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Frequency Combs and the Precise Control of Ultrafast Pulses

Optical waveform generation and control is an exciting research area in laser physics. The development of radio-frequency waveform generators during the early part of the 20th century helped lay the foundation for the global electronics industry. The development ofoptical waveform generators promises at least a thousandfold improvement in the speed and processing capacity of today’s best electronic systems. New optical devices will produce designer optical signals that are both high fidelity and coherent. Optical signal processing will rapidly find applications in high-capacity and covert communications, surveillance, remote sensing, high-fidelity data generation and transmission, as well as high-speed computing. In scientific research, optical signal processing will lead to improvements in spectroscopy, precision quantum control, and the control of ultrafast processes in atoms and molecules as well as chemical reactions.

The Steve Cundiff group uses high-resolution, frequency comb-based control for research on arbitrary optical waveform generation. The quest for arbitrary optical waveform generation is daunting. Femtosecond combs produce upwards of four million comb lines, or colors. In arbitrary optical waveform generation, it is necessary to control the shape of each pulse in a stream of pulses with line-by-line pulse shaping. The group has demonstrated how the process works in the laboratory with repeated patterns of a small number of comb lines. However, its goal is to extend their technique to all four million comb lines and meet the significant challenge of achieving computer control of each pixel as simple patterns are created and evolve. Thus far, the group has achieved the highest reported resolution of line-by-line pulse shaping. As their work progresses, however, the researchers must address fundamental tradeoffs between response time and waveform fidelity as they work to optimize their pulse-shaping technology.

Frequency Combs

Optical frequency combs are at the heart of research into optical waveform generation. Their invention led to the awarding of the 2005 Nobel Prize in Physics to long-time JILAn John (Jan) L. Hall and his colleague Ted Hänsch of the Max Planck Institute for Quantum Optics. Thanks to the work of Hänsch, Hall, Steve Cundiff, Jun Ye, and their colleagues, it is now possible to build lasers with extremely sharp colors. The frequency comb technique makes it possible to precisely measure the frequency of literally millions of colors of light. Initially, frequency comb technology became the foundation for precision optical atomic clocks. Today, it is transforming radiowave- and microwave-based technologies throughout the world.

The Ye group demonstrated the first direct frequency-comb spectroscopy with a femtosecond optical frequency comb. The group then expanded its research on coherent spectroscopy by extending the precise control of femtosecond combs into mid-IR and XUV regions of the electromagnetic spectrum. Ye and his colleagues are investigating frequency comb-based spectroscopy across wider regions of the spectrum with the goal of achieving high-resolution quantum control of atomic and molecular states. Their coherent spectroscopic techniques also promise important applications in homeland security, atmospheric research (for the monitoring of greenhouse gases), and medical analysis.

The Ye group continues to push the frontiers of frequency-comb precision control to longer and shorter wavelength radiation in the electromagnetic spectrum. Initially, frequency combs operated only in the visible and near-infrared regions. Starting in 2005 and continuing through 2009, however, the group demonstrated and improved a vacuum ultraviolet (VUV) comb. Then in 2012, the group achieved its long-range goal of building an even shorter-wavelength extreme ultraviolet (XUV) frequency comb. The wavelengths of XUV colors range from about 120 nm down to about 50 nm. “Seeing” these colors requires special laboratory instruments. With them, the XUV comb has opened up entire new vistas of research. For instance, the first XUV ruler allowed Ye group to precisely measure electronic transitions between different quantum states in argon and neon atoms. It also opened the door to exploring the internal quantum states of many different atoms and molecules.

In 2014, the Ye group built a second XUV frequency comb and figured out how to use the two devices together to produce a signal observable with laboratory electronics. With this setup, researchers were able to prove that the two rulers had extraordinarily long phase-coherence time. 

Like their optical counterparst, XUV frequency combs produce a spectrum consisting of a series of equally spaced "teeth," which are like tics on a ruler, but much closer together than the teeth in an optical frequency comb. Since no person or machine can “see” XUV frequency-comb teeth, XUV comb teeth are detected via “beating” together two combs, resulting in precisely and evenly spaced electronic “beat notes” observable with laboratory electronics. The beat notes occur when the two lasers are slightly offset from one another in frequency, but overlapped in space and time. The lasers are then detected simultaneously, which causes the beat notes to appear. The pair of precision XUV combs will be able to make precision measurements of atomic nuclei, atoms, charged ions, and simple molecules. 

The Ye group has also developed a new laser that shifts light from near-IR frequencies to the mid-IR (3–5 µm) range. The laser includes an optical parametric oscillator (OPO) in a new IR cavity. By combining the OPO-based laser with Fourier-transform infrared spectroscopy (FTIR), the group was able to devise a system that works much faster than conventional FTIR spectrometers. The new technique can identify and measure the concentrations of many molecules in less than one minute—as compared to hours.  The speed is crucial for many practical applications, including molecular fingerprinting.

The Ye group is working on extending the reach of frequency combs to wavelengths centered around 10 µm, an important region for molecular fingerprinting. The group is also working on modifying the equipment to make it portable—a key development for such practical applications as airborne or satellite-based chemical analysis of the atmosphere. JILA’s chemical physicists are looking forward to both developments, which will give them new tools for their investigations.

 Quantum Limits of a Frequency Comb

There are still some interesting questions to answer about frequency combs. One is understanding the quantum mechanical limits that determine how narrow comb lines can be. In a modelocked laser, for example, photons can be spontaneously emitted into all the modes of a titanium:sapphire laser. To understand the effect of the spontaneous emissions, the Cundiff group developed a method to characterize the behavior of an ultrafast pulse in response to "tickling" the pump laser with small power changes. Using this characterization, the group was able to predict the quantum-limited linewidth of the comb. Researchers were also able to predict the performance of an optical atomic clock that used the same laser as the gear work connecting optical and microwave frequencies.

The Cundiff group recently worked on a similar study of modelocked fiber lasers. The new study required the development of a new method to tickle the  fiber laser. This method  involved injecting a continuous wave laser into the fiber laser to modify the fiber laser's gain. The researchers then measured the response of the fiber laser and were able to determine the coupling between the pulse energy, frequency, pulse time, and phase. With this kind of information, it is possible to predict the uncertainty of frequency measurements made with the exact same fiber laser. 

The Search for an eEDM

In an exciting collaboration with Eric Cornell, the Ye group is applying its expertise in comb-based spectroscopy to the search for the elusive electron electric dipole moment (eEDM). An electric dipole moment is a measure of the separation of positive and negative charges in a system. If the electron does have an electric dipole moment, it's a pretty darn small one. So small, in fact, that if the electron were the size of the Earth, its eEDM would only alter the planet’s roundness by less than the width of a virus.

But even a very small eEDM would have large implications for our understanding of fundamental physics. That’s why the Cornell and Ye groups used comb-based spectroscopy to scan for electronic transitions in hafnium fluoride ion (HfF+), identified by Cornell and Bohn as a good candidate for the eEDM search. The comb spectroscopy is so powerful that the collaborating team has already precisely identified these transitions. The team is now developing and testing new technology to perform precision spectroscopy for eEDM.

The new technology uses a trapped ball of ~1000 halfnium fluoride ions (HfF+) that are aligned with a rotating electric field. This procedure causes the axes of all the molecular ions to point in the same direction, while keeping them securely in a trap. This configuration allows the researchers to probe the interaction of an electron inside an ion with the large electric fields inside the ion itself. The identically aligned electrons in all 1000 ions will help to magnify an eEDM signal, if there is one. The experimentalists are now working on further improvements of their apparatus in anticipation of looking for an eEDM at the level predicted by one major theory. The eEDM search is supported by theorist John Bohn, JILA’s itinerant theoretical molecular spectroscopist.

Molecular Fingerprinting and Control

The heart of molecular fingerprinting is frequency comb-based spectroscopy. Comb spectroscopy combines broad bandwidth with a high spectral-resolution frequency comb with ultrahigh detection sensitivity from an optical enhancement cavity. This combination makes it possible to readily identify many individual kinds of molecules in a mixture such as a human breath. Because fundamental molecular vibration frequencies are located in the mid-IR, molecular fingerprinting works most efficiently at those wavelengths. In 2009, the Ye group developed a mid-IR frequency comb that produces broadband radiation with wavelengths tunable between 3 and 5 µm and power levels greater than 1 W. In 2011, the group began work on reaching wavelengths of up to 10 µm.

Unlike in the visible and near-IR, where frequency combs can be generated directly from a laser, the production of a mid-IR comb requires an optical parametric oscillator. In this device, pump photons from a near-IR frequency comb laser are down-converted to pairs of lower energy photons that maintain the comb structure. One challenge in designing the new cavity-enhanced comb-spectroscopy system was getting low-loss mirrors that were highly reflective to mid-IR light. The group spent many hours working with mirror-coating specialists at local Boulder companies to perfect the new mirrors.

While enhancing molecular fingerprinting technology, the Ye group continued working on its many important applications. First, the group has extended its studies of molecular-fingerprinting-based breath analysis by collaborating with Dr. John Repine at the CU Health Sciences Center and researchers at the CU Cancer Center. The medical school departments are providing breath samples from patients or research animals with severe lung diseases such as cancer or emphysema. By comparing the spectra of breath samples from very sick patients or lab animals with those of healthy student volunteers, the group plans to demonstrate that its new technology can discriminate between people who are healthy or sick via a simple, noninvasive test. Next, its medical collaborators will launch double-blind studies of the new technology.

Second, the group has worked with a local semiconductor manufacturer on the detection of contaminants (e.g., water or carbon dioxide) in arsine gas used to make laser diodes for laser pointers, CD-ROM drives, and DVDs. Even at very low levels, these contaminants can greatly influence the performance of the diodes. The researchers have already used comb spectroscopy to detect water contamination in the semiconductor process gas. They were able to scan a broad region of the spectrum and zoom in on single water absorption lines. Their job was complicated by the fact that the arsine gas absorbed throughout the region under study, making it very difficult to find the water absorptions and impossible to "see" absorption from carbon dioxide. The results of this study reinforced the group’s goal of developing a mid-IR comb, which should make it much easier to resolve the spectra of individual molecules.

Finally, the group has demonstrated the ultrahigh resolution possible with a comb spectroscopy system by probing cold molecules. The experiment used an erbium-doped fiber laser comb to characterize a supersonic jet expansion of acetylene in argon gas inside an optical enhancement cavity. In this high-resolution broadband system, it is possible to detect all of the acetylene absorption lines at once while also looking closely at how the shape of each line changes. By moving the nozzle in two spatial dimensions, the system can extract different information about the molecules under study and perform 3D mapping of the rotational temperature (basically, how fast the molecules are spinning) and density of the supersonic jet. The system has not only made it easier to perform high-resolution spectroscopy, but also much faster since the comb lines act as thousands of independent detection channels. The independent detection channels allow for the processing of multiple molecular absorption lines in an instant.

These results are so encouraging that the researchers believe this technology could allow them to "see" and measure short-lived intermediates in chemical reactions. The Ye group is now using the new technology to investigate a variety of chemical reaction intermediates. The group also envisions the application of comb-based spectroscopy to the study of cold chemistry and cold collisions, the investigation of greenhouse gases in the atmosphere, and the identification of explosives, toxins, or industrial contaminants.

The Most Stable Clock Laser in the World

A central feature of the Jun Ye group’s optical atomic clock is one of the most stable and “quiet” clock lasers ever built. Since 2004, this laser has evolved to become so stable that it can now detect and measure quantum fluctuations in the optical atomic clock and the group’s new quantum simulation. It is a stunning achievement capping eight years of precision laser development in the Ye labs.

In 2009, Mike Martin launched the most recent effort to build the world’s most stable laser for the Sr-lattice optical atomic clock. He had some serious help:

  1. Nobel Laureate Jan Hall donated about half the space in his quiet, climate-controlled lab deep inside the Ye labs.
  2. Lisheng Chen (a specialist in laser stabilization) came to JILA from China for six months as a visiting scholar. And,
  3. Todd Asnicar, head of JILA’s instrument shop, agreed to help construct the most stable optics table ever built at JILA.

The requirements for the new laser included air filters to purify the air in the lab, a vacuum system to keep the laser cavity at a pressure of 10-8 Torr or less, and a mechanism to isolate and insulate the laser cavity so it could be cooled to about 10 degrees below room temperature and isolated from ordinary lab vibration noise.

In just two months, the new optics table (consisting of an actively stabilized table on top of a regular optics table with short legs) was in place. Because the new optics table could actually sense vibrations and cancel them out, it was (and remains) vibration free. The optics structure and shelving were installed at the same time as the new table. One shelf housed a HEPA filter to keep the air clean around the optical cavity.

The optics structure consists of an optical cavity inside a copper box, which provides a passive temperature shield. The copper box, in turn, sits inside a small aluminum vacuum chamber that, in turn, sits inside a larger aluminum vacuum chamber. The larger chamber can lower the pressure to about 10-3 Torr, and the smaller chamber lowers the pressure to at least 10-8 Torr. The entire optics structure has been able to accommodate different laser cavities whose accuracy and noise were recently evaluated.

Two of the most promising cavities are (1) an optical cavity made of a single crystal of silicon and (2) a cavity designed by the Ye Labs that uses a space made of ultralow expansion glass and mirror substrates made from fused-silica glass. The Ye Labs and researchers from a German national laboratory, the Physikalisch-Technische Bundesanstalt (PTB) are collaborating on tests of the single-crystal silicon cavity. Early results on this cavity looked very promising for the single-crystal cavity, which outperformed two other cavities in initial tests. PTB and JILA scientists are now working to improve the performance of the single-crystal silicon cavity. A new laser using this kind of cavity is due to arrive at JILA in 2013 for use in the Sr-lattice clocks.  

Today, the ultrastable precision clock laser exits a thermos-bottle-like optical cavity at 698 nm on a state-of-the-art optics table enclosed by plastic curtains. It is currently undergoing testing and refinement.

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