Ti:S crystal in an optical frequency comb laser in the Cundiff lab.

Credit: Greg Kuebler

John Hall, Jun Ye, and Steve Cundiff have made JILA a world leader in the development of frequency-stabilized lasers and precision optical frequency metrology (see Optical Frequency Combs ). The researchers use ultrafast lasers, fiber optics, and optical cavities to create highly correlated light pulses. The pulses have a spectrum that consists of an evenly spaced frequency comb of thousands of sharp spectral lines. Each laser pulse consists of a unique carrier (wave)-envelope phase that can be described mathematically in a way that allows scientists to know the frequency of every single comb line with one or two measurements. The researchers are investigating exactly what femtosecond combs look like, the optical processes that create them, and how the combs evolve through a succession of pulses. Research on frequency combs has made it possible to accomplish precise optical frequency measurements in a matter of minutes. The combs also provide a direct bi-directional link between optical and microwave frequencies.

Because precision femtosecond lasers are crucial for precision measurement, Ye and Cundiff continue to collaborate on their development. In addition to a goal of developing the best possible atomic clock oscillators, they want to identify the devices that provide the best stability with the least complexity. In other words, they are working to create new technologies that offer the greatest benefit for the most reasonable cost. One exciting application of ultrafast precision lasers would be for their use in the next generation of GRACE satellites to measure global gravity variations, particularly those associated with ocean heights and climate change. On the research front, researchers are exploring the quantum limit of a frequency comb and extending combs into higher energy regions of the electromagnetic spectrum such as the new XUV comb that covers the extreme ultraviolet region.

In a creative union of precision optical frequency metrology with laser-cooled atoms, James Thompson is exploring ways to use coherence properties of cold atoms to extract more information about these atoms than physicists had previously thought possible.

The Quantum Limit of a Frequency Comb

Optical frequency comb laser used by the Cundiff group to determine the fundamental limit of a comb line.

Credit: Greg Kuebler

The Steve Cundiff group is working with theorist Curtis Menyuk of the University of Maryland Baltimore County to explore the quantum limits of a frequency comb. Their goal was to discover the fundamental limit on the width of a comb line (due to quantum mechanics in the gain medium) from a mode-locked Ti:sapphire laser. This limit is similar to the Schawlow-Townes limit, which defines the quantum-limited linewidth of a continuous-wave laser. However, the nonlinear dynamics in mode-locked lasers complicate the story. The theoretical calculation to answer this question was relatively straightforward, but it required poorly understood parameters, such as the gain and nonlinear optical properties of the titanium sapphire crystal, as inputs.

The Cundiff group designed a set of simple experiments to determine the needed experimental parameters. As part of its efforts, the group developed a new technique for measuring timing and phase dynamics. The technique used frequency combs from two lasers, the first as a reference laser and the second for the study. The lasers were loosely locked, and a beat was generated by interfering the two combs. The researchers perturbed the second laser, and by measuring the beat at both ends of the comb, they were able to determine how much of the change they observed was due to timing and how much to phase dynamics.

By combining their experimental observations with Menyuk’s calculations, the researchers were able to determine that the frequency comb limit due to quantum noise is extremely small — a few mHz in the center of the laser spectrum. In other words, the phase of a comb line with respect to a perfectly stable source would take minutes to drift noticeably. This value is too small to be a limiting factor for current mode-locked laser technology, though it could become more significant in the future as technology improves.

XUV Comb

The Jun Ye group is well on the way to expanding precision "optical" frequency metrology into the ultrahigh-frequency vacuum ultraviolet (VUV) region of the electromagnetic spectrum. The group has just demonstrated a new VUV frequency comb, which is a key milestone in its long-range goal of one day building an X-ray frequency comb (XUV). The new VUV comb is produced by a high-performance, ultrastable precision laser (frequency comb) linked with a femtosecond enhancement cavity.

To create coherent comblike VUV radiation, the fiber frequency comb’s output of a few hundred thousand colors (comb lines) is routed into a passive broadband enhancement cavity. This cavity enhances pulse energies as much as a thousandfold while maintaining the shape of the pulses. The peak intensities of the enhanced pulses can rip electrons out of xenon atoms fed into the cavity by a gas jet. When these electrons recollide with the xenon atoms, short-wavelength VUV radiation is emitted, in a process known as high-harmonic generation, or HHG.

The group recently proved it was possible to get the VUV light out of the cavity by etching a diffraction grating onto the surface of one the cavity mirrors, which cannot reflect or transmit such high frequency light. The grating routs the VUV radiation out of the cavity; at the same time, the mirrors continue to reflect the fiber laser’s light pulses around the cavity. The light exiting the cavity has a wavelength between 50 and 100 nm. Recent experiments have confirmed that the VUV light exiting the cavity is coherent. They’ve also shown that connecting the enhancement cavity to a high-intensity infrared laser also produces coherent VUV light even though this light source can’t ionize the xenon atoms in the cavity. In this case, the VUV light is produced by interference between different two electron excitation pathways and has a somewhat longer wavelength between 150 and 200 nm.

The good news is that the VUV comb lines are close to being narrow enough to begin developing precision spectroscopy experiments. The challenge is to refine the system enough to close this gap. To meet the challenge, the group plans to build two separate VUV combs fed by the same laser that overlap on a photodetector. This system will allow the researchers to investigate and reduce the phase noise of the comb. At the same time, the group plans to purchase a new, higher power laser for the VUV comb. These improvements are expected to make it possible to begin spectroscopy experiments with individual comb lines produced by the enhanced VUV comb. Initial VUV spectroscopy experiments will probe electronic transitions in argon, helium, and carbon atoms, with the goal of controlling the quantum coherence of these transitions.

In the future, X-ray comb-based spectroscopy will not only make it possible to probe structures in atomic nuclei, but also develop a new generation of atomic clocks based on transitions in those nuclei. The future of precision "optical" frequency metrology looks quite promising.

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