAs substrate. The In co-doped GaAs substrate used in this embodiment can be manufactured by a method of co-doping In during GaAs crystal growth by the horizontal / vertical Bridgman method or the pulling method.

In the structure of this embodiment, the light output characteristics as a laser and the electrical resistance value are not changed as compared with the case where In is not doped, and no adverse effect on static characteristics due to In codoping is observed. It was Further, in this embodiment,
In addition to the narrow vertical emission angle and low threshold characteristics in each of the above-mentioned examples, dislocations are hard to enter due to the crystal hardening effect of In co-doping, etc., and a highly stable and high output laser which was unprecedented in the long term was obtained. .

[0035]

As described above, in the semiconductor laser epitaxial crystal laminate of the present invention, by introducing a layer having a lower refractive index than the cladding layer between the cladding layer and the guide layer, the active layer can be formed. The leakage of light can be equivalently spread to the cladding layer without reducing the light confinement. Therefore, the vertical narrowing radiation angle can be realized without causing the increase of the oscillation threshold and the deterioration of the temperature characteristics which are usually seen in the past. Coupling to a single mode fiber is also dramatically improved by reducing the radiation angle and improving the ellipticity of the beam. Further, due to the barrier effect of the low refractive index layer, carrier overflow can be suppressed during high current operation, and higher light output can be obtained as compared with the conventional case. In addition, by co-doping In on all epitaxial layers including the substrate, the above features are maintained even during high light output, and further, the propagation of dislocations in the crystal is suppressed, and a more reliable laser is realized. it can.

[Brief description of drawings]

FIG. 1A is a sectional view showing a first embodiment of a semiconductor laser epitaxial crystal laminate of the present invention, and FIG.
3 is a graph showing Al composition and In composition in the structure (a).

2A is a cross-sectional view of a conventional semiconductor laser epitaxial crystal laminate, and FIG. 2B is a graph showing Al composition and In composition in the structure of FIG.

FIG. 3 is a cross-sectional view showing a laser configuration example using an epitaxial film having a structure of a semiconductor laser epitaxial crystal laminate of the present invention.

FIG. 4 is a graph showing calculated values of light intensity distribution in a waveguide centered on an active layer in a semiconductor laser having a conventional epitaxial structure.

FIG. 5 is a graph showing calculated values of light intensity distribution in a waveguide centered on an active layer in a semiconductor laser having an epitaxial structure of the present invention.

FIG. 6 is a far-field image (calculated value) corresponding to the near-field image of FIG.
It is a graph which shows.

FIG. 7 is a far-field image (calculated value) corresponding to the near-field image of FIG.
It is a graph which shows.

FIG. 8A is a graph showing the relationship between the light confinement count and the light vertical emission angle, and FIG. 8B is a graph showing the relationship between the light confinement coefficient, the threshold current value, and the characteristic temperature.

FIG. 9A is a sectional view showing a second embodiment of the semiconductor laser epitaxial crystal layered body of the present invention, showing a structure having an asymmetric refractive index distribution and thickness,
(B) is a graph showing Al composition and In composition in the structure of (a).

FIG. 10 (a) is a sectional view showing a third embodiment of the semiconductor laser epitaxial crystal laminate of the present invention, wherein I
FIG. 3B is a graph showing a laser epistructure formed on an n-codoped substrate and doped with In in all layers, and FIG. 6B is a graph showing Al composition and In composition in the structure of FIG.

[Explanation of symbols]

1 n ⁺ -GaAs substrate 2 n ⁺ -GaAs buffer layer 3 n-Al _x Ga _1-x As clad layer (lower clad layer) 4 n-Al _z Ga _1-z As low refractive index layer (lower low refractive index layer _{) 5 n-Al y Ga 1} -y As guide layer (lower guide layer) 6 AlGaAs SCH layer

[Outside 5] 8 AlGaAs barrier layer 10 AlGaAs CSH layer _{11 p-Al y Ga 1-} y As guide layer (upper guide _{layer) 12 p-Al z Ga 1} -z As low refractive index layer (upper low refractive index layer) 13 p-Al _x Ga _1-x As clad layer (upper clad layer) 14 p ⁺ -GaAs cap layer 15 insulating layer 16 p electrode 17 n electrode 18 In co-doped n-GaAs substrate

Claims (8)

[Claims] 1. An active layer, an upper clad layer and a lower clad layer sandwiching the active layer from above and below, and an upper guide layer and a lower guide layer provided between the both clad layers and the active layer, respectively. In the semiconductor epitaxial crystal layered body including, a low refractive index having a smaller refractive index than the neighboring clad layers between the upper clad layer and the upper guide layer and between the lower clad layer and the lower guide layer, respectively. A semiconductor laser epitaxial crystal laminate provided with an index layer. 2. The semiconductor laser epitaxial crystal laminate according to claim 1, wherein the active layer is at least one layer. The upper or lower clad layer is made of Al _x Ga _1-x As, and the upper or lower guide layer is made of Al _y G _1-y As (y <x). , The low refractive index layer is Al _z Ga _1-z As (z
> X), which is a semiconductor laser epitaxial crystal laminated body. 3. The semiconductor laser epitaxial crystal laminate according to claim 2, wherein the upper cladding layer and the lower cladding layer have different Al compositions. 4. The semiconductor laser epitaxial crystal laminate according to claim 2, wherein the upper guide layer and the lower guide layer have different Al compositions. 5. The semiconductor laser epitaxial crystal laminated body according to claim 2, wherein the upper clad layer and the lower clad layer have different thicknesses. 6. The semiconductor laser epitaxial crystal laminated body according to claim 2, wherein the upper guide layer and the lower guide layer have different thicknesses. 7. The semiconductor laser epitaxial crystal laminated body according to claim 2, wherein each layer is doped with In at a concentration of 1 × 10 ¹⁹ to 2 × 10 ²⁰ cm ⁻³ , and The semiconductor laser epitaxial crystal laminated body, wherein the laminated body composed of the respective layers is provided on an In co-doped GaAs substrate having an In concentration in the above range. 8. A semiconductor laser comprising the semiconductor laser epitaxial crystal laminate according to any one of claims 1 to 7.