Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-20
2004-02-24
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012, C372S049010, C372S049010, C372S050121, C372S026000
Reexamination Certificate
active
06697405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a surface emitting laser device having a vertical cavity, and an optical module and an optical system using the same.
2. Statement of the Related Art
Along with explosive increase of Internet users in recent years, high speed information transmission has been required in local area networks (LAN). It is expected that transmission rate at Gb/s level for end users and in excess of 10 Gb/s level for backbones connecting between HUBs will be required in the next 5 to 10 years. Therefore, it is considered that optical communications using optical fibers as far as end users are necessary in the near future. Usually, for optical communication, semiconductor lasers, photo-detectors and optical modules incorporating driving circuits therein are used. In optical modules to be used in future LAN, it is indispensable that these optical modules are provided at a reduced cost for use by a great number of users. In addition, high speed transmission performance in excess of 10 Gb/s must be possible.
FIG. 1
shows a schematic view for a high speed optical module in excess of 10 Gb/s known so far.
As shown in
FIG. 1
, the optical module comprises a semiconductor laser device
401
, a laser driving circuit
402
, an external modulator
403
, a TEC (Thermo Electric Cooler) for stabilizing the temperature for the device
404
, a photodetector
405
, a photodetector driving circuit
406
, an entire optical module package
407
, an external circuit
408
for operating the optical module, and an optical fiber
409
. The optical module generates a laser beam from the semiconductor laser device
401
in accordance with the external circuit
408
. The high-speed modulated light in excess of 10 Gb/s is transmitted through the external modulator
403
. Further, optical signals transmitted from a mating optical module is received by the photodetector
405
. All the optical signals are transmitted and received through the optical fiber
409
. As the semiconductor laser device
401
, an edge emitting laser using gallium indium phospho arsenide (GaInPAs) series semiconductor material for the active layer is mainly used. The laser beam wavelength is at 1.3 &mgr;m or 1.55 &mgr;m applicable to a single mode fiber capable of long distance and high speed transmission.
Generally, the GaInPAs series laser has a drawback that a threshold current increases remarkably when a device temperature increases. Accordingly, it has been necessary to incorporate a temperature stabilizing thermoelectric cooler. As described above, the number of parts constituting the optical module is large and, therefore, the size of the module is large and the cost of the optical module itself is expensive. This is a concern in that the existent level of transmission rate of 10 Gb/s has been mainly used in trunk transmission networks in which performance rather than the cost is emphasized. In view of the above, existent 10 Gb/s optical module is not essentially suitable for the application to future LANs which require cost reduction. The dotted lines in
FIG. 1
denote partitioning between the light transmission side in which a semiconductor laser device is disposed and a light receiving side in which the photodetector is disposed, and each of the portions may serve as an optical transmission module and an optical receiving module independently. Further, the photodetector for optical output monitor of semiconductor laser device is omitted.
Recently, a surface emitting laser has attracted attention as a light source suitable for use as a high speed optical module for future LANs. The surface emitting laser has a cavity length as small as several &mgr;m which is much shorter when compared with the cavity length (several hundreds &mgr;m) of the edge emitting laser and is basically excellent in high speed characteristics. Further, the surface emitting laser also has excellent features in that (1) the beam shape is nearly circular which is easily coupled with an optical fiber (2) a cleaving step is not necessary in the production step and device check is possible on the wafer and (3) the laser oscillation is conducted at a low threshold current and consumes less electric power to reduce the cost. As for the lasing wavelength, the lasing operation at 1.3 &mgr;m range by using new semiconductor materials which can be formed on a gallium arsenide (GaAs) substrate such as of gallium indium nitrogen arsenide (GaInNAs) or gallium arsenic antimonate (GaAsSb) have been reported successively in recent years.
For the semiconductor laser devices, it has been expected that more and more practical surface emitting lasers in a long wavelength range are adaptable to a single mode fiber for long distance and high speed transmission. Particularly, it is expected that when GaInNAs is used for the active layer, electrons can be confined in a deep potential well in the conduction band and the stability of the temperature characteristics can also be improved drastically. It has been expected for the long wave range surface emitting laser device using GaInNAs as the active layer that it can provide an optical module of high performance, at a reduced cost and suitable to use in LANs based on the foregoing advantages.
The surface emitting laser basically comprises an active layer for generating light, a current confinement layer for injecting current to a minute region of the active layer and an optical cavity comprising a pair of reflectors disposed so as to vertically put the active layer therebetween. Usually a semiconductor Distributed Bragg Reflector (DBR) is used as a reflector and the current is injected by way of a semiconductor DBR layer into the active layer.
Since the semiconductor distributed Bragg reflector (DBR) has high resistance, a surface emitting laser of a different structure in which current is injected not by way of the reflector has also been studied. An example is a surface emitting laser as described in Japanese Patent Laid-Open Hei 11-204875 (laid-open on Jul. 30, 1999).
FIG. 2
shows a device structural view of a surface emitting laser. As shown in
FIG. 2
, the surface emitting laser comprises a lower electrode
501
, a semiconductor substrate
502
, a lower DBR
503
, a first spacer layer
504
, an active layer
505
, a 20 second spacer layer
506
, a current confinement layer
507
, a current induced layer
508
, a third spacer layer
509
, an upper electrode
510
and an upper DBR
511
. Since the upper electrode
510
is disposed on the side of the upper DBR
511
, the induced current from above is introduced from the third spacer layer
509
through the current induced layer
508
to the aperture restricted by the current confinement layer
507
and then introduced into the active layer
505
. That is, since the current is induced not by way of the upper DBR
511
, the device resistance can be reduced. Further, in this structure, the current induced layer
508
with an increased doping concentration is adopted intending to reduce the resistance to the horizontal direction relative to the substrate between the electrode and the aperture (hereinafter referred to as a resistance to the lateral direction).
OBJECT AND THE SUMMARY OF THE INVENTION
This invention intends to provide a surface emitting semiconductor laser device capable of high speed operation. High speed operation, for example, above 10 Gb/s is attained in accordance with this invention.
This invention further intends to provide a surface emitting semiconductor laser device capable of high speed operation and reduction in cost.
This invention further provides an optical module incorporating the surface emitting semiconductor laser device capable of higher speed operation.
For coping with such technical subjects, it is necessary to overcome the foregoing problems in the surface emitting laser. At first, a surface emitting laser device structure capable of injecting current to an active region not by way of an upper DBR of high resistance should be adopted. For this purpose, it is necessary to provide a
Kitatani Takeshi
Kondow Masahiko
Tanaka Toshiaki
Antonelli Terry Stout & Kraus LLP
Ip Paul
Rodriguez Armando
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