10 kHz accuracy of an optical frequency reference based on ^12C_2H_2-filled large-core kagome photonic crystal fibers

Abstract

Saturated absorption spectroscopy reveals the narrowest features so far in molecular gas-filled hollow-core photonic crystal fiber. The 48-68 mum core diameter of the kagome-structured fiber used here allows for 8 MHz full-width half-maximum sub-Doppler features, and its wavelength-insensitive transmission is suitable for high-accuracy frequency measurements. A fiber laser is locked to the (12)C2H2 nu(1); + nu(3) P(13) transition inside kagome fiber, and compared with frequency combs based on both a carbon nanotube fiber laser and a Cr:forsterite laser, each of which are referenced to a GPS-disciplined Rb oscillator. The absolute frequency of the measured line center agrees with those measured in power build-up cavities to within 9.3 kHz (1 sigma error), and the fractional frequency instability is less than 1.2 x 10(-11) at 1 s averaging time.

Optical frequency standard using acetylene-filled hollow-core photonic crystal fibers

“…The performance achieved improves previously published results [12,15] for gas-filled HC fibers both in terms of frequency stability and lock-point repeatability, reducing the gap with respect to the performance achieved with bulk glass cells [7].…”

“…Previously published works ascribe this effect to the presence of high order modes (HOMs) [12], surface modes [15] inside the band gap or polarization instability between the pump and probe beams [16]. The mechanism ascribed to the lock-point inaccuracy presented in [12] suffers a lack of universality and it cannot explain some of the effects presented in these paper. This work aims at demonstrating that most of these effects are directly connected to the interaction between different spatial modes inside the fiber.…”

“…During the last decade, I 2 , acetylene and Rb vapour cells have been widely investigated and implemented in laser stabilization using the saturated absorption spectroscopy (SAS) technique [2], often combined with frequency modulated spectroscopy [3][4][5][6][7]. Since the demonstration of the first photonic band gap guidance in air [8], the increased availability of fibers with an hollow core (HC) allowed to investigate the realization of a portable optical frequency reference based on a gas-filled HC fiber [9][10][11][12][13].…”

“…However, developing a portable and user-friendly optical frequency standard is not free of challenges; although fiber gas-filling and other main technical issues have already been characterized [14], targeting the most suitable fiber for this technology is still under debate. State of the art research suggest that kagome fibers produce better performance than more common hollow core photonic crystal fibers (HC-PCF), both in terms of lock-point repeatability and stability [12,15]. Limited transit-time broadening due to larger core size and absence of surface modes are the key-features of the kagome fibers.…”

“…Limited transit-time broadening due to larger core size and absence of surface modes are the key-features of the kagome fibers. On the other hand, they are highly multi-mode, which could create an issue for long term frequency stability, as mentioned in [12].…”