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17 Jun 2002
Wilson Sibbett, the father of femtosecond laser physics, talks to Jacqueline Hewett about his quest to generate ultrashort pulses of light and put them to good use.
From Opto & Laser Europe June 2002
When Wilson Sibbett was embarking on his academic career in the mid-1960s, the idea that femtosecond lasers would one day be put to use as precision scalpels for cutting microscopic holes in materials must have seemed far-fetched.
Little did Sibbett realize that some 30 years later he would become the first person to generate femtosecond pulses using Kerr-Lens modelocking (KLM), a technique that has now been adopted worldwide and has earned him international recognition in the laser community.
Sibbett describes the history of ultrashort pulse-generation as resembling a staircase. "You make a
step up and then you consolidate, making incremental developments and thinking up lateral applications,"
he explained. "And then some bright spark makes a quantum leap into the next regime and you go up
again. Today the staircase is still moving. I have every expectation that it will continue to deliver
absolutely fascinating science in the future." Current theories suggest that it might be possible to exploit the polarization
properties of petawatt lasers to enter the zeptosecond regime. Once there, studying the inner workings of
atomic nuclei becomes a real possibility. "It's really starting to become very exciting fundamental
physics," commented Sibbett. "Depending on the spectral region in which we generate the pulses, a
variety of new and different experiments will emerge." During the past 30 years, Sibbett has earned
himself an international reputation in the field of ultrashort pulses, and has built up an enormous bank of
knowledge. Asked if he has ever considered other career avenues, he gives a surprising response. "I was
tempted to go back to farming and in fact did go back for a short time," he admitted. "One of my original
career objectives was to be a science teacher and a farmer as a sort of joint occupation. The farming
instinct didn't leave, but I was persuaded to return to science." Today, lasers that produce ultrashort
pulses are finding applications in biology and materials processing. Most of these applications hinge on
the KLM discovery that Sibbett made in 1990. This gave researchers the ability to generate picosecond
and femtosecond pulses with high average and peak powers over a broad spectral range. At the time, this
was a great improvement on the femtosecond dye lasers that were then commonplace. Sibbett's research into short-pulse continuous-wave dye lasers at Imperial College
continued for 14 years. But there was a problem. These lasers did not have the tunability to access the
wavelengths needed for optical communications. To overcome this problem, Sibbett began exploring
colour-centre lasers working at liquid-helium temperatures and emitting in the telecoms band between 1.4
and 1.6 µm. But as this research continued, other considerations entered the equation. From a
domestic point of view, Sibbett found that his quality of life was suffering in London. He had never
intended to remain in the capital, and moved north in 1985 to continue his research at St Andrews
University in Scotland. In Scotland Sibbett managed to raise the working temperature of his lasers
to match that of liquid nitrogen (77 K), and this achievement transformed his work. For the first time,
Sibbett's research on so-called "coupled-cavity modelocking" and colour-centre lasers was yielding
results that were competitive with those of the leading US research centres, such as the Bell laboratories
in New Jersey. Tempted by the lure of greater bandwidth and tunability, however, Sibbett shifted
the focus of his research to a newer gain medium, Ti:sapphire, and began duplicating some of his
coupled-cavity modelocking experiments that relied on pieces of customized optical fibre. The discovery
that was looming would be revolutionary. "We made the amazing observation that we didn't need
the fibre at all to get the modelocking effect working," explained Sibbett. "We found that the nonlinearity
was in the Ti:sapphire crystal itself." This simplified everything: combining the gain of the crystal and its
inherent nonlinearity yielded modelocking in one resonator working at room temperature. "It was
such a staggering breakthrough that it was almost unbelievable," said Sibbett. "Some people called it
magic because it was just so different." Sibbett and colleagues had witnessed the first generation of
femtosecond pulses using KLM. Today, the optical nonlinearities that give rise to modelocking
can be accessed using tiny, compact lasers. Femtosecond pulses with average powers of 20 mW are
readily achieved by pumping a CR:LiSAF crystal using the everyday red diodes that are found in the
DVD industry. Such a system also has the inherent advantages of portability and self-containment. The
latest compact versions of these diode-pumped femtosecond lasers can be powered with "AA" batteries
and operate at a total input electrical energy of just 1 W. Sibbett has plans to use this sort of
system to mimic the UVA and UVB components of sunlight. "We can apply this in areas of oncology
where you might want to look at the damage caused within tissue in that part of the spectrum," he
explained. Other medical applications include corneal surgery for vision correction, in which this
compact system could replace conventional excimer laser systems. "We are thinking that the combination of quantum
confinement with various semiconductor structures, and also the combination of photonic band-gap
structures, might enable us to get both optical and optoelectronic hybrid devices that would allow us to
take this technology into the next regime," said Sibbett. If integration can be achieved, this opens the door
for femtosecond networks. Sibbett recalls a time during his days at Imperial College when
someone to whom he was introduced asked him: "I believe you work with short pulses?" Wanting to
impress, he replied that pulses of around 100 fs were now feasible. His enquirer's response was
somewhat unexpected: "That's a bit long for us. What would be really nice would be if you could get
down to 10_38 s. That gets you into some of the theories that relate to the big
bang." Even if light pulses never reach that milestone, one thing is for sure: as long as physicists'
fascination with ever-shorter pulses continues, there will be more breakthroughs, leading to more
chapters of physics textbooks being rewritten.
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