D. Seidel

Kerr frequency comb generation in overmoded resonators

We show that scattering-based interaction among nearly degenerate optical modes is the key factor in low threshold generation of Kerr frequency combs in nonlinear optical resonators with small group velocity dispersion (GVD). Mode interaction is capable of producing drastic changes in the local GVD, resulting in either a significant reduction, or an increase, in the oscillation threshold. Furthermore, we show that mode interaction is also responsible for majority of observed optical frequency combs in resonators characterized with large normal GVD. We present results of our numerical simulations together with supporting experimental data.

Phase noise of a high performance OEO and an ultra low noise floor cross-correlation microwave photonic homodyne system

This paper describes two recent types of opto-electronic oscillators. The first is a long fiber opto-electronic oscillator, utilizing a high power laser with long delay, and consisting of low noise components. This oscillator generates a stable 10 GHz signal with phase noise of -163 dBc/Hz at 6 kHz offset from the carrier. The second is a low noise 10 GHz compact opto-electronic oscillator. This latter oscillator consists of coupled optical and microwave loops utilizing a short fiber. We also report on an automatic ultra-low noise floor measurement system, designed and built to measure the phase noise of the above (and other) oscillators. This delay line cross-correlation measurement system utilizes microwave-photonic links, eliminating the need for a second oscillator. This system provides quick and reliable measurement of the oscillator under test.

Whispering-gallery mode based opto-electronic oscillators

We present our recent results related to the development of ultra-compact opto-electronic oscillators based on high-Q whispering gallery mode (WGM) resonators. These oscillators operate in X-, Ku-, and Ka-bands and are able to generate spectrally pure RF signals characterized with less than −120 dBc/Hz phase noise at 100 kHz. The floor of the phase noise (< −140 dBc/Hz) is limited by the shot noise of the signal received at the photodiode. Both tunable and fixed frequency oscillators are demonstrated.

High performance, miniature hyper-parametric microwave photonic oscillator

We report on the realization of a packaged microwave photonic hyperparametric oscillator based on a crystalline whispering gallery mode (WGM) resonator. The oscillator produces spectrally pure microwave signals in the X frequency band, characterized with single sideband phase noise of −120 dBc/Hz at 10 kHz. We show that self-injection locking mechanism of the laser frequency to an optical WGM is vitally important for the steady operation of the device. We theoretically study dynamics for formation of the optical frequency comb generated by the hyperparametric oscillator and show that the oscillation threshold coincides with the threshold of the comb generation.

Resonant Widely Tunable Opto-Electronic Oscillator

We experimentally demonstrate a widely tunable opto-electronic oscillator (TOEO) based on a tunable optical resonant filter and a wideband phase modulator. The oscillator can be tuned in the range of 2-15 GHz, limited by the bandwidth of the RF components utilized in the loop, with a tuning speed greater than 1 GHz/μs. The TOEO, with 220 m fiber link used as the high Q element, is characterized with phase noise less than -100 dBc measured at 10 kHz frequency offset. The agile tunable optical filter in the TOEO is a whispering gallery mode resonator made of lithium tantalate crystal. We develop a theoretical model for the TOEO and find a good agreement between the measurement results and the theoretical predictions.

All-Optical Integrated rubidium Atomic Clock

We report on progress in the development of an All-Optical Integrated Micro-Primary Atomic Clock (AO-IMPAC). The operattion of the clock is based on an inhomogeneously broadened rubidium optical transition used for stabilization of a Kerr optical frequency comb generated in a high-Q whispering gallery mode (WGM) microresonator. We have demonstrated the initial operation of this clock built in a physics package with dimensions 5 × 6 × 1.2 cm. The clocks long term stability approaches 3 × 10−13 at about 2 × 104 s integration time. The measured value is limited by the noise floor of the measurement setup.

All-optical integrated atomic clock

We describe a novel architecture for realization of a miniature all-optical atomic clock. We show theoretically that a hyperparametric oscillator based on a whispering gallery mode resonator can be used as a miniature frequency divider for the clock. Such a frequency divider allows for transferring the long term frequency stability from the optical frequency domain to the RF frequency domain. We present some details for the clock that utilizes the thermally stable isoclinic point of the inhomogeneously broadened D1 line of 87Rb for frequency stabilization, and report on progress towards its experimental realization.

RACE: laser-cooled Rb microgravity clock

RACE is a high performance Rb clock slated to fly on the International Space Station. RACE aims to realize high accuracy and short-term stability. The cold collision shift and multiple launching (juggling) have important implications for the design and the resulting clock accuracy and stability. We present and discuss the double clock design for RACE. This design reduces the noise contributions of the local oscillator and simplifies and enhances an accuracy evaluation of the clock.

On fundamental quantum noises of whispering gallery mode electro-optic modulators

Using the example of whispering gallery mode (WGM) electro-optic modulator (EOM) we show that the majority of phase EOMs, particularly the resonant types, introduce additional quantum noise to the modulated light. The noise power grows quadratically with the optical power and results from the unavoidable spontaneous emission process originating from the strongly nondegenerate parametric interaction. This latter process is the physical basis for modulation.

Laser-cooled microgravity clocks

The principle advantage of microgravity for atomic clocks is interrogation times longer than 1 s. With a 10 s interrogation time, a clock has a 50 mHz linewidth suggesting that accuracies may potentially exceed 10/sup -16/. However, to achieve greater accuracy within the same averaging time, greater stability is needed. Achieving greater stability in a microgravity clock constrains the design differently than for earth based fountains. In this paper, we discuss the design considerations for laser-cooled microgravity clocks highlighting the considerations that differ from those for earth-based fountains. As in earthbased fountains, the frequency shift due to cold collisions plays an important role in the design of the clock. Given our predictions (and measurements) for the shift in laser-cooled Rb clocks, we currently anticipate building a high performance Rb clock and discuss the relative merits of Rb and Cs microgravity clocks. Finally, we present our tentative designs for two microgravity clocks.