Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-03-07
2001-11-20
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06320891
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and a surface emitting semiconductor laser array which are used as a light source for optical disks, laser printers and laser displays.
2. Description of the Related Art
As the method of monitoring the light output of a surface emitting laser, there is proposed a method of monitoring light split by a beam splitter provided at the light emission window of a laser element package with light receiving elements provided in the package (Japanese Published Unexamined Patent Application No. Hei 8-330661). However, in this method, the laser package has a special structure and a general-purpose product cannot be used, thereby increasing costs. In addition, the beam splitter and the light receiving elements having a relatively large area are used, thereby boosting costs. In this method, when a surface emitting laser generates multiple beams, beam outputs cannot be monitored independently and the whole size of the laser cannot be made compact. Therefore, a technology for forming a photodetector on a surface emitting laser monolithically is desired.
As the method of forming a photodetector monolithically, there are proposed a method of forming light receiving elements on the structure layer of a surface emitting laser and a method of forming light receiving elements in parallel next to a surface emitting laser.
The method of forming light receiving elements on the structure layer of a surface emitting laser is first proposed by U.S. Pat. No. 5,136,603. This method is to monitor the light output of a surface emitting laser by forming an i type layer and an n type layer on a p type DBR (distributed Bragg reflector) mirror layer on the front side of the laser to form a columnar pin photodetector and causing the laser to emit light through the pin photodetector. However, in this structure, the pin photodetector destroys the cyclic structure of the DBR mirror, thereby reducing reflectance. Since an p type electrode formed in the p type DBR is shaped like a ring surrounding the columnar pin photodetector, a current injected from this electrode concentrates on the peripheral portion of the light emitting part of an active layer, whereby an output light intensity distribution hardly becomes a unimodal convex but has a high-order horizontal mode. This causes such a problem that the laser cannot be used as a light source for optical disks and laser printers.
U.S. Pat. No. 5,757,837 improves the above invention of U.S. Pat. No. 5,136,603 and proposes a structure that an intra-cavity type pin photodetector is embedded in a DBR mirror on the front side of an intra-cavity type surface emitting laser. Since this laser has an intra-cavity structure, as shown in
FIG. 15A
, a laser drive electrode (p type electrode
50
) on the front side of a substrate is formed on an upper spacer layer
16
or at an intermediate position of a DBR mirror
17
. An n type electrode
21
for driving a laser is formed on the rear side of the substrate.
The pin photodetector is constituted such that a quantum well which is an optical absorber layer is made from 80 &lgr;- Ga
0.8
In
0.2
As, sandwiched between GaAs spacer layers and further sandwiched between laminatestructured DBR mirrors having a thickness of &lgr;/4. This photodetector shares a p type electrode
50
for driving a laser as the p type electrode of the photodetector, and a reverse bias is applied by a power source
54
to a space between an n type electrode
52
for a photodetector formed on the top surface and the p type electrode
50
to monitor the quantity of laser light with an ammeter
56
.
In this patent, the structure of the surface emitting laser is formed of an intra-cavity type to prevent a laser drive current from running into the photodetector. However, this intra-cavity type surface emitting laser having the photodetector formed monolithically has the following problems.
Since the p type electrode
50
is formed outside thepost-shaped upperDBRmirror in the intra-cavity type surface emitting laser, the hole of the ring of the p type electrode
50
becomes large. Therefore, a current injected into the active layer from the p type electrode
50
concentrates upon the peripheral portion of the light emitting part. Accordingly, the horizontal mode is apt to become a high-order mode and a unimodal light intensity distribution is hardly obtained. To prevent these to some degree, a current strangulation structure for reducing the diameter of a current path is generally formed in a spacer layer or a DBR mirror near the spacer layer. In spite of this, a current concentrates upon a portion around the current path and a light emitting part concentrates upon that portion. A current injected into the active layer from the p type electrode moves in a horizontal direction, resulting in large electric resistance. Particularly when the p type electrode is formed on the spacer layer, a current moves horizontally in the narrow spacer layer, resulting in a large resistance value.
In this intra-cavity type surface emitting laser, since the pin structure of the photodetector is a quantum well structure, photocarriers generated in the quantum well by optical absorption are kept in the quantum well and hardly taken out to the outside. Therefore, the current value greatly depends on a reverse bias voltage applied to the photodetector. Therefore, to monitor laser light properly, voltage applied to the p type electrode
50
and the n type electrode
52
of the photodetector must be maintained at a fixed level. That is, as shown in
FIG. 15A
, while the potential of the p type electrode
50
shared by the laser and the photodetector is grounded and a fixed reverse bias voltage is applied to the n type electrode
52
of the photodetector, the laser is driven by operating the potential of the n type electrode
21
for driving a laser.
FIG. 3B
shows an equivalent circuit for a surface emitting laser having this photodetector. In the case of a single-beam laser, this drive method has no problem. However, in the case of a multi-beam laser, since an n type DBR mirror is generally electrically connected, all the lasers can be driven simultaneously and hence, this drive method cannot be employed. To eliminate this problem, as shown in
FIG. 15B
, lasers which are grown on a semi-insulating substrate
58
must be isolated from one another by etching. However, this case involves serious problems that the production process becomes complicated and that surface emitting lasers cannot be arranged at a small pitch.
U.S. Pat. No. 5,748,661 proposes a method of forming light receiving elements in parallel next to a surface emitting laser. In this patent, a cylindrical photodetector is formed to surround a columnar surface emitting laser so as to detect natural emission light leaked from the surface emitting laser. Since natural emission light not contributing to laser oscillation and not laser oscillation light is detected in this method, the quantity of light detected increases as the amount of an injection current becomes larger until a laser oscillates. However, once the laser oscillates, the quantity of natural emission light rarely increases even when the amount of an injection current grows. Therefore, an increase in the current value for monitoring the quantity of light is extremely small though the quantity of laser light increases, thereby making it impossible to carry out monitoring properly. Further, since the photodetector has the same pin structure as the surface emitting laser, carriers generated in the active layer which is an i layer are kept in a potential well and hardly taken out to the outside. As the intensity of natural emission light is low, the value of current generated in the photodetector by the light is small, thereby making difficult the detection of the natural emission light.
As described above, in the method of forming a pin photodetector structure in the laminate structure of an intra-cavity type surface emitting laser monolithically
Davie James W.
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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