Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-04-28
2004-08-24
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029011, C372S009000, C372S018000, C372S032000, C372S028000, C359S247000, C359S260000
Reexamination Certificate
active
06782017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength locker for locking the wavelength of a laser beam outputted from a laser source, at a specific wavelength and, more particularly, to a wavelength locker capable of arbitrarily setting a specific wavelength to be locked. The invention relates further to a multi-constant wavelength light source and a wavelength-division multiplexing light source using that wavelength locker. Further related is a wavelength discriminating apparatus for discriminating the wavelength of a laser beam to identify what the wavelength is of.
Especially, the WDM light source to be used in the WDM system has to output a laser beam in a plurality of wavelengths, whose spaces have to be given a grid determined for each channel (as will be abbreviated into the “ch”) according to the advice of ITU-T. For this necessity, there has been investigated and developed the WDM light source.
2. Description of the Related Art
In the case of a WDM optical communication system, for example, in the prior art for wavelength-multiplexing eight optical signals, the WDM light source is provided with eight semiconductor lasers for oscillating laser beams with wavelengths different from each other. As these semiconductor lasers, moreover, there is used a distributed feedback laser (as will be abbreviated into the “DFB”) and a distributed Bragg reflector laser (as will be abbreviated into the “DBR”).
On the other hand, these semiconductor lasers are designed on the pitch of a diffraction grating so that a single-mode laser beam of a specific wavelength may be oscillated in a steady state. The semiconductor lasers will not always oscillate in the specific wavelength when they are ignited. In a steady state, too, some fluctuations are present so that the oscillation wavelength is not always locked at the specific wavelength.
In order to lock the oscillation wavelength at the specific wavelength, therefore, a wavelength locker is employed in the WDM light source.
In
FIG. 38A
, a laser beam outputted from a DFB laser
710
is inputted into a coupler
711
in a wavelength locker
700
for branching an input beam into two. After a portion of the laser beam was branched, the remaining laser beam is outputted from the wavelength locker
700
. The DFB laser
710
is a semiconductor laser for oscillating, when in the steady state, a laser beam having a wavelength corresponding to a ch
0
.
In the wavelength locker
700
, a portion of the laser beam branched by the coupler
711
is inputted into and branched by a coupler
712
for branching an input beam into two. One laser beam branched by the coupler
712
is inputted through a Fabry-Perot Etalon filter (as will be abbreviated into the “ET filter”) into a first photodiode (as will be abbreviated into the “PD”)
714
for outputting an electric current in accordance with a light-intensity so that its intensity is detected by the PD
714
. This first PD
714
has an output value PDo
1
. On the other hand, the other laser beam branched by the coupler
712
is inputted into a second PD
715
for outputting an electric current in accordance with the light-intensity so that its intensity is detected by the PD
715
. This second PD
715
has an output value PDo
2
.
The wavelength of the ET filter
713
for giving the maximal value of a light transmittance is so set that the value PDo
1
standardized with the PDo
2
in the wavelength to be locked, that is, the value PDo
1
/PDo
2
may become a target value of 0.5.
And, a controlling CPU
716
receives those values PDo
1
and PDo
2
and sends a control signal for locking the oscillation wavelength of the DFB laser
710
at a specific wavelength in accordance with those detected values, to the DFB laser
710
.
The wavelength locker
700
thus constructed operates in the following manners to lock the oscillation wavelength of the DFB laser
710
at the ch
0
.
After having ignited the DFB laser
710
, the controlling CPU
716
receives the values PDo
1
and PDo
2
and calculates the value PDo
1
/PDo
2
. When this value PDo
1
/PDo
2
is larger than the target value of 0.5, moreover, the controlling CPU
716
controls the DFB laser
710
by adjusting the drive current or temperature of the DFB laser
710
so that the oscillation wavelength may become longer. When the value PDo
1
/PDo
2
at the time of igniting the DFB laser
710
is smaller than the target value of 0.5, on the other hand, the controlling CPU
716
controls the DFB
710
so that the oscillation wavelength may become shorter. Thus, the DFB laser
710
is controlled so that the value PDo
1
/PDo
2
may always become 0.5, and the oscillation wavelength is locked at the ch
0
.
By the way, the controlling CPU
716
controls the oscillation wavelength merely by comparing the magnitudes of the value PDo
1
/PDo
2
and the target value of 0.5. So, when the DFB laser
710
is ignited at wavelengths of points a, b, c and d of
FIG. 38B
, the oscillation wavelength can be locked at the desired ch
0
, but when the DFB laser
710
is ignited at wavelengths of points e and f, the oscillation wavelength is locked at wavelengths other than the ch
0
.
Considering the wavelength range at the ignition time of the DFB laser, therefore, the wavelength locker is designed to one specific wave to be locked.
Here, the wavelength range within which the wavelength locker can lock the oscillation wavelength of the laser at a desired wavelength with respect to the wavelength at the laser igniting time will be called the “locking range”. Moreover, this locking range is determined by the free spectrum range (as will be abbreviated as “FSR”) of the ET filter and can be widened by enlarging the FSR.
When the DFB laser
710
in
FIG. 38A
is replaced by a tunable wavelength laser capable of oscillating a single mode and making the oscillation wavelength continuously variable, on the other hand, there is employed a wavelength locker capable of oscillating a specific wavelength from a plurality of wavelengths to lock the oscillation wavelength.
In
FIG. 39A
, on the other hand, the construction of a wavelength locker
750
is similar to that of
FIG. 38A
excepting that the ET filter
713
of
FIG. 38A
is replaced by an ET filter
754
having an FSR conforming to the wavelength space to be locked, and its description will be omitted.
This locking range of the wavelength locker
750
is determined with the wavelength space of the WDM system, as shown in
FIG. 39B
, when it is used in the WDM tunable wavelength laser. This is because the ET filter is a periodic filter so that the value PDo
1
/PDo
2
takes an identical value for every constant periods and because a controlling CPU
757
merely receives the values PDo
1
and PDo
2
so that it cannot discriminate points g and h, for example.
The locking range is about ±30 GHz in the wavelength locker which is used when each ch is located having wavelength spaces of 0.8 nm.
Thus, the WDM light source is constructed to include semiconductor lasers of different oscillation wavelengths in a number corresponding to that of wavelengths to be multiplexed. When preparatory semiconductor lasers are to be prepared for a breakage of the semiconductor lasers, therefore, they have to be prepared for every semiconductor lasers of different oscillation wavelengths. In the WDM light source for thirty two waves, for example, thirty two ordinary semiconductor lasers of different oscillation wavelengths are provided for every thirty two ch so that thirty two semiconductor lasers have to be individually prepared for the preparatory ones.
This means that a number of semiconductor lasers will become necessary when the degree of multiplicity increases with the future increase in the traffic.
By preparing a tunable wavelength laser capable of outputting a plurality of wavelengths, on the other hand, the number of semiconductor lasers could be reduced. In this case, however, there is no suitable wavelength locker.
Specifically, this light source outputs the plurality of wavelengths so that it can
Kai Yutaka
Miyata Hideyuki
Onaka Hiroshi
Fujitsu Limited
Rodriguez Armando
Wong Don
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