Mode Selective Coupler Research Articles

Tunable multi-wavelength EDF laser based on Sagnac interferometer with weakly-coupled FMF delay line

A simply manufactured and broadly tunable multi-wavelength Erbium-doped fiber (EDF) ring laser based on Sagnac interferometer with weakly-coupled few-mode fiber (FMF) delay line is proposed and experimentally demonstrated. The incorporation of FMF is realized with all-fiber mode multiplexer/demultiplexer (MUX/DEMUX) consisted of mode-selective couplers (MSC). The FMF and the mode MUX can simultaneously generate higher-order modes and fundamental mode. Single-, dual- and triple-wavelength lasing are obtained with broad tuning over 30-nm and smallest tuning step of 0.52-nm by adjusting polarization controller (PC) in the Sagnac interferometer. Optical signal-to-noise ratio (OSNR) higher than 30-dB and stability in 30-min are also measured.

Passively Mode-Locked Ytterbium-Doped Fiber Laser With Cylindrical Vector Beam Generation Based on Mode Selective Coupler

We propose and demonstrate, for the first time to the best of our knowledge, a passively mode-locked ytterbium-doped fiber laser generating cylindrical vector beams (CVBs) using a mode selective coupler (MSC) as the transverse mode converter and splitter, and a semiconductor saturable absorber mirror for mode-locking. According to the coupling mode theory and the phase matching principle, the MSC was fabricated by weakly fused technology, having a low loss of about 0.5 dB and achieving LP11 mode with a high purity of >96%. The CVB fiber laser operates at a center wavelength of 1042.3 nm, with a 3 dB spectral width of 1.5 nm. The repetition rate of the mode-locked laser pulses is 18.58 MHz. The radially and azimuthally polarized vector beams can be switched by adjusting the polarization controllers in the fiber ring cavity, with a high mode purity measured to be >95%. The mode-locked CVB ytterbium-doped fiber laser has potential applications in optical tweezers, optical imaging, etc.

Switchable Dual-Wavelength Cylindrical Vector Beam Generation From a Passively Mode-Locked Fiber Laser Based on Carbon Nanotubes

Cylindrical vector beams (CVBs) with axial symmetry in both polarization and field intensity have attracted much attention because of their unique optical properties. Conventional methods to obtain CVBs including direct modulation of light beams in free space and high-order mode excitation by offset splicing single-mode fiber with few-mode fiber usually works at single wavelength with rather narrow bandwidth. Here, for the first time to the best of our knowledge, we demonstrate switchable dual-wavelength CVB generation from a passively mode-locked fiber laser using carbon nanotubes as saturable absorber for mode-locking and a home-made mode-selective coupler as both mode converter and birefringence filter. In experiments, the mode-locked fiber laser delivers CVB pulses of dual-wavelength (1532.5 nm and 1555.5 nm) and corresponding single wavelength with duration of hundreds of femtosecond, respectively. Moreover, the output polarization status is switchable between radially and azimuthally polarized states. The mode-locked CVBs with wavelength and polarization flexibility may have potential applications in mode-division multiplexing optical fiber communication, nanoparticle manipulation, material processing, nonlinear optics, and so on.

High-order mode direct oscillation of few-mode fiber laser for high-quality cylindrical vector beams

Generation of high-order modes with high quality is important for the application of cylindrical vector beams in fibers. We experimentally demonstrated high-order LP11 mode generation and amplification with a broad bandwidth in an all few-mode fiber laser. A wavelength-division-multiplexing (WDM) mode selective coupler (MSC) is proposed to achieve efficient mode conversion from LP01 mode to LP11 mode, but also combine high-order LP11 modes at the wavelengths of 980/1550 nm. To the best of our knowledge, this is the first report on the high-order mode oscillation in an all few-mode fiber laser. LP11 mode and cylindrical vector beams including radially and azimuthally polarized beams are obtained with high modal purity. The purity of the generated high-order modes are all in excess of 95%.

Weakly-coupled 4-mode step-index FMF and demonstration of IM/DD MDM transmission.

Weakly coupled-mode division multiplexing (MDM) over few-mode fibers (FMF) for short-reach transmission has attracted great interest, which can avoid multiple-input-multiple-output digital signal processing (MIMO-DSP) by greatly suppressing modal crosstalk. In this paper, step-index FMF supporting 4 linearity polarization (LP) modes for MIMO-free transmission is designed and fabricated for the first time, to our knowledge. Modal crosstalk of the fiber is suppressed by increasing the mode effective refractive index differences. The same fabrication method as standard single-mode fiber is adopted so that it is practical and cost-effective. The mode multiplexer/demultiplexer (MUX/DEMUX) consists of cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). The mode MUX and DEMUX achieve very low modal crosstalk not only for the multiplexing/demultiplexing but also for the coupling to/from the FMF. Based on the fabricated FMF and mode MUX/DEMUX, we successfully demonstrate the first simultaneous 4-modes (LP01, LP11, LP21 & LP31) 10-km FMF transmission with 10-Gb/s intensity modulation and MIMO-free direct detection (IM/DD). The modal crosstalk of the whole transmission link is successfully suppressed to less than -16.5 dB. The experimental results indicate that FMF with simple step-index structure supporting 4 weakly-coupled modes is feasible.

A 1- $\mu$ m Cylindrical Vector Beam Fiber Ring Laser Based on a Mode Selective Coupler

We propose and demonstrate a continuous-wave all-fiber ring laser generating cylindrical vector beams (CVBs) using a mode selective coupler (MSC) as transverse mode converter and mode splitter. The MSC is fabricated by a novel method free of pre-tapering, achieving LP11 mode with a high purity of >96% near the wavelength of 1064 nm. The CVB fiber laser operates at a center wavelength of 1053.9 nm, with a 3-dB linewidth of less than 0.04 nm and a signal-to-background ratio of >60 dB. The laser slope efficiency is >9%. The radially and azimuthally polarized beams can be switched by adjusting the polarization controllers in the fiber ring cavity, with a high mode purity measured to be >96%.

Fiber-based mode converter for generating optical vortex beams

In this work, an all-fiber-based mode converter for generating orbital angular momentum (OAM) beams is proposed and numerically investigated. Its structure is constructed by cascading a mode selective coupler (MSC) and an inner elliptical cladding fiber (IECF). OAM modes refer to a combination of two orthogonal LPlm modes with a phase difference of ±π/2. By adjusting the parameters and controlling the splicing angle of MSC and IECF appropriately, higher-order OAM modes with topological charges of l = ±1, ±2, ±3 can be obtained with the injection of the fundamental mode LP01, resulting in a mode-conversion efficiency of almost 100%. This achievement may pave the way towards the realization of a compact, all-fiber, and high-efficiency device for increasing the transmission capacity and spectral efficiency in optical communication systems with OAM mode multiplexing.

Generation of cylindrical vector beams in a mode-locked fiber laser using a mode-selective coupler

We experimentally obtain cylindrical vector beams (CVBs) in a passively mode-locked fiber laser based on nonlinear polarization rotation. A mode-selective coupler composed of both a single-mode fiber (SMF) and a two-mode fiber (TMF) is incorporated into the cavity to act as a mode converter from LP01 mode to LP11 mode with broad spectral bandwidth. CVBs in different mode-locked states including single-pulse, multi-pulse, and bound pulse are obtained, for the first time to our best knowledge. The ultrafast CVBs with different operation states have potential applications in many fields such as laser beam machining, nanoparticle manipulation, and so on.

Spatial-mode switchable ring fiber laser based on low mode-crosstalk all-fiber mode MUX/DEMUX

We report an all-fiber ring laser that emits linearly polarized (LP) modes based on the intracavity all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). Multiple LP modes in ring fiber laser are generated by taking advantage of mode MUX/DEMUX. The all-fiber mode MUX/DEMUX are composed of cascaded mode-selective couplers (MSCs). The output lasing mode of the ring fiber laser can be switched among the three lowest-order LP modes by employing combination of a mode MUX and a simple N × 1 optical switch. The slope efficiencies, optical spectra and mode profiles are measured.

Fundamental-mode MMF transmission enabled by mode conversion

Modal dispersion in conventional multi-mode fiber (MMF) will cause serious signal degradation and an effective solution is to restrict the signal transmission in the fundamental mode of MMF. In this paper, unlike previous methods by filtering out higher-order modes, we propose to adopt low-modal-crosstalk mode converters to realize fundamental-mode MMF transmission. We design and fabricate all-fiber mode-selective couplers (MSC), which perform mode conversion between the fundamental mode in single-mode fiber (SMF) and fundamental mode in MMF. The proposed scheme is experimentally compared with center launching method under different MMF links and then its wavelength division multiplexing (WDM) transmission performance is investigated. Experimental results indicate that the proposed mode conversion scheme could achieve better transmission performance and works well for the whole C-band.

Orbital angular momentum beam excitation using an all-fiber weakly fused mode selective coupler.

Orbital angular momentum beam excitation through direct phase-matched coupling is experimentally demonstrated using an all-fiber weakly fused mode selective coupler consisting of a single-mode fiber and a ring-core fiber. Experimental results showing the excited OAM mode purity of up to 75% measured through the standard ring technique not only demonstrate the proof of concept but also provide a baseline for further improvement.

Multiwavelength mode-locked cylindrical vector beam fiber laser based on mode selective coupler

We propose and demonstrate a multiwavelength mode-locked fiber laser with cylindrical vector beam generation for the first time, to the best of our knowledge. The mode-locking mechanism is based on a nonlinear polarization rotation effect in fiber, and the multiwavelength operation is contributed to by an in-line birefringence fiber filter with periodic multiple passbands, formed by incorporating a section of polarization maintaining fiber into the laser cavity with a fiber polarizer. Furthermore, by using a home-made mode selective coupler, which acts as both a mode converter from fundamental mode to higher-order mode and an output coupler, multiwavelength mode-locked cylindrical vector beams have been obtained. This may have potential applications in mode-division multiplexing optical fiber communication and material processing.

All-fiber-based selective mode multiplexer and demultiplexer for weakly-coupled mode-division multiplexed systems

We show all-fiber-based selective mode multiplexers and demultiplexers for weakly-coupled mode-division multiplexed systems. We fabricate a set of six-mode multiplexer and demultiplexer based on fiber mode selective couplers, and experimentally evaluate the performance for the six-mode dual-polarization (DP) quadrature phase shift keying (QPSK) optical signals. In the mode multiplexer and demultiplexer, the mode couplings between the lower three modes and the higher three modes are suppressed to be less than −20 dB, which enables us to apply partial 6×6 MIMO equalizers even for the six-mode demultiplexing. For the six-mode DP-QPSK signals, the penalty of optical signal-to-noise ratio by replacing the full 12×12MIMO to the partial 6×6 MIMO is suppressed by less than 1 dB.

High efficiency mode-locked, cylindrical vector beam fiber laser based on a mode selective coupler.

We propose and demonstrate an all-fiber passively mode-locked laser with a figure-8 cavity, which generates pulsed cylindrical vector beam output based on a mode selective coupler (MSC). The MSC made of a two mode fiber and a standard single mode fiber is used as both the intracavity transverse mode converter and mode splitter with a low insertion loss of about 0.65 dB. The slope efficiency of the fiber laser is > 3%. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity which are measured to be > 94%. The laser operates at 1556.3 nm with a spectral bandwidth of 3.2 nm. The mode-locked pulses have duration of 17 ns and a repetition rate of 0.66 MHz.

All-fiber optical mode switching based on cascaded mode selective couplers for short-reach MDM networks

We propose and experimentally demonstrate an all-fiber optical mode switching structure supporting independent switching, exchanging, adding, and dropping functionalities in which each mode can be switched individually. The mode switching structure consists of cascaded mode selective couplers (MSCs) capable of exciting and selecting specific higher order modes in few-mode fibers with high efficiency and one multiport optical switch routing the independent spatial modes to their destinations. The data carried on three different spatial modes can be switched, exchanged, added, and dropped through this all-fiber structure. For this experimental demonstration, optical on-off-keying (OOK) signals at 10-Gb/s carried on three spatial modes are successfully processed with open and clear eye diagrams. The mode switch exhibits power penalties of less than 3.1 dB after through operation, less than 2.7 dB after exchange operation, less than 2.8 dB after switching operation, and less than 1.6 dB after mode adding and dropping operations at the bit-error rate (BER) of 10−3, while all three channels carried on three spatial modes are simultaneously routed. The proposed structure, compatible with current optical switching networks based on single-mode fibers, can potentially be used to expand the switching scalability in advanced and flexible short-reach mode-division multiplexing-based networks.

All-fiber 6-mode multiplexers based on fiber mode selective couplers.

All-fiber 6-mode multiplexer composed of two consecutive LP11-mode selective couplers (MSC), two LP21-MSCs and an LP02-MSC is fully characterized by wavelength-swept interferometer technique. The MSCs are fabricated by polished-type fiber couplers coupling LP01 mode of a single mode fiber into a higher-order mode of a few mode fiber. A pair of the mode multiplexers has minimum mode dependent loss of 4 dB and high mode group selectivity of over 15 dB. Mode division multiplexed transmission enabled by the all-fiber mode multiplexers is demonstrated over fiber spans of 117 km employing an in-line multi-mode optical amplifier. 6 modes of 120 Gb/s dual polarization quadrature phase shift keying signals combined with 30 wavelength channels are successfully transmitted.

Generation of Femtosecond Optical Vortex Beams in All-Fiber Mode-Locked Fiber Laser Using Mode Selective Coupler

We experimentally demonstrated a high-order optical vortex pulsed laser based on a mode selective all-fiber fused coupler composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity not only acts as mode converter from LP01 mode to LP11 or LP21 modes with a broadband width over 100 nm, but also directly delivers femtosecond vortex pulses out of the mode locked cavity. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in mode-locked fiber laser. The generated 140 femtosecond vortex beam has a spectral width of 67 nm centered at 1544 nm.

Mode selective coupler for optical vortices generation (Invited Paper)

In this Letter, an effective method using a mode selective coupler (MSC), which is composed of a three-core fiber is presented to generate optical vortices (OVs). The conversions of OVs with different topological charges, 0→±1 and 0→±3, are simulated in detail. We also prove that a higher-order topological charge can be obtained simply by changing the parameters of the fiber to increase the number of modes in the fiber. The polarization of OVs can be controlled as well.

Three-mode mode-division-multiplexing passive optical network over 12-km low mode-crosstalk FMF using all-fiber mode MUX/DEMUX

We propose three-mode mode-division-multiplexing passive optical network (MDM-PON) based on low mode-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). The FMF with step-index profile is designed and fabricated for effectively three-independent-spatial-mode transmission and low mode-crosstalk for MDM-PON transmission. The all-fiber mode MUX/DEMUX are composed of cascaded mode selective couplers (MSCs), which simultaneously multiplex or demultiplex multiple modes. Based on the low mode-crosstalk of the FMF and all-fiber mode MUX/DEMUX, each optical network unit (ONU) communicates with the optical line terminal (OLT) independently utilizing a different optical linearly polarized (LP) spatial mode in MDM-PON system. We experimentally demonstrate MDM-PON transmission of three independent-spatial-modes over 12-km FMF with 10-Gb/s optical on-off keying (OOK) signal and direct detection.

Recent progress in mode-division multiplexed passive optical networks with low modal crosstalk

Abstract Recently, mode-division multiplexing (MDM) has been investigated in transmission systems and optical access networks for capacity enhancement. In this paper, recent progress in MDM optical passive networks (PONs) enabled by low-modal crosstalk few-mode fibers (FMFs) and optical components is reviewed. All-fiber mode multiplexer/demultiplexers (MUX/DeMUX) composed of mode-selective couplers (MSCs) are introduced to simultaneously multiplex or demultiplex multiple modes. Few-mode circulators (FMCs) are employed to realize bidirectional MDM-PON transmission. Direct detection without complex multiple-input–multiple-output (MIMO) digital signal processing (DSP) is applied. Moreover, multidimensional PONs by cascading MDM optical distribution network (ODN) with conventional time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) ODNs are proposed and experimentally demonstrated.