Illustrated Physics
physics, drawn
Concept

Chirped pulse amplification — where it appears

A way of amplifying ultrashort laser pulses by stretching each into a long chirp, amplifying it at low peak power, and compressing it back with opposite dispersion. It avoids damaging the amplifier and drives every petawatt laser, winning the 2018 Nobel Prize in physics.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Colours that leave a grating on different roads. A pair of parallel diffraction gratings with 1,200 lines per millimetre and light arriving at 30°, drawn to scale for three wavelengths, 760, 800 and 840 nm. The first grating sends each colour off at its own angle — 24.3° for 760 nm, 27.4° for 800 nm, 30.5° for 840 nm — and the second turns them all back parallel to the input, side by side. Between the gratings the red travels the longest road: for gratings 10 mm apart the slant paths are 10.97 mm, 11.26 mm, 11.61 mm. Measured to a common wavefront after the second grating, the longer road still wins, so red arrives last: the geometry has given the pulse the opposite ordering to the one glass gives it.

The dispersion made of angles

Glass delays blue light more than red, so a short pulse that passes through glass comes out stretched with its red end first. At the wavelengths where most short-pulse lasers work, no transparent glass does the opposite. Two diffraction gratings do it with no material at all. The first spreads the colours in angle, the second turns them back parallel, and in between the red has travelled the longer road. A spread in angle, however it is made, always adds this kind of dispersion, of the sign glass cannot give. It is how every high-power ultrafast laser squeezes its pulse back after amplifying it.

optics · Dispersion The long pulse that arrives as a spike. The output of the matched filter for the chirp, in decibels against delay in units of 1/B, beside the same filter's output for an unmodulated pulse of the same duration (dashed). The plain pulse's echo is a broad triangle 100 units wide; the chirp's is a spike 0.878/B wide at its half-power points — compressed by the time-bandwidth product, 50. The compression is not free of shape: the spike has sidelobes, the first at −13.6 dB, from the abrupt edges of the band, and a weak echo sitting in a strong echo's sidelobe can be hidden by it. The energy is all there — the filter has concentrated the whole pulse's echo into one instant of output.

The long pulse that arrives as a spike

A radar that wants to see far must send a lot of energy, and a transmitter limited in peak power can only do that with a long pulse; a radar that wants to tell close targets apart needs a short one. A chirp — a pulse whose pitch rises steadily from start to finish — is both. Received through a filter that delays its low notes more than its high ones, every part of the echo arrives at the same instant, and fifty microseconds of pulse leave the receiver as a spike one microsecond wide. Resolution turns out to belong to bandwidth, not to duration, and the price of the trick is paid in sidelobes and in a target's speed being mistaken for its range.

waves · Wave packets

Named alongside it

The objects these essays reach for when they reach for this one.

Angular dispersionBandwidthCorrelationDiffraction gratingDispersionGroup delay dispersionGroup velocityPrismPulse front tiltRadarResolutionSidelobes

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