Coherent light generators – Particular resonant cavity
Reexamination Certificate
2002-05-28
2004-08-24
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
C372S045013, C372S043010
Reexamination Certificate
active
06782032
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser, a optical module using the same, and a optical communication system. More particularly, the invention relates to a semiconductor laser having, on a substrate crystal, an active layer which emits light and a cavity structure for obtaining a laser beam from the light generated from the active layer, in which a regrown layer is formed near the active layer, a optical module having the semiconductor laser as a component, and a optical communication system using the semiconductor laser or optical module.
The speed of information transfer of recent years is requested to be rapidly increased. Consequently, a optical communication of transfer speed of 10 Gb/s or higher is being developed. For a optical communication, usually, a optical module in which a semiconductor laser, a detector, driving circuits for the semiconductor laser, and the like are assembled is used.
As a optical communication system using the optical module and whose transfer speed exceeds 10 Gb/s, a system as shown in
FIG. 9
is known. A optical module
907
transmits signal light from a semiconductor laser
901
in accordance with an external circuit
908
which operates the optical module
907
. A optical signal transmitted from a optical module on the other side is received by a detector
905
driven by a detector driving circuit
906
. All of optical signals are transferred at high speed via optical fibers
909
.
As the semiconductor laser
901
, an edge emitting laser in which a gallium indium phosphide arsenide (GaInPAs) semiconductor material is used for an active layer is mainly used. Generally, a GaInPAs laser has a drawback such that when device temperature increases, a threshold current largely increases. It is therefore necessary to assemble a thermoelectronic device
904
for stabilizing temperature and an automatic power control (APC) circuit for always measuring fluctuations in a optical output from the semiconductor laser
901
by a detector
903
for monitoring and feeding back the measurement value to a laser driving circuit
902
.
Consequently, the number of parts constructing the optical module
907
is large, the driving circuit is complicated and has a large size and, accordingly, the cost of the optical module itself is high.
On the other hand, attention is being paid to a vertical cavity surface emitting laser (VCSEL) as a light source suitable for a high-speed optical module. The vertical cavity surface emitting laser is constructed by an active layer for generating light and an optical resonator taking the form of a pair of reflecting mirrors disposed so as to sandwich a current blocking layer for injecting current to a very small region in the active layer and the active layer. The cavity length of the vertical cavity surface emitting laser is only a few &mgr;m which is much shorter than the cavity length (few hundreds &mgr;m) of an edge emitting laser, and the volume of the active region is small, so that the vertical cavity surface emitting laser has an excellent high-speed characteristic. Further, the vertical cavity surface emitting laser has excellent advantages such that due to an almost circular beam shape, the beam is easily coupled to a optical fiber, a cleavage process is unnecessary, a device inspection on a wafer unit basis can be made, laser oscillation is carried out with a low-threshold current, the power consumption is low, and the cost is also low.
With respect to laser oscillation wavelength, in recent years, oscillation of a vertical cavity surface emitting laser of a 1.3 &mgr;m band made of a new semiconductor material which can be formed on a substrate made of gallium arsenide (GaAs) such as gallium indium nitride arsenide (GaInNAs) or gallium arsenide antimonide (GaAsSb) has been reported one after another. Expectations are running high for practical use of a longer wavelength region vertical cavity surface emitting laser adapted to a single mode fiber capable of performing long-distance high-speed transfer. Particularly, in the case of using GaInNAs for an active layer, electrons can be confined in a deep potential well in a conduction band, and it is expected that the stability of characteristics with respect to temperature can be largely improved.
By the above advantages, if the longer wavelength region vertical cavity surface emitting laser is realized, it is expected that a higher-performance and lower-cost optical module suitable for use in a LAN can be achieved.
The length of the optical resonator of the vertical cavity surface emitting laser is remarkably short. To generate laser oscillation, it is necessary to set the reflectance of upper and lower reflecting mirrors to an extremely high value (99.5% or higher). As a reflecting mirror, a multilayer reflecting mirror obtained by alternately stacking two kinds of semiconductors of different refractive indices having a thickness of a quarter of the wavelength (&lgr;/4 n: &lgr; denotes a wavelength, and n denotes a refractive index of a semiconductor material) is mainly used.
To obtain high reflectance by the smaller number of layers stacked, it is desired that the difference between the refractive indices of the two kinds of semiconductor materials used for the multi-layer reflecting mirror is as large as possible. In the case where the material is semiconductor crystal, to suppress misfit dislocation, it is preferred that the semiconductor crystal is lattice-matched with the substrate material. Under present conditions, a multilayer reflecting mirror made of a GaAs/aluminum arsenide (AlAs) semiconductor material or a dielectric material such as silicon dioxide (SiO
2
) or titanium dioxide (TiO
2
) is mainly used. The current blocking layer is indispensable to lower the threshold current, disposed between the active layer and an electrode for passing a current, and plays the role of limiting the current injected to the active layer to a very small region (hereinbelow, described as an aperture). To realize a single lateral mode, the diameter of the aperture has to be small as 2 to 3 &mgr;m at an oscillation wavelength of 850 nm or 5 to 6 &mgr;m at an oscillation wavelength of 1300 nm. Concretely, a method of selectively oxidizing an AlAs layer introduced into a device structure in the lateral direction to change the layer to an aluminum oxide (Al
x
O
y
) insulating layer, thereby blocking current only by a very small AlAs region remained in the center is in the mainstream at present. There is also a method of blocking current by burying either a semiconductor material having a large band gap or a material doped with an impurity of a conduction type opposite to the conduction type of the device.
On the other hand, in order to realize a optical module having a high speed characteristic over 10 Gb/s, a vertical cavity surface emitting laser used as a light source has to achieve a high speed characteristic over 10 Gb/s. Consequently, it is indispensable to reduce resistance (R) and capacity (C) of the vertical cavity surface emitting laser.
FIG. 5
shows the relations of resistance, capacity, and modulation characteristic. The capacity of a vertical cavity surface emitting laser is generally a few hundreds fF. To achieve high-speed modulation over 10 Gb/s, the device resistance has to be reduced to at least 50 &OHgr; or lower.
For a vertical cavity surface emitting laser, as described above, a multi-layer reflecting mirror made of an AlAs/GaAs semiconductor is mainly used. In a conventional device, an electrode is disposed on an upper p-type AlAs/GaAs semiconductor multilayer reflecting mirror and a current is injected to an active layer via the semiconductor multilayer reflecting mirror. At this time, there is a problem that the energy difference of a valence band of the AlAs/GaAs semiconductor becomes a large resistance component in a heterojunction for holes having a heavy effective mass and increases device resistance. As countermeasures against the problem, attempts to, for example, reduce resistance components in the heterojun
Kitatani Takeshi
Kondow Masahiko
Kudo Makoto
Hitachi , Ltd.
Miles & Stockbridge P.C.
Nguyen Dung T
Wong Don
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