Light source used in wavelength multiplexing

Details Light source used in wavelength multiplexing Light source used in wavelength multiplexing

2001-01-19

2003-10-21

Lee, John D. (Department: 2874)

Optical: systems and elements

Optical frequency converter

C385S024000, C398S082000

Reexamination Certificate

active

06636342

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to light sources used in wavelength multiplexing and, more specifically, to a light source for outputting a light for wavelength division multiplexing transmission where a plurality of lights varied in wavelength are multiplexed.
2. Description of the Background Art
In optical fiber communications, a wavelength division multiplexing (WDM) technique by using a broad band characteristic of an optical fiber is utilized. With the WDM technique, a plurality of optical signals varied in wavelength from one another can be transmitted at one time through a single optical fiber. Therefore, the WDM technique is quite useful in terms of effective use of optical wavelengths. In this WDM technique, however, each of a plurality of laser light sources has to be controlled in oscillation wavelength in order to prevent crosstalk between optical signals varied in wavelength from each other.
One example of conventional light sources used in the WDM technique is shown in FIG.
4
. One example of how to control each semiconductor laser diode in oscillation wavelength is shown in FIG.
5
.
As shown in
FIG. 4
, in the conventional light source used in wavelength multiplexing, a plurality of lights varied in wavelength (&lgr;
1
, &lgr;
2
, &lgr;
3
, . . . , &lgr;n−1, &lgr;n) are outputted from a group of semiconductor laser diodes
100
, and then provided through a guiding part
5
, which is a waveguide or fiber, to an optical multiplexer
6
for multiplexing. A light outputted from the optical multiplexer
6
is transmitted through a transmission medium
7
. The oscillation wavelength of each diode in the group of semiconductor laser diodes
100
is controlled based on temperature dependence thereof.
With reference to
FIG. 5
, how to control the oscillation wavelength of each semiconductor diode
101
included in the group of semiconductor laser diodes
100
is now described. First, a light outputted from the semiconductor diode
101
that oscillates in the vicinity of a wavelength &lgr;k is branched by an optical branched
110
. One branched light is provided to an optical band-pass filter
111
that passes a light of the wavelength &lgr;k. A light intensity detector
112
detects the intensity of the light after being passed through the optical band-pass filter
111
. In this case, the intensity detected by the light intensity detector
112
varies according to the difference between the oscillation wavelength of the semiconductor laser diode
101
and the wavelength that can be passed through the optical band-pass filter
111
. For example, the light intensity detected by the light intensity detector
112
becomes maximum when the wavelength of the light outputted from the semiconductor laser diode
101
is equal to the wavelength that can be passed through the optical band-pass filter
111
.
According to the detection result, a control signal generator
113
outputs a control signal to a temperature changer
114
implemented by, for example, a Peatier element. The temperature at the temperature changer
114
is changed based on the control signal. With this change, the temperature at the semiconductor laser diode
101
is controlled so that the intensity of the light detected by the light intensity detector
112
becomes maximum. In general, as the temperature becomes higher, the oscillation wavelength of the semiconductor laser diode is uniquely shifted to a longer side. Therefore, with temperature control at the semiconductor laser diode
101
, the oscillation wavelength thereof is changed. As such, feed-back control is carried out so that the semiconductor laser diode
101
is always equal in wavelength to the optical band-pass filter
111
. Thus, the semiconductor laser diode
101
can be controlled in oscillation wavelength.
For local networks, such as optical subscriber networks, optical LANs, and optical CATV, a wavelength band of 1.3 &mgr;m or 1.55 &mgr;m is typically used. In such wavelength band, a low-loss characteristic can be observed in a silica optical single-mode fiber. For private networks in which transmission is carried out within several tens of meters at most, a polymethyl methacrylate (PMMA) plastic optical fiber is useful because it is easy to handle and more economical. In the PMMA plastic optical fiber, 0.65 &mgr;m is used as the wavelength band, where the low-loss characteristic can be observed.
FIG. 6
a
shows a relation between a loss characteristic of the silica optical single-mode fiber and an optical spectrum of the conventional light source used in wavelength multiplexing.
FIG. 6
b
shows a relation between a loss characteristic of the PMMA plastic optical fiber and the optical spectrum of the conventional light source used in wavelength multiplexing.
In
FIG. 6
a,
assume that a silica single-mode optical fiber whose low-loss window in a long wavelength band is several hundreds nm or longer is used. If ten optical signals are multiplexed within a wavelength band of 100 nm, for example, each wavelength interval &Dgr; becomes 10 nm, if constant. Also, each semiconductor laser diode is controlled in wavelength with such accuracy as that the oscillation wavelength thereof is on the order of 1 nm, that is, at least 10% of the wavelength interval &Dgr;.
As shown in
FIG. 6
b,
on the other hand, as for the PMMA plastic optical fiber, its low-loss window in a wavelength band of 0.65 &mgr;m is about 20 nm, which is extremely narrower compared with that of the silica optical fiber. If, for example, ten optical signals are multiplexed within the wavelength band of 0.65 &mgr;m, the wavelength interval &Dgr; becomes 2 nm. Therefore, high accuracy, on the order of 0.2 nm or shorter, is required for controlling each semiconductor laser diode. Thus, to carry out a large amount of wavelength multiplexing transmission in such wavelength band, higher accuracy is required for controlling each semiconductor laser diode in oscillation wavelength, thereby causing complication in structure and increase in cost.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a light source used in wavelength multiplexing capable of outputting a light controlled in wavelength with higher accuracy without controlling a semiconductor laser diode in oscillation wavelength with higher accuracy even when a large amount of wavelength multiplexing transmission is carried out over a transmission medium with a limited low-loss window, such as a PMMA plastic optical fiber.
One aspect of the present invention is directed to a light source for outputting a light for wavelength division multiplexing transmission where a plurality of lights varied in wavelength are multiplexed, the light source comprising:
a light emitter for emitting fundamental-wave lights each controlled to be k (k>1) times longer in wavelength than lights to be multiplexed; and
a wavelength converter for receiving the fundamental-wave lights and outputting lights including lights of one-kth a wavelength of each of the fundamental-wave lights.
As described above, according to the above-stated aspect, wavelength control can be carried out with higher accuracy without controlling the light emitter in oscillation wavelength with higher accuracy. Therefore, the light source can output a light suitable for high-density wavelength multiplexing transmission.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 5436757 (1995-07-01), Okazaki et al.
patent: 5838709 (1998-11-01), Owa
patent: 5923683 (1999-07-01), Morioka et al.
patent: 6097540 (2000-08-01), Neuberger et al.
patent: 2-23732 (1990-01-01), None

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