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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a wavelenght/output power stabilizing apparatus
of a semiconductor laser which is capable of stabilizing the
wavelength/output power of a single-mode semiconductor laser (it is also
referred to as a laser diode; LD).
In recent years, a semiconductor laser has been used for various equipment
equipped with an optical system since the converting efficiency of an
output energy with respect to an input energy is large. This semiconductor
laser has such character as that its wavelength and output power are
varied depending on variation of the operating temperature of the
semiconductor laser. The wavelength/output power are also varied according
to variation of injection current of a current supply source for supplying
an injection current to the semiconductor laser (see a search report under
the title of "Frequency and Power Stabilizations of GaAlAs Diode Laser on
an Opto galvanic Effect" of Shingaku Giho Vol. 82 No. 218, OQE-99 of
Technical Search Report of Denshitsunshin Gakkai; OQE82-95 to 106 (issued
on Jan. 17, 1983)).
More specifically, the variation .DELTA..lambda. of the wavelength .lambda.
of a semiconductor laser and the variation .DELTA.P of its output power P
are expressed by a function of the variation of .DELTA.I of an injection
current I and the variation .DELTA.T of the operating temperature T.sub.T
of the semiconductor laser.
The relations therebetween are as follows;
##EQU1##
In the above relations, dT/dI represents the increase of temperature of the
laser as a function of the injection current I injected by the
semiconductor laser.
In the above research report, in order to stabilize the wavelength/output
power, there are shown means for feeding back the variation of the
wavelength output power to the operating temperature and feeding back the
variation of the wavelength to the injection current and means for feeding
back the variation of the output power to the injection current and
feeding back the wavelength to the operating temperature. However,
according to the afore-mentioned means, since the output power or
wavelength is controlled by the feed-back to the operating temperature,
with an environmental temperature variation, the system is slow in its
responsive speed, and temporary variation of wavelength/output power is
allowed.
If T equals constant, .DELTA.T=0, dT/dI=0,
.differential.P/.differential.T=O are obtained. Therefore, the relations
(A) and (B) can be rewritten as follows;
##EQU2##
These relations principly indicate that if the variation .DELTA.P of the
output power P is constant, the variation .DELTA.I of the injection
current I is restricted, and if the variation .DELTA.I of the injection
current I is restricted, the variation .DELTA..lambda. of the wavelength
is restricted, thus stabilizing the wavelength of the semiconductor.
Because of the above-mentioned phenomena, in order to stabilize the
wavelength/output power of the semiconductor laser, it is preferred that
the variation .DELTA.I of the injection current I is restricted while
maintaining the operating temperature T.sub.T of the injection current I
constant.
In view of the above, in order to maintain the operating temperature of the
semiconductor laser at a specified temperature, it can be considered that
a temperature controlling apparatus (see Japanese Patent Early Laid-open
Publication No. 53-1782) having a Peltier element as a thermoelectric
element is used. However, the employment of the temperature controlling
apparatus has the following problems. That is, when a semiconductor laser
is used, the semiconductor laser itself is heated by the injection
current. Therefore, if the thermoelectric element is controlled in such a
way as to bring the operating temperature close to the specified
temperature based on the temperature difference between the operating
temperature of the semiconductor laser and the specified temperature, the
operating temperature differs from the specified temperature due to
variations of the environmental temperature caused by the heating of the
semiconductor laser. In addition, the operating temperature is difficult
to be stabilized for a long time due to seccessive variation of the
thermal resistance of the semiconductor laser element and/or successive
variation of the element forming the temperature controlling apparatus.
The above-mentioned problems will be described more in detail with
reference to FIG. 1 and 2.
FIG. 1 is a schematic view of an operating temperature controlling portion
for stabilizing the operating temperature of a semiconductor laser 1. FIG.
2 is a schematic view of a thermoelectric converting apparatus 6 for
stabilizing the operating temperature of the semiconductor laser 1. The
thermoelectric converting apparatus 6 is provided with the semiconductor
laser 1 on one side of a Peltier element 7 and with a radiating plate 8 on
the other side thereof. The thermoelectric converting apparatus 6 contains
a thermistor 9.
The thermistor 9 detects the operating temperature T.sub.T of the
semiconductor laser 1, and the operating temperature T.sub.T is
voltage-converted into an operating temperature converting voltage E.sub.T
by a temperature voltage converting circuit 32. This operating temperature
converting voltage E.sub.T is inputted into one terminal of an operational
amplifier 10. The other terminal of the operational amplifier 10 is
inputted with a reference voltage E.sub.S corresponding to a specified
temperature T.sub.S by a reference power source 11. The operational
amplifier 10 compares the reference voltage E.sub.S and the operating
temperature converting voltage E.sub.T and outputs the different output
towards a transistor 12. The transistor 12 comprises a transistor 12a and
a transistor 12b. The current flowing direction of the Peltier element 7
is converted by the transistor 12 so that when E.sub.T >E.sub.S and
T.sub.T >T.sub.S, the transistor 12 is controlled so that the
semiconductor laser 1 is cooled by the Peltier element 7, whereas when
E.sub.T <E.sub.S (T.sub.T <T.sub.S), the transistor 12 is controlled so
that the semiconductor laser 1 is heated by the Peltier element 7. As a
result, the operating temperature T.sub.T of the semiconductor laser 1 is
controlled as as to approach to the specified temperature T.sub.S, reaches
a balance-state and becomes a balanced-temperature Te.
In the operating temperature controlling portion, however, the operating
temperature T.sub.T is varied based on variation of the environmental
temperature T.sub.h and heat value of the semiconductor laser 1.
For example, the Peltier element 7 has such characteristics as shown in
FIG. 3. The illustration of FIG. 3 showing the characteristics is based on
the Peltier element 7 of KSM-0211 manufactured by Komatsu Electronics. In
this FIG. 3, the vertical axis shows a heat value Q as a heating load
incurred to the Peltier element 7, whereas the horizontal axis shows a
balanced-current I.sub.P which flows in the Peltier element 7. The
parameter T is a temperature difference between the operating temperature
T.sub.T when it reaches the balanced-state (T.sub.T =T.sub.T at this time)
and the environmental temperature T.sub.h as a temperature at the
radiating side of the Peltier element 7 and is represented by the
following relation;
.DELTA.T.ident.Te-T.sub.h
The temperature difference T=0 means that the balanced-temperature Te is
equal to the environmental temperature T.sub.h.
As is apparent from FIG. 3, when a heating element (Q.noteq.0) is used,
even when, for example, the temperature difference T=0.degree. C., a
balanced-current I.sub.p flows in the Peltier element 7 so as to release a
heat value Q. In this case, the operating temperature converting voltage
E.sub.T when the operating temperature T.sub.T reaches the
balanced-temperature Te is a balanced-temperature corresponding voltage
Ee. Furthermore, if the voltage/current converting coefficient of the
operating temperature contolling portion shown in FIG. 1 is represented by
.alpha., the balanced-temperature corresponding voltage Ee corresponding
to the balanced-temperature of the semiconductor laser 1 when the heat is
balanced can be obtained by the following relation;
##EQU3##
which is obtained by rewriting the relation of I.sub.1.sup.e
=(Ee-E.sub.S).
In the above relation, I.sub.1 represents a current flowing in the Peltier
element 7 when the specified temperature T.sub.S and the environmental
temperature T.sub.h are equally controlled. At that time, a relation of
E.sub.S =E.sub.h is obtained between the reference voltage E.sub.S and the
environmental temperature corresponding voltage E.sub.h.
Furthermore, since the balanced current I.sub.p.sup.e and the heat value Q
are in linear relation when the heat value Q is in the small range as
shown in FIG. 3, the heat value Q can be represented by the following
relation;
Q=.beta..multidot.I.sub.1.sup.e ( 2)
wherein .beta. is a converting coefficient.
Therefore, based on the relations (1) and (2), the balanced-temperature
corresponding voltage Ee can be obtained by the following relation;
##EQU4##
This relation (3) shows that it becomes Ee=E.sub.S (.DELTA.T=0) since the
controlling circuit controls so as to make E.sub.S -E.sub.T =E.sub.S -Ee=0
when the reference voltage E.sub.S is set to be equal to the environmental
temperature corresponding voltage E.sub.h when Q=0. It also shows that it
becomes as follow;
Ee.noteq.E.sub.S ( 4)
even if it is set as E.sub.S =E.sub.h so as to set the specifiedting
temperature T.sub.S and environmental temperature T.sub.h equally when
Q.noteq.0. That is, this relation shows that when a heating element such
as the semiconductor laser 1 is used, the balanced-temperature
corresponding voltage Ee corresponding to the balanced temperature Te is
not equal to the reference voltage E.sub.S corresponding to the specified
temperature T.sub.S and that in this operating temperature stabilizing
circuit, the balanced-temperature Te is shifted with respect to the
specified temperature T.sub.S by a value which is in proportion to the
size of the heat value Q, i.e., a value which is equal to
Q/(.alpha..multidot..beta.). The heat value Q is in proportion to the
injection current I of the semiconductor laser 1.
The environmental temperature T.sub.h is not constant but variable. The
specified temperature T.sub.S is not necessarily in agreement with the
environmental temperature T.sub.h. When the balanced-temperature Te is
different from the environmental temperature T.sub.h (.DELTA.T=Te-T.sub.h
.noteq.0), a balanced-current I.sub.2 is flowed in the Peltier element 7
even when the heating element is not used as shown in FIG. 3. FIG. 4 is an
illustration of the characteristics of the Peltier element 7 in which the
relation between .DELTA.T=Te-T.sub.h and the balanced-current
I.sub.2.sup.e when Q=0. The balanced temperature corresponding voltage Ee
can be obtained from the following relation;
##EQU5##
which is obtained by rewriting I.sub.2.sup.e =.alpha.(Ee-E.sub.S). In the
portion where the temperature T between the balanced temperature Te and
the environmental temperature T.sub.h is small (.DELTA.T.ltoreq.15.degree.
C.), the temperature difference .DELTA.T and the balanced-current
I.sub.2.sup.e are in linear relation. Therefore, the temperature
difference .DELTA.T can be represented by the following relation;
.DELTA.T=-.gamma..multidot.I.sub.2.sup.e ( 6)
wherein the flowing direction of the balanced-current I.sub.2.sup.e flowing
in the Peltier element 7 is normal when it flows in the direction for
cooling the semiconductor laser 1 as a test sample, and .gamma. is a
converting coefficient.
By using this relation (6), if the balanced-temperature corresponding
voltage Ee is shown as a function of the temperature difference .DELTA.T
by rewriting the relation (5), it can be represented by the following
relation;
##EQU6##
Accordingly, if the operating temperature controlling portion which is
shown in FIG. 1 is used, the balanced-temperature corresponding voltage Ee
corresponding to the balanced-temperature Te is not in accord with the
reference voltage E.sub.S when the specified temperature T.sub.S and the
environmental temperature T.sub.h are not in accord. The differnece
Ee-E.sub.S means that the balanced-temperature Te is shifted by a value
which is in proportion to .DELTA.T with respect to the specified
temperature T.sub.S.
More specifically, even if the specified temperature T.sub.S is constant,
the temperature difference T varies when the environmental temperature
T.sub.h varies. Accordingly, since the balanced-temperature Te is varied
by the influence of the environmental temperature T.sub.h, the operating
temperature T.sub.T is not constant.
Next, when a heating element is used, and the environmental temperature
T.sub.h and the specified temperature T.sub.S are not in agreement with
respect to each other, the balanced-current I.sub.p is represented
according to the principle of superimposition by the following relation;
##EQU7##
This relation (8) can be rewritten according to the relation (1) as
follows;
##EQU8##
and the following relation can then be obtained;
##EQU9##
In this way, the balanced-temperature Te (corresponding voltage Ee) varies
according to the heat value Q of the semiconductor laser and variation of
the difference T between the specified temperature T.sub.S and the
environmental temperature T.sub.h.
Accordingly, it is unexpected that the operating temperature T.sub.T is
stabilized for a long time by such operating temperature controlling
portion as mentioned. Furthermore, even if it is assumed that the
operating temperature T.sub.T at the place where the thermistor 9 is
disposed is constant, there is no guarantee that the operating temperature
T.sub.T of the semiconductor laser 1 is stabilized for a long time due to
successive variation of thermal resistance between the thermistor 9 and
the semiconductor laser 1 and successive variation of the thermistor 9
itself. There is no guarantee either that the output power P itself is
constant since the operating temperature T.sub.T is not controlled by
considering the variation of the output power P.
Therefore, it is difficult to stabilize both the wavelength of the
semiconductor laser 1 and output power thereof.
SUMMARY OF THE INVENTION
It is therefore the object of the present invention to provide an apparatus
for stabilizing the wavelength/output power of a semiconductor laser in
which the wavelength and the output power of a semiconductor laser can be
stabilized for a long time.
In order to achieve the above object, there is essentially provided an
apparatus for stabilizing the wavelength/output power of a semiconductor
laser including an injection current supplying source for supplying an
injection current to a single-mode semiconductor laser, an output power
variation detecting portion for detecting the variation of the output
power by receiving a part of the output power of the semiconductor laser;
a wavelength variation detecting portion which has a light receiving
portion for receiving a part of the output power of the semiconductor
laser through an optical element whose spectral characteristics are varied
in the wavelength area of the semiconductor laser and a processing portion
for obtaining a variation of the wavelength of the semiconductor laser
based on the output of the light receiving portion and the output of the
output power variation detecting portion; a heat value detecting portion
for detecting a heat value which said semiconductor laser generates, an
operating temperature stabilizing portion which comprises an operating
temperature detecting portion provided to the semiconductor laser and
adpated to detect an operating temperature thereof, a thermoelectric
element for forwarding and receiving heat to and from the semiconductor
laser, and an operating temperature controlling portion for controlling
the thermoelectric element so as to bring the operating temperature in
accord with a specified temperature while maintaining the output power
constant based on a reference signal corresponding to the specified
temperature, an output of the heat value detecting portion, and an output
of the output power variation detecting portion; and an injection current
controlling portion for controlling the injection current of the injection
current source so as to maintain the wavelength constant based on an
output of the wavelength variation detecting portion.
According to an apparatus for stabilizing the wavelength/output power of a
semiconductor laser of the present invention, an operating temperature
stabilizing circuit controls a thermoelectric element so as to bring an
operating temperature in accord with a specified temperature while
maintaining the output power constant based on the output of a wavelength
output power variation detecting portion for detecting the variation of
the output power by receiving a part of the output power of the
semiconductor laser, a reference voltage corresponding to the specified
temperature and the output of the heat value detecting portion, and an
injection current controlling portion controls the injection current of
the injection current supplying source so as to maintain the wavelength
constant based on the output of the wavelength variation detecting portion
for detecting the variation of the wavelength by receiving a part of the
wavelength output power of the semiconductor laser.
The above object and features of the present invention will become apparent
to those skilled in the art upon reading the following detailed
description with reference to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing the construction of a conventional
operating temperature controlling portion;
FIG. 2 is a schematic view showing the construction of a conventional
thermoelectric converter;
FIG. 3 is a characteristic illustration showing the relation between a heat
value of a Peltier element and a balanced-current flowing in the Peltier
element;
FIG. 4 is a characteristic illustration showing the relation between a
balanced-current and a temperature difference when there is the
temperature difference between an operating temperature and an
environmental temperature under the condition in which the heat value is
zero;
FIG. 5 is a schematic view showing the whole construction of an apparatus
for stabilizing the wavelength/output power of a semiconductor laser
according to the present invention;
FIGS. 6 and 7 are illustrations showing the mode jump characteristics of
the semiconductor laser according to the present invention;
FIG. 8 is an illustration showing the construction of the operating
temperature controlling portion of FIG. 5 in detail;
FIG. 9 is a characteristic illustration showing the relation between an
injection current and a heat value of the semiconductor laser of FIG. 5;
FIG. 10 is an illustration showing the construction of a thermoelectric
converter according to the present invention; and
FIG. 11 is a characteristic illustration of a transmittance curve of an
optical element of FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
One preferred embodiment of an apparatus for stabilizing the
wavelength/output power of a semiconductor laser according to the present
invention will be described hereunder with reference to the accompanying
drawings.
FIG. 5 is an illustration showing the construction of an important part of
an apparatus for stabilizing the wavelength/output power of a
semiconductor laser. The apparatus for stabilizing the wavelength/output
power of a semiconductor laser includes an operating temperature
stabilizing portion 13, an injection current controlling portion 14, an
injection current supplying source 15, an output power variation detecting
portion 16, and a wavelength variation detecting portion 44. The
wavelength output power variation detecting portion 16 has such function
as to detect the variation of the wavelength output power by receiving a
part of the wavelength output power of the semiconductor laser 1. The
output power variation detecting portion 16 generally comprises a beam
splitter 18, a condenser lens 19, a light receiving element 20, and an
operational amplifier 21. One terminal of the operational amplifier 21 is
connected to a reference voltage V.sub.x of a reference power source 22,
and the other terminal thereof is connected to an output V.sub.y of the
light receiving element 20. The reference voltage V.sub.x corresponds to a
predetermined output level of the semiconductor laser 1 and is set as such
that the output of the operational amplifier 21 becomes zero when the
semiconductor laser outputs a predetermined level. In the case of an
ordinary semiconductor laser having a Fabry-Perot resonance structure, a
mode jump is produced based on the variation of an operating temperature
T.sub.T as shown in FIG. 6, and a wavelength .lambda. is shifted. The mode
jump characteristic of this semiconductor laser has a hysteresis.
Therefore, in order to stabilize the osciallatin of the semiconductor
laser 1, it is preferred that a specified temperature T.sub.S is selected
in a stable area in which this mode jump is difficult to occur. The
operational amplifier 21 outputs an output V which is equal to the
difference between the reference voltage V.sub.x and the output V.sub.y
towards an operating temperature controlling portion 17.
The operating temperature stabilizing portion 13 has an operating
temperature controlling portion 17, a Peltier element 7, and thermistors
33 and 35 which are equivalent to the operating temperature detecting
portion. The operating temperature controlling portion 17, as shown in
FIG. 8, includes a difference output power generating circuit 23 having a
function which will be described hereinafter, a heat correcting output
generating circuit 24 having a function which will be described
hereinafter, and a thermoelectric element controlling portion 25 for
controlling the Peltier element 7 acting as the thermoelectric element.
The thermoelectric element controlling portion 25 includes an operational
amplifier 26a, an operational amplifier 26, an operational amplifier 27
and an operational amplifier 28a. One terminal of the operational
amplifier 26a is grounded, and the other terminal thereof is inputted with
a heat correcting voltage E'c and a difference correcting voltage E"c to
output a correcting voltage Ec. This correcting voltage Ec has a physical
quantity which is in proportion to the heat value of the semiconductor
laser 1 and the temperature difference .DELTA.T between the environmental
temperature T.sub.h and the specified temperature T.sub.S. The details of
the correcting voltage Ec will be described hereinafter. One terminal of
the operational amplifier 28a is inputted with an output .DELTA.V of the
operational amplifier 21, and the other terminal thereof is inputted with
a reference voltage Vz of a standard power source 28b. A comparator 28a
transmits an output E.sub.S1 which is equivalent to the difference between
the reference voltage Vz and the output .DELTA.V of the operational
amplifier 21 to the operational amplifier 26b. This reference voltage Vz
transmits the output E.sub.S1 corresponding to the specified temperature
Ts to the operational amplifier 26b. The operational amplifier 26b outputs
a correcting reference voltage E.sub.S2 which is equivalent to the
difference E.sub.S1 -E.sub.C between the output E.sub.S1 and the
correcting voltage E.sub.C towards the other terminal of the operational
amplifier 27. The operational amplifier 27 compares the operating
temperature converting voltage E.sub.T1 which is inputted into one
terminal thereof and the correcting reference voltage E.sub.S2, controls
the transistor 12 by the output of the difference between the voltages
E.sub.T1 and E.sub.S2 and controls the Peltier element 7 so that the
operating temperature T.sub.T reaches the balanced-state by the transistor
12.
Assuming that the operating temperature T.sub.T reached the balanced-state
by the control just mentioned, the relation (9) can expressed using the
correcting reference voltage E.sub.S2 as follows;
##EQU10##
Since E.sub.S2 =E.sub.S1 -E.sub.C, the relation (10) can be rewritten as
follows;
##EQU11##
In order to bring the balanced-temperature Te in agreement with the
specified temperature T.sub.S, the difference between the reference
voltage E.sub.S1 and the balanced-temperature corresponding voltage Ee
must be "0".
Under the above-mentioned condition, the relation (11) can be rewritten as
follows;
##EQU12##
The following relation can then be obtained from the above relation;
##EQU13##
Assuming that the following conditions are given to the relation (12);
##EQU14##
E.sub.C =E.sub.C '+E.sub.C " is obtained.
E.sub.C ' physically means a heat correcting voltage which is required to
correct the gap between the operating temperature T.sub.T and the
specified temperature T.sub.S based on the heat of the semiconductor laser
1, whereas E.sub.C " physically means a difference correcting voltage
which is required to correct the gap between the operating temperature
T.sub.T and the specified temperature T.sub.S based on the difference
between the operating temperature T.sub.T and the specified temperature
T.sub.S. Therefore, if the correcting voltages E.sub.C ' and E.sub.C " are
added as the controlling voltage E.sub.C, the balanced-temperature Te when
the operating temperature T.sub.T reaches the balanced-state can be
brought to be in agreement with the specified temperature T.sub.S.
The heat correcting output generating circuit 24 has such function as to
generate the heat correcting voltage E.sub.C ' and includes operational
amplifiers 29a and 29b. The operational amplifier 29a is inputted with a
detecting output from a heat value detecting portion 30. The heat value
detecting portion 30 includes a stationary resistor R. Assuming that the
heat value Q of the semiconductor laser 1, as shown in FIG. 10, is in
proportion to the injection current I and that the stationary resistor
R.sub.F is provided in the midway of a serial circuit including an
injection current supplying source 15 for supplying the injection current
I to the semiconductor laser 1, the potential fall V of the stationary
resistor R.sub.F is in proportion to the injection current I.
The above can be expressed using a numerical formula as follows;
since Q=C.times.I.sub.LD and V=R.sub.F .times.I.sub.LD,
##EQU15##
wherein symbolic character C is a converting coefficient.
The voltage V is inputted into one terminal of the operational amplifiers
29a and 29b, and the amplification factor m is adjusted.
Since the output voltage outputted from the operational amplifier 29b is
used as the heat correcting voltage E.sub.C ', E.sub.C '=mV is obtained.
According to this relation E.sub.C '=mV and the afore-mentioned relations
(13) and (15), the amplification factor m can be expressed as follows;
##EQU16##
Since all of the physicalquantities which are included in the right term
in the above relation (16) can be regarded as constants, the amplification
factor m is constant.
Since this amplification factor m is M<1 and the nonreverse amplification
cannot be performed directly, the reverse amplification is repeated twice
in this embodiment.
The difference correcting output generating circuit 23 has such function as
to generate the difference correcting voltage E.sub.C ". This difference
correcting output generating circuit 23 includes an operational amplifier
31. The detecting output of the thermistor 33 is converted to a voltage
E.sub.T1 by a temperature-voltage converting circuit 22 and then inputted
into one end of the operational amplifier 31, whereas the detecting output
of the thermistor 35 is converted to a voltage E.sub.T2 by a
temperature-voltage converting circuit 34 and then inputted into the other
end of the operational amplifier 31. The thermistor 33, as shown in FIG.
10, is contained in the semiconductor laser 1 and mounted on a radiating
plate 8, thereby constituting the thermoelectric convertor 6. The
thermistor 33 acts as the operating temperature detecting portion for
detecting the operating temperature T.sub.T of the simiconductor laser 1.
The thermistor 35 acts as the environmental temperature detecting portion
for detecting the environmental temperature T. The detecting output
E.sub.T1 correpsonds to the operating temperature T.sub.T, whereas the
detecting output E.sub.T2 corresponds to the environmental temperature
T.sub.h. The operational amplifier 31 has such function as to generate a
voltage V.sub.D which is in proportion to the temperature difference T
between the environmental temperature T.sub.h and the operating
temperature T.sub.T.
If the temperature/voltage converting coefficient is represented by n, the
relation between T and voltage V.sub.D can be expressed as follows;
V.sub.D =n.multidot..DELTA.T (17)
Therefore, if the amplification factor m' is adjusted by a variable
resistor R.sub.V ' which is connected to the operational amplifier 31, the
following relation is obtained;
##EQU17##
Thus, the amplification factor m' is expressed as follows;
##EQU18##
Therefore, if the amplification factors m and m' are adjusted, the
variation of the operating temperature T.sub.T based on the heat value Q
of the semiconductor laser 1 and the temperature difference .DELTA.T
between the environmental temperature T.sub.h and the operating
temperature T.sub.T can be removed.
In the operating temperature stabilizing circuit 13, when the output power
P of the semiconductor laser 1 is successively varied, the output E.sub.S1
is varied according to variation of the output power P by the operational
amplifiers 21 and 28a. Based on the output E.sub.S1, the operational
amplifiers 26 and 27 controls the transistor 12 so as to maintain the
output power P constant while taking the correction between the heat and
the temperature difference T into consideration.
In FIG. 5, a wavelength variation detecting portion 44 includes a light
receiving portion 45 and a processing portion 41.
The light receiving portion 45 includes a beam splitter 37, an optical
element 38 such as an interference filter in which the spectal
characteristics are varied in the wavelength bandwidth of the
semiconductor laser, a condenser lens 39, and a light receiving element
40. This light receiving element 40 receives a part of the output power of
the semiconductor laser 1 through the optical element 38. The processing
portion 41 comprises a divider, etc. It receives an output Vx of the light
receiving element 20 which receives a part of the output power of the
semiconductor laser 1 and an output Va of the light receiving element 40
which receives a part of the output power of the semiconductor laser 1
through the optical element 38, performs a calculation of Va/Vx (=Vc),
obtains the variation of the wavelength of the semiconductor laser 1, and
then outputs it as Vc.
In this embodiment, an interference filter is used as the optical element
38. FIG. 11 is an illustration showing a transmittance curve with respect
to the wavelength of this interference filter. The interference filter has
such function as to pass a light of a predetermined wavelength bandwidth,
and an interference filter having a sharp leading edge and a sharp
trailing edge. In FIG. 11, .lambda..sub.1 .about..lambda..sub.2 indicates
a wavelength bandwidth within the range of the leading edge, whereas
.lambda..sub.1 '.about..lambda..sub.2 ' indicates a wavelength bandwidth
within the range of the trailing edge. The interference filter is
generally linearly raised and downed within the wavelength bandwidths
.lambda..sub.1 .about..lambda..sub.2 and .lambda..sub.1
'.about..lambda..sub.2 '. .DELTA..lambda..ident..lambda..sub.1
.about..lambda..sub.2 and .DELTA..lambda..ident..lambda..sub.1
'.about..lambda..sub.2 ' are 50 to 90 A. Therefore, if the wavelength of
the semiconductor laser 1 is set as follows;
##EQU19##
when the wavelength is varied, the output power P which is guided to the
light receiving element 40 after passing through the optical element 38
is largely varied. Accordingly, the variation of the wavelength can be
very accurately monitored by detecting the variation of the output power P
passing through the optical element 38.
The optical element 38 is set in such a manner as will be described
hereinafter. The afore-mentioned mode jump is also occurred due to
variation of the injection current I.
Therefore, the injection current I.sub.O is set in the area in which such
mode jump does not occur. In a system in which a wavelength is locked
using a Fabry-Perot etalon plate and an atomic/molecular absorption
spectrum, the wavelength is sometimes locked in the area in which the mode
jump is readily occurred. However, according to a wavelength/output power
stabilizing apparatus of the present invention, the area in which the mode
jump is difficult to occur can be selected. Since the semiconductor laser
1 is already set as such that the injecion current I and the operating
temperature T.sub.T are oscillated in the stabilizing area, the
semiconductor laser 1 i oscillated, and the output V.sub.B of the light
receiving element 40 is obtained under the condition that the optical
element 38 is not inserted between the beam splitter 37 and the condenser
lens 39. The optical element 38 is then inserted between the beam splitter
37 and the condenser lens 39. The optical element 38 is designed
beforehand as such that the following item;
##EQU20##
is slightly closer to the long wavelength than the wavelength
.lambda..sub.0 of the semiconductor laser. Upon insertion, if the optical
element 38 is gradually inclined with respect to an optical path, the
transmittance curve is shifted toward the side in which the wavelength
becomes shorter while maintaining its configuration. Accordingly, by
inclining the optical element 38, a place in which the output V.sub.B of
the light receiving element 40 becomes h.sub.o P.sub.o can be selected.
H.sub.o used herein is a value in the output power P becomes approximately
1/2, and the output power P.sub.o of the light receiving element 40 is
measured by other means. In that state, the reference power source 43 is
adjusted so that the output of the operational amplifier becomes zero. One
which uses this optical element 38 has such function as almost similar to
the etalon plate, atomic/molecular absorption spectrum, etc. without using
a large-sized wavelength reference such as etalon plate, atomic/molecular
absorption spectrum as a wavelength reference. In addition, it has such
advantages as that it can be made small and at low cost. Even when a
semiconductor laser 1 having a different wavelength is used, it can be
properly set merely by altering the design value of the optical element 38
and inclining the optical element 38.
Although the leading edge portion at the short wavelength side of the
transmittance curve is used in this embodiment, the leading edge portion
at the long wavelength side can be used.
The transmittance curve of the optical element 38 is also varied according
to variation of environmental temperature, moisture, etc., this variation
is negligibly small compared with variation of the wavelength of the
semiconductor laser 1 based on variation of operating temperature. When
the variation of this transmittance curve is eagerly restricted, the
temperature of the optical element 38 is maintained constant using a
temperature stabilizing circuit, or otherwise the moisture may be
prevented by a covering glass.
The injection current controlling portion 14 comprises an operational
amplifier and a reference power source 43. It controls the injection
current I of the injection current source 15 based on the output Vc of the
processing portion 41 so that the wavelength becomes constant.
One terminal of the operational amplifier 42 is inputted with the output Vc
of the processing portion 41, and the other terminal thereof is inputted
with the reference voltage V.sub.A of the reference power source 43.
This reference voltage V.sub.A is adjusted to the level equal to the output
Vc of the processing portion 41 when the semiconductor laser maintains a
predetermined wavelength and a predetermined output.
The operational amplifier 42 outputs the difference of the output Vc of the
processing portion 41 and the reference voltage V.sub.A to the injection
current supplying source 15.
The injection current supplying source 15 supplies an injection current of
the value according to the output Vc of the operational amplifier 42 to
the semiconductor laser 1. Therefore, when the wavelength of the
semiconductor laser is varied, the operational amplifier 42 transmits the
output for restricting the variation to the injection current supplying
source 15 and rapidly controls the injection current so that the variation
of the wavelength becomes smaller.
Therefore, when the wavelength is varied for some reasons, the
wavelength/output power stabilizing apparatus of a semiconductor laser
according to the present invention increases or decreases the injection
current I according to the variation thereof, and the wavelength is
maintained in its stabilized state. At the time, although the output power
P is varied by variation of the injection current I, the variation of the
output power P is detected by the output power variation detecting portion
16 and the operating temperature controlling portion 17 controls the
operating temperature T.sub.T so that the variation becomes smaller. In
this case, since a bias is added to the operating temperature controlling
portion 17 so as to approach to the specified temperature T.sub.S while
correcting at least the heat based on injection current I, the output
power P can be smoothly maintained in its stabilized state even if the
responding speed of the Peltier element 7 is slow.
If the output power of the semiconductor laser 1 is varied by some reasons,
the operating temperature stabilizing portion 13 controls the operating
temperature T.sub.T so as to maintain the output power P in its stabilized
state. Since the operating temperature controlling portion 17 is added
with the bias so as to approach to the specified temperature T.sub.S, the
output power P can be smoothly maintained in its stabilized state even if
the responding speed of the Peltier element 7 is slow. Although the
variation of the operating temperature T.sub.T adversely affects the
wavelength .lambda., the injection current controlling portion 14 rapidly
controls the injection current I so as to maintain the predetermined
wavelength.
As described in the foregoing, an apparatus for stabilizing the
wavelength/output power of a semiconductor laser according to the present
invention includes an injection current supplying source for supplying an
injection current to a single-modesemiconductor laser; an output power
variation detecting portion for detecting the variation of the output
power by receiving a part of the output power of semiconductor laser; a
wavelength variation detecting portion which has a light receiving portion
for receiving a part of the output power of the semiconductor laser
through an optical element whose spectral characteristics are varied in
the wavelength area of the semiconductor laser and a processing portion
for obtaining a variation of the wavelength of the semiconductor laser
based on the output of the light receiving portion and the output of the
output power variation detecting portion; a heat value detecting portion
for detecting a heat value which said semiconductor laser generates, an
operating temperature stabilizing portion which comprises an operating
temperature detecting portion provided to the semiconductor laser and
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