Versatile femtosecond optical parametric oscillator frequency combs for metrology

This thesis addresses the development of broadly tunable, high repetition rate frequency combs in the mid-IR region. A novel PPKTP crystal design was used to provide phasematching for parametric oscillation and simultaneously give efficient pump+idler sum-frequency generation (SFG). This innovation enabled a fully stabilized idler comb from a 333-MHz femtosecond optical parametric oscillator to be generated in which the carrier envelope offset frequency fCEO together with the repetition frequency fREP were stabilised. This OPO platform was then extended to demonstrate, via harmonic pumping, a fully stabilized 1-GHz OPO frequency comb from a 333-MHz pump laser. Next, an alternative route to a 1-GHz OPO comb was investigated by synchronously pumping an OPO directly with a 1-GHz Ti:sapphire laser. Here the comb was fully stabilized for the signal, idler and pump pulses by using a narrow linewidth CW diode laser developed for the project and whose design is also presented. A further increase in the comb mode spacing was performed with a Fabry-Pérot cavity. A stabilised cavity was used to filter 1.5 m signal pulses from a 333-MHz repetition rate OPO frequency comb to yield a 10-GHz comb. The length of the Fabry-Pérot cavity was dither locked to a single-frequency ECDL and later on directly to the OPO frequency comb. Finally the 333-MHz OPO comb was demonstrated in an optical frequency metrology experiment. The frequency comb mode number and the absolute frequency of a narrow-linewidth CW laser were measured and the performance of the OPO comb was found to be comparable to that of a commercial fibre laser comb used as a benchmark in the experiment

Communications with guaranteed bandwidth and low latency using frequency-referenced multiplexing

Emerging cloud applications such as virtual reality and connected car fleets demand guaranteed connections, as well as low and stable latency, to edge data centres. Currently, user–cloud communications rely on time-scheduled data frames through tree-topology fibre networks, which are incapable of providing guaranteed connections with low or stable latency and cannot be scaled to a larger number of users. Here we show that a frequency-referenced multiplexing method can provide guaranteed bandwidth and low latency for time-critical applications. We use clock and optical frequency synchronization, enabled by frequency comb and signal processing techniques, to provide each user with dedicated optical bandwidth, creating scalable user–cloud upstream communications. As a proof of concept, we demonstrate a frequency-division multiplexing system servicing up to 64 users with an aggregate bandwidth of 160 GHz, exhibiting a data rate of up to 4.3 Gbps per user (240.0 Gbps aggregated capacity considering a 200 GHz wavelength band) with a high receiver sensitivity of –35 dBm

Optical Frequency Comb Generation in Monolithic Microresonators

This thesis presents an entirely novel approach for frequency comb generation based on nonlinear frequency conversion in micrometer sized optical resonators. Here, the comb generation process can be directly described in frequency domain as energy conserving interactions between four photons (four-photon mixing). This process is a result of extremely high light intensities that build up in microresonators with long photon storage times. The thesis is composed of four main parts that answer fundamental questions in the context of microresonator-based frequency comb generation as well as providing insights in the control and possible applications of this type of comb generators

A 920 km optical fiber link for frequency metrology at the 19th decimal place

With residual uncertainties at the 10^-18 level, modern atomic frequency standards constitute extremely precise measurement devices. Besides frequency and time metrology, they provide valuable tools to investigate the validity of Einstein's theory of general relativity, to test a possible time variation of the fundamental constants, and to verify predictions of quantum electrodynamics. Furthermore, applications as diverse as geodesy, satellite navigation, and very long base-line interferometry may benefit from steadily improving precision of both microwave and optical atomic clocks. Clocks ticking at optical frequencies slice time into much finer intervals than microwave clocks and thus provide increased stability. It is expected that this will result in a redefinition of the second in the International System of Units (SI). However, any frequency measurement is based on a comparison to a second, ideally more precise frequency. A single clock, as highly developed as it may be, is useless if it is not accessible for applications. Unfortunately, the most precise optical clocks or frequency standards can not be readily transported. Hence, in order to link the increasing number of world-wide precision laboratories engaged in state-of-the-art optical frequency standards, a suitable infrastructure is of crucial importance. Today, the stabilities of current satellite based dissemination techniques using global satellite navigation systems (such as GPS, GLONASS) or two way satellite time and frequency transfer reach an uncertainty level of 10^-15 after one day of comparison . While this is sufficient for the comparison of most microwave clock systems, the exploitation of the full potential of optical clocks requires more advanced techniques. This work demonstrates that the transmission of an optical carrier phase via telecommunication fiber links can provide a highly accurate means for clock comparisons reaching continental scales: Two 920 km long fibers are used to connect MPQ (Max-Planck- Institut für Quantenoptik, Garching, Germany) and PTB (Physikalisch-Technische Bundesanstalt, Braunschweig, Germany) separated by a geographical distance of 600 km. The fibers run in a cable duct next to a gas pipeline and are actively compensated for fluctuations of their optical path length that lead to frequency offsets via the Doppler effect. Together with specially designed and remotely controllable in-line amplication this enables the transfer of an ultra-stable optical signal across a large part of Germany with a stability of 5 x 10^-15 after one second, reaching 10^-18 after less than 1000 seconds of integration time. Any frequency deviation induced by the transmission can be constrained to be smaller than 4 x 10^-19. As a first application, the fiber link was used to measure the 1S-2S two photon transition frequency in atomic hydrogen at MPQ referenced to PTB's primary Cs-fountain clock (CSF1). Hydrogen allows for precise theoretical analysis and the named transition possesses a narrow natural line width of 1.3 Hz. Hence, this experiment constitutes a very accurate test bed for quantum electrodynamics and has been performed at MPQ with ever increasing accuracy. The latest measurement has reached a level of precision at which satellite-based referencing to a remote primary clock is limiting the experiment. Using the fiber link, a frequency measurement can be carried out directly since the transmission via the optical carrier phase provides orders of magnitude better stability than state-of-the-art microwave clocks. The achieved results demonstrate that high-precision optical frequency dissemination via optical fibers can be employed in real world applications. Embedded in an existing telecommunication network and passing several urban agglomerations the fiber link now permanently connects MPQ and PTB and is operated routinely. It represents far more than a proof-of-principle experiment conducted under optimized laboratory conditions. Rather it constitutes a solution for the topical issue of remote optical clock comparison. This opens a variety of applications in fundamental physics such as tests of general and special relativity as well as quantum electrodynamics. Beyond that, such a link will enable clock-based, relativistic geodesy at the sub-decimeter level. Further applications in navigation, geology, dynamic ocean topography and seismology are currently being discussed. In the future, this link will serve as a backbone of a Europe-wide optical frequency dissemination network

TiSapphire frequency combs

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis. Vita.Includes bibliographical references (p. 179-186).Femtosecond mode-locked lasers are a unique laser technology due to their broad optical bandwidth and potential for linking the optical and radio frequency domains when these lasers are configured as frequency combs. Ti:Sapphire based mode-locked lasers offer considerable advantages over other laser systems by generating both the broadest optical spectrum and highest fundamental pulse repetition rates directly from the laser cavity. Recent advances in laser diode technology have reduced the cost of pump lasers for Ti:Sapphire based frequency combs considerably, and the recent demonstration of direct diode pumping of a narrowband mode-locked Ti:Sapphire laser suggests that Ti:Sapphire frequency combs may finally be ready to make the transition from an indispensible research tool to a wider set of industrial applications. In this thesis, several applications and fundamental properties of Ti:Sapphire based mode-locked lasers are investigated. To enable more widespread use of Ti:Sapphire based frequency combs, a frequency comb based on an octave spanning 1 GHz Ti:Sapphire laser is demonstrated. The I GHz Ti:Sapphire laser is referenced to a methane stabilized HeNe laser, resulting in a frequency comb with a fractional frequency stability of its optical spectrum of 2x1 0-14 on a 20 second timescale. A recently identified frequency comb application is the calibration of astronomical spectrographs to enable detection of Earth-like planets which are orbiting Sun-like stars. In support of this application, a second frequency comb system was constructed which ultimately was characterized by a 51 GHz pulse repetition rate and 12 nm bandwidth centered at 410 nm. This "astro-comb" system was deployed to the Fred Lawrence Whipple Observatory where preliminary results indicate a 40-fold increase in the spectrograph stability due to calibration by the astro-comb. Finally, the stability of the optical pulse train emitted from femtosecond mode-locked lasers is expected to exhibit the lowest phase noise of any oscillator, with theoretical predictions of phase noise levels below -190 dBc for offset frequencies exceeding 1 kHz. A comparison between the pulse trains of two nearly identical mode-locked lasers resulted in a measured timing error of less than 13 attoseconds measured over the entire Nyquist bandwidth.by Andrew John Benedick.Ph.D

Chip-scale optical frequency comb sources for terabit communications

The number of devices connected to the internet and the required data transmission speeds are increasing exponentially. To keep up with this trend, data center interconnects should scale up to provide multi-Tbit/s connectivity. With typical distances from a few kilometers to 100 km, these links require the use of a high number of WDM channels. The associated transceivers should have low cost and footprint. The scalability of the number of channels, however, is still limited by the lack of adequate optical sources. In this book, we investigate novel chip-scale frequency comb generators as multi-wavelength light sources in WDM links. With a holistic model, we estimate the performance of comb-based WDM links, and we compare the transmission performance of different comb generator types, namely a quantum-dash mode-locked laser diode and a microresonator-based Kerr comb generator. We characterize their potential for massively-parallel WDM transmission with various transmission experiments. Combined with photonic integrated circuits, these comb sources offer a path towards highly scalable, compact, and energy-efficient Tbit/s transceivers