Lying between optical and microwave ranges, the terahertz band in the electromagnetic spectrum is attracting increased attention. Optical fibers are essential for developing the full potential of complex terahertz systems. In this manuscript, we review the optimal materials, the guiding mechanisms, the fabrication methodologies, the characterization methods and the applications of such terahertz waveguides. We examine various optical fiber types including tube fibers, solid core fiber, hollow-core photonic bandgap, anti-resonant fibers, porous-core fibers, metamaterial-based fibers, and their guiding mechanisms. The optimal materials for terahertz applications are discussed. The past and present trends of fabrication methods, including drilling, stacking, extrusion and 3D printing, are elaborated. Fiber characterization methods including different optics for terahertz time-domain spectroscopy (THz-TDS) setups are reviewed and application areas including short-distance data transmission, imaging, sensing, and spectroscopy are discussed.

Terahertz optical fibers [Invited].

A novel single-polarization single-mode double-ring hollow-core anti-resonant fiber with two single-polarization regions (1545–1553 nm and 1591–1596 nm) is proposed. Single-polarization guidance is achieved by coupling a polarized fundamental mode and silica mode by using different tube thicknesses. Specifically, when the wavelength is 1550 nm, only a single x-polarized fundamental mode with a low loss of 0.04 dB/m is propagated by a polarization extinction ratio of 17662 and minimum higher-order mode extinction ratio of 393 by optimizing the structural parameters. Furthermore, this fiber also exhibits high-performance bend resistance. The x-polarized FM loss is as low as 0.11 dB/m with single-polarization single-mode guidance when the proposed fiber was bent at a bend radius of 8 cm toward the x-direction.

Single-polarization single-mode double-ring hollow-core anti-resonant fiber.

A strain sensor based on cascaded Sagnac loops with two polarization-maintaining photonic crystal fibers (PMPCFs) has been experimentally demonstrated. Air holes distributed on the cross section of PMPCF go through the whole fiber, which makes the special fiber more sensitive to ambient force compared with conventional optical fiber. The lengths of the two PMPCFs are close, but not the same, to form envelopes on the total output spectrum and then obtain high resolution to the detected parameter by the Vernier effect. The influences of the difference in length of the two PMPCFS on spectral envelopes are discussed. As the length of the PMPCF in reference loop increases, free spectrum range of the cascaded Sagnac loops (or envelope period) and magnification decrease. The sensitivities of this strain sensor are calculated by tracking upper and lower envelopes. Results reveal that its average sensitivity is up to 45.15 pm/ $\mu \varepsilon $ and the corresponding resolution is $0.44~\mu \varepsilon $ as strain varies from 0 to $2000~\mu \varepsilon $ . Compared with a single Sagnac loop, the sensitivity magnification is about 28.94. The sensing characteristics of a conventional polarization-maintaining fiber (PMF) are studied to compare with the PMPCF. Moreover, reversibility has also been proved to possess low error rate. In short, the fiber strain sensor possesses high sensitivity and great reversibility.

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Cascaded Sagnac Loops Embedded With Two Polarization Maintaining Photonic Crystal Fibers for Highly Sensitive Strain Measurement

Distributed optical fiber pressure sensors

We present a numerical investigation on the effect of introducing the second ring of antiresonant tubes on the guiding properties of the negative-curvature fiber. We determine the range of structural parameters for achieving the optimum light guidance in the double-ring geometry. Our study shows that the double-ring negative-curvature fiber can improve the confinement loss by up to four orders of magnitude with considerably better bending and single-mode performance when compared to its single-ring counterpart.

Effect of the second ring of antiresonant tubes in negative-curvature fibers.

A novel high-birefringence hollow-core anti-resonant THz fiber is proposed in this paper. This fiber has a simple structure which consists of only ten Topas tubes. High birefringence is achieved by introducing two large tubes. The first two resonant frequencies are 1.44 and 2.88 THz by fixing tube thickness at 0.09 mm, which makes two low-loss transmission windows exist in the frequency range from 0.8 to 3.0 THz. The lowest loss is 2.10 dB/m occurring at 1.2 THz in the first transmission window and 1.68 dB/m at 2.34 THz in the second transmission window. By optimizing the structure parameters, high birefringence above 7 × 10−4 in the frequency range from 1.0 to 1.24 THz are obtained. The highest birefringence is up to 8.7 × 10−4 at 1.04 THz. Birefringence can be further increased to the order of 10−3 by adjusting the structure parameters at the cost of loss increasing and the bandwidth decreasing. In addition, bent performance of this fiber is also discussed. In addition, this fiber can keep good performance when it is bent for x-direction. At the bend radius of 15 cm, the loss and birefringence has a more slightly change in the first transmission window than the second transmission window. The first transmission window own much better bent-insensitive characteristics.

High-birefringence hollow-core anti-resonant THz fiber

An in-line Mach–Zehnder interferometer based on a dual-tapered photonic crystal fiber (PCF) for bending vector and strain sensing is proposed and experimentally demonstrated. This sensor consists of a 25.7-cm homemade asymmetric twin-core PCF sandwiched between two tapers. Due to the asymmetry of twin cores, two opposite bending directions can be distinguished with the maximum bending sensitivities of 18.29 and −18.13 nm/ $\text{m}^{-1}$ , respectively. By applying axial strain to the sensor, the strain sensitivity reaches up to −10.65 pm/ $\mu \varepsilon $ . Because of the excellent linear response, the effect of temperature on strain and bending sensing can be eliminated by using a measurement matrix. The advantages of bending direction recognition, high strain and bending sensitivities, and compact structure make this sensor attractive for structural deformation monitoring.

High-Performance Bending Vector and Strain Sensor Using a Dual-Tapered Photonic Crystal Fiber Mach–Zehnder Interferometer

We propose a new method to achieve single-polarization guidance in a hollow-core negative-curvature fiber. Simulation results show that coating a layer of high-index material with a suitable thickness on the inner surface of one vertical cladding tube in a hollow-core negative-curvature fiber can effectively suppress $y$ -polarized fundamental mode. Silicon is chosen as the high-index material in this paper. In the five-nested-tube silicon-coated negative-curvature fiber, the single-polarization region is located in the wavelength range of 1512–1587 nm with a broad bandwidth of 75 nm. The polarization extinction ratio is up to 1732 at the wavelength of 1550 nm. Moreover, the mode evolutions of $x$ - and $y$ -polarized fundamental modes in the five-nested-tube silicon-coated negative-curvature fiber are investigated to explain why $y$ -polarized fundamental mode is suppressed. Compared with the traditional method of using different tube thicknesses to achieve single-polarization guidance in a negative-curvature fiber, this method possesses the advantages of broad single-polarization bandwidth and high error tolerance.

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A New Method to Achieve Single-Polarization Guidance in Hollow-Core Negative- Curvature Fibers

Basing on the mode coupling mechanism of symmetrical triple-core photonic crystal fiber (STCPCF), a highly sensitive strain sensor is proposed and experimentally demonstrated. Particularly, the triple-cores are arranged in an equilateral triangle on the pure silica host STCPCF. As a result, the coupling coefficient among the triple-cores of the STCPCF can increase as the fiber stretches. Moreover, the proposed sensor is designed by monitoring the wavelength shift of the coupling output spectrum. The experimental results show that the maximum strain sensitivity can be as high as −29.8 pm/μϵ, which outperforms most of mode interference theory based fiber sensors. In addition, the strain sensitivity can be predicted by using mode-coupling mechanism in triple-core fiber. Theoretical results reveal that the sensitivity can be further improved up to −58.15 pm/μϵ by optimizing the fiber structure parameters. Finally, the experimental results also show a high repeatability and stability, and a low temperature-strain cross-sensitivity of the proposed sensor.

Shibo Yan

Papers

Single-polarization single-mode double-ring hollow-core anti-resonant fiber.

High-birefringence hollow-core anti-resonant THz fiber

High-Performance Bending Vector and Strain Sensor Using a Dual-Tapered Photonic Crystal Fiber Mach–Zehnder Interferometer

A New Method to Achieve Single-Polarization Guidance in Hollow-Core Negative- Curvature Fibers

Using Mode Coupling Mechanism in Symmetrical Triple-Core Photonic Crystal Fiber for High Performance Strain Sensing