Silica Hollow-Core Negative Curvature Fibers Enable Ultrasensitive Mid-Infrared Absorption Spectroscopy

Abstract

Ultrasensitive mid-infrared absorption spectroscopy was demonstrated by the use of a novel silica-based hollow-core negative curvature fiber (HC-NCF). The HC-NCF used in this work consists of a single ring of six nontouching cladding capillaries around the hollow core, thus forming a unique core boundary with a negative curvature. Such a silica HC-NCF enables the broadband single-mode transmission in the mid-infrared. By using the HC-NCF as a compact gas cell, a proof-of-principle experiment was conducted to detect the N2O line at 2778.37 cm -1 with a distributed-feedback interband cascade laser emitting at 3.6 μm. A minimum detectable absorbance of 3×10 -5 was achieved for a fiber length of 120 cm, corresponding to a noise equivalent absorption (NEA) coefficient of 2.5×10 -7 cm -1 . Silica HC-NCFs offer a new opportunity of developing sensitive and compact gas sensors using mid-infrared absorption spectroscopy.Index Term-Mid-infrared absorption spectroscopy, Gas sensor, Optical fiber sensor, Photonic crystal fiber, Hollow core fiber I. INTRODUCTIONollow-core microstructured optical fibers (HC-MOFs) typically comprise a glass microstructure with a tiny hollow core in the central region where light and gas/liquid samples could interact perfectly. Flexible HC-MOFs with a low optical transmission loss provide opportunities for developing sensitive and compact optical sensors that would otherwise be difficult for free-space optics. The use of HC-MOFs for gas sensing has been investigated by several spectroscopic methods such as direct absorption spectroscopy (DAS) [1-5], wavelength modulation spectroscopy (WMS) [6-8] and photothermal spectroscopy (PTS) [9,10].

A Review of Antiresonant Hollow-Core Fiber-Assisted Spectroscopy of Gases

“…Therefore, the application of the HCFs, especially the ARHCFs, seems to be a promising way to deliver low-volume, robust, and long optical paths. A successful demonstration of TDLAS-based gas sensors aided with ARHCFs has been already demonstrated by several research groups justifying the viability of this approach [21,24,35,38]. Nikodem et al reported in [21] a very simple carbon dioxide (CO 2 ) sensor configuration utilizing a silica-based 7 capillary ARHCF as depicted in Figure 3.…”

“…The interaction leads to the absorption of light by the gas molecules, which are excited at the wavelength corresponding to the selected molecular transition. This phenomenon is governed by the Beer-Lambert law and expressed by the following formula [34,35]:…”

Laser-Based Trace Gas Detection inside Hollow-Core Fibers: A Review

“…A piece of standard fiber was coiled around a piezo transducer (PZT) and was used to make sure that the interferometer operated at the quadrature point. In [73], the same group presented another system, operating at 3.6 µm. The fiber was 1.2-m-long, had a core diameter of 65 µm, and had a transmission band from 1.6 µm to the mid-infrared region.…”

“…The sample absorption spectrum measured when the laser wavelength was scanned across the nitrous oxide transition near 2203.75 cm -1 is presented in Figure 14b. When the WMS technique was implemented, the detection limit of a few parts-per-billion was obtained and the response time was 23 s. In [73], the same group presented another system, operating at 3.6 µm. The fiber was 1.2-m-long, had a core diameter of 65 µm, and had a transmission band from 1.6 µm to the mid-infrared region.…”

“…In [ 73 ], the same group presented another system, operating at 3.6 µm. The fiber was 1.2-m-long, had a core diameter of 65 µm, and had a transmission band from 1.6 µm to the mid-infrared region.…”