Mode Selective Coupler Research Articles

Switchable multi-wavelength pulse fiber laser with mode conversion

A switchable multi-wavelength pulse fiber laser based on dual-Sagnac comb filter, nonlinear optical loop mirror (NOLM) and a homemade mode selective coupler (MSC) is proposed and experimentally demonstrated. The dual-Sagnac comb filter and the NOLM are adopted as wavelength selective filter and saturable absorber (SA) in this work, respectively. This proposed fiber laser has a maximum output wavelength number of four. And the fiber laser can output multi-wavelength pulse lasers within a certain interval of the integer times of 1 nm and 1.9 nm. Moreover, when the pump power is 150 mW, the mode-locked fiber laser with the pulse repetition frequency of 120 MHz, a pulse duration of 4 ns and pulse interval of 8.3 ns have been realized via carefully adjusting the PC. And the fiber laser has a maximum output power of 226 μW under 300 mW pump power. In addition, the homemade MSC is utilized as mode converter to realize mode conversion up to LP31 and the mode purity is higher than 92 %. This work can find applications in wavelength division multiplexing (WDM) and the optical communication.

Cylindrical Vector Beams with an MOPA Amplifier Based on Nonlinear Polarization Rotation Mode-Locking

CVBs (cylindrical vector beams) are widely used in optical imaging, optical trapping, material processing, etc. In this study, based on mode-selective couplers and passive mode-locking fiber technology, a cylindrical vector fiber amplifier with an MOPA (master oscillator power amplifier) structure was developed. In the experiment, the pre-amp stage reached 19.87 mW output power and a CVB output with a mode purity greater than 97%. The measured beam quality factor was M2 = 2.1. The CVB output power obtained by the first-amp stage was 152.4 mW, and the mode purity was greater than 92%. The measured beam quality factor was M2 = 1.99. The internal inhomogeneity and external effects of the isotropic LMA (large-mode-area) fiber led to a decrease in beam quality and mode purity. After amplification, the gain of the fundamental mode was higher and the power was greater, resulting in a greatly reduced mode purity. This CVB fiber amplifier yielded important research value in expanding the applications of high-power fiber lasers.

High gain distributed Raman amplifier for orbital angular momentum mode-division multiplexing and wavelength-division multiplexing in a 104-km ring-core fiber over the C + L band.

Mode-division multiplexing (MDM) and wavelength-division multiplexing (WDM) are regarded as effective solutions for enhancing the communication capacity of fiber-optic transmission systems. To extend the transmission distance, a suitable amplification technique is required to amplify the signal light, with distributed Raman amplifier (DRA) offering a high-flat gain. In this paper, we experimentally demonstrate the transmission of 3 orbital angular momentum (OAM) mode-division multiplexing and 9 wavelength-division multiplexing ranging from 1530 nm to 1610 nm over a 104-km self-designed ring-core fiber (RCF) with the assistance of DRA. The homemade all-fiber mode selective couplers (MSCs) can (de)multiplex OAM modes (OAM01, OAM11, OAM21) efficiently in long-distance MDM + WDM fiber-optic transmission systems. The OAM-DRA provides high gain (with an on-off gain greater than 10 dB) as well as flat gain (with a differential wavelength gain of less than 0.88 dB and a differential mode gain of less than 0.28 dB) over the C + L band.

All-fiber spatial and wavelength gain-flattening of few-mode EDFA via mode selective coupler

The mode division multiplexing (MDM) technique together with wavelength division multiplexing (WDM) shows the great prospect to solve the capacity crunch of current standard single mode fiber (SSMF) transmission system. However, the modal and wavelength gain fluctuation fundamentally limit both the capacity and the reach of hybrid MDM-WDM transmission system. Here, we propose an all-fiber few-mode gain-flattening filter (FM-GFF) based on mode selective coupler (MSC), to effectively mitigate the gain fluctuation of few-mode erbium doped fiber amplifier (FM-EDFA) in both spatial and wavelength dimensions. With the help of genetic algorithm (GA), variable mode dependent loss spectra of cascaded MSCs can be efficiently obtained, according to the gain profile of the used FM-EDFA. Consequently, gain-flattening among all guided linearly polarized (LP) mode groups over the C-band can be realized. Simulation results reveal that, as for the spatial and wavelength gain flattening among 4/6-LP mode groups, only 5/7 cascaded MSCs are good enough to mitigate the modal and wavelength gain fluctuation from 9.89 dB and 11.79-dB to less than 0.38 dB and 0.46 dB over the C-band. Finally, we carry out experimental verification based on 2-LP mode groups gain flattening over C-band. When 3 cascaded MSCs are involved, the gain fluctuation of FM-EDFA over the C-band can be flattened from 2.46 dB to less than 0.96 dB.

Mode-selective power splitters for mode-division multiplexing optical networks

We propose an all-fiber mode-selective power splitter (MSPS) for non-circular-symmetric LP lm (l = 1, 2, …) modes, which is suitable for multicasting and optical performance monitoring in mode-division multiplexing optical fiber networks. The MSPSs are asymmetric two-core few-mode directional couplers composed of a few-mode fiber and a two-mode fiber. We theoretically studied the three conditions required by the MSPSs. By carefully choosing the core-to-core distance and coupling length, the MSPS can achieve arbitrary splitting ratio regardless of the modal field orientation of the input non-circular-symmetric LP mode. By using an asymmetric structure, the MSPS can ensure the power splitting only happens on the target non-circular-symmetric LP mode when the phase matching condition is satisfied. In addition, we designed and numerically simulated LP31 MSPSs with four kinds of splitting ratios, among which the one with 90/10 splitting ratio was fabricated based on tapering and polishing method. The fabricated LP31 MSPS is characterized and the results show that its splitting ratio is much more stable than regular LP31 mode-selective coupler.

All-Fiber Low-Modal-Crosstalk Demultiplexers for DSP-Free IM/DD LP-Mode MDM Transmission

Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two degenerate-mode reception schemes. Firstly, a low-modal-crosstalk orthogonal combined reception method for degenerate modes is proposed based on all-fiber mode-selective couplers, in which signals in both degenerate modes are demultiplexed into the LP01 mode of single-mode fibers and then are multiplexed into the mutually orthogonal LP01 and LP11 modes of a two-mode fiber (TMF) for simultaneous detection. Secondly, a novel degenerate-mode-selective coupler consisting of an input few-mode fiber and an output TMF is proposed, which could demultiplex degenerate LP modes without any digital signal processing (DSP). Both demultiplexers are achieved based on the taper and polish process. The fabricated devices are characterized and compared. The results show that the proposed schemes can pave the way to the practical implementation of DSP-free IM/DD LP-mode MDM transmission systems.

Ultra-low-loss all-fiber orbital angular momentum mode-division multiplexer based on cascaded fused-biconical mode selective couplers

Ultra-low-loss all-fiber orbital angular momentum mode-division multiplexer based on cascaded fused-biconical mode selective couplers

Wavelength insensitive mode selective coupler based on ultra-wideband resonant effect

Wavelength insensitive mode selective coupler based on ultra-wideband resonant effect

Analysis of guided optical eigenmodes in magneto-optical fibers with applications to OAM mode selective couplers

Mode division multiplexing technology can regard each eigenmode in optical fibers as an independent channel to carry different data information, helpful for improving the transmission capacity of optical fiber communication systems. The spatial distribution, state of polarization and propagation constant of guided optical eigenmodes in the magneto-optical fiber are derived theoretically by the analysis of linear polarization and exact modes. The transverse distribution of the eigenmodes in the MO fiber can be constructed by those of exact modes in the conventional fiber. The effect of applied magnetic field on the eigenmodes and their effective refractive indices is studied. Simulation results show that, the eigenmodes in the MO fiber may be represented by the pseudo-modes of circular polarization () or the true modes of orbital angular momentum () when applied magnetic field is sufficiently large. The measurement method for weak magnetic field based on Stokes parameters is proposed. The magnetic tunability of OAM mode selective couplers is also analyzed.

Fiber-based broadband detection of a rotational object with superposed vortices

Recently, the rotational Doppler effect has attracted broad attention in detecting rotational motion. However, the presently proposed detection techniques based on the rotational Doppler effect are generally configured relying on discrete components in free space, resulting in cumbersome and inflexible systems, which brings challenges to practical applications. In this paper, we demonstrate a fiber-based configuration on rotational Doppler measurements for the detection of a rotational object using an ultra-broadband mode-selective coupler to convert the superposed vortices. Remarkably, the results show the broadband operating range of the fiber-based measurement system intuitively through wavelength scanning. The refinement of rotational Doppler detection techniques is of great significance for lowering the cost, reducing system complexity, improving system integration, and industrial manufacturing. This fiber-based scheme might be a promising candidate for facilitating the rotational Doppler effect applied as novel motion monitoring and sensing equipment in engineering and industry.

Wavelength-Tunable Pulsed Cylindrical Vector Beams in a 1.7-μm Mode-Locking Thulium-Doped All-Fiber Laser

Because of the special absorption peak, pulsed lasers at 1.7 μm have been rapidly developed in medical treatment, biological imaging and so on. Introducing the cylindrical vector beam (CVB) may further promote these special applications due to its unique intensity, phase and polarization characteristics. Herein, we have experimentally demonstrated the generation of wavelength-tunable pulsed CVBs at 1.7 μm based on a thulium-doped all-fiber laser. A bandpass filter with a wide bandwidth combined with nonlinear polarization rotation technology is used to obtain pulsed laser emission at 1.7 μm. By taking advantage of a home-made Lyot filter and mode selective coupler (MSC), pulsed CVBs can be obtained with a wavelength tuning range of 66 nm (1720–1786 nm). The development of wavelength-tunable pulsed CVBs at the 1.7 μm waveband has significant potential applications in deep bioimaging and laser processing.

Multi-wavelength fiber laser with high-purity LP11 output mode based on a symmetrical fiber mode selective coupler and Sagnac interferometer

In this work, a multi-wavelength fiber laser with a high-purity LP11 output mode was proposed and experimentally demonstrated. More specifically, a novel symmetrical fiber mode selective coupler (MSC) was utilized for mode conversion and a Sagnac interferometer is fabricated as a multi-channel filter for wavelength selection. From the experimental results, it was shown that the laser was able to emit stable single-, dual-, and triple-wavelength outputs both around 1530 nm and 1550 nm, and the purity of the LP11 mode was above 97 % and 98 % at the two bands respectively. In addition, the laser was capable of simultaneous dual-wavelength emission around 1530 nm and 1550 nm, resulting in an overall four-wavelength output with an LP11 mode purity of above 98 %. The proposed high-order mode multi-wavelength fiber laser has potential applications in various fields including optical communications, optical sensing, and materials processing.

1.7 µm sub-200 fs vortex beams generation from a thulium-doped all-fiber laser.

The pulsed 1.7 µm vortex beams (VBs) has significant research prospects in the fields of imaging and material processing. We experimentally demonstrate the generation of sub-200 fs pulsed VBs at 1.7 µm based on a home-made mode-selective coupler (MSC). Through dispersion management technology in a thulium-doped fiber laser, the stable linearly polarized VBs pulse directly emitting from the cavity is measured to be 186 fs with central wavelength of 1721.2 nm. By controlling the linear superposition of LP11 modes, cylindrical vector beams (CVBs) can also be obtained. In addition, a variety of bound states pulsed VBs at 1.7 µm can also be observed. Our finding provides an effective way to generate ultrashort pulsed VBs and CVBs at 1.7 µm waveband.

Efficient Integrated Multimode Amplifiers for Scalable Long-Haul SDM Transmission

Scaling up cable capacity in a power- and cost-efficient manner is a goal of spatial-division multiplexing (SDM) in long-haul optical communication systems. Integration is a key to scaling SDM beyond the parallel single-mode fiber paradigm. Multicore or multimode fiber systems promise higher integration, but native multicore or multimode amplifiers pose challenges in efficient coupling of pump light into the amplifier, and in control of mode-dependent gain (MDG). We propose an integrated multimode amplifier design for signals in six spatial modes (12 spatial and polarization modes) that relies on excitation of only four pump spatial modes at 980 nm. Simulations of an exemplary amplifier predict a relatively flat gain above 10 dB, noise figure below 5 dB, and MDG standard deviation below 0.1 dB over the C-band (1530–1565 nm), with a power-conversion efficiency close to 30%. Key design features include a graded-index multimode fiber amplifier with optimized ring erbium doping profile, length and pump mode powers; and a cascade of wavelength- and mode-selective couplers that efficiently couples four pump diodes to four pump modes, while passing signal modes with minimal loss. Our design approach, which uses fewer pump diodes per signal mode than parallel single-mode systems, may provide an option for efficient, economical scaling of SDM long-haul systems.

High Energy Singular Beams Generation From a Dissipative Soliton Resonance Raman Fiber Laser

We demonstrated the high energy 1.65 μm singular beam generation from a dissipative soliton resonance (DSR) all-fiber Raman laser experimentally. By utilizing advantages of the high nonlinearity and home-made mode-selective coupler (MSC), different DSR pulsed singular beams, including cylindrical vector beam (CVB) and vortex beams (VB), have been generated experimentally. The maximum pulse energy can be enhanced up to 698.19 nJ with high signal to noise ratio of 80 dB by optimizing the intra-cavity nonlinearity. Our research may provide a new method to generate high energy pulsed singular beams with a flexible operation wavelength.

Five-LP-Mode IM/DD MDM Transmission Based on Degenerate-Mode-Selective Couplers With Side-Polishing Processing

Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) have attracted great attention for capacity enhancement of short-reach applications such as optical interconnections, for which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss (IL) and modal crosstalk are the key components. In this paper, we firstly propose the design method for degenerate–mode-selective couplers (DMSC) with side-polishing processing. Three kinds of DMSCs for different degenerate linearly-polarized (LP) modes (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sub> ) in a weakly-coupled few-mode fiber (FMF) are designed. Then a pair of MMUX and MDEMUX consisting of cascaded mode-selective couplers (MSC) and DMSCs for all the 5 LP modes of the FMF are fabricated with side-polishing processing. The measurement results show that the MMUX and MDEMUX achieve low back-to-back IL lower than 5.3 dB for all the 5 modes. Finally, stable digital-signal-processing (DSP)-free 5-mode IM/DD MDM transmission utilizing conventional optical modules over 10-km FMF are successfully demonstrated. The proposed low-IL MMUX/MDEMUX is scalable to support more modes and can promote the practical application of MDM techniques.

Ultra-low-loss 5-LP mode selective coupler based on fused biconical taper technique.

Trapped in the stringent adiabatic transmission condition of high-order modes, low-loss fused biconical taper mode selective coupler (FBT-MSC) has long been challenging to achieve. We identify the adiabatic predicament of high-order modes to stem from the rapid variation of the eigenmode field diameter, which is caused by the large core-cladding diameter difference of few-mode fiber (FMF). We demonstrate that introducing a positive-index inner cladding in FMF is an effective approach to address this predicament. The optimized FMF can be used as dedicated fiber for FBT-MSC fabrication, and exhibits good compatibility with the original fibers, which is critical for the wide adoption of MSC. As an example, we add inner cladding in a step-index FMF to achieve excellent adiabatic high-order mode characteristics. The optimized fiber is used to manufacture ultra-low-loss 5-LP MSC. The insertion losses of the fabricated LP01, LP11, LP21, LP02 and LP12 MSCs are 0.13 dB at 1541 nm, 0.02 dB at 1553 nm, 0.08 dB at 1538 nm, 0.20 dB at 1523 nm, and 0.15 dB at 1539 nm, respectively, with smoothly varying insertion loss across the wavelength domain. Additional loss is less than 0.20 dB from 1465.00 nm to 1639.31 nm, and the 90% conversion bandwidth exceeds 68.03 nm, 166.68 nm, 174.31 nm, 132.83 nm, and 84.17 nm, respectively. MSCs are manufactured using commercial equipment and a standardized process that takes just 15 minutes, making them a potential candidate for low-cost batch manufacturing in a space division multiplexing system.

Direct Emission of Two-Color Vortex Beams at 1.0 μm and 1.5 μm Wavebands From an All-Fiber Laser

In this letter, we demonstrated an all-fiber laser that can simultaneously radiate dual-color vortex beams by incorporating a dual-channel mode-selective coupler (DC-MSC) into the laser cavity. By optimizing the phase-matching condition between LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes, wavelength-dependent mode coupling could be observed. In this case, a broadband mode conversion with two isolated bands can be achieved, which totally covers the gain bands of ytterbium and erbium ion, enabling the simultaneous generation of vortex beams (VBs) at 1.06 μm and 1.55 μm wavebands. With intra-cavity mode conversion and polarization controlling, a coaxial two-color VB is transmitted directly from the laser cavity, providing a more compact dual-beam light source for subsequent vortex nonlinear frequency conversions, such as wavelength extension to the red light and the mid-infrared (3 μm) wavebands.

Sensitivity Improvement of an Optical Fiber Sensor Based on Surface Plasmon Resonance with Pure Higher-Order Modes

In this paper, we propose an approach to improve the sensitivity of an optical fiber surface plasmon resonance (SPR) sensor with a pure higher-order mode excited by a designed mode selective coupler (MSC). We calculate the proportion of the power of the higher-order mode in the cladding. Compared to the LP01 mode, the power proportion of the LP11 mode (LP21 mode) in the cladding theoretically improves by 100% (150%). To generate a relatively pure LP11 mode or LP21 mode, a mode selective coupler (MSC, 430–580 nm) is designed. The coupling efficiency of the LP01–LP11 mode coupler is over 80%, and that of the LP01–LP21 mode coupler is over 50%. The simulation results show that the sensitivity of the LP11 mode and the LP21 mode increases by approximately 330% and 360%, respectively, using the intensity modulation (n = 1.33–1.38, 430–580 nm); the resolution of the refractive indices of our sensor, using the LP11 mode (LP21 mode), is 2.6×10−4 RIU (2.4×10−4 RIU). The higher sensitivity and resolution of our presented fiber SPR sensor containing a visible MSC make it a promising candidate for the measurement of refractive indices.

Achromatic Broadband Multi‐Layer Diffraction Mode Multiplexing

AbstractEfforts have been made in recent years to achieve broadband mode‐division multiplexing (MDM) for wavelength and mode hybrid multiplexing optical interconnect. However, most multiplexers are suffering from either strong chromatic aberration or material dispersion, like planar lightwave circuits, mode selective couplers, and single‐layer diffraction elements. The multi‐layer diffraction structure provides a method for achieving both mode conversion and broadband properties. However, the reported multi‐layer diffraction MDM can usually only achieve high efficiency working at a specific narrow bandwidth due to the monochromatic coherent light design. Here, a general achromatic broadband multi‐layer diffraction model is proposed to compensate the dispersion by wavefront matching of discrete multi‐wavelength optical fields. Using this model, an achromatic orbital angular momentum beam generator and an achromatic linearly polarized mode multiplexer for coarse wavelength‐division multiplexing (CWDM) optical interconnections are designed. Compared with the monochromatic designs, the generator exhibits lower wavelength dependence and the multiplexer doubles the bandwidth with mode purity exceeding 91%. Additionally, signal transmission over a 5‐km fiber is demonstrated experimentally in the CWDM‐MDM hybrid communication system. This work represents a solid step toward practical implementation of achromatic MDM and may find applications in the design of various achromatic devices.