Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-03-30
2002-11-05
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S050121, C372S092000, C372S102000, C372S044010, C372S036000, C372S096000, C372S045013, C372S043010, C372S075000
Reexamination Certificate
active
06477191
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor laser devices which have a waveguide structure with a diffraction grating for controlling-the longitudinal mode, like dynamic single-mode semiconductor laser devices such as a distributed feedback semiconductor laser and a distributed bragg reflector semiconductor laser, and also relates to semiconductor laser modules, rare-earth-element-doped optical fiber amplifiers and fiber lasers using the same.
2. Description of the Related Art
There are known distributed feedback (DFB) semiconductor lasers, distributed bragg reflector (DBR) semiconductor lasers and the like as a semiconductor laser (laser diode(LD)) realizing a dynamic single-mode oscillation. Any one of these laser diodes has a waveguide structure incorporating therein a diffraction grating with a wavelength selecting function. Waveguide structures based on a stepped refractive index profile, in general, comprise a waveguide layer having a higher refractive index sandwiched between cladding layers having a lower refractive index. Prior art technologies will be summarized with attention given to the location of the diffraction grating in the waveguide structure.
An example of a first prior art technology is disclosed in Japanese Unexamined Patent Publication JP-A 8-316566 (1996) in which a diffraction grating is formed at the interface between a waveguide layer and a cladding layer.
FIG. 16
is a sectional view, taken along the resonance cavity of a DFB laser diode, of the art shown in the Publication.
FIG. 17
is a schematic view of the refractive index profile, as viewed vertically, of the waveguide structure. In this prior art reference, an unevenness is provided at the interface between an upper waveguide layer
41
and an upper cladding layer
42
to form an index modulation diffraction grating
43
. This diffraction grating
43
is formed in the following manner: crystal growth is performed up to the upper waveguide layer; subsequently, the unevenness is formed on the surface by ordinary two-beam holographic lithography process and wet etching process; and crystal growth is performed again to form the upper cladding layer and its succeeding layers, thereby burying the unevenness to complete the grating.
An example of a second prior art technology is disclosed in Journal of Lightwave Technology, Vol. 7, No. 12, pp. 2072-2077, 1989, “1.3-&mgr;m Distributed Feedback Laser Diode with a Grating Accurately Controlled by a New Fabrication Technique”, in which a diffraction grating is buried within a cladding layer.
FIG. 18
is a sectional view, taken along the resonance cavity, of this art.
FIG. 19
is a schematic diagram of the refractive index profile, as viewed vertically, of the waveguide structure. Within a cladding layer
53
of n-InP, a diffraction grating comprising a diffraction grating layer
52
of n-InGaAsP having a higher refractive index than the cladding layer is buried. In this art, crystal growth is performed to form a barrier layer
51
of n-InP and the diffraction grating layer
52
of n-InGaAsP; subsequently, the resultant stacked structure is subjected to a two-beam holographic lithography process and a wet etching process to form a plurality of trenches having a depth reaching the barrier layer
51
, the trenches being oriented perpendicular to the resonance cavity to form a striped structure; and finally, this striped structure is covered with the cladding layer
53
of n-InP that is the same material as that of the barrier layer, thereby completing the diffraction grating
54
.
The coupling efficiency of a buried diffraction grating is determined by the following factors: sectional configuration of the diffraction grating, thickness, distance between the diffraction grating and the center of the waveguide structure, refractive indices of the diffraction grating layer and the layer in which the diffraction grating layer is buried, and the like. The literature of the second prior art mentions some advantages of the art including: reduced influence on the guided mode due to the cladding layer and barrier layer of the same composition, higher thickness controllability of the diffraction grating layer in the crystal growth, and like merits.
The coupling efficiency of the unevenness-type diffraction grating like the first prior art technology can be designed by adjusting the factors such as configuration of the unevenness, depth, distance between the diffraction grating and the center of the waveguide structure, refractive index of each of the layers lying on and under the diffraction grating. However, the design of the waveguide structure, including the location of the waveguide layer/cladding layer interface, is largely limited by the guided mode configuration and the beam-divergence angle. Further, the material (refractive index) of the waveguide layer is also limited to keep satisfactory the crystal quality of a portion adjacent the crystal re-growth interface. For this reason, the number of factors based on which the coupling efficiency of the unevenness-type diffraction grating can be designed independently of the waveguide structure is small, and thus, the design freedom has been largely restricted. Furthermore, the depth of the unevenness formed by wet etching is required to be controlled uniformly and accurately so as to form the unevenness-type diffraction grating having the coupling efficiency in conformity with the design. It is, however, difficult to control the wet-etching depth with precision and, hence, difficult to secure the uniformity and reproducibility of the coupling efficiency.
On the other hand, the guided mode propagating within the waveguide structure based on a stepped refractive index profile is configured concentrated in the waveguide layer having a higher refractive index, while on the other hand the intensity of the guided mode is rapidly attenuated in a exponential function fashion within the cladding layer having a lower refractive index. Since the coupling efficiency is determined by the overlap between the guided mode and the diffraction grating, it is required that the diffraction grating buried within the cladding layer as in the second prior art technology be located with a higher precision to provide the diffraction grating with a predetermined coupling efficiency. For this reason, strict limitation is imposed on both the design and the manufacture, resulting in a limited allowance. In semiconductor laser diodes of which the oscillation wavelength is about 1 &mgr;m or smaller, in particular, Al-containing materials such as AlGaAs are frequently used for the cladding layer having a lower refractive index. In the case of the diffraction grating buried in the cladding layer formed of such an Al-containing material, it is very difficult to clean a surface of the cladding layer that has been oxidized during the formation of the diffraction grating prior to the crystal re-growth. For this reason, the crystal quality of a portion adjacent the crystal re-growth interface may be deteriorated, resulting in a danger that the reliability of the resultant device is lowered.
As described above, there has been a strong demand for a waveguide structure to which Al-containing materials are applicable, and which has a diffraction grating offering a wider design freedom in terms of the coupling efficiency and a wider manufacture freedom.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to realize a waveguide structure to which Al-containing materials are applicable, and which has a diffraction grating offering a wider design freedom in terms of the coupling efficiency and a wider manufacture freedom, thereby providing a dynamic single-mode semiconductor laser device easily with higher reproducibility, yield and reliability.
It is another object of the present invention to provide a semiconductor laser module capable of being easily and efficiently connected to an optical fiber amplifier, an optical fiber laser and the like having an optical fiber as
Fujimoto Tsuyoshi
Oeda Yasuo
Okada Satoru
Birch & Stewart Kolasch & Birch, LLP
Flores Ruiz Delma R.
Ip Paul
Mitsui Chemicals Inc.
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