JPH10215022A - Semiconductor laser and generation of single polarized mode light - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to obtain the single polarization characteristic by utilizing the difference of the thermal expansion coefficients of a mounting member and a semiconductor laser element, generating the single polarization- mode light, and intentionally generating the non-strain state or compression/ tensile strain in a semiconductor laser. SOLUTION: A ridge-type semiconductor laser element 10 sequentially has an SiN insulating film 11, a P-side Au electrode 12, a P-type GaAs cap layer 13, a P-type AlGaAs clad layer 14, an AlGaAs barrier layer 15, a GaAs active layer 16, an AlGaAs barrier layer 17, an N-type AlGaAs clad layer 18, an N- type AlGaAs buffer layer 19, an N-type GaAs substrate 20 and an N-type Au electrode 21 from the lower side. The single quantum well laser without strain having the GaAs active layer 16 of 200Å in this ridge-type semiconductor element 10 is assembled by using the mounting member of SiC, GaAs and Cu. By using the mounting member such as this, the low-noise laser is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser and a method for generating single polarization mode light, and more particularly to a method for assembling a semiconductor laser by using a difference in thermal expansion characteristics between the semiconductor laser and a mounting material thereof. The present invention relates to a semiconductor laser and a method for generating low-noise single polarization mode light.

[0002]

2. Description of the Related Art An ordinary semiconductor laser device is assembled on a mount material having a high thermal conductivity, and is configured to suppress heat generation of the laser itself during operation.

[0003]

In such a case, it is known that the emission characteristics of the semiconductor laser are affected by differences in various characteristics between the semiconductor laser element and its mounting material. For example, [Ref. W. Epperlei
n, G. Hunziker, K .; Gatwyler,
U. Deutsch, H .; P. Dietrich an
d D. J. Webb: “Mechanical st
Less in AlGaAs lid case
rs: it's measurements and e
effect on the optical near
field ", Inst. Symp.
d Semicond. , San Diego, 18-
22 September. 1994, pp 483-4
[Reference document 2: K. 88]; Shigihara, Y .; Naga
i, S. Karakida, M .; Aiga, M .; Ots
ubo and K. Ikeda: “Estimati
on of strain arising from
the assembling process a
second influence of assemblin
g materials on performance
eoflaser diodes ", J. Appl.
Phys. 78 (3), 1995, pp1419-14
23].

In particular, the difference in the coefficient of thermal expansion between the semiconductor laser element and its mounting material causes distortion in the semiconductor laser, which has a large effect on the oscillation wavelength, threshold current, and polarization characteristics. For example, when a semiconductor laser is subjected to a two-dimensional tensile strain exceeding a certain force, oscillation
The mode changes from the TE mode to the TM mode, and polarization competition noise of the laser light occurs.

[0005] This is described in [Ref.
TANAKA: "780 nm BAND TM-MO
DE LASER OPERATION OF GaAs
sP / AlGaAs TENSILE-STRAIN
D QUANTUM-WELL LASERS ", EL
ECTRONICS LETTERS 1993, 2
9, 18, pp 1611-1613] [Reference 4: K.I. Magari, M .; Okamot
o, H .; Yasaka, K .; Sato, Y .; Noguc
hi and O.M. Mikami: "Polariza
Tion Insensitive Travelin
g wave type amplifier enhancer usi
ng strained multiple quan
tun well structure ”, IEEE
Photonics Technology. Lett. , 1
990, 2, pp 556-558].

According to the present invention, by selecting an appropriate mounting material, distortion is prevented from occurring in a semiconductor laser, or compression / tensile distortion is intentionally generated, and control is performed by mechanical distortion which can obtain a single polarization characteristic. It is an object of the present invention to provide a semiconductor laser and a method of generating single polarization mode light.

[0007]

In order to achieve the above object, the present invention provides: [1] a semiconductor laser, a mounting material, a semiconductor laser element formed on the mounting material; Means for applying mechanical strain to the semiconductor laser element, wherein a single polarization mode light is generated by utilizing a difference in thermal expansion coefficient between the mounting material and the semiconductor laser element.

[2] In the method of generating a single polarization mode light, by making use of a difference in thermal expansion coefficient between the semiconductor laser element and the mounting material, the laser is made to have no distortion, or a compression / tensile distortion is intentionally generated. A single polarization mode light is generated irrespective of the operating conditions of the laser so as to have low noise. [3] The method for generating single polarization mode light according to [2], wherein the active layer is at or near the bulk, and the subband of the valence band is degenerated or the separation is small. The thermal expansion coefficient of the semiconductor laser device is greater than or equal to the thermal expansion coefficient of the main material of the semiconductor laser device.
It is designed to receive the dimensional compression distortion, suppress the TM polarized light, generate the TE polarized mode light, and have low noise.

[4] In the method for generating single polarization mode light according to the above [2], in a semiconductor laser device having an active layer having a quantum well structure, a thermal expansion coefficient of a mount material is set to a thermal expansion coefficient of a main material of the semiconductor laser device. The coefficient of expansion is close to or higher than the coefficient of expansion. During the operation of the laser, the active layer of the semiconductor laser element is subjected to no distortion or two-dimensional compressive distortion so that the valence band sub-band structure of the active layer is not changed.
M polarization light is suppressed, TE polarization mode light is generated, and low noise is achieved.

[5] In the method for generating single polarization mode light according to the above [2], the active layer may be in the bulk or close to the bulk, and the valence band sub-band may be degenerated or the separation thereof may be small. The thermal expansion coefficient of the mounting material is smaller than the thermal expansion coefficient of the main material of the semiconductor laser element, so that the semiconductor laser element is subjected to two-dimensional tensile strain during laser operation to suppress TE polarized light, Light is generated and low noise is generated.

[6] In the method for generating single polarization mode light according to the above [2], in a semiconductor laser device having an active layer having a quantum well structure, the thermal expansion coefficient of a mount material is set to the thermal expansion coefficient of the main material of the semiconductor laser device. Smaller than the expansion coefficient, the active layer of the semiconductor laser device is subjected to two-dimensional tensile strain during laser operation, and changes the valence band sub-band structure of the active layer to suppress TE polarized light, The polarization mode light is generated so as to have low noise.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a sectional view showing a configuration of a semiconductor laser according to an embodiment of the present invention. In this figure, reference numeral 1 denotes a heat sink mounting material, on which a ridge type semiconductor laser element 10 is mounted face down on the heat sink mounting material 1.

That is, the ridge type semiconductor laser device 10 includes a SiN insulating film 11, a p-side Au electrode 12, a p-type GaAs cap layer 13, and a p-type AlGaAs
s cladding layer 14, AlGaAs barrier layer 15, GaAs
s active layer (200 °) 16, AlGaAs barrier layer 1
7, n-type AlGaAs cladding layer 18, n-type AlGaAs
It has an s buffer layer 19, an n-type GaAs substrate 20, and an n-side Au electrode 21.

Such a strain-free single quantum well laser having a 200 ° GaAs active layer 16 of a ridge-type semiconductor laser device is fabricated by using SiC, GaAs as shown in FIG.
Assembling was performed using s and Cu mounting materials (details will be described later). FIG. 2 shows the temperature dependence of the TE and TM mode polarization intensity ratios at the time of current injection twice the current threshold of this semiconductor laser.

In the laser mounted on SiC as shown in FIG. 3 (c), the TE and TM mode ratios are inverted at around 150K. In addition, when GaAs and Cu are used as the mounting material, the inversion of the polarization mode intensity ratio does not occur. Comparing the coefficients of thermal expansion of the respective mounting materials, SiC, GaAs, and Cu were 3.7, respectively.
6.5 and 17.0 (10 ⁻⁶ / ° C.). Since the semiconductor laser element is bonded onto the heat sink mount material at the melting point of the solder material, the operating temperature is naturally lower than this.
Assuming that the thermal expansion coefficient of the laser is equal to GaAs, and this is cooled to a low temperature, as shown in FIG.
In the mount, tensile strain, as shown in FIG.
The GaAs mount has no distortion, and as shown in FIG.
In Cu mount, compressive strain is generated in the laser.

In the band structure of an unstrained GaAs active layer having a quantum well structure, a band (hh) having a heavy hole in the valence band exists on the conduction band side from a band (lh) having a light hole, and the conduction band is higher. The electrons in the band and the holes in the hh band transition preferentially and contribute to laser emission. At this time, the polarization mode of the emitted light becomes TE mode from the relationship with the reflectivity of the laser end face, and the TM mode oscillation becomes It is forbidden. When two-dimensional compressive strain occurs in this quantum well structure, a similar transition occurs between bands, and TE mode oscillation becomes dominant.

However, when a two-dimensional tensile strain exceeding a certain level occurs, the lh band of the valence band is located closer to the conduction band than the hh band [Ref. 5: Kenichi Iga, "Semiconductor Laser""1994, pp113
(Ohmsha)]; [Ref. Ikegami:
“Reflectivity of Mode atF
acet and Oscillation Mode
in Double-Heterostructure
e Injection Lasers ", IEEE
Quantum Electron. 1972, QE-
8, 6, pp 470-476].

The above-described 200 ° quantum well laser is replaced with Si.
When mounted on a material that generates tensile strain at low temperature such as C, the upper end of the valence band in bulk and lh and hh
FIG. 4 is an explanatory diagram of the temperature change of the energy difference between the bands and the valence band structure at that time. Below a certain temperature, the lh band is located closer to the conduction band than the hh band, and at this time, the transition between the electrons in the conduction band and the holes in the lh band has priority. Polarization in the transition at this time is allowed in both the TE mode and the TM mode. However, the TM mode oscillates because the gain of the TM mode in the waveguide and the reflection surface in the laser exceeds the gain of the TE mode.

As described above, even the originally distortion-free laser oscillates in the TM mode under certain operating conditions depending on the mounting material, and the laser light source cannot maintain a single TE polarization mode. Therefore, in order to control the TM oscillation and obtain a low-noise laser with no polarization competition in the single polarization mode, the thermal expansion coefficient between the laser and the mounting material is taken into consideration,
It is necessary to select a mounting material.

Table 1 shows the TE / TM intensity ratio and the noise measurement results of each laser at about 100K.

[0021]

[Table 1] The noise is a minimum value based on the shot noise level. Similar to the laser having an active layer of 200 ° shown in FIG. 2, the laser having an active layer of 600 ° thick also has a strong TM polarization mode when SiC is used as a mounting material, and the C polarization mode is high.
When u is used, the TE polarization mode becomes strong. The noise at this time was 2.5 dB when the SiC mount was used, whereas it was 1.3 dB when the Cu mount was used, and the noise was reduced.

From this, it was found that the two-dimensional compressive strain caused by the mounting material on the semiconductor laser element suppressed the TM mode polarization, the polarization competitive noise was suppressed, and a low-noise laser was obtained. In Table 1, the lower row is 60 °
A laser having a quantum well active layer thickness of was mounted on a SiC mount material, and noise was strongly suppressed.
This is because the lh and hh bands of the valence band are separated by being strongly quantized, and the band structure such as bulk does not change.

From the above, it has been found that a low-noise laser can be obtained by using a mount material in which TM polarized light is suppressed such that excessive tensile strain occurs in the laser during operation. As a mounting material which does not generate tensile strain with respect to an AlGaAs-based laser, W-Cu or Mo having a larger thermal expansion coefficient than GaAs is used.
-Cu, Al-Si alloy or Al ₂ O _3, BeO, Cu or the like.

By appropriately selecting these mounting materials, polarization competition noise of laser light is suppressed, and a low-noise laser can be obtained. As an effect on the industry expected from the present invention, appropriate selection of a mount material is an effective means for reducing the noise of a semiconductor laser, and is effective for application to a semiconductor laser for optical fiber communication, and furthermore, at low temperatures, polarization competition noise. Is suppressed, which is effective as a method of assembling a laser serving as a light source of squeezed light.

It should be noted that the present invention is not limited to the above-described embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0026]

As described in detail above, according to the present invention, by selecting an appropriate mounting material, it is possible to reduce the distortion in the semiconductor laser and to intentionally generate compression and tensile strain, One polarization characteristic can be obtained. It is effective as a method for assembling a laser serving as a light source of squeezed light, because it is applied to a semiconductor laser for optical fiber communication having high reliability, and in particular, polarization competition noise is suppressed at low temperatures.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of a semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a diagram showing the temperature dependence of the ratio of polarization intensity between TE and TM modes at the time of injecting current twice the current threshold of the semiconductor laser light source of the present invention.

FIG. 3 is a view showing various mount materials showing an embodiment of the present invention.

FIG. 4 shows the calculation results of the temperature dependence of the energy difference between the upper end of the valence band and the lh and hh subbands when the quantum well laser is mounted on a material that generates tensile strain at a low temperature such as SiC. FIG. 4 is an explanatory diagram of a valence band structure at that time.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heat sink mount material 10 Ridge type semiconductor laser element 11 SiN insulating film 12 p-side Au electrode 13 p-type GaAs cap layer 14 p-type AlGaAs cladding layer 15, 17 AlGaAs barrier layer 16 GaAs active layer (200 °) 18 n-type AlGaAs cladding layer 19 n-type AlGaAs buffer layer 20 n-type GaAs substrate 21 n-side Au electrode

Claims (6)

[Claims] 1. A semiconductor laser device comprising: (a) a mounting material; (b) a semiconductor laser device formed on the mounting material; and (c) means for applying mechanical strain to the mounting material and the semiconductor laser device. d) A semiconductor laser, wherein a single polarization mode light is generated by utilizing a difference in thermal expansion coefficient between the mounting material and the semiconductor laser element. 2. A single polarized light regardless of the operating conditions of the laser, by using the difference in thermal expansion coefficient between the semiconductor laser element and the mounting material to de-strain the laser or deliberately generate compressive / tensile strain. A method for generating single polarization mode light, characterized by generating mode light and having low noise. 3. The method for generating single polarization mode light according to claim 2, wherein the active layer is at or near the bulk, the thermal expansion coefficient of the mount material is equal to or greater than the thermal expansion coefficient of the main material of the semiconductor laser device, and A method for generating single polarization mode light, wherein distortion applied to an active layer of a semiconductor laser element at a temperature suppresses TM polarization light, generates TE polarization mode light, and has low noise. 4. The semiconductor laser device according to claim 2, wherein the active layer has a quantum well structure, and the mount member has a thermal expansion coefficient of a main material of the semiconductor laser device. Characterized in that the strain applied to the active layer of the semiconductor laser device suppresses the TM polarized light, generates the TE polarized mode light, and is low noise. . 5. The method according to claim 2, wherein the active layer is at or near the bulk, the thermal expansion coefficient of the mounting material is smaller than the thermal expansion coefficient of the main material of the semiconductor laser device, and the operating temperature is lower. Wherein the strain applied to the active layer of the semiconductor laser element suppresses TE polarized light, generates TM polarized mode light, and has low noise. 6. The semiconductor laser device according to claim 2, wherein the active layer has a quantum well structure, wherein the mount material has a thermal expansion coefficient of a main material of the semiconductor laser device. A method for generating single polarization mode light, wherein the distortion applied to the active layer of the semiconductor laser element is smaller than that of the first embodiment, and the TE polarization light is suppressed, the TM polarization mode light is generated, and the noise is low.