Mode Selective Coupler Research Articles

Experimental Investigation of LP11Mode to OAM Conversion in Few Mode-Polarization Maintaining Fiber and the Usage for All Fiber OAM Generator

In this paper, the LP11 mode evolution in a few mode-polarization maintaining fiber is experimentally investigated. Selective conversion from the LP11a/LP11b mode to the LP11b/LP11a , OAM+1, and OAM−1 modes are demonstrated with varying wavelengths and fiber lengths. This property may have nice applications in spatial division multiplexing (SDM) systems, and one of them, i.e., the all-fiber compact and robust scalar orbital angular momentum (OAM) generator, which is cascaded by a mode-selective coupler and a few mode-polarization maintaining fiber, is also demonstrated with expected performance for conversion between LP01 mode and OAM+1/OAM−1.

All-Fiber Broadband Degenerate Mode Rotator for Mode-Division Multiplexing Systems

An all-fiber broadband degenerate mode rotator for the rotation between arbitrary degenerate modes is proposed and investigated numerically. Furthermore, the rotator combined with a mode selective coupler (MSC) can be used to realize mode multiplexing. The mode multiplexing can be implemented in one physical dimension, which is considered easier than 3-D cascading MSCs. It is found that the rotator for any two degenerate modes rotation possesses a high rotating efficiency from 1450 to 1650 nm, and the insertion loss at the interconnection of a fiber-based rotator and a normal fiber is lower than 3 dB for any modes. Moreover, the rotators in performance are not sensitive to the four geometrical parameters of design. Finally, a six-mode multiplexer using the proposed rotator combined with MSC is modeled and analyzed. It is shown that multiplexing efficiencies of higher than 70% and mode extinction ratios of more than 20 dB for mode LP01, LP11a/LP11b, LP21a/LP21b, and LP02 are achieved in the $C$ -band.

Loss Cause Identification by Evaluating Backscattered Modal Loss Ratio Obtained With 1-μm-Band Mode-Detection OTDR

We propose a novel diagnostic technique that uses 1-μm-band mode-detection optical time domain reflectometry (1 μm-OTDR) to identify the loss causes such as macrobends and fusion splices, of single-mode fibers. 1 μm-OTDR, which employs 1-μm-band probe pulses and a mode selective coupler operating in the same band, enables us to individually observe the fundamental (LP01) and the second-order (LP11) modes of the backscattered light generated in the two-mode region of single-mode fibers. Evaluating the ratio of losses occurring in the LP01 to LP11 modes of the backscattered light allows us to determine whether the cause of loss is a macrobend or a fusion splice. A proof-of-concept demonstration is performed for two kinds of widely used single-mode fibers, and the results show that macrobending and fusion splice losses can be clearly identified.

Removing the Directional Degeneracy of LP11Mode in a Fused-Type Mode Selective Coupler

The removal of polarization and modal degeneracy in the excitation of higher order modes is realised in all-fiber mode-selective weakly-fused-couplers by controlling the coupler geometry. Experiments carried out with short couplers show strong polarization dependency, while long couplers with a slow varying cross-section have shown to remove polarization and spatial degeneracy in the first high order mode.

Mode-Selective Couplers for Two-Mode Transmission at 850 nm in Standard SMF

The optimal design of a low-loss fused fiber mode-selective coupler for two-mode fiber transmission in the 850-nm band is presented. The coupler is based on precise phase matching of the propagation constants in each arm of a weakly fused fiber coupler. The designed component permits both mode converter and mode multiplexer/demultiplexer operation, thus enabling modal multiplexing transmission in this band with no additional component. The presented design is evaluated by simulation considering two types of structures, leading to asymmetric and symmetric coupler configurations. Mode converter and mode multiplexer operation is achieved with 93.5% efficiency in the band of 845–855 nm. Mode demultiplexer operation is achieved with an extinction ratio better than 20.4 dB in the same band.

Broadband mode division multiplexer using all-fiber mode selective couplers.

We report an all-fiber mode division multiplexer formed with cascaded mode selective couplers with significantly broadened bandwidth potentially spanning S, C and L band. This was achieved by matching the effective refractive indices over a wide wavelength range for the few mode fiber and the single mode fiber used in the coupler. The multiplexer provides high coupling efficiency (>55% for the worst case) for the 4 spatial modes over the entire wavelength range of 1515-1590 nm. The all-fiber construction provides mechanical stability. Experimental results for the coupling efficiency and the mode extinction ratio for each spatial mode are presented along with the far field radiation patterns.

Cascaded Mode-Division-Multiplexing and Time-Division-Multiplexing Passive Optical Network Based on Low Mode-Crosstalk FMF and Mode MUX/DEMUX

We propose cascaded mode-division-multiplexing and time-division-multiplexing passive optical network (MDM-TDM-PON) based on low mode-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX), in which optical network units communicate with the optical line terminal utilizing different time slots and specific optical linearly polarized spatial modes. An MDM optical distribution network (ODN) is cascaded with multiple conventional TDM ODNs to effectively extend a larger scale of current commercial PON systems based on TDM. The upgrade from TDM-PON to the cascaded MDM-TDM-PON is simple and cost-effective. No multiple-input–multiple-output (MIMO) digital signal processing is required to eliminate the mode crosstalk. The all-fiber mode MUX/DEMUXs are composed of mode selective couplers, which simultaneously multiplex or demultiplex multiple modes. We experimentally demonstrate MDM-PON transmission over 10 and 55 km two-mode FMFs and cascaded MDM-TDM-PON transmission over a 10-km two-mode FMF and a 10-km standard single-mode fiber with 10-Gb/s optical on–off keying (OOK) signal and direct detection.

High-Sensitivity Detection of Fiber Bends: 1-μm-Band Mode-Detection OTDR

We propose a measurement system based on optical time-domain reflectometry (OTDR) that offers high-sensitivity detection of bend location in a standard single-mode (SM) fiber. This technique, which employs 1-μm-band probe pulse, i.e., below the cut-off wavelength of SM fiber, enables us to individually observe the fundamental (LP01 ) and the second-order (LP11) modes of the backscattered light generated in the two-mode region of the SM fiber, using a key technical advance is a mode-selective coupler designed for optimal 1-μm-band operation. Analyzing the loss distribution of the backscattered LP11 mode realizes the high-sensitivity detection of the fiber bend location without suffering degradations from the modal dispersion. We perform experiments for the four kinds of widely-used SM fibers, and demonstrate the proof-of-concept; a comparison is made with the conventional OTDR operating at the wavelength of 1650 nm.

Grating assisted optical waveguide coupler to excite individual modes of a multi-mode waveguide

Spatial division multiplexing (SDM) in the form of mode division multiplexing (MDM) in multi-mode (MM) waveguides is currently explored to overcome the capacity limitation of single-mode (SM) waveguides in data transmission technology. In this work a new approach towards mode selective optical waveguide couplers to multiplex and demultiplex individual modes of MM waveguides is presented. We discuss a grating assisted mode selective optical waveguide coupler and evaluate numerically its coupling efficiency. The approach relies on a grating structure in a SM waveguide which is used to excite individual modes of an adjacent unmodified MM waveguide via evanescent field coupling. The simulations verify that by using the grating structure and tailoring the grating period, light from the SM waveguide can be coupled selectively into the fundamental mode or any higher-order mode of a MM waveguide with high efficiency and low crosstalk to adjacent mode-channels. The results indicate the potential of the grating assisted waveguide coupler approach for future applications in on-chip photonic networks and the (de)multiplexing of individual modes of MM waveguides.

Mode converter based on dual-core all-solid photonic bandgap fiber

In this paper, we present a mode-selective coupler based on a dual-core all-solid photonic bandgap fiber (AS-PBGF). Because they are all-solid, AS-PBGF-based mode converters are easier to splice to other fibers than those based on air-hole photonic crystal fibers. Mode conversions between the LP01 and LP11 modes, LP01 and LP21 modes, and LP01 and LP02 modes are obtained at the wavelength λ=1550 nm. The 3 dB wavelength bandwidth of these mode converters are 47.8, 20.3, and 20.3 nm, respectively.

Holey fiber mode-selective couplers.

Directional mode coupling in an asymmetric holey fiber coupler is demonstrated both numerically and experimentally for the first time. The holey fiber mode couplers have interesting spectral characteristics and are also found to exhibit increased dimensional tolerances. Following a design based on numerical investigations, a dual-core polymer holey fiber coupler for LP(01) and LP(11) mode multiplexing was fabricated via a drilling and drawing technique. The measurements are compared with the simulation results.

Controlling guided modes in plasmonic metal/dielectric multilayer waveguides

We investigate the mode properties of planar dielectric aluminum-quinoline (Alq3) multilayer waveguides comprising one single or three equally spaced embedded nanometer-thin (∼10 nm thick) Alq3-Mg0.9:Ag0.1 composite metal-island layers. The plasmonic waveguides were fabricated by organic molecular beam deposition. Transverse magnetic (TM) and transverse electric (TE) modes were selectively excited using the m-line method. The symmetric plasmonic TM0 mode was launched in all waveguides and—in addition—two higher order plasmonic TM1 and TM2 modes were generated in waveguides comprising three metal layers. Other TM modes have hybrid dielectric-plasmonic characters, showing an increased effective refractive index when one electric field antinode is close to a metallic layer. TM modes which have all their antinode(s) in the dielectric layers propagate essentially like dielectric modes. TE modes with antinode(s) at the position of the metal layer(s) are strongly damped while the losses are low for TE modes comprising a node at the position of the composite metal film(s). The possibility to control the effective refractive index and the losses for individual hybrid plasmonic-dielectric TM and dielectric TE modes opens new design opportunities for mode selective waveguides and TM-TE mode couplers.

Mode- and wavelength-division multiplexed transmission using all-fiber mode multiplexer based on mode selective couplers.

We propose all-fiber mode multiplexer composed of two consecutive LP₁₁ mode selective couplers that allows for the multiplexing of LP₀₁ mode and two-fold degenerate LP₁₁ modes. We demonstrate WDM transmission of 32 wavelength channels with 100 GHz spacing, each carrying 3 modes of 120 Gb/s polarization division multiplexed quadrature phase shifted keying (PDM-QPSK) signal, over 560 km of few-mode fiber (FMF). Long distance transmission is achieved by 6×6 multiple-input multiple-output digital signal processing and modal differential group delay compensated link of FMF. The all-fiber mode multiplexer has considerable potential to be used in mode- and wavelength-division multiplexed transmission.

Broadband Mode Converter Based on Photonic Crystal Fiber

In this letter, the scheme of a tapered mode-selective coupler based on photonic crystal fiber (PCF) is proposed. Three typical mode converters, such as $\mathrm{LP}_{\text {01}}$ to $\mathrm{LP}_{\text {11a}}$ , $\mathrm{LP}_{\text {01}}$ to $\mathrm{LP}_{\text {11b}}$ , and $\mathrm{LP}_{\text {01}}$ to $\mathrm{LP}_{\text {21}}$ , are described and simulated in detail. The coupling loss and bandwidth is calculated to evaluate those mode converters. The results demonstrate that those mode converters are broadband and low-loss devices. In addition, those mode converters are insensitive to the orthogonal polarizations. They can be fabricated using PCF postprocessing technique and are useful for future mode-division multiplexing optical fiber communication systems.

Femtosecond direct-written integrated mode couplers.

We report the design and fabrication of three-dimensional integrated mode couplers operating in the C-band. These mode-selective couplers were inscribed into a boro-aluminosilicate photonic chip using the femtosecond laser direct-write technique. Horizontally and vertically written two-core couplers are shown to allow for the multiplexing of the LP11a and LP11b spatial modes of an optical fiber, respectively, with excellent mode extinction ratios (25-37 + dB) and low loss (~1 dB) between 1500 and 1580 nm. Furthermore, optimized fabrication parameters enable coupling ratios close to 100%. When written in sequence, the couplers allow for the multiplexing of all LP01, LP11a and LP11b modes. This is also shown to be possible using a single 3-dimensional three-core coupler. These integrated mode couplers have considerable potential to be used in mode-division multiplexing for increasing optical fiber capacity. The three-dimensional capability of the femtosecond direct-write technique provides the versatility to write linear cascades of such two- and three-core couplers into a single compact glass chip, with arbitrary routing of waveguides to ensure a small footprint. This technology could be used for high-performance, compact and cost-effective multiplexing of large numbers of modes of an optical fiber.

Three-dimensional ultra-broadband integrated tapered mode multiplexers

The demonstration of a three-dimensional tapered mode-selective coupler in a photonic chip is reported. This waveguide-based, ultra-broadband mode multiplexer was fabricated using the femtosecond laser direct-write technique in a boro-aluminosilicate glass chip. A three-core coupler has been shown to enable the multiplexing of the LP01, LP and LP spatial modes of a multimode waveguide, across an extremely wide bandwidth exceeding 400 nm, with low loss, high mode extinction ratios and negligible mode crosstalk. Linear cascades of such devices on a single photonic chip have the potential to become a definitive technology in the realization of broadband mode-division multiplexing for increasing optical fiber capacity.

Mapping of intermodal beat length distribution in an elliptical-core two-mode fiber based on Brillouin dynamic grating.

Distributed measurement and characterization of the intermodal beat length between LP(01) and LP(11) modes in an elliptical-core (e-core) two-mode fiber (TMF) are demonstrated by the analysis of Brillouin dynamic grating (BDG) spectra. The BDG is generated by the stimulated Brillouin scattering (SBS) of the LP(01) mode and probed by the LP(11) mode, with four different pairs of the spatial and polarization modes in the e-core TMF applied for the pump and the probe (LP(01)x-LP(11)y, LP(01)y-LP(11)y, LP(01)x-LP(11)x, and LP(01)y-LP(11)x). A mode selective coupler (MSC) is used for selective launch and retrieval of the LP(01) and the LP(11) modes in the BDG operation, and the local reflection spectra from the BDG are obtained by an optical time-domain analysis. A distribution map of the intermodal beat length is acquired for each pair of the pump-probe modes with a 1.5 m spatial resolution along a 75 m e-core TMF. Temperature- and strain-dependence of the BDG spectrum is also evaluated for each case.

Securitized Optical Fiber Communication and Sensor Systems Using Mode-Selective Couplers

In this paper, we report the results of testing a securitized hybrid sensing/communication system. A long-period grating sensor operating in the 1550 nm range along a bidirectional communication link at 1310 nm is securitized against intrusion by launching visible light at 650 nm through a splitter. The arising few-mode interference pattern changes are monitored and the responses to a variety of deformations on the fiber link are investigated. It is shown that the intrusion sensing channel is activated by even the weakest deformations imposed on the fiber with no losses on the channel being incurred. Analysis of the intermodal pattern changes make it possible to discern the types of deformations imposed on the sensor-communication channel.

Mode division multiplexed optical transmission enabled by all-fiber mode multiplexer.

Mode division multiplexed optical transmission enabled by all-fiber mode multiplexer is investigated. The proposed all-fiber mode multiplexer is composed of consecutive mode selective couplers. It multiplexes or demultiplexes LP01, LP11, LP21, and LP02 modes simultaneously. We demonstrate successful transmission of three spatial modes with 120 Gb/s PDM-QPSK signals over 15 km of four mode fiber by using 6x6 MIMO digital signal processing.

All-fiber fused directional coupler for highly efficient spatial mode conversion.

We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP(01) mode is coupled into the higher order mode (LP(11), LP(21), LP(02)) in the other arm, over a broadband wave-length range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry.