Yanyi Jiang

Generation of ultrastable microwaves via optical frequency division

Researchers demonstrate a microwave generator based on a high-Q optical resonator and a frequency comb functioning as an optical-to-microwave divider. They generate 10 GHz electrical signals with a fractional frequency instability of ≤8 × 10−16 at 1 s.

Making optical atomic clocks more stable with 10 −16 -level laser stabilization

Scientists demonstrate a cavity-stabilized laser system with a reduced thermal noise floor, exhibiting a fractional frequency instability of 2 × 10−16. They use this system as a stable optical source in an ytterbium optical lattice clock to resolve an ultranarrow 1 Hz linewidth for the 518 THz clock transition. Consistent measurements with a clock instability of 5 × 10−16/√τ are reported.

Ultralow phase noise microwave generation with an Er:fiber-based optical frequency divider

We present an optical frequency divider based on a 200 MHz repetition rate Er:fiber mode-locked laser that, when locked to a stable optical frequency reference, generates microwave signals with absolute phase noise that is equal to or better than cryogenic microwave oscillators. At 1 Hz offset from a 10 GHz carrier, the phase noise is below -100 dBc/Hz, limited by the optical reference. For offset frequencies >10 kHz, the phase noise is shot noise limited at -145 dBc/Hz. An analysis of the contribution of the residual noise from the Er:fiber optical frequency divider is also presented.

A collinear self-referencing set-up for control of the carrier-envelope offset frequency in Ti : sapphire femtosecond laser frequency combs

We demonstrate a simplified set-up for control of the carrier-envelope offset frequency (fceo) in a Ti : sapphire femtosecond laser frequency comb. A periodically poled KTiOPO4 crystal is used for second harmonic generation with zero walk-off angle, which enables collinear propagation of beating beams in the self-referencing set-up. A beat signal with a signal-to-noise ratio of more than 40 dB can be obtained within a 300 kHz bandwidth. Using this signal, fceo could be tracked to a frequency synthesizer within a few millihertz for an averaging time of 1 s, which contributed a relative stability of 8 × 10−18 in the visible region around 532 nm.

Optical frequency comb with an absolute linewidth of 0.6 Hz–1.2 Hz over an octave spectrum

We demonstrate a narrow-linewidth optical frequency comb based on a femtosecond Ti:sapphire laser by precisely phase-locking it to a subhertz-linewidth Nd:YAG laser at 1064 nm. Each comb tooth inherits the phase coherence and frequency stability of the subhertz-linewidth laser. By comparing against other independent narrow-linewidth lasers, we measured the absolute linewidth of the comb teeth to be 0.6 Hz–1.2 Hz over an octave spectrum.

0.26-Hz-linewidth ultrastable lasers at 1557 nm

Narrow-linewidth ultrastable lasers at 1.5 μm are essential in many applications such as coherent transfer of light through fiber and precision spectroscopy. Those applications all rely on the ultimate performance of the lasers. Here we demonstrate two ultrastable lasers at 1557 nm with a most probable linewidth of 0.26 Hz by independently frequency-stabilizing to the resonance of 10-cm-long ultrastable Fabry-Pérot cavities at room temperature. The fractional frequency instability of each laser system is nearly 8 × 10−16 at 1–30 s averaging time, approaching the thermal noise limit of the reference cavities. A remarkable frequency instability of 1 × 10−15 is achieved on the long time scale of 100–4000 s.

Thermal analysis of optical reference cavities for low sensitivity to environmental temperature fluctuations

The temperature stability of optical reference cavities is significant in state-of-the-art ultra-stable narrow-linewidth laser systems. In this paper, the thermal time constant and thermal sensitivity of reference cavities are analyzed when reference cavities respond to environmental perturbations via heat transfer of thermal conduction and thermal radiation separately. The analysis as well as simulation results indicate that a reference cavity enclosed in multiple layers of thermal shields with larger mass, higher thermal capacity and lower emissivity is found to have a larger thermal time constant and thus a smaller sensitivity to environmental temperature perturbations. The design of thermal shields for reference cavities may vary according to experimentally achievable temperature stability and the coefficient of thermal expansion of reference cavities. A temperature fluctuation-induced length instability of reference cavities as low as 6 × 10(-16) on a day timescale can be achieved if a two-layer thermal shield is inserted between a cavity with the coefficient of thermal expansion of 1 × 10(-10) /K and an outer vacuum chamber with temperature fluctuation amplitude of 1 mK and period of 24 hours.

Study on the clock-transition spectrum of cold 171Yb ytterbium atoms

We present a detailed study of the clock-transition spectrum of cold 171Yb ytterbium atoms in a 1D optical lattice. A typical clock-transition spectrum with a carrier-sideband structure is observed. After minimizing the power broadening effect and compensating the stray magnetic field, the carrier linewidth is narrowed to about 16 Hz for a 60 ms interrogation time. By increasing the interrogation time to 150 ms, the linewidth is further reduced to 6.8 Hz. By applying the bias magnetic field parallel to the clock-laser polarization, a two-peak spectrum corresponding to two π transitions is obtained. Finally, spin polarization of atoms to a single desired Zeeman sublevel of the ground state is also demonstrated. The presented results will be very useful for developing better optical lattice clocks.

Coherence transfer of subhertz-linewidth laser light via an 82-km fiber link

We demonstrate optical coherence transfer of subhertz-linewidth laser light through fiber links by actively compensating random fiber phase noise induced by environmental perturbations. The relative linewidth of laser light after transferring through a 32-km urban fiber link is suppressed within 1 mHz (resolution bandwidth limited), and the absolute linewidth of the transferred laser light is less than 0.36 Hz. For an 82-km fiber link, a repeater station is constructed between a 32-km urban fiber and a 50-km spooled fiber to recover the spectral purity. A relative linewidth of 1 mHz is also demonstrated for light transferring through the 82-km cascaded fiber. Such an optical signal distribution network based on repeater stations allows optical coherence and synchronization available over spatially separated places.

A low noise optical frequency synthesizer at 700–990 nm

Optical frequency synthesizers can generate single-frequency laser light with high precision and accuracy at any desired wavelength over a wide optical region. Here, we demonstrate such an optical frequency synthesizer, which yields coherent light at any wavelength within 700–990 nm with more than 500 mW of power. The relative fractional frequency instability and uncertainty between the output light and the reference light of the optical frequency synthesizer are 6 × 10−19 at 1 s averaging time and 2 × 10−21, respectively. This synthesis noise is two orders of magnitude better than the frequency stability and accuracy provided by optical clocks, supporting optical frequency synthesis from the most accurate optical clocks. When the optical frequency synthesizer is referenced to a cavity-stabilized laser at 1064 nm, the output of the optical frequency synthesizer is tested to have an average linewidth of 1 Hz and frequency instability of 1.5 × 10−15 at 1 s, limited by the reference laser.