Two-mode Fiber Research Articles

Statistical analysis of nonlinear coupling in a WDM system over a two mode fiber.

We study the effect of nonlinear coupling in a WDM configuration over a two-mode fiber. A statistical analysis is presented that takes into account the effect of the random phase-sensitive amplification or depletion. Our results show high nonlinear coupling between the modes. We have quantified the channel power fluctuations, due to the wave phase random variations, at the output of the fiber. We also investigate the effect of random linear mode coupling on the nonlinear mode coupling.

A stable and tunable linear polarization single longitudinal mode fiber ring laser

A stable and tunable linear polarization single longitudinal mode fiber ring laser is proposed and demonstrated. A high extinction ratio spatial mode interference filter utilizing two-mode graded-index fiber supresses irregular side modes effectively, the combination of fiber polarizer and polarization maintaining fiber act as a polarization dependent comb filter, and tunable bandpass filter is adopted to select proper lasing wavelength. The power fluctuation and wavelength fluctuation at 1549.88 nm are less than 0.05 dB and 0.02 nm in 1 h, respectively. Only one longitudinal mode appears within a free spectral range of 7.5 GHz, and the degree of polarization in percentage is approximately as high as 99.07%.

High efficiency all-fiber cylindrical vector beam laser using a long-period fiber grating.

We demonstrate a novel all-fiber laser generating cylindrical vector beams with high slope efficiency and low threshold by introducing a long-period fiber grating into the laser cavity. Highly efficient mode conversion is realized by an LPFG at ∼1548 nm. Mode selection and spectrum filtering are achieved in combination with a two-mode fiber Bragg grating (TM-FBG). The fiber laser operates at a single wavelength of 1547.95nm with a 30dB linewidth of less than 0.18nm and a side-mode suppression ratio (SMSR) of more than 56dB. The lasing threshold and slope efficiency of the laser are 24.5mW and 35.41%, respectively. The output power is 72mW with an absorbed pump power of 225mW. The variation of slope efficiency with respect to the reflectivity of the TM-FBG is investigated. Through adjusting the intra-cavity polarization controller, high-purity radially and azimuthally polarized beams are both obtained.

Effects of intermode nonlinearity and intramode nonlinearity on modulation instability in randomly birefringent two-mode optical fibers

We analyze in detail the effects of the intermode nonlinearity (IEMN) and intramode nonlinearity (IRMN) on modulation instability (MI) in randomly birefringent two-mode optical fibers (RB-TMFs). In the anomalous dispersion regime, the MI gain enhances significantly as the IEMN and IRMN coefficients increases. In the normal dispersion regime, MI can be generated without the differential mode group delay (DMGD) effect, as long as the IEMN coefficient between two distinct modes is above a critical value, or the IRMN coefficient inside a mode is below a critical value. This critical IEMN (IRMN) coefficient depends strongly on the given IRMN (IEMN) coefficient and DMGD for a given nonlinear RB-TMF structure, and is independent on the input total power, the power ratio distribution and the group velocity dispersion (GVD) ratio between the two modes. On the other hand, in contrast to the MI band arising from the pure effect of DMGD in the normal dispersion regime, where MI vanishes after a critical total power, the generated MI band under the combined effects of IEMN and IRMN without DMGD exists for any total power and enhances with the total power. The MI analysis is verified numerically by launching perturbed continuous waves (CWs) with wave propagation method.

Tunable Orbital Angular Momentum Generation Using All-Fiber Fused Coupler

We propose an effective approach to generate tunable orbital angular momentum (OAM) beam based on all-fiber fused coupler consisting of a single mode fiber (SMF) and a two-mode fiber. The fundamental mode in the SMF is directly coupled to the LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode group (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,1</sub> , HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,1</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">even,odd</sup> , TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,1</sub> ) by appropriately phase matching the modes in ,the fiber coupler. The experimental results demonstrate that the OAM beam is produced by combining HEeven 2,1 and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,1</sub> (or HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,1</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">odd</sup> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,1</sub> ) with a π/2 phase shift and the OAM topological charge could be tuned from l = -1 to l = +1 by adjusting a polarizer set at the output end of fiber.

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.

Formation of enhanced regenerated grating in few-mode fiber by CO_2 laser pretreatment

In this work, we experimentally investigated the effect of CO2 laser pretreatment on regenerated grating formulated in few-mode fibers (FMFs) (two-mode and four-mode step-index fibers). In the preparation, the pristine FMFs were first treated with direct CO2 laser annealing followed by a slow cooling procedure to minimize the frozen-in stresses and the thermal stresses in the fibers before the grating inscription. After the thermal regeneration process, the produced regenerated gratings (RGs) were then subject to a thermal durability test at 1050°C for 10 h. In the comparison against the results from the RGs formulated in non-treated FMFs, the finding indicates that RGs in treated fibers have a better regeneration ratio and thermal resilience. It is believed that the thermal stress relaxation and structural rearrangement in the fiber glass are the major factors that lead to the lower grating recovery during the state of thermal regeneration process and higher degradation in the grating strength in a longer exposure period. The elimination and reduction of stresses in the fibers by the laser pretreatment in the earliest stage enables the grating to be formed in a more thermal stable condition, hence, RGs with prolonged lifespan can be produced.

Indium Tin Oxide Coated Two-Mode Fiber for Enhanced SPR Sensor in Near-Infrared Region

A surface plasmon resonance (SPR) sensor based on side-polished two-mode fiber (TMF) coated with indium tin oxide (ITO) film is proposed. ITO enables tuning of resonance wavelength of the sensor through changing its carrier concentration. The advantages of the TMF and ITO are combined in the sensor, which can achieve high sensitivity and plasma frequencies in the near-infrared region. The impact of ITO film thickness and residual fiber thickness on the sensing performance are numerically investigated with LP01 and LP11a modes coupled to specific surface plasmon modes. The results show that the resonance wavelength locates in 1500–2200 nm and an average sensitivity of ∼10149 and ∼10400 nm/RIU can be achieved for LP01 and LP11a modes, respectively, when the environmental refractive index varies between 1.33 and 1.39. The penetration depths of evanescent wave for the LP01 and LP11a modes are 460 and 502 nm, respectively. Both the sensitivity and penetration depth of the sensor are larger than those of single-mode fiber and multimode fiber based SPR sensors. The proposed sensor can play an important role in various applications, including environment, medicine, and security.

Polarization properties of fiber-based orbital angular momentum modes

Abstract Generally, orbital angular momentum (OAM) can be generated by combining two vector modes (VMs) or combining two linearly polarized modes (LPMs). The vortices generated by the first method are circularly polarized and carry spin angular momentum (SAM). While, the vortices generated by the second method are linearly polarized with no SAM. Although these two methods are closely related, they are essentially different. The difference and relation between the two methods are theoretically clarified in this paper. Two different kinds of OAM modes are experimentally demonstrated and validated in a two-mode fiber.

Geometric phase due to orbit–orbit interaction: rotating LP11 modes in a two-mode fiber

Accumulation of geometric phase due to non-coplanar propagation of higher-order modes in an optical fiber is experimentally demonstrated. Vertically-polarized LP11 fiber mode, excited in a horizontally-held, torsion-free, step-index, two-mode optical fiber, rotates due to asymmetry in the propagating k-vectors, arising due to off-centered beam location at the fiber input. Perceiving the process as due to rotation of the fiber about the off-axis launch position, the orbital Berry phase accumulation upon scanning the launch position in a closed-loop around the fiber axis manifests as rotational Doppler effect, a consequence of orbit–orbit interaction. The anticipated phase accumulation as a function of the input launch position, observed through interferometry is connected to the mode rotation angle, quantified using the autocorrelation method.

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

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 0.6dB. Furthermore, the channel spacing can be freely tailored by adjusting the distance between two stress points.

Switchable narrow linewidth fiber laser with [formula omitted] transverse mode output

We experimentally demonstrate a switchable narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser with LP11 transverse mode output. The laser is based on a mode selective all-fiber fused coupler which is composed of a single-mode fiber (SMF) and a two-mode fiber (TMF). By controlling the polarization state of the output light, the laser can provide narrow linewidth SLM output with LP11 transverse mode at two specific wavelengths, which correspond to two transmission peaks of the chirped moiré fiber grating (CMFBG). The 20dB linewidth of the fiber laser for each wavelength is approximately 7.2 and 6.4kHz.

Optical time-domain reflectometry based on a Brillouin dynamic grating in an elliptical-core two-mode fiber.

Optical time-domain reflectometry based on a Brillouin dynamic grating (BDG-OTDR) in an elliptical-core two-mode fiber (e-core TMF) is proposed and experimentally demonstrated for the discriminative measurement of strain and temperature distributions. Acoustic gratings are generated by the spontaneous Brillouin scattering of a pump pulse, which is used to reflect a probe pulse. Two orthogonal polarizations of the LP01 mode are sequentially used as the pump, while single polarization of the LP11 mode is used as the probe for intermodal BDG operation. Distribution maps of intermodal and polarization birefringence are acquired by analyzing the two BDG spectra, which are used to separately measure the temperature and strain distributions. In experiments, distributed measurement of intermodal BDG spectra along a 95m e-core TMF is performed with a 2m spatial resolution, where the strain and temperature coefficients of the BDG spectral shift are measured to be -0.085 MHz/με, +7.6 MHz/°C for the pump-probe of the LP01y-LP11x mode, and -0.093 MHz/με, +4.3 MHz/°C for the LP01x-LP11x mode, respectively. Distribution maps of the strain and temperature variation along the TMF are separately obtained by matrix calculation using the four coefficients with an error of ±105 με in strain and ±1.6°C in temperature.

LP11–LP01Mode Conversion Based on an Angled-Facet Two-Mode Fiber

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.

Characteristics of chiral long-period fiber gratings written in the twisted two-mode fiber by CO2 laser.

We demonstrated the fabrication of a chiral long-period grating (CLPG) by twisting a two-mode fiber (TMF) when a CO2 laser beam was sweeping along the fiber axis. The torsion, temperature, and surrounding refractive index characteristics of the fabricated TMF-CLPG were investigated experimentally. The fabricated TMF-CLPG has a high torsion sensitivity [0.7768nm/(rad/m)], and can measure the twist rate and twist direction simultaneously. This kind of CLPG would have great potential applications in high-sensitivity optical sensors.

Simultaneous Measurement of Chromatic and Modal Dispersion in FMFs Using Microwave Photonic Techniques

A microwave-photonic technique for measuring dispersion characteristics of few-mode fibers is proposed and experimentally demonstrated. We extract these dispersion parameters by sweeping the frequency of microwave signals modulated on an optical carrier. For the first time, simultaneous high-precision measurement of chromatic dispersion and modal dispersion are made possible. Experimental results obtained on a two-mode group fiber match well with simulation results, as well as those obtained using time-domain methods. The proposed scheme is scalable to measuring fibers with a large number of modes.

Design of Highly Nonlinear Few-Mode Fiber for C-Band Optical Parametric Amplification

We present a thorough description of the dominating intramodal and intermodal four-wave mixing interactions occurring in a highly nonlinear few-mode fiber and describe their phase matching conditions. Those interactions that result in few-mode parametric amplification using a single-frequency pump are of particular interest. Thus, based on the phase matching conditions of such interactions, we outline the dispersion properties that a fiber should possess in order to achieve few-mode parametric amplification, while having minimal modal crosstalk. Accordingly, we design and optimize two fibers such that they meet the dispersion requirements for parametric amplification of two and four spatial modes, respectively. The two-mode fiber provides a maximum differential modal gain (DMG) of 0.21 dB across the C-band with a minimum gain of 9.5 dB per mode, while the four-mode fiber provides a maximum DMG of 1.51 dB with a minimum gain of 6.5 dB per mode over a 19 nm bandwidth in the C-band. The designed fibers are highly nonlinear dispersion-shifted few-mode fibers that provide both high nonlinearity and low dispersion for several modes in the C-band, which have not been demonstrated simultaneously to date. We also take practical fabrication issues into account and analyze and compare the tolerances of the structural parameters of both fibers to small deviations from their optimal values.

Bound-state solutions, Lax pair and conservation laws for the coupled higher-order nonlinear Schrödinger equations in the birefringent or two-mode fiber

For describing the propagation of ultrashort pulses in the birefringent or two-mode fiber, we consider the coupled nonlinear Schrödinger equations with higher-order effects. According to the Ablowitz–Kaup–Newell–Segur system, 3[Formula: see text][Formula: see text]3 Lax pair for such equations is derived. Based on the Lax pair, we construct the conservation laws and Darboux transformation (DT). One- and two-soliton solutions are obtained via the DT, and we graphically present the one soliton and bound-state two solitons.

Multiwavelength Brillouin fibre laser in two-mode fiber

In this paper, a multiwavelength two-mode Brillouin fibre laser generated by two-mode optical fibre (TMOF) is proposed and demonstrated experimentally. The system comprises a 500 m long TMOF. A tunable laser source amplified by a high power erbium doped fibre amplifier acts as the Brillouin pump (BP) followed by a single mode fibre Bragg grating as a reflector in the cavity with a centre wavelength of 1544 nm. The BP is observed in the superposition of and modes in the few-mode fibre. The set-up is capable of generating a maximum of 12 clearly defined Stokes lines at the pump power of 28.6 dBm, spanning from 1543.88 to 1544.92 nm.

Linearly polarized orbital angular momentum mode purity measurement in optical fibers.

We presented a simple method for measuring the mode purity of linearly polarized orbital angular momentum (OAM) modes in optical fibers. The method is based on the analysis of OAM beam projections filtered by a polarizer. The amplitude spectrum and phase spectrum of a data ring derived from the beam pattern are obtained by Fourier transform. Then the coefficients of the mixed electric field expression can be determined and the mode purity can be obtained. The proposed method is validated and it is experimentally demonstrated in a two-mode fiber.