Frequently Asked Questions

Glossary

Streak Camera

A streak camera is a device to measure the time dependence of very fast and very weak optical signals. Its main component is a vacuum electron tube which integrates:

A photocathode to transform the incident photons into electrons;
Electron optics to accelerate those electrons to an energy of 15 keV (which corresponds to a velocity of more than 260 million km/h or 25% of the speed of light) and focus them on a screen;
A phosphor screen to transform each electron into many visible photons which can then be read on a photographic film or a CCD camera;
A set of deflection plates to apply a linearly increasing voltage while the electrons travel between them. The temporal information contained in the electron pulse (therefore in the incident optical signal) is transformed into spatial information on the screen.

Click here to see an animated schematic.

A 3-dimension image is obtained for a every optical pulse incident on the photocathode. The intensity of the signal collected on the screen corresponds to the intensity of the incident signal, the horizontal and vertical axis correspond respectively to the time and the position along the cathode.

In some applications, one can replace the spatial dimension by a spectral dimension by placing a spectrometer in front of the streak camera.

Lasers

CPA Lasers

Laser systems using the Chirped Pulse Amplification (CPA) technique to amplify femtosecond pulses to Terawatt level.

Chirped Pulse Amplification

The typical characteristics of this type of lasers are given in the following table:

Repetition Rate
(pulses / second) <0.03 10 1000

Energy per pulse 0.5-10 J 50-500 mJ 0.1-1 mJ

pulse duration 300 fs - 1 ps 100-500 fs 50-200 fs

Peak Power 0.5- 50 TW 0.1-5 TW 0.5-20 GW

Repetition Rate (pulses / second)	<0.03	10	1000
Energy per pulse	0.5-10 J	50-500 mJ	0.1-1 mJ
pulse duration	300 fs - 1 ps	100-500 fs	50-200 fs
Peak Power	0.5- 50 TW	0.1-5 TW	0.5-20 GW

Time Units

Short pulse: In this research field, "short" corresponds to time a duration typically of the order of 1 picosecond or shorter.
Picosecond: 1 picosecond (ps) =10^-12 second, or 1 millionth of 1 millionth of a second.

1 picosecond (ps) = 1000 femtosecond (fs)

In 1 picosecond, light travelling at 300 000 km/s, travels only 0.3 mm.
Subpicosecond: A time duration shorter than 1 picosecond and typically longer than 100 femtosecond.
Femtosecond: 1 femtosecond (fs) = 10^-15 second, or 1 millionth of 1 billionth of a second.

Other Units

Energy: Energy is measured in Joules.

1 Joule (J) = 1000 millijoules (mJ)
Power: Power is measured in Watts which corresponds to 1 Joule per second.

1 Terawatt (TW) = 1000 Gigawatt (GW) =10¹² Watts
Intensity: Intensity of light is measured in W/cm². When focussed on a target, a femtosecond laser pulse can produce an intensity up to 10¹⁹ W/cm². As a comparison, the intensity of sunlight received on earth is about 0.1 W/cm².