Winters are cold in Rochester, N.Y. The city sits on the southern shore of Lake Ontario, and tunnels connect buildings at the nearby university to shelter the students from the temperatures.
But the snow, gloomy skies and chill make for great skiing.
Gérard Mourou, Ph.D., was riding a chairlift at a local ski mountain in the 1980s. There wasn’t much to do on a lift in those days, before the cell phone became our newest appendage.
His mind drifted to his work in high-energy physics at the University of Rochester. As a professor and member of the Laboratory for Laser Energetics, he was trying to figure out how to make lasers more powerful.
Mourou’s goal was to develop an ultra-short, high-intensity laser pulse without destroying the equipment used to produce it.
That lift ride produced perhaps his greatest breakthrough.
Rochester, New York skyline
American scientist Theodore Maiman invented the laser in 1960. After that, the power of lasers grew slowly. Scientists struggled to create more intense pulses without the light obliterating the amplifying material.
Sitting on the chairlift, Mourou had a moment of inspiration.
“I picked up my car and went immediately to the lab in Rochester,” Mourou said. “My students were working. I said, ‘Morris, stop everything you are doing – this is what we are going to do.’ ”
Mourou came up with a way to make a wave stretcher using gratings that would become a key component of his and his graduate student Donna Strickland’s idea to amplify short-pulsed lasers. They called it chirped pulse amplification (CPA).
CPA, established in 1985, is a way of amplifying a laser pulse without destroying the amplifying medium. It’s elegantly simple in concept: stretch the beam, amplify it, then compress it.
Stretch
An initial short pulse of light from a laser travels through a pair of diffraction gratings. The gratings send the lower frequency components of the pulse along a shorter path than the higher frequency components. The gratings spread out the pulse’s light in time by a factor of 1,000 to 100,000 times.
That means the diffraction grating decomposes the pulse into all of its constituent colors, and sends the longer wavelengths first and the shorter wavelengths last. It results in a longer pulse duration. This reduces the intensity of the pulse.
Amplify
Now that the pulse is longer and of lower power, it’s safe to amplify without losing any of its signal or information. Once amplified, there is a higher energy pulse, but still long in duration. That way it can pass through its amplifier medium safely.
Compress
This long, amplified, higher energy pulse passes through a second pair of gratings that reverse the work of the first pair. These undo the delay between the longer and shorter-wavelengths of the pulse. The gratings arrange all the wavelengths together into a single pulse of the same shape and length as the original pulse. The gratings recompresses the pulse in time, resulting in an amplified version of the laser pulse’s original state.
The resulting pulse has a much higher amplified power. The intensities would be impossible to achieve safely using direct amplification of the pulse.
Copyright: ©Phil Saunders Graphics/ Project École Polytechnique
The laser light’s frequency changes when scientists stretch the pulse. That produces a chirp, a term coined after the changes in sound frequency over time when a bird makes a chirp. The high frequency components lag behind, resulting in a longer pulse duration and a chirp.
The result of CPA was much stronger, short-pulsed lasers. The strength of lasers grew dramatically once scientists perfected the technique.
Mourou didn’t have a sense of the enormity of his 1985 accomplishment at the time.
“When we started, the stretcher and compressor wasn’t matched, so it was not a great achievement,” he said. “We didn’t know what to do with it. But we knew we had some way to go.”
The first stretcher was a fiber cable. That caused problems.
“In the first CPA we had used fiber for stretching and gratings for compressing,” Mourou said. “If the stretcher and compressor is not perfectly matched, the shape won’t match.”
Mourou and Strickland looked for the perfect stretcher and compressor for a year or so.
On the chairlift that winter day, Mourou’s thoughts drifted to the work of Oscar Martinez, Ph.D. Martinez was a researcher working on a communications application.
“He did exactly the converse of what we were doing,” Mourou said. “We were communicating together.”
The lack of an exact match between the fiber stretcher and gratings compressor interfered with the output.
“That was a big problem. I solved it by using a stretcher of two gratings, the same thing as the compressor. It took a year or two to think about it. The fiber for stretching was easy. The clue was Oscar Martinez. Oscar Martinez wanted to compress this pulse. I realized his compressor was exactly the stretcher I was looking for.”
As a result, Mourou and Strickland began using diffraction gratings for the stretcher that were perfectly matched to the compressor gratings.
Prof. Gérard Mourou and one of his initial diffraction gratings
Mourou and Strickland’s 1985 demonstration of CPA at the University of Rochester was a proof of concept. They based the experiment on fiber for a stretcher and small diffraction gratings for the compressor. That limited the output of the laser pulse intensity and efficiency of the light conversion process.
When Mourou returned to France, he proposed to upgrade what was at the time one of the oldest lasers in the country. In order to make that upgrade, he needed much larger gratings that would produce more power and higher efficiency output. Efficiency is the percentage of energy projected on the gratings that bounces off it.
That produced more problems.
“When you compress the pulses, then you have a huge energy over a very short amount of time,” Olivier Nicolle, Gratings and OEM Director for HORIBA France said. “And you take the risk of damaging the coated grating surface.”
Mourou worked with HORIBA France, then a separate company called Jobin Yvon, to develop larger gratings to produce higher power output and greater efficiencies. Jobin Yvon also developed gold-coated coatings that the compressed beam wouldn’t damage.
The gratings Jobin Yvon and Mourou developed were huge by the day’s standards - 190 mm by 350 mm (7.5 inches by 13.8 inches). These were much larger than the standard 50 mm by 50 mm (two-inch square) commercial grating commonly used. There was a good reason why.
“A 190 by 350 mm grating is about 25 times larger than a 50 by 50 mm grating and can thus compress a much higher energy laser pulse,” Nicolle said. “It allows an increase of energy by 25 times. Additionally, there is a significant amount of energy that is saved using a highly efficient grating design.”
Efficiency is important, because the beam bounces off four gratings when being stretched and then compressed. So any loss of efficiency is increased to the fourth power.
There is a significant amount of energy saved by using higher efficiency grating by an order of magnitude. The intensity, thanks to the size, immediately made the new CPA design a system that could provide the kind of intensity researchers were looking for.
Thirty-three years after the 1985 discovery of CPA, the Nobel committee awarded Mourou and Strickland, Ph.D., a share of the 2018 Nobel Prize in Physics. The dissertation detailing CPA was Strickland’s doctoral thesis and first published paper. They titled the paper Compression of amplified chirped optical pulses, and submitted it for release on July 5, 1985.
By the time they received the Nobel Prize, both had moved on from the University of Rochester.
Mourou joined the University of Michigan in Ann Arbor in 1988, where he founded the Center for Ultrafast Optical Science. He returned to France in 2005 and became director of the Laboratory of Applied Optics at the École Polytechnique. He now works with a three-country European Union project to apply high-energy physics in a number of fields.
Strickland, a Canadian, became a professor of optical physics at the University of Waterloo, in Ontario, Canada. She was the third woman awarded a Nobel prize in Physics.
There’s a small museum in Stockholm, Sweden dedicated to the Nobel Prize. It recognizes the greatest achievements in science.
The Nobel Museum’s directors asked Mourou what he wanted to display to commemorate the discovery of CPA.
Mourou chose one of the original Jobin Yvon (HORIBA) diffraction gratings used for his experiments with larger gratings. It opened the doors to even greater short-pulse intensities and power. The breakthrough was the diffraction grating’s robustness and size within a high short-pulse energy environment.
HORIBA Diffraction grating exhibited at the Nobel Museum
Strickland and Mourou generated pulses that lasted 500 femtoseconds. With that speed, it became possible to study chemical reactions and the activities inside individual atoms.
CPA became the new standard for high-intensity lasers. Since the discovery of CPA, the intensity delivered in a short laser pulse has increased to the petawatt range. One Petawatt equals 1015 watts. And the duration of a pulse has decreased to the femtosecond range, a quadrillionth of a second. It’s a lot of power, but the power lasts ever so briefly.
CPA technology also resulted in more compact laser-producing equipment.
Laser eye surgery became one of the best-known uses of CPA. Yet, it took a freak accident in 1993 to grasp the technology’s potential for the procedure.
It happened at Mourou’s University of Michigan lab in the early 1990s. A student was adjusting a CPA laser without wearing the required protective goggles. The laser shined in the student’s eye. Mourou rushed the student to the university’s Kellogg Eye Center for treatment.
The ophthalmologist examining the student was stunned.
“This is fantastic, he said. “What kind of laser were you using?”
The student said, “It’s a new type of laser. Why do you ask?”
“It’s strange,” the doctor said, “because the damage you have in your eye is perfect.”
The CPA laser caused a flawlessly round injured area on the student’s retina. There was no collateral damage to the surrounding tissue. This peaked the medical center’s interest in the technology, and it eventually invested in it.
"Immediately, we realized we had something,” Mourou said. "The ophthalmologist called me a few days later and said he wanted to work in our group to do femtosecond ophthalmology.”
Femtosecond ophthalmology is laser eye surgery using ultra-short bursts of energy as a laser scalpel that doesn’t damage the surrounding tissue.
That sequence of events eventually led to the development of laser eye surgery. Doctors initially introduced CPA-based lasers for corneal refractive surgery in flap creation during LASIK in 2008, and subsequently for cataract surgery.
Scientists use CPA technology in a variety of settings, including research, industry, medicine, and commercial manufacturing. Almost all the most powerful lasers use CPA. That involves those with more than 100 terawatts of power - 1 trillion watts.
Those applications include:
Mourou is on a campaign to devise ways to change the life span of radioactive waste, the byproduct of nuclear fission. He told The Conversation, a French scientific journal:
“Take the nucleus of an atom. It is made up of protons and neutrons. If we add or take away a neutron, it changes absolutely everything. It is no longer the same atom, and its properties will completely change. The lifespan of nuclear waste is fundamentally changed, and we could cut this from a million years to 30 minutes.”
“We are already able to irradiate large quantities of material in one go with a high-power laser, so the technique is perfectly applicable and, in theory, nothing prevents us from scaling it up to an industrial level. This is the project that I am launching in partnership with the Alternative Energies and Atomic Energy Commission, or CEA, in France. We think that in 10 or 15 years' time we will have something we can demonstrate. This is what really allows me to dream, thinking of all the future applications of our invention."
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