Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Reexamination Certificate
2000-04-03
2001-11-20
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
C324S092000, C324S765010, C324S754120, C029S874000, C029S876000, C340S870280
Reexamination Certificate
active
06320367
ABSTRACT:
TECHNICAL FIELD
The invention relates to the technical field of electrical samplers, in particular for pulses having short or very short durations.
The metrology of pulses makes it possible to describe the variation over time “time” of a signal or of an electrical pulse, in particular the time variation of its voltage or of its energy, when the signal or pulse is single (non-repetitive) and very short (i.e. having a duration of about a few tens of picoseconds).
Such pulses to be measured are generally output by very fast radiation detectors which convert into electrical pulses the energy from a radiation pulse that they receive, e.g. an X, gamma, visible, or infrared radiation pulse. Such radiations can be emitted by ultra-fast radiation sources, such as lasers or synchrotron radiation sources, or they can result from a laser-matter interaction caused by an ultra-fast laser (i.e. a laser whose pulse duration is in the domain of the picosecond or of the femtosecond).
The invention is applicable to any measurement of a short and non-repetitive electrical signal, in particular in event physics, or in event measurement, the events in question being generated by picosecond phenomena.
STATE OF THE ART
Sampling oscilloscopes for measuring signals of spectrum extending to 50 GHz or 70 GHz are currently available on the market. Such instruments make it possible to measure repetitive frequencies. The sampling frequency is variable, typically over the range 250 kHz to 1 GHz.
Instruments for measuring single pulses are also commercially available. They make it possible to yield a spectrum up to 10 GHz.
However, it is necessary, in particular for oscilloscopes, to have a picosecond light source that is synchronous with the electrical pulse to be analyzed, which restricts the field of applications.
Devices are also known that are based on the principle of spatial sampling of a pulse propagating along a propagation line. There results a spatial equivalence of the time variation of the pulse which propagates along the line at a speed dependent on the physical characteristics of said line. At a determined instant t, if the line is of sufficient length, the entire pulse is distributed spatially along the line.
If samplers are disposed along the propagation line, by actuating them simultaneously, it is possible to perform full sampling of the pulse, with a time pitch equal to the spatial pitch of the samplers, divided by the speed of propagation.
In particular, document EP-327 420 describes an opto-sampler device that measures signals of passband up to 35 GHz. That device is shown in FIG.
1
. It includes a propagation line
2
in which a pulse signal
4
to be measured is introduced and along which said signal propagates. Sampling gates
6
made of a photoconductive material (CdTe) are distributed uniformly along the propagation line. The sampling gates are associated with tapping lines
8
, each of which is itself followed by means for reading the charges. All of the means for reading the charges are placed together in a device
10
for reading the charges. The means for reading the charges are connected to a computer
12
programmed to measure the charges relating to each channel and to analyze the pulse
4
. Each sampling gate
6
is closed by means of a triggering light pulse
14
: as many triggering light pulses as there are sampling gates are necessary. That device thus requires a picosecond optical flash of a few tens of nanojoules in order to trigger the sampling.
Each of the samplers (photoconductors) of that device thus taps a portion of the signal present at its level along the line. They are placed in parallel with the propagation line.
An optoelectronic sampling system incorporating a device of the above-described type is disclosed in the thesis by Vincent Gerbe (Université Joseph Fourier, Grenoble, Sep. 24, 1993). That system is shown diagrammatically in FIG.
2
. It includes a sampler
1
having 16 photo-switches and operating on the principle described above with reference to
FIG. 1. A
picosecond laser (not shown in the figure) delivers pulses
16
at a rate of 0.2 hertz, and at a wavelength of 0.53 &mgr;m. A set of mirrors
18
-
24
constitutes an optical delay. The beam is then focussed by means of a cylindrical lens
26
onto the set of photoconductors. The irradiated area is thus about 4 cm×100 &mgr;m.
A pulse to be analyzed (of width at half height equal to about 150 picoseconds) is generated by a detector
28
of the n-preirradiated GaAs type lit by a portion of the beam. The pulse can be viewed, after sampling, on a viewing device
30
.
The sampling device
1
is provided with 16 sampling channels, the sampling pitch being 18 picoseconds. The main line has a length L=40 mm for a working length L
u
=32 mm.
Such a device is fixed as regards the total length of the pulse that the device is capable of analyzing. Similarly, it is fixed as regards the sampling pitch and thus the accuracy with which the pulse is to be analyzed: the sampling interval &Dgr;T is equal to
p
V
c
,
where p is the distance between two photoconductors and V
c
is the speed of propagation of the signal in the line
2
.
In addition, the ultra-fast laser pulse (time width lying in the range 1.2 picoseconds to 1.6 picoseconds, and energy per pulse lying in the range 200 &mgr;joules to 300 &mgr;joules) has an elliptical shape (43×0.4 mm
2
), i.e. an area of about 7 mm
2
. Each photoconductor tab has dimensions of about 120 &mgr;m
2
×20 &mgr;m
2
, i.e. an area of about 2×10
−3
mm
2
per photoconductor. For each photoconductor, the efficiency, as defined by the ratio between the injected power and the working power, is thus very low.
It is thus desirable to find an optical system that makes it possible to increase the efficiency of the injected power/working power.
Another problem related to the optical interface of that system is the use of a cylindrical lens
26
: the laser, which in that example is a frequency-doubled YAG laser, has poor stability from one shot to another, and that instability is added to the instability created by the use of the lens. The photoconductor tabs do not therefore receive the same energy each time the laser is fired. It would thus be desirable to have an optical interface making it possible to attenuate the influence of the spatial instabilities, while also being simple to implement.
STATEMENT OF THE INVENTION
The invention firstly provides a device for analyzing a single electrical pulse, which device comprises an electrical circuit made up of a propagation line, including, in uniformly-distributed manner, optoelectronic switches constituted by photoconductor tabs, each of them being connected to a corresponding analysis line, and an optical device making it possible to light the switches with a laser beam or with portions of a laser beam, any two adjacent switches being lit successively and not simultaneously.
The photoconductors are thus sampled successively, which, with the same propagation structure, makes it possible to sample with a pitch that is smaller or larger than the pitch corresponding to the spacing between the lines on the structure.
Thus, the optical device makes it possible to use the analysis device in variable ranges as regards both the total length of the pulse that it is capable of analyzing, and also the sampling pitch (and thus the accuracy) with which the pulse is to be analyzed.
The invention also provides a device for analyzing a single electrical pulse, which device comprises an electrical circuit made up of a propagation line, including, in uniformly-distributed manner, optoelectronic switches constituted by photoconductor tabs, each of them being connected to a corresponding analysis line, and an optical device making it possible to light the switches with portions of a laser beam, each portion of the beam corresponding to a respective switch, said optical device making it possible to impart an optical delay between two adjacent portions of laser beam corresponding to two adjacent photoconductors of t
Cuzin Marc
Gentet Marie-Claude
Brown Glenn W.
Commissariat A l'Energie Atomique
Hamdan Wasseem H.
Harness Dickey & Pierce
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