Nondestructive measurement of the roughness of the inner surface of hollow core-photonic bandgap fibers

Abstract

Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX We present optical and atomic force microscopy measurements of the roughness of the core wall surface within a Hollow Core Photonic Bandgap Fiber (HC-PBGF) over the [3×10 -2 µm -1 -30 µm -1 ] spatial frequency range. A recently developed immersion optical profilometry technique with picometer-scale sensitivity was used to measure the roughness of air-glass surfaces inside the fiber at unprecedentedly low spatial frequencies, which are known to have the highest impact on HC-PBGF scattering loss and thus determine their loss limit. Optical access to the inner surface of the core was obtained by selective filling of the cladding holes with index matching liquid using techniques borrowed from micro-fluidics. Both measurement techniques reveal ultra-low roughness levels exhibiting a 1/f spectral power density dependency characteristic of frozen surface capillary waves over a broad spatial frequency range. However, a deviation from this behavior at low spatial frequencies was observed for the first time. © 2016 Optical Society of America Hollow core-photonic bandgap fibers (HC-PBGFs) represent a potential candidate for optical communication applications that require ultra-low latency. HC-PBGFs are essentially formed by a periodic cladding made of air holes in a silica glass matrix which confine light in a central air core. Guidance in an air core allows for a much lower nonlinearity and latency compared to classical solidcore fibers [1, 2]. Furthermore, HC-PBGFs offer reduced losses by Rayleigh scattering, impurities, or multiphonon absorption relative to solid fibers. However, the lowest loss values reported on HCPBGFs so far are still one order of magnitude higher than those of solid single mode fibers [3]. Losses in HC-PBGFs are fundamentally limited by scattering at the numerous and intrinsically "rough" air/glass interfaces present in these structures [3]. ehttct eu gc te tac sgtuhec teguaecss eew eecet ht hem s ht hh utc gcee cs( .c. wchem1mw -1 ) is understood to provide the dominant contribution to the overall loss [4]. ac stg r eu teguaecss e ta s utc gceer theuc tc g tcs tce sc sgtuhec wchsgtcwcets ecct stheec sehhcs eu scccthh age tc s eu w etewctcts, ma ea s eet ess whc m ta west itew e setec i etesee cs (isis).The structure of glasses is amorphous, mostly owing to their past liquid state: to a first approximation, glass can be viewed as a liquid abruptly frozen at the glass transition temperature TG. Surface capillary waves (SCWs) that are thermally excited in the liquid state are suddenly frozen during the fiber drawing process [3, 5, 6]: the glass surface structure therefore reflects the thermal fluctuations of the liquid surface at TG.. These SCWs result from the interplay of two opposing effect: thermal noise (which tends to destabilize the surface), and interface tension (which tends to dampen the oscillations and flatten the surface). A simple estimate of the variance...

Hollow-core fibers with reduced surface roughness and ultralow loss in the short-wavelength range

“…Under the SCW framework, the Fourier spectrum of the surface height profiles, which is typically assessed via their power spectral density (PSD) functions calculated over 1D surface profiles, is expected to display a 1/ f behavior, where f is the spatial frequency. The 1/ f -trended behavior of PSD functions accounted from HCPCFs core surface profiles has been observed in previous works 2 , 21 – 23 . For low spatial frequency values (<5 × 10 −2 µm −1 ), however, deviation from the 1/ f trend has been observed, and PSD functions following a 1/ f 3 -trend have been detected 23 .…”

Local Surface Chemistry Dynamically Monitored by Quantitative Phase Microscopy

“…Likewise, reflectivity, ellipsometry, and optical profilometry provide rich information about interfaces, but their ability to quantitatively analyze surface modification is strongly model‐dependent, making them less adapted to track inhomogeneous grafting reactions over large surfaces. [ 19–22 ]…”

Anisotropic Superattenuation of Capillary Waves on Driven Glass Interfaces

“…Optical fibres were obtained using a two-step stack and draw process [17,20]. In a first step, cylindrical fused silica capillaries of millimetric diameter were stacked in a triangular lattice with 37 capillaries omitted to form the inner hollow core.…”

“…The quantitative characterization of the roughness spectrum of glass interfaces has recently been the subject of a growing interest in the context of the development of Hollow-Core Photonic Band-Gap Fibres (HC-PBGF) [14][15][16][17]. In such microstructured fibres, light propagates through air within the hollow core and the ultimate losses are expected to be determined by the scattering from the inner interfaces of the fibre [18].…”

“…Attenuated frozen capillary waves on inner glass interfaces of Photonic Band Gap Fibres -Surface roughness was measured on the core interface of a 37 cell HC-PBGF. Optical fibres were obtained using a two-step stack and draw process [17,20]. In a first step, cylindrical fused silica capillaries of millimetric diameter were stacked in a triangular lattice with 37 capillaries omitted to form the inner hollow core.…”