An oscilloscope constructed for use in real time resolving an ultrafast voltage signal includes a streak camera having a transmission line photocarthode, a constant light source for illuminating the photocathode, an input coupled to the transmission line photocathode, an accellerating mesh, a pair of sweep electrodes, electron multiplication means, a phosphor screen and a DC high voltage source.
In use, the voltage signal to be examined is applied to the input. When the voltage signal on propagating through the photocathode intersects in time and space on the photocathode with the light from the light source, a number of electrons proportional to the intensity of the voltage signal are emitted from the photocathode.
These electrons are then accelerated, deflected by the pair of sweep electrodes, multiplied by the microchannel plate and then impinge upon the phosphor screen, creating an optical image having an intensity proportional to the number of impinging electrons. The image on the phosphor screen is recorded by a video camera, processed by a computer and then displayed on a monitor.
In another embodiment, the streak camera includes a second photocathode with input signals being applied to either one or both photocathodes. In other embodiments, a photomultiplier tube having a pair of transmission line photocathodes are described.
This application relates generally to oscilloscopes and more particularly to an oscilloscope for use in time resolving an ultrafast voltage signal.
Many photoelectric devices, most notable of which are the photomultiplier tube and the streak camera, have been used in the past to time resolve very short optical pulses.
Basically, a photomultiplier tube comprises a photocathode, an electron multiplier and an anode, all disposed in an evacuated glass housing, with potential differences set up between the electrodes and the electron multiplier to cause photoelectrons emitted by the photocathode when it is illuminated to pass through the electron multiplier and on to the anode.
In the operation of a photomultiplier tube a beam of light strikes the photocathode, causing a number of electrons, the number being proportional to the intensity of the incident light beam, to be emitted into the evacuated housing. These electrons are then multiplied by the electron multiplier to produce a stronger signal and, thereafter, transmitted through the housing to the anode where the electrons are collected to produce an output voltage signal.
Because of the electron multiplication, photomultiplier tubes are especially well adapted among photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. Photomultiplier tubes also feature relatively fast time response.
The photocathode in a photomultiplier tube is generally arranged in either a side-on or a head-on configuration. In the side-on type configuration the photocathode receives incident light through the side of the glass housing, while, in the head-on type, light is received through the end of the glass housing.
In general, the side-on type photomultiplier tube is widely used for spectrophotometers and general photometric systems. Most of the side-on types employ an opaque photocathode (reflection-mode photocathode) and a circular-cage structure electron multiplier which has good sensitivity and high amplification at relatively low supply voltage.
The head-on type photomultiplier tube has a semitransparent photocathode (transmission-mode photocathode) deposited upon the inner surface of the entrance window while in the side-on type, the photocathode is a separate structure. Because the head-on type provides better uniformity and lower noise, it is frequently used in scintillation and photon counting applications.
The electron multiplier in a photomultiplier tube is usually in the form of either a series of electrodes, called dynodes, or a microchannel plate. As is known, a microchannel plate (MCP) is a form of secondary electron multiplier consisting of an array of millions of glass capillaries (channels) having an internal diameter ranging from 10 um to 20 um fused into the form of a thin disk less than 1 mm thick.
The inside wall of each channel is coated with a secondary electron emissive material having a proper resistance, and both ends of the channel are covered with a metal thin film which acts as electrodes, thus each channel becomes an independent secondary electron multiplier.
When a voltage is applied between both sides of the MCP, an electric field is generated in the direction of the channel axis. When an electron hits the entrance wall of the channel, secondary electrons are produced. These secondary electrons are accelerated by the electric field and travel along the parabolic trajectories determined by their initial velocity.
Then they strike the opposite wall and produce other secondary electrons. This process is repeated many times along the channel, and, as a result, the electron current increases exponentially towards the output end of the channel.
The photocathode in a head-on type photomultiplier tube is generally circularly shaped and in a side-on photomultiplier tube is usually in the shape of a portion of a cylinder.
One of the limitations of photomultiplier tubes is that although they have a relatively fast time response, they are not capable of time resolving luminous events in the picosecond time frame. On the other hand, an optoelectric device that does have the capability of time resolving luminous events in the picosecond time frame is the streak camera.
Streak cameras are over fifteen years old in the art and have been used, hitherto, to directly measure the time dynamics of luminous events (i.e. 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, electron accellerating means, 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 local plane is then either photographed by a conventional still camera or by a video camera. The streak camera tube generally includes a photocathode, an accelerating mesh, sweeping electrodes, and a 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 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 may comprise a single lens.
U.S. Pat. No. 4,659,921 to Alfano, a streak camera which can be gated on and off over an ultrashort time window, such as in picoseconds or femtoseconds, is disclosed. The device 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 typical streak camera.
Swept electrons strike the microchannel plate where electron multiplication is accomplished. Exciting electrons which pass through the variable aperture and which strike the first dynode (cathode) in the dynode chain are further multiplied and outputed 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 design all of the dynodes in the chain except for the last dynode are replaced by a second microchannel plate.
In U.S. Pat. No. 4,467,189 to Tsuchiya, a streak camera is disclosed which includes a cylindrical airtight vacuum tube, a shutter plate, and a ramp generator. The container has a photocathode at one end thereof and a flourescent screen at the other end thereof which is opposite to the photocathode.
The shutter plate is disposed between and parallel to the surface of the photocathode and fluorescent screen and has a multiplicity of through holes perforated perpendicular to its surface. The shutter plate also carries at least three electrodes that are disposed perpendicular to the axis of the through holes and spaced parallel to each other.
The electrodes divide the surface of the shutter plate into a plurality of sections. The ramp generator is connected to the electrodes. The ramp voltage generated changes in such a manner as to reverse its polarity, producing a time lag between the individual electrode. Developing an electric field across the axis of the through holes in the shutter screen, the ramp voltage controls the passage of the electron beams from the photocathode through the through holes.
A framing camera includes the above-described framing tube and an optical system. The optical system includes a semitransparent mirror that breaks up the light from the object under observation into a plurality of light components and a focusing lens disposed in the path through which each of the light components travels.
Each of the light components corresponds to each of the sections on the shutter plate. The images of a rapidly changing object are produced, at extremely short time intervals, on different parts of the fluorescent screen.
As can be appreciated, the streak camera devices described above are useful only for time resolution of optical pulses.
In copending application by Alfano et al., Ser. No. 091,123, filed Aug. 31, 1987, there is disclosed a photomultiplier tube constructed for use in either time resolving an ultrafast test voltage signal or time resolving an ultrafast optical pulse. The photomultiplier tube comprises a housing having therein a photocathode for receiving incident light and producing emission of electrons in proportion to the intensity of the light, the photocathode having a transmission strip line configuration, an accelerating mesh for accelerating electrons emitted by the photocathode, a microchannel plate for performing electron multiplication on the electrons emitted from the accelerating means, an anode for receiving electrons from the microchannel plate and producing an analog electrical signal output, a power supply for use in applying a biasing voltage across the photomultiplier tube so that electrons emitted by the photocathode will be conveyed through the accelerating mesh and the microchannel plate and onto the anode, and cables connected to the photocathode for receiving and transmitting an ultrafast voltage signal.
It is an object of this design to provide a new and improved oscilloscope.
It is another object of this design to provide an oscilloscope for use in time resolving an ultrafast test voltage signal.
It is a further object of this design to provide an oscilloscope as described above which includes a streak camera.
It is still a further object of this design to provide an oscilloscope as described above which includes a specially designed photomultiplier tube.
It is yet still a further object of this design to provide an oscilloscope as described above for use in time resolving an ultrafast voltage signal with minimal background noise.
It is another object of this design to provide a photomultiplier tube having a photocathode in the form of transmission stripline whose width is matched to the output slit size of a spectrometer whose output is detected by the photomultiplier tube or matched to the cross sectional size and shape of a fiber optics bundle connecting the spectrometer to the photomultiplier tube.
An oscilloscope for use in time resolving an ultrafast voltage signal constructed according to one embodiment of the present design comprises a streak camera including a housing in which there is disposed a photocathode having a transmission line configuration, means connected to the photocathode for receiving a voltage signal to be examined, a constant DC light source for illuminating the photocathode, a number of electrons being emitted from the photocathode when the beam of light from the constant DC light source and the voltage signal intersect in place and time on the photocathode, the number of electrons being proportional to the intensity of the voltage signal, an accelerating mesh deflection means for angularly deflecting the electrons as a function of time, electron multiplication means for performing electron multiplication on the electrons passed through the deflection means, a phosphor screen for receiving the electrons from the electron multiplication means and for producing a light image proportional in intensity to the number of electrons impinging thereon, and accellerating means for causing electrons emitted by the photocathode to move through the accelerating mesh, the deflection means, the electron multiplication means, and onto the phosphor screen.
The oscilloscope also includes a video camera, such as a vidicon or a SIT camera or a CCD camera, disposed in proximity to and focussed on the phosphor screen, the video camera being used to record the streak image on the phosphor screen, a computer coupled to the output of the video camera, the computer being used to convert the streak image into digital information to process the information, to generate a time resolved profile based on the digital information, and a monitor electrically coupled to the computer to display the time resolved profile.
In another embodiment of the design, the oscilloscope is specifically constructed so as to minimize voltage readings attributable to background noise and dark current. More specifically, the streak camera portion oscilloscope further comprises a second transmission line photocathode and a second phosphor screen, both of which are disposed within the housing in proximity to the first photocathode and phosphor screen, respectively.
The second photocathode is not arranged to receive the voltage signal. Consequently, only a background level of electrons, representing background noise are emitted from the second transmission line photocathode. These background electrons travel through the housing and form a light image on the second phosphor screen just as the electrons emitted from the first photocathode travel through the housing and form an image on the first phosphor screen.
A second TV camera, focused on the second phosphor screen, records the image formed on the second phosphor screen and conveys this information to the computer. The computer then digitizes this light image, and then subtracts it from the light image from the first phosphor screen to yield a background-corrected time resolved profile of the voltage signal.
In still another embodiment of the design the photoelectric portion of the oscilloscope includes a photomultiplier tube having a pair of transmission line photocathodes rather than a streak camera tube having a pair of transmission line photocathodes.
Various features and objects will appear from the description to follow. In the description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration, specific embodiments for practicing the design. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the design, and it is understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the design.
The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present design is best defined by the appended claims.
The present design is directed to a new and novel oscilloscope for use in time resolving an ultrafast voltage signal. The voltage signal may be as short as one picosecond. The present design accomplishes this in one embodiment by propagating the voltage signal through the photocathode in a streak camera, the photocathode being illuminated with light from a constant or long pulse light source.
When the optical pulse and the electrical signal intersect in time and space on the photocathode, a number of electrons proportional to the intensity of the voltage signal are emitted from the photocathode. These electrons are then accelerated through the streak camera tube (or housing) towards a phosphor screen.
Before the electrons reach the phosphor screen, they are swept (or deflected) by a pair of deflection (sweeping) electrodes. The sweeping temporally resolves the electrons since the electrons are angularly deflected as a function of time. From the sweeping electrodes, the electrons continue to travel through the housing along their respective trajectories until they reach a microchannel plate.
The microchannel plate, which is made up of secondary emissive material, multiplies the electrical signal along each trajectory by emitting a number of electrons for each electron impinging thereon. Finally, the electrons impinge upon the phosphor screen, creating an optical image having an intensity proportional to the number of impinging electrons.
The image is recorded by a video camera and processed by a computer. The end result is a time resolved profile of the test voltage signal which may then be displayed on a monitor.
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