Hollow-core photonic crystal fibers for efficient terahertz transmission
Introduction
Terahertz (THz) or T-ray radiation falls in the spectral region that is a connection between microwave electronics and infrared optics. The domain of the T-ray starts from the end of the microwaves and continues to midrange of the infrared waves. In fact, its frequency spans from 10 to 10 (similarly, the wavelength range is from to 3 mm) as shown in Fig. 1 [1], [2]. The energy of optical photons in the THz region is less than the bandgap of non-conductive material; therefore, T-ray can penetrate to these substances. Moreover, THz radiation may be sent to materials for investigating their characteristics and can be regarded as a good alternative to the X-ray for obtaining high-resolution images of solid objects [3], [4], [5], [6].
By increasing applications of THz waves, it is essential to design and fabricate new waveguides with minimum loss in THz region. Metallic waveguides were the first types which guided THz waves; and based on their geometry include four types known as circular, parallel plate, bare metal wire, and slit waveguide (see Fig. 2) [7], [8], [9], [10], [11], [12]. Dielectric waveguides can also support THz waves and based on their guidance mechanism are divided into two main subgroups: index-guiding and photonic-bandgap (PBG) fibers. An Index-guiding fiber consists of a solid core with higher refractive index than the cladding where light is guided by total internal reflection in the core [13]. As shown in Fig. 3, there are three types of index-guiding fibers with different cross sections known as: step-index fiber, solid-core photonic crystal fiber and porous-core fiber [14], [15], [16], [17]. Since light mainly propagates in the solid core of the fiber, therefore, transmission of light is accompanied by the material absorption. However, by introducing porous-core, researchers are able to engineer and reduce the material absorption and consequently the transmission loss.
PBG fibers generally consist of the regular array of microscopic air holes in the cladding and a central air hole as a defect in the core region. Light is transmitted in the fiber using the photonic-bandgap mechanism [18]. As depicted in Fig. 4, there are two kinds of photonic bandgap fibers, the so-called Bragg-fiber [19], [20] and hollow-core photonic crystal fiber (HC-PCF) [18], [21], [22]. The HC-PCF can transmit light at the limited range of frequencies located in the PBG. It should be noted that since the light is transmitted in the hollow core consisted of air, the material absorption is almost zero and hence the transmission loss is decreased.
Flexibility in design is one of the most desirable features of HC-PCFs that make them very attractive in comparison with the conventional (step-index) optical fibers. By manipulating the structural parameters like the air-hole diameter (d), the hole-to-hole distance (or lattice pitch) , core diameter (d), number of the air-hole rings (N) and the background material, one can control linear and nonlinear characteristics of the PCF [23], [24], [25]. Particularly, by the control of these structural parameters, we can find a suitable structure of the HC-PCF to transmit the light with a minimum loss in the THz region [26], [27].
In this article, we propose and simulate HC-PCFs with the minimum transmission loss in the THz region. To this end, we manipulate structural parameters of the HC-PCF and find a suitable structure for efficient THz transmission. The method which is used for simulations is based on the finite-difference time domain (FDTD) to calculate the transmission loss, mode effective area and dispersion.
Section snippets
Proposed fiber structure
We consider a HC-PCF where its background material is composed of a medium with higher refractive index (such as Teflon, PMMA, silica or chalcogenide) than its core which is filled with air (a). The size of the core is chosen approximately 3 times greater than that of the air holes in cladding region. As it is shown in Fig. 5, the core in the proposed HC-PCF is commonly constructed by removing the central air hole together with the first ring of the air holes surrounding the central hole.
Results and discussion
In this section, we investigate impacts of structural parameters on the transmission spectrum, dispersion curve and modal field of HC-PCFs and try to find a suitable HC-PCF with the lowest transmission loss in THz region. We start our study by considering four different common optical materials including Teflon, PMMA, silica and chalcogenide glass as a background material. First, we use Polytetrafluoroethylene (PTFE) known as Teflon, as the background material for the HC-PCF. PTFE is a plastic
Conclusion
Using the FDTD method, we have simulated HC-PCFs for efficient transmission in the THz region and investigated impacts of structural parameters on the THz transmission window. Four types of practical substances including Teflon, silica, PMMA and chalcogenide were used as background materials and we found that Teflon has the lowest transmission loss in the range of 1.55–1.85 THz. We have also examined the influence of the core size (d), the number of air-hole rings (N) and the filling
Acknowledgment
The authors would like to thank Dr. Hamed Saghaei for helpful discussions.
References (60)
- et al.
Fabrication of microstructured polymer optical fibres
Opt. Fiber Technol., Mater. Devices Syst.
(2004) A perfectly matched layer for the absorption of electromagnetic waves
J. Comput. Phys.
(1994)- et al.
Choice of the perfectly matched layer boundary condition for frequency-domain Maxwell’s equations solvers
J. Comput. Phys.
(2012) - et al.
Highly birefringent and low effective material loss microstructure fiber for THz wave guidance
Opt. Commun.
(2018) - et al.
Preparation and characterization of transparent PMMA–cellulose-based nanocomposites
Carbohydr. Polymers
(2015) - et al.
Extracting accurate optical parameters from glasses usingterahertz time-domain spectroscopy
J. Non-Cryst. Solids
(2009) Terahertz wave imaging: horizons and hurdles
Phys. Med. Biol.
(2002)- et al.
Special issue on T-ray imaging, sensing, and retection
Proc. IEEE
(2007) - et al.
High-resolution terahertz reflective imaging and image restoration
Appl. Opt.
(2010) - et al.
A high precision terahertz wave image reconstruction algorithm
Sensors
(2016)