Dielectric coating of optical fibers has been widely applied to various sensing applications and has been shown to improve the performance of fibers. Optical fibers coated with non-uniform metal coatings of gold, silver, copper or palladium exhibit better sensitivity to various forms of perturbations [1
]. In particular, palladium (Pd) coated optical fiber sensors demonstrate an ultrahigh sensitivity for detecting hydrogen [2
]. Polyimide coated optical fibers can effectively respond to humidity changes [3
]. Zinc oxide (ZnO) coated long-period fiber gratings (LPFG) have improved refractive index (RI) sensing properties [4
]. In certain cases, the accuracy of the coating thickness is very important for the qualitative study of the physical properties. Various methods for the accurate measurement of the coating thickness have been widely studied. A method has been proposed for measuring thin film thickness using total internal reflection fluorescence microscopy with the use of evanescent wave illumination, which can overcome the drawbacks of the optical methods that are insufficient for measuring the thickness of a thin film with curvature [5
]. A scanning angle (SA) Raman spectroscopy method enables the non-destructive measurement of the polymer thickness [6
]. High frequency ultrasonic transducers can be used to characterize the variation of thickness in polymer thin films (<1 µm) [8
]. Reflection high-energy electron diffraction, piezoelectricity, interferometry, and gravimetric methods can be used to measure the thickness of films in the range of 1 nm‒1 µm during film deposition or on a finished product [9
]. Scanning electron microscopy (SEM), which has a good resolution and simple operation, is most commonly used to detect the thickness of a coating material. However, SEM is expensive, and measurement is usually limited by the sample conductivity (for dielectrics, a thin layer metal coating is required) and requires high vacuum conditions [10
]. Fourier transform-based structured-illumination microscopy (FTSIM) and modulation-based structured-illumination microscopy (MSIM) can also detect the surface topography and thickness distribution [11
]. Fourier-domain optical coherence tomography (FDOCT) is an alternative method [13
], however, this method is again costly and relatively complicated, and can only achieve micrometer-scale detection, which has a relatively low resolution.
Hollow-core fiber (HCF) has attracted a lot of research interest in the field of optical fiber sensors. For instance, a simple structure is demonstrated for the refractive index sensing by splicing a segment of capillary with two segments of single mode fibers (SMFs) [14
]. An air-gap microcavity is incorporated in the HCF to form a Fabry–Pérot interferometer (FPI) for sensing applications [15
]. In FPI-based sensors, the size of the microcavity needs to be very accurately controlled to the order of microns, as subtle variations in the length of the cavity will result in significant changes in the actual spectrum. The resolution and detection limits for whispering gallery mode-based refractometric sensor devices can be quickly estimated by a simple numerical relationship [18
]. Previous reports show that HCF-based interferometer structures have been used for temperature [19
], vibration [20
], and humidity sensing [21
], demonstrating its wide application prospects.
In this paper, we propose and investigate a new way to use a single HCF-based multiple beam interferometer for thin-layer thickness measurement, in which a short section of HCF is fusion spliced between two SMFs. The proposed method can achieve an ultrahigh resolution with sub-nanometer thickness detection. The demonstrated sensor has the advantages of being low cost, having good repeatability, and an ease of fabrication.