Showing papers on "Mode scrambler published in 2017"

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

Abstract: 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.

Broadband high-order mode pass filter based on mode conversion.

Abstract: We report a unique concept to implement a high-order mode pass filter using mode converters. Our proposed design method implements a high-order mode pass filter of any order, uses different mode converters available, and applies to a variety of planar lightwave circuit material platforms. We fabricate a broadband fundamental mode filter device using a Mach–Zehnder interferometer and Y-junctions to demonstrate our idea. The performance of the fabricated device is demonstrated experimentally in the wavelength range of 1.530–1.565 μm (C-band). This filter exhibits a simulated extinction ratio of 37 dB with an excess loss of 0.52 dB for the first-order mode transmission.

Mode Control in a Multimode Fiber Through Acquiring its Transmission Matrix from a Reference-less Optical System

Abstract: A simple imaging system together with complex semidefinite programming is used to generate the transmission matrix of a multimode fiber. Once the transmission matrix is acquired, we can modulate the phase of the input signal to induce strong mode interference at the fiber output. The optical design does not contain a reference arm and no interferometric measurements are required. We use a phase-only spatial light modulator to shape the profile of the propagating modes and the output intensity at an individual pixel is monitored. The semidefinite program uses a convex optimization algorithm to generate the transmission matrix of the optical system using intensity only measurements. This simple yet powerful method can be used to compensate for modal dispersion in multimode fiber communication systems. It also yields great promises for the next generation biomedical imaging, quantum communication, and cryptography.

Transverse mode interaction via stimulated Raman scattering comb in a silica microcavity

Abstract: Comb generation in different mode families via a stimulated Raman scattering (SRS) process is studied using a silica toroid microcavity. The broad gain bandwidth of SRS in silica allows us to excite longitudinal modes at long wavelengths belonging to mode families that are either the same as or different from the pump mode. We found through experiment and numerical analysis, that an SRS comb in a different mode family with a high quality factor (Q) is excited when we pump in a low-Q mode. No transverse mode interaction occurs when we excite in a high-Q mode resulting the generation of a single comb family. We studied the condition of the transverse mode interaction while varying the mode overlap and Q of the Raman mode. Our experimental results are in good agreement with the analysis and this enables us to control the generation of one- and two-mode combs.

Fiber cavities with integrated mode matching optics.

Abstract: In fiber based Fabry-Perot Cavities (FFPCs), limited spatial mode matching between the cavity mode and input/output modes has been the main hindrance for many applications. We have demonstrated a versatile mode matching method for FFPCs. Our novel design employs an assembly of a graded-index and large core multimode fiber directly spliced to a single mode fiber. This all-fiber assembly transforms the propagating mode of the single mode fiber to match with the mode of a FFPC. As a result, we have measured a mode matching of 90% for a cavity length of ~400 μm. This is a significant improvement compared to conventional FFPCs coupled with just a single mode fiber, especially at long cavity lengths. Adjusting the parameters of the assembly, the fundamental cavity mode can be matched with the mode of almost any single mode fiber, making this approach highly versatile and integrable.

Multi-channel mode converter based on a modal interferometer in a two-mode fiber

Abstract: In this Letter, we propose a multi-channel mode converter with the concept of a modal interferometer in a two-mode fiber (TMF). Two lateral stress points in a TMF function as in-line fiber mode couplers to construct the modal interferometer, and both transmission spectra and near-field patterns confirm that the LP01 mode is successfully converted into an LP11 mode at the multiple channels. The measured mode conversion efficiency almost completely follows the theoretical tendency. Finally, the mode conversion is realized at 20 channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss as low as 0.6 dB. Furthermore, the channel spacing can be freely tailored by adjusting the distance between two stress points.

Fiber laser for on-demand mode generation in 1550 nm band

Abstract: Transverse mode characteristics of a laser are related to a variety of interesting applications An on-demand mode solid laser in the 1064 nm band was proposed previously In this paper, we provide a fiber laser for on-demand modes in the 1550 nm band to prescribe the pure and high-quality emission of a higher-order transverse laser mode, based on a simple construction with one spatial light modulator (SLM) and a single-mode erbium-doped fiber (SM-EDF) The SLM is designated to generate the desired higher-order mode and separate the higher-order mode and the fundamental mode The fundamental mode oscillates in the fiber ring laser, and therefore the SM-EDF can be pumped with a single-mode 980 nm laser, no matter what higher-order mode is prescribed In this proof-of-principle experiment, high-quality higher-order modes are observed from LP01 to LP105 Stable emission and real-time switching between modes can be easily realized by altering the phase on the SLM In addition, the propagation of the LP01, LP11, LP21, and LP02 modes from the fiber laser is also demonstrated in a four-mode few-mode fiber

Rectangular versus circular fiber core designs: New opportunities for mode division multiplexing?

Abstract: The properties of rectangular core fiber are investigated for mode division multiplexing. Polarization degenerate mode groups, favorable mode profiles for device coupling, modal area uniformity, and good splice performance suggest it's a good candidate.

Influence of the linear mode coupling on the nonlinear impairments in few-mode fibers

Abstract: This paper is focused on the influence of the linear mode coupling caused by the fiber bending on the nonlinear distortions in a mode-division multiplexed system. The system under test utilizes the fundamental Gaussian mode and the conjugated first-order vortex modes propagating in the step-index fiber at the same wavelength. For such kind of system, the nonlinear impairments are caused mainly by the cross-phase and self-phase modulations. Propagation of the modal composition is described by the system of generalized coupled nonlinear Schrodinger equations, which serves as a basis of our simulations. Considering the nonlinear operator analytically, we show that it reaches its maximum value due to the power transfer between mode channels caused by the linear mode coupling. Simulation results for equal initial powers in NRZ-coded mode channels demonstrate that nonlinear signal impairments increase significantly for all mode channels in the case of strong linear mode coupling. In the case of weak linear coupling, the increase of nonlinear impairments was also observed, but this effect was appreciably weaker. Moreover, simulations show that the effect described above is stronger for the first-order modes than for the fundamental mode.

Physical evidence of mode conversion along mode-selective transmission line

Abstract: This work investigates mode conversion along a longitudinally uniform mode-selective transmission line (MSTL) and demonstrates physical evidence of the mode conversion. Mode conversion of the fundamental mode is observed through examining the field distributions of the MSTL. Characteristic mode conversion frequency is defined based on the distribution properties of the longitudinal magnetic field component and the intrinsic physical implications. The accuracy and effectiveness of this definition are verified numerically and experimentally. The result indicates that a quasi-TEM fundamental mode and a quasi-TE10 fundamental mode dominate in the MSTL below and above this frequency, respectively. This mode conversion could have specific applications in the design of microwave, millimeter-wave, and THz wideband and multi-band components and systems.

Integrated fiber-based transverse mode converter

Abstract: A transverse mode converter based on a binary micro relief implemented directly on the endface of a few-mode fiber was numerically investigat ed. The results of numerical simulation demonstrated the converter to form LP-11 and LP-21 modes with high efficiency, providing a more-than 92 % mode purity. Transformations of modes excited by a fiber microbending were also numerically investigated. The excited beams were sh own to save their mode purity even in a strong bending as the arising parasitical modes wer e mostly unguided by the fiber. The resulting beam power and mode content were also demonstrated o depend on the beam and bending mutual orientation for beams with strong rotational symmet ry.

Reduced-symmetry LMA rod-type fiber for enhanced higher-order mode delocalization.

Abstract: We present a novel design of a micro-structured large-pitch, large-mode-area (LMA) asymmetric rod-type fiber. By reducing the cladding symmetry through six high-refractive index germanium-doped silica inclusions, the fiber features strong higher-order mode (HOM) delocalization, leading to a potentially enhanced preferential gain for the fundamental mode in active fibers. In addition, high resolution spatially and spectrally (S2) resolved mode analysis measurements confirm HOM contributions below 1% and LP1m-like HOM contributions below the detection limit. This proposed fiber design enables single-mode operation, with near-diffraction-limited beam quality of M2=1.3 and an effective mode area of 2560 μm2 at 1064 nm. This design opens new insights into improving the threshold-like onset of modal instabilities in high-power fiber lasers and fiber amplifiers by efficiently suppressing LP11 modes.

Characterizing a 14 × 14 OAM mode transfer matrix of a ring-core fiber based on quadrature phase-shift interference.

Abstract: The transfer matrix of light propagating in fibers can quantitatively elucidate the mechanisms of mode coupling, thus having important implications for the knowledge such as the mode division multiplexing communication link characteristics in fibers. However, most methods for measuring the transfer matrix require a prior knowledge of the launched modes at the input and a complex optical system for the characterization at the output of the fiber. In this Letter, we use an interferometric approach for decomposing orbital angular momentum (OAM) modes of the output beams from a ring-core fiber, thereby processing a 14×14 OAM mode transfer matrix of the fiber with merely a camera imaging the mode field at the output of the fiber. The suitability of such a method is validated by the beam reconstruction. Thus, this method is crucial for characterizing the fiber transfer matrix with promising features of fast response and simple operation.

Long-term mode shape degradation in large mode area Yb-doped pulsed fiber amplifiers

Abstract: Experimental observation of long-term output mode shape degradation in pulsed Yb-doped fiber amplifiers is reported for the first time. The process occurs in large mode area Yb-doped fibers and is caused by the formation of the long period grating responsible for power transfer from the fundamental mode to the first high-order mode. The linkage between the process and photodarkening was revealed.

Vector mode conversion based on an asymmetric fiber Bragg grating in few-mode fibers

Abstract: We propose a vector mode conversion approach based on asymmetric fiber Bragg gratings (AFBGs) written in step-index fiber and vortex fiber, respectively. The mode coupling properties of AFBGs are numerically investigated. Compared to step-index fiber, the large mode separation in the vortex fiber is beneficial to extracting the desired vector mode at specific wavelengths. In addition, the polarization of incident light and the attenuation coefficient of index change distribution of the AFBG play critical roles in the mode coupling process. The proposed AFBG provides an efficient method to realize high-order vector mode conversion, and it shows great potential for orbital angular momentum multiplexing and fiber lasers with vortex beam output.

Theoretical study of gain-induced mode coupling and mode beating in few-mode optical fiber amplifiers.

Abstract: We developed a generalized field-propagating model for active optical fibers that takes into account mode beating and mode coupling through the amplifying medium. We applied the model to the particular case of a few-mode erbium doped fiber amplifier. Results from the model predict that mode coupling mediated by the amplifying medium is very low. Furthermore, we applied the model to a typical amplifier configuration. In this particular configuration, the new model predicts much lower differential modal gain than that predicted by a classical intensity model.

Phase-Sensitive Mode Conversion and Equalization in a Few Mode Fiber Through Parametric Interactions

Abstract: The parametric interaction in few mode fibers is theoretically and numerically studied in the particular case in which the signal and the idler waves are frequency degenerate but mode nondegenerate. Under simplifying hypotheses, we derive analytical formulas for the phase-insensitive and phase-sensitive amplification gain and conversion efficiency. The analytical formulas are in very good agreement with the numerical solutions of a full vectorial model that takes into account losses, mode coupling, and all possible four-wave mixing interactions. In the phase-sensitive regime, we predict that for small input pump powers, a large and tunable phase-sensitive extinction ratio can be achieved on one mode, whereas the other mode power remains essentially unaffected. Finally, in the high-gain regime, the self-equalization of the output power on different modes can be also achieved.

Reduction of modal evolution fluctuation in 2-LP mode optical time domain reflectometry.

Abstract: This paper describes theoretical and experimental investigations of an optical time domain reflectometry (OTDR) that conducts measurements in the 2-LP mode region of the fiber under test. Rayleigh backscattering amplitude coefficients from the HE11 mode to higher-order vector modes are analyzed to clarify the evolution in the state of polarization of the backscattered vector modes with respect to that of the probe HE11 mode. Based on the analysis, we then propose a technique for reducing amplitude fluctuations in the OTDR traces attributed to intensity distribution evolution of the LP11 mode. A proof-of-concept demonstration is performed on optical fibers in the laboratory and field environments, and the measured results confirm its usefulness.

Design of mode conversion waveguides based on adiabatical mode evolution for mode division multiplexing

Abstract: Mode conversion based on adiabatical mode evolution in a two-core configuration is proposed. The mode conversion feature is only dependent on the relationship between the effective mode indexes of the two cores in the configuration, which shows the highly flexible characteristics of the configuration and large fabrication tolerance. A mode wide bandwidth multiplexer/demultiplexer which is achieved by cascading the configuration is demonstrated numerically.

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

Abstract: 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.

Design of MDM channel precision positioning scheme at the end of a few-mode optical fiber with enlarged core diameter

Abstract: This work presents method for design of precision positioning scheme for mode division multiplexing (MDM) system channels at the end of link few-mode optical fiber with enlarged core diameter. Proposed solution is based on combination of well known overlap integral method and earlier on developed Gaussian approximation modification generalized for analysis of silica weakly guiding optical fibers with one outer solid cladding and arbitrary axially-symmetric refractive index profile. The last one provides a passage to analytical expressions for any order mode coupling coefficients under taking into account set precision radial misalignment. By using presented method we computed launching parameters of 5-mode MDM multiplexer channels to the end of few-mode optical fiber samples with enlarged core diameter up to 42 μm and reduced differential mode delay provided by specially designed graded refractive index profile. According to computation results, it is able to transmit the almost total one MDM channel mode power to only one optical fiber particular mode with the same azimuthal order by corresponding combination between mode field diameter and precision radial misalignment.

Design of low DMD few-mode optical fibers with extremely enlarged core diameter providing nonlinearity suppression for operating over “C”-band central region

Abstract: We present method for design of refractive index profile for silica GeO2-doped graded-index few-mode optical fibers with reduced differential mode delay and suppressed mode nonlinearity due to extremely enlarged core diameter up to 42 μm. Proposed solution is based on earlier on developed modified Gaussian approximation generalized for analysis of silica weakly guiding optical fibers with single outer cladding and arbitrary axially-symmetric refractive index profile, that provides ability to derive analytical expressions for guided mode delays. Therefore the objective function is represented as a sum of squares of deviations between mode staff delays and reference value corresponding to refractive index profile of current iteration. Based on developed method we performed computation for various combination of the reference mode delay and first iteration profile parameter. Some results of desired graded-index profile for a few-mode optical fibers 42/125 providing differential mode delay minimization over central region of "C"-band are represented.

Optical fiber span with low differential mode delay

Abstract: A fiber span comprising: a first optical fiber and a second optical fiber coupled to the first optical fiber, both fibers comprising the an inner core region with maximum refractive index delta, Δ 0 ≦01% and an outer radius R 1 >45 μm, an outer core region with an outer radius R 2 and a minimum refractive index delta Δ 1 and alpha value α≧5, wherein Δ 1 Δ R,MIN ; the first fiber introducing differential mode delay DMD 1 for wavelengths between 1525 and 1570 nm such that |DMD 1 |≦100 ps/km, and a first differential mode delay slope DMDS 1 ; the second fiber introducing differential mode delay DMD 2 for wavelengths between 1525 and 1570 nm such that |DMD 2 |≦100 ps/km, and a second differential mode delay slope DMDS 2 that has an opposite sign from the first dispersion slope DMDS 1 ; wherein total differential mode delay provided by the first fiber in conjunction with the second fiber is DMD tot =DMD 1 +DMD 2 , and −10 ps/km

An SMS (single mode – multi mode – single mode) fiber structure for vibration sensing

Abstract: We describe an SMS (single mode - multi mode - single mode) fiber structure to be used in a vibration sensing system. The fiber structure was fabricated by splicing a section (about 300 mm in length) of a step index multi mode fiber between two single mode fibers obtained from a communication grade fiber patchcord. Interference between higher order modes occurs while light from a narrow band light source travels along the multi mode fiber. When the multi mode fiber vibrates, the refractive index profile is changed because of the photo-elastics effect and the amplitude of the interference pattern is changed accordingly. To simulate a vibrating structure we used a loudspeaker to vibrate a wooden table. By using a digital oscilloscope, we recorded and analysed the vibrating signals obtained from the SMS fiber structure as well as from a GS-32CT geophone for referencing. We observed that this SMS fiber structure was potential to be used in a vibration sensing system with a measurement range from 30 to 180 Hz with inherent optical fiber sensor advantages such as light weight, immune to electromagnetic interference, and no electricity in the sensing part.

LP 11 –LP 01 Mode Conversion Based on an Angled-Facet Two-Mode Fiber

Abstract: We propose and experimentally demonstrate LP11–LP01 mode conversion based on an angled-facet two-mode fiber (AFTMF) It is a simple device based on a standard two-mode fiber with an output facet angle of 43° At a specific orientation, an incident LP11 beam on the angled-facet can be transformed into a transmitted beam with single-lobe mode profile The single-lobe beam coupled into a target few-mode fiber (FMF) has successfully excited LP01 mode in the target FMF The feasibility of the AFTMF to operate as a mode converter is demonstrated The proposed device is a simple and low-cost mode conversion tool for the space division multiplexing system

Simulation and analysis of mode staff excitation during “O”-band optical signal launching to graded multimode fiber with large central defect of refractive index profile via standard singlemode fiber

Abstract: We present modified technique for differential mode delay map measurement. Here according to well-known methods a fast laser pulse is also launched into a tested multimode fiber (MMF) via single mode fiber (SMF), which scans core of MMF under precision offset positions. However unlike known technique formalized in ratified standards, proposed modification differs by addition scanning of the output end of tested MMF by short tail of SMF. Therefore for each radial offset position at the input/output MMF ends, the shape of pulse response of launched optical signal is recorded, that provides to get more informative differential mode delay map. This work presents some results of experimental approbation of proposed modified technique for differential mode delay map measurement.

Elliptical-core mode-selective photonic lanterns for MIMO-free mode division multiplexing systems

Abstract: We design a three mode elliptical-core mode-selective photonic lantern (EC-MSPL) which can match well with the elliptical core few mode fiber in MIMO-free MDM systems. The mode crosstalk of the EC-MSPL are below −20dB for all three modes.