Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2002-08-27
2003-11-18
Davie, James (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S046012
Reexamination Certificate
active
06650683
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser, and more particularly to a surface emitting semiconductor laser with which oscillation of high-output, fundamental lateral mode light is possible.
The present invention further relates to a surface emitting semiconductor laser. More particularly, the present invention relates to a surface emitting semiconductor laser which can oscillate high-output fundamental lateral mode light, and a structure of an element in which polarization of light is controlled and which has high utility value as a light source employed for optical information processing or optical communication or a light source for a xerographic image forming apparatus.
2. Description of Related Art
In comparison with an edge emitting laser, a vertical cavity surface emitting laser (hereinafter referred to as “VCSEL”) has numerous merits. For example, manufacturing costs of the VCSEL are low, productivity is high, and achieving a two-dimensional array is easy, coupling efficiency with an optical fiber is high, and electric power consumption is low. For these reasons, the use of the VCSEL for a number of purposes has been investigated in recent years. For example, VCSEL structures, laser characteristics, applications and such VCSEL are described by Kenichi Iga. Fumio Koyama, and Susumu Kinoshita in “Surface emitting Semiconductor Lasers”,
IEEE Journal of Quantum Electronics,
1988, 24, pp. 1845-1855.
However, fundamental lateral made optical output power of conventional VCSELs remains small, or roughly 1 mW at most. Therefore, the application range of conventional VCSELs has been limited to narrow fields, such as a light pick-up used in a CD-ROM drive. With conventional VCSELs, because laser oscillation by the fundamental lateral mode has been obtained by narrowing the diameter of an optical emission region to roughly several &mgr;m, the volume of an active region has been resultantly small, and fundamental lateral mode optical output power has been low.
On the other hand, when the fundamental lateral mode optical output power of the VCSEL increases to 5 mW or more, for example, it becomes possible to use the VCSEL for an image writing apparatus like a laser printer, an optical magnetic disk apparatus and the like.
In Japanese Patent Application Laid-Open (JP-A) No. 10-56233, a VCSEL that has a high-intensity fundamental lateral mode optical output power has been proposed. In this proposal, raising an output of the fundamental lateral mode optical output power is realized by selectively suppressing a laser oscillation condition in a higher-order lateral mode that is secondarily generated in addition to the lateral mode. Namely, fundamental lateral mode oscillation in the VCSEL is generated at the center of an optical waveguide (close to an optical axis), and a higher-order lateral mode oscillation is generated at a remote position separate from the optical axis. Consequently, optical loss of a cavity gradually increases as the distance of separation from the optical axis increases, whereby it becomes possible to suppress a shift to a multi-mode oscillation while increasing an injection current value, and the fundamental lateral mode optical output power can be increased.
As shown in
FIG. 12
, the VCSEL is structured by a conductive semiconductor substrate
171
, a lower DBR (Distributed Bragg Refrector) layer
172
, an upper DBR layer
174
having a conductive type 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 or the like, a loss determination element
176
, and electrodes
177
and
178
. A laser beam is emitted along an optical axis
179
.
The loss determination element
176
is formed in a concave shape, in order to gradually increase the optical loss of the cavity in accordance with an increase in the distance from the optical axis
179
in a direction orthogonal to the optical axis
179
. The concave loss determination element
176
has an operation to refract the cavity laser beam and an operation to either disperse the cavity laser beam sideways or deviate the focus. Consequently, together with increasing the distance from the optical axis
179
in a direction orthogonal to the optical axis
179
, the loss determination element
176
can increase the refractive loss and enlarge the optical loss of the cavity. Moreover, the fundamental lateral mode oscillation in the VCSEL is generated close to the optical axis
179
, and the higher-order lateral mode oscillation is generated at a remote position separate from the optical axis
179
.
As a result, with respect to a higher-order lateral mode, optical loss in a cavity (optical cavity) increases, and a threshold current required for laser oscillation in the higher-order lateral mode increases. On the other hand, with respect to a fundamental lateral mode, variation in the optical loss of the cavity is small, and the threshold current does not vary, thus resulting in an increase of maximum optical output power in the fundamental lateral mode.
Japanese National Publication No. 7-507183 (WO 93/22813) discloses a gain-guiding type surface emitting semiconductor laser that, as shown in
FIG. 14
, forms a metal contact layer
260
having an optical aperture
265
with a diameter smaller than the diameter of an optical gain region
235
, and that suppresses higher-order lateral mode oscillation. With this structure, the optical aperture
265
shields a primary-order lateral mode having a high optical intensity nearer the periphery of the optical aperture than of at the center, or a further higher-order lateral mode having an optical intensity peak at the periphery in addition to at the center, from a fundamental lateral mode having a high optical intensity near the center of the optical aperture
265
within a horizontal surface on a substrate
200
. Thus, only the fundamental lateral mode optical output power is selectively taken out, whereby the fundamental lateral mode optical output power is increased.
U.S. Pat. No. 5,753,941 discloses a gain-guiding type surface emitting semiconductor laser. As shown in
FIG. 15
, an auxiliary layer
38
for lowering cavity optical reflectance is formed beneath an electrode layer
40
used for current injection, whereby higher-order mode generated near the electrode layer of an emission aperture
46
is suppressed. In this structure also, the basic principle is selective suppression of oscillation in a primary-order lateral mode or a further higher-order lateral mode. The method by which suppression is carried out is as follows. Depending on the presence or absence of the auxiliary layer
38
, a distribution of optical reflectance is formed within a horizontal surface on a substrate
30
. A high reflectance is maintained near the center of the optical aperture
46
within the horizontal plane on the substrate, and a reflectance is effectively lowered near the periphery of the optical aperture
46
by the existence of the auxiliary layer
38
. Thus, it becomes easier to oscillate in the fundamental lateral mode by providing a difference in the ease of both oscillations.
As described above, according to the technology disclosed in JP-A No. 10-56233, it becomes possible in principle to raise fundamental lateral mode output. However, at the same time, there are also problems in that a negative influence is exerted on fundamental lateral mode characteristics, and it is considerably difficult to stably form the loss determination element
176
of a predetermined configuration.
As Kenichi Iga and Fumio Koyama describe in “Surface emitting Laser” (Ohm, 1990), because it is difficult for the surface emitting semiconductor laser to earn gains necessary for the laser oscillation due to the active region being small, generally a high reflectance is necessary for the cavity. In actuality, VCSEL cavities currently being researched have a reflectance of 99% or h
Nakayama Hideo
Otoma Hiromi
Sakamoto Akira
Ueki Nobuaki
Yoshikawa Masahiro
Davie James
Fuji Xerox Co, Ltd.
Oliff & Berridg,e PLC
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