As_2S_3–silica double-nanospike waveguide for mid-infrared supercontinuum generation

Abstract

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2 dB at 1550 nm in the case of free-space in-coupling, which is ∼7 dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.

Octave-spanning Supercontinuum From As2S3-silica Double-nanospike Waveguide Pumped by Femtosecond Cr:ZnS Laser at 2.35 μm

“…The waveguides were produced using a modified pressure-assisted melt-filling technique [3]. Adjusting the waveguide parameters at the fabrication stage, namely d, L NS1 , d NS1 and L NS2 , allows the waveguide GVD to be controlled, together with the near-field mode radii and divergence angles for input and output light.…”

“…High modal confinement in such waveguides increases the effective nonlinearity, while varying the core diameter allows the group velocity dispersion (GVD) to be engineered, thus giving the opportunity to create structures with outstanding potential for SC generation. In order to improve the coupling efficiency into such ultrahigh-NA waveguide structures, a "double-nanospike" structure with inverse nanotapers at both ends of the waveguide has been implemented [3]. By pumping the double-nanospike waveguide with an Er-doped femtosecond fiber laser, an octave-spanning supercontinuum reaching 2.5m was demonstrated.…”

Coherent octave-spanning mid-infrared supercontinuum generated in As_2S_3-silica double-nanospike waveguide pumped by femtosecond Cr:ZnS laser

“…In order to improve the coupling efficiency into such ultrahigh numerical aperture waveguides, a "double-nanospike" structure with inverse nanotapers integrated at both ends of the waveguide has been implemented [17], as shown in Fig. 1.…”

“…In the current experiment, the lengths of the input NS (NS1) were ~350 and ~150 μm respectively for the 3.2 and 1.3 μm core diameter samples, the lengths of NS2 being ~300 μm in both cases. Previously the double-nanospike waveguide was pumped with pulses from an Er-doped femtosecond fiber laser, resulting in generation of an octavespanning supercontinuum reaching to 2.5 μm [17]. In order to fully exploit the long wavelength transparency of the chalcogenide waveguide, as well as shifting the generated spectrum away from the absorption edge of the As 2 S 3 glass so as to avoid thermal effects, it is beneficial to further red-shift the pump source.…”

Tapered silicon core fibers with nano-spikes for optical coupling via spliced silica fibers

“…Free space coupling even with the highest numerical aperture objective lens is not efficient enough to couple light into subwavelength-sized silicon cores due to diffraction limited focusing and high Fresnel reflection at the silicon/air interface. Similar to the nano-tapers commonly used in planar silicon waveguides [18] and photonic crystal waveguides [19], inverse tapering of an As 2 S 3 (n = 2.4) core inside a silica clad fiber has been utilized to achieve free space optical coupling with a loss of 2 dB [20] and supercontinuum generation [21]. Direct splicing of silica-clad SCFs to conventional single mode fibers (SMFs) has also been demonstrated [9], and micro-structuring of the silicon core surface via chemical etching before splicing has been proposed to reduce Fresnel reflection [22].…”