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Soliton Kerr Physics in microresonator-bases frequency combs

Final Report Summary - SOLICOMB (Soliton Kerr Physics in microresonator-bases frequency combs)

The advent of optical frequency combs (a source of evenly spaced frequency markers of coherent radiation) has had a tremendous impact on a wide range of applications that require high precision in time and/or frequency. Discovered in 2007 by the host group, microresonator frequency combs triggered a second revolution in metrology by enabling a dramatic miniaturization of frequency comb sources due to the small dimensions of the resonator, giving access to very high repetition rates (1 GHz-1 THz). They are generated by coupling a continuous wave laser to a high quality (Q) factor microresonator with a Kerr nonlinearity which converts an input pump laser to an optical frequency comb via parametric processes. The field has seen significant advances in the last 6 years. It is now possible to generate temporal dissipative solitons in microresonators, thus to obtain ultrashort pulses from microresonators. This success paves the way to generate broadband and entirely coherent combs. IEF proposal (“Soliton Kerr physics in microresonator-based frequency combs”) takes aim at this new threshold to address novel endeavors. The project builds upon the new method developed by the host group of T. J. Kippenberg at EPFL to generate frequency combs based on temporal solitons in optical microresonators. It is proposed to explore soliton physics from crystalline microresonators (Objective 1). Second, employing crystalline microresonators with external broadening technique, it is proposed to achieve a RF-to-Optical link for the first time (Objective 2). Third, using crystalline microresonator and quantum cascade laser (QCL), it is proposed to demonstrate, for the first time, Kerr frequency combs in a new spectral range such as the mid-infrared (mid-IR) (Objective 3).

At the beginning of the project, Objective 1 was devoted to fabricate ultra-high Q factor (>10^8) crystalline microresonators suitable for soliton generation in the near-infrared and mid-IR Kerr comb generation. Using a narrow-linewidth fiber laser at lambda = 1.55 micron, their optical quality factors were measured to be typically in the order of 10^9. We observed Kerr combs and soliton formation in the near-infrared. The fellow developed ultra-high Q magnesium fluoride crystalline resonators suitable for soliton generation in the near-infrared in a reproducible way. Using the same fabrication procedure but with different crystalline fluoride materials, the fellow fabricated ultra-high Q crystalline microresonators that are transparent in the mid-IR window.

The development of ultra-high Q crystalline microresonators suitable for soliton generation in the near-infrared was a prerequisite to Objective 2. To obtain high peak power ultrashort pulses, the fellow and colleagues of the host institute used a microresonator with a low repetition rate of around 14 GHz. After creating a single soliton inside the resonator, they pursued the route to self-referencing via the 2f-3f method using a broadening technique. They showed coherence of the generated spectra by heterodyning reference lasers with the comb at the ends of the spectrum. They measured the pulse repetition rate of 14.09 GHz and the overall offset from zero called the carrier envelope offset frequency. Knowing these two values create a self-referenced optical frequency comb suitable for optical frequency metrology purposes and provides a phase coherent link between the microwave and the optical domain [1]. In terms of soliton dynamics, this constitutes the first measurement of the carrier envelope offset frequency of a temporal dissipative soliton. The above results, obtained in the first year of the fellowship, produced a satellite project that was pursued during the second year of the fellowship. Highlights of this project include all-optical stabilization of a soliton frequency comb in a crystalline microresonator [2].

Early in the development of project, it became obvious that Objective 3 was more challenging than expected. The fellow and colleagues developed an efficient coupling technique based on an optical tapered fiber made out of chalcogenide glass. It enabled that light from a QCL could be evanescently coupled to a crystalline microresonator via a chalcogenide uncoated tapered fiber. They showed that critical coupling was achieved with high ideality, necessary for faithful Q factor measurements, and extending this technique for the first time to the mid-IR. The fellow and colleagues at EPFL studied systematically the optical Q factors of four crystalline materials transparent in the mid-IR window of the alkaline earth metal fluoride XF2 family (where X = Ca, Mg, Ba, Sr). They measured a critical factor of Qc around 1.10^7 of the MgF2 microresonator, a value close to the theoretical limit of multiphonon absorption at this wavelength. They demonstrated that BaF2 and SrF2 microresonators featured ultra-high optical quality factors of Q > 1.4 10^8 at 4.4 micron (more than a ten-fold improvement compared to previous results), exhibiting the highest observed finesse of F around 4.10^4 for any cavity in the mid-IR so far [3].

There are two principal impacts of the project result: First, the achievement of a phase-coherent link from the microwave to the optical domain using a microresonator demonstrate that microresonator-based frequency combs have now the potential to provide accurate and precise absolute optical frequency standards for a wide range of applications in optical frequency metrology, optical atomic clocks, optical frequency synthesis, or low-noise microwave generation by frequency division. Second, results in the mid-IR pave the way to the next generation of ultra-stable sources and ultra-precise spectrometers in the molecular fingerprint region and can further leverage QCL technology, by e.g. enabling injection locked QCL similar to technology developed in the near-IR. Results are a further important step in the replacement of existing bulky and complicated frequency comb sources in the mid-infrared. Despite differences with near-IR, these results prove that the mid-IR region is not limited to the high-Q regime when proper crystalline fluoride materials are used. In addition, combining QCL with mid-IR ultra-high Q crystalline microresonators opens a route for mid-IR Kerr comb or soliton generation.

Figure 1: Mid-IR ultra-high Q factor crystalline microresonators. Measurements for different fluoride crystals of the XF2 family (where X = Ca, Mg, Ba, Sr) prove the possibility of attaining the ultra-high Q regime in the mid-IR (Q >10^8) around 4.5 micron. The lines represent the theoretical multiphonon absorption limit of Q with respect to the wavelength.
The circles represent our experimental values.


[1] J. D. Jost, T. Herr, C. Lecaplain, V. Brasch, M. H. Pfeiffer, T. J. Kippenberg, "Counting the cycles of light using a self-referenced optical microresonator", Optica 2, 706–711 (2015).

[2] J. D. Jost, E. Lucas, T. Herr, C. Lecaplain, V. Brasch, M. H. Pfeiffer, T. J. Kippenberg, "All-optical stabilization of a soliton frequency comb in a crystalline microresonator", Optics Letters 40, 4723 (2015).

[3] C. Lecaplain, C. Javerzac-Galy, M. Gorodetski, T. J. Kippenberg, "Mid-Infrared ultra-high-Q resonators based on fluoride crystalline materials", arXiv preprint arXiv:1603.07305 (2016) (submitted/in review in Nature Materials).