Ultrahigh Resolution Thickness Measurement Technique Based on a Hollow Core Optical Fiber Structure

Abstract

An ultrahigh resolution thickness measurement sensor was proposed based on a single mode-hollow core-single mode (SMF-HCF-SMF) fiber structure by coating a thin layer of material on the HCF surface. Theoretical analysis shows that the SMF-HCF-SMF fiber structure can measure coating thickness down to sub-nanometers. An experimental study was carried out by coating a thin layer of graphene oxide (GO) on the HCF surface of the fabricated SMF-HCF-SMF fiber structure. The experimental results show that the fiber sensor structure can detect a thin layer with a thickness down to 0.21 nanometers, which agrees well with the simulation results. The proposed sensing technology has the advantages of simple configuration, ease of fabrication, low cost, high resolution, and good repeatability, which offer great potential for practical thickness measurement applications.2 of 8 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 [11,12]. 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][16][17]. 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 ...

Investigation of Relative Humidity Sensing Using Tapered No-Core Fiber Coated With Graphene Oxide Film

“…The thickness of GO nanofilms depend on the concentration of GO solution [27]. The film thickness was measured using field emission scanning electron microscope (FESEM, FEI Nano SEM450*).…”

Singlemode-Multimode-Singlemode Fiber Structures for Sensing Applications—A Review

“…Cai et al cascaded multiple HCFs with different cladding thicknesses to achieve multipoint temperature sensing [113]. The SHCS fiber structure can also be functionalized for sensing of magnetic field strength [114], humidity [115], hydrogen detection [116], displacement [117], liquid level with sub-micrometer resolution [118] and ultrahigh sensitivity thickness measurement with nanometer resolution [119]. A tailored SHCS fiber structure for RI sensing [120] and Fano resonance achieved by inserting a microsphere into the HCF have also been observed and studied recently [121][122].…”

U-Shape Panda Polarization-Maintaining Microfiber Sensor Coated With Graphene Oxide for Relative Humidity Measurement

“…Liu et al proposed to functionalize a thin layer of GO nanosheets (49.2 nm) on a specific designed long-period grating for ultrasensitive specific binding detection between antibody and antigen [26]. In addition, GO also has been applied in ultrahigh resolution thickness measurement and mode locked lasers [27,28].…”