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By the mid-1980s, laser physics had hit a wall. Short laser pulses could not be made much more intense, because the light itself would destroy the amplifier that boosted it. A graduate student and her supervisor found a way around the wall, and their solution now sits inside laser eye surgery and many physics labs. This post covers Donna Strickland's path, how chirped pulse amplification works, and a calculation that shows why it matters.
Who is Donna Strickland?
Her Nobel Prize facts page records that she was born on 27 May 1959 in Guelph, Ontario, Canada. She studied at McMaster University and did her doctoral work at the University of Rochester in the United States, receiving her PhD in 1989. She later worked at Princeton University and, since 1997, at the University of Waterloo in Canada.
In her Nobel autobiography, Strickland recalls that when she told a high school teacher she wanted to take maths and physics, the teacher said they were boys' subjects. She "could not believe she would say such a thing." She chose McMaster's engineering physics programme because one of its streams was about lasers and electro-optics, and graduated in 1981.
Why couldn't lasers simply be made stronger?
The Nobel Committee's popular science background explains the problem. By the mid-1980s, for short pulses, it was no longer practical to increase the intensity of the light without destroying the amplifying material.
The trick is that damage depends on peak power, the energy divided by the time over which it arrives. A short, energetic pulse has an enormous peak power.
How does chirped pulse amplification work?
The method, in the Nobel Committee's words, is to take a short laser pulse, stretch it in time, amplify it, and squeeze it together again. Stretched, the pulse has a much lower peak power, so it can be amplified hugely without harming the amplifier. Compressed again, the same energy arrives in a tiny time and the intensity rises dramatically.
"Chirped" describes the stretched pulse. Strickland explains in her autobiography that chirping means the frequency changes through the pulse, as the pitch changes through a bird's chirp. She credits the idea of stretching by chirping to Gérard Mourou, her supervisor. Her job was to make it work: it took her about a year to build the amplification system.
Worked example: why stretching protects the amplifier
Use round, illustrative numbers. Take a pulse of energy 1 mJ (10^-3 J) lasting 100 fs (100 × 10^-15 s = 10^-13 s). Peak power is roughly energy ÷ duration.
Before stretching: P = 10^-3 ÷ 10^-13 = 10^10 W, or 10 GW.
Stretch it to 100 ps (10^-10 s), 1000 times longer: P = 10^-3 ÷ 10^-10 = 10^7 W, or 10 MW. The amplifier now sees a thousand times less power.
Amplify the stretched pulse to 1 J: P = 1 ÷ 10^-10 = 10^10 W. The amplifier sees 10 GW, the same as the original short pulse, but the pulse now carries a thousand times more energy.
Compress back to 100 fs: P = 1 ÷ 10^-13 = 10^13 W, or 10 TW.
Without stretching, amplifying the short pulse to 1 J would put 10 TW through the amplifier. Common mistakes: forgetting that 1 fs is 10^-15 s and 1 ps is 10^-12 s, and confusing energy (joules) with power (watts).
If unit prefixes and powers of ten slow you down, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise the one idea that is blocking you.
What went wrong along the way?
Real experiments rarely work first time. The Nobel background records that the pulse was to be stretched in a newly acquired 2.5 km optical fibre, but no light came out because the cable had broken somewhere in the middle; 1.4 km had to be enough. Matching the stretcher to the compressor was another challenge. Strickland writes that she had no way to measure the compressed pulses until a colleague, Steve Williamson, brought his streak camera into the lab one night. "CPA worked."
The paper, Compression of amplified chirped optical pulses, appeared in Optics Communications on 1 December 1985. It was her first published paper and, she notes, only three pages long. She also notes it was not highly cited at first; the method spread once titanium-doped sapphire made CPA systems possible in small academic labs.
What came of it?
The Royal Swedish Academy of Sciences awarded half of the 2018 Nobel Prize in Physics jointly to Mourou and Strickland "for their method of generating high-intensity, ultra-short optical pulses". The other half went to Arthur Ashkin for optical tweezers; read about him in our post on Arthur Ashkin and optical tweezers. The press release notes that CPA became standard for later high-intensity lasers, and that its uses include the millions of corrective eye surgeries performed every year.
What can a student take from how Strickland worked?
She chose her field because lasers looked fun, picked a supervisor whose lab excited her, and spent a year getting someone else's idea to actually work, broken fibre and all. She writes that she was simply trying to do one of the world's best PhDs. The lesson is that building and debugging an experiment is real science, not a side task.
Try it now: repeat the worked example for a 2 mJ, 50 fs pulse stretched 2000 times, and find the peak power at each stage.
