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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.

We theoretically model and numerically analyze the linear behavior of distributed mode crosstalk in a step-index weakly-coupled 7-mode fiber. This fiber is assumed to be used for one-polarization uncoupled mode-division multiplexing (MDM) systems with: (1) sparse MIMO equalizers which are performed over only degenerate LP modes, or (2) one single differential mode delay-independent MIMO equalizer which is performed over all non-degenerate LP modes. For the above two low-complexity MIMO configuration schemes, the impacts of distributed mode coupling on the multi-path interference-dependent achievable distance and system quality are empirically evaluated, through the numerical simulations for uncoupled MDM transmissions of a single-channel 28 GBaud QPSK signal over the fiber.

We present a theoretical study on the mode coupling between core modes and cladding modes of the tilted long period fiber grating (TLPFG) in dual-mode fiber. The TLPFG can be applied as mode filters at several specific wavelengths. Using the full vector complex coupled mode theory, numerical simulation shows the maximum coupling coefficients between core modes (LP01 and LP11) and cladding modes can be found near the tilted angle of 86°. The transmission peaks of modes coupling are sensitive to the titled angle. Furthermore, several peak amplitude between the LP11 mode and same azimuthal order modes increase with the growth of the titled angle. With the band-stop characteristic of specific mode, the TLPFG written in dual-mode fiber could be exploited as mode filters.

We experimentally demonstrate an all optical space-to-time mapping process using modal dispersion of large core high numerical aperture step-index multimode fiber in this paper. We use light beam with different input angle to excite various modes in a span of multimode fiber. The input optical pulses are stretched in time by modal dispersion and received by a large area, high speed photodiode. Through this process, the spatial information is directly mapped into device's temporal response. It has high speed, broad bandwidth and low system latency. Comparing with the widely used spectral imaging technology, this device is weak dependent of the input signal wavelength and optical carrier bandwidth.

Recently developed PLC-based mode multi/demultiplexers (MUX/DEMUXs) for mode division multiplexing (MDM) transmission are reviewed. We firstly show the operation principle and basic characteristics of PLC-based MUX/DEMUXs with an asymmetric directional coupler (ADC). We then demonstrate the 3-mode (2LP-mode) multiplexing of the LP01, LP11a, and LP11b modes by using fabricated PLC-based mode MUX/DEMUX on one chip. In order to excite LP11b mode in the same plane, a PLC-based LP11 mode rotator is introduced. Finally, we show the PLC-based 6-mode (4LP-mode) MUX/DEMUX with a uniform height by using ADCs, LP11 mode rotators, and tapered waveguides. It is shown that the LP21a mode can be excited from the LP11b mode by using ADC, and the two nearly degenerated LP21b and LP02 modes can be (de)multiplexed separately by using tapered mode converter from E13 (E31) mode to LP21b (LP02) mode.

This work presents alternative fast and simple method for design refractive index profile of silica multimode optical fibers (MMFs) providing effective mode bandwidth improvement under laser-based multi-Gigabit data transmission and few-mode regime operation in comparison with commercially available MMFs. Optimized profile equalizes delays of selected particular order guided modes being the main mode components of multi-Gigabit network transceiver laser excited optical signal transferred over proposed MMF in a few-mode regime. We present some results of refractive index profiles synthesis for MMFs with core diameters 50 and 100 μm under various combinations of the reference mode delay and the first iteration profile grade parameter as well as corresponding to these samples curves of differential mode delay spectral characteristics computed over “O”-band

This work presents fast and simple method for design and computing parameters of precision spatial positioning scheme for mode division multiplexing (MDM) system channels placement over optical fiber core end. Proposed solution is based on combination of well known mode field overlap integral method and earlier on developed modified Gaussian approximation. Computation results showed that fitting of MDM channel mode field radius and its corresponding offset launching position in relation to optical fiber core center provides transmission of the almost total channel injected mode power to only one particular excited fiber mode with the same azimuthal order. This work presents results of proposed method approbation for computation of spatial positioning scheme for modes of 5-channel MDM system placement over few-mode optical fiber 42/125 core end.

A Single-mode flat fiber investigated by mode matching filtering technique is presented. This class of rectangular optical fiber employed a mode filtering technique to filter high order modes. Fresnel leakage loss induced the propagation modal loss favors fundamental mode, whereas significantly filtered higher-order mode due to modal loss. This resulting mode operation can be expanded via high-aspect-ratio. We numerically analyze the via full-vectorical numerical analysis. We also cover the dispersion analysis to be estimated around wavelength of 1268 nm.

We test Slemon and Wells's function and recently reported Hurand et al.'s (Appl. Opt., 50, 492–499, 2011) function for calculation of coupling characteristics in step-index optical fibers against experimental measurements and against calculations by a related method that is based on the power flow equation. Compared are the coupling length Lc (which is the fiber length where the equilibrium mode distribution is achieved) and length zs (where steady-state distribution is achieved) in three step index glass optical fibers as well as a liquid core optical fiber. The two functions, while simpler to apply being just algebraic formulas, are less accurate over a wide range of numerical apertures. It is also shown that fibers with same coupling coefficient can have much different coupling characteristics.

The 2D FDTD method is used to design a tapered photonic crystal fiber, where large mode area is prominently coupled to optical fiber. For the first time, we observed that the mode area is stabilized with respect to possible fabrication tolerance by implementing a secondary surrounding waveguide with configuration of large air holes in the outer ring. The mode field area can be obtained in tapered region using a full vectorial H field technique. Beside these, the coupling coefficient can be optimizing by using least square boundary residual technique.

Using the power flow equation and experimental measurements, investigated in this article is the state of mode coupling in low NA (0.3) step-index plastic optical fibers for laser beam launch. Results show the coupling length L c at which the equilibrium mode distribution is achieved and the length z s of the fiber required for achieving the steady-state mode distribution. Numerical results obtained using the power flow equation agree well with experimental measurements.

This paper investigates influence of large-signal modulation on mode oscillation in InGaAsP/InP lasers. The mode competition dynamics under modulation are examined in terms of the temporal trajectories of the total output and the strongest modes as well as their Fourier spectra. The mode coupling induced by the strong spectral gain suppression is evaluated by both the correlation coefficients among the strongest modes and their signal distortion. We show that small and moderate modulations at the multimode hopping of the laser modulate the mode hopping along with modulation of each mode. The mode coupling is characterized by anti-correlation among the modes like as the non-modulated laser. Under modulation with the resonance frequency, the increase in the modulation depth changes mode coupling from anti-correlation to positive correlation and then to complete coupling that correspond to emission of periodic pulses. The mode coupling is characterized by mode competition distortion, which measures the amount of power carried by the mode signal at the frequency of multimode-hopping.

The main source of light transmission loss between single-mode fiber (SMF) and large mode area (LMA) fiber is from the mode field mismatch. Thermally expanded core (TEC) technique was introduced to realize mode field adaptation between them. The SMF was heated by a H2–O2 flame to increase the mode field diameter (MFD) which can result in a mode field match with LMA. Both numerical calculation and experiment results show it is workable to realize mode field adaptation and get a low transmission loss by this method. For HI1060 (d=5 μm, NA=0.14) and LMA-GDF-15/130(d=15 μm, NA=0.08), a low-loss with 0.4 dB was obtained for SMF–LMA–SMF from 6.4 dB with TEC processing experimentally.

Approximate empirical analysis of mode power distribution (MPD) carried by the fundamental mode is newly investigated based on a broadband dispersion compensating microstructured fiber (MF). The fraction of modal power in the core region, η, is defined with the help of extending the applicability of well-established classical optical fiber theories to MFs. In doing so, the influences of structural parameters and wavelength on MPD characteristics of the fundamental mode are systematically analyzed in detail based on simple physically consistent concepts of conventional fibers. As a result, it is shown that for the optimum MF design in Ref. [11] as a multimode fiber, when the number of guiding modes increases, the mode power ratio of the fundamental mode constantly increases from 69.4%; in addition, we find that as wavelength increases within 1.2–1.6 μm, mode power confinement ability of the fundamental mode is lessened for single-mode propagation, whereas for multimode propagation it becomes enhanced.

The propagation of higher modes, such as the LP 11 mode, in optical nanofibers using the exponentially tapered optical fiber as a basic model is investigated. In order to preserve the LP 11 mode in the downtaper as far as the nanofiber waist, the effect of varying the cladding-core radius ratio on the LP 11 adiabatic criterion is modeled. A Laguerre–Gaussian beam is created in free space using a spatial light modulator (SLM) and coupled to a few-mode fiber. This device allows convenient switching between the fundamental and LP 11 fiber modes. By selecting a few-mode fiber with a relatively low cladding-core ratio, the propagation of the LP 11 mode down to a submicron waist has been maintained. Furthermore, by observing the transmission profile during tapering, it is possible to decisively terminate the pulling process in order to eliminate the two degenerate HE 21 modes of the LP 11 mode. As a result, a nanofiber can be fabricated through which only the TE 01 and TM 01 components of the LP 11 mode propagate. Such a nanofiber has promising applications in the area of mode interference for controlled particle trapping sites.

Considering the optical stability of solution, the sugar-solution is infused into the outer core ring of dual-concentric-core photonic crystal fiber (DCCPCF). The influences of structure parameters and solution concentration on the phase and loss matching are comprehensively analyzed. By choosing the appropriate outer core mode to completely couple with the inner core fundamental mode, the large negative dispersion PCF around 1.55 μm is designed, which has the dispersion value of − 39,500 ps/km/nm as well as bandwidth of 7.4 nm and effective mode area of 28.3 μm 2 . The designed PCF with hybrid cladding structure can effectively compensate the positive dispersion of conventional single mode fiber, and suppress the system perturbation caused by a series of nonlinear effects. Considering the mode field mismatching between the DCCPCF and the tapered fiber, the calculated connection loss around 1.55 μm is below 3 dB. In addition, the equivalent propagation constants of two leaky modes are deduced from the coupled-mode theory, and the complete mode coupling case can be well predicted by comparing the real and imaginary parts of propagation constants.

This paper describes the group delay difference and mode coupling of few-mode fiber for “uncoupled” mode division multiplexing. Optical mode separation is compatible with few-mode fiber. It has complementary characteristics with the MIMO technique.

Using the time-independent power flow equation, we have examined the mode coupling caused by intrinsic perturbation effects in step-index As2Se3 chalcogenide-glass multimode optical fiber in the mid-infrared region. Results show that the coupling length where the equilibrium mode distribution is achieved depends on the wavelength and is of the order of kilometers.

In this paper, we report the modal dispersion of silica graded-index optical fibers as a function of the input mode parameters and lunching conditions in local area network (LAN) context. In that, we examine the mode-depending parameters, namely, modal delay, modal attenuation and mode-coupling effects as a function of wavelength. We show that the number of excited mode groups depends strongly on the spot beam radius when the fiber is excited with an axial Gaussian beam where we find an optimal axial diameter exciting only two mode groups. We present a comparison of the number of excited mode groups, the optimal spot radius beam, the signal penalty and the 3-dB baseband bandwidth enhancement for the optimal axial launching compared with full mode excitation, offset launching and mode-field matched axial launching.

We present a novel mode filter operating for the 1060-nm band, which selectively removes the power of the high-order mode (HOM) from a conventional single-mode fiber (C-SMF) that supports two modes at the short-wavelength band. Our mode filter is fabricated very easily by fusion-splicing a short section of short-wavelength single-mode fiber in the middle of the C-SMF link. By using our spliced-fiber mode filter (SF-MF), a high HOM suppression performance of > 18 dB has been achieved with a low insertion loss of

In large mode area fiber amplifier, it is difficult to avoid the bend induced mode distortion. With the existence of mode distortion taken into account, a previous transverse mode competition model has been improved. Based on this model and the finite element difference-beam propagation method, the behaviors of bent mode competition are demonstrated and compared with the straight mode competition where the mode distortion is neglected, the comparison and quantitative results show that serious mode distortion can change the appearance of transverse mode competition in bent fiber amplifiers and even drastically degrade the amplifier performance.

3D beam propagation numerical code and a newly developed optical mode solver were implemented for studies on the recently experimentally realized Yb fiber amplifier with 7-core hexagonal structure. It was found that the launched seven single-frequency beams matched with the cores converge to a wave field with permanently diminishing space beating patterns, but the result is sensitive to phases of separate beams. Namely, at a spread of phases of individual beams within a few tenths of radian, the convergence of the wave field to the in-phase mode is observed, while at phases random spread on the order of π the out-of-phase mode is dominating. Such behavior is observed at strong gain saturation in the cores. Both effects take place even if the guiding index of the cores fluctuates within a limited range. The mode solver predicts that the in-phase mode has the minimal small signal gain, and the gain of the out-of-phase mode is maximal one. This fact proves that there is no strict correlation between modal gains and amplification in strong saturated multi-core fiber. The smaller core index step significantly improves the convergence of the amplified radiation to the in-phase mode. An approach to the waveguide structure tailoring is described which assures the single in-phase mode stability.

The spectrum of a Bragg grating in a multimode optical fiber can be controlled by the number of irradiation pulses during the writing procedure. Hence, a single mode grating can be fabrication readily within a multimode fiber. The width of the stop band of the grating can also be controlled by means of the number of irradiation pulses. A tilted grating leads to the coupling of core mode to cladding mode, and when the tilt is sufficiently large, eventually to radiation mode. In the case of small tilt, the grating spectrum is the sum of the core–core mode spectrum and the core–cladding mode spectrum.

A ring launching scheme using a mode converter based on hollow optical fibre (HOF) is theoretically investigated for the purpose of increasing bandwidth for multimode Gigabit Ethernet communication. The mode converter based on HOF transforms the fundamental mode of a single-mode fibre (SMF) to a ring-shaped mode. It has the characteristic to improve the bandwidth by exciting higher-order modes of the multimode fibre (MMF) selectively. The HOF launch is studied under various aspects such as induced coupling efficiency, mode power distribution, and bandwidth-length product and is compared to centre and offset launching schemes. It is shown that the ring launching scheme is expected to support 10-Gb Ethernet applications using conventional multimode fibres.

In this paper, an efficient multi-to-single mode converter based on a tapered multimode fiber is presented. This novel tapered multimode fiber has a multimode input end and a single mode output end, which is fabricated by a reproducible chemical etching process. The core diameter of the thicker end of the fiber is around 150 μm (a typical multimode fiber case) while the core diameter of the thinner end of the fiber is around 10 μm (a typical single mode fiber case). By launching the light into the tapered multimode fiber from the thicker end, the multimode light field distribution at the thicker input end becomes a single mode light field distribution at the thinner output end. Thus, a unique multi-to-single mode conversion is achieved. The major advantages of this mode converter include: (1) high conversion efficiency (>95% based on theoretical analysis), (2) compact size and (3) low cost, which make it very useful for a variety of applications, such as: (1) efficiently coupling the light from a laser, in particular a semiconductor diode laser, into a single mode fiber; (2) highly sensitive evanescent wave based fiber optic sensors and (3) effective mode conversion and connection between multimode and single mode fibers. Experimental results have confirmed this theoretical prediction (i.e., a multi-to-single mode conversion can be realized via a special tapered multimode fiber).

In an FEM oscillator with a Bragg resonator excitation of an additional starting mode can be exploited as a source of an initial signal for the operating mode. In such a regime the starting mode has a rather small detuning from synchronism resulting in a high temporal gain but a rather small electron efficiency. With the starting mode growth the situation becomes favorable for excitation of the operating mode. Afterwards due to a non-linear competition mechanism the operating mode suppresses the starting mode and a single-mode regime of oscillations is realized with a rather high efficiency.

This paper shows, theoretically and experimentally, how a polarization scrambler increases the response stability of a fiber Mach–Zehnder interferometer. We have studied the situation when the scrambler is placed in one of the interferometer branches and also when it is placed at the interferometer input. The measurements carried out using a scrambler based on the Faraday effect, agree with the simulation results.

This paper details the conditions for which a c.w. OPO pumped by a single mode field is working in a single or multimode regime. It shows that c.w. OPOs are intrinsically single mode systems in the steady state regime, both for the longitudinal and transverse modes, unless special conditions of mode degeneracy or mode coupling are met. Complex optical patterns may appear in such cicumstances, that we have recently observed. Furthermore, if one adds a linear coupling between the signal and idler modes, these two modes can phase-lock on each other: one then obtains a perfect degenerate single mode operation for the c.w. OPO.

We review our recent work toward designing large mode area fibers for high power applications. We show that appropriately designed doped multi-mode fibers can be used to provide robust single-mode output when used in fiber laser cavities. Single-mode (SM) fiber mode field diameters of ~35µm are demonstrated with record SM pulse energies of 0.5mJ at 1550nm with a repetition rate of 200Hz. Energies approaching 1mJ are obtained with a slight compromise in mode quality. A modification in the laser cavity results in a passively modelocked laser giving femtosecond pulses with nanojoule energies.

A new single-mode fiber polarization scrambler based upon a simple fiber-ring structure and usable with a narrowband light source is described. The degree of polarization decreases by more than −20 dB when the input is 100% polarized. Design criteria and principles are discussed. We demonstrate that the polarization scrambler eliminates the effects of induced polarization fluctuation in a polarization-sensitive fiber-optic system.

An analysis of modal second-order dispersion β2 and third-order dispersion β3 characteristics from singly-clad multimode optical fibers is presented. We show, for the first time, that for some step-index fiber with GeO2SiO2 core and B2O3SiO2 cladding, negative values of β2 and β3 are simultaneously achieved for the LP51 mode in the vicinity of 1.2 μm region. Possible application of this result to bright soliton transmission through this mode in femtosecond regime can be foreseen.

Effects of random mode coupling are experimentally characterized on polarization mode dispersion and on system penalty. The virtual optical fiber links are proposed and constructed by the use of highly birefringent (B = 6.0 × 10-4) polarization-maintaining fiber to stabilize the state of polarization during propagation and by the introduction of three different numbers of fusion splices (0, 10, and 99 splices) to simulate stable mode coupling. Measurements are conducted on polarization mode dispersion and transmission experiments at 2.5 Gbit/s for the constructed fiber links to see the effects of random mode coupling on polarization mode dispersion and on power penalty. It is confirmed that polarization mode dispersion and system penalty vary inversely with k and inversely with k, respectively, where parameter k characterizes how many times the number of coupling points is increased. The constructed fiber links, which realize stable polarization mode dispersion with random mode coupling, enable this characterization to be successfully conducted.

This paper proposes a new type of filter, that is, a dominant mode filter (DMF) which uses the rotor speed fluctuations for dynamic security assessment and stability enhancement of power systems. The proposed filter can be used in detecting the degradation of damping power of the system and in improving the stabilizing ability of a control device such as PSS. The basic concept of the DMF design is based on the inherent characteristics of the power system network and the application of principal component analysis to a covariance matrix of the rotor speed fluctuations. The coefficients of the DMF are easily computed from the eigenvector of the covariance matrix. The weak damping modes which are useful for generating the control signals of the stabilizing devices, are satisfactorily estimated from the rotor speed fluctuations by means of the DMF. In the computing process of the DMF coefficients the ‘mode energy’ and ‘mode profile’ are also obtained from the eigenvalue and the eigenvector of the covariance matrix respectively. The mode energy is the strength of each mode contained in the fluctuations. The mode profile expresses the distribution of the mode oscillation with the rotor speed responses of all machines. Since, in the DMF design, the on-line fluctuation data are used, the weak damping modes, the mode energy and the mode profile can be estimated adaptively according to the system situation. In this paper, applications of the DMF are also discussed. One application is to use the mode energy as an on-line stability index. If the damping power of the system becomes weak, a system operator can detect signs of an unstable condition by observing the time variation of the mode energy. Another is a disturbance identifier to find the location and severity of a disturbance. This identifier utilizes the mode profile and mode energy. The third application of the DMF is to supply the slower mode component as a global control signal for HPSS (Hybrid PSS)11. The mode energy can be also utilized for adapting the parameters of the HPSS.

TE-TM mode conversion, in magneto-optic guides for integrated optics, requires TE-TM mode overlap and phasematching. This two properties are analyzed with circular and linear magnetic birefringence dependence. In application an experimental 95% TE-TM mode conversion efficiency is presented for an uniform longitudinal magnetization.

A new mode scrambler with sinusoidally serpentine bends is experimentally studied by analyzing both the half width and the average contrast of the far-field speckle pattern of an emitting light from a short multimode fiber. It is found that, as the number of bendings increases, the speckle contrast approaches a certain constant low level where the steady state mode power distribution is achieved.

A numerical solution of the coupled-power equation for graded index optical fiber is derived for any launching condition. As an application of the formulation the fiber frequency response is determined as a function of the index gradient α. Results show that the optimum α value departs slightly from the optimum obtained without mode coupling.

We describe a method for determining the V -value from the far-field radiation pattern of dual mode optical fibers (this method is an extension of the work by Gambling et al. for the LP 11 modes). The method involves measurements of the maximum radiation angle and the angle of the first minimum. Curves are presented which can be used with dual mode fibers, determining the core diameter, the refractive index difference between core and cladding, as well as the V -value in the single mode regime for longer wavelength operation with the help of dispersion curves of the dopant materials.

Modal noise may arise in single mode fiber optic networks from the random coupling of the fundamental model LP01 and the higher order mode LP11. The higher mode LP11 is generated in the fiber optic link from imperfect connectors. A model for the modal noise generated in a single mode fiber optic network consisting of a number of connectors or splices is presented. The effect of modal noise on the link is discussed and expression for signal to noise ratio has been obtained. Suggestions for the reduction of modal noise are given.

The results are described of an experimental investigation of mode coupling in a multimode graded-index fiber as a function of the periodic microbend amplitude. Computer-generated holograms were used as mode selecting spatial filters for measurements of the modal power distribution. The experimental data are in a good qualitative agreement with theoretical predictions. The results obtained may be helpful in the development of amplitude-type graded-index fiber sensors with high sensitivity.