Comparing ultrastable lasers at 7 × 10−17 fractional frequency instability through a 2220 km optical fibre network

Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. Here, we report a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10−17 for averaging times between 30 s and 200 s. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks.

Optical Frequency Comb Generation based on Erbium Fiber Lasers

Optical frequency combs have revolutionized optical frequency metrology and are being actively investigated in a number of applications outside of pure optical frequency metrology. For reasons of cost, robustness, performance, and flexibility, the erbium fiber laser frequency comb has emerged as the most commonly used frequency comb system and many different designs of erbium fiber frequency combs have been demonstrated. We review the different approaches taken in the design of erbium fiber frequency combs, including the major building blocks of the underlying mode-locked laser, amplifier, supercontinuum generation and actuators for stabilization of the frequency comb.

Supercontinuum Generation in Nonperiodic Photonic Crystal Fibers and its Application in Frequency Metrology

We generated supercontinuum by utilizing photonic crystal fibers with randomly distributed air holes in the cladding and successfully applied it to optical frequency metrology. Ultra-broad spectra covering a range from the UV to the near infrared were obtained by launching 80 fs Ti: sapphire laser pulses into nonperiodic photonic crystal fibers. In the meanwhile, the experimental results revealed that the distribution of the output spectrum depended on the polarization state of the seeding pulses. Besides, taking advantage of this supercontinuum, we also acquired the clear beat signals by using a new scheme in optical frequency metrology.