GB2186075A

GB2186075A - Light pulse measuring instrument

Info

Publication number: GB2186075A
Application number: GB08629985A
Authority: GB
Inventors: Akira Takeshima; Shinichiro Aoshima
Original assignee: Hamamatsu Photonics KK
Current assignee: Hamamatsu Photonics KK
Priority date: 1985-12-16
Filing date: 1986-12-16
Publication date: 1987-08-05
Also published as: JPS62142234A; GB2186075B; GB8629985D0; JPH0466298B2

Abstract

A fast recurrent light pulse measuring instrument comprises a source 12 for generating a light pulse train for exciting a sample 1; an interrupter 13; an electron tube 3 for receiving fluorescence from the sample and including a photocathode, a deflecting electrode and a slit electrode; a lock-in amplifier 4 for amplifying the output of the electron tube 3; and data reforming means 6 for integrating the output of the amplifier 4. A delay sweep means 7 applies pulses to a sweep voltage generator 5 with gradually greater delay so that the output of the tube represents a series of samples of the waveform of the fluorescent pulses taken at gradually later times. The interrupter 13 is switched with a period sufficiently longer than the recurrent period of the light pulse train. The lock-in amplifier 4 is interlocked with the interrupter 13 so that it amplifies only for a period during which light reaches the electron tube 3. This makes the instrument particularly effective for examining weak light outputs from the sample 1. <IMAGE>

Description

SPECIFICATION Light pulse measuring instrument This invention relates to a fast recurrent light pulse measuring instrument suitable for measuring light pulses that are repeated at high speed and have substantially the same waveform and period.

Various instruments have been proposed previously to detect a fast recurrent light pulse. JP-A-134538/84 discloses two kinds of electron tubes for measuring fast recurrent light pulses; with the use of the principle of a streak tube, both of them are intended to reproduce the waveform of the fast recurrent light pulse by cutting out while slightly shifting the short time in the direction of the time axis of the streak tube.

The present inventors attempted to detect weak light emissions using the above fast recurrent light pulse measuring instrument and found limitations in the measurement of weak light emissions because it is difficult to separate the dark current of the tube from the signal current at weak light emissions.

According to this invention a fast recurrent light pulse measuring instrument comprises a light source for generating a train of light pulses for exciting a sample to be examined and for causing the sample to emit light in response to the light pulses; an electron tube including photoelectron generating means responsive to light, deflecting means for deflecting electrons emitted from the photoelectron generating means and slit means perpendicular to the sweeping direction of the deflecting means; optical means for coupling the light emitted from the sample to be examined to the photoelectron generating means; an interrupting means arranged between the light source and the photoelectron generating means and interrupting the light pulse train at regular intervals; a deflection voltage generating means for supplying a deflection voltage to the deflecting means;; a delay sweep means for receiving a signal corresponding to the light pulse train and outputting a delayed signal train to the deflecting voltage generating means which, in response to the delayed signal pulse train, causes the deflection voltage to be applied to the deflecting means; amplifying means for amplifying the output of the electron tube; data reforming means for integrating the output of the amplifying means; and, a power supply for supplying actuating power to the electron tube.

A particular example of a measuring instrument in accordance with this invention will now be described with reference to the accompanying drawings; in which: Figure 1 is a block diagram of the instrument; Figure 2 is a circuit diagram of the electron tube, a power supply and delay sweep means; Figure 3 is a graph of output against time illustrating the operation of the delay sweep means; Figure 4 is a waveform graph illustrating the relationship between the incident light and sweep timing in the electron tube; Figure 5 is a circuit diagram of a lock-in amplifier; and, Figure 6 is a waveform chart illustrating the operation of the instrument as a whole.

Fig. 1 is a block diagram showing a fast recurrent light pulse measuring instrument embodying the present invention. An exciting light source 12 for generating a pulse train for exciting a sample cell is generating a group of pulses at fixed intervals. The pulse train is incident on a chopper 10 through an optical means 9. As the chopper 10, a mechanical chopper 10 capable of operating at 1Hz-lO KHz is used. The chopper 10 is operated by a driving signal supplied by a chopper driver 13.

Part of the beam for forming a clock signal is separated by a half mirror 91 of the optical means 9. Total reflection mirrors 92, 93, 94, 95 are used to adjust the light path length, the total reflection mirrors 94, 95 as a pair being vertically moved to adjust the light path length. The chopper 10 is operated with a period sufficiently longer than that of the laser pulse produced at fixed intervals by the exciting light source 12. A plurality of laser pulses are caused to be incident on the sample cell 1 and induce fluorescence from the sample for the period during which the chopper 10 is opened, the fluorescence lasting for a period longer than the pulse width but shorter than the pulse period. The fluorescence is also projected on the photocathode of an electron tube 3 through a lens system 2. Fig. 2 shows the electron tube 3 for use in detecting the fast recurrent light pulse.A photocathode 31 is formed on the inner surface of an incident side of the vacuum tube of the electron tube 3 for measuring the fast recurrent light pulse.

In the vacuum tube 30 are provided a mesh electrode 32 installed close to the photocathode 31, subsequently, a focusing electrode 33, a deflecting electrode 34, a slit electrode 35, a group of dynodes 36 for multiplying the electrons passed through the slit of the slit electrode 35, and an anode 37 for taking out the electrons thus multiplied. The slit of the slit electrode 35 is provided perpendicular to the deflecting direciton of the deflection electrode 34. The sweep voltage from a sweep voltage generator 5 is supplied to the deflecting electrode 34 of the electron tube 3 for detecting the fast recurrent light pulse, whereas the actuating voltage from the power supply 11 is supplied to the other electrodes.

The power supply 11 consists of electric cells E1, E2 and voltage dividing resistors R,-R,.

Voltage higher than what is supplied to the photocathode 31 is supplied to the mesh electrode to accelerate the photoelectrons emitted therefrom. The photoelectrons are focused by the focusing electrode 33 to which further higher voltage has been applied and caused to be incident to the deflecting electrode 34. Of the electrons forming the flow deflected by the deflecting electrode 34, as described hereinafter, the electrons allowed to pass through the slit of the slit electrode 35 are multiplied by the dynodes 36 and taken out through the anode 37 to which the highest voltage has been applied. The current thus taken out of the anode 37 is amplified by a lock-in amplifier 4 to be actuated interlockingly with the chopper 10.

The laser pulse train thus separated by a half mirror 91 of the optical means 9 shown in Fig. 1 is detected by a pin photodiode 8 and a current pulse equivalent to the laser pulse train is formed. The current pulse is connected to a delay sweep means 7. Fig. 2 shows the detailed construction of the delay sweep means 7.

The delay sweep means 7 produces a delay pulse for determining the sweep voltage generation timing and a signal setting up the time axis of an X-Y recorder 6 as described hereinafter with the above pulse as a reference.

An inductance L, variable capacitive diodes D1-D4, resistors R7 ,-Re 2 in the delay sweep means 7 generate the delay pulse.

The signal setting up the time axis of the X-Y recorder 6 is produced by an X-Y recorder sweep voltage generator 71 and supplied to the X axis of the X-Y recorder 6, the output of a lock-in amplifier 4 being connected to the Y axis of the X-Y recorder 6.

As shown in Fig. 3, the delay pulse is a pulse train (Fig. 3(B)) obtained by delaying the pulse train (Fig. 3(A)) detected by the pin photodiode 8 by AT second. The AT second is variable and, in this embodiment, the pulse train (10 ns) detected by the pin photodiode 8 is so arranged that it is time-delay swept continuously at a rate of 0.1 ns/sec. By changing the voltage (A) applied to the variable capacitive diode, the capacitance is made variable, whereas the delay quantity AT is also made variable by changing the phase characteristics of the filter constituted by L1, D1-D4.

In the above embodiment, a starting timepoint of the sweeping operation is determined by the optical means 9, and the delay quantity AT is changed by the delay sweep means with the light path length adjusted by the optical means 9 constant. However, the delay quantity AT may be changed by changing the light path length.

However, the delay time is not changed every pulse but, assuming the frequency of the chopper 10 to be 1KHz, the delay quantity is increased at the rate of 0.1 ns every 1,000 periods of the chopper 10 because of the rate of 0.1 ns/sec.

Fig. 4 is a waveform chart mainly illustrating the relation between the incident light and sweep timing in the electron tube. When a sample cell 1 is excited by the laser pulse (A), it produces fluorescence, which is subjected by the photocathode 31 of the electron tube 3 to photoelectric conversion, and an electron flow shown in Fig. 3(B) is generated.

On the other hand, the delay sweep means 7 produces a pulse (D) whose input (C) has been delayed by AT. The sweep voltage gen erator 5 applies the sweep voltage to the deflecting electrode 34 of the electron tube.

Only the electron flow thus deflected by the voltage and allowed to pass through the slit of the slit electrode 35 is sampled and multi plied by the dynodes 36 before being sent out (Fig. 4(F)).

Since the point of time the sweep voltage is generated is determined by the delay quantity AT in the delay sweep means 7, the sampling portion of the fluorescent emission may be changed sequentially by gradually changing AT. As the incident laser pulse to the sample cell is delayed by the optical means 9, the time difference occurs between the laser pulse and the fluorescent emission generated thereby.

Fig. 5 shows the construction of the lock-in amplifier for amplifying the output of the elec tron tube 3 interlockingly with the operation of the chopper 10. The output of the anode 37 of the electron tube 3 is amplified by an A.C.

amplifier of the lock-in amplifier 4. Conse quently, the D.C. component as a noise com ponent is not amplified. A signal correspond ing to a period suring which the chopper 10 is opened is formed by a reference signal gen erator 42 and applied to a multiplier circuit 43, whereby the portion equivalent to the signal is extracted from the output of the A.C.

amplifier 41.

The output of the multiplier circuit 43 is applied to a D.C. amplifier 45 through a low pass filter 44 and D.C.-amplified. The output of the D.C. amplifier 45 corresponds to the intensity of the extracted portion of fluores cence.

Referring to numerical values by way of example, the whole operation of the above measuring instrument according to the present invention will subsequently be described. The period of the pulse train of the light source 1 2 is assumed 10 ns. As shown in Fig. 6(A), the chopper 10 is operated with an opening time of 500 xs and a period of 1 ms. Fig. 6(B) is a current waveform corresponding to the fluo rescent emission of the sample cell. The re current period of the fluorescent emission is the same as the period 10 ns of the pulse train of the light source 12.

Fig. 6(C) shows the output of the electron tube 3. A sampling outputs amounting to 500 Cls/lOns=Sx 104 are available every period of the chopper 10.

Fig. 6(D), (E) shows the expanded time axis and enlarged amplitude of those shown in Fig.

6(B), (C). As shown in Fig. 6(E), signal components 1a, 12 are superposed on the direct component Id (dark current ) of the electron tube.

Fig. 6(F) is an extremely compressed version of the time axis of Fig. 6(C). In order to facilitate the understanding of the above drawing, the number of sampled outputs has been made smaller than what is actually is.

The output shown in Fig. 6(F) is, when amplified by the A.C. amplifier 41 of the lock-in amplifier 4, caused to bear a waveform shown in Fig. 6(G). The output of the lock-in, amplifier 4 becomes that is shown in Fig. 6(H). The amplitude shown in Fig. 6(H) corresponds to what has been sampled with the period of the chopper. When the output of the lock-in amplifier 4 is connected to the Y input terminal of the X-Y recorder 6 shown in Fig. 1, a waveform reproduced in Fig. 6(1) is obtained since the output voltage of the X-Y recorder sweep voltage generator 71 of the delay sweep 7 has been applied to X input terminal of the X-Y recorder 6.

Fig. 6(1) shows a further compressed version of the time axis of Fig. 6(H).

The above embodiment thus detailed can be modified in various ways within the scope of the present invention.

Although a mechanical chopper has been described by way of example, an Acoustic Optical Modulator (AOM) may be used. The AOM is capable of chopping quicker than the above mechanical one with a wider period (1 MHz from direct current), so that data can be processed faster.

Although the electron tube incorporating the group of dynodes has been used in the above embodiment, it is also possible to employ a combination of a streak tube incorporating the slit and a photomultiplier.

In addition, a personal computer as a datareforming means may be used.

As described above in detail, the fast recurrent light pulse measuring instrument according to the present invention employs the electron tube for detecting a fast recurrent light pulse, the exciting light source for generating a pulse train for exciting the sample cell and the optical means for connecting the light derived from the sample cell to the photocathode of the electron tube for measuring the fast recurrent light pulse.

In the output from the electron tube composed of the signal component (I) synchronous with the ON/OFF modulation of the incident light and the dark current component, only the signal component is amplified, whereas the normal dark signal component from the electron tube, free from modulation, is not amplified.

The signal thus amplified is detected and converted into d.c. voltage before being outputted.

Accordingly, weak light emission can be detected with excellent S/N and the expansion of the dynamic range during the measurement of fluorescent light is performed by lessening the marginal quantity of light for measurement, assuming lights can be obtained.

About 100 times improvement in total can be achieved, provided that the limitation of detection is assumed 20 times improved because of the improvement resulting from the separation of dark currrent and five times improved because of the improvement in the narrow-band amplification characteristics of the lock-in amplifier.

In consequence, a weak light buried in the dark current and hardly detectable by the conventional instrument can be detected with accuracy.

Consideration may also be given to a method deemed effective likewise in that the photocathode in the instrument is cooled or otherwise the whole instrument is cooled. The dark current may be reduced effectively by cooling the photocathode in the conventional instrument; dark current may be reduced to 1/10 at the room temperature (about 25"C) if it is cooled at about 0 C.

Notwithstanding, greater effect can be achieved less costly according to the present invention.

Claims

1. A fast recurrent light pulse measuring instrument comprising: a light source for generating a train of light pulses for exciting a sample to be examined and for causing the sample to emit light in response to the light pulses; an electron tube including photoelectron generating means responsive to light, deflecting means for deflecting electrons emitted from the photoelectron generating means and slit means perpendicular to the sweeping direction of the deflecting means; optical means for coupling the light emitted from the sample to be examined to the photoelectron generating means; an interrupting means arranged between the light source and the photoelectron generating means and interrupting the light pulse train at regular intervals; a deflection voltage generating means for supplying a deflection voltage to the deflecting means;; a delay sweep means for receiving a signal corresponding to the light pulse train and outputting a delayed signal train to the deflecting voltage generating means which, in response to the delayed signal pulse train, causes the deflection voltage to be applied to the deflecting means; amplifying means for amplifying the output of the electron tube; data reforming means for integrating the output of the amplifying means; and, a power supply for supplying actuating power to the electron tube.

2. A fast recurrent light pulse measuring instrument according to claim 1, wherein the interrupting means has an ON-OFF period longer than the recurrent period of the light pulse train.

3. A fast recurrent light pulse measuring instrument according to claim 1 or 2, wherein the delayed signal pulse train is obtained by delaying the signal pulse train for a time interval.

4. A fast recurrent light pulse measuring instrument according to any one of the preceding claims, wherein the deflecting voltage phase changes sequentially.

5. A fast recurrent light pulse measuring instrument according to any one of the preceding claims, wherein the amplifying means is interlocked with the interrupting means.

6. A fast recurrent light pulse measuring instrument according to any one of the preceding claims, wherein the amplifying means comprises an A.C. amplifier for amplifying the output of the electron tube, a reference signal generator for forming a signal representing a ON-state of the interrupting means, a multiplier circuit for receiving the signal from the reference signal generator and the output of the A.C. amplifier and for extrating from the output of the A.C. amplifier signals corresponding to the period during which the interrupting means is in ON-state, a low-pass filter, and a D.C. amplifier for D.C. amplifying the signals of the multiplier circuit.

7. A fast recurrent light pulse measuring instrument according to any one of the preceding claims, wherein the photoelectron generator means, the deflecting means, the interrupting means and the slit means are formed by a photocathode, a deflecting electrode, a chopper and a slit electrode, respectively.

8. A fast recurrent light pulse measuring instrument according to claim 7, wherein the electron tube further comprises a mesh electrode, a focusing electrode, a group of dynodes for multiplying the electrons passed by the slit electrode, and an anode for collecting the electrons thus multiplied by the dynodes.

9. A fast recurrent light pulse measuring instrument according to any one of the preceding claims, wherein the interrupting means is a mechanical chopper.

10. A fast recurrent light pulse measuring instrument according to any one of claims 1 to 6, wherein the interrupting means includes an accousto-optic modulator.

11. A fast recurrent light pulse measuring instrument according to any one of the preceding claims, wherein the data reforming means is an X-Y recorder.

12. A fast recurrent light pulse measuring instrument according to any one of claims 1 to 10, wherein the data reforming means is a personal computer.

13. A fast recurrent light pulse measuring instrument substantially as described with reference to the accompanying drawings.