EP0418965B1 - Cathode ray tube having a photodeflector - Google Patents

Description

The invention relates to a cathode ray tube, provided with means for electrostatic deflection of the path of an electron beam e _f coming from an electron source.

In a cathode ray tube it is usual to deflect the path of the electron beam using an electrostatic deflection formed by plates joined to different potentials. Usually the tube has a pair of plates for horizontal deflection to which a time base is applied and a pair of plates for vertical deflection to which the electrical signal to be processed is applied. This electrical signal is introduced into the tube using connectors and cables which are connected to a signal generator. These signals can be generated initially in forms that are not electrical. Conversion to an electrical signal is therefore necessary, which in certain situations can be a disadvantage.

On the other hand, these signals can have various speeds. In the field of fast signals when it is desired to produce, for example, an oscilloscope having a pass band covering several hundred megahertz, this is difficult to achieve with such means of electrostatic deflection. Solutions have been proposed using wave propagation techniques.

Thus the document entitled "Cathode ray tubes with very wide band wave propagation" by C. LOTY, Acta Electronica vol. 10 No. 4 1966 p.351-361, reveals a solution using a helical line. In this case, there is a line with distributed constants consisting of a folded conducting wire, along which the wave propagates at the speed of light in a three-dimensional structure. An oscilloscope made on such bases has a very high bandwidth. But the signals that are to be analyzed and which act on the electrostatic deflection of the electron beam are to be introduced in an electrical form using connecting cables which have significant capacities. In practice, we are always faced with a sensitivity problem and the designer is led to establish a compromise between the speed and the sensitivity of the deflection of the electron beam.

However, when analyzing light phenomena, which can be of excessively short duration, a large part of the rapid information which they contain can be masked or even lost by these difficulties of introducing electrical signals into the cathode ray tube. which aggravates the disadvantages.

The problem then arises of avoiding the conversion of the optical signals. In addition, it may be desired to keep both great sensitivity and rapidity in the tube for the analysis of such light signals when they are rapid.

The solution proposed by the invention is that the electrostatic deflection means comprise at least one electrostatic photodeviator including a photodetector which, under the action of incident light radiation, creates electric charges e _p which modify the electric deflection field of the photodeviator.

Advantageously, the light radiation is not converted into an electrical signal prior to its introduction into the cathode ray tube and the information it contains is thus better preserved. There is therefore direct intervention of light radiation on the electron beam.

This is very useful not only in devices which must respond quickly to the action of light radiation, but also in slower devices which take advantage of the absence of transformation of light radiation into an electrical signal outside said device. .

The principle of the invention is to send the light radiation to be detected directly to one of the deflection plates through a window placed on the side of the tube. This deflection plate can be coated with a photodetector which depends on the spectral range of the light radiation to be detected. When this photodetector receives light radiation, a quantity of charges is created in proportion to the intensity of the light radiation. If a positive electrode is placed nearby, these charges will transit and develop a positive potential on the deflection plate. This is equivalent to placing a photodetector inside the cathode ray tube. The deflection plates and the photodetector constitute the photodevector. The photodetector can be a photocathode which, under the action of incident light radiation, creates charges under vacuum, or a photoelectric element such as a photodiode, which, under the action of incident light radiation, creates charges in the material of the photoelectric element. This removes connecting cables, connectors and by-passes between the photodetector and the deflection plate of the cathode ray tube. This results in great freedom in the choice of the load impedance Z.

In particular, there is no longer any need to have an impedance adapted to that of a connecting cable (typically Z = 50Ω), and it is possible to adopt an impedance of high value and to increase significantly. vertical detection sensitivity. Thus, if the impedance Z is a resistance of 1000Ω accompanied by a stray capacitance of C = 0, 1pF, we obtain a gain in deflection sensitivity in the ratio 1000/50 = 20 for an excessively short rise time (100ps ) of the photodetector.

The photodevector can comprise 3 electrodes and for this comprises a first and second extreme electrode between which a central electrode is interposed, the central electrode separating on one side a first space through which the electron beam e _f passes and on the other side a second space where the photodetector is located.

When the photodetector is a photocathode, according to one embodiment, the photocathode is deposited on the most negative electrode of the electrodes delimiting the second space, the electric charges e _p moving from the photocathode to the positive electrode and the electron beam e _f crossing the first space in a substantially perpendicular direction.

It is possible according to embodiments that the central electrode is, as the case may be, brought to an intermediate potential higher or lower than the potentials of the first and second extreme electrodes.

When the photodetector is a photodiode, this can consist of a piece of silicon placed between the extreme positive electrode and the central electrode, the electron beam e _f passing through the space delimited by the central electrode and l '' extreme negative electrode.

Il est clair que la sensibilité de déflexion est alors inversement proportionnelle à la capacité C donc proportionnelle à la distance entre le photodétecteur et l'électrode positive divisée par la surface active du photodétecteur. Par ailleurs une augmentation de cette distance photodétecteur-électrode positive allonge le temps de vol des électrons, c'est-à-dire le temps de montée propre de cet espace interélectrode. Il y a donc une distance optimale à déterminer en fonction de l'application envisagée.It is also possible to want to adopt a very high load resistance, of almost infinite value for example 10 MΩ, to increase the detection sensitivity. In this case the time constant becomes large in front of the rise time of the optical signals to be detected and this time the vertical deviation V _y is no longer proportional to the instantaneous light signal but to the integral of this signal as a function of time: $${\text{V}}_{\text{y}}\text{= 1 / C ∫i dt}$$

It is clear that the deflection sensitivity is then inversely proportional to the capacitance C therefore proportional to the distance between the photodetector and the positive electrode divided by the active surface of the photodetector. Furthermore, an increase in this positive photodetector-electrode distance lengthens the flight time of the electrons, that is to say the clean rise time of this interelectrode space. There is therefore an optimal distance to be determined depending on the intended application.

In all the embodiments, in particular the 3-electrode modes, it is advantageous to reduce the capacity clean of the photodetector.

When the photodetector is a photocathode, one way to reduce the capacitance between the photocathode and the deflection electrodes is to remove one of the electrodes. There then exists a single interelectrode space where the electron beam ef and the electric charges e _p act. In this case the photodevector is with 2 electrodes, joined respectively to a positive and negative potential, the photocathode being deposited on the face of the negative electrode directed towards the positive electrode, the negative electrode being joined to the negative potential GND by a impedance Z, the electric charges e _p moving from the photocathode to the positive electrode and the electron beam crossing the same interelectrode space in a substantially perpendicular direction.

The light radiation must reach the photodetector to create the electric charges e _p . Depending on the orientation of the light radiation, it may be necessary for at least one of the electrodes to be transparent in order to transmit the light radiation to the photodetector. It can be a transparent support, such as a metallized glass, for receiving the photocathode. Or the electrode facing the photocathode may be a tight mesh grid. Or, when it is a photodiode, the piece of silicon can be covered with a transparent metal oxide.

When the photodevector has 2 electrodes with a single space for the electron beam e _f and the electric charges generated e _p , there is at rest a permanent deviation which it is normally necessary to compensate. This deflection at rest of the path of the electron beam e _f is then compensated by means of corrections, for example correcting coils or an electrostatic deflector.

The various embodiments which have just been described relate to a photodevector whose basic structure comprises three electrodes or two electrodes. By electrode you must hear a plate or an element of appropriate shape which deflects the beam. The fact that the photodetector is incorporated into the deflection means to form a photodevector makes it possible to increase the speed of response to a rapid light signal. However, it is still possible to increase this speed of response by producing a distributed photodevector which comprises several photodetectors arranged along the path of the electron beam e _f , the light radiation being successively deflected from a photocathode or from a photodiode to the next one using reflectors. The best results are obtained when the distances which separate the photocathodes or the photodiodes from the reflectors on the one hand, and the distances which separate two consecutive central electrodes on the other hand, are determined to ensure a synchronized action on the electron beam e _f .

The photodeviator or the distributed photodeviator can be placed inside a single enclosure, in which a vacuum has been created and which contains all the elements of a cathode ray tube. But in the case of an embodiment with 3 electrodes, to facilitate industrial production, it is possible to isolate the enclosure containing the photodeviator from the enclosure containing the other elements of the cathode ray tube. Thus when it is a photocathode, it is possible to independently carry out the heat treatments which are necessary for the formation of the photocathode on the one hand and of the cathode of the electron gun (source of electrons) of on the other hand so as not to damage them mutually. After mounting, these two enclosures can remain non-communicating but become mechanically integral after their adapted arrangement.

figure 1 :

un tube à rayons cathodiques connu.

figures 2A, 2B :

deux schémas d'un photodéviateur à 3 électrodes muni d'une photocathode selon l'invention.

figure 3 :

un schéma d'un photodéviateur à 2 électrodes muni d'une photocathode selon l'invention.

figures 4A, 4B :

un schéma d'un photodéviateur muni d'une photodiode.

figures 5A à 5D :

des schémas de réalisation d'un photodéviateur distribué.

figures 6A, 6B :

un exemple de réalisation du photodéviateur distribué selon le schéma optique de la figure 5B.

figures 7A, 7B :

un exemple de réalisation du photodéviateur distribué selon le schéma électrique de la figure 5A.

figure 8 :

un exemple de réalisation d'un tube à rayons cathodiques selon l'invention formé de deux enceintes séparées.

The invention will be better understood using the following figures given by way of non-limiting example and which represent:

figure 1 :

a known cathode ray tube.

Figures 2A, 2B:

two diagrams of a 3-electrode photodevector provided with a photocathode according to the invention.

figure 3:

a diagram of a 2-electrode photodevector provided with a photocathode according to the invention.

Figures 4A, 4B:

a diagram of a photodevector provided with a photodiode.

Figures 5A to 5D:

diagrams of making a distributed photodevector

Figures 6A, 6B:

an exemplary embodiment of the photodevector distributed according to the optical diagram of FIG. 5B.

Figures 7A, 7B:

an exemplary embodiment of the photodevector distributed according to the electrical diagram of FIG. 5A.

figure 8:

an exemplary embodiment of a cathode ray tube according to the invention formed of two separate enclosures.

Figure 1 shows a cathode ray tube according to the known art. It comprises a vacuum chamber 10 in which an electron gun 11 emits an electron beam e _f which is deflected (beam 14) by vertical deflection plates 12 and horizontal deflection plates 13. The deflection plates can be formed by helical lines according to the prior art to increase the speed of deflection of the beam. The rapid electrical signals to be analyzed are introduced by electrical connectors which are not shown.

According to the invention, at least one of the deflection means is replaced by a photodeviator.

FIG. 2A represents a photodeviator with 3 electrodes comprising a first extreme electrode 20, a second extreme electrode 21 and a central electrode 22. The electron beam e _f passes in the space between the electrodes 21 and 22. The first extreme electrode 20 is brought to a positive potential HT, the second extreme electrode 21 is brought to a negative potential GND, and the central electrode 22 is brought to an intermediate potential. On the most negative electrode of electrodes 20 and 22, the central electrode, a photocathode 24 is deposited on the side of electrode 20. The central electrode is connected to the negative potential GND by a charge impedance Z. Under the influence of light radiation 25₁, 25₂, 25₃ the photocathode emits electrons which are captured by the first extreme electrode 20. Under the influence of the electric current thus created, the potential of the central electrode 22 varies and the electric deflection field between the electrodes 21 and 22 also varies, which makes it possible to deflect the electron beam e _f .

FIG. 2B shows another arrangement of the elements of a photodeviator with 3 electrodes. The first extreme electrode 20 is brought to a negative potential GND, the second extreme electrode 21 is brought to a positive potential HT, and the central electrode 22 is brought to an intermediate potential, being connected to the positive potential HT by a load impedance Z. The electron beam e _f passes between the electrodes 21 and 22. The photocathode is deposited on the negative electrode 20 opposite the central electrode 22 which is at a more positive potential. The same mechanism as before occurs to deflect the beam.

In other embodiments, the central electrode can be brought to a potential lower or higher than the potentials of the first and second extreme electrodes, with the photocathode deposited on the most negative electrode of the electrodes 20 and 22.

• soit en inclinant à priori le faisceau d'électrons avant son entrée dans le photodéviateur,
• soit en plaçant un second déviateur électrostatique agissant en sens opposé et placé soit en amont, soit en aval du photodéviateur,
• soit en utilisant un déviateur magnétique convenablement disposé pour que la trace du faisceau vienne se former à l'endroit désiré sur l'écran.

FIG. 3 represents a photodeviator with 2 electrodes. The electron beam e _f and the electric charges e _p move in the same interelectrode space. The photocathode 24 is deposited on the electrode 22 which is connected to the negative potential GND by the impedance Z. In this case the DC bias voltage between the photocathode and the positive electrode causes the electron beam e _{f to be} strongly deviated at rest. This deviation at rest must be compensated by means of corrections:

• either by tilting the electron beam a priori before it enters the photodeviator,
• either by placing a second electrostatic deflector acting in the opposite direction and placed either upstream or downstream of the photodevector,
• or by using a magnetic deflector suitably arranged so that the beam trace comes to form at the desired location on the screen.

FIG. 4A represents the electrical diagram of the principle of a photodeviator provided with a photodiode. The photodiode 40 is connected on the one hand to a positive potential V _p (lower than the high voltage HT in the case of a photocathode) and on the other hand to the central electrode 22 connected to ground through an impedance Z The electron beam e _f passes between the central electrode 22 and the second extreme electrode 21 brought to a negative potential. FIG. 4B represents an embodiment diagram. The photodiode is formed from a piece of silicon 41 placed between the first extreme electrode 20 brought to a positive potential and the central electrode 22. To capture the light radiation 25₁, 25₂ at least one of the electrodes must be transparent.

FIG. 5A represents an electrical diagram of a distributed photodevector. It comprises a first extreme electrode 20 brought to a positive potential, a second extreme electrode 21 brought to a negative potential and a plurality of central electrodes 22₁ to 22₆. Each of these central electrodes carries a photocathode such as 24₁ for the 22₁ electrode. Each central electrode is connected to the negative potential by an impedance 2.

où
   e est la charge électrique,
   m la masse de l'électron,
   V le potentiel appliqué,
   les distances d entre les photocathodes sont ainsi déterminées en fonction du potentiel appliqué.FIG. 5B represents the optical path followed by the light radiation 50. It begins by striking the first photocathode 24₁. Part of the radiation is absorbed and generates electrons (electrical charges e _p ) which act on the potential of the central electrode 22₁ according to the mechanisms already exposed. The other part of the radiation is reflected towards the first extreme electrode 20 which in turn sends it back to the second photocathode and so on. The light radiation is thus absorbed after its action on some photocathodes. To keep all the interest in the distributed photodevector it is desirable to distribute the absorption of light radiation between all the photocathodes concerned without favoring the former by adapting their absorption rate.
But for the actions of all the individual photodevectors to be in phase, it is necessary to determine the distance d separating two consecutive individual photodevectors to adapt the optical path, followed by the light radiation between two consecutive photocathodes, to the distance separating a photocathode ( for example 24₁) of the first extreme electrode 20. The speed of the electrons being: $${\text{v (m / s) = (2e.V / m)}}^{\text{½}}{\text{= 5,932.10⁵. (V)}}^{\text{½}}$$

or
e is the electric charge,
m the mass of the electron,
V the applied potential,
the distances d between the photocathodes are thus determined as a function of the potential applied.

To lengthen the optical path, it is possible to use not the first end electrode 20 but side reflectors as shown in FIGS. 6A and 6B.

To lengthen the optical path followed by the light radiation, it is also possible to produce a distributed photodevector according to FIG. 5C. Each central electrode 22₁- 22₆ is connected to the negative potential by an impedance Z (see FIG. 5A). The photocathode 24 is in this case deposited on a transparent support 53 which receives beforehand the first extreme semi-transparent electrode 20 brought to a negative potential. The electron beam e _f passes between these central electrodes and the second extreme electrode 21 brought to a positive potential. Thus the light radiation passes through the transparent support 53 and the semi-transparent electrode 20, is partially absorbed and is reflected on the photocathode 24, crosses the same elements and is reflected again on a reflector 55. The successive reflection mechanisms are then produce in the same way as before. In this case the optical path can be adapted to the distance d by positioning the reflector 55.

It is also possible to modify the diagram in FIG. 5C by making the transparent support 53 sufficiently thick so that the light radiation does not leave the support 53 through the face 56 in the direction of the reflector 55, while having an optical path. quite long (Figure 5D). The reflection can be carried out either on the reflector 55 when such a reflector is attached to the support 53, or without reflector 55 on the face 56 itself by total reflection. The thicknesses and the positions of these various elements depend on the characteristics of speed which it is desired to give to the distributed photodeviator.

FIGS. 6A, 6B represent an exemplary embodiment of a photodevector according to the diagram of FIG. 5B but with lateral reflectors 61, 62.

The light radiation 50 arrives in a direction very different from the direction of propagation of the electron beam e _f on the first photocathode 24₁, deposited on the first central electrode 22₁, is partially absorbed and generates electric charges e _p which are picked up by the first extreme electrode 20. The other part of the light radiation is reflected on the lateral reflector 61 which returns the radiation to the second photocathode. At each photocathode, the radiation which is not absorbed is thus reflected towards the next photocathode, alternately by one and the other side reflector. FIG. 6B represents a top view of the photodevector of FIG. 6A where the extreme electrodes have been omitted so as not to weigh down the drawing. The same elements are represented with the same references.

In FIG. 5A, the central electrodes 22₁ to 22₆ constitute independent conductive surfaces each connected by an impedance Z to the negative potential GND. The electrical potential of each central electrode is thus controlled by the electrical charges e _p which are created by each photocathode. It is possible to realize different ways this plurality of central conductive electrodes. Figures 7A and 7B show an exemplary embodiment. For this, an insulating support 70 is used on which the central electrodes 22₁ to 22₆ are placed individually and consecutively in the direction of propagation of the electron beam e _f . Each central electrode passes through the insulating support 70 so that it appears on both sides of the support. The upper face (in FIG. 7B) receives the photocathode and the lower face serves to deflect the beam. Each photocathode (for example 24₁) is connected by an impedance Z (for example 71₁) to the negative potential GND. The conductive electrodes as well as the Z impedances can be produced by conventional thin film or thick film technologies. The photocathodes are deposited by the usual methods.

The other arrangements described with the photocathodes deposited on the negative electrodes can use the same production methods.

FIG. 8 represents an exemplary embodiment of a cathode ray tube provided with a photodeviator with 3 electrodes according to the invention. We find the essential elements already described in Figure 1 but one of the deflectors is replaced here by a photodeviator.

The cathode ray tube is shown formed of two independent vacuum chambers 10 and 80.

The enclosure 80 is formed of an empty air bulb. It contains the first extreme electrode 20 and the central electrode 22 _a provided with the photocathode 24. Thus this enclosure 80 can be treated independently for all the processes of formation of the photocathode which otherwise could receive a slight pollution of the other parts of the tube to cathode rays. The enclosure 80 can receive the window which is used to introduce the light radiation therein.

The enclosure 10 is provided with the second extreme electrode 21 as well as with another central electrode 22 _b which is accessible from the outside. So during assembly, the central electrodes 22 _a and 22 _b are electrically connected to each other (for example welded) and constitute the single central electrode 22 of the photodevector. The central electrode _22b of the vacuum enclosure 10 can be placed in a re-entrant part of the vacuum enclosure 10 in order to reduce the distance which separates it from the electron beam e _f , and therefore the capacities, and facilitate the positioning of the vacuum chamber 80.

Obviously it is possible not to adopt this constitution with two separate enclosures and to place all the elements in the vacuum enclosure 10. The embodiments of the photodeviator described above can be mounted in a cathode ray tube according to similar principles accessible to the skilled person without departing from the scope of the invention.

Such a tube can be used to make an oscilloscope.

Claims (19)

1. A cathode ray tube comprising electrostatic deflection means for deflecting the path of an electron beam e_f issuing from an electron source, characterized in that the said deflection means comprise at least an electrostatic photodeflector including a photodetector which under the action of an incident light radiation creates electric charges e_p which modify the electric deflection field of the photodeflector.
2. A tube as claimed in Claim 1, characterized in that the photodeflector comprises a first and a second outer electrode between which a central electrode is interposed, a first space through which the electron beam e_f passes being defined by the central electrode and the second outer electrode, and a second space which comprises the photodetector being defined by the central electrode and the first outer electrode.
3. A tube as claimed in Claim 2, characterized in that the photodetector is a photocathode deposited on the electrode which is the most negative of the electrodes defining the second space, the electric charges e_p moving from the photocathode towards the positive electrode and the electron beam e_f traversing the first space in a substantially perpendicular direction.
4. A tube as claimed in Claim 2 or 3, characterized in that the first outer electrode is brought to a negative potential, the second outer electrode is brought to a positive potential and the central electrode is brought to an intermediate potential.
5. A tube as claimed in Claim 2 or 3, characterized in that the first outer electrode is brought to a positive potential, the second outer electrode is brought to a negative potential, and the central electrode is brought to an intermediate potential.
6. A tube as claimed in Claim 2 or 3, characterized in that the central electrode is brought to a potential which is higher than the potentials of the first and the second outer electrodes.
7. A tube as claimed in Claim 2 or 3, characterized in that the central electrode is brought to a potential which is lower than the potentials of the first and the second outer electrodes.
8. A tube as claimed in Claim 1, characterized in that the photodeflector comprises 2 electrodes which are brought to respectively a positive and negative potential, a photocathode being deposited on the face of the negative electrode directed towards the positive electrode, the negative electrode being brought to the negative potential GND via an impedance Z, the electric charges e_p moving from the photocathode towards the positive electrode and the electron beam traversing the same interelectrode space in a substantially perpendicular direction.
9. A tube as claimed in Claim 8, characterized in that the deflection in the quiescent state of the path of the electron beam e_f is compensated for by correction means.
10. A tube as claimed in Claim 1 or 2, characterized in that the photodetector is a photodiode.
11. A tube as claimed in Claim 10, in so far as Claim 10 depends on Claim 2, characterized in that the photodiode is constituted by a silicon piece located between the positive outer electrode and the central electrode, the electron beam e_f traversing the space defined by the central electrode and the negative outer electrode.
12. A tube as claimed in any one of the Claims 1 to 11, characterized in that the electron beam e_f is deflected by the combination of an electric signal applied to at least one of the electrodes and an optical signal applied to the photodetector.
13. A tube as claimed in any one of the Claims 2 to 12, characterized in that at least one of the electrodes is transparent to transmit the light radiation to the photodetector.
14. A tube as claimed in Claim 13, characterized in that the transparent electrode is constituted by a close-meshed grid.
15. A tube as claimed in any one of the Claims 1 to 14, characterized in that it comprises several photodeflectors forming a distributed photodeflector along the path of the electron beam e_f, the light radiation being successively deflected from one photocathode or one photodiode to the next by means of reflectors.
16. A tube as claimed in Claim 15, characterized in that the distances which separate the photocathodes or the photodiodes from the reflectors on the one hand, and the distances which separate two consecutive central electrodes on the other hand are chosen so as to ensure a synchronized action on the electron beam e_f.
17. A tube as claimed in any one of the Claims 1 to 7 or 10 to 16 in so far as any one of Claims 10 to 16 depends on Claims 1 to 7, characterized in that it comprises a first evacuated space which comprises the photodeflector and a second evacuated space formed integrally with the first space and which comprises the other elements of the tube.
18. A tube as claimed in Claim 17, characterized in that prior to assembly the first evacuated space forms an independent element.
19. An oscilloscope, characterized in that it comprises a cathode ray tube as claimed in any of the Claims 1 to 18.

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