Coherent light generators – Particular component circuitry – Having feedback circuitry
Reexamination Certificate
1998-11-10
2001-05-08
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
Having feedback circuitry
C372S029020, C372S029011, C372S038020, C372S033000
Reexamination Certificate
active
06229832
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength stability control apparatus for stabilizing an optical wavelength output from a laser diode (hereinafter, LD). In particular, the present invention relates to an optical wavelength stability control apparatus suitable for an optical multiple wavelength transmission.
2. Description of the Related Art
Due to the development of an advanced information society, an optical communication system to which an optical signal is transmitted by using an optical fiber requires an enlarged transmission capacity. The optical multiple wavelength transmission is implemented to realize an increase in transmission capacity. A plurality of channels are transmitted through a common transmission path by assigning respective signals to different optical wavelengths. The precision stabilization of the optical wavelength within ±0.2 nm has long been required so that adjacent wavelengths do not interfere with each other.
FIG. 2
illustrates a conventional apparatus for optical wavelength stabilization. In general, it is known that a temperature fluctuation as well as a drive current fluctuation of a semiconductor laser cause a fluctuation of the optical transmitter.
FIG. 2
illustrates an apparatus used for stabilizing an optical wavelength by keeping the temperature of a semiconductor laser
5
constant. A temperature monitor
10
detects the temperature of LD using a thermistor
9
and a reference voltage generator
3
b
outputs a reference temperature voltage which is a target value for controlling a temperature. An output voltage (Vth) of the temperature monitor
10
and an output voltage (Vref
1
) of the reference voltage generator
3
b
are compared at a comparator
8
, and the difference between Vth and Vref
1
is calculated. In a current controller
11
, the stabilization of the optical wavelength is done by determining a drive current value of a thermoelectric cooler
12
so that an output value at the comparator
8
becomes zero. A semiconductor laser apparatus described in a Japanese laid-open patent No 57-186383 also employs the same method.
However, the electric power consumption (an input electric power to the semiconductor laser) required to obtain the identical optical power output gradually increases over time with the age of a semiconductor laser. Thus, the temperature at an active layer of the semiconductor laser rises and thereby causes an optical wavelength to fluctuate.
Japanese laid open patent 6-283797 describes a control method for keeping an optical power output and the temperature of the active layer constant. According to this method the temperature of a heat sink is controlled to negate a temperature rise of the active layer caused by an increase of the electric power consumption to gain an identical optical power with respect to an age related change of the semiconductor laser. Based upon this control, the temperature of the laser can be constantly controlled for a long period of time.
However, even if the temperature of the laser could be made constant, there is a problem that the optical wavelength of the laser changes in accordance with the fluctuation of the drive current when it is varied. In other words, as shown in
FIG. 3
, efficiency decreases with the age of the LD. To compensate for this deterioration, the LD drive current is controlled by an auto power control circuit (hereinafter, APC) so that the optical output of the LD becomes constant. Therefore, as shown in
FIG. 4
, the LD drive current value If (t) increases. The relation between optical wavelength and LD drive current is shown in FIG.
5
.
Then, the fluctuation of the LD drive current causes fluctuation of the wavelength. A timing chart of an operation and a wavelength fluctuation in the conventional art is shown in FIGS.
6
(
a
)-
6
(
d
).
When the LD drive current If (t) fluctuates with respect to an aging deterioration as shown in FIG.
6
(
a
), a quantity of the wavelength fluctuation increases and the fluctuation cannot be compensated because a reference voltage (Vref
1
) is a fixed value as shown in FIG.
6
(
b
).
The above-mentioned characteristics are explained using the following equations. A quantity of the wavelength drift (&Dgr;&lgr;1) causing an increase/decrease of the LD drive current is given as equation 1.
&Dgr;&lgr;1=&agr;·{
If
(
tn
)−
If
(
t
0
)} (1)
where
&agr;=Drive current-wavelength fluctuation conversion constant,
If(t
0
)=Drive current value at initial time t
0
, and
If(tn)=Drive current after passing time tn.
On the other hand, a quantity of the wavelength drift (&Dgr;&lgr;2) caused by a control loop error of a current controller is given as equation 2.
&Dgr;&lgr;2=(1
/G
)·
Vatc·&bgr;·&ggr;
(2),
where
G=Feedback loop gain,
Vatc=Normalization portion output voltage value,
&bgr;=Temperature of the laser—Wavelength conversion constant, and
&ggr;=Temperature in a circuit—Voltage conversion constant.
Accordingly, a quantity of the wavelength drift (&Dgr;&lgr;) in the optical wavelength stability control method of the conventional art is given as equation 3.
&Dgr;&lgr;=&Dgr;&lgr;1+&Dgr;&lgr;2=&agr;·{
If
(
tn
)−
If
(
t
0
)}+(1
/G
)·
Vatc·&bgr;·&ggr;
(3)
Equation 4 is obtained from a feedback stability condition.
Vatc=G·
(
Vth−Vref
1
) (4)
where
Vth=Temperature monitor output (LD temperature), and
Vref=Reference voltage generator output (initial set temperature).
When the equation 4 is substituted into the equation 3, &Dgr;&lgr; is given as equation 5.
&Dgr;&lgr;=&agr;·{
If
(
tn
)−
If
(
t
0
)}+(
Vth−Vref
1
)·&bgr;·&ggr; (5)
From the equation 5, it is confirmed that it is impossible to compensate the wavelength drift &agr;·{If(tn)−If(t
0
)} causing an increase/decrease of the LD drive current, even though the thermal detection voltage Vth and the reference voltage Vref
1
can be controlled.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical wavelength stability control apparatus for stabilizing the wavelength precisely by compensating for the wavelength drift over a long period of time.
An object of the present invention is to provide an optical wavelength stability control apparatus for stabilizing the optical wavelength output from a LD. This apparatus includes a current detector for detecting the LD drive current driving the LD, and a thermal controller including a compensated reference voltage generator
3
a
, a comparator
8
, a thermistor
9
, a temperature monitor
10
, a current controller
11
and a thermoelectric cooler
12
, for controlling a temperature of the LD to be a control target value. The thermal controller includes a reference generator means for setting the control target value in response to the LD drive current detected by the current detector.
Another object of the present invention is to provide an optical transmitter including a plurality of optical wavelength stability control apparatus, each of the optical wavelength stability control apparatus including a laser diode module having the LD, a photo-diode (hereinafter, PD), a thermoelectric cooler and a thermistor built-in. The apparatus includes an APC capable of controlling the stability of the optical power output by varying the LD drive current driving the LD,
a LD drive current detector for detecting the LD drive current,
a LD drive current increase/decrease normalization unit for outputting the increased or decreased LD drive current value being normalized, based upon the detected LD drive current value,
a compensated reference voltage generator for generating the LD temperature control target value in response to an increase or a decrease of the LD drive current value being normalized,
a temperature monitor circuit for detecting a temperature of the LD based on the thermistor,
a comparator for detecting
Baba Naohiko
Chujo Norio
Hatano Tadashi
Hayami Akihiro
Serizawa Hideyuki
Font Frank G.
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Rodriguez Armando
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