Long-Sheng Ma

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.

Ultrasensitive spectroscopy, the ultrastable lasers, the ultrafast lasers, and the seriously nonlinear fiber: a new alliance for physics and metrology

We now appreciate the fruit of decades of development in the independent fields of ultrasensitive spectroscopy, ultrastable lasers, ultrafast lasers, and nonlinear optics. But a new feature of the past two or three years is the explosion of interconnectedness between these fields, opening remarkable and unexpected progress in each, due to advances in the other fields. For brevity, we here focus mainly on the new possibilities in the field of optical frequency measurement.

A new method to determine the absolute mode number of a mode-locked femtosecond-laser comb used for absolute optical frequency measurements

The generation of optical frequency combs, directly referenced to the SI second, can be used to make measurements of optical frequencies. This provides a supreme method for the realization of the meter. However, an approximate knowledge of the frequency of the radiation is normally needed for such measurements in order to determine the integral order of the comb component used. Such information is usually obtained by prior wavelength measurements of the radiation under study. This paper demonstrates a new method to determine the absolute mode number in optical frequency measurements using mode-locked femtosecond lasers, thus eliminating the need for complementary wavelength measurements. Measurements of the frequency of an iodine-stabilized He-Ne laser at /spl lambda/=633 nm and a Nd:YAG laser at /spl lambda/=532nm are given as examples.

Frequency Uncertainty for Optically Referenced Femtosecond Laser Frequency Combs

We present measurements and analysis of the currently known relative frequency uncertainty of femtosecond laser frequency combs (FLFCs) based on Kerr-lens mode-locked Ti:sapphire lasers. Broadband frequency combs generated directly from the laser oscillator, as well as octave-spanning combs generated with nonlinear optical fiber are compared. The relative frequency uncertainty introduced by an optically referenced FLFC is measured for both its optical and microwave outputs. We find that the relative frequency uncertainty of the optical and microwave outputs of the FLFC can be as low as 8times10-20 and 1.7times10-18, with a confidence level of 95%, respectively. Photo-detection of the optical pulse train introduces a small amount of excess noise, which degrades the stability and subsequent relative frequency uncertainty limit of the microwave output to 2.6times10-17

Ultra-stable optical frequency reference at 1.064 /spl mu/m using a C/sub 2/HD molecular overtone transition

Recently we have developed a new frequency modulation technique for detection of weak resonances of molecules located inside a high finesse cavity. We match the FM sideband frequency to the exact splitting of the cavity free spectral range. With this method we are now detecting saturated molecular overtone transitions with very high sensitivities. The resulting excellent signal-noise ratio for these weak but narrow overtone lines has opened the door for realizing high quality optical frequency references in many spectral regions in the visible and near infrared. Here we demonstrate its potential by presenting our recent frequency stabilization result of locking a diode-laser-pumped Nd:YAG laser to the P(5) line of the HCCD (/spl nu//sub 2/+3/spl nu//sub 3/) band at 1064 nm.

First international comparison of femtosecond laser combs at the International Bureau of Weights and Measures

The first international comparison of femtosecond laser combs has been carried out at the International Bureau of Weights and Measures (BIPM). Three comb systems were involved: BIPM-C1 and BIPM-C2 from the BIPM and ECNU-C1 from the East China Normal University (ECNU). The agreement among the three combs was found to be on the subhertz level in the vicinity of 563 THz. A frequency difference measurement scheme was demonstrated that is suitable for general comb comparisons.

International comparisons of femtosecond laser frequency combs

Two types of international comparisons of femtosecond laser frequency combs have been performed in France and the USA. Five combs were involved in the comparisons. Three combs, of which two are transportable, employ nonlinear photonic crystal fiber (PCF) to obtain a wide spectrum covering a full optical octave. The other two are based on broadband femtosecond lasers and require no PCF. The comparisons were performed by counting the optical heterodyne beats between pairs of combs. The frequency agreement among three combs was at the subhertz level in the 563 THz part of the comb spectrum when the combs were referenced to a hydrogen maser. When the combs were referenced to an optical standard, the frequency agreement among four combs was much improved and found to be at the /spl sim/10/sup -19/ level in the spectral range of 333-473 THz. The fact that this result is obtained by five independent measurement systems (combs) strengthens the conclusion that no systematic effects are present at this level.

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.

Direct measurement of the absolute frequency of the international reference laser BIPM4

Direct phase-coherent measurements of the absolute frequency of the 11-5, R(127) transition in 127I2 at 633 nm of the BIPM4 He–Ne laser have been made. This laser has been used as a reference laser for a large number of international comparisons over the years and, as such, has served as the connecting point for the national implementation of the SI metre in the member states of the Metre Convention. The frequency value found for the f-component was 473 612 353 605.4 kHz with a combined uncertainty of 1.8 kHz, a value in close agreement with the value adopted for this radiation in the list of recommended radiations for the realization of the metre, which is 473 612 353 604 kHz.

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.