Optical signal detector incorporating means for eluminating background light

A first part of input light containing both background light and signal light is extracted by a sampling streak tube and converted to a first electric signal by a photodetector. A second part of the input light containing only background light is extracted by the same sampling streak tube by offsetting the sampling timing and converted to a second electric signal by the photodetector. A signal component corresponding to the signal light is extracted by taking the difference between the first and second electric signals. In one embodiment, the sampling timing is offset by applying a chopping voltage to chopping electrodes provided in the sampling streak tube.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical signal detector for detecting 
signal light superposed on a part of input light which contains background 
light. More particularly, the present invention relates to an optical 
signal detector with which an optical signal superposed on intense 
background light can be observed at high time resolution. 
The transient behaviors of ultra-fast optical phenomena can be measured by 
a variety of means. One method is the use of a streak camera which is 
operated by the following principle: incident light is focused on a 
photocathode where the photons are converted to electrons; the 
photoelectron beam emitted from the photocathode is then swept at high 
speed by applying a deflection voltage; and thereby the temporal change in 
the intensity of the incident light is measured as the change in 
brightness associated with position on a phosphor screen. 
The heart of the streak camera is a streak tube which is generally 
indicated as 13 in FIG. 19 and which includes the following main 
components: a photocathode 14 where an optical image (slit image) focused 
at an entrance window through a slit plate 10 and a lens 12 in an input 
optical system is converted to an electron image; an accelerating 
electrode 16, typically in mesh form, which accelerates the electron image 
produced on the photocathode 14; (main) deflecting electrodes 22 by which 
the accelerated photoelectrons are swept at high speed in a direction 
perpendicular to the slit length direction (either upward or downward as 
viewed in the drawing); and a phosphor screen 26 on which the deflected 
photoelectron image is converted to an optical image (called "streak 
image" which carries brightness information with the lapse of time being 
expressed by position on the vertical axis), which emerges from an exit 
window. 
The other components shown in FIG. 19 are as follows: a focusing electrode 
18 by which the photoelectrons accelerated by the electrode 16 are 
converged to have a specified cross-sectional area; an anode 20 having an 
opening area in the center; a (main) sweep voltage generator circuit 23 
which applies a predetermined (main) sweep voltage to the deflecting 
electrodes 22 in synchronism with the passage of electrons; a microchannel 
plate (MCP) 24 by which the electrons passing through the deflecting 
electrodes 22 are multiplied before they arrive at the phosphor screen 26; 
a cone-shaped shielding electrode 25 that is provided on the input side of 
MCP 24 and which improves the precision of measurements by blocking the 
electrons deflected to go outside the effective sweep range of phosphor 
screen 26; and an image pickup device 28 such as a high-sensitivity TV 
camera, e.g., a silicon intensified target (SIT) camera or a CCD camera, 
which picks up the streak image via a lens 27 in an output optical system. 
Operating by the principles described above, streak cameras are roughly 
divided into two types according to the method of sweeping; namely, a 
single scan type and a synchroscan type. A single scan streak camera 
performs linear sweep with an ultra-fast sawtoothed wave repetitive at a 
rate not higher than several kilohertz in synchronism with pulse laser 
light. A synchroscan streak camera performs high-speed repetitive sweep 
with a sinusoidal wave synchronized with laser light pulses repetitive at 
80-160 MHz. An improved version called a synchronous blanking streak 
camera has also been developed. As shown in FIG. 20, this camera has 
sub-deflecting electrodes 29 crossed with the main deflecting electrode 22 
and performs elliptical sweep in such a way that retrace sweep is 
laterally shifted to avoid scanning on the phosphor screen 26, thereby 
insuring that a signal associated with main sweep is selectively measured 
in a correct way. 
These prior art streak cameras are described in many patent documents such 
as Japanese Patent Nos. 1,149,098, 1,149,120 and 1,099,753, JP-A-59-58745 
(the term "JP-A" as used herein means an "unexamined published Japanese 
patent application"), JP-A-61-183857, U.S. Pat. Nos. 4,232,333, 4,352,127, 
4,611,920 and 4,661,694, British Patent Nos. 2,042,163, 2,044,588 and 
2,131,165. 
The use of streak cameras in measuring the transient behaviors of 
ultra-fast optical phenomena has the following advantages: first, it 
provides a purely electronic direct method having fast time resolution and 
high detection sensitivity; second it is capable of measuring single 
(non-repetitive) phenomena; third, the streak image which is inherently a 
two-dimensional image helps provide two-dimensional measurements such as 
time-resolved spectral measurements and space- and time-resolved 
measurements, as well as multi-channel measurements; and fourth, by 
properly selecting the materials of which the photocathode and entrance 
window are made, measurements over a broad spectral range extending from 
the near-infrared region through the vacuum ultraviolet region up to the 
X-ray region can be realized. 
A sampling optical oscilloscope has also been commercialized. As shown in 
FIG. 21, the streak image is electronically sampled with a sampling streak 
camera 30 in which a slit plate 32 having an electronic sampling slit 32A 
that limits said streak image spatially is provided typically in the 
streak tube. FIG. 21 also shows a photodetector 34 that detects the light 
emission intensity caused by electrons impinging on the phosphor screen 26 
and may be composed of a photomultiplier tube, a high-sensitivity 
photodiode, an avalanche photodiode, a PIN photodiode, etc. This sampling 
optical oscilloscope is described in such patent documents as 
JP-A-59-104519, JP-A-59-134538, JP-A-59-135330, U.S. Pat. Nos. 4,645,918 
and 4,694,154, and British Patent No. 2,133,875. 
However, all of the prior art streak cameras described above perform 
immediate photoelectric conversion on input light and it has been 
difficult to observe with these cameras the waveform of signal light 
superposed on intense background light (dc light). 
SUMMARY OF THE INVENTION 
The principal object, therefore, of the present invention is to solve the 
aforementioned problem of the-prior art by providing an optical signal 
detector with which signal light superposed on a part of input light which 
contains background light can be detected for observation purposes. 
This object of the present invention can be attained by an optical signal 
detector for detecting signal light superposed on a part of input light 
which contains background light, which detector includes first 
photoelectric converting means that extracts the part of input light 
containing both background light and signal light at high time resolution 
and which converts the extracted part to an electric signal, second 
photoelectric converting means that extracts the part consisting of only 
background light and not containing signal light at high time resolution 
and which converts the extracted part to an electric signal, and means for 
extracting the component solely composed of signal light on the basis of 
the difference between the electric signal for the part containing signal 
light and the electric signal for the part consisting of only background 
light. 
In one embodiment, the first -photoelectric converting means comprises a 
sampling streak tube and the second photoelectric converting means 
produces the electric signal for the part of input light consisting of 
only background light by offsetting the timing of sampling with the 
sampling streak tube. 
In another embodiment, the timing of sampling with the sampling streak tube 
is offset by applying a chopping voltage to chopping deflecting 
electrodes. 
In still another embodiment, the timing of sampling with the sampling 
streak tube is offset by applying a chopping voltage in superposition on a 
sub-sweep voltage applied to subsweep deflecting electrodes. 
In a further embodiment, the timing of sampling with the sampling streak 
tube is offset by changing the phase of a sub-sweep voltage applied to 
sub-sweep deflecting electrodes. 
In another embodiment, the timing of sampling with the sampling streak tube 
is offset by changing the amount of delay of a main sweep voltage to be 
applied to main sweep deflecting electrodes. 
In yet anther embodiment, the first and second photoelectric converting 
means are a plurality of electronic sampling slits provided independently 
of one another in a sampling streak tube. 
In accordance with the present invention, the part of input light 
containing both background light and signal light is extracted at high 
time resolution and converted to an electric signal. At the same time, the 
part of input light consisting of only background light and not containing 
signal light is also extracted at high time resolution and converted to an 
electric signal. Hence, the component of input light solely composed of 
signal light can be extracted on the basis of the difference between the 
electric signal for the part containing signal light and the electric 
signal for the part consisting of only background light, and this insures 
that even signal light superposed on intense background light (dc light) 
can be observed at high time resolution. As a result, the optical signal 
detector of the present invention enables measurements of light absorption 
and many other optical measurements that have previously been considered 
difficult to perform. 
When a sampling streak tube is used as the first photoelectric converting 
means, the electric signal for the part of input light which consists of 
only background light can be readily obtained by offsetting the timing of 
sampling with this sampling streak tube. 
The timing of sampling with the sampling streak tube may be offset by 
applying a chopping voltage to chopping deflecting electrodes and this 
offers the advantage of eliminating the need to alter a sub-sweep voltage. 
If the timing of sampling with the sampling streak tube is offset by 
applying a chopping voltage in superposition on a sub-sweep voltage 
applied to a sub-sweep deflecting electrodes, this eliminates the need to 
provide chopping deflecting electrodes. 
The timing of sampling with the sampling streak tube may also be offset by 
changing the phase of a sub-sweep voltage applied to sub-sweep deflecting 
electrodes, and this eliminates the need to generate a chopping voltage. 
If the timing of sampling with the sampling streak tube is offset by 
changing the amount of delay of a main weep voltage to be applied to main 
sweep deflecting electrodes, the optical signal detector of the present 
invention can be easily constructed using a commercially available 
sampling optical oscilloscope. 
In another embodiment, the first and second photoelectric converting means 
may be a plurality of electronic sampling slits provided independently of 
one another in a sampling streak tube. In this case, the electric signal 
for the part of input light containing signal light and the electric 
signal for the part consisting of only background light can be obtained 
independently of each other without performing chopping.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Several embodiments of the present invention are described below in detail 
with reference to the accompanying drawings. 
A first embodiment of the present invention relates to a sampling streak 
camera which is generally indicated by 30 in FIG. 1 and which contains a 
photocathode 14, an anode 18, main deflecting electrode 22, sub-deflecting 
electrodes 29, a slit plate 32 and a phosphor screen as main components. 
According to the first embodiment, chopping deflecting electrodes 40 are 
provided in the tube and a chopping voltage V.sub.3 as supplied from a 
chopping voltage generator circuit 42 is applied to the electrodes 40 so 
as to offset the timing of sampling with an electronic sampling slit 32A 
in the slit plate 32. By so doing, an electric signal for the part of 
input light containing signal light S and an electric signal consisting of 
only background light N can both be obtained by a single photodetector 
(e.g., photomultiplier tube) 34. 
The system shown in FIG. 1 also includes a main sweep voltage generator 
circuit 23 for generating in response to a trigger signal a main sweep 
voltage V.sub.1 to be applied to the main deflecting electrodes 22 in 
synchronism with the input light to be measured, and a sub-sweep voltage 
generator circuit 33 for generating a sub-sweep voltage V.sub.2 whose 
phase is different from the main sweep voltage by a predetermined 
difference .DELTA..phi.. 
The output of the photodetector 34 is processed with a signal processing 
circuit which, as shown in FIG. 2, may comprise the following components: 
a switching circuit 44 for switching the output of the photodetector 34 in 
synchronism with the timing of the change in the chopping voltage produced 
by the chopping voltage generator circuit 42; integrator circuits 46 and 
48 by which an electric signal for the part of input light containing 
signal light S and an electric signal for the part consisting of only 
background light N and not containing signal light are respectively 
integrated in accordance with the switching state of the switching circuit 
44; a differential amplifier 50 for amplifying the difference between 
outputs A and B from the respective integrators 46 and 48; and a lock-in 
amplifier 52 that amplifies an output (A-B) from the differential 
amplifier 50 at the frequency (chopping frequency) of the timing change in 
the chopping voltage being supplied from the chopping voltage generator 
circuit 42 with a narrow bandwidth and outputs the amplified signal. 
The optical signal detector according to the first embodiment of the 
present invention will operate in the following manner. 
An example of the relationship, as applicable in the first embodiment, 
among the main sweep voltage V.sub.1, sub-sweep voltage V.sub.2, chopping 
voltage V.sub.3, light to be measured (input light), and the timing of 
sampling is shown in FIG. 3. In zone A where the chopping voltage V.sub.3 
is not applied (V.sub.3= 0), the photodetector 34 receives an optical 
signal for the part containing both the background light N and signal 
light S that has been sampled with the electronic sampling slit 32A as in 
the prior art (see FIG. 4A). In zone B where the chopping voltage V.sub.3 
is applied, the photodetector 34 receives an optical signal for the part 
consisting of only the background light N and not containing the signal 
light (see FIG. 4B). 
The outputs produced by the photodetector 34 in zones A and B are 
integrated by the integrator circuits 46 and 48, respectively, and are 
then amplified by the differential amplifier 50, in the signal processing 
circuit. Thus, unwanted signal of the background light (dc component) N is 
rejected from the signal of the light to be measured. The output of the 
differential amplifier 50 is fed to the lock-in amplifier 52, where the 
chopping frequency component of the signal is amplified with a narrow 
bandwidth. Thus, the noise component is rejected from the output of the 
photodetector 34 to thereby provide an improved S/N ratio. These 
operations, accompanied by gradual change in the timing of sampling, will 
insure that the entire waveform of the input light can be obtained at high 
time resolution. 
In the first embodiment where the chopping deflecting electrodes 40 are 
used, one need only apply sweep voltages to the main deflecting electrodes 
22 and the sub-deflecting electrodes 29 in the usual manner. The use of 
lock-in amplifier 52 contributes to a higher S/N ratio but this may be 
omitted if so desired. 
A specific configuration of the signal processing circuit for processing 
the output of the photodetector 34 is shown in FIG. 5 and may comprise the 
following components: a switching circuit 44; two integrator circuits 46 
and 48; two sample-and-hold circuits 54 and 56; a subtractor 55; and four 
AND gates 58, 60, 62 and 64 for supplying appropriate circuits with a 
clock signal .phi..sub.1 having the same waveform as the chopping voltage 
V.sub.3 and a clock signal .phi..sub.2 delayed in phase by 90.degree. with 
respect to the clock signal .phi..sub.1. 
As shown in FIG. 6, the switching circuit 44 may be composed of an invertor 
44A and two multipliers 44B and 44C. 
An example of the waveforms of signals generated at various parts of the 
signal processing circuit when the chopping voltage has a duty factor of 
50% is shown in FIG. 7. 
A second embodiment of the present invention is described below in detail. 
As shown in FIG. 8, this embodiment is the same as the first embodiment 
except that no chopping deflecting electrode is provided in the sampling 
streak tube 30 and instead a chopping voltage supplied from the chopping 
voltage generator circuit 42 is applied in superposition on the sub-sweep 
voltage being applied to the sub-deflecting electrodes 29. If desired, the 
chopping voltage generator circuit 42 may be replaced by a rectangular 
wave generator circuit 43 as indicated by a dashed line in FIG. 8. 
An example of the relationship between the main sweep voltage V.sub.1 and 
sub-sweep voltage V.sub.2 as applied in the second embodiment is shown in 
FIG. 9. 
The other features of the second embodiment are the same as those of the 
first embodiment and hence will not be described here. According to this 
second embodiment, the timing of sampling with the sampling streak tube 
can be offset without providing separate chopping deflecting electrodes. 
A third embodiment of the present invention is described below in detail. 
As shown in FIG. 10, this embodiment is the same as the second embodiment 
except that the chopping voltage generator circuit is replaced by a phase 
inverting circuit 70 and a switching circuit 72 which are separately 
connected to the sub-sweep voltage generator circuit 33. The 
sub-deflecting electrodes 29 are supplied with the output of the sub-sweep 
voltage generator circuit 33 through the phase inverting circuit 70, where 
the phase is inverted (180.degree. change) at the timing determined by the 
switching circuit 72. This technique is also effective in producing two 
electric signals, one for the part of input light containing signal light 
S and the other for the part consisting of only background light N. 
An example of the relationship between the main sweep voltage V.sub.1 and 
sub-sweep voltage V.sub.2 as applied in the third embodiment is shown in 
FIG. 11. 
The other features, including operational aspects, of the third embodiment 
are essentially the same as those of the first and second embodiments and 
hence will not be described here. The major advantage of the third 
embodiment is that a separate chopping voltage generator circuit need not 
be provided. 
A fourth embodiment of the present invention is described below in detail. 
As shown in FIG. 12, this embodiment is the same as the second embodiment 
except that the chopping voltage generator is replaced by a variable delay 
circuit 74 provided before the main sweep voltage generator circuit 23. 
The amount of delay provided by the variable delay circuit 74 is changed 
by a switching circuit 72 to produce both an electric signal for the part 
of input light containing signal light S and an electric signal for the 
part consisting of only background light N. 
An example of the relationship between D.sub.A or the amount of delay of 
timing with which light containing signal light S is obtained, and D.sub.B 
or the amount of delay of timing with which background light N is 
obtained, as applicable in the fourth embodiment, is shown in FIG. 13. If 
the input light is repetitive at 100 MHz, a change of the amount of delay, 
.DELTA.D, may be adjusted to 5 nanoseconds and this permits the system to 
be operated in the same manner as the previous embodiments. 
The fourth embodiment offers the advantage of realizing the present 
invention in an easy way since the components of the sampling streak tube 
30 downstream of the main sweep voltage generator circuit 23 are identical 
to those of a conventional sampling optical oscilloscope. If desired, the 
variable delay circuit 74 may be incorporated in the main sweep voltage 
generator circuit 23 to form an integral unit. 
A fifth embodiment of the present invention is described below in detail. 
As shown in FIG. 14, this embodiment is the same as the first embodiment 
except that the slit plate 32 in the sampling streak tube 30 is provided 
with two electronic sampling slits 32A and 32B by which the part of input 
light containing signal light S and the part consisting of only background 
light N can be extracted independently of each other. These sampling slits 
are respectively associated with two photodetectors 34A and 34B. 
As shown in FIG. 15, a signal processing circuit of use in the fifth 
embodiment may be composed of a subtractor 76 and an amplifier 78, which 
may be combined as an integral unit in the form of a differential 
amplifier. 
This embodiment, where the light containing the signal component S can be 
sampled and detected independently of the light consisting of only the 
background component N, obviates the need to perform chopping, and hence. 
signal processing can be accomplished with a very simple circuit of the 
configuration shown in FIG. 15. 
If desired, the signal obtained in the circuit shown in FIG. 15 may be 
chopped for lock-in detection. 
FIG. 16 shows a modification of the fifth embodiment in which a variable 
gain circuit 79A is provided subsequent to the first photodetector 34A. In 
this modification, dc light such as that from a flashlight is supplied to 
the optical signal detector while it is operated and the variable gain 
circuit 79A is adjusted in such a way that its output signal A becomes 
equal to the output signal B of the second photodetector 34B. 
FIG. 17 shows another modification of the fifth embodiment in which the 
variable gain circuit 79A is automatically adjusted with an automatic gain 
control circuit 79B. 
In the two modifications, the variable gain circuit 79A is provided 
subsequent to the first photodetector 34A but it may be provided 
subsequent to the second photodetector 34B. If desired, the variable gain 
circuit 79A may be provided subsequent to each of the first and second 
photodetectors. 
Further, each of the first to fourth embodiments described above may be so 
modified that a variable gain circuit is additionally provided, for 
example, subsequent to either one or both of the integrator circuits 46 
and 48 shown in FIG. 2. A variable gain circuit may also be provided 
subsequent to the photodetector 34 so that gain control is performed in 
synchronism with the chopping voltage. 
The optical signal detector according to the various embodiments of the 
present invention may be used as a sampling high-speed photodetector in a 
voltage detecting apparatus of a type that utilizes an electrooptical 
effect in which the refractive index varies with the voltage in a selected 
site of an object to be measured. An example of the use of the optical 
signal detector in this application is shown in FIG. 18 where the optical 
signal detector is indicated by 80. The other components of the system 
shown in FIG. 18 are as follows: a CW light source 82; an optical 
modulator 84 comprising a polarizer 84A, an electrooptic crystal 84B which 
is either placed in the neighborhood of an object to be measured so as to 
cause a change in its refractive index in response to the voltage in a 
selected site of said object or which is directly supplied with the 
voltage to be measured, a compensator 84C and an analyzer 84D; a branching 
device 86 which divides the electric signal to be measured into two parts, 
one of which is supplied to the electrooptic crystal 84 in the optical 
modulator 84 while the other part is supplied into a trigger signal 
generator 88; and a delay circuit 90 which drives the sweep voltage 
generator circuit in the sampling high-speed photodetector 80 according to 
the present invention after delaying the trigger signal from the trigger 
signal generator 88 by a predetermined amount. 
The foregoing description of the present invention assumes that it is 
applied to a synchroscan photometer or a sampling optical oscilloscope 
which are provided with a sampling streak tube, but it will be readily 
apparent to one skilled in the art that these are not the sole examples of 
the use of the present invention and that it can equally be applied to 
ordinary streak tubes having no slit plate, as well as to an apparatus 
having other photoelectric converting means.