Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Having graded composition
Reexamination Certificate
2003-01-03
2004-11-30
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Having graded composition
C257S076000, C257S183000, C257S200000, C257S201000, C257S437000, C257S184000, C438S060000, C438S048000, C438S072000, C438S636000
Reexamination Certificate
active
06825505
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase-shifted distributed feedback (DFB) type semiconductor laser diode (DFB-LD) and its manufacturing method.
2. Description of the Related Art
DFB-LDs have been used as light sources in high-speed, long-distance and large-capacity optical fiber communications. In directly-modulated DFB-LDs, whose output amplitude is modulated by a pump circuit, the carrier density within an active layer and the equivalent refractive index fluctuate, which induces a spectrum spread called a dynamic wavelength shift or a wavelength chirp.
In order to suppress the wavelength chirp, a prior art phase-shifted DFB-LD has been suggested (see: JP-A-2000-077774 and JP-A-2000-277851). That is, a &lgr;
phase shift (n>4, preferably, n=5~8) where &lgr; is an oscillation wavelength is located at a diffraction grating of a waveguide.
Generally, in a DFB-LD, the fluctuation of a Bragg deviation amount &Dgr;&bgr; is opposite in phase to the fluctuation of the optical output. In this case, note that a Bragg deviation amount &Dgr;&bgr; is defined by
&Dgr;&bgr;=2
n
eq
&pgr;(1/&lgr;−1/&lgr;
B
)
where n
eq
is an equivalent refractive index;
&lgr; is an oscillation wavelength; and
&lgr;
B
is a Bragg wavelength determined by the period of the diffraction grating, i.e., twice the period of the diffraction grating.
Also, assume that the phase shift value is less than &lgr;/4, for example, &lgr;/5~&lgr;/8. In this case, the larger the Bragg deviation &Dgr;&bgr;, the smaller the mirror loss &agr;
m
. Note that the Bragg deviation &Dgr;&bgr; and the mirror loss &agr;
m
determine an oscillation mode. Further, the smaller the mirror loss &agr;
m
, the larger the optical output. Therefore, when the optical output is increased by the external reflection return light, the Bragg deviation &Dgr;&bgr; is decreased so that the mirror loss &agr;
m
is increased, thus decreasing the optical output. Contrary to this, when the optical output is decreased by the external reflection return light, the Bragg deviation &Dgr;&bgr; is increased so that the mirror loss &agr;
m
is decreased, thus increasing the optical output. Therefore, a negative feedback control by the external reflection return light is performed upon the optical output, so that the fluctuation of the optical output can be suppressed, which also suppresses the wavelength chirp.
Note that JP-A-2000-277851 provides a &lgr;/5 to &lgr;/8 phase-shifted DFB-LD including a multiple quantum well (MQW) active layer formed by a tensile-strained well layer, thus realizing the above-mentioned negative feedback control.
In the above-described prior art phase-shifted DFB-LD, however, since the wavelength chirping characteristics and the transmission characteristics strongly depend on parameters of the DFB-LD, the wavelength chirping and transmission characteristics cannot be improved. Note that the wavelength chirping characteristics dominates the transmission characteristics.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a phase-shifted DFB-LD capable of improving the wavelength chirping and transmission characteristics.
Another object is to provide a method for manufacturing such a DFB-LD.
According to the present invention in a DFB-LD including a semiconductor substrate, an optical guide layer formed on the semiconductor substrate, a diffraction grating having a phase shift region being formed between the semiconductor substrate and the optical guide layer, and an active layer formed on the optical guide layer,
&kgr;
L+A·&Dgr;&lgr;≧B
where &kgr; is a coupling coefficient of the diffraction grating, L is a cavity length of the diode, &Dgr;&lgr; is a detuning amount denoted by &Dgr;=&lgr;
g
−&lgr; where &lgr;
g
is a gain peak wavelength of the diode and &lgr; is an oscillation wavelength of the diode, A is a constant from 0.04 nm
−1
to 0.06 nm
−1
, and B is a constant from 3.0 to 5.0.
Also, in a method for manufacturing a phase-shifted DFB-LD, a plurality of samples of the phase-shifted DFB-LD having different normalized coupling coefficients &kgr;L and different detuning amounts &Dgr;&lgr; are formed. Next, power penalties of the samples connected to an optical fiber are measured. Next, values of the normalized coupling coefficients &kgr;L and the detuning amounts &Dgr;&lgr; of the samples with the power penalties are plotted in a graph. Next, &kgr;L+A·&Dgr;&lgr;=B is determined where A and B are constants in order to divide the samples into first and seconds areas in the graph, so that most of the samples belonging to the first area have power penalties smaller than a definite value and most of the samples belonging to the second area have power penalties not smaller than the definite value. Finally, a new phase-shifted DFB-LD having a normalized coupling coefficient &kgr;L and a detuning amount &Dgr;&lgr; satisfying &kgr;L+A·&Dgr;&lgr;≧B is formed.
BRIEF DESCRIPTION OF THE DRAWINGS
REFERENCES:
patent: 5825047 (1998-10-01), Ajisawa et al.
patent: 2000-077774 (2000-03-01), None
patent: 2000-277851 (2000-10-01), None
Huynh Andy
Katten Muchin Zavis & Rosenman
Nelms David
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