Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2001-06-22
2004-02-17
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S020000, C372S026000, C372S029020, C372S029011, C372S038010, C372S034000
Reexamination Certificate
active
06693932
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength control method which makes possible architecture of a high density wavelength division multiplexing (WDM) system by controlling with high accuracy the optical wavelength of optical signals supplied by an optical transmitter used in the WDM system and thereby stabilizing the optical wavelength. The invention further enables optical communication systems to be reduced in size. The invention is particularly suitable for a wavelength control method by which, where a wavelength monitoring section for constantly monitoring the wavelength is integrated into a light source section, wavelength fluctuations due to degradation or any other factor are restrained.
2. Description of the Related Art
In optical fiber communication, wavelength multiplexing is used for economical transmission of a greater quantity of information.
FIG. 1
illustrates a model of this method. An optical transmission system
1
on the sending side uses an optical transmitter
6
having different optical wavelengths &lgr;
1
through &lgr;N and an optical multiplexer
2
to wavelength-multiplex individual optical signals onto a single fiber
3
. An optical transmission system
5
on the receiving side uses an optical demultiplexer
4
to demultiplex the optical signals. In this manner, the capacity of an existing transmission line can be economically increased using existing optical fibers. In the drawing, reference numeral
7
denotes an optical receiver.
A further increase in transmission capacity can be achieved by broadening the band of wavelengths used. However, in order for optical fibers to transmit optical signals efficiently with little loss, the range of usable wavelengths is limited. Therefore, for multiplexing more optical signals, the wavelength spacing should be narrowed to achieve a higher density.
In today's high density wavelength multiplexing systems, the oscillation wavelength of the semiconductor laser ranges between 1530 nm and 1560 nm. If adjoining wavelengths are spaced at 0.8 nm, about 40 waves can be wavelength-multiplexed. To achieve an even higher density, the wavelength spacing may be narrowed to 0.4 nm or even 0.2 nm to make possible wavelength multiplexing of 80 or 160 waves, respectively.
An important factor in increasing the density of wavelength multiplexing is the stability of wavelengths. If they are unstable, optical signals which are supposed to be independent from other optical signals of different wavelengths leak into signals of adjoining wavelengths, making it impossible to achieve a desired qualitative level of communication. Usually, the tolerable fluctuation in a wavelength multiplexing system of 0.8 nm in wavelength spacing is not more than 0.1 nm, and in a system of 0.4 nm or 0.2 nm the tolerance is about 0.05 nm or 0.025 nm, respectively.
The wavelength of a semiconductor laser used in a wavelength multiplexing system is heavily dependent on the temperature of the active layer of the semiconductor laser. Therefore, in order to enhance the stability of wavelengths, more accurate temperature control of the active layer of the semiconductor laser device is required.
Also, an optical transmitter used in optical communication is required not to fluctuate substantially in its average output intensity, and to realize this function there is used an auto-power control (APC) circuit. Even where the efficiency of light emission that can be expressed in the ratio between the output intensity of and the injected current to the semiconductor laser varies on account of the degradation of the laser or any other factor, this APC circuit controls the current injected to the laser so as to keep the average output intensity constant, resulting in a stable output intensity.
However, a variation in the current injected into the semiconductor laser device invites a variation in power consumption in the active layer of the semiconductor laser, resulting in a temperature change in the active layer. This temperature change in the active layer invites a variation in optical wavelength, posing a serious impediment to architecture of a high density wavelength multiplexing system.
Therefore, in order to realize a high density wavelength multiplexing system, a major requirement is to establish a temperature control method for keeping the temperature of this active layer constant.
FIG. 2
is a conceptual diagram illustrating an example of wavelength control method according to the prior art. The output optical signals of a light source
20
of an optical transmitter are branched into two segments using an optical splitter
11
. Optical signals of one of these branched lights are further separated into two directions by another optical splitter
12
. This semiconductor laser device is controlled in light intensity by an APC circuit. Optical signals in one of the directions into which separation was done by the optical splitter
12
, after passing a wavelength filter
13
whose optical transmissivity is dependent on the optical wavelength, are received by a light receiving element (PD
1
)
14
. Optical signals in the other direction are received directly by a light receiving element (PD
2
)
15
without going through the wavelength filter. A voltage Vpd
1
generated by a photo current generated by the PD
1
is similarly wavelength-dependent as shown in FIG.
3
. Another voltage Vpd
2
generated by the photo current generated by the PD
2
is not wavelength-dependent because it does not go through the wavelength filter.
FIG. 3
illustrates an example of the wavelength-dependence of the wavelength monitoring output, wherein the horizontal axis represents the wavelength, and the longitudinal axis represents the output Vpd.
By setting as desired the voltage Vpd
2
to be generated by the PD
2
and keeping the difference between Vpd
1
and Vpd
2
constant, the output optical wavelength of the laser can be set to a desired optical wavelength &lgr;
1
. Vpd
1
and Vpd
2
are received by a comparator
16
, and the resultant error signal is fed back to a temperature control circuit
17
for controlling the laser temperature. The laser is mounted on a thermoelectric cooler (TEC)
19
, and the temperature of the TEC
19
is controlled with a signal from the temperature control circuit
17
. The temperature of the active layer of the semiconductor laser device is thereby kept constant. An example of optical wavelength stabilization system for keeping the optical wavelength of a laser constant by using a wavelength monitor circuit in this manner is disclosed in, for instance, a laid-open patent JP-A No. H11-251673.
According to the prior art, the section for controlling the temperature of the laser and that for monitoring its wavelength used to be configured as separate components. However, in response to a demand for smaller optical communication systems, integration of these laser mounting section and the wavelength monitoring section has been attempted.
FIG. 4
illustrates how these components are packaged when they are integrated.
In an optical transmitter
40
illustrated in
FIG. 4
, a laser element
30
, an optical splitter
31
, a wavelength filter
32
, a light receiving element
33
(PD
3
) and another light receiving element
34
(PD
4
) are mounted on a TEC
35
for controlling the temperature of the semiconductor laser element
30
. The laser module with a wavelength monitor
39
is formed by comprising the laser element
30
, the optical splitter
31
, the wavelength filter
32
, the light receiving element
33
,
34
and the TEC. The light intensity of the semiconductor laser is controlled by an APC circuit
36
. Control of the optical wavelength, as in the case of
FIG. 2
, is accomplished by comparing with a comparator
38
Vpd
3
and Vpd
4
generated by photo current flowing in the PD
3
and the PD
4
, and feeding back the resultant error voltage to a temperature control circuit
37
of the laser. This serves to stabilize the wavelength. As the light source section and the wavelength mo
Akashi Mitsuo
Honzawa Yoichi
Sharma Sunil
Tokita Shigeru
Flores-Ruiz Delma R.
Ip Paul
Opnext Japan, Inc.
Sofer & Haroun LLP
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