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Optomechanics at the Quantum Level

Final Report Summary - OQL (Optomechanics at the Quantum Level)

Microresonators have become an important platform for studying optomechanical effects on a quantum level. They have enabled several important accomplishments [1]. In pursuing this line of research, the group of Tobias Kippenberg (where the fellow is working) made an unexpected discovery in 2007 [1]. It was found microresonators, in part based on their high quality factors and material properties could be used to produce frequency combs. Frequency combs have been acknowledged as a major breakthrough in physics. This is evidenced by the fact that the original discovers were awarded the 2005 Nobel Prize in physics for its discovery. These original experiments took up entire optical tables. Now it appeared this could be done with microresonators ranging in size from a hundred micrometers to a few millimeters in diameter. At first it appeared that this would immediately revolutionize the field. However, in order to have a practical frequency comb it needs to be self-referenced [3] and this requires the frequency comb to be low noise. Most of the early microresonator based frequency combs were too noisy to be useful [4]. Right before the fellow joined the team another discovery was made in the group of Tobias Kippenberg. Modelocking via soliton formation was demonstrated for the first time [5]. This allowed the generation of truly low noise frequency combs and also ultrashort pulses of light in a tiny package. Already having a background in frequency combs the fellow was uniquely position help pursue this new discovery, which is why the fellow changed direction from the originally proposal. The goal was to pursue this avenue of research and make the first self-referenced optical microresonator frequency comb, which was a long standing goal of the field.
An optical frequency comb is created by a train of femtosecond optical pulses, which in the frequency domain creates an equidistant array of frequency markers, and thus the name a frequency comb. Knowing only two parameters allows for the frequency comb to be used as a frequency reference. These parameters are the repetition rate of the pulses and an overall offset frequency. The key difficulty to making a self-reference system is to measure the offset frequency which is a challenging goal.
The first challenge to overcome was to use the soliton pulses from the microresonator to generate a broad enough spectrum to allow for self-referencing via harmonic interferometry [3]. This is traditionally accomplished with high energy laser pulses (>1nJ) and super continuum generation. However, the soliton pulses produced by microresonators are less than a 1 pJ in energy. To overcome this challenge the fellow had to master elements of dispersion management in optical fibers, ultrafast pulse characterization techniques, and chirped pulse amplification. This allowed the pulse energy to be increased 1000 fold and produce a pulse that was 300 fs in duration. During the first period of the grant this challenge was successfully overcome and a spectrum spanning two-thirds of an octave centered at 1550 nm was generated.
That breakthrough paved the way for the work in second period of the grant. The next challenge was to use this spectrum to measure the offset frequency using a 2f-3f harmonic interferometer. To meet this goal, first the coherence of the generated spectrum had to be verified, because this is a difficult thing to achieve with low pulse energies. This was accomplished by optically heterodyning the generated light with two transfer lasers at 1908 nm and 1272 nm. A 2f-3f interferometer also was constructed using the two transfer lasers. After phase locking the 1272 nm laser to the generated frequency comb and phase locking the 1908 nm laser to the 1272 laser via the 2f-3f interferometer it was possibly to directly measure the offset frequency of the microresonator frequency comb. This marks the first time a microresonator based frequency comb has been self-referenced as well as the first measurement of the carrier envelope phase of dissipative temporal cavity soliton. This opens the door to many applications requiring precision optical measurements. For example, referencing the frequency comb to an atomic clock allows it to be used as an absolute frequency reference. Another exciting new application in addition to spectroscopy and telecommunications is the ability to create ultra-low phase noise microwaves, which could vastly outperform existing commercial sources.
References for Report
[1] T.J. Kippenberg and K.J. Vahala, "Cavity Optomechanics: Back-Action at the Mesoscale"
Science 321, 1172 (2008)

[2] P. Del'Haye, et al. Optical frequency comb generation from a monolithic microresonator. Nature 450, 1214-1217 (2007).

[3] S. T. Cundiff and J. Ye, Colloquium: Femtosecond optical frequency combs,
Rev. Mod. Phys. 75, 325 (2003)

[4] T. Herr, .et al., Universal formation dynamics and noise of Kerr-frequency combs in microresonators. Nature Photonics 6,480-487 (2012).
[5] T. Herr, .et al., Universal Dynamics of Kerr Frequency Comb Formation in Microresonators
arXiv:1111.3071 [physics.optics]