Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-12-27
2003-04-08
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S050121
Reexamination Certificate
active
06546034
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device for optical communication. A single mode oscillation semiconductor laser device according to the present invention is useful for dense wavelength division multiplexing transmission.
With the spread and development of the Internet, the volume of information transmitted to each household is increased. The demand for greater volume has accelerated development of the dense wavelength division multiplexing (hereinafter abbreviated to DWDM) transmission system that enables high-volume transmission. In this system, the number of wavelengths of light transmitted through a single optical fiber is increased for higher transmission volume. On the other hand, an increase in the number of wavelengths contributes to narrowing of a channel spacing. This is because there is a limit to loss characteristics of fibers applicable to optical communication, and there is accordingly a limit to a transmission wavelength band for practical use. The narrowing of a channel spacing tends to cause crosstalk between neighboring channels. In order to avoid such crosstalk, required accuracy of transmission wavelengths is made stricter. For example, when transmission wavelengths are arranged in a wavelength band of 60 nm at a spacing of 0.8 nm, information for 64 channels or 80 channels can be transmitted. In this case, the stability of the wavelengths required in transmission is ±0.01 nm. Thus, the required yield of wavelengths of semiconductor laser devices serving as optical sources is now extremely high. Therefore, transmitting optical sources that meet such wavelength specifications are fabricated at extremely high cost.
On the other hand, in a future DWDM system, the number of transmitting optical sources required will be further increased. Accordingly, it is desired that cost for a single transmission channel be further reduced. Thus, with regard to transmitting optical sources, it is necessary to realize a semiconductor laser array in which low-cost, strongly-built, and compact semiconductor lasers with different oscillation wavelengths from each other are integrated.
A transmitting optical source used for DWDM has a wavelength selecting function for providing a single wavelength. For example, a distributed feedback laser (hereinafter abbreviated to a DFB laser), a typical example of a single mode oscillation laser, has a diffraction grating structure shaped like a saw blade in the vicinity of its optical waveguide layer. The periodicity of refractive indexes of the diffraction grating structure has an optical effect on light propagating in the waveguide. When this optical effect is specifically described, the oscillation wavelength (&lgr;) of a single mode semiconductor laser is determined by the following equation:
&lgr;=(2×
n
&Lgr;)/
m
where n is the equivalent refractive index of transmitting waveguide structures, A is the period of the grating, and m is a degree. It is understood from this equation that in order to control oscillation wavelength accurately, it is desired to suppress variations in the equivalent refractive index (n). In order to achieve this, it is desirable to be able to control the equivalent refractive index (n) readily and accurately in device fabrication.
The equivalent refractive index (n) in the above equation is determined not only by the refractive index possessed by the material of the active layer where light propagates, but also by the shape and dimensions of the active layer and the refractive index of a structure around the active layer. Therefore, in order to control oscillation wavelength accurately, it is also necessary to control the shape and dimensions of the active layer of a semiconductor laser device and suppress variations in the refractive index of the structure around the active layer.
Basic structures of conventional semiconductor lasers are roughly divided into a gain-guide type structure and a refractive index waveguide type structure. Atypical example of a gain-guide type semiconductor laser is a ridge waveguide semiconductor laser. In fabricating a ridge type semiconductor laser, a semiconductor laminate structure that serves as a base is formed by a single crystal growth process. Thereafter, while leaving a light emitting region, an upper cladding layer, which is a region that sandwiches the light emitting region, is etched, and is then buried in a polyimide resin.
On the other hand, a refractive index waveguide type semiconductor laser, which is typified by a buried heterostructure laser device, has a buried heterostructure in which only the waveguiding region for light in the semiconductor laminate is made to remain, and the other regions are buried in substrate material. In a process of fabricating this single mode oscillation semiconductor laser, a current blocking layer is formed by a regrowth process step after etching.
SUMMARY OF THE INVENTION
The process of fabricating the structure of the above-mentioned ridge waveguide semiconductor laser is simple, and therefore, in the case of Fabry-Perot lasers, the yield of their fabrication is very high. The side walls of the ridge shape of the upper cladding layer are covered with polyimide. Therefore, a relative refractive index difference between the active layer formed by semiconductor material and the ridge sides formed by non-semiconductor material is very large. Thus, reflecting variations in the equivalent refractive index (n) caused by variations in active region width on the ridge side, variations in the oscillation wavelength of the single mode oscillation laser become significant.
On the other hand, in the case of the buried heterostructure semiconductor laser, nonuniformity in the structure of its active region is significant, as compared with the ridge waveguide semiconductor laser. Thus, variations in the equivalent refractive index of the buried heterostructure semiconductor laser proper become significant, thereby making it difficult to control the oscillation wavelength accurately.
Moreover, DWDM transmitting optical sources present a problem other than that of the precision of oscillation wavelength in device fabrication. The problem is a drift of lasing wavelength resulting from secular changes of a transmitting optical source itself mounted in a system. In order to deal with this problem, development of wavelength variable lasers to be used as transmitting optical sources has been conducted. The wavelength variable laser is a single mode oscillation semiconductor laser mounted with a heater, so that its oscillation wavelength is changed by heating the active layer. The heating of the active layer, however, impairs characteristics of the semiconductor laser. Therefore, in the development of DWDM transmitting optical sources, it is essential to fabricate a single mode oscillation semiconductor laser that can oscillate at a required wavelength with accuracy and to improve temperature characteristics of the semiconductor laser itself for use as a wavelength variable laser. Factors in the impairment of temperature characteristics of conventional semiconductor lasers will be described in the following. In the above-mentioned ridge waveguide semiconductor laser, carriers injected into the active layer are diffused laterally as the temperature of the active layer rises. Therefore, it becomes necessary to inject an excess amount of carriers to compensate for a decrease in gain resulting from the temperature rise.
In the case of the buried heterostructure semiconductor laser, there is a decrease in electric resistance in the vicinity of an interface between the etched active layer and the semiconductor material for burying the active layer. Thus, as the temperature of the active layer rises, carriers flow out through this region, and therefore are not effectively injected into the active layer.
In view of the technical background described above, it is a first object of the present invention to provide semiconductor laser devices and semiconductor laser array devices that can ensur
Aoki Masahiro
Hosomi Kazuhiko
Komori Masaaki
Uomi Kazuhisa
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