Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-07-30
2001-01-16
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06175581
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser and more particularly to a phase-shifted distributed feedback (DFB) semiconductor laser which is high in mode stability and utilized in a digital Optical transmission system.
2. Description of the Prior Art
In the conventional digital optical transmission system, a semiconductor laser called a &lgr;/4 phase-shifted DFB semiconductor laser and high in a single-mode quality is utilized, wherein the phase of a diffraction grating is shifted by a half period at the center of a laser resonance cavity. The &lgr;/4 phase-shift structure is a known structure and is described, for example, in “Semiconductor, Japan Society of Applied Physics(ed.), Ohmsha Ltd., 1994, pp. 272, FIG.
12
—
12
”.
A &lgr;/4 phase-shifted DFB semiconductor laser has, as
FIG. 5
shows a cross-sectional view thereof, a &lgr;/4 phase-shift structure
45
by which the phase between a first grating
43
and a second grating
44
is shifted by a half period, at the center of the laser cavity. In such a structure, a side mode suppression ratio may be advantageously high, since the laser oscillation wavelength therein is equal to the Bragg wavelength &lgr;
B
, determined by the grating period &Lgr; thereof, that is
&lgr;
B
32 2
&Lgr;n
eff
where n
eff
is the effective refractive index.
However, this structure has a problem that the forward lasing output power cannot be monitored by the backward lasing output power (tracking error), because the ratio of the forward to the backward lasing output power varies with the bias current. Further, there is another problem that wavelength shifts at the time of modulation (chirping) are large so that code errors may be brought about in the long distance transmission. These problems arise from the fact that, because the &lgr;/4 phase-shift structure is located at the center of the laser cavity, the electric field in this phase-shift region becomes very strong. And, as the bias current is increased, the non-uniformity of the distribution of the internal electric field extremely increases. The resulting fluctuation of carrier distribution causes the difference of the refractive index changes along locations in the laser cavity.
Further, there is another problem that the conversion efficiency of the output light against the input current is low, since it becomes difficult for the light to go out as the output power due to the concentration of the electric field in the central region of the laser cavity.
In Japanese Patent Application Laid-open No. 025086/1990, an example structure to solve the above problems is described, wherein, instead of positioning a &lgr;/4 phase shift structure at the center of the laser cavity, as FIG.
6
(A) shows across-sectional view thereof, a second diffraction grating
54
, a period of which differs slightly from a period of a first grating
53
and a third grating
55
, is set in a central region of about 100 &mgr;m of the cavity and thereby the phase between the gratings
53
and
55
in end sections is shifted by a total of &lgr;/4. This structure is characterized by the flatter longitudinal intensity distribution of electric field in the cavity, in comparison with the &lgr;/4 phase-shifted DFB semiconductor laser described above. However, in this structure, because grating periods are not constant within the cavity, the laser does not oscillate at Bragg wavelength, which causes a problem of a low stability in the lasing mode.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser which does not show the variations in the ratio of the forward to the backward lasing output power, even when the bias current is altered, by making the longitudinal intensity distribution of electric field in the laser cavity uniform and which is easy to monitor the lasing output power on the system. A further object of the present invention, by making the longitudinal intensity distribution of electric field in the laser cavity uniform, is to provide a semiconductor laser which shows less wavelength shift at the time of modulation, is capable of a stable operation even at the time of modulation and has a high conversion efficiency (output light against input current).
A further object of the present invention is to provide a semiconductor laser which can reduce rates of code errors at the time of digital modulation by raising the stability of the lasing mode, compared with the conventional semiconductor lasers.
A further object of the present invention is to provide a semiconductor laser which can be manufactured with reduced fluctuations of characteristics among lasers and therefore have high yields in mass production.
The present invention is directed to a DFB semiconductor laser with a diffraction grating which carrys out optical feedback, wherein this diffraction grating has, at the center of the laser cavity, a phase-shift structure of a half of the basic grating period which is the period averaged over the whole grating, and a section of period variation having grating periods different from the basic grating period is incorporated in the central region of the grating. This section of period variation is set in such a manner as the amounts of variation from the basic period spread from the phase-shift structure at the center towards both facets of the laser cavity with equal absolute values but the reversed sign to each other.
Further, the present invention relates to a DFB semiconductor laser with a diffraction grating which carrys out optical feedback, wherein this diffraction grating has, at the center of the laser cavity, a phase-shift structure of a half of the basic grating period which is the period averaged over the whole grating. A section without a grating is introduced into the central region of the cavity. On both sides of this section are incorporated sections of period variation having grating periods different from the basic grating period and these sections of period variation are set in such a manner as the amounts of variation from the basic period spread from the phase-shift structure at the center towards both facets of the laser cavity with equal absolute values but the reversed sign to each other.
Further, the present invention relates to a DFB semiconductor laser with a diffraction grating which carries out optical feedback, wherein this diffraction grating has, at the center of the laser cavity, a phase-shift structure of a half period, and a section without a grating is introduced into the central region of the cavity and on both sides of this section each portion of grating with an equal periods set symmetrically towards each facet of the laser cavity with respect to the phase-shift structure at the center.
REFERENCES:
patent: 4847856 (1989-07-01), Sugimura et al.
patent: 4847857 (1989-07-01), Ohkura
patent: 5170402 (1992-12-01), Ogita et al.
patent: 5185759 (1993-02-01), Matsuyama
patent: 5386433 (1995-01-01), Ohkura et al.
patent: 5926497 (1999-07-01), Nitta et al.
patent: 5936994 (1999-08-01), Hong et al.
patent: 61-47685 (1986-03-01), None
patent: 61-283192 (1986-12-01), None
patent: 62-155584 (1987-07-01), None
patent: 1-238181 (1989-09-01), None
patent: 3-76291 (1991-04-01), None
patent: 4-100287 (1992-04-01), None
patent: 6338659 (1994-12-01), None
patent: 7-335971 (1995-12-01), None
Arroyo Teresa M.
Leung Quyen P.
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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