Wavelength monitor and semiconductor laser device

Details Wavelength monitor and semiconductor laser device Wavelength monitor and semiconductor laser device

2001-11-20

2004-10-05

Wong, Don (Department: 2828)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S029020

Reexamination Certificate

active

06801553

ABSTRACT:

The present application is based upon Japanese Patent Application No. 2000-371471 filed Dec. 6, 2000 and Japanese Patent Application No. 2001-132746 filed Apr. 27, 2001, the entire disclosures of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength monitor and a semiconductor laser device in which a wavelength of a laser beam outputted from a semiconductor laser is monitored.
2. Description of Related Art
A dense wavelength division multiplexing (DWDM) optical communication has been performed in an optical communication field using optical fibers. In this DWDM optical communication, laser beams, which are emitted from a number of semiconductor lasers and have various wavelengths, pass through a plurality of optical fibers and are multiplexed to produce a multiplexed laser beam, the multiplexed laser beam is lead to an optical fiber, and the multiplexed laser beam is transmitted to a destination. Thereafter, the multiplexed laser beam is demultiplexed to a plurality of laser beams, and the laser beams are used for various purposes.
In recent years, a technique in which laser beams are multiplexed at high density has been proposed in order to efficiently transmit the laser beams. In this technique, wavelength intervals of the laser beams to be multiplexed are narrowed (for example, wavelength intervals of the laser beams are set to specific wavelength intervals equivalent to 50 GHz). Therefore, to multiplex the laser beams without interfering with each other, it is required for each semiconductor laser device to set a wavelength of the laser beam with high stability. To achieve this requirement, an intensity and wavelength of a backward-directed laser beam (also referred to as a backward laser beam), which is emitted from a semiconductor laser simultaneously with a forward-directed laser beam (also referred to as a forward laser beam) to an optical fiber, is detected and monitored, and the wavelength of the backward laser beam is controlled according to the intensity of the backward laser beam to adjust an wavelength of the forward laser beam. Also, in a laser beam measuring field, an intensity and wavelength of a backward laser beam emitted from a semiconductor laser is monitored, and a wavelength of a beam of homogeneous light emitted from the semiconductor laser is measured with high precision.
FIG. 23
is a view schematically showing the configuration of a conventional wavelength monitor in which an intensity and varying wavelength of a backward laser beam emitted from a semiconductor laser is monitored. The conventional wavelength monitor shown in
FIG. 23
is disclosed in Published Unexamined Japanese Patent Application H10-79551 (1998). As shown in
FIG. 23
, a backward laser beam emitted from a semiconductor laser
126
is collimated in a lens
127
, and the collimated laser beam transmits through a quarter wavelength plate
128
to transform a linear polarization of the laser beam into a circular polarization. Thereafter, the circularly polarized laser beam is incident on a first polarized beam splitter
129
to divide the incident laser beam into a first laser beam
130
and a second laser beam
131
. The first polarized beam splitter
129
has a band pass filter film
132
placed on a first output end face. The first laser beam
130
transmits through the band pass filter film
132
and is received in a first photodiode
133
. An output of electric current of the first laser beam
130
detected in the first photodiode
133
fluctuates with a varying wavelength of the backward laser beam emitted from the semiconductor laser
126
. The second laser beam
131
is incident on a second polarized beam splitter
134
to divide the incident laser beam into a third laser beam
135
and a fourth laser beam
136
. The second polarized beam splitter
134
has a band pass filter film
137
placed on a third output end face. The third laser beam
135
transmits through a band pass filter film
137
and is received in a second photodiode
138
. An output of electric current of the third laser beam
135
detected in the second photodiode
138
fluctuates with a varying wavelength of the backward laser beam emitted from the semiconductor laser
126
. The fourth laser beam
136
is received in a third photodiode
139
to detect an output of electric current of the fourth laser beam
136
. In the conventional wavelength monitor, the outputs of electric current detected in both the first photodiode
133
and the second photodiode
138
are used to monitor the wavelength of the backward laser beam emitted from the semiconductor laser
126
, and the output of electric current detected in the third photodiode
139
is used to monitor the intensity of the backward laser beam emitted from the semiconductor laser
126
. Therefore, the wavelength and intensity of a forward laser beam emitted from the semiconductor laser
126
can be stabilized.
However, because the conventional wavelength monitor has the above-described configuration, two polarized beam splitters
129
and
134
and two band pass filters
132
and
137
are required. Therefore, a problem has arisen that the number of parts is increased in the conventional wavelength monitor so as to heighten a product cost.
Also, because the backward laser beam emitted from the semiconductor laser
126
is split to propagate in three directions, optical elements such as the lens
127
adjusting the convergence of the backward laser beam emitted from the semiconductor laser
126
, the polarized beam splitters
129
and
134
and the photodiodes
133
,
138
and
139
are widely separated in a plane. In this case, another problem has arisen that it is difficult to accurately arrange the optical elements with respect to the backward laser beam propagated in three directions.
Also, because three plates, on which the photodiodes
133
,
138
and
139
are arranged respectively, separately move in different directions due to a temperature variation and/or a mechanical variation occurring over a long period of time, a positional relationship among the semiconductor laser
126
, the lens
127
and the photodiodes
133
,
138
and
139
undergoes variation. In this case, another problem has arisen that an intensity of the laser beam detected in each photodiode fluctuates even though an intensity of the laser beam emitted from the semiconductor laser
126
is constant.
Also, because the second and third photodiodes
138
and
139
are arranged on two planes positioned orthogonal to each other, the planes separately move in different directions due to a temperature variation and/or a mechanical variation occurring over a long period of time. Therefore, another problem has arisen that an output of electric current of the laser beam detected in each photodiode is not stabilized.
In Published Unexamined Japanese Patent Application H5-149793 (1993), a conventional semiconductor laser device is disclosed. In this device, a semiconductor laser and a wavelength monitor for detecting a varying wavelength of a laser beam emitted from the semiconductor laser are arranged. Also, in Published Unexamined Japanese Patent Application S58-12831 (1983), a wavelength measuring device for detecting a wavelength of a laser beam is disclosed. In these devices, a laser beam emitted from a beam source is directly received by one photodetector. Also, the laser beam is received by another photodetector through a filter. These photodetectors are placed on a carrier. In this case, though the precision of the position of the photodetectors is relatively high with respect to a vertical direction, it is difficult to precisely arrange the photodetectors in horizontal directions. Therefore, a problem has arisen that it is difficult to precisely arrange the photodetectors in horizontal directions such that the whole laser beam is correctly detected in the photodetectors or such that a preset part of the laser beam is correctly detected in each photodetector. Also, positions of the photodetectors s

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