JP2001060739A - Surface emission laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance stability of polarizing direction while keeping a high coupling efficiency with an optical fiber by employing a multiple quantum well having a well layer of a specified substance introduced with compressive distortion in an active layer. SOLUTION: The surface emission laser is fabricated by epitaxially growing an n-type distributed feedback reflector 2, an AlGaAs spacer layer 3, an InAlGaAs/AlGaAs multiple quantum well active layer 4, an AlGaAs spacer layer 5, and a p-type distributed feedback reflector 6 sequentially on an n-type GaAs substrate 1. The AlGaAs spacer layer 3 comprises a nondoped Al0.6Ga0.4As spacer layer and the InAlGaAs/AlGaAs multiple quantum well active layer 4 comprises a multiple quantum well layer of three well layers including a well layer of nondoped In0.6Al0.156Ga0.65As and a barrier layer of nondoped Al0.3Ga0.7As. When a current is injected to a surface emission laser having such a structure under room temperature, laser oscillation is confirmed at an oscillation wavelength of 0.85 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-emitting laser device having a controlled polarization used for optical interconnection and two-dimensional parallel signal processing.

[0002]

2. Description of the Related Art A surface emitting laser device is capable of two-dimensional high-density integration and has a circular light-emitting pattern, which facilitates coupling with an optical fiber. Important as a light source.

However, the polarization direction of oscillation is random due to good symmetry in the light emitting surface, which causes excessive noise when high-speed transmission is performed. The random polarization direction also causes transmission errors when applied to a system using free space or a polarization direction-dependent system such as an optical memory.

[0004] Thus, there have been reported many attempts to control the polarization direction of laser light in a certain direction within a plane. For example, T. Mukaihara et.al., IEEE Photonics Tec
hnology Letters, vol 5, PP. 133-135 (1993), a method of applying anisotropic stress in the (100) plane, and KDChoquetto et.al., IEEE Photonics Te.
As disclosed in Chnology Letters, vol. 6, PP. 40-42 (1994), there is a type in which a waveguide mode is controlled by giving anisotropy to a cross-sectional structure in a waveguide direction. In other cases, the off-substrate whose substrate orientation is inclined several degrees from the (100) plane to the (011) or (011 ~) (1 to 1 represents a bar on 1) plane direction And anisotropic optical gain, and furthermore, by fabricating on an inclined substrate such as an inclined (n11) plane or (1nn) plane, the difference in in-plane optical gain obtained from crystal anisotropy. There are things that use anisotropic increase. Among them, the manufacturing method is the same as that on the (100) plane, and a surface emitting laser device manufactured on an inclined substrate that can make the emission shape circular when the coupling with the optical fiber is considered is promising. It is considered to be.

In addition, there is no crystal anisotropy on a plane equivalent to the (111) plane inclined at about 55 degrees from the (100) plane toward the (011) or (011 ~) plane.
On a plane equivalent to the inclined (1nn) plane (n> 1), it is difficult to cleave in a rectangular shape due to the cleavage characteristics of the crystal of the zinc blende structure. Therefore, the inclination angle is preferably 15 ° to 40 °. it is conceivable that.

A polarization controlled surface emitting laser using such an inclined substrate is disclosed in M. Takahashi et.al., IEEE Photonics Tes.
chnology Letters, vol 8, PP737-739 (1996) and A. Mizu
taniet.al., Japan Journal of Applied Phisics, vol 3
7, PP1408 (1998).
m band and K. Tateno et.al., Applied Phisics Lett
ers, vol 70, PP3395 (1997), there is a 0.85 μm band, both of which are about 2 from the (100) plane to the (011) plane or the (011 ~) plane.
It is fabricated on a GaAs (311) substrate inclined at 5 °.

[0007] Then, the notation of the exponential plane is described in Tateno e
According to the report of t.al., the waveguide structure is 2
Even in the case of a circular shape symmetric with respect to the axis,
2 to 33 to show oscillation characteristics polarized in the> direction. Here, the polarization direction and the direction orthogonal thereto, in the above example, (31
1 ~) The <2-33 ~> direction in the plane and the direction orthogonal thereto <
The light intensity ratio in the 01 ~ 1 ~> direction is called the orthogonal polarization suppression ratio.

In general, even when the laser beam is on a normal (100) plane, it is difficult to form an active layer using a lattice matching system when a 0.98 μm band laser is fabricated on a substrate which can be easily obtained at low cost. , A GaAs substrate having a large lattice constant InGa
As is used as a well layer to form a strained quantum well structure. That is, in the case of simple fabrication, strained quantum wells must be used for the active layer. On the other hand, a laser having an oscillation wavelength of 0.75 μm to 0.9 μm can emit light of the wavelength without using a strained quantum well and has a sufficient optical gain, so that the lattice matching G
Generally, an aAs / AlGaAs quantum well is used as an active layer, and even the reported polarization control type surface emitting laser device uses only a GaAs / AlGaAs quantum well.

[0009]

However, the aforementioned G
The polarization control type surface emitting laser device using the aAs / AlGaAs quantum well is polarized in a <2 to 33> direction (2 to 3 indicate a bar with a bar on 2, 3 respectively). However, since the gain difference from the <01 ~ 1 ~> directions is small,
Light emission was also observed in the 1-1 to 1-> directions, and the orthogonal polarization suppression ratio remained at 15 dB at the maximum. This <01 ~ 1 ~>
Light emission in the direction has an effect on dynamic characteristics, and it has not been possible to sufficiently reduce excessive noise. In addition, it may cause an error in transmission characteristics in a polarization-dependent system.

This is due to the dynamic characteristics of the semiconductor laser.
At the time of carrier injection, the carrier density exceeds the oscillation threshold in the <2-33 ~> direction and relaxes and oscillates, and is higher than the <2-33 ~> direction, but the difference is small, so the threshold in the <01-1 ~> direction is reached. And oscillation in the <01 to 1 to> directions is allowed. In order to solve this, the <2 ~ 33 ~> direction and <01 ~ 1 ~>
It is necessary to further increase the threshold carrier density difference in the direction.

An object of the present invention is to further improve the dynamic characteristics by increasing the difference in threshold carrier density between the <2 to 33> direction and the <01 to 1> direction while keeping the waveguide structure circular. An object of the present invention is to provide a surface emitting laser device exhibiting polarization characteristics.

The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0013]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

According to the present invention, the plane orientation is changed from (100) plane to (0) plane.
11) 15 ° to 40 ° in plane or (011 ~) plane direction
In a surface emitting laser device having a distributed feedback mirror formed of upper and lower multilayer films formed on a tilted GaAs substrate by epitaxial growth and an active layer formed of a quantum well sandwiched therebetween, the active layer has a compressive strain. It is a multiple quantum well having a well layer made of InAlGaAs or InGaAsP.

With this configuration, the difference in threshold carrier density between the <2 to 33> direction and the <01 to 1> direction is further increased while keeping the waveguide structure circular, so that dynamic characteristics are also stable. A surface-emitting laser device exhibiting improved polarization characteristics can be obtained.

That is, when the conventional strain-free GaAs quantum well active layer is used, the in-plane of heavy holes in the valence band is due to the crystal anisotropy on the (n11) plane. 2
When compressive strain is applied to the well layer in addition to the anisotropy of the optical gain with respect to the axis, the optical gain anisotropy further increases. This is because the state function of heavy holes in the valence band changes due to compressive strain, and the direction of the change acts in a direction that makes the anisotropy of optical gain redundant. FIG. 3 shows a photoluminescence (PL) spectrum showing experimental results for verifying the effect. The sample is a (311) B substrate (B has a surface atomic layer of A
s) (on the substrate). Ar having a wavelength of 514.5 nm is used as an excitation light source.
A laser is used. In FIG. 3, light emitted from the quantum well is polarized and separated by a polarizer, and shows the magnitude of the optical gain in a specific polarization direction. FIG. 3A shows a PL spectrum of a strained quantum well of a three-well layer having non-doped In _0.2 Al _0.15 Ga _0.65 As as a well layer and a non-doped Al _0.3 Ga _0.7 As layer as a barrier layer, and FIG.
Uses undoped GaAs as a well layer and undoped Al _0.3
5 is a PL spectrum of a strained quantum well of a three-well layer having a Ga _0.7 As layer as a barrier layer. The intensity at the peak wavelength of the PL spectrum changes depending on the angle of the polarizer, and both are <2.
The maximum value was obtained in the <33 ~> direction, and the minimum value was obtained in the <01 ~ 1 ~> direction. In the case of the non-strained quantum well shown in FIG. 3B, the emission intensity polarized in the <01 to 1> directions is reduced by only 5.9% compared to the <2 to 33> directions. In the strained quantum well shown in FIG. 3A, the decrease is 7.4%, and the difference in emission intensity is large. By providing such a distortion, the anisotropy of the optical gain is enhanced. Then, the (311) plane, another (211) plane, and (41)
The enhancement of the optical gain anisotropy can be obtained even in the quantum wells on the GaAs substrate of the (1) and (511) planes and on the off-substrate shifted several degrees from the planes. As a result, the threshold carrier density difference due to the in-plane polarization direction becomes larger than in the case of no distortion, the orthogonal polarization suppression ratio increases in the static characteristics of the surface emitting laser device, and the dynamic characteristics exceed the threshold carrier density of the main axis. However, the transmission does not reach the threshold carrier density in the direction orthogonal thereto, transmission can be performed with stable polarization, and excessive noise reduction can be realized.

Hereinafter, the present invention will be described in detail together with embodiments (examples) with reference to the drawings.

[0018]

(Embodiment 1) FIG. 1 is a sectional view showing a schematic configuration of a surface emitting laser device according to an embodiment of the present invention. As shown in FIG. 1, a surface emitting laser device according to the present embodiment has an n-type distributed feedback mirror 2, an AlGaAs spacer layer 3, an InAlGaAs
s / AlGaAs multiple quantum well (MQW) active layer 4, A
lGaAs spacer layer 5 and p-type distributed feedback mirror 6
Are sequentially epitaxially grown.

The n-type GaAs substrate 1 is composed of, for example, an n-type GaAs (311) B substrate, and the n-type distributed feedback mirror 2 is composed of, for example, Al _0.15 G doped with n-type.
a _0.85 As layer and an AlAs layer are formed of a distributed feedback mirror having 38 pairs of multilayer films with a film thickness of 1/4 of the optical wavelength.

The AlGaAs spacer layer 3 is made of, for example, a non-doped Al _0.6 Ga _0.4 As spacer layer, and has an InAlGaAs / AlGaAs multiple quantum well (M
QW) The active layer 4 is made of, for example, non-doped In _0.2 Al
_0.15 Ga _0.65 As is used as a well layer and non-doped Al _0.3 G
It is composed of a three-well multiple quantum well layer having an a _0.7 As layer as a barrier layer.

The AlGaAs spacer layer 5 comprises, for example, a non-doped Al _0.6 Ga _0.4 As spacer layer, and the p-type distributed feedback mirror 6 comprises, for example, a p-type doped Al _0.15 Ga _0.85 As layer and an AlAs layer. Are distributed feedback reflectors having a thickness of １ ／ of the optical wavelength and 21 pairs of multilayer films.

The epitaxial growth substrate has a diameter of 20 μm.
An m-shaped circular mesa is formed, and is planarized by polyimide 7. AuZnN is applied to the exposed outermost GaAs layer.
A ring-shaped p-type ohmic electrode 8 made of i is provided, and an n-type ohmic electrode 9 made of AuGeNi is provided on the back surface.

When a current was injected into the surface emitting laser device having such a configuration at room temperature, laser oscillation having an oscillation wavelength of 0.85 μm was confirmed. FIG. 2 shows the polarization direction dependence of the measured current-light output characteristics. In this case, GaAs (31
1) Since it is produced on the B-plane (B indicates that the surface atomic layer is composed of As), the surface index of the surface is represented as (311１１). The light is efficiently polarized in the <2 to 33> directions with respect to the two orthogonal directions shown in FIG. Then, the maximum value of the light output ratio of 25 dB was obtained at a current value of 17 mA. This is because the active layer is made of GaAs / Al
The orthogonal polarization suppression ratio is improved by 10 dB as compared with the conventional surface emitting laser device of GaAs.

In this embodiment, InAlG is used for the active layer.
Although an aAs / AlGaAs quantum well was used, InGaAs was used.
InGaAsP using sP / InGaP quantum wells or GaAsP or InGaP layers with compressive strain introduced into the barrier layer
It goes without saying that the same effect can be obtained also in the / InGaP or InGaAsP / GaAsP strain compensation quantum well.

In this embodiment, n-type GaAs (31
1) A B substrate (B indicates that a surface atomic layer is composed of As) was used, but a p-type GaAs (311) B substrate and an n-type GaAs having a surface atomic layer composed of Ga were used.
s (311) A substrate (A indicates that the surface atomic layer is composed of Ga) and p-type GaAs (311) A, the plane orientation of which is (011) or (011) from the (100) plane
~) N-type (n11) B substrate, p-type (n11) B substrate, n-type (n11) A substrate, p-type such as (n11) substrate (2 ≦ n ≦ 5) inclined at 15 ° to 40 ° in the plane direction (N1
1) Needless to say, the same effect can be obtained by using the A substrate and its off substrate.

As described above, the present invention has been specifically described based on the embodiments. However, the present invention is not limited to the above embodiments, and it is needless to say that various modifications can be made without departing from the gist of the present invention. is there.

[0027]

As described above, according to the present invention,
The orthogonal polarization suppression ratio can be improved as compared with the conventional one, and the stability of the polarization direction can be increased. In order to increase the stability of the polarization direction, it is possible to make the waveguide cross-sectional shape of the surface emitting laser device anisotropic in the conventional method, but even without using such a method, the present invention can be applied. If the method is used, the polarization stability can be improved while keeping the waveguide sectional shape circular. That is, if the waveguide cross-section has anisotropy, the light emission shape deviates from a circular shape and the coupling efficiency with the optical fiber is reduced, but the surface emitting laser device of the present invention maintains high coupling efficiency with the optical fiber. It is possible to improve the stability of the polarization direction as it is.

Even when the surface emitting laser device according to the present invention having such a large orthogonal polarization suppression ratio is dynamically driven, oscillation perpendicular to the polarization axis can be suppressed and excessive noise can be reduced. it can. Further, it is possible to reduce errors in transmission characteristics in a system using a free space having polarization dependence.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a schematic configuration of a surface emitting laser device according to one embodiment (example) of the present invention.

FIG. 2 is a diagram showing polarization-separated current-light intensity characteristics of a surface emitting laser device having an InAlGaAs / AlGaAs strained active layer on a GaAs (311) B surface according to the present embodiment.

FIG. 3 is a diagram showing a photoluminescence (PL) spectrum of an active layer formed on a GaAs (311) B plane, which is separated by polarization;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... n-type GaAs substrate, 2 ... n-type distributed feedback mirror, 3
... AlGaAs spacer layer, 4 ... InAlGaAs / A
lGaAs multiple quantum well (MQW) active layer, 5 ... AlG
aAs spacer layer, 6 ... p-type distributed feedback mirror, 7 ... polyimide, 8 ... p-type ohmic electrode 9 ... n-type ohmic electrode 9.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Uenohara 2-3-1 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (72) Inventor Toshiaki Kagawa 2-chome Otemachi, Chiyoda-ku, Tokyo No.3-1 Nippon Telegraph and Telephone Co., Ltd. (72) Inventor Amano Main Tax 2-3-1 Otemachi, Chiyoda-ku, Tokyo F-Term within Nippon Telegraph and Telephone Co., Ltd. 5F073 AA74 AB17 AB28 CA13 CA15 CB02 EA22

Claims (1)

[Claims] 1. A GaAs substrate whose plane orientation is inclined from the (100) plane to the (011) plane or the (011 ~) (1 to 1 represents a bar on 1) plane direction by 15 ° to 40 °. In a surface emitting laser device having a distributed feedback mirror formed of upper and lower multilayer films formed by epitaxial growth thereon and an active layer formed of a quantum well interposed therebetween, the active layer is formed of a compression-strained InAlGaAs or A surface emitting laser device comprising a multiple quantum well having a well layer made of InGaAsP.