Low-loss Transmission Window Research Articles

Transmission and Confocal Imaging Characteristics of Bendable ABS/Ag-Coated Hollow Waveguide at Low THz Band

A bendable Ag-coated hollow waveguide (HWG) based on the acrylonitrile-butadiene-styrene (ABS) tube is fabricated for low-loss transmission and imaging of low-frequency terahertz (THz) waves. ABS plastic tubing is selected since it allows growing a smooth silver (Ag) reflective film with good adhesion to the inner wall and contributes to constructing a bendable THz HWG structure. The low-loss transmission window of the ABS/Ag-coated HWG is at 0.2-1.0 THz. The attenuation characteristics of the as-prepared waveguide samples with different bore sizes (2.3, 3.2, and 4.2 mm) at 100 and 300 GHz are investigated. The 3.2-mm-bore THz waveguide exhibits good bendability and transmission performances. Its measured straight transmission loss is 1.02 dB/m at 300 GHz, and the bending loss reaches 1.56 dB/m at 150° (the curvature radius is 10 cm). Moreover, a far-field confocal waveguide-based THz imaging system is established based on the ABS/Ag-coated HWG. High-quality imaging with a super-resolution of 1.12 mm was achieved using the 3.2-mm-bore waveguide samples at 100 GHz.

Ultralow confinement loss anti-resonant fibers designed by swarm intelligence

Anti-resonant fibers featured with multiple cladding tubes are promising light guiding media with a broadband low-loss transmission window in the optical spectrum. Herein, we numerically demonstrate anti-resonant fibers with ultra-low confinement loss and good single mode property, which is automatically designed by swarm intelligence. The basic idea of the particle swarm optimization algorithm is to find the optimal solution by the cooperation and information sharing among particles in the group. It is demonstrated that the particle swarm optimization algorithm can be utilized to design such anti-resonant fibers with multiple geometry parameters. By breaking the mirror symmetry of the fiber’s cross section, the coupling between the fundamental mode and cladding mode is further suppressed than the mirror symmetry one. As a result, ultra-low confinement loss as small as 2.2 × 10−8 dB/m can be achieved in simulation. Our results might facilitate the advanced applications of nested anti-resonant fibers in optical communication and high energy pulsed laser, etc.

Single-ring hollow-core anti-resonant fiber with a record low loss (4.3 dB/km) for high-power laser delivery at 1 µm.

We report a 7-tube single-ring hollow-core anti-resonant fiber (SR-ARF) with a record low transmission loss of 4.3 dB/km @1080 nm, which is almost half of the current lowest loss record of an SR-ARF (7.7 dB/km @750 nm). The 7-tube SR-ARF has a large core diameter of 43 µm and a wide low-loss transmission window exceeding 270 nm for the 3-dB bandwidth. Moreover, it exhibits an excellent beam quality with an M2 factor of 1.05 after 10-m-long transmission. The robust single-mode operation, ultralow loss, and wide bandwidth make the fiber an ideal candidate for short-distance Yb and Nd:YAG high-power laser delivery.

Flexible chalcogenide glass large-core multimode fibers for hundred-watt-level mid-infrared 2-5 µm laser transmission

The rapidly-developed high-power mid-infrared 2-5 µm laser technology requires a compact, flexible low-loss glass fiber for power delivery or laser generation. With the broadest bandwidth of low-loss transmission window in mid-infrared region amongst all mid-infrared glass fibers, chalcogenide glass fiber is the best candidate covering the whole 2-5 µm range. Multi-hundred-watt high-power delivery for 5.4-µm CO laser was previously demonstrated in a multimode chalcogenide fiber with a 1-mm-diameter large core, at the cost of giving up one of the most desirable fiber advantages, the flexibility. Indeed, chalcogenide glass fibers with decent flexibility have never exhibited hundred-watt-level power transmitting capability in the 2-5 µm range. In this paper, we have experimentally demonstrated 100-watt-level power transmission in multimode As2S3 chalcogenide fibers, using a customized high-power 2-µm thulium doped silica fiber laser source. With effective forced cooling, the multimode As2S3 fiber with 200 µm core diameter can resist incident laser power of 120 W and deliver transmitted power of 63 W. Nano-sized scattering center related laser damage mechanism and the cylindrical heat transfer model have been proposed to explain the high-power damage process of chalcogenide glass fibers. The calculation is in good agreement with the experiments. It is promising to further enhance the transmitted power above 100 W in flexible chalcogenide glass large-core fibers.

Single Microwire Optical Autocorrelator at 2-μm Wavelength

In recent years, significant efforts have been made to develop ultrafast laser source around 2- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> wavelength. Meanwhile, the characterization of the ultrashort pulses within ultracompact footprints has been attracting growing interests. Here relying on the transverse second-harmonic generation in a single CdTe microwire, we successfully demonstrate an optical autocorrelator for femtosecond pulse characterization at 2- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> wavelength. With 220-fs laser pulses (38 pJ/pulse) as input light, the measured pulse width after calibration is 236 fs, showing a good accuracy of this microwire autocorrelator. Due to the low-loss transmission window and high nonlinearity of CdTe in the mid-infrared (MIR) region, the microwire-based autocorrelator shows great potential for MIR applications ranging from on-chip optical communication to ultracompact laser spectroscopy.

Terahertz Hollow-Core Optical Fibers for Efficient Transmission of Orbital Angular Momentum Modes

Terahertz (THz) radiation has recently received a lot of interests in science and technology. The potential of orbital angular momentum (OAM) of light in the THz region is a novel idea for researchers with increasing applications in broadband communications. THz waveguides have rarely been reported to generate and guide OAM modes. In this paper, we propose for the first time hollow-core optical fibers consist of circular air-hole arrays for guiding THz OAM modes with the topological charge order up to l = 3. Using numerical simulations, two low-loss transmission windows over 0.47 - 0.65 THz (window1) and 0.95 - 1.20 THz (window2) are recognized to guide OAM modes. Results show that both windows of the THz fiber exhibit the flat near-zero dispersion as well as low confinement loss (~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> dB/cm) with simultaneously high purity of the OAM modes (higher than 90%). Finally, two designs of THz optical fibers consist of one and two rings of air-hole arrays in their cladding are examined and it is shown that difference of the confinement loss and the modal purity between two designs are negligible. This implies that hollow-core optical fibers consist of one ring of air-hole array in their cladding are very promising for efficient transmission of OAM modes in the THz region.

100 m full-duplex underwater wireless optical communication based on blue and green lasers and high sensitivity detectors

In this paper, in order to improve the practicability and performance of the underwater wireless optical communication (UWOC), we propose a full-duplex (FD) UWOC system based on blue–green wavelength division, which make full use of the low-loss transmission window of water. Narrowband optical filters corresponding to the blue and green wavelengths are employed to eliminate self-interference caused by the backscattering. Single-photon sensitivity photomultipliers (PMTs) are exploited to extend the communication range. The impairment introduced by limited bandwidth and nonlinearity of the devices is compensated by post equalizers. Finally, an FD UWOC system reaching a data rate up to 200-Mbps at a distance of 100 m is demonstrated in a standard swimming pool, which is competitive both in range and data rate among the reported explorations of FD UWOC system. By using a nonlinear equalizer based on the memory polynomial model, the data rates are enhanced by 60-Mbps and 140 Mbps for the blue and green links compared with hard decision, respectively. By measuring the minimum optical power at which the bit error ratio (BER) is lower than the forward error correction (FEC) threshold, and combining with the attenuation coefficient of water as well as the geometric loss caused by the beam divergence, we predict that the proposed FD UWOC system can achieve 200 Mbps over 120 m or 100 Mbps over 139 m underwater transmission. Finally, we analyze and estimate the performance of the FD UWOC system in practical seawater.

Low-Loss Dual-Polarized Frequency-Selective Rasorber With Graphene-Based Planar Resistor

A novel dual-polarized frequency-selective rasorber (FSR) using an omnidirectional planar resistor based on graphene flake is presented in this article. The FSR exhibits a low-loss transmission window above an absorption band. It consists of a lossy layer and a lossless bandpass frequency-selective surface (FSS). In the lossy layer, a central symmetric graphene-metal hybrid structure is formed by connecting four metal strip-based resonant circuits with a square graphene flake. A low insertion loss transmission band is obtained by removing the resistance out of the parallel resonant circuits. Moreover, equivalent circuits are utilized to demonstrate the working mechanism. The proposed FSR exhibits a good absorption performance from 8.15 to 13.8 GHz, and a low insertion loss of 0.15 dB obtained at 16 GHz. Finally, a prototype FSR is fabricated and good agreement between the simulated and the measured results is obtained. The FSR has further advantages of avoiding the process of welding multiple lumped resistors per unit, thereby greatly reducing the number of lumped components and improving the planar integration of the structure.

300 GHz bending transmission of silver/polypropylene hollow terahertz waveguide

A bendable metal/dielectric hollow waveguide for low-loss transmission of 300 GHz terahertz (THz) radiation was proposed based on polypropylene (PP) tubing materials. The PP tube has low absorption at 300 GHz and the silver reflective film grows well on the surface of the tube via simple wet chemical method. The external silver film coated PP tube constructs a leaky-type hollow waveguide structure. The PP tube acts as the waveguide substrate providing support as well as the thick dielectric layer (>100 μm) to regulate the transmission losses for low-THz frequencies. Moreover, the low loss transmission window has been shifted at 300 GHz by optimizing the wall thickness of the PP tube. The measured straight loss of the waveguide sample at 300 GHz reaches to 1.05 dB/m, and the bending loss raises to 1.34 dB/m at a bending angle of 270° and curvature radius of 15 cm.

Nonlinear dynamic of picosecond pulse propagation in atmospheric air-filled hollow core fibers

Atmospheric air-filled hollow core (HC) fibers, representing the simplest yet reliable form of gas-filled hollow core fiber, show remarkable nonlinear properties and have several interesting applications such as pulse compression, frequency conversion and supercontinuum generation. Although the propagation of sub-picosecond and few hundred picosecond pulses are well-studied in air-filled fibers, the nonlinear response of air to pulses with a duration of a few picoseconds has interesting features that have not yet been explored fully. Here, we experimentally and theoretically study the nonlinear propagation of ~6 ps pulses in three different types of atmospheric air-filled HC fiber. With this pulse length, we were able to explore different nonlinear characteristics of air at different power levels. Using in-house-fabricated, state-of-the-art HC photonic bandgap, HC tubular and HC Kagomé fibers, we were able to associate the origin of the initial pulse broadening process in these fibers to rotational Raman scattering (RRS) at low power levels. Due to the broadband and low loss transmission window of the HC Kagomé fiber we used, we observed the transition from initial pulse broadening (by RRS) at lower powers, through long-range frequency conversion (2330 cm-1) with the help of vibrational Raman scattering, to broadband (~700 nm) supercontinuum generation at high power levels. To model such a wide range of nonlinear processes in a unified approach, we have implemented a semi-quantum model for air into the generalized nonlinear Schrodinger equation, which surpasses the limits of the common single damping oscillator model in this pulse length regime. The model has been validated by comparison with experimental results and provides a powerful tool for the design, modeling and optimization of nonlinear processes in air-filled HC fibers.

High-birefringence hollow-core anti-resonant THz fiber

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.

Broadband Terahertz Refraction Index Dispersion and Loss of Polymeric Dielectric Substrate and Packaging Materials

In the effort to push the high-frequency performance of electronic circuits and signal interconnects from millimeter waves to beyond 1 THz, a quantitative knowledge of complex refraction index values and dispersion in potential dielectric substrate, encapsulation, waveguide, and packaging materials becomes critical. Here we present very broadband measurements of the real and imaginary index spectra of four polymeric dielectric materials considered for use in high-frequency electronics: benzocyclobutene (BCB), polyethylene naphthalate (PEN), the photoresist SU-8, and polydimethylsiloxane (PDMS). Reflectance and transmittance spectra from ~ 3 to 75 THz were made using a Fourier transform spectrometer on freestanding material samples. These data were quantitatively analyzed, taking into account multiple partial reflections from front and back surfaces and molecular bond resonances, where applicable, to generate real and imaginary parts of the refraction index as a function of frequency. All materials showed signatures of infrared active organic molecular bond resonances between 10 and 50 THz. Low-loss transmission windows as well as anti-window bands of high dispersion and loss can be readily identified and incorporated into high-frequency design models.

Low frequency Raman scattering for high resolution low temperature optical fiber sensors

Raman distributed optical fiber temperature sensors are based on the intensity ratio of the anti-Stokes to the Stokes Raman band at 440cm−1 of silica. In this paper we predict that the sensitivity of the Raman measurements for low temperatures can be improved by considering the Boson peak in the low frequency Raman scattering domain at 60cm−1. In this way Raman temperature sensors can be performed down to cryogenic temperatures. It is further shown that the Boson peak is less dependent than the 440cm−1 band to the polarization of light. For the usual excitation at 1550nm the anti-Stokes Boson peak at 1536nm is in the low loss transmission window of the silica fibers.

Guiding terahertz orbital angular momentum beams in multimode Kagome hollow-core fibers.

We explore terahertz (THz) orbital angular momentum (OAM) modes supported in multimode Kagome hollow-core fibers. Numerical models are adopted to characterize the effective indices and confinement losses of vector modes over 0.2-0.9THz, where two low-loss transmission windows are observed. Linearly combining the vector modes, THz OAM states can be generated. Covering a broad bandwidth of 0.25THz, the purity values of OAM modes are beyond 0.9. Using numerical simulations, the hollow-core THz fibers with one and two rings of Kagome structures are also comparably investigated. We reveal that the OAM purity is dependent upon the confinement performance of THz fiber.

Antiresonant Hollow Core Fiber With an Octave Spanning Bandwidth for Short Haul Data Communications

We report an effectively single mode tubular antiresonant hollow core fiber with minimum loss of ∼25 dB/km at ∼1200 nm, and an extremely wide low-loss transmission window (lower than 30 dB/km loss from 1000 to 1400 nm and 6 dB bandwidth exceeding 1000 nm). Despite the relatively large mode field diameter of 32 µm, the fiber can be interfaced to SMF28 to produce fully connectorized samples. Exploiting an excellent modal purity arising from large modal differential loss and low intermodal coupling, we demonstrate penalty-free 10G on–off keying data transmission through 100 m of fiber, at wavelengths of 1065, 1565, and 1963 nm.

40 Gb/s WDM Transmission Over 1.15-km HC-PBGF Using an InP-Based Mach-Zehnder Modulator at 2 μm

The 2-μm wavelength range has emerged as a low-loss and low-latency optical transmission window when using hollow-core photonic band gap fiber (HC-PBGF) and high-gain thulium-doped fiber amplifiers (TDFA). Various single and multichannel transmission experiments at these wavelengths have been implemented using directly modulated lasers and LiNbO3-based modulators. Here, we report the transmission performance of an externally modulated 4 × 10 Gb/s NRZ-OOK WDM signal over 1.15 km of low-loss HC-PBGF employing an InP-based Mach–Zehnder modulator (MZM) in the transmitter for the first time. An OSNR of 25 dB on 100-GHz spaced channels is required using a direct detection scheme. Furthermore, we demonstrate the lowest Vπ InP-based MZM operating at 2 μm by increasing the electro-optical overlap in the optical waveguide. The peak–peak modulation voltage is reduced significantly from 4 to 2.7 V with an electro-optic bandwidth of 9 GHz.

Fabrication of an IR hollow-core Bragg fiber based on chalcogenide glass extrusion

The theoretical analysis and experimental preparation of a hollow-core Bragg fiber based on chalcogenide glasses are demonstrated. The fiber has potential applications in bio-sensing and IR energy transmission. Two chalcogenide glasses with, respectively, high and low refractive indexes are investigated in detail for the fabrication of hollow-core Bragg fibers. The most appropriate structure is selected; this structure is composed of four concentric rings and a center air hole . Its band gap for the Bragg fiber is analyzed by the plane wave method. The chalcogenide glasses Ge15Sb20S58.5I13 and Ge15Sb10Se75 are chosen to extrude the robust multi-material glass preform with a specialized punch and glass container. The glass preform is simultaneously protected with a polyetherimide polymer. The hollow-core Bragg fibers are finally obtained after glass preform extrusion, fiber preform fabrication, and fiber drawing. Results showed that the fiber has a transparency window from 2.5 to 14 μm, including a low-loss transmission window from 10.5 to 12 μm. The location of this low-loss transmission window matches the predicted photonic band gap in the simulation

Highly photostable organic distributed feedback laser emitting at 573 nm

An efficient, low-cost, and highly photostable second-order distributed feedback (DFB) laser, fabricated by thermal nanoimprint lithography and based on a polymer active film containing a perylenediimide derivative, is reported. It shows a photostability half-life of 3.1×105 pump pulses (&amp;gt;8 h), when pumped at the same spot of the film. This value is the highest reported to date for organic DFB lasers measured under ambient conditions. The device emits at 573 nm, matching the second low-loss transmission window of poly(methylmethacrylate) (460–590 nm), thus offering potential for applications in data communications based on polymer optical fibers.

Effect of structural modifications in the laser properties of polymer films doped with perylenebisimide derivatives

In this work we study the effect of different types of modifications of the chemical structure of perylenebisimide derivatives (PBIs) in the laser properties of PBI-doped polystyrene films at various concentrations. In particular, we focus on controlling the wavelength of emission, in order to tune the laser wavelength, as well as on increasing the amount of PBI in the films, aiming to decrease the laser thresholds, while keeping a good photostability. Amplified spontaneous emission (ASE) was observed for all compounds, the best performance being obtained for films doped with PBIs symmetrically substituted at the imide positions, that emitted at 580 nm with a threshold of 20 kW/cm 2 and a photostability half-life of 30,000 pump pulses. Substitution at the bay positions of the PBI core and replacement of imide functions by anhydride groups allow to red-shift the emission wavelength up to 645 nm, but the thresholds increase considerably and the photostability is reduced. PBIs are among the most photostable materials reported in the literature and show very reasonable thresholds. In addition they have a great potential for their application in the field of data communications based on poly(methyl methacrylate) optical fibers, with low-loss transmission windows between 530 and 590 nm and at 650 nm.

High-power Yb-doped photonic bandgap fiber amplifier at 1150-1200 nm

Ytterbium-doped solid-core photonic bandgap fiber amplifiers operating at the long-wavelength edge of the ytterbium gain band are reported. The low-loss bandgap transmission window is formed in the very low gain region, whilst outside the bandgap, large attenuation inhibits the exponential growth of amplified spontaneous emission in the huge-gain 1030-1100 nm region. Hence parasitic-lasing-free, high-power amplification with a marked efficiency is enabled. A 32 W output at 1156 nm with a 66% slope efficiency and 30 W output at 1178 nm with a 58% slope efficiency were successfully obtained. To our knowledge, these are the highest output powers generating from active photonic bandgap fibers, as well as from ytterbium-doped fiber lasers at these wavelengths.