Generation and Photonic Guidance of Multi-Octave Optical-Frequency Combs

Abstract

Ultrabroad coherent comb-like optical spectra spanning several octaves are a chief ingredient in the emerging field of attoscience. We demonstrate generation and guidance of a three-octave spectral comb, spanning wavelengths from 325 to 2300 nanometers, in a hydrogen-filled hollow-core photonic crystal fiber. The waveguidance results not from a photonic band gap but from the inhibited coupling between the core and cladding modes. The spectrum consists of up to 45 high-order Stokes and anti-Stokes lines and is generated by driving the confined gas with a single, moderately powerful (10-kilowatt) infrared laser, producing 12-nanosecond-duration pulses. This represents a reduction by six orders of magnitude in the required laser powers over previous equivalent techniques and opens up a robust and much simplified route to synthesizing attosecond pulses.

Bound states in the continuum

“…5f). While such fiber modes are not true BICs because there can be residual radiation (the propagation loss is typically 1 dB/m), they enable broadband guidance in air and have found many applications including multiple-octave frequency comb generations 229 (shown in Fig. 5d), all-fiber gas cells 230,231 , particle transportation and levitation 232 , and Raman sensing 233 .…”

“…A type of hollow-core Kagome-lattice PhC fiber (shown in Fig. 5f) can provide wave guiding inside the continuum without a bandgap 228,229 . Its mechanism, referred to as "inhibited coupling" by some, is due to the dissimilar azimuthal dependence of the core and cladding modes: the core mode varies slowly with angle, while the cladding mode is fast oscillating (see Fig.…”

Hollow-core photonic crystal fibre for high power laser beam delivery

“…Guidance mechanism HC-PCF can guide either by a photonic bandgap or, in the absence of a bandgap, via inhibited coupling between the core-guided modes and modes associated with the cladding [26] . Bandgap guidance mechanism has been well explained by several intuitive pictures such as the 'Anti-Resonant Reflective Optical Waveguides (ARROW)' model [38] , 'cellular method' [39] and the 'tight binding model' [40] .…”

“…The optical guidance in this type of HC-PCF, sometimes called Kagome-like HC-PCF, does not rely on the PBG effect but rather on a radically new mechanism called 'inhibited coupling' (IC) guidance [26] . Kagome-lattice HC-PCF, first reported in 2002 by Benabid et al [17] , is an example of such a fibre.…”

“…At the birth of this fibre, its guidance mechanism was not well understood, which hindered further advances in terms of its propagation loss and transmission bandwidth. Since 2006, Kagome-type HC-PCF has experienced a strong development in both experimental fabrication [26][27][28] and physical understanding [29,30] . It was shown that this type of fibre favours a large pitch and guides via inhibited coupling between the core modes and the cladding modes.…”