GB2172991A - Concurrently measuring a plurality of light signals - Google Patents

Abstract

An instrument capable of concurrently measuring ultra-high-speed light signals on a plurality of channels comprises a streaking tube (7), having in order a photocathode, deflection electrodes 75, a slit plate and a phosphor layer, electrodes 75 being arranged to deflect electrons on application of a sweeping voltage (15), the slit plate having its slit perpendicular to the direction of deflection. Optical means 4, 5, 6 transient repetitive light signals to a section of the photocathode perpendicular to the direction of deflection. The deflection voltage is synchronized (9, 10, 12, 14) with the light signals, its phase being successively incremented. A photodiode array 8 perpendicular to the deflecting direction feeds processing means (17, 18, 30, 32, 34). The instrument can also measure periodically the mechanical distortion of an object on which a pulsed laser beam is incident and time-serial data may be output as parallel data on a parallel time base. <IMAGE>

Description

SPECIFICATION Light signal measuring instrument The present invention relates to a light signal measuring instrument, an instrument for measuring two-dimensional light images which are repeated at ultra-high speed, and especially an instrument for concurrently measuring two-dimensional multichannel light images which may change at ultrahigh speed, e.g. an instrument for the time-resolving spectroscopic measurement of laser pulse stimulated fluorescence, or for spatial time-resolving measurement in the pico second range.

The streak camera is used as an instrument to observe optical intensity distributions which change at high speed.

The streaking tube used in the streak camera is an electron tube wherein a pair of deflection electrodes are arranged in a space between a photocathode and a phosphor layer.

When a light pulse is incident on the photocathode of the streaking tube, the photocathode emits photoelectrons.

If an electric field is applied across the pair of deflection electrodes while the photoelectrons move toward the phosphor layer, the incident light intensity change can be detected on the phosphor layer as a linear light intensity distribution along the sweeping trace on the phosphor screen (in the direction of scanning with time).

This light intensity distribution is called a streaking image.

The streak camera consists of a streaking tube, an optical system to project the light pulse to be measured onto the photocathode of the streaking tube, and a power supply to feed the necessary voltages to the streaking tube.

High-repetition-rate pulse light measuring instruments (see Japanese Patent Application LaidOpen Nos. 104519/1984 and 135330/1984) and electron tube devices for high-repetition-rate pulse light measuring instruments (see Japanese Patent Application Laid-Open No. 134538/1984) have already been proposed by the inventor of the present invention and two other persons, and these instruments and devices operate in response to light emitted at high repetition rate in accordance with the principle of operation of the streaking tube aforementioned.

These are suitable for measuring extremely low intensity light changing with time at a high repetition rate. Data arranged on the streaking tube in a direction perpendicular to the sweeping of the streaking tube cannot be output at the same time for more than a single incident image. That is, data on a plurality of input channels cannot be output concurrently.

The streaking image on the phosphor layer of the streaking tube can be analyzed by picking it up on a television camera. Highly repetitive streak images can be superimposed during one frame time (1/30 to 1/60 sec.) and this is picked up by a television camera. In this way an image signal with greater level can be obtained. The dynamic range, however, is limited to a value much lower than 104 to 106 because of the following reasons: First, the image is stored as charges in the target capacitance of the pick up tube and the dynamic range of an image for one field period is of the order of 100 to 1.

Second, charges caused by the dark current of the pick up tube are stored during the above one field and the measurement accuracy of faint streaking images is low.

Third, the dynamic range of the streaking image for a plurality of fields is also limited to 1000:1 by both the target capacitance and dark current.

The dynamic range of the streaking image can be improved by feeding the streaking tube output to a photomultiplier tube when an electron tube device for measuring the high-speed light pulses such as is disclosed in Japanese Patent Application Laid-Open No.

134538/1984 is used. The streaking images on a plurality of channels, however, cannot concurrently be measured.

The streaking images on a plurality of channels can only be measured in sequence, and this elongatges the measuring time. It is thus an objective of the present invention to provide an instrument capable of measuring streaking images on a plurality of channels concurrently.

According to the present invention there is provided a light signal measuring instrument for concurrently measuring a plurality of light signals, said instrument comprising: a streaking tube having, in order, a photocathode, deflection electrodes, a slit plate and a phosphor layer, said photocathode being arranged to emit an electron image on the incidence thereon of a light image, said deflection electrodes being arranged on the application thereto of a sweeping voltage to deflect electrons of a said electron image, said slit plate being disposed between said deflection electrodes and said phosphor layer and having therein a slit arranged perpendicular to the deflecting direction of said deflection electrodes and through which electrons may pass;; optical means arranged to receive repetitive light signals and to transmit said signals to a section of said photocathode, said section lying in a direction perpendicular to said deflecting direction; deflection voltage generating means arranged to generate said sweeping deflection voltage synchronized with said repetitive light signals, and with the phase of the sweeping deflection voltage successively incremented for successive incident light signals; a photodiode array consisting of a plurality of photodiodes arranged facing the phosphor layer of said streaking tube in a line perpendicular to said deflecting direction; and processing means to process the outputs of said photodiode array.

The instrument of the invention may be used to measure concurrently ultra-high-speed optical phenomena on a plurality of channels.

The deflection voltage generating means in the instrument of the invention is conveniently operated in synchronization with a light pulse generating or detecting means, e.g. a pulsed laser source used to excite an object under study, and conveniently is provided with a delay circuit whereby the deflection voltage sweep may be delayed by a time incremented for successive light pulses. In this way the portion of the electron image passing through the slit in the streaking tube corresponds to a successively later portion of the repetitive light pulses incident on the streaking tube.

Preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a first embodiment of the instrument according to the present invention; Figure 2 is a cross-sectional view of the streaking tube used in the embodiment of Figure 1 and includes the circuit diagram of the power supply for the streaking tube; Figure 3 is an exploded view of the electrodes arranged in the streaking tube and the photodiode array of Figure 1; Figure 4 shows waveforms illustrating the change of the intensity of a single light pulse picked up from the repetitive light emission consisting of a plurality of wavelength components to be measured by the embodiment of Figure 1;; Figure 5 shows the waveforms of light pulses incident on the instrument of Figure 1 illustrating repetitive light emission and sampling; Figure 6 shows a cross-sectional diagram qf another streaking tube used in the instrument of the invention and includes the circuit diagram of the power supply for the streaking tube; Figure 7 shows a perspective view of a further streaking tube used in the instrument of the invention and shows optical means to project the streaking image on the streaking tube in a direction perpendicular to the deflecting direction; Figure 8 shows a partial cross-sectional view of optical means for the streaking tube shown in Figure 7, and shows particularly a cross-sectional view of the junction between the optical means and the photocathode; Figure 9 shows the arrangement of optical means in a second embodiment of the instrument of the invention;; Figure 10 shows a sketch of interference fringes which are produced using the instrument of Figure 9.

Referring to Figure 1, a sample 3 is set on the instrument and is periodically stimulated by laser 1 to cause fluorescent light emission in the sample. The light emitted is spectroscopically measured.

Dye laser source 1 generates a repetitive pulse train. The repetitive pulse train branches into two paths at half mirror 2 and the light beam passing through half mirror 2 irradiates object 3. A light pulse train is generated from object 3 in response to the incident light pulse train. The light beam obtained by periodical light emission is projected onto the photocathode of the streaking tube in a direction perpendicular to the deflecting direction of the deflection electrodes.

The optical means in this embodiment consists of a spectroscope 4, a slit 5 to pass the light beam components analyzed (e.g. wavelength separated) by spectroscope 4, and a lens 6.

If some means to accomplish the same function are built into spectroscope 4, no independent slit plate is required. Thus if the light beam obtained by periodical light emission is fed to the spectroscope via a pin-holelike structure, and if the incident light beam is then projected onto the photocathode of the streaking tube as a linear image extending in a direction perpendicular to the deflecting direction, the slit 5 and lens 6 become unnecessary.

The linear images in Figure 1, formed in the plane of the drawing, contain wavelength components A1, A2, ... An from bottom to top.

Figure 2 shows a cross-sectional view of the streaking tube used in the instrument to concurrently measure the ultra-high-speed streaking images on a plurality of channels, and the circuit diagram of the power supply for the streaking tube.

The cross-section of streaking tube 7 in Figure 2 is taken in a direction perpendicular to the plane of the drawing of the streaking tube in Figure 1.

Figure 3 shows an exploded view of the electrodes arranged in the streaking tube and the photodiode array.

Photocathode 71 is formed on the inner surface of the faceplate of streaking tube 7.

Mesh electrodes 72 are arranged against photocathode 71. Electrons emitted by photocathode 71 on the incidence of light thereon are accelerated by mesh electrodes 72, are focussed by focussing electrode 73 and, after passing through aperture plate 74, pass into the electric field formed hy deflection electrode 75.

A sweep voltage synchronizing with the light pulse caused by fluorescence light emission is applied to deflection electrode 75.

Synchronization will be described hereafter.

Those electrons deflected by deflection electrode 75 within a given deflection range pass through the slit of slit plate 77 and are incident on phosphor layer 78 formed on the inner surface of fiberplate 79.

The slit of slit plate 77 in Figure 3 is perpendicular to the direction of the deflection in streaking tube 7. The operating voltages are fed from power supply 20 and sweep voltage generator 15 to the respective electrodes of streaking tube 7. The sweep voltage will be described later.

Power supply 20 consists of power regulators 21, 26 and 27, and voltage dividers 22, 23 and 24. An acceleration voltage of 3 kV to 5 kV from power supply 27 is applied between slit plate 77 and phosphor layer 78.

Part of the light pulse train from dye laser pulse source 1, reflected from half mirror 2, is detected by PIN photodiode 9, amplified with amplifier 10, and then fed to tuned amplifier 14 through delay circuit 12.

The delay time in delay circuit 12 is controlled by using delay control circuit 16.

Tuned amplifier 14 is used to generate a sine-wave signal voltage tuned to the tuned pulse signal and the sine-wave signal voltage is amplified with sweep voltage generator 15.

The amplified signal is fed to a pair of deflection electrodes 75.

The linear range of a sine-wave signal voltage can be used to generate the sweep signal.

The deflection voltages and electrons arriving at the phosphor layer will be described referring to Figures 4 and 5.

Figure 4 shows the waveform illustrating the change of the intensity of a single light pulse picked up from the repetitive light emission and consisting of a plurality of wavelength components to be measured by this embodiment of the measuring instrument.

In Figure 4, the time axis is t, the wavelength axis is A, and the intensity axis is I; thus the graph is represented in a three-dimensional space.

Thus, A4 and A6 wavelength components, which are bound by the relation in Figure 4, are incident on the photocathode of the streaking tube at time t1; A3 through A6 wavelength components, which are bound by the relation in Figure 4, are incident on the photocathode of the streaking tube at time t2; etc.

Figure 5 shows waveforms illustrating repetitive light emission and sampling.

(1) through (m) in Figure 5 show the waveforms of the Aj and Ak wavelength components of the fluorescent emission light pulses which are incident on photocathode 71 of streaking tube 7 in the first through m-th time slots.

The light pulses due to light emission by fluorescence are analyzed by spectroscope 4, and the location on photocathode 71 where light is incident depends on the wavelength of that light.

Aj and Ak represent the light beam intensities on locations j and k of photocathode 71.

The vertical lines, the "black stripes", in Figure 5 represent the portion of the electron pulses corresponding to incident light pulses Aj and Ak deflected by deflection electrode 75 through slit plate 77 and ultimately detected by photodiodes 8-j and 8-k. It will be seen that for the first through m-th time slots the image portion selected to pass through slit plate 77 is a progressively later portion of the incident pulses.

Photodiode array 8 consists of a number of photodiodes arranged to form a linear array in a direction perpendicular to the deflecting direction of streaking tube 7.

A single photodiode, having an appropriate size which can be specified in accordance with the objectives is conveniently 100,um X 5 mm in dimension.

Each photodiode is arranged to receive light emitted from phosphor layer 78 corresponding to a specific wavelength of the incident light.

Each photodiode output is amplified with each amplifier in amplifier group 17.

The output of each amplifier in amplifier group 17 is converted into the corresponding digital signal by using each A/D converter in A/D converter group 18.

The output of each A/D converter in A/D converter group 18 is stored into memory 30.

Information indicated by the black stripe of each incident wavelength component of light emission in the first time slot, information indicated by the black stripe of each incident wavelength component of light emission in the second time slot, and finally information indicated by the black stripe of each incident wavelength component of light emission in the m-th time slot can be stored in memory 30.

The outputs of A/D converter group 18, picked up in the first through m-th time slots, are brought together to construct a waveform due to fluorescence light emission in the respective wavelength components.

The contents of memory 30 are read out to convert them into the corresponding time-serial analog signal by using D/A converter 32, and then the obtained analog signal is output from output device 34, e.g. a VDT, synchronizing with the output of delay controller 16.

Then, the output signal can be observed in real time.

Figure 6 shows a cross-sectional diagram of another streaking tube which may be used in the instrument of the invention and includes the circuit diagram of the power supply for the streaking tube.

The streaking tube in Figure 6 differs from that of Figures 2 and 3 in providing a microchannel plate 76 between deflection electrodes 75 and slit plate 77 so that the deflected photoelectrons are multiplied before being incident on phosphor layer 78.

Power supply 26 is used to supply the operating voltage to microchannel plate 76, power supply 25 is used to generate an acceleration field between microchannel plate 76 and slit plate 77, and power supply 27 is used to generate an acceleration field between slit plate 77 and phosphor layer 78.

Figure 7 shows a perspective view of a further streaking tube which may be used in the instrument of the invention and shows optical means used to project the streaking image onto the streaking tube in a direction perpendicular to the deflecting direction of the deflection electrodes.

Figure 8 shows a cross-sectional view of the optical means for the streaking tube shown in Figure 7, and in particular shows a cross-sectional view of the junction between the optical means and the photocathode.

Optical fibers 804-1 through 804-x lead into faceplate 702 and are retained along the diameter thereof perpendicular to the deflecting direction with their light output ends directed at the photocathode 71 which is formed on the inner surface of the faceplate. The light beams are incident on the outer ends of each fiber and the optical means thus serve to project the linear light beam image onto a section of the photocathode which is perpendicular to the deflecting direction.

A glass plate may be used to construct faceplate 702. Thus, faceplate 702 made of Kovar glass is fastened to main glass cylinder 701 via ring 703, which can also be used as a metal electrode.

The inner ends of optical fibers 804-1 through 804-x are preferably flush with the inner surface of Kovar glass faceplate 702 and photocathode 71 is formed on that inner surface.

Each of optical fibers 804-1 through 804-x consists of a clad and a core formed within the clad, the clad generally being 125 ,um to 200 pm in diameter.

To construct this assembly, the Kovar glass faceplate 702 is first cut into two segments along its diameter then grooves are formed at the linear edges of the two segments and optical fibers 804-1 to 804-x are set in these grooves. The circumference of each fiber is placed on a groove formed at the linear edge of a segment, and by bringing the segments together each fiber is placed between a pair of grooves in the pair of segments. Glass powder is then laid on the junctions between the fibers and grooves and it is heated with the fibers and faceplate segments so as to join them together.

After the fibers and faceplate segments are joined together, the inner surface whereon the photocathode can be formed is polished.

Photocathode 71 is formed on the inner surface of faceplate 702 and mesh electrode 72 is provided facing photocathode 71.

Aside from projections of the fibers 804-1 through 804-x from faceplate 702, shielding layer 705 of black paint is formed on the outer surface of faceplate 702 so that light entry from any other portions than fibers 8041 through 804-x is blocked.

The outer ends of the optical fibers 804-1 through 804-x are arranged in such locations that the periodical light signals of light emission can be received from the sample, and then the light signals of light emission in a plurality of locations can be measured concurrently.

Diodes in photodiode array 8 are related (in one-to-one correspondence) to optical fibers 804-1 through 804-x, respectively.

As described above, the faceplate of the streaking tube is divided into two sections to form the pairs of grooves for connecting the fibers and faceplate. The faceplate can alternatively be made by other methods; e.g. holes may be bored through the faceplate by using a diamond drill, the fibers may be set into the holes, and the fibers may be fastened there by glass powder. Similarly, rather than a glass plate, a fiber plate can be used as the faceplate.

A shielding slit plate made of aluminum, having a slit with a slit width narrower than the fiber core diameter, may be placed over the inner surface of the faceplate before the photocathode is formed thereon-in this way the width of the section of photocathode used for generating electron images may be made less than the fibre core diameter.

With the optical fibers and faceplate of the streaking tube assembled together in this way, alignment of the photocathode to the fibers is unnecessary. This simplifies operation and facilitates maintenance including replacement of the streaking tube.

Turning now to the embodiment shown in Figures 9 and 10, when a material is stimulted by a laser pulse, the excited material is distorted in accordance with the excitation.

The interference fringe due to this type of distortion can be used to observe the change of the interference fringe with time.

The instrument of this embodiment can be used for measurement of ultra-high-speed displacement and for analysis of vibration.

A light pulse train from laser pulse light source 1 is divided into an excitation light pulse I and another light pulse by half mirror 51.

Excitation light pulse I is reflected from total reflection mirror 52 and the light pulse reflected from the total reflection mirror 52 stimulates object material 58.

The other light pulse from half mirror 51 is divided into exposure light pulse II and reference light pulse Ill by a second half mirror 54.

The light pulse signal reflected from the surface of material 58 when the material 58 is exposed to exposure light pulse II, and the reference light pulse Ill are combined together by using half mirror 56 so as to form an interference image corresponding to the structure of the material surface.

Figure 10 shows an example of the interference image.

Slit plate 5 is arranged in front of streaking tube 7 and part of an interference fringe is projected onto the photocathode of streaking tube 7 so as to form a linear image thereof (perpendicular to the deflecting direction of the deflection electrodes in streaking tube 7) when the light beam passes through the slit plate 5.

Photodiode array 8 and succeeding stages, and the sweep voltage generator are essentially as described in connection with the first embodiment of the present invention.

The material structure is distorted when exposed to the laser pulse beam, and thus the material surface is displaced a little.

The memory stores the change of the structure with time.

The instruments described heretobefore in detail can partly be modified within the scope of the present invention.

For instance, a mesh electrode to accelerate electrons emitted from the photocathode may be a planar electrode with a slit-like aperture.

As described above, the instrument to concurrently measure the ultra-high-speed light signals on a plurality of channels in accordance with the present invention consists of: a streaking tube wherein a slit plate with a slit is arranged succeeding the deflection electrodes in such a manner that the slit is arranged in a direction perpendicular to the deflection carried out by the deflection electrodes; optical means whereby the periodical light pulse signal sent from the sample is developed and caused to impinge on the photocathode of the streaking tube in a direction perpendicular to the deflection; deflection voltage generation means to generate a series of deflection voltages whose phases are shifted a little each time scintillation has occurred while operated synchronizing with the scintillation of the sample; and a photodiode array used to parallelly pick up a series of streaking images obtained by the scintillation. Thus, the instrument built in accordance with the present invention can concurrently measure the ultra-high-speed light signals on a plurality of channels.

The size of each diode in a photodiode array can be set arbitrarily.

No scanning circuit is required to read out the streaking images because the streaking images are parallelly read out, and this leads to high speed processing of data read out of the instrument.

If serial read-out operations are carried out, a shiftregister or a demultiplexer is needed and this makes the circuit configuration complicated.

Such linesensors as Reticon RTM and CCD photosensors, which are available on the market, are not used but a diode array is used in the present invention. This results in the following features: (Spikes, Noises and Dynamic Range) Parallel read-out operations eliminate spikes and reduce noises; this leads to high S/N value and high dynamic range.

Noises are mainly caused by amplifiers and the S/N value for a diode array is n112 times greater than the scanned sensor. ('n' indicates the number of picture elements.) The minimum signal level which can be read out of the sensor is limited by spikes. The diode array completely eliminates spikes, and the minimum signal level of the diode array is extended to a value limited by the amplifier noise or photocurrent shot noise.

(Read-out Time) Assume that n=100. The line sensor requires a read-out time of 't/n' for each element, and the diode array requires 't' because data is parallelly read out of all diode array elements. The frequency bandwidths for the former and latter are 'n/(2t)' and '1/(2t)', respectively. The bandwidth for the diode array is n-times greater than that for the linear sensor.

(Others) The photodiode array used in the present invention permits a readout operation in specific picture elements, and also permits a variable readout operation.

The linear array sensor has an upper bound to the scanning frequency. The scanning frequency is limited by the shiftregister performance. The shiftregister generally limits the readout time to 100 ns. The diode array uses amplifiers arranged in parallel with the respective diode outputs, and this reduces the sampling time to 100 ns/'n'. This reduces the equivalent amplifier bandwidth.

The sampling can be done at a speed ntimes faster than that of the line sensor.

Claims (9)

1. A light signal measuring instrument for concurrently measuring a plurality of light signals, said instrument comprising: a streaking tube having, in order, a photocathode, deflection electrodes, a slit plate and a phosphor layer, said photocathode being arranged to emit an electron image on the incidence thereon of a light image, said deflection electrodes being arranged on the application thereto of a sweeping voltage to deflect electrons of a said electron image, said slit plate being disposed between said deflection electrodes and said phosphor layer and having therein a slit arranged perpendicular to the deflecting direction of said deflection elec trodes and through which electrons may pass;; optical means arranged to receive repetitive light signals and to transmit said signals to a section of said photocathode, said section lying in a direction perpendicular to said-deflecting direction;, deflection voltage generating means arranged to generate said sweeping deflection voltage synchronized with said repetitive light signals, and with the phase of the sweeping deflection voltage successively incremented for successive incident light signals; a photodiode array consisting of a plurality of photodiodes arranged facing the phosphor layer of said streaking tube in a line perpendicular to said deflecting direction; and processing means to process the outputs of said photodiode array.

2. An instrument as claimed in claim 1 further comprising a laser beam source capable of emitting a pulsed laser beam capable of causing a sample to emit said repetitive light signals.

3. An instrument as claimed in claim 2 wherein said laser beam source is a dye laser device operable at high repetition rate.

4. An instrument as claimed in any one of claims 1 to 3 wherein said optical means comprises: a spectroscope arranged to separate by wavelength said repetitive light signals; a second slit plate with a slit arranged in parallel with the direction of scanning in the spectrum analysis performed by said spectroscope; and imaging means for causing an image of the light signal passing through said second slit plate to form on said photocathode.

5. An instrument as claimed in claim 4 wherein said slit plate is provided as part of said spectroscope.

6. An instrument as claimed in any one of claims 1 to 5 wherein said optical means comprises an optical element to feed said repetitive light signals to a spectroscope through a pin hole; and a said spectroscope arranged to separate by wavelength said repetitive light signals received thereby and to project the wavelength separated light signals onto said section of said photocathode.

7. An instrument as claimed in any one of claims 1 to 6 wherein said optical means comprise a plurality of fibers arranged to receive a plurality of light signals and to transmit said light signals to a corresponding plurality of locations arranged along said section of said photocathode.

8. An instrument as claimed in any one of claims 1 to 3 for measuring light signals from a periodically displaced object, wherein said optical means comprises: a laser beam source capable of generating light pulses; light combining means for superposing light pulses from two light paths; means for defining a light path from said source to a said object for excitation of said object with said light pulses; means for defining a light path from said source to a surface of said object wherefrom light pulses may be reflected to said light combining means; means for defining a light path from said source to said light combining means whereby light pulses from said source may be caused to interfere with the light pulses reflected from said object;; means for defining a light path from said combining means to said streaking tube whereby light pulses combined at said combining means may be caused to be incident on said streaking tube; and slit means for restricting the light pulses combined at said combining means to incidence on said streaking tube in the form of an elongate slit image perpendicular to said deflecting direction.

9. An instrument as claimed in any one of claims 1 to 8 wherein said optical means comprises light transmitting means arranged concurrently to transmit a plurality of repetitive light signals to a respective plurality of discrete locations of said photocathode, said locations being disposed in a line along said photocathode in a direction perpendicular to said deflecting direction and wherein said photodiode array comprises a corresponding plurality of photodiodes each arranged to receive an electron image corresponding to a portion of the electron image emitted by one said location of said photocathode.