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Optical frequency combs span new wavelengths

02 Jun 2008

Optical frequency combs are breaking new ground in applications ranging from precision timing to breath analysis. Marie Freebody speaks to Jun Ye from JILA to find out more.

Jun Ye is a fellow of JILA, the National Institute of Standards and Technology (NIST) and the University of Colorado, US. He currently heads up a research group in JILA that works on precision measurement, ultracold atoms and molecules, and ultrafast science. The group has played a leading role in finding many novel applications for optical frequency combs (OFCs). Most recently, his research group has pioneered the development of optical atomic clocks, generating frequency combs in the far infrared and extreme ultraviolet (XUV) spectral regions, and high-resolution quantum control with a frequency comb.

Can you summarize how optical frequency combs work?
Modelocked lasers produce regular trains of ultrashort pulses of light that can last anywhere from 10 to 100 fs. Corresponding to the train of the phase-coherent optical pulses in the time domain, a wide bandwidth OFC is formed in the spectral domain. Under adequate frequency stabilization, the spectrum consists of narrow spikes at integer multiples of the repetition rate of the laser.

The pulses emitted by a frequency comb laser are not identical – they are the product of a sinusoidal carrier wave and the envelope around the pulse. A small difference exists between the peak of the envelope and the peak of the light wave, which is called the carrier-envelope phase. This phase can vary from pulse to pulse because the carrier wave and the envelope travel at different speeds inside the laser. This pulse-to-pulse change in the carrier-envelope phase causes a small frequency shift in the entire comb in the spectral domain, also known as the carrier-envelope offset frequency.

In the same way as the repetition rate, the comb offset frequency is determined by the specific characteristics of an individual modelocked laser. The measurement of these two parameters makes it possible to completely characterize and subsequently control a frequency comb in terms of both time and frequency.

Why is developing OFCs important?
Phase-coherent links between optical and microwave/radio frequency spectral domains have now been established, which allows one to transfer signal from one spectral area to another without losing coherence. This is invaluable for frequency metrology and precision measurement, as we can now directly measure, control and visualize optical frequencies.

Most stable, precise and accurate timing signals and frequency standards are being developed in the optical domain and thanks to the frequency comb these optical signals are now available in the radio domain. For ultrafast science, the frequency comb-based precise control of the pulse trains, in terms of both the repetition rate and the pulse-to-pulse carrier-envelope phase, allows scientists to perform attosecond metrology, i.e. generation and control of ultrafast phenomena below 1 fs. This has brought some very exciting opportunities to develop better understanding of electron dynamics within an atomic or molecular system.

What are the main applications and when do you expect them to occur?
There are many applications of OFCs besides measuring optical frequencies. The development of optical atomic clocks is one of them. Another powerful application is direct frequency comb spectroscopy pioneered in my lab. This approach uses an OFC to gain information about atomic and molecular structure with a broad bandwidth (the entire forest) and extremely high resolution (individual trees). Subsequent developments by our lab and others have pushed this approach to practical applications, such as breath analysis. We are also using OFCs to achieve exquisite quantum control at very high spectral resolution. This is now having a great impact in merging the active fields of ultrafast science and ultracold matter research.

Extending frequency comb technology across the electromagnetic spectrum is also a focus of intense research. At JILA we have created combs at infrared wavelengths using difference frequency generation. The creation of a frequency comb spanning the microwave through visible is now established. In 2005, teams at JILA and MPQ, Germany, generated precise frequency combs in the XUV using a combination of a near infrared modelocked laser and a high-finesse optical cavity. The cavity enhanced the intensity of the original laser light by nearly a factor of several hundred and allowed high harmonic generation into the XUV without an active amplifier. These short wavelength comb sources will allow scientists to perform novel high-resolution spectroscopy and quantum control of atoms and molecules with coherent XUV light.

What would you say is the most important recent advance?
The most important recent advances are the development of optical atomic clocks, direct frequency comb spectroscopy and XUV frequency combs.

What do you think the next big breakthrough will be?
The key challenges left to overcome are extending the coverage of frequency combs to much longer as well as much shorter wavelengths, and enhancing the power as well as performing spectroscopy work at these extreme wavelengths. In optical metrology we continue to develop more coherent optical local oscillators with the aim of preserving optical phase coherence beyond the 10 s time scale.

• This article originally appeared in the June 2008 issue of Optics & Laser Europe magazine.

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