John Hall

Building on JILA’s Legacy of Laser Precision

Steven Burrows — Fri, 12 Jan 2024 18:39:52 +0000

Building on JILA’s Legacy of Laser Precision Steven Burrows Fri, 01/12/2024 - 11:39

Kenna Hughes-Castleberry / JILA Science Communicator

A schematic of a laser going through an AOM, which sends sound waves into a silicon cavity. Image Credit: Kenna Hughes-Castleberry / JILA

Within atomic and laser physics communities, scientist John “Jan” Hall is a key figure in the history of laser frequency stabilization and precision measurement using lasers. Hall's work revolved around understanding and manipulating stable lasers in ways that were revolutionary for their time. His work laid a technical foundation for measuring a tiny fractional distance change brought by a passing gravitational wave. His work in laser arrays awarded him the Nobel Prize in Physics in 2005.

Building on this foundation, JILA and NIST Fellow Jun Ye and his team embarked on an ambitious journey to push the boundaries of precision measurement even further. This time, their focus turned to a specialized technique known as the Pound-Drever-Hall (PDH) method (developed by scientists R. V. Pound, Ronald Drever, and Jan Hall himself), which plays a large role in precision optical interferometry and laser frequency stabilization.

While physicists have used the PDH method for decades in ensuring their laser frequency is stably “locked” to an artificial or quantum reference, a limitation arising from the frequency modulation process itself, called residual amplitude modulation (RAM), can still affect the stability and accuracy of the laser’s measurements.

In a new Optica paper, Ye’s team, working with JILA electronic staff member Ivan Ryger and Hall, describe implementing a new approach for the PDH method, reducing RAM to never-before-seen minimal levels while simultaneously making the system more robust and simpler.

As the PDH technique is implemented in various experiments, from gravitational wave interferometers to optical clocks, improving it further offers advancements to a range of scientific fields.

A Dive into Laser “Locking”

Since its publication in 1983, the PDH method has been cited and utilized thousands of times. “Setting up a PDH lock is something you might learn in an undergraduate lab course; that's just how central it is doing all the experiments we do in atomic physics,” explained recent Ph.D. graduate Dhruv Kedar, the paper’s co-first author.

The PDH method uses a frequency modulation approach to precisely measure the laser frequency or phase fluctuations. The frequency modulation adds special “sidebands” (additional light signals) around a main light beam, known as the “carrier.” Comparison of these sidebands against the main carrier helps measure any slight changes in the frequency, or phase, of the main light beam relative to a reference. This technique is especially useful because it's very sensitive and can reduce unwanted noise and errors.

Physicists can then use these combined light beams to interrogate different environments, such as an optical cavity made of mirrors. To do this, the researchers must “lock” the laser to the cavity, that is, have the laser probe the cavity at a particular frequency.

“What that means is that you're trying to lock your laser to the center of your resonance,” Kedar added. This allows the laser to reach state-of-the-art levels of stability, which is especially important when trying to tease out tiny changes in the optical length or when monitoring quantum dynamics, such as energy shifts or spin changes in atoms and molecules.

Unfortunately, “locking” a laser doesn’t always mean it stays stable or “in resonance with the center of the optical cavity, as noise like RAM can change the relative offsets of the reference light beams and introduces frequency shift,” co-first author and JILA Postdoc Zhibin Yao elaborated. “The RAM can contaminate your PDH error signal.”

As the JILA researchers quickly realized, along with the rest of the laser physics community, reducing this RAM is crucial for improving the stability of the PDH technique and, in turn, their laser measurements. Overcoming the RAM problem has been a long struggle, but the new approach would make the fight much easier.

Reducing RAM via EOMs and AOMs

The two-reference-light "sidebands" are essential for the PDH locking method. To generate the ‘sidebands,’ the JILA researchers needed to use a frequency modulator, either an electro-optic modulator (EOM) or an acousto-optic modulator (AOM).

Historically, EOMs have been employed in various optical systems by applying electric fields to optical crystals to change the phase of laser light coming through the crystal. When an electric field is applied to certain types of crystals, it modulates the laser phase by altering the crystal's refractive index. This process allows EOMs to add sidebands to the carrier beam easily.

However, the effective phase modulation of the crystal used in EOMs is easily altered by environmental fluctuations, introducing RAM into the PDH error signal and, consequently, making it less stable. In contexts where ultra-high precision is required, such as running an optical timescale or operating an atomic clock, even minuscule amounts of RAM can introduce fluctuations at undesired levels.

“EOMs add sidebands to the carrier laser in the optical domain, which is more challenging for us to control,” Kedar explained. “So instead, we can try to generate these sidebands in the electronic domain and translate them to the optical by using an AOM.”

AOMs represent a newer approach to reducing RAM by using sound waves to modulate the laser light. When a sound wave propagates through a crystal or a transparent medium, it creates a diffraction pattern that bends the laser light in various amounts. As a light beam passes through this sound wave-altered medium, the variations in refractive index act like a series of tiny prisms, altering the path and, thus, the frequency of the light.

Kedar added, "If you want to control the amplitude of each sideband, you control the amplitude of the main tone that you're generating in the microwave domain via the AOM.” Because the AOM doesn’t modulate the laser frequency based on the electro-optic effect, it produces much less RAM noise than EOM, reducing the overall RAM level of the system. All of the beams coming out of the AOM crystal can be combined in a single optical fiber, and putting all frequency shift beams into a single, common spatial mode profile.

Comparing EOM and AOM

To measure the advantages of this new PDH approach, Kedar, Yao, Ye, and the rest of the team ran an experiment using both the traditional EOM and their improved AOM setup and compared the results. They found that with the AOM, they could reduce the RAM levels from parts per million to a small fraction of parts per million. Equally important, this approach allows much more flexibility in controlling relative strength between the carrier and two sidebands. The AOM advantage is much more obvious when the carrier becomes vanishingly small.

“Instead of parts per million, you can do like 0.2 parts per million, which seems like a small improvement, but that's kind of toeing the line for acceptable levels of RAM for us,” Kedar elaborated. “Even though this RAM level is so small, it's still a significant roadblock to improving our cavities and making them slightly better. That extra factor of two or three is enormously helpful in pushing the frontiers of state-of-the-art laser stabilization.”

Expanding on the Legacy

The simple implementation of AOM instead of EOM suggests an answer even Hall would be proud of. “It's simple enough that, in principle, someone can look at this scheme and see it as a natural method to interrogate a spectral feature,” Kedar elaborated. “In the end, this speaks to the research style that Jan and Jun both create: a very elegant, simple solution.”

In a new Optica paper, Ye’s team, working with JILA electronic staff member Ivan Ryger and John "Jan" Hall, describe implementing a new approach for the PDH method, reducing RAM to never-before-seen minimal levels while simultaneously making the system more robust and simpler.

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Celebrating 60 Years of JILA

Steven Burrows — Tue, 12 Jul 2022 19:46:03 +0000

Celebrating 60 Years of JILA Steven Burrows Tue, 07/12/2022 - 13:46

Kenna Hughes-Castleberry / JILA Science Communicator

Groundbreaking ceremony for the new JILA laboratory wing and the 10-story office tower, February 25, 1965 (l-r) Lewis Branscomb, Chair of JILA; Donald Hornig, Science Advisor to President Lyndon Johnson; Joseph Smiley, CU President, and Robert Huntoon, Director of the Institute for Basic Standards at NBS. Credit: University of Colorado Publications Service

This year, JILA celebrates its 60th anniversary. Officially established on April 13, 1962, as a joint institution between the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), JILA has become a world leader in physics research. Its rich history includes three Nobel laureates, groundbreaking work in laser development, atomic clocks, underlying dedication to precision measurement, and even competitive sports leagues. The process of creating this science goliath was not always straightforward and took the dedication and hard work of many individuals.

The idea for JILA came from a 1958 meeting of the International Astronomical Union in Moscow. Dr. Lewis Branscombe, a founding member and the head of the atomic physics department of the National Bureau of Standards (NBS, which would later become NIST) proposed an institution for laboratory astrophysics to co-founder, and professor of astrophysics at ��Ҵ�ý�ƽ��, Dr. Richard Thomas. As Branscombe was directly funded by the government, and Thomas by the university, they realized that the best option for such an institution would be a joint establishment between the two entities. Together with the third founding member, Dr. Michael Seaton, a theorist at University College London, they toured nine universities in 1960 and 1961 to find a suitable home for the institution. Finally, the trio settled on ��Ҵ�ý�ƽ�� as the location for their new institution. This was in part due to the President of the university at the time, Quigg Newton, who was supportive of their cause.

In April of 1962, JILA was founded, standing for the Joint Institute of Laboratory Astrophysics. Laboratory astrophysics was of particular interest to the International Astronomical Union as it focused on topics ranging from studying the Sun’s visible light spectrum to developing retroreflecting mirrors.

Aerial view of the newly completed JILA tower situated on the University of Colorado at Boulder campus, 1967. Credit: University of Colorado Publications Service

Trying to find a building on the campus to house JILA, ��Ҵ�ý�ƽ��'s Chief Financial Officer Leo Hill worked with both the NBS and National Science Foundation to pay rent for two floors of the old State Armory building. The NBS also provided funds for laboratory equipment. JILA began construction for its own building shortly after, with the first part, the B-wing, completed in 1966, and the JILA tower finished in 1967. JILA added two more wings to its building, the S-wing (dedicated in 1988), and the X-wing in 2011. There are plans for further expansion with a Y-wing to be built, but nothing is currently in process.

Setting up in the Old Armory building, the JILA scientists (by the early 1960s there were seven scientists at JILA) established several rules that would help JILA function properly. These rules centered around leadership, funding, and fellowships. It was negotiated that with JILA's creation, the NBS would provide instruments and laboratories, while ��Ҵ�ý�ƽ�� would provide researchers and land for the institution. With its unique agreements and roles, JILA’s institute was relatively free to make its own way scientifically. In 1961, ��Ҵ�ý�ƽ��'s Board of Regents approved the title of professor adjoint for any NBS faculty who taught classes at the University. This further solidified the connection between the university and the NBS and made it easier for JILA to attract new scientists.

One of these scientists was Dr. John “Jan” Hall, who was an expert in laser systems and who had previously worked at the NBS location in Washington DC. Though JILA was created during the height of the space race, with the idea being to help the U.S. win this race, Hall helped move JILA in a new direction with laser development. JILA still had ties to astrophysics and astronomy, such as developing lunar lasers for the space race, but the times were changing, and JILA was shifting its research focus to other topics.

Close up of entrance to the old State Armory Building, JILA’s first home on the University of Colorado campus. Credit: JILA

By the late 1960s into the 1970s, JILA's fields were expanding to include laser physics, atomic physics, and others. Hall, at the helm of this shift, helped develop the first high-precision lasers at JILA. His work on these systems would later garner him a Nobel Prize in Physics in 2005.

The 1970s brought a deeper sense of community within JILA, as it was described as a “fun, fast, and free-spirited place.” It was during this time that, along with rafting or ski trips, JILAns also created their own sports leagues, including softball and volleyball. In 1974, JILA elected its first female chair, Katharine Gebbie. Gebbie would later move over to NIST and become their Chief of Quantum Physics Division in 1988, but before she did, she helped recruit and support other female JILA Fellows in JILA. The fields of study within the institution also diversified, as in 1977, the NBS changed the name of their JILA division to the “Quantum Physics Division,” predicting the role that quantum physics would play in JILA'S future.

In the 1980s, JILA was beginning to modernize with the help of the early internet. Thanks to JILA fellow Judah Levine and colleagues the Automated Computer Time Service was brought online, accessible through dial-up modems. This was a monumental first step in modernizing time transfer, as users had access to atomic clock time. By 1988, JILA’s population consisted of more than 200 people, including 23 Fellows. It was also the year that the National Bureau of Standards (NBS)became the National Institute of Standards and Technology (NIST), changing its infrastructure and goals.

More breakthroughs occurred in the 1990s, as JILA once more shifted its mission to reflect NIST's mandate for developing precision measurement, and educating graduate students in future technology. In 1994, JILA had become more than its previous name implied, and dropped the definition of its acronym as the Joint Institute of Laboratory Astrophysics in acknowledgement of the broader scope of science conducted there. In 1995, Nobel-prize winning research was performed by JILA Fellows Carl Weiman and Eric Cornell, as they discovered the Bose-Einstein-Condensate (BEC), a special state of matter helpful for studying quantum dynamics. Nineteen ninety-six brought the 500th Fellows’ meeting, as well as diversity initiatives to make the community more inclusive.

JILA scientists studying in the library on the 10th floor of the JILA tower. Credit: JILA

The 1990s was also an important decade for laser physics at JILA. By 1997, JILA identified seven fields of physics that researchers were studying: atomic physics, chemical physics, materials physics, optical physics, molecular physics, precision measurement, and astrophysics. Laser physics was an underlying study in many of these fields. In 1999, JILA Fellows Margaret Murnane and her husband Henry Kapteyn created what was then the fastest tabletop laser system. That same year, Fellows Jan Hall and Jun Ye developed the first optical frequency comb laser, a tool used by researchers to study optical physics. With these important developments, JILA was quickly establishing a reputation as a world leader in physics research. This reputation boosted JILA's success, as, by the late 1990s, the institution was producing 5–10% of the nation's new Ph.D. graduates in atomic, molecular, and optical (AMO) physics.

The success continued into the 2000s, as the decade brought three Nobel Prizes to JILA. In 2001, Eric Cornell and Carl Weiman were awarded the Nobel Prize in Physics for their work in 1995 on the BEC. The State of Colorado established March 6th as “Carl Weinman and Eric Cornell day” to honor the scientists. A few years later in 2005, Jan Hall also received the Nobel Prize in Physics for his work on laser systems and for developing the first optical frequency comb. JILA also added biophysics as a new field of study, which was helped by the addition of JILA Fellow Thomas Perkins, who worked in this area.

Three JILA Fellows were honored during the 2010s by being selected by then-President Obama to fill important leadership positions within scientific governing groups, including the White House Office of Science and Technology Policy. These Fellows included Carl Weinman, Margaret Murnane, and Carl Lineberger. JILA also celebrated its 50th birthday on April 13th, 2012.

JILA tower (circa 1966) under construction in front of the recently completed laboratory wing, now known as the B-wing of the Duane Physical Laboratories complex. Credit: JILA

Since then, JILA Fellows have received many prestigious scientific awards and grants. The decades of graduate students and postdoctoral researchers who have worked at the institution have gone on to lead successful careers and scientific efforts for other institutions around the world. JILA has also helped spawn many spin-off companies, including 12 companies based in Colorado. These companies range in their products and technology and include companies such as ColdQuanta, Hall Stable Lasers, High Precision Devices, KM Labs, Vescent, to name a few.

With 60 years of scientific research and groundbreaking discoveries, and many successful scientific careers launched, hundreds of lives impacted, it is no surprise that JILA continues to be a global leader in physics research and a pillar within the scientific community. As JILA celebrates its 60th anniversary this year, we look not only to past accomplishments but also to the future, excited to be carrying on such a rich and fulfilling legacy.

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