Surface emitting laser and surface emitting laser array

Details Surface emitting laser and surface emitting laser array Surface emitting laser and surface emitting laser array

1999-05-28

2003-07-08

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

Reexamination Certificate

active

06590917

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface emitting laser and a surface emitting laser array. More particularly, the present invention relates to a vertical cavity surface emitting laser and a surface emitting laser array with a high-luminance optical output in fundamental transverse mode.
2. Description of the Related Art
The vertical cavity surface emitting laser (“VCSEL” for short hereinafter) is expected to find use in a wide range of applications because of its many advantages over the edge emitting laser, such as lower production cost and higher yields and capability of being arranged easily in a two-dimensional array.
The structure, characteristics, and applications of VCSEL were mentioned in IEEE Journal of Quantum Electronics, 1988, 24, pp. 1845-1855, “Surface Emitting Semiconductor Lasers” by Kenichi Iga, Fumio Koyama, and Susumu Kinoshita. Since then, VCSEL has been greatly improved in characteristic properties and put to practical use in the area of optical communications.
However, the conventional VCSEL still has a small optical output in fundamental transverse mode (2-3 mW at the highest) and hence is limited in applications. If it has an increased optical output in fundamental transverse mode (say, 5 mW and above), then it will find use in the image-writing unit (such as laser beam printer) and magneto-optical disk unit.
One way to increase the optical out of VCSEL in fundamental transverse mode was described in IEEE Photonics Technology Letters, 4, pp. 374-377, 1993, “Transverse Mode Control of Vertical-Cavity Top-Surface-Emitting Lasers” by R. A. Morgan et al. According to this literature, the object is achieved if the emitting region for laser beam has an adequate opening.
This VCSEL has the general structure of proton implantation type, as shown in FIG.
16
. It is composed of several layers formed on an n-type GaAs substrate (not shown). On the substrate is formed a lower n-type DBR (Distributed Bragg Reflector) layer
161
including layers of AlAs and Al
0.16
Ga
0.84
As deposited alternately 28.5 periods, having a carrier density of 3×10
18
cm
−3
. On this lower DBR layer
161
is formed an undoped active layer region
162
including an active layer of quantum well structure and a spacer layer. On this active layer region
162
is formed an upper p-type DBR layer
163
including layers of AlAs and Al
0.16
Ga
0.84
As deposited alternately 20 periods, with Al
0.58
Ga
0.42
As placed at interface, having a carrier density of 3×10
18
~2×10
19
cm
−3
. On this upper p-type DBR layer
163
is formed a p-side electrode
164
with an opening
166
which defines the laser beam emitting region with a diameter W. The upper p-type DBR layer
163
is surrounded by a high-resistance region
165
formed by proton implantation which limits the region for current confinement into the active layer.
Incidentally, an n-side electrode (not shown) is formed on the underside of the substrate (not shown).
The VCSEL constructed as mentioned above is said to increase in the optical output in fundamental transverse mode if the size (or diameter g) of the current injection region and the opening W of the emitting region
166
are optimized. However, the optical output in fundamental transverse mode is still only 1.5 mW at the maximum. This output is too small for the laser to be used satisfactorily for the magneto-optical disk unit.
To address this problem, there has been proposed a VCSEL with a high-luminance optical output in fundamental transverse mode (Japanese Patent Laid-open H10-56233). According to this disclosure, the object of increasing the optical output in fundamental transverse mode is achieved by selectively controlling the lasing condition that permits the high-order transverse mode to occur secondarily in addition to the fundamental transverse mode. Because the fundamental transverse mode oscillation in VCSEL occurs at the center of the optical waveguide (or in the vicinity of the optical axis) and the high-order transverse mode oscillation occurs at a place away from the optical axis, it is possible to increase the optical output in fundamental transverse mode if the lasing condition is controlled such that the optical loss of the cavity gradually increases with the increasing distance from the optical axis and the injection current increases accordingly and multiple mode oscillation is suppressed.
To be more specific, the VCSEL is explained with reference to FIG.
17
. It consists of a conductivity-type semiconductor substrate
171
, a lower DBR layer
172
, an upper DBR layer
174
whose conduction mode is opposite to that of the lower DBR layer
172
, an active layer region
173
interposed between the lower DBR layer
172
and the upper DBR layer
174
, a low reflectance zone
175
formed by ion implantation, a loss-determining element
176
, and electrodes
177
and
178
. It emits the laser beam along the optical axis
179
.
The loss-determining element
176
has a concave shape so that the optical loss of the cavity gradually increases in going away from the optical axis
179
in the direction perpendicular to the optical axis
179
. The concave loss-determining element
176
both diffracts the laser beam from the cavity and diffuses sideward (or defocuses) the laser beam from the cavity.
Therefore, this loss-determining element
176
causes the refraction loss to increase with the increasing distance from the optical axis
179
in the direction perpendicular to the optical axis
179
, and the optical loss of the cavity increases accordingly. On the other hand, in VCSEL, the fundamental transverse mode oscillation occurs in the vicinity of the optical axis
179
and the high-order transverse mode oscillation occurs at a position away from the optical axis
179
.
As the result, the optical loss of the cavity increases for the high-order transverse mode, the threshold current density necessary for the laser oscillation of high-order transverse mode to start increases, and the maximum fundamental transverse mode optical output greatly increases.
As mentioned above, the technology disclosed in Japanese Patent Laid-open H10-56233, in principle, makes it possible to increase the output in fundamental transverse mode. However, it also has the disadvantage of adversely affecting the fundamental transverse mode characteristics and presenting difficulties in forming stably the loss-determining element
176
of prescribed shape.
In other words, the technology disclosed in Japanese Patent Laid-open H10-56233 utilizes the fact that, in VCSEL, the fundamental transverse mode oscillation occurs at the center of the optical waveguide (in the vicinity of the optical axis) and the high-order transverse mode oscillation occurs at a position away from the optical axis, thereby causing the reflectivity of the cavity to gradually decrease in going from the center to the periphery. That is, it causes the optical loss to increase gradually and thereby suppresses the laser oscillation in high-order transverse mode.
On the other hand, VCSEL is usually has a small active region, as explained in “Surface Emitting Laser” by K. Iga and F. Koyama, issued by Ohm-sha, 1990. Therefore, it requires that the cavity have a high reflectance. In fact, the cavity for VCSEL under study today has a reflectance greater than 99%. Conversely, if the reflectance of the cavity is low, the threshold current density increases, making it difficult for laser oscillation to take place.
As matter of fact, the VCSEL disclosed in Japanese Patent Laid-open H10-56233 is constructed such that the reflectance of the cavity decreases at a position only slightly away from the optical axis
179
. This suppresses not only the laser oscillation of the high-order transverse mode but also the laser oscillation of the fundamental transverse mode. As the result, this VCSEL does not provide a sufficiently high luminance fundamental transverse mode optical output.
In addition, the VCSEL disclosed in Japanese Patent Laid-open No. 56233/1998 is c

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