Highly birefringent terahertz polarization maintaining plastic photonic crystal fibers

Continuous fabrication of polarization maintaining fibers via an annealing improved infinity additive manufacturing technique for THz communications

We report the design and fabrication of a polarization-maintaining fiber for applications in fiber-assisted THz communications. The fiber features a subwavelength square core suspended in the middle of a hexagonal over-cladding tube by four bridges. The fiber is designed to have low transmission losses, high birefringence, high flexibility, and near-zero dispersion at the carrier frequency of 128 GHz. An infinity 3D printing technique is used to continuously fabricate a 5 m-long polypropylene fiber of ∼6.8 mm diameter. The fiber transmission losses are furthermore reduced by as high as ∼4.4 dB/m via post-fabrication annealing. Cutback measurements using 3 m-long annealed fibers show ∼6.5-11 dB/m and ∼6.9-13.5 dB/m losses (by power) over a 110-150 GHz window for the two orthogonally polarized modes. Signal transmission with bit error rates of ∼10^-11-10^-5 is achieved at 128 GHz for 1-6 Gbps data rates using a 1.6 m-long fiber link. The average polarization crosstalk values of ∼14.5 dB and ∼12.7 dB are demonstrated for the two orthogonal polarizations in fiber lengths of 1.6-2 m, which confirms the polarization-maintaining property of the fiber at ∼1-2 meter lengths. Finally, THz imaging of the fiber near-field is performed and shows strong modal confinement of the two orthogonal modes in the suspended-core region well inside of the hexagonal over-cladding. We believe that this work shows a strong potential of the infinity 3D printing technique augmented with post-fabrication annealing to continuously produce high-performance fibers of complex geometries for demanding THz communications applications.

Terahertz optical fibers [Invited]

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 detection of alcohol using a photonic crystal fiber sensor

Ethanol is widely used in chemical industrial processes as well as in the food and beverage industry. Therefore, methods of detecting alcohol must be accurate, precise, and reliable. In this content, a novel Zeonex-based photonic crystal fiber (PCF) has been modeled and analyzed for ethanol detection in terahertz frequency range. A finite-element-method-based simulation of the PCF sensor shows a high relative sensitivity of 68.87% with negligible confinement loss of 7.79×10^-12  cm^-1 at 1 THz frequency and x-polarization mode. Moreover, the core power fraction, birefringence, effective material loss, dispersion, and numerical aperture are also determined in the terahertz frequency range. Owing to the simple fiber structure, existing fabrication methods are feasible. With the outstanding waveguiding properties, the proposed sensor can potentially be used in ethanol detection, as well as polarization-preserving applications of terahertz waves.

Design and numerical analysis of a THz square porous-core photonic crystal fiber for low flattened dispersion, ultrahigh birefringence

We propose a kind of square porous-core photonic crystal fiber (PCF) for polarization-maintaining terahertz (THz) wave guidance. An asymmetry is introduced by implementing rectangular array air holes in the porous core of the PCF, and ultrahigh birefringence and low effective material loss (EML) can be achieved simultaneously. The properties of THz wave propagation are analyzed numerically in detail. The numerical results indicate that the proposed fiber offers a high birefringence of 0.063 and a low EML of 0.081  cm^-1 at 1 THz. Moreover, a very low flattened dispersion profile is observed over a wide frequency domain of 0.85-1.9 THz. The zero flattened dispersion can be controlled. It is predicted that this PCF would be used potentially in polarization maintaining and dispersion management of THz waves.

Broadband terahertz dispersion control in hybrid waveguides

Dispersion control is a key objective in the field of photonics and spectroscopy, since it enhances non-linear effects by both enabling phase matching and offering slow light generation. In addition, it is essential for frequency comb generation, which requires a phase-lock mechanism that is provided by broadband compensation of group velocity dispersion (GVD). At optical frequencies, there are several well-established concepts for dispersion control such as prism or grating pairs. However, terahertz dispersion control is still a challenge, thus hindering further progress in the field of terahertz science and technology. In this work, we present a hybrid waveguide with both broadband, tuneable positive and more than octave-spanning negative terahertz GVD on the order of 10^-22 s²/m, which is suitable for either intra- or extra cavity operation. This new terahertz device will enable ultra-short pulse compression, allow soliton propagation, improve frequency comb operation and foster the development of novel non-linear applications.

In-line polarization rotator based on the quantum-optical analogy

An in-line polarization rotator (PR) is proposed based on the quantum-optical analogy (QOA). The proposed PR possesses an auxiliary E7 liquid crystal (LC) waveguide in the vicinity of the single-mode fiber (SMF) core. Because of the matched core size, the PR demonstrates good compatibility with the established backbone networks which are composed of conventional SMFs. With optimized parameters for the auxiliary waveguide, the PR offers a near 100% polarization conversion efficiency at the 1550 nm band with a bandwidth of ∼30  nm, a length of ∼4625.9  μm with a large tolerance of ∼550  μm, and a tolerance of the input light polarization angle and rotation angle of the E7 LC of ∼π/30 and ∼π/36  rad, respectively. The performance was verified by the full-vector finite-element method. The proposed PR can be easily fabricated based on the existing photonics crystal fiber manufacturing process, making it a potentially inexpensive device for applications in modern communication systems. Moreover, the QOA, compared with the previous supermode-theory design method, allows a designer to consider several waveguides separately. Therefore, various unique characteristics can be met simultaneously which is consistent with the trend of modern fiber design.

Novel porous fiber based on dual-asymmetry for low-loss polarization maintaining THz wave guidance

In this Letter, we suggest a novel kind of porous-core photonic crystal fiber (PCF) (to the best of our knowledge) for efficient transportation of polarization maintaining (PM) terahertz (THz) waves. We introduce an asymmetry in both the porous-core and the porous-cladding of the structure to achieve an ultra-high birefringence. Besides, only circular air holes have been used to represent the structure, which makes the fiber remarkably simple. The transmission characteristics have been numerically examined based on an efficient finite element method (FEM). The numerical results confirm a high birefringence of ∼0.045 and a very low effective absorption loss of 0.08  cm(-1) for optimal design parameters at 1 THz. We have also thoroughly investigated some important modal properties such as bending loss, power fraction, dispersion, and fabrication possibilities to completely analyze the structure's usability in a multitude of THz appliances. Moreover, physical insights of the proposed fiber have also been discussed.

Elliptical metallic hollow fiber inner-coated with non-uniform dielectric layer

We report on the fabrication and characterization of an elliptical hollow fiber inner coated with a silver layer and a dielectric layer for polarization maintaining and low loss transmission of terahertz (THz) radiation. The primary purpose of adding the dielectric layer is to prevent the silver layer from oxidation. The thickness of the dielectric layer is non-uniform owing to the surface tension of the coating, which was initially applied as a liquid. Transmission loss and polarization maintenance are experimentally characterized. Effects of the dielectric layer on transmission properties are analyzed by comparing the fiber to Ag-only fiber. Results show that a dielectric layer with thickness less than λ/10 can effectively decreases the power distributed on the metal surface and thus can practically reduce loss resulting from roughness of the silver layer. Bending effects on transmission loss and polarization maintenance are also investigated.

Graded index porous optical fibers – dispersion management in terahertz range

A graded index porous optical fiber incorporating an air-hole array featuring variable air-hole diameters and inter-hole separations is proposed, fabricated, and characterized in a view of the fiber potential applications in low-loss, low-dispersion terahertz guidance. The proposed fiber features simultaneously low modal and intermodal dispersions, as well as low loss in the terahertz spectral range. We experimentally demonstrate that graded index porous fibers exhibit smaller pulse distortion, larger bandwidth, and higher excitation efficiency when compared to fibers with uniform porosity.

Design and optimization of low-loss high-birefringence hollow fiber at terahertz frequency

Transmission characteristics at terahertz (THz) frequencies are numerically analyzed for elliptical dielectric-coated metallic hollow fiber (DMHF). Attenuation constants, group velocity, modal birefringence, and modal power fraction in the air core are presented. Optimization of the fiber geometry is investigated to reduce the attenuation and to increase the birefringence simultaneously. Modal birefringence of 3.3 × 10 -2 and attenuation of 2.4 dB/m are expected. It is found that a desirable ellipticity of the air core is around 3. And both the modal birefringence and the attenuation constant are inversely proportional to the cube of the core size. Multiple dielectric layers significantly reduce the attenuation and meanwhile have little influence on the modal birefringence.

A terahertz broadband 3 dB directional coupler based on bridged PPDW

In this paper, a novel broadband 3 dB directional coupler with very flat coupling based on bridged parallel plate dielectric waveguide (PPDW) is proposed and demonstrated. In the uniform coupling section, a bridge structure between the two PPDWs is employed to obtain accurate coupling value and achieve a broadband coupling. It is found that this new type of coupling structure exhibits excellent performance at terahertz frequencies. In order to achieve strong isolation between the adjacent ports and reduce the power reflection in all ports, two quarter-circle bend arms are introduced as the curved transition sections to connect the uniform coupling section. For this bridged coupler, it only needs the value of the uniform coupling length as short as 400 μm to achieve a broadband 3 dB coupling. In this case, the coupler's average return loss is greater than 28 dB, average isolation is better than 27 dB and average coupler loss is only 0.9 dB, over a percentage bandwidth of 12.5% at 1 THz. Compared to the conventional PPDW coupler, the bridged PPDW coupler shows significantly greater bandwidth (about 4.2 times), compact and mechanically stable with a much shorter uniform coupling length (reduced about 61%), which may have potential applications for terahertz integrated circuits and systems.

THz porous fibers: design, fabrication and experimental characterization

Porous fibers have been identified as a means of achieving low losses, low dispersion and high birefringence among THz polymer fibers. By exploiting optical fiber fabrication techniques, two types of THz polymer porous fibers--spider-web and rectangular porous fibers--with 57% and 65% porosity have been fabricated. The effective refractive index measured by terahertz time domain spectroscopy shows a good agreement between the theoretical and experimental results indicating a lower dispersion for THz porous fiber compared to THz microwires. A birefringence of 0.012 at 0.65 THz is also reported for rectangular porous fiber.

Squeezed lattice elliptical-hole terahertz fiber with high birefringence

A highly birefringent elliptical-hole terahertz (THz) fiber with a squeezed lattice is proposed. This THz fiber is formed through regular linear deformation of a circular porous fiber by squeezing it in one direction, which may be fabricated with the existing fabrication technology. By numerical analysis we show that, with a moderate amount of deformation, the proposed THz fiber can exhibit high birefringence on a level of 10(-2) over a wide THz frequency range. And the THz fiber guiding loss caused by material absorption can be reduced effectively, since a dominant fraction of modal power is distributed in the airholes inside the dielectric material.

Low-loss air-core polarization maintaining terahertz fiber

We propose a low-loss air-core polarization maintaining polymer fiber for terahertz (THz) wave guiding. The periodic arrangement of square holes with round corners in the cladding offers a bandgap effect for mode guiding. Numerical simulations show that the bandgap effect repels the modal power from the absorbent background polymers, resulting in a significant suppression of absorption loss of the polymers by a factor of more than 25. The phase-index birefringence of the proposed THz fiber is in the order of 10(-3).