W-type Fiber Research Articles

Analysis on acoustic modes and Brillouin gain spectra of backward stimulated Brillouin scattering in W-type acoustic waveguide optical fibers

The acoustic Helmholtz equations of W-type acoustic waveguide fibers, including WI- and WII-type acoustic velocity of vl,2 < vl,3 < vl,1 and vl,2 < vl,1 < vl,3, separately are solved by using the method of separation of variables, and their characteristic equations are derived according to the boundary condition and the acoustic Helmholtz equations. The distribution and cut-off of acoustic modes are analyzed by introducing acoustic normalized frequencies. The dependence of the acoustic inner core radius, the acoustic velocities in the acoustic inner and acoustic outer core on acoustic modes, and Brillouin gain spectra (BGS) is investigated. The results indicate that two significant peaks of the BGS are induced by acousto-optic coupling of L01 and L02 acoustic modes with LP01 optical mode. The acousto-optic overlap factor of the L01 acoustic mode decreases first and then increases, and that of the L02 acoustic mode increases first and then decreases with the rise of acoustic inner core radius. Due to the difference of the acoustic mode field distribution, the gain peak induced by L01 mode is sensitive to the acoustic velocity of the acoustic inner core, and that of the L02 mode is sensitive to the acoustic velocity of the acoustic outer core. These conclusions agree well with the reported experimental results, and provide theoretical references for research and application of backward stimulated Brillouin scattering of acoustic waveguide optical fibers.

Estimation of propagation characteristics of l1 series of LP modes of few-mode W-type fibers using numerical and analytical approach

In this work, single and two parameter variational fields for the l1 series of linearly polarised (LPl1) modes of a graded index few-mode W-type fibers have been proposed. The propagation characteristics, such as, the propagation constant, the normalised dispersion parameter and the normalised group delay have been computed for lower order LPl1 modes of such fibers using these variational fields. The exact results were derived by numerically solving the scalar wave equation for the LPl1 modes of W-type fibers. Further, the cut-off frequencies for the first few lower order modes have been computed numerically. Accuracies of the estimated propagation parameters have been investigated by comparing it with the numerical results. Compared to single-parameter variational fields, the two-parameter variational fields provide superior accuracy. Additionally, it has been found that the accuracy increases for lower order LPl1 modes. Thus, few-mode graded-index W-type fiber based photonic devices may utilise these proposed trial fields.

Effects of fiber cross-section shape on thermal comfort properties of polyester interlock fabrics

In this study, some dimensional and thermal comfort properties, i.e. thermal resistance, water vapor permeability, and air permeability properties of interlock fabrics knitted from different cross-section shaped polyester fibers were investigated. Four polyester yarns of different fiber cross-section shapes (hollow-round, round, triangular, and W-shape) were used to produce the samples. From the results, it was found that the triangular and W-type fibers had higher fabric density values due to the denser and closer fiber settlement. The highest bursting strength value belonged to the sample knitted from hollow-round yarn, which had also the highest yarn-breaking load value. The samples knitted from both round cross-section polyester yarns had significantly higher air permeability and thermal resistance values than the other samples due to the lower fabric density values of these fabrics. On the other hand, the water vapor permeability values of the fabrics from triangular and W-type fibers were slightly higher than the others because of their lower thicknesses.

A dehydrated W-type fluorotellurite fiber for mid-infrared supercontinuum generation

A W-type fluorotellurite glass fiber was fabricated by rotational casting method. The OH absorption coefficient of the fluorotellurite glass is 0.004 cm−1, and the OH-induced attenuation of the fiber is 8.5 dB/m at 3.1 μm. The fiber has a small core of 5 μm in diameter with the lowest loss of 0.9 dB/m at 2.7 μm. 1–2.4 µm supercontinuum (SC) has been generated from a 30 cm-long dehydrated fluorotellurite glass fiber pumped by a 1560 nm femtosecond fiber laser. The numerical simulation is in good agreement with experiment result based on the generalized nonlinear Schrödinger equation. The experiment and numerical simulation show that the dehydrated fluorotellurite fiber is a promising candidate for SC in 1–4 µm region.

Triple-clad W-type fiber mitigates multipath artifacts in multimodal optical coherence tomography.

Multimodal endoscopic optical coherence tomography (OCT) can be implemented with double-clad fiber by using the presumed single-mode core for OCT and the higher numerical aperture cladding for a secondary modality. However, the quality of OCT in double-clad fiber (DCF) based systems is compromised by the introduction of multipath artifacts that are nt present in single-mode fiber OCT systems. Herein, the mechanisms for multipath artifacts in DCF are linked to its modal contents using a commercial software package and experimental measurement. A triple-clad W-type fiber is proposed as a method for achieving multimodal imaging with single-mode quality OCT in an endoscopic system. Simulations of the modal contents of a W-type fiber are compared to DCF and single-mode fiber. Finally, a W-Type fiber rotary catheter is used in a DCF-based endoscopic OCT and autofluorescence imaging (AFI) system to demonstrate multipath artifact free OCT and AFI of a human fingertip.

Single-mode operation and spectral depuration by W-type Yb-doped fibers in Q-switched fiber lasers

We report a W-type fiber design for improving the beam quality and spectral purity of the Q-switched Yb-doped fiber lasers (YDFLs). Our W-type fiber exhibited higher losses in the high order modes (>100 dB/m) and lower losses in the fundamental modes (<0.1 dB/m). A 20/130 μm Yb-doped W-type fiber was fabricated by employing a modified chemical vapor deposition (MCVD) process and a solution-doping technique. The Yb-doped W-type fiber was examined in an acousto-optic Q-switched YDFL. A robust single-mode operation with a 1.19 M2 factor was obtained by optimizing the bending radii. The suppression of mixed mode four-wave mixing (FWM) and amplified spontaneous emission (ASE) was observed in the output spectral with an 18 dB signal to noise ratio (SNR) enhancement at 1064 nm and a 5.3 dB ASE inhibition ratio at 1030 nm. The Yb-doped W-type fiber exhibited enhanced beam quality and output spectra than the conventional Yb-doped step-type refractive index fiber. Therefore the W-type fiber suits the Q-switched fiber laser applications.

Wavelength dependent equilibrium mode distribution and steady-state distribution in double-clad W-type microstructured polymer optical fibers

This paper investigates wavelength dependence of equilibrium mode distribution and steady-state distribution in W-type (double-clad) microstructured polymer optical fibers (mPOFs) with a solid core for parametrically varied refractive index and width of the intermediate layer (IL) (inner cladding) by solving the time-independent power flow equation (TI PFE). In the case of wider IL, independent of wavelength, the lengths for establishing the equilibrium mode distribution and steady-state distribution are larger. We have demonstrated that the wavelength has no effect on these lengths for IL’s width that is larger. These lengths drop in a wavelength-dependent manner as the IL's width decreases. Equilibrium mode distribution and steady-state distribution occur at shorter optical fiber lengths as the depth of the IL diminishes, which is due to the similarly declining number of leaky modes. The smaller the depth of the IL, the shorter the fiber length is required for completion of the coupling process. These programmable characteristics allow double-clad W-type mPOFs to be easier customized for a particular use in optical fiber sensors and communications at various wavelengths.

Investigation of bandwidth in multimode W-type microstructured plastic optical fibers

By employing the time-dependent power flow equation (TD PFE), we examine the bandwidth in multimode W-type (double clad) microstructured plastic optical fibers (mPOFs) with a PMMA (polymethyl methacrylate) solid core for parametrically varying depth and width of the intermediate layer (IL) (inner cladding). The investigated W-type mPOF's bandwidth was calculated for various configurations of the air-holes in the inner cladding and varied launch excitations. We obtained that results for smaller inner cladding air-holes at longer fiber lengths exhibit greater bandwidth. On the other hand, for shorter fibers, the launch beam which only excites guided modes, has no effect on the bandwidth due to the air-hole size. The W-type mPOF with a narrower inner cladding has a greater bandwidth. The bandwidth is also larger for a narrow launch that only excites guided modes as opposed to a wider launch that excites both guided and leaky modes. Therefore, the bandwidth increases as the width of the IL is reduced, or by decreasing the diameter of air-holes in the IL, or by exciting only guided modes. W-type mPOFs can be more easily tailored for a specific use in optical fiber sensors and communications because to their programmable properties.

Transmission performance of multimode W-type microstructured polymer optical fibers.

By solving the time-independent power flow equation (TI PFE), we study mode coupling in a multimode W-type microstructured polymer optical fiber (mPOF) with a solid-core. The multimode W-type mPOF is created by modifying the cladding layer and reducing the core of a multimode singly clad (SC) mPOF. For such optical fiber, the angular power distributions, the length Lc at which an equilibrium mode distribution (EMD) is achieved, and the length zs for establishing a steady state distribution (SSD) are determined for various arrangements of the inner cladding's air-holes and different launch excitations. This information is useful for the implement of multimode W-type mPOFs in telecommunications and optical fiber sensors.

W-type and Graded-index bismuth-doped fibers for efficient lasers and amplifiers operating in E-band.

In this paper, we focus on the fabrication and investigation of optical properties of W-type and Graded-index single-mode Bi-doped germanosilicate fibers. The laser and gain characteristics of Bi-doped fibers of new designs were studied. It was shown that by variation of doping profile, it is possible to change characteristic parameters (active absorption, unsaturable loss level) of the active medium and, as a consequence, achieve an improvement of the performance of the optical devices based on these types of fibers. As a progress one can consider the creation of a Bi-doped fiber laser operating at 1460 nm with a record efficiency of 72% using a relatively short active fiber (L = 75 m); and a 20-dB Bi-doped fiber amplifier (L = 120 m) with a pump power of 45 mW (for the input signal powers lower than 30 µW) having a high gain efficiency of 0.52 dB/mW. We suggest that the obtained results could be a driver for further investigation in this direction.

Theoretical Investigation of the Influence of Wavelength on the Bandwidth in Multimode W-Type Plastic Optical Fibers with Graded-Index Core Distribution.

The bandwidth of multimode W-type plastic optical fibers (POFs) with graded-index (GI) core distribution is investigated by solving the time-dependent power flow equation. The multimode W-type GI POF is designed from a multimode single-clad (SC) GI POF fiber upon modification of the cladding layer of the latter. Results show how the bandwidth in W-type GI POFs can be enhanced by increasing the wavelength for different widths of the intermediate layer and refractive indices of the outer cladding. These fibers are characterized according to their apparent efficiency to reduce modal dispersion and increase bandwidth.

Amplification of femtosecond and picosecond pulses using large mode area Erbium-doped fiber at 1530 nm wavelength

We present amplification of short and ultrashort laser pulses at 1530-nm center wavelength using a large mode area Er-doped fiber (LMA-EDF). The mode area of about 1700 μm2 tremendously increases the power handling capacity of the fiber and helps in achieving up to ∼ 17 dB/m gain with 10 W power of 980-nm wavelength pump. The LMA fiber used here is a 3-layer W-type fiber designed to have effective single mode operation with higher order mode discrimination and sufficient mode spacing between the first few modes to avoid any possible mode coupling. In particular, we numerically demonstrate the amplification of 300 fs, 1 kW peak power pulses to ∼ 20 fs, ∼2.6 MW peak power pulses in 1.5-m propagation distance, and of 50 ps, 1 kW peak power pulses to ∼ 65 ps, ∼26 MW peak power pulses in 3-m propagation distance of the LMA-EDF. The generated pulses would be useful in applications such as eye-safe remote sensing and 3D laser processing of silicon.

Wavelength dependent transmission in W-type plastic optical fibers with graded index core distribution

This paper investigates the wavelength dependence of the equilibrium mode distribution (EMD) and steady state distribution (SSD) in W-type plastic optical fibers (POFs) with graded index (GI) core distribution for parametrically varied width of the fiber's intermediate optical layer and refractive index of the outer cladding. The numerical solution of the time-independent power flow equation is used to determine the transmission characteristics of the W-type GI POF. We demonstrated that the coupling length Lc required to achieve an EMD in W-type GI POF is shorter than the length determined experimentally for the original SC GI POF at 633 nm. We also demonstrated that as the wavelength increases, the EMD and steady-state distribution (SSD) are achieved at shorter W-type GI POF lengths. This is explained by the increase in leaky mode losses as wavelength increases. This makes it easier to tailor W-type GI POFs to a specific application at different wavelengths.

W-type normal dispersion thulium-doped fiber-based high-energy all-fiber femtosecond laser at 1.7 µm.

We propose a parabolic W-type thulium-doped fiber for the 1.7 µm high-energy femtosecond pulsed laser. Despite its attractive normal dispersion, the fiber offers high gain in 1.7 µm region thanks to its distributed short-pass filtering effect. With a proper dispersion management in an all-fiber chirped pulse amplification (CPA) scheme, we demonstrate so far the highest pulse energy of 128.0 nJ in a stable pulse of 174 fs in the 1.7-1.8 µm region, which marks above an order of magnitude improvement in pulse energy while exhibiting the shortest pulse duration among fiber-based CPA works at 1.7 µm. Hence, we provide a pathway to an energy scalable and efficient femtosecond laser at 1.7 µm via a compact and elegant all-fiber solution.

Influence of width of launch beam distribution and mode coupling on transmission characteristics of W-type plastic optical fibers

We show strong effects of width of the launch beam’s angular distribution on bandwidth and mode coupling in W-type plastic optical fibers (POFs). In general, the broader this width, the stronger the coupling and the lower the bandwidth—but the bandwidth drop due to this effect tapers off with distance from the input end and ceases at lengths over 25 m as the steady state distribution is reached. With increasing the strength of mode coupling, the equilibrium mode distribution is reached at shorter fiber lengths, which results in slower bandwidth decrease and finally in higher bandwidth at longer fiber lengths for fibers with larger coupling coefficients D. These results are aimed at aiding designers and application integrators of W-type POFs, particularly in employing laser, LED or vertical cavity surface emitting laser (VCSEL) light source.

Power flow in multimode W-type plastic optical fibers with graded index core distribution

A method is proposed for investigation of transmission along a multimode W-type (doubly clad) plastic optical fiber (POF) with graded index (GI) distribution of the core. The multimode W-type GI POF is designed from a multimode singly clad (SC) GI POF fiber upon modification of the cladding layer of the latter. The transmission characteristics of such proposed W-type GI POF are determined from the numerical solution of the power flow equation. We have shown that coupling length Lc at which an equilibrium mode distribution (EMD) is achieved in W-type GI POF is shorter than this length experimentally determined for the original SC GI POF. This is a consequence of leaky mode losses which reduce the number of higher guided modes involved in the coupling process, thus reducing the length Lc. We have shown that the shorter the length Lc, the earlier the bandwidth switches from the functional dependence of 1/z to 1/z1/2. We found that bandwidth improves with thinner and shallower intermediate layers. Finally, the bandwidth of the W-type GI POF analyzed in this work is greater if compared to the bandwidth of the original SC GI POF.

Method for calculating coupling coefficient in W-type multimode optical fibers

A simple method is proposed for determining the mode coupling coefficient D in W-type (double clad) multimode optical fibers. It only requires the observation of the far-field output pattern for one fiber length with the input light launched centrally along the fiber axis (θ 0= 0). For illustration, the coupling coefficient determined by this simple method for a W-type multimode glass optical fiber was used to calculate the coupling length L c at which the equilibrium mode distribution is achieved.

Effect of Modified Deformed Steel Fiber on Mechanical Properties of Artificial Granite

In order to explore the influence of the shape and surface treatment of steel fiber on the mechanical properties of artificial granite matrix, the drawing models of W‐type, L‐type, V‐type, and I‐type steel fiber are established, the pull‐out force of different shapes of steel fiber is calculated theoretically, and a large number of experimental specimens are made for the pull‐out test of steel fiber and material strength test. Both theory and experiment show that W‐type steel fiber has the greatest influence in artificial granite. The steel fibers with different shapes were treated with KH‐550 silane coupling agent. Then the steel fibers with different shapes before and after treatment were put into the artificial granite matrix for pull‐out test and material strength test. The results showed that, compared with L‐type, V‐type, and I‐type steel fibers before and after KH‐550 silane coupling agent treatment, W‐type steel fibers after KH‐550 silane coupling agent treatment have the best strength enhancement effect in artificial granite matrix.

Generation of femtosecond pulse train by pulse splitting in a large mode area fiber at 2 μm wavelength

Abstract We numerically demonstrate the generation of a very high peak power femtosecond pulse train by splitting a 1 ps, 100 kW pulse of central wavelength 2 μm using a few meters length of a large mode area fiber. The input 1 ps pulse during the propagation splits into multiple femtosecond pulses of very high peak powers. Concurrently, the spectral domain of the pulse also witnesses multiple peak formation. The used fiber is a W-type large mode area fiber, whose effective mode area can be varied from ~700 μm2 to ~1700 μm2 at 2 μm wavelength by varying the fiber parameters. The fiber design parameters were optimized for the efficient formation of femtosecond pulse train of peak powers ranging from ~100 kW to ~600 kW and FWHM ranging from ~100 fs to ~450 fs.

Influence of wavelength on equilibrium mode distribution and steady state distribution in W-type plastic optical fibers

Wavelength dependence of equilibrium mode distribution (EMD) and steady state distribution (SSD) in W-type plastic optical fibers (POFs) is investigated in this paper for parametrically varied width of the fiber’s intermediate optical layer and refractive index of the outer cladding. We have shown that with increasing the wavelength EMD and SSD are achieved at shorter W-type POF lengths. This is explained by the rise of the leaky mode losses with increasing wavelength. This facilitates tailoring W-fibers to a specific application at hand at different wavelengths.