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CHAPTER 2: STREAK CAMERA DESIGN. CO NSTRUCTIO N AND

Q PER ATIQ.N

2 .0 In t r o d u c t i o n

With the ready availability of subpicosecond, indeed femtosecond light pulses, there is a vital need for the development of linear monitoring instruments to accommodate the ultrashort optical pulse regime if such laser sources are to be fuUy understood and exploited. Photodiode technology employing HI-V and II-VI semiconductors has now extended linear electronic diagnostics into the sub-10 picosecond region [il - the measured results of the latter reference were in fact generated on the CPM laser discussed in this thesis. However, since there is as yet no commercially available oscilloscope with

sufficient sampling speed or bandwidth to display the output from such devices, they are f not useable directly as linear monitors of ultrashort events; relying more on the rather

circuitous methods of Fourier harmonic analysis Ifi, electro-optical sampling [2] or cross- correlation techniques P»4],

Ultrahigh speed cameras, on the other hand, based on the principles of electron optical

image tubes, have consistently been at the forefront of linear optical pulse measurement | applications. Recent years have seen the limiting temporal resolution, % of these cameras

improve by over an order of magnitude to the present state of the art of < 300 fs They

have been shown to offer high sensitivity and can provide temporal and spectral | information over time windows of hundreds of picoseconds - as a consequence these

instruments are the mainstay of the developing field of ultrqfast chronoscopy

In the following chapters the important design criteria of these streak tubes are described in respect of their modes of operation and the limitations on their performance. The final chapters of this section are devoted to the development of solid-state readout systems for these cameras (both an external format and an internal system incorporated within the tube envelope) and is referenced to a specific spacebome laser ranging application.

2.1 Im a g e t u b e d e s i g n a n d o p e r a t i o n 2.1.1 Co n s t r u c t i o n

There is a wide variety of picosecond image tube designs currently available but all operate on the same basic principle. Figure 2.1 illustrates the two modes of operation of these instruments namely that of streak mode and framing mode. Figure 2.1a demonstrates the operation of a streak camera which records ultrashort events in time and one spatial direction (along the length of the input slit), while figure 2.1b demonstrates the application of a framing camera M which can record one or more snapshots of very short duration events (~100ps) with two spatial dimensions.

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Slit Photocathode Phosphor

"Optical events"

Deflector plates

Lens _{Electrostatic lens}

:

Photocathode

Figure 2.1a: Schematic of a Streak Camera

Phosphor

Fusion _Deflector

plates

Electrostatic lens Compensating_plates

Figure 2.1b: Schematic of a Framing Camera

Both types of tube consist of a photocathode (with a spectral response tailored to the operating wavelengths of the mode-locked lasers or ultrashort events that are to be

analysed), a electrostatic leasing section, a deflection region in which deflector plates of differing design and number are enclosed, a drift region and finally an image recording section which sometimes includes a proximity focused microchannel plate (MCP) intensifier in front of a phosphor if high detection sensitivity is required. The outer tube envelope is constructed from glass or ceramic with Nilo flanges sectioning the different tube regions. High vacuum is imperative for avoiding internal discharge, so-called muMpactor discharge during dynamic operation and maintenance of photocathode life.

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2.1.2 Dy n a m i c o p e r a t i o n o f i m a g e t u b e s

The mechanism whereby a streak camera works is basically quite straightforward

(complicated theoretical analyses are, however, necessary to describe the details of the .! streaking process). Light incident at an input slit is focused onto the photocathode causing

the liberation of photoelectrons which are accelerated down the tube. The electrostatic lens

arrangement modulates the electron momenta such that all electron trajectories see an equal 4 time of flight (an analogy with Fermat's Principle which will be discussed later) and are

imaged under static operation at the phosphor screen where the image information is converted back to optical data. The electron-optical image tube is designed to have good leasing characteristics in static mode with high spatial resolution and minimal distortion of image information.

In streak mode as the electrons pass through the deflector plates a voltage ramp is ^ applied which deflects or streaks the electrons along a direction perpendicular to the input

slit length. If we imagine a short optical event, ie) an optical pulse incident on the camera,

then this will generate at the photocathode a distribution of electrons whose number density < along the axis of the tube is linearly proportional to the intensity distribution across the

optical pulse. On streaking at the deflection plates the electron distribution is mapped out spatially on the phosphor, in other words one spatial axis of the image tube has been converted into a temporal axis. The light emanating from the phosphor will be linearly

proportional to the incident electron number density and consequently recording the I intensity of light across the phosphor as a function of distance thus provides a linear record j

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