Conventional Step-index Fiber Research Articles

Seeing the strong suppression of higher order modes in single trench fiber using the S2 technique.

The single trench fiber (STF) is a promising fiber design for mode area scaling and higher order mode (HOM) suppression. In this Letter, we experimentally demonstrate the strong HOM-suppression in a homemade STF using the spatially and spectrally resolved imaging (S2) technique. This STF has a 20-µm core and its performance is compared to a conventional step-index fiber with almost the same parameter. Results show that the bending loss of the HOM in STF is 8-times larger than conventional fiber at a bend radius of 7 cm. In addition, when severe coupling mismatch is introduced at the input end of the fiber, the STF can keep the fundamental-mode output while the conventional fiber cannot. To the best of our knowledge, this is the first time to experimentally analyze the HOM content in an STF and compare its performance with that of a conventional fiber. Our results indicate the great potential of the STF for filtering the HOM in fiber laser applications.

Highly birefringent microstructured optical fiber for distributed hydrostatic pressure sensing with sub-bar resolution.

We demonstrate distributed optical fiber-based pressure measurements with sub-bar pressure resolution and 1 m spatial resolution over a ∼100 m distance using a phase-sensitive optical time-domain reflectometry technique. To do so, we have designed a novel highly birefringent microstructured optical fiber that features a high pressure to temperature sensitivity ratio, a high birefringence and a mode field diameter that is comparable to that of conventional step-index single mode fibers. Our experiments with two fibers fabricated according to the design confirm the high polarimetric pressure sensitivities (-62.4 rad×MPa-1×m-1 and -40.1 rad×MPa-1×m-1) and simultaneously low polarimetric temperature sensitivities (0.09 rad×K-1×m-1 and 0.2 rad×K-1×m-1), at a wavelength of 1550 nm. The fiber features a sufficiently uniform birefringence over its entire length (2.17×10-4 ± 7.65×10-6) and low propagation loss (∼3 dB/km), which allows envisaging pressure measurements along distances up to several kilometers.

Nested compound negative curvature hollow-core fiber for single-mode operation in the infrared region

In this study, a novel tubular hollow-core fiber design with extended cladding structures aiming low transmission losses and dominant single-mode guidance in the infrared region is proposed. The fiber parameters are optimized to achieve low-loss operation at the target wavelength of 2μm, at which the emission spectrum of thulium-doped fiber lasers are centered. The confinement loss of the fundamental mode is found to be less than 0.02 dB/km in the near- and mid-infrared region, which makes the proposed design a strong alternative for conventional step-index silica optical fibers. Strong suppression of the higher order modes, and low bending sensitivity are also observed in the proposed design. The proposed fiber shows promising results to achieve low-loss transmission in the infrared spectrum.

Thermally Induced Mode Amplification Characteristics of Large Mode Area Segmented Cladding Fiber

A model of amplifier based on large mode area (LMA) segmented cladding fiber (SCF) is proposed. High order modes (HOMs) filtering effect and thermal-induced modal evolution characteristics are theoretically investigated by the few-mode steady state rate equations and heat conduction equation. Simulation results show that an amplifier based on ytterbium doped SCF presents much better HOMs suppression and mode beam quality than the amplifier based on conventional step-index fiber when achieving single-mode operation by coiling fiber. Furthermore, heat load generating in the core of ytterbium doped SCF can change the pre-designed refractive index profile through the thermo-optic effect, which induces evolution of mode amplification characteristics, including mode loss decreasing and mode area shrinking. And the co-pumping amplifier ytterbium doped SCF is more advantageous to get large mode area of fundamental mode at the output end, while single-mode operation is achieved much more effectively in the counter-pumping amplifier. These results can provide guidance in the design of high-power single-mode fiber amplifier based on rare-earth doped LMA-SCF.

Application and analysis of three-layer-core structure in single-mode large-mode-area fiber with low bending loss

A three-layer-core single-mode large-mode-area fiber with low bending loss is investigated in this paper. The three-layer structure in the core which is comprised of core-index layer, cladding-index layer and depression-index layer, can achieve large-effective-area Aeff while maintaining low-bending-loss without deteriorating cutoff behaviors. The large-mode-area of 100–330 μm<sup>2</sup> can be achieved in the fiber. The effective area Aeff can be further enlarged by adjusting the layer parameters. Furthermore, the bending property can be improved in this three-layer-core structure. The bending loss can decrease by 2–4 orders of magnitude compared with the bending loss of the conventional step-index fiber with the same Aeff. These characteristics of three-layer-core fiber suggest that it can be used in large-mode-area wide-bandwidth high-capacity transmission, or high-power optical fiber laser and amplifier in the optical communications, which can be conducive to studying the basic physical layer structure of big data storage, reading, calculation and transmission applications and so on.

Inverse photonic-crystal-fiber design through geometrical and material scalings

Geometrical and material — i.e., external and internal — scaling symmetries are exploited to obtain approximated analytical expressions for the mode effective index, group index, and chromatic dispersion of a scaled fiber. Our results include material refractive index scaling that changes the numerical aperture. First, the analytical expressions are successfully tested with a conventional step index fiber in a broadband range of wavelengths, from 1 to 2 μm. Then, we establish a procedure to adapt the analytical expressions to photonic crystal fibers (PCFs) and illustrate its application in a triangular PCF with circular holes. These adapted analytical expressions show good agreement with a rigorous numerical solution of the fundamental fiber mode. Finally, we demonstrate how powerful these expressions are for the design of PCFs. In particular, we illustrate our approach designing, in four iterations or less, PCFs with flattened dispersion profile over 300 nm or high dispersion slope over 40 nm, with different chromatic dispersion values.

Anti-resonance, inhibited coupling and mode transition in depressed core fibers.

The depressed core fiber (DCF), consisting of a low-index solid core, a high-index cladding and air surrounding, is in effect a bridge between the conventional step-index fiber and the tube-type hollow-core fiber from the point of view of the index profile. In this paper the dispersion diagram of a DCF is obtained by solving the full-vector eigenvalue equations and analyzed using the theory of anti-resonant and the inhibited coupling mechanisms. While light propagation in tube-type hollow-core fibers is commonly described by the symmetric planar waveguide model, here we propose an asymmetric planar waveguide for the DCFs in an anti-resonant reflecting optical waveguide (ARROW) model. It is found that the anti-resonant core modes in the DCFs have real effective indices, compared to the anti-resonant core modes with complex effective indices in the tube-type hollow-core fibers. The anti-resonant core modes in the DCFs exhibit similar qualitative and quantitative behavior as the core modes in the conventional step-index fibers. The full-vector analytical results for the simple-structure DCFs can contribute to a better understanding of the anti-resonant and inhibited coupling guidance mechanisms in other complex inversed index fibers.

High Birefringence and Negative Dispersion Based Modified Decagonal Photonic Crystal Fibers: A Numerical Study

Abstract In this paper, an extremely birefringent PCF based on a modified decagonal (MD-PCF) arrangement is studied for broadband compensation covering the S-, C- and L-communication bands wavelength ranging from 1460 to 1625 nm. It is made known in theory that it is conceivable to attain negative dispersion coefficient about − 448 to − 835 ps/nm/km covering S-, C- and L-communication bands as well as a relative dispersion slope near to single mode fiber (SMF) of 0.0036 nm−1. On the basis of simulation results incorporating finite-element method based COMSOL multiphysics software, birefringence is obtained as high as 1.7 × 10−2, which is definately greater than conventional step-index fiber (SIF) and circular air- holes PCF so far. We also discuss the characteristics of chromatic dispersion, effective area and confinement loss of the designed PCF.

All-fiber mode converter based on LP11 coupled-mode for mode division multiplexing systems

In this paper, two LP11 mode converters were reported and compared; one, employing two-mode conventional step index fiber (CSIF) and the other, using two identical multi-step two-mode fiber (MSTMF). In each case, the two fibers constituting the proposed devices were laterally misaligned. With the appropriate adjustments of the lateral displacements (h) and separation distance (d) between the axis of the identical CSIF and MSTMF, all-fiber converters with the coupling efficiencies of 69% and 94% were achieved, respectively. The performance of the two devices is compared in terms of the coupling efficiency (CE) and mode purity (MP) at the outputs. The results based on the coupling power indicate that MSTMF-based device offers higher CE and batter MP than the CSIF-converter. An MP of 84% and 58.8% were respectively determined for the MSTMF and CSIF converters. Therefore, the MSTMF device can be more suitable in mode conversion during mode division multiplexing (MDM) systems due to its high CE and MP.

Modal group-velocity mismatch induced intermodal modulation instability in step-index fiber

We present detailed experimental study on noise-seeded intermodal modulation instability (IM-MI) in the normal dispersion region of a conventional step-index fiber. The sharp refractive index contrast between core and cladding leads to large group velocity mismatch between the spatial modes, coaxing to efficient IM-MI and generation of multiple spectral peaks along with Raman peaks. Evolution of the spectrum with pump powers and fiber lengths are observed. Experimental findings are well supported with the theoretical framework based on a bimodal-MI model, considering the distinct dispersion parameters of the participating modes.

Mode Division Multiplexing Based on Ring Core Optical Fibers

The unique modal characteristics of ring core fibers (RCFs) potentially enable the implementation of mode-division multiplexing (MDM) schemes that can increase optical data transmission capacity with either low-complexity modular multi-input multi-output (MIMO) equalization or no MIMO equalization. This paper attempts to present a comprehensive review of recent research on the key aspects of RCF-based MDM transmission. Starting from fundamental fiber modal structures, a theoretical comparison between RCFs and conventional step-index and graded-index multi-mode fibers in terms of their MDM capacity and the associated MIMO complexity is given first as the underlining rationale behind RCF-MDM. This is followed by a discussion of RCF design considerations for achieving high-mode channel count and low crosstalk performances in either MIMO-free or modular MIMO transmission schemes. The principles and implementations of RCF mode (de-)multiplexing devices are discussed in detail, followed by RCF-based optical amplifiers culminating in MIMO-free or modular-MIMO RCF-MDM data transmission schemes. A discussion on further research directions is also given.

Design and analysis of three-layer-core optical fiber

A three-layer-core single-mode large-mode-area fiber is investigated. The three-layer structure in the core, which is composed of a core-index layer, a cladding-index layer, and a depression-index layer, could achieve a large effective area Aeff while maintaining an ultralow bending loss without deteriorating cutoff behaviors. The single-mode large mode area of 100 to 330 μm2 could be achieved in the fiber. The effective area Aeff can be further enlarged by adjusting the layer parameters. Furthermore, the bending property could be improved in this three-layer-core structure. The bending loss could decrease by 2 to 4 orders of magnitude compared with the conventional step-index fiber with the same Aeff. These characteristics of three-layer-core fiber suggest that it can be used in large-mode-area wide-bandwidth high-capacity transmission or high-power optical fiber laser and amplifier in optical communications, which could be used for the basic physical layer structure of big data storage, reading, calculation, and transmission applications.

Ultralow transmission loss in inhibited-coupling guiding hollow fibers

Attenuation in photonic bandgap guiding hollow-core photonic crystal fiber (HC-PCF) has not beaten the fundamental silica Rayleigh scattering limit (SRSL) of conventional step-index fibers due to strong core-cladding optical overlap, surface roughness at the silica cladding struts, and the presence of interface modes. Hope has been revived recently by the introduction of hypocycloid core contour (i.e., negative curvature) in inhibited-coupling guiding HC-PCF. We report on several fibers with a hypocycloid core contour and a cladding structure made of a single ring from a tubular amorphous lattice, including one with a record transmission loss of 7.7 dB/km at ∼750 nm (only a factor ∼2 above the SRSL) and a second with an ultrabroad fundamental band with loss in the range of 10–20 dB/km, spanning from 600 to 1200 nm. The reduction in confinement loss makes these fibers serious contenders for light transmission below the SRSL in the UV–VIS–NIR spectral range and could find application in high-energy pulse laser beam delivery or gas-based coherent and nonlinear optics.

Modal content of living human cone photoreceptors.

Decades of experimental and theoretical investigations have established that photoreceptors capture light based on the principles of optical waveguiding. Yet considerable uncertainty remains, even for the most basic prediction as to whether photoreceptors support more than a single waveguide mode. To test for modal behavior in human cone photoreceptors in the near infrared, we took advantage of adaptive-optics optical coherence tomography (AO-OCT, λc = 785 nm) to noninvasively image in three dimensions the reflectance profile of cones. Modal content of reflections generated at the cone inner segment and outer segment junction (IS/OS) and cone outer segment tip (COST) was examined over a range of cone diameters in 1,802 cones from 0.6° to 10° retinal eccentricity. Second moment analysis in conjunction with theoretical predictions indicate cone IS and OS have optical properties consistent of waveguides, which depend on segment diameter and refractive index. Cone IS was found to support a single mode near the fovea (≤3°) and multiple modes further away (>4°). In contrast, no evidence of multiple modes was found in the cone OSs. The IS/OS and COST reflections share a common optical aperture, are most circular near the fovea, show no orientation preference, and are temporally stable. We tested mode predictions of a conventional step-index fiber model and found that in order to fit our AO-OCT results required a lower estimate of the IS refractive index and introduction of an IS focusing/tapering effect.

Multiring large-mode-area delivery fiber for high power

The study presents a novel design of multiring delivery fiber with large mode area for high power. Using a FiberCAD method, we investigated a fiber whose core is surrounded by alternative low- and high-index rings. Based on our calculation, the effective area is 400 μm2 at 1.08 μm, larger than the ~280 μm2 of conventional step-index fiber (20/400). The macrobending loss at 1.08 μm is estimated to be 1×10(-3) dB/m, approximately one-third that of conventional step-index fiber (20/400). The single-mode operation can be achieved by the macrobending loss contrast between the fundamental mode (<1 dB/m) and high-order mode (>100 dB/m). The results indicate that multiring delivery fiber fabricated by modified chemical vapor deposition process is a promising candidate for high-power transmission.

All-Digital Holographic Tool for Mode Excitation and Analysis in Optical Fibers

A procedure for the multiplexing and demultiplexing of modes in optical fibers with digital holograms is presented. By using a spatial light modulator (SLM) to encode a digital hologram, the desired complex field is shaped and injected into the fiber. The SLM's ability to rapidly refresh the encoded transmission function enables one to excite pure single modes, as well as arbitrary coherent mode superpositions, in real-time. The modes from the output of the fiber are subsequently demultipexed by applying a correlation filter for modal decomposition, thus allowing for an all-digital-hologram approach to modal analysis of fibers. The working principle is tested using conventional step-index large mode area fibers being excited with higher-order single modes and superpositions.

1:61 photonic lanterns for astrophotometry: a performance study

In this paper, we study how the spectral information is transmitted through a 1:61 photonic lantern (PL) when illuminating the multimode (MM) end with a point source. We found that the spectrum undergoes two major effects between the MM input and the independent single-mode (SM) outputs of the PL. First, at resolution R = 775 (at 1550 nm), the overall output spectrum envelope strongly differs from SM to SM outputs. The spectra measured at the 61 SM outputs also vary when the small point object (10 μm in diameter, compared with the 100 μm diameter MM core) is moved across the MM fibre end-face of the PL. However, the sum of all the spectra measured at the SM outputs is very close to the spectrum of the point source, and is insensitive to the position of the point source. Secondly, at resolution R = 77 500 a fine 150 pm periodic structure was observed at the SM outputs. We found that a PL in the MM–SM configuration (i.e. MM input/SM outputs) is a very stable system regarding variations of the transmitted spectra when it is left mechanically non-perturbated and only submitted to environment-related changes, compared to a conventional step-index MM fibre of same core diameter. The latter clearly shows characteristic modal noise. Furthermore, we showed that at resolution R = 77 500 the sum of the spectra measured at 61 the SM outputs is very close to the spectrum of the point source, and does not present the 150 pm periodic oscillations. We also studied two PLs spliced together in the MM–SM–MM configuration and the results showed that the spliced PLs perform very well as mode scramblers, especially under conditions where conventional step-index MM fibres usually fail at performing effective scrambling.

Simple and complete formulation to compute propagation constants of photonic crystal fibers and predict their total chromatic dispersion

Within the scalar framework, a simple and complete formulation for the normalized propagation constants of the infinite cladding region of a photonic crystal fiber (PCF) with triangular lattice of air-holes is presented, which is dependent only on the ratio of air-hole diameters and their separation. The accuracy of the proposed formulation is depicted by comparing our results with those obtained by Russell. Then the refractive indices of the fundamental space-filling mode (nFSM) in the cladding region of the PCF from Russell's equation and the proposed relations are evaluated and the two indices are observed to match quite excellently for different values of relative air-hole size and wavelength. An equivalence between the two approaches of Russell and Saitoh is also sought. Finally, in order to check the validity of the formulation in problems of practical interest, the proposed relations are applied to evaluate the total chromatic dispersion in a PCF, treating it as a conventional step index fiber having its core and cladding indices as those of silica and nFSM, respectively. On comparison with the available results of Saitoh, the results match nicely.

Limitation on Effective Area of Bent Large-Mode-Area Leakage Channel Fibers

We investigate the bending characteristics of leakage channel fibers (LCFs) to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for compact Yb-doped fiber applications. Through numerical simulations, carried by the full-vectorial finite-element method, we present the limitations on the effective area of LCFs under bent condition and compare their limits with that of conventional step-index LMA fibers. Due to a better controllability of the low numerical aperture and a large value of the differential bending loss (~20 dB/m) between the fundamental and higher order modes in LCFs, the LMA of ~500 μm2 (core diameter of ~36 μm) at 1064 nm can be achieved when the optimized LCF is bent into a 10 cm bending radius.

Classification of the Core Modes of Hollow-Core Photonic-Bandgap Fibers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Using a new full-vectorial finite-difference mode solver utilizing a hexagonal Yee's cell, we calculated the dispersion diagram of a slightly multimode (16 modes) air-core photonic-bandgap fiber (PBF) and the electric-field profiles of all of its core modes. Careful comparison shows striking similarities between these properties and those of the hybrid modes of a conventional step-index fiber, in terms of the modes' field profiles, the modes' degeneracy, the order in which the modes mode cut off in the wavelength space, and the maximum number of modes. Based on these similarities, we propose for the first time a systematic nomenclature for the modes of a PBF, namely hybrid HE and EH modes and of quasi-TE and quasi-TM modes. Other small but relevant similarities and differences between the modes of these two types of fibers are also discussed. </para>