Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1999-02-11
2003-03-04
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S020000, C372S034000
Reexamination Certificate
active
06529534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a laser wavelength controlling circuit for controlling the wavelength of laser light, and more particularly to a wavelength controlling circuit for a laser signal having a rough adjustment function of an oscillation wavelength of laser light.
2. Description of the Relates Art
Investigations of a wavelength multiplex method as an optical transmission system are performed energetically at present, and a demand for a technique for transmitting many wavelength signals is increasing. However, the wavelength band of a signal which can be used by an optical amplifier is restricted, and in order to allow the restricted wavelength band to have a number of channels as great as possible, it is necessary to narrow the wavelength interval and stabilize the oscillation frequency of a laser signal with a high degree of accuracy.
Conventionally, as a method of stabilizing the oscillation frequency of a laser signal of an optical transmitter, a method of controlling the temperature of a laser module is used. For example, according to Japanese Patent Laid-Open No. 302948/1995, a wavelength stabilization apparatus is disclosed wherein, as shown in
FIG. 1
, temperature sensor
20
and Peltier element
22
are disposed in the proximity of semiconductor laser
14
, and temperature controller
26
which drives Peltier element
22
in response to an output of temperature sensor
20
is provided. Further, polarizing beam splitter
28
, ¼ wavelength plate
32
, etalon
34
and photodiode
36
are disposed in order in front of light source unit
12
, and current controller
38
which supplies inrush current to semiconductor laser
14
in response to an output from photodiode
36
is provided to stabilize the wavelength of light to be emitted.
Meanwhile, according to Japanese Patent Laid-Open No. 37334/1996, a laser wavelength controlling apparatus is disclosed wherein, as shown in
FIG. 2
, laser diode
42
and Peltier effect element
41
which controls the temperature of laser diode
42
are disposed in LD module
43
. Laser light
45
generated from LD module
43
is introduced into optical narrow band high-pass filter
46
and optical narrow band low-pass filter
51
, and transmission light through them is detected by photodiodes
48
,
53
, respectively. Filters
46
,
51
transmit light components at wavelength positions displaced a little by an equal wavelength to the longer wavelength side and the shorter wavelength side from a desired oscillation center wavelength of the laser light. Accordingly, if Peltier effect element
41
is controlled by temperature adjustment circuit
58
so that the light components detected by photodiodes
48
,
53
may be equal to each other, the laser light can be set to the desired oscillation center frequency.
However, the former wavelength stabilization apparatus has a problem in that, even if the temperature is controlled to a fixed value, the wavelength suffers from a drift by secular degradation of the semiconductor laser or the temperature sensor.
Meanwhile, with the latter laser wavelength controlling apparatus, although it is improved against a drift of the wavelength by secular degradation of the laser diode or the temperature sensor, it is not easy to set it to an intended wavelength.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a wavelength controlling circuit for a laser signal which can set a laser signal to a target wavelength readily upon wavelength control of the laser signal and eliminates the problem of a drift.
According to an aspect of the present invention, a wavelength controlling circuit for a laser signal comprises a laser diode module including a semiconductor laser diode, a temperature detection element and a temperature adjustment element, wavelength controlling means including an optical band-pass filter used as a wavelength discrimination element for controlling the wavelength of a laser signal within a range of fine adjustment within which the wavelength can be discriminated by the optical band-pass filter, and temperature controlling means for adjusting, when the wavelength of the laser signal is outside the range within which the wavelength can be discriminated with a wavelength discrimination characteristic by the optical band-pass filter, the wavelength of the laser signal roughly so that the wavelength may be included in the discrimination allowing range.
According to another aspect of the present invention, a wavelength controlling circuit for a laser signal comprises a laser diode including a semiconductor laser diode, a temperature detection element and a temperature adjustment element, monitoring means including an element which branches part of a laser signal output of the semiconductor laser diode and a wavelength discrimination element for monitoring the wavelength of the laser signal, first adjustment means for generating a signal for finely adjusting the wavelength of the laser signal in response to an output from the monitoring means, second adjustment means for generating a signal for roughly adjusting the wavelength of the laser signal in response to a result of monitoring of the temperature which is monitored by the temperature detection element of the laser diode module, switching means for discriminating, in response to the result of the temperature monitoring of the laser diode module, that a predetermined wavelength is reached and performing switching from the second adjustment means to the first adjustment means, and driving means for driving the temperature adjustment element of the laser diode module with the adjustment signals of the first and second adjustment means.
According to a further aspect of the present invention, a wavelength controlling circuit for a laser signal comprises a laser diode including a semiconductor laser diode, a temperature detection element and a temperature adjustment element, an element for branching part of a laser signal output from the semiconductor laser diode, a wavelength monitoring circuit including a beam splitter for splitting the branched laser signal into two laser signals, an optical band-pass filter having a different transmission factor of an optical wavelength for receiving one of the split laser signals, a first optical detector for receiving the laser signal having transmitted through the optical band-pass filter, and a second optical detector for receiving the other of the split laser signals, the wavelength monitoring circuit monitoring a range within which the wavelength of the laser signal can be discriminated by the optical band-pass filter, a first controlling circuit for generating a signal for finely adjusting the wavelength of the laser signal, the first controlling circuit including a divider for dividing an output from the second optical detector of the wavelength monitoring circuit by an output from the first optical detector and a first amplifier for amplifying a difference voltage between an output signal from the divider and a first reference voltage, a second controlling circuit for generating a signal for roughly adjusting the wavelength of the laser signal so that the wavelength of the laser signal may come within a discrimination allowing range, the second controlling circuit including a temperature monitoring circuit connected to a thermistor of the laser diode module for detecting a voltage signal proportional to the temperature after operation is started and a second amplifier for amplifying a difference between an output from the temperature monitoring circuit and a second reference voltage, a switching circuit for performing switching from the second controlling circuit which roughly adjusts the wavelength of the laser signal to the first controlling circuit which finely adjusts the wavelength of the laser signal, the switching circuit including a comparison circuit for receiving the voltage proportional to the temperature by the rough adjustment of the wavelength of the laser signal and generating an activation signal with
Dickstein Shapiro Morin & Oshinsky LLP.
Ip Paul
Monbleau Davienne
NEC Corporation
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