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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to an apparatus for detecting the change of light
intensity. More particularly, it relates to an apparatus for detecting the
change of light intensity that is equipped with a light intensity changing
means such as an electrooptic crystal that allows the intensity of
incident light to vary in accordance with the change in a signal to be
measured such as an electric field.
An example of the apparatus for detecting the change of light intensity is
an electrooptic (E/O) voltage detector which uses an electrooptic material
whose refractive index is changed by the strength of an applied electric
field. Conventional versions of this E/O voltage detector are disclosed in
U.S. Pat. No. 4,446,425, Japanese Patent Application Unexamined
Publication Nos. 300969/1988, 9370/1989, etc.
These voltage detectors have an optical probe that is made of an
electrooptic material having a light-reflecting surface on its tip. The
light reflected from said reflecting surface is received by a
photodetector and subjected to photoelectric conversion. However, the
dynamic range of the photodetector is not wide enough to allow a small
change in light intensity (voltage change) to be detected with high S/N
ratio. To overcome this difficulty, it has been necessary that the
electric signal to be measured be turned on and off, and the signal of
such a modulation frequency be amplified over a narrow band.
In the inventions described in the prior patents listed above, this need
can be easily met if the electric signal to be measured is an output
signal from a photodetector, since one only need to turn the incident
pulse light on and off. However, the problem is that the device that can
be measured (i.e., the source of the electric signal to be measured) is
limited to one that generates an electric signal in response to input
light.
As an alternative, the electric signal to be measured may be directly
turned on and off by means of an electric chopper, but, in this case, the
waveform of a signal to be measured will be distorted when it passes
through the electric chopper, whereby it becomes difficult to achieve
correct waveform measurements.
Considering the measurement of a dc voltage, the intensity of light passing
through an E/O probe will change if a voltage is applied to it, so that
the measurement of the dc voltage should be made possible by monitoring
the quantity of transmitted light with a photodetector. In fact, however,
the E/O probe has such a low sensitivity that if the applied voltage is
only a few volts, the resulting change in the quantity of light is too
small to be detected by an optical power meter having an ordinary dynamic
range.
It has generally been proposed that an electric signal of interest be
chopped as shown in FIG. 14, with only the modulated component being
efficiently detected with a lock-in amplifier. This technique, however, is
not effective when a dc voltage is to be measured, since the dc electric
signal cannot be chopped. Instead, the input light to the optical probe
may be chopped as shown in FIG. 15. However, if the output light is simply
subjected to lock-in detection, it is the light intensity itself that is
measured as shown in FIG. 15(C), and the change in light intensity
resulting from the application of a voltage to the E/O probe cannot be
detected.
Further, the light-reflecting surface is formed at the tip of the optical
probe and it is practically impossible to achieve 100% reflection by this
surface. This inevitably causes part of the incident light to leak toward
a source of the signal to be measured. If part of the incident light leaks
toward the signal source, the light reflected from the latter will be
transmitted again through the light-reflecting surface to enter the
optical probe, and detected as part of the output signal. If, even in this
case, the reflectance of the signal source is constant, correct
measurements can be accomplished by processing the output signal in
consideration of said constant reflectance and the transmittance of the
light-reflecting surface. However, if the signal source is, for example,
an LSI, its reflectance will change in accordance with its circuit pattern
and this cannot be easily compensated.
SUMMARY OF THE INVENTION
The present invention has been accomplished under these circumstances of
the prior art and the principal object of this invention is to provide an
apparatus for detecting the change of light intensity which is capable of
correctly detecting the change of the signal to be measured without
turning it on or off.
Another object of the present invention is to provide an apparatus for
detecting the change of light intensity that is also capable of measuring
an unchanging signal such as a dc voltage.
A further object of the present invention is to provide an apparatus for
detecting the change of light intensity that has a light intensity
changing means with a light-reflecting surface and which is capable of
compensating for the noise caused by the light transmitted through the
reflecting surface.
Yet another object of the present invention is to provide an apparatus for
detecting the change of light intensity that is capable of compensating
for the noise variation that is caused by the light transmitted through
the reflecting surface and the variation in the reflectance of the surface
of a signal source.
These objects of the present invention can be attained by an apparatus for
detecting a signal through a change of light intensity which comprises:
light source means including:
a light source for emitting a light beam;
light chopping means for chopping the light beam at a predetermined
chopping frequency; and
chopping control means for controlling the light chopping means in
synchronism with the signal to be detected;
light intensity changing means for changing an intensity of part of the
light beam from the light source means in accordance with a change in the
signal being applied thereto;
first photodetecting means for converting an output light beam of the light
intensity changing means into a first electrical signal;
second photodetecting means for converting part of the light beam which is
emitted from the light source means and not subjected to the light
intensity changing by the light intensity changing means into a second
electrical signal; and
narrow-band detecting means for detecting a difference between the first
and second electrical signals in a narrow band at the chopping frequency.
In the present invention, the light to be incident on the light intensity
changing means, rather than the signal to be measured, is turned on and
off by means of the light chopping means, so that compared to the case
where the signal to be measured is chopped, the waveform of said signal
can be measured in a desired correct way. The present invention also
enables the measurement of an unchanging signal such as a dc voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an apparatus for detecting the change of
light intensity according to a first embodiment of the present invention;
FIG. 2 is a block diagram showing an apparatus for detecting the change of
light intensity according to a second embodiment of the present invention;
FIGS. 3(A-D) are a diagram illustrating the operation of the apparatus
according to the second embodiment;
FIG. 4 is a block diagram showing the essential part of an apparatus for
detecting the change of light intensity according to a third embodiment of
the present invention;
FIG. 5 is an enlarged cross-sectional view of an optical probe used in the
third embodiment and the nearby area;
FIGS. 6(A-D) are a diagram showing the results of measurement in accordance
with the third embodiment;
FIGS. 7-13 are block diagrams showing modifications of various parts of the
apparatus according to the second embodiment;
FIGS. 14(A-C) are a diagram showing the result of detection with a prior
art voltage detector when an electric signal to be measured is chopped;
and
FIGS. 15(A-C) are a diagram showing the result of detection with another
prior art voltage detector when input light is chopped.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Various embodiments of the present invention are described below with
reference to FIGS. 1-13.
As shown in FIG. 1, an apparatus for detecting the change of light
intensity according to a first embodiment of the present invention is
generally indicated by numeral 30 and comprises the following components:
a light-intensity changing means 12 such as an electrooptic device, a
Faraday rotation device, an accoustooptic device or a certain type of
fiber optics that allow the intensity of incident light to vary in
accordance with the change of a signal to be measured from a signal source
10 such as an electric signal source that produces a change in voltage, an
ultrasonic signal source that is accompanied by a change in intensity or
frequency, or a similar optical magnetic signal source.
Thus, the light intensity changing means 12 effectively converts variations
in a parameter represented by an input signal into light intensity
variations which make it possible to measure the parameter variations in
accordance with the present invention. A light source unit 20 including a
short pulse light source 14 composed of a semiconductor laser, light
chopper 16 that allows the light emitted from said short pulse light
source 14 to be repeatedly turned on and off at high speed, and light
chopper control circuit 18 that controls said light chopper 16 in
synchronism with said signal source 10; a first photodetector 22 that
performs photoelectric conversion on the component of light that is
emitted from said light source unit 20 and transmitted through said light
intensity changing means 12; a beam splitter 24 for separating part of the
light from said light source unit 20 to be deflected sidewise before the
light intensity changing means 12; a second photodetector 26 that receives
the thus separated light component and performs photoelectric conversion
on it; and a narrow-band detecting means 28 that calculates the difference
between the outputs of said first and second photodetectors 22 and 26 and
which performs narrow-band detection on the calculated difference at the
on/off frequency of said light chopper 16.
The narrow-band detecting means 28 is composed of a computing circuit 28A
that calculates the difference between the outputs of said first and
second photodetectors 22 and 26, a narrow-band filter 28B that detects the
output signal from the computing circuit 28A in a narrow band at the
on/off frequency of said light chopper 16, an amplifier 28C for amplifying
the output of this narrow-band filter 28B, and an output regulator 28D
that adjusts said computing circuit 28A in such a way that the difference
between the outputs of the first and second photodetectors 22 and 26 will
be zero in the absence of any change in light intensity that will occur in
the light intensity changing means 12 on account of the signal to be
measured. This output regulator 28D adjusts either the output of the first
photodetector 22 or the output of the second photodetector 26, or both of
those.
In the first embodiment of the present invention described above, the
signal from the signal source 10 is not directly turned on or off but the
incident light is turned on and off by the light chopper 16, and yet the
result is the same as what would be attained by turning the signal of
interest on and off. Accordingly, this embodiment offers the advantage
that the signal waveform distortion which would otherwise occur if the
signal to be measured were turned on and off, whereby the waveform of that
signal can be measured correctly.
If the short pulse light source 14 is operated in synchronism with the
signal to be measured, the change in light intensity that occur in the
light intensity changing means 12 on account of the change in the signal
to be measured can be detected with a gating time that is comparable to
the duration of each light pulse emitted from the short pulse light source
14. In this case, neither of the first and second photodetectors 22 and 26
need to have high-speed responding characteristics.
In the first embodiment described above, a short pulse light source is
used, but this may be replaced by a cw light source if there is no need to
detect the signal of interest with a short gating time.
The second photodetector 26 used in the first embodiment receives part of
the incident light as separated by the beam splitter 24 before it is input
to the light intensity changing means 12. However, it suffices that this
second photodetector is capable of receiving light the intensity of which
has not yet been changed by the light-intensity changing means 12. Thus, a
beam splitter may be provided on the exit side of the light-intensity
changing means 12 so that the second photodetector receives the component
of the output light that has not been changed in intensity on account of
the signal to be measured.
In the first embodiment, the light chopper 16 is a mechanical light
chopper, an ultrasonic optical modulator or a light source drive circuit.
Further, the narrow-band filter used in the narrow-band detecting means 28
may be replaced by a spectrum analyzer, a lock-in amplifier or a
narrow-bandpass amplifier.
FIG. 2 is a block diagram showing schematically an apparatus for detecting
the change of light intensity according to a second embodiment of the
present invention, in which said apparatus is used as a voltage detector
which is generally indicated by numeral 32. As shown, the voltage detector
32 comprises an optical probe 34 that is made of an electrooptic material
whose refractive index varies in accordance with an electric field
developing in a selected part of an object to be measured and which has a
light reflecting surface 34A formed at the tip, a light source unit 36 for
generating a light beam to be input to said optical probe 34, and a first
photodetector 38 that performs photoelectric conversion on a light beam
that is emitted from the light source unit 36, then launched into the
optical probe 34, reflected by the light-reflecting surface 34A, and
finally output from the optical probe 34. The voltage detector 32 is
adapted to measure the voltage of the object of interest on the basis of
the change in the output of the first photodetector 38. According to the
second embodiment of the present invention, the light source unit 36 is
equipped with an ultrasonic optical modulator 36A that repeatedly turns on
and off the light beam. The voltage detector 32 further includes a second
photodetector 40 that performs photoelectric conversion on the light beam
component that has not been influenced by the change in refractive index
by the electric field on the optical probe 34 and which is received before
said optical probe 34, and a narrow-band detecting means 42 that detects
the difference between the outputs of the first and second photodetectors
38 and 40 in a narrow band at the on/off frequency of the ultrasonic
optical modulator 36A.
The light source unit 36 is composed of the ultrasonic optical modulator
36A, a semiconductor laser 36B as a light source, a condenser lens 36C and
an optical isolator provided between the semi-conductor laser 36B and the
ultrasonic optical modulator 36A, a semiconductor laser drive device 36E
for driving the semiconductor laser 36B in synchronism with an electronic
device 46 which is the object to be measured, and an optical modulator
control device 36F for controlling the ultrasonic optical modulator 36A.
Shown by numeral 48 is an electronic device control device for driving the
electronic device 46, which is adapted to supply the semiconductor laser
drive device 36E with a signal for achieving synchronization with the
electronic device 46.
The ultrasonic optical modulator 36A which is controlled by the optical
modulator control device 36F is designed s that zeroth-order
(non-modulated) light the optical probe 34 but travels toward a light
stopper 50, and that when the light is modulated by ultrasonic waves, the
first-order light passes through a collimator lens 52 to travel toward the
optical probe 34.
Between the collimator lens 52 and the optical probe 34 are provided a
polarizing beam splitter 54, a beam splitter 56, a compensator 58 and a
condenser lens 60 in the order written. The light that has passed through
these components to be input to the optical probe 34 and which is
thereafter reflected by the light-reflecting surface 34A is separated by
the polarizing beam splitter 54 to be deflected sidewise and input to the
first photodetector 38. On the other hand, the beam the splitter 56
separates part of the first-order light before the optical probe 34, and
deflects it sidewise to be directed to the second photodetector 40. As in
the case of the first embodiment, it suffices that the second
photodetector 40 is capable of receiving light the intensity of which has
not yet been changed by the electric field applied to the optical probe
34. Therefore, the beam splitter 56 may be disposed so as to deflect
sidewise part of the light beam reflected from the optical probe 34 to
introduce it to the second photodetector 40. This is because the intensity
change of the reflected light beam from the optical probe 34, which is due
to the applied electric field, can be produced by passing it through a
polarizing element such as the polarizing beam splitter 54, but a mere
beam splitter does not produce such a light intensity change.
The narrow-band detecting means 42 is composed of a variable attenuator 42A
for adjusting the output signal of the second photodetector 40, a variable
attenuator control circuit 42B that controls this variable attenuator 42A
in response to the outputs of the first and second photodetectors 38 and
40 so that the outputs of the first photodetector 38 and the variable
attenuator 42A become equal to each other in the absence of any electric
field applied to said optical probe 34, and a lock-in amplifier 42C that
amplifies the difference between the output from said first photodetector
38 and the output from the variable attenuator 42A in a narrow band at the
on/off frequency of the ultrasonic optical modulator 36A on the basis of a
reference signal from said optical modulator control circuit 36F.
Shown by numeral 55 in FIG. 2 is a recording unit that records the output
from said narrow band detecting means 42 together with the amount of
change in the timing of the light pulse generation in the semiconductor
laser 36B on the basis of a signal from said semiconductor laser drive
device 36E. Shown by numeral 57 is a device for displaying and analyzing
recording data in the recording device 55.
The operation of the apparatus according to the second embodiment is
described below.
The electronic device 46 to be measured is driven by the electronic device
drive device 48. Based on the signal from this electronic device drive
device 48, the semiconductor laser drive device 36E drives the
semiconductor laser 36B in synchronism with the electronic device 46.
The light issuing from the semiconductor laser 36B is input to the
ultrasonic optical modulator 36A via the condenser lens 36C and the
optical isolator 36D. The ultrasonic optical modulator 36A is driven by
the optical modulator control device 36F and the first-order light
produced by modulating the incident light is incident on the optical probe
34 via the collimator lens 52, polarizing beam splitter 54, beam splitter
56, compensator 58 and condenser lens 60.
An electric field created by the voltage on the electronic device 46 acts
on the optical probe 34 and the polarization state of light is changed in
said optical probe 34 in accordance with this electric field. The light
input to the optical probe 34 is reflected by the light-reflecting surface
34A and thence passes through the condenser lens 60, compensator 58 and
beam splitter 56 to arrive at the polarizing beam splitter 54, by which it
is reflected toward the first photodetector 38.
The first-order light from the ultrasonic optical modulator 36A passes
through the beam splitter 56 but part of it is reflected to arrive at the
second photodetector 40.
The output A of the first photodetector 38 is supplied to the lock-in
amplifier 42C. The output B of the second photodetector 40 (i.e., the
output of the variable attenuator 42A) is automatically and preliminarily
adjusted by means of the variable attenuator 42A and the variable
attenuator control circuit 42B so that B becomes equal to A in the absence
of any electric field applied to the optical probe 34. After adjustment by
the variable attenuator 42A, the output B is also supplied to the lock-in
amplifier 42C. Based on the reference signal from the optical modulator
control device 36F, the component of the signal A-B which corresponds to
the on/off modulation frequency of the ultrasonic optical modulator 36A is
amplified in a narrow band by the lock-in amplifier 42C.
The signal A-B=C which is the output from the lock-in amplifier 42C is
supplied to the recording unit 55, which records this output of the
lock-in amplifier 42C together with the amount of change in the timing of
the light pulse generation in the semiconductor laser drive device 36E,
and the recording data is displayed and analyzed by the device 57.
Thus, in the second embodiment of the present invention, the signal in the
electronic device 46 to be measured is not turned on and off, but instead
the light to be input to the optical probe 34 is turned on and off by the
ultrasonic optical modulator 36A and the result is the same as what would
be attained if the very signal to be measured were turned on and off. In
addition, the distortion of the signal waveform that would otherwise occur
if the signal were turned on and off in the electronic device 46 can be
avoided.
In this second embodiment, the semiconductor laser 36B is driven in
synchronism with the signal of the electronic device 46, so that by
changing the timing of successive generation of light pulses, the waveform
of the signal in the electronic device 46 can be correctly measured.
As described above, the second embodiment of the present invention enables
the measurement of rapidly changing electric waveforms by synchronizing
the semiconductor laser 36B with the operation of the electronic device
46. It should, however, be noted that the present invention is in no way
limited to this particular case alone and that it is also applicable to
the case where a dc voltage is applied to the electronic device 46. In
this case, the light source in the light source unit need not be a pulse
light source and may be a cw light source such as a He-Ne laser.
Measurements of dc voltages have been impossible in the prior art as shown
in FIGS. 14 and 15. However, in accordance with the second embodiment of
the present invention, the input light is chopped as shown in FIG. 3 and
only the change in output light due to the application of a voltage to the
optical probe 34 can be detected by the lock-in detection of the
differential signal (A-B) in the lock-in amplifier 42C, as shown in FIG.
3(D). Thus, even an absolute voltage can be measured in accordance with
the second embodiment of the present invention.
FIG. 4 is a block diagram showing schematically an apparatus for detecting
the change of light intensity in accordance with a third embodiment of the
present invention. This third embodiment differs from the already
described second embodiment in that a compensator circuit 44 is provided
on the output side of the lock-in amplifier 42C. The other components of
the third embodiment are the same as those used in the second embodiment,
so like parts are identified by like numerals and will not be described in
detail.
The compensator circuit 44 performs the following two functions: first, it
computes the difference between the output of the lock-in amplifier 42C
for the case where no electric field is applied to the optical probe 34
and the output for the case where an electric field is applied; second, it
divides, by a divider, the computed difference by the output that is
produced from the first photodetector 38 when no electric field is applied
to the optical probe 34.
The operation of the apparatus according to the third embodiment of the
present invention is described below.
It is generally impossible to achieve 100% reflection by the
light-reflecting surface 34A at the tip of the optical probe 34. Hence,
part of the light input to the optical probe 34 passes through the
reflecting surface 34A to arrive at the surface of the object to be
measured and is thence reflected by said surface to make another entry
into the optical probe via the reflecting surface 34A. This reentry light
is detected by the photodetector as noise to cause an error in
measurement. Further, if there is any variation in the reflectance of the
surface of the object to be measured, the error component due to the light
reflected from said surface also varies with the change in its
reflectance, whereby it becomes more difficult to achieve correct
measurements.
The compensator circuit 44 is intended to enable correct measurements by
eliminating not only the error caused by the light transmitted through the
light-reflecting surface 34A because of such imperfection in its
reflectance, but also the error due to the variation in the reflectance of
the surface of the object to be measured.
As shown in FIG. 5, suppose that the surface of the object to be measured,
say, the electronic device 46, has two areas 46A and 46B having different
reflectances R.sub.1 and R.sub.2, respectively. Also suppose that the
incident light has an intensity of 2I.sub.0 and that the light-reflecting
surface 34A has a reflectance of R.sub.0.
If R.sub.0 =1, the fundamental output I of the optical probe 34 is
expressed by:
I=I.sub.0 {1+(.pi./V.sub..pi.)v} (1)
where V.sub..pi. is the half-wave voltage and v is a voltage to be
measured.
If R.sub.0 <1, the output I.sub.x of light reflected by the reflecting
surface 34A and the output I.sub.y of light transmitted through said
surface are respectively expressed by:
I.sub.x =R.sub.0 I.sub.0 {1+(.pi./V.sub..pi.)v} (2)
I.sub.y =(1-R.sub.0).sup.2 R.sub.1 I.sub.0 {1+(.pi./V.sub..pi.)v}(3).
Since I expressed by equation (1) is nearly equal to the sum of I.sub.x and
I.sub.y, equation (1) can be rewritten as:
##EQU1##
where Ix and Iy are each supposed to be decreased by half by means of the
polarizing beam splitter 54.
As already mentioned in connection with the second embodiment, the
narrow-band detecting means 42 includes the variable attenuator 42A and
the variable attenuator control circuit 42B and is pre-adjusted in such a
way that the outputs of the first and second photodetectors 38 and 40
become equal to each other in the absence of any electric field applied to
the optical probe 34. In this pre-adjusted state, the lock-in amplifier
42C produces a differential output I.sub.A -I.sub.B, or A-B =C as shown in
FIG. 2.
When the first area 46A and the second area 46B are measured, I.sub.A,
I.sub.B and I.sub.A-B for the respective areas may be expressed as follows
depending on whether a voltage is applied to the optical probe 34.
As for the first area 46A, they can be expressed by equations (5)-(9),
where (.smallcircle.) denotes the presence of a voltage applied and (x)
denotes the absence of voltage application:
##EQU2##
As for the second area 46B, I.sub.A, I.sub.B and I.sub.A-B can be expressed
by equations (10)-(13):
##EQU3##
I.sub.B is expressed by equation (7).
The difference between the differential output (A-B) in the presence of a
voltage applied to the optical probe 34 and the differential output (A-B)
in the absence of an applied voltage may be expressed as follows for the
area 46A:
##EQU4##
Since R.sub.1 in equation (14) differs from R.sub.2 in equation (15), it
cannot be said that the outputs are completely compensated (see FIG. 6).
If equations (14) and (15) are each divided by the output I.sub.A (x) in
the absence of voltage change, the following equation (16) is derived for
each case, and the difference in the differential output (A-B) is not
dependent on the reflectance R1 of the first area 46A and the reflectance
R2 of the second area 46B but is proportional to the applied voltage v:
{I.sub.A-B (.smallcircle.)-I.sub.A-B (x)}/I.sub.A
(x)=(.pi./V.sub..pi.)v(16)
As already mentioned, the compensator circuit 44 divides the differential
output from the lock-in amplifier 42C by the output of the first
photodetector 38 for the case where no voltage is applied to the optical
probe 34, and this eventually provides equation (16).
Suppose here that the light-reflecting surface 34A has a reflectance
(R.sub.0) of 90% and that, in the worse case of the electronic device 46,
the first area 46A has a reflectance (R.sub.1) of 0% and the second area
46B has a reflectance (R.sub.2) of 100%. Then, R.sub.0 +(1-R.sub.0).sup.2
R.sub.1 =R.sub.0 =0.90 in the area 46A and R.sub.0 +(1-R.sub.0).sup.2
R.sub.2 =R.sub.0 +(1-R.sub.0)=0.91 in the area 46B. In other words, the
output compensated by equations (14) and (15) differs by about 1% from the
output compensated by equation (16). Hence, the precision of measurement
that is attained in accordance with the third embodiment is about 1%.
In the third embodiment described above, the variable attenuator 42A is
pre-adjusted in such a way that I.sub.A-B (x) becomes zero for the first
area 46A. It is noted that the same pre-adjustment may be performed for
the second area 46B instead of the first area 46A.
In the second and third embodiments described above, the semiconductor
laser drive device 36E is driven on the basis of the signal from the
electronic device drive device 48. If desired, a reference clock generator
62 may be provide which, as shown in FIG. 7, supplies a clock signal to
both the semiconductor laser drive device 36E and the electronic device
drive device 48.
In the second and third embodiments, the first photodetector 38 and the
second photodetector 40 receive light beams that are reflected by the
polarilizing beam splitter 54 and the beam splitter 56, respectively. If
desired, a half mirror 64 may be provided on the optical axis of the
incident light as shown in FIG. 8 so that part of the incident light
passing through a polarizer 66 is reflected to the second photodetector 40
and part of the return light from the optical probe 34 is input to the
first photodetector 38 via an analyzer 68. Alternatively, the half mirror
64 may be replaced by the polarizing beam splitter 54 as shown in FIG. 9.
In the second and third embodiments, the output of the second photodetector
40 is controlled by means of the variable attenuator 42A and the variable
attenuator control circuit 42B in such a way that it becomes equal to the
output of the first photodetector 38 when no voltage is applied to the
optical probe 34. If desired, the same result can be attained by the
circuit configuration shown in FIG. 10 which includes a variable amplifier
70 for amplifying the output of the first photodetector 38 and a variable
amplifier control circuit 72 that controls said variable amplifier 70 in
accordance with the outputs of the first and second photodetectors 38 and
40.
Alternatively, the outputs of the first and second photodetectors 38 and 40
may be supplied to a differential amplifier 74 as shown in FIG. 11, with
its output being supplied to the lock-in amplifier 42C. In this case, the
differential amplifier 74 should be preadjusted in such a way that the
output signals of the first and second photodetectors 38 and 40 will
become equal to each other when no voltage is applied to the optical probe
34.
Further, a divider circuit 76 may be provided on the output side of the
lock-in amplifier 42C as shown in FIG. 12. The divider circuit 76 divides
the differential output of the lock-in amplifier 42C by part of the output
from the second photodetector 40, so that any operational variation in the
light source unit 36 can be canceled out.
In the embodiments described above, the emitted light is turned on and off
by means of the ultrasonic optical modulator 36A or the light chopper 16.
If desired, for example, the semiconductor laser 36B may be electrically
turned on and off by the semiconductor laser drive device 36E so as to
produce the same result as is attained by turning on and off the light by
the optical modulator 36A. In this case, the on/off signal in the
semiconductor laser drive device 36E may be used as a reference signal for
the lock-in amplifier 42C as shown in FIG. 13.
Having the construction described above, the apparatus of the present
invention is adapted to turn the incident light on and off and yet it
produces results comparable to those which would be attained by directly
turning on and off the signal to be measured. Further, the apparatus has
the advantage of preventing the occurrence of waveform distortion in the
signal to be measured.
* * * * *
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