Coherent light generators – Optical fiber laser
Reexamination Certificate
2002-02-13
2004-11-16
Wong, Don (Department: 2828)
Coherent light generators
Optical fiber laser
C372S043010, C372S045013
Reexamination Certificate
active
06819688
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device for controlling changes in the emission wavelength of laser light emitted out of a semiconductor laser by means of an optical fiber grating. The present invention also relates to an optical fiber amplifier which uses the semiconductor laser device.
2. Description of the Prior Art
FIG. 11
is a diagram showing the structure of a prior art semiconductor laser device. In
FIG. 11
, reference numeral
110
denotes a pump laser module that emits laser light, reference numeral
120
denotes an optical fiber for guiding the laser light from the pump laser module
110
, and reference numeral
130
denotes an optical fiber grating formed in the optical fiber
120
.
Furthermore, in the pump laser module
110
of
FIG. 11
, reference numeral
111
denotes a 980-nm band semiconductor laser (i.e. laser diode), reference numeral
112
denotes a temperature monitor for monitoring the temperature of the pump laser module
110
, reference numeral
113
denotes a cooler for keeping the temperature of the pump laser module
110
constant according to the monitoring result of the temperature monitor
112
, and reference numeral
115
denotes a coupling optical system for coupling light emitted out of the semiconductor laser
111
into an optical fiber
120
.
980-nm band laser light is used for the excitation of an erbium-doped fiber amplifier (EDFA). Since the gain-wavelength characteristic of EDFA changes when the emission wavelength of the laser light changes during the excitation, an optical fiber grating
130
is disposed at the output of the pump laser module
110
as measures against changes in the gain-wavelength characteristic.
FIG. 12
is a diagram showing an example of the structure of the semiconductor laser
111
. In
FIG. 12
, reference numeral
111
a
denotes an n-type electrode, reference numeral
111
b
denotes a GaAs substrate, reference numeral
111
c
denotes an n-type cladding layer, reference numeral
111
d
denotes a multiple quantum well (MQW) active layer, reference numeral
111
e
denotes a p-type cladding layer, and reference numeral
111
f
denotes a p-type electrode. In the prior art semiconductor laser device, the semiconductor laser
111
having the MQW active layer
111
d
is used.
FIG. 13
is a diagram showing an energy band structure in the vicinity of the MQW active layer
11
d
of the semiconductor laser
111
. In
FIG. 13
, reference numeral
142
denotes a conduction band, reference numeral
143
denotes a valence band, reference numerals
146
A and
146
B denote quantum wells, respectively, reference numeral
147
denotes a barrier layer, reference numeral
144
denotes a guide layer, and reference numeral
145
denotes a cladding layer. Each of the two quantum wells
146
A and
146
B is composed of InGaAs of In chemical composition of 0.2. The barrier layer
147
is composed of AlGaAs of Al chemical composition of 0.2. The guide layer
144
is composed of AlGaAs of Al chemical composition of 0.2. The cladding layer
145
is composed of AlGaAs of Al chemical composition of 0.48.
In general, the number of wells included in the MQW active layer
111
d
ranges from 2 to 4. Each of the two quantum wells
146
A and
146
B has a thickness Lz ranging from 5 nm to 15 nm, the barrier layer
147
has a thickness Lb ranging from 10 nm to 50 nm, and the guide layer
144
has a thickness ranging from 10 nm to 500 nm. The Al chemical composition of the above-mentioned AlGaAs is adjusted between 0.0 and 0.5 from the viewpoint of optical confinement.
Population inversion is formed by an electric current's flowing in a forward direction between the p-type electrode
111
f
and the n-type electrode
111
a
, and hence injecting electrons and holes into the MQW active layer
111
d
. As a result, the semiconductor laser
111
oscillates at a 980-nm band of emission wavelengths determined by the bandgap of the MQW active layer
111
d
, and emits laser light to the optical fiber
120
by way of the coupling optical system
115
.
In general, since the semiconductor laser uses interband transitions, it has a gain over a wide wavelength range (e.g., ten-odd nm). The emission wavelength of the semiconductor laser
111
differs and changes according to chip-to-chip variations and change in temperature. Therefore, the change in the emission wavelength of the semiconductor laser device is controlled by the optical fiber grating
130
disposed as an external resonator in the prior art semiconductor laser device. For example, details of the semiconductor laser device provided with the optical fiber grating
130
are disclosed in <Reference 1>.
<Reference 1>: Martin Achtenhagen, et al.: “L-I Characteristics of Fiber Bragg Grating Stabilized 980-nm Pump Lasers”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 5, MAY 2001.
When the temperature of the pump laser module
110
changes greatly because of a self heating of the semiconductor laser
111
and change in ambient temperature, the wavelength characteristic of the threshold gain distribution also changes. On the other hand, since the wavelength characteristic of the optical fiber grating
130
remains fixed, the semiconductor laser
111
does not oscillate in external resonance mode and therefore the emission wavelength cannot be kept constant.
To avoid this problem, a temperature control mechanism is disposed in the semiconductor laser device of FIG.
11
. In other words, the prior art semiconductor laser device is so constructed as to monitor the temperature of the pump laser module
110
by means of the temperature monitor
112
, to control an electric current flowing through the cooler
113
by means of a temperature controller not shown in the figure, and to keep the temperature of the pump laser module
110
constant. Thus, the semiconductor laser device can stabilize the emission wavelength, and can control the change in the gain-wavelength characteristic when applied to EDFA. Japanese patent application publication No. 2000-353856 discloses a prior art technology associated with the semiconductor laser device mentioned above, for example.
A problem with a prior art semiconductor laser device constructed as mentioned above is that to keep the emission wavelength constant the semiconductor laser device has to have a temperature control mechanism that consists of a temperature monitor, a temperature controller, a cooler, etc., and the structure of the semiconductor laser device therefore becomes complex.
SUMMARY OF THE INVENTION
The present invention is proposed to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a semiconductor laser device having a simple structure and capable of keeping the emission wavelength constant without having to use a temperature control mechanism.
It is another object of the present invention to provide a semiconductor laser device capable of controlling the change in the emission wavelength by means of a temperature control mechanism with low control resolution or low control performance.
It is a further object of the present invention to provide an optical fiber amplifier provided with such a semiconductor laser device as a source of pumping light, and capable of controlling the change in the gain-wavelength characteristic.
In accordance with an aspect of the present invention, there is provided a semiconductor laser device, comprising: an optical fiber having an optical fiber grating; a semiconductor laser having an active layer with a single quantum well, for emitting laser light; and a coupling optical system for coupling the laser light emitted out of the semiconductor laser into the optical fiber.
In accordance with another aspect of the present invention, the coupling optical system includes a narrow-band filter for adjusting an incident angle of the laser light emitted out of the semiconductor laser.
In accordance with a further aspect of the present invention, the optical fiber grating has a reflection bandwidth w
Fujita Takeshi
Hamada Yuuhiko
Hatta Tatsuo
Sakai Kiyohide
Shigihara Kimio
Burns Doane Swecker & Mathis L.L.P.
Jackson Cornelius H.
Mitsubishi Denki & Kabushiki Kaisha
Wong Don
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