Ultrafast gated light detector

A light detector which can be gated on and off over an ultrashort time window, such as in picoseconds or femtoseconds, is disclosed. The light detector includes, in one embodiment, an input slit for receiving a light signal, relay optics, a sweep generator and a tubular housing, the tubular housing having therein a photocathode, an accelerating mesh, a pair of sweeping electrodes, a microchannel plate, a variable aperture and a dynode chain. Light received at the input slit is imaged by the relay optics onto the photocathode. Electrons emitted by the photocathode are conducted by the accelerating mesh to the sweeping electrodes where they are swept transversely across the tubular housing at a rate defined by the sweep generator over an angular distance defined by the sweeping electrodes, in a similar manner as in a streak camera. Swept electrons strike the microchannel plate where electron multiplication is accomplished. Exiting electrons which pass through the variable aperture and which strike the first dynode (cathode) in the dynode chain are further multiplied and outputted from the last dynode anode in the dynode chain as an analog electrical signal, the analog electrical signal corresponding to the intensity of the light signal during the time window over which swept electrons are picked up by the first dynode. In another embodiment of the invention all of the dynodes in the chain except for the last dynode are replaced by a second microchannel plate.

BACKGROUND OF THE INVENTION 
The present invention relates to light detectors and more particularly to a 
light detector having an ultrafast gated input. 
There is a need for a light detector having an input that can be gated on 
and off over an ultrashort time window, such as in picoseconds. For 
example, it is well known that Raman scattering signals produced when a 
sample is excited by a light source respond instantaneously following the 
shape of the impinging light signal, which may be a pulse on the order of 
picoseconds, while the fluorescence emission times are generally greater 
than a few nanoseconds. In order to measure the intensity of the Raman 
signals it would therefore be desirable to provide a light detector that 
can be gated on and off for a time period that is not longer than the 
excitation pulse. 
Photomultiplier tubes are well known in the art and commonly used as light 
detectors to measure the intensity of light inpinging thereon. These tubes 
generally include a photocathode which receives a light signal and 
produces emission of electrons in proportion to the intensity of the 
impinging light and some form of electron multiplication means, such as a 
dynode chain for amplifying the emitted electrons. 
Streak cameras are short about ten years old in the art and have been used, 
hitherto, to directly measure the time dynamics of luminous events, that 
is to time resolve a light signal. A typical streak camera includes an 
entrance slit which is usually rectangular, a streak camera tube, input 
relay optics for imaging the entrance slit onto the streak camera tube, 
appropriate sweep generating electronics and output-relay optics for 
imaging the streak image formed at the output end of the streak camera 
tube onto an external focal plane. The image at the external focal plane 
is then either photographed by a conventional still camera or a television 
camera. The streak camera tube generally includes a photocathode screen, 
an accelerating mesh, sweeping electrodes and an phosphor screen. The 
streak camera tube may also include a microchannel plate. Light incident 
on the entrance of the streak camera is converted into a streak image 
which is formed on the phosphor screen with the intensity of the streak 
image from the start of the streak to the end of the streak corresponding 
to the intensity of the light incident thereon during the time window of 
the streak. The time during which the electrons are swept to form the 
streak image is controlled by a sweep generator which supplies a very fast 
sweep signal to the sweeping electrodes. The input optics of the streak 
camera, in the past, has been a single lens. 
In an article entitled "An Ultrafast Streak Camera System" by N. H. 
Schiller, Y. Tsuchiya, E. Inuzuka, Y. Suzuki, K. Kinoshita, K. Kamiya, H. 
Iida and R. Alfano appearing in the June, 1980, Edition of Optical 
Sprectra, various known streak camera systems are discussed. The article 
is incorporated herein by reference. 
In U.S. Pat. No. 4,232,333 to T. Hiruma et al there is disclosed a streak 
image analyzing device in which the output streak image of a streak camera 
is fed into a television camera. The output of the television camera is 
fed through a videomixing circuit to a monitor. The output of the 
television camera is also fed to an integrating circuit through a gate 
circuit. The output of the integrating circuit is fed to a memory through 
an analog to digital converter. The output of the memory is displayed by a 
display unit and/or fed back into the videomixing circuit. 
In an article entitled Picosecond Characteristics Of A Spectrograph 
Measured By A Streak Camera/Video Readout System by N. H. Schiller and R. 
R. Alfano appearing in the December 1980, issue of Optical Communications, 
Volume 35, No. 3. pp. 451-454, a streak camera/video readout system is 
disclosed. 
Another article pertaining to streak cameras and spectrographs is Coupling 
An Ultraviolet Spectrograph To A Scloma For three Dimensional Picosecond 
Flurorescent Measurements by C. W. Robinson et al in Multichannel Image 
Detectors pp. 199-213 ACS Symposium Series 102, Amercian Chemical Society. 
Known patents of interest include U.S. Pat. No. 2,823,577 to R. C. Machler; 
U.S. Pat. No. 3,385,160 to J. B. Dawson et al; U.S. Pat. No. 2,436,104 to 
A. W. Fisher et al; U. S. Pat. No. 3,765,769 to E. B. Treacy; U. S. Pat. 
No. 4,060,327 to Jacobowitz et al; U. S. Pat. No. 4,162,851 to A. Wade; U. 
S. Pat. No. 4,299,488 to W. J. Tomlinson and U.S. Pat. No. 4,320,971 to N. 
Hashimato et al. 
It is the general purpose of this invention to provide a light detector 
having an ultrafast input gate. 
Accordingly, it is an object of this invention to provide a new and 
improved light detector. 
It is another object of this invention to provide a light detector having 
an input that can be gated on and off. 
It is still another object of this invention to provide a light detector 
having an input that can be gated on and off in picoseconds. 
It is yet still another object of this invention to provide a light 
detector which utilizes sweeping electronic such as found in a streak 
camera as a mechanism for gating on and off a light detector. 
The foregoing and other objects and advantages will appear from the 
description to follow. In the description, reference is made to the 
accompanying drawing which forms a part thereof, and in which is shown by 
way of illustration, a specific embodiment for practicing the invention. 
This embodiment will be described in sufficient detail to enable those 
skilled in the art to practice the invention, and it is to be understood 
that other embodiments may be utilized and that structural changes may be 
made without departing from the scope of the invention. The following 
detailed description is, therefore, not to be taken in a limiting sense, 
and the scope of the present invention is best defined by the appended 
claims. 
SUMMARY OF THE INVENTION 
A light detector constructed according to the teachings of the present 
ivnention comprises a photocathode for receiving a light signal and 
producing emission of electrons in proportion to the intensity of the 
light signal, an accelerating mesh for conducting the electrons emitted by 
the photocathode into a deflection field, sweeping electronic means for 
sweeping the electrons in the deflection field over a defined angular 
distance at a defined rate, electron multiplication means for receiving 
the electrons swept over at least a portion of the defined angular 
distance, performing electron multiplication thereon and producing an 
analog electric signal output, whereby the analog electrical signal output 
of the electron multiplication means will correspond to the intensity of 
the light incident on the photocathode over a time window corresponding to 
the time during which the electrons are received by the electron 
multiplication means. 
In order that the invention may be more fully understood, it will now be 
described, by way of example, with reference to the accompanying drawing 
in which like reference numerals or characters represent like parts:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to a light detector which can be gated on 
and off over an ultrashort time window, such as in picoseconds or 
femtoseconds. 
The present invention accomplishes this by providing a light detector 
which, in essence, combines into a single device parts of a streak camera 
and parts of a photomultiplier tube, with the streak portion of the device 
being used not to time resolve a light signal spatially as in the past but 
rather to gate the light signal over a very short time window. 
Referring now to the drawings, there is illustrated in FIG. 1 a diagram of 
a light detector constructed according to the teachings of the present 
invention and identified generally by reference numeral 11. 
Light detector 11 includes an input section 13 and a tube 15, tube 15, 
which will hereinafter be described, being basically a modified streak 
camera tube. Input section 13 images light incident thereon into the input 
end of tube 15. Tube 15 receives the light incident thereon and produces 
an analog electrical signal whose intensity is proportional to the 
intensity of the incident light over an ultrashort time window as will 
hereinafter be explained. 
Input section 13 includes an input slit 17 and a relay lens system 19, the 
lens sytem 19 being made up of a first lens 19-1 at the focal distance 
from input slit 17, and a second lens 19-2 at the focal distance to a 
photocathode in tube 15, hereinafter is described. Input slit 17 is 
preferably rectangular in cross section but may be a pinhole or any other 
shape which produces the equivalent of a point source. 
Tube 15 comprises a tubular housing 21 having an input end 23 and an output 
end 25. Disposed in housing 21 going from input end 23 to output end 25 
are a photocathode 27, and accelerating mesh 29, a pair of sweeping 
electrodes 31, a microchannel plate 33, a variable aperture 35 and a 
dynode chain 37, the dynode chain 37 comprising a first dynode or cathode 
37-1 a plurality of intermediate dynodes 37-2 through 37-10 and a last 
dynode or anode 37-11. Light detector 11 also includes a power supply 38 
for providing operating power to the appropriate elements. Input section 
13 may be housed in a separate tubular housing 13-1, as shown, or may be 
housed in the front of housing 21. Power supply 38 is connected to each 
dynode 37-1 thorugh 37-10 through a conventional resistive and capacitive 
circuit (not shown) used for the dynode chain in a photomultiplier tube so 
that each dynode will be negatively biased. 
As can be appreciated, tube 15 is basically the equivalent of streak camera 
tube of the type having a microchannel plate in which the phosphor screen 
at the output end of the tube is replaced by a dynode chain and a variable 
aperture is provided between the micro channel plate and the dynode chain. 
Microchannel plate 33 may be of the type used in Hamamatsu TV Co Ltd, 
Hamamatsu, Japan, streak camera tube model No. N895. The voltage applied 
to microchannel plate is controlled by a switch 33-1. 
In the operation of light detector 11, light received at input slit 17 is 
collected by relay lens system 19 and brought to focus on photocathode 27. 
First lens 19-1 is at a distance from input slit 17 equal to its local 
length and second lens 19-2 is at a distance from photocathode 27 equal to 
its focal length. 
Light striking photocathode 27 produces emission of electrons in proportion 
to the intensity of light incident thereon. The electrons so emitted are 
accellerated into tube 15 by accelerating mesh 29 and electrostatically 
swept transversely across tube 15 over a predetermined angular distance at 
a predetermined velocity (rate) by sweeping electrodes 31. The sweep 
generator 39 which is connected to sweeping electrodes 31. The particular 
signal supplied by sweep generator 39 (i.e. the particular speed of one 
complete sweep) is controlled by a switch 39-1. Variable sweep generator 
39 receives a trigger signal from a trigger 41, such as a pin diode, which 
is coupled to sweep generator 39 through a variable delay unit 43. Delay 
unit 43 delays the time of arrival the trigger signal to sweep generator 
39. The swept electrons strike microchannel plate 33 producing electron 
multiplication through secondary emission. At least some of the electrons 
emitted by microchannel plate 33 and passing through vairable aperture 35 
strike first dynode 37-1 in dynode chain 37. The electrons which actually 
pass through variable aperture 35 depend on the size of the opening in 
variable aperture and the electrons passed through which actually strike 
first dynode 37-1 depend on the size and position of first dynode 37-1. 
Electrons striking first dynode (i.e. cathode) 37-1 are deflected 
successively through intermediate dynode 37-2 through 37-10 to last dynode 
(i.e. anode) 37-11 during which additional electron multiplication is 
produced and are outputted as an analog electrical signal from last dynode 
37-11. 
As can be appreciated, the streak portion of tube 15, namely, accellerating 
mesh 29 and sweeping electrodes 31, causes electrons emitted from 
photocathode 27 to be swept over a predetermined angular distance is 
dependent mainly on the space between the two seeping electrodes 31 and 
the distance from sweeping electrodes to microchannel plate 33 and the 
sweep generator 39. The sweep rate and angular distance, together, define 
a time window produced by the streak portion of tube 15 over which the 
incident light signal is gated, the time window being equal to the time of 
one complete sweep. For example, if sweep generator 39 causes electrons to 
be swept at a rate of 25 picoseconds per millimeter and the defined 
angular distance of a complete sweep is 15 millimeters then the time 
window or gate produced by the streak portion of tube 15 for one complete 
sweep is 375 picoseconds. 
The ultimate time window or gate of detector 11 is also dependent on and 
may be further limited by the size of dynode 37-1 and the openign of 
variable aperture 35. 
If dynode 37-1 is sized to intercept all of the electrons emitted from 
microchannel plate 33, then all electrons emitted therefrom will impinge 
thereon. However, if dynode 37-1 is sized to intercept only a portion of 
the electrons emitted from microchannel plate 33, then the resulting time 
gate for detector 11 will be proportionally reduced. 
Variable aperture 35 is made of electrical shielding material and controls 
the portion of the electrons emitted from microchannel plate 33 that 
strike first dynode 37-1. The size of the opening of variable aperture 35 
is controlled by a knob 35-1. For example, if dynode 37-1 is sized to 
receive all electrons emitted from microchannel plate 33 and variable 
aperture is adjusted so that only one-quarter of the electrons so emitted 
will strike dynode 37-1 then the overall time window or gate produced by 
detector 11 will be one-quarter of the time during which a complete sweept 
is made. 
As can thus be appreciated, the time window is initially determined by the 
time over which a single sweep is made but may be further fractionalized 
or reduced by providing a first dynode that is sized to intercept only a 
portion of the electrons emitted by microchannel plate 33 and/or by 
reducing the opening of aperture 35. 
Sweep generator 39 may be of the type used in Hamamatsu TV Co. Ltd, 
Hamamatsu, Japan, streak camera Model No. C1370 which provides sweep 
speeds for a single complete sweep of 375 picoseconds, 1000 picoseconds, 
2000 picoseconds, 5000 picoseconds and 1 nanosecond. 
Thus, a time window on the order of as small as 375 picoseconds may be 
produced and by selecting the size of first dynode 37-1 and/or adjusting 
the size of opening of variable aperture 35, the time window may be 
reduced by a factor on the order of around ten or greater. 
In the absence of a sweep signal, the accelerated electrons passed between 
sweeping electrodes 31 and striking microchannel plate 33 will not strike 
first dynode 37-1. 
Delay unit 43 delays the arrival of the trigger signal to sweep generator 
39. Delay unit may provide step delays such as 30, 50, 100, 200 and 500 
picoseconds and, as such, effectively shift the time window temporally by 
any one of these amounts or may of a type which provides continuous delays 
and/or may be programmable. 
As can be appreciated, time windows or gates on the order of around 30 
picoseconds of an incident light signal may be produced, with the size of 
the time window being limited only by the jitter caused by the sweep 
generators. 
Light detector 11 amy be enclosed by a main housing (not shown). 
Referring now to FIG. 2, there is illustrated a diagram of a system 51 
using picosecond light detector 11. 
Light from a laser light source 53 which may be a train of picosecond 
pulses from a mode locked continuous wave laser or a single pulse mode 
locked laser is deflected by a mirror 55, passed through a chopper 57 and 
brought to focus by a lens 59 on a sample S to be tested. Light outputed 
from sample S is collected and brought to focus by a relay lens system 61 
at the input of a spectrometer 62. Light from the spectral region of 
interest from spectrometer 63 is passed into light detector 11 which 
measures the intensity of the light over a time window set by streak 
camera 13. Light from laser 53 is also used to provide a light signal for 
triggering trigger 49 which may be a photodiode. The trigger signal from 
trigger 49 is fed ito the sweep generator in streak camera 13 of light 
detector 11 through delay unit 66 which may be the same as delay unit 47. 
Alternatively, delay unit 55 may be programmable and controlled by the 
computer hereinafter described over a line (not shown). The analog 
electrical output signal from light detector 11 is fed through a lock-in 
amplifier 69 to a computer 71 where it may be stored and/or displayed on a 
monitor 73. Computer 71 is also coupled to spectrometer 63 for controlling 
the spectral region of interest that is outputted from spectrometer 63 to 
picosecond light detector 11. If light source 53 is a single shot or a low 
repetition rate laser, chopper 57 is eliminated. 
In FIG. 3 there is illustrated a modified version of the tube portion of 
the invention. In the FIG. 3 version, dynodes 37-1 through 37-10 are 
replaced by a second microchannel plate 91 and the tube identified by 
reference numeral 15-1. The FIG. 3 embodiment operates in a similar manner 
as the FIG. 1 embodiment with the difference being that electrons 
outputted from microchannel plate 33 are apertured by aperture 35, passed 
and further amplified by a second microchannel plate 91 rather than a 
chain of dynodes. The anode (dynode 37-11) collects the swept electron 
beam passing through aperture 35 and outputs a signal 91-2. 
The embodiments of the present invention are intended to be merely 
exemplary and those skilled in the art shall be able to make numerous 
variations and modifications to it without departing from the spirit of 
the present invention. All such variations and modifications are intended 
to be within the scope of the present invention as defined in the appended 
claims.