---
title: "CPA: amplify the energy while the pulse is long"
description: "How did stretching a pulse make a shorter, more energetic output practical?"
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Apparatus · Observation · Inference

# CPA: amplify the energy while the pulse is long

Sandbox Physics · 2026-10-22

How did stretching a pulse make a shorter, more energetic output practical?

1. Broadband seed
2. Dispersive stretching
3. Energy amplification
4. Recompression and pulse measurement

The compressor reverses spectral delay within its usable bandwidth; it does not supply the amplifier’s energy.

## Peak power constrained amplification

A short energetic pulse can drive unwanted nonlinear phase and damage while crossing an amplifier. Strickland and Mourou’s approach spread the pulse in time before extracting more energy, then compensated the chirp afterward. The sequence separated energy gain from the highest peak power.

The corrected 1985 paper reports millijoule-level pulses at 1.06 micrometres with a duration of 2 picoseconds. It did not report today’s petawatt systems. The Rochester institutional history identifies the corrected publication and the work’s later influence.

Sources: [1] [3] [4]

## Stretching rearranges arrival times

A pulse with enough spectral bandwidth can become long when different frequencies acquire different group delays. Strickland’s account describes fibre broadening and stretching, Nd:glass amplification, and grating compression in the original demonstration. Modern CPA architectures need not use the same stretcher.

For a fixed Gaussian intensity pulse shape, energy divided by intensity full-width at half-maximum sets peak power with the factor below. Increasing duration at fixed energy reduces peak power in inverse proportion. This is an illustrative pulse-shape calculation, not the original apparatus fit.

$$
P_{\mathrm{peak}}=\frac{2\sqrt{\ln2}}{\sqrt{\pi}}\frac{\mathcal E}{\tau_{\mathrm{FWHM}}}
$$

Sources: [2] [1]

## Compression must repair the phase

Amplification supplies energy, but it can also reshape the spectrum and add phase. A compressor compensating only one delay term may leave higher-order phase, temporal wings or satellite pulses. A broad spectrum alone is therefore not a measurement of a short pulse.

Spectral phase and a suitable temporal diagnostic are needed to distinguish transform-limited compression from a long chirped output. An intensity autocorrelation also needs an assumed pulse shape or additional reconstruction information to yield a unique duration.

Sources: [2]

## Low peak power does not remove every limit

Gain bandwidth, amplified spontaneous emission, nonlinear phase, damage fluence and compressor efficiency still constrain a CPA system. Compressing at the end moves the highest intensity to optics that must withstand it. Stretching is not unlimited protection against damage.

The 2018 prize recognizes a method for generating intense ultrashort pulses. The original picosecond demonstration, later femtosecond systems and attosecond metrology belong to distinct experimental steps.

Sources: [1] [2]

## Require energy and duration at the output

In the linked model compare the stretched pulse, amplified pulse and recompressed pulse while keeping track of energy. A successful compression claim needs both a temporal result and a spectral-phase explanation. Matching a desired peak power by changing a label is not sufficient.

Sources: [2]

## Explore the related models

Each link states what the existing model can explain and where the historical measurement goes further.

### [Follow the pulse through CPA](https://sandboxphysics.com/labs/chirped-pulse-amplification/)

Compare duration, spectral phase and energy between stages.

This teaching pipeline does not certify damage margins or reproduce the 1985 measurement.

## Sources and reading notes

The corrected paper abstract, author lecture passage and Rochester attribution were read. The full lecture was inaccessible; modern pulse-shape calculations are explicitly separate from reported measurements.

1. [D. Strickland & G. Mourou, Compression of amplified chirped optical pulses, corrected version, Opt.Commun.56,219–221 (1985)](https://www.sciencedirect.com/science/article/pii/0030401885901208)

   Author abstract and correction note read: millijoule pulses, 2 ps, 1.06 micrometres. This version corrects a figure printed in volume55.

2. [D. Strickland, Generating High-Intensity Ultrashort Optical Pulses (2018), original CPA apparatus passage](https://www.nobelprize.org/uploads/2018/10/strickland-lecture.pdf)

   Indexed author passage read: fibre broadening/stretching, Nd:glass amplification and grating compression. Full lecture endpoint returned 403.

3. [University of Rochester LLE, 1985 timeline](https://www.lle.rochester.edu/timeline/1985/)

   Institutional attribution and corrected publication reference read; later petawatt operation is not assigned to the 1985 demonstration.

4. [Royal Swedish Academy of Sciences, Physics Prize 2018: official award announcement](https://www.nobelprize.org/prizes/physics/2018/press-release/)

   Official announcement read for the separate Ashkin biological-tweezer contribution and Strickland–Mourou pulse-generation contribution; experimental details come from the cited original accounts.

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