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

Ultrasensitive mid-infrared absorption spectroscopy is demonstrated by the use of a novel silica-based hollow-core negative curvature fiber (HC-NCF). The HC-NCF used in this article 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 is conducted to detect the N ₂ O 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 is 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.