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Diffraction-limited fundamental mode amplification is demonstrated in Er-doped fibers with mode areas ranging from 900 to 1800 mum2. The amplifiers are core-pumped with Raman fiber lasers with both signal and pump selectively launched into the fundamental mode. This scheme results in differential gain for the fundamental mode and stabilizes it against mode mixing caused by perturbations in the core. Gains that are greater than 30 dB are demonstrated from a single stage without significant amplified spontaneous emission. The low nonlinearity of the large mode areas enables amplification to high peak powers without resorting to unconventional microstructured or higher order mode fibers.

This paper describes the physics and properties of a novel optical fiber that would be attractive for building high-power fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian-shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra-short pulses - a capability that is practically nonexistent in conventional fibers. Particularly interesting is the tact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter-length fibers (as in high-power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm 2 , and propagate signals with negligible mode distortions over up to 50-meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.

A wavelength-encoded electrical current sensor for high voltage using ultra-bright green (525 nm) light-emitting diode (LED) transducer and plastic optical fibre (POF), linking to remote location, has been developed. The wavelength receiver is based on passive filtering by employing an OG530 edge filter. LEDs from three different manufactures were characterized in their optical chirp response range, aiming for a sensor resolution improvement. The sensor precision (stability) was improved against external perturbations on the POF link by using an in-line mode-scrambler. The environment temperature of the LED transducer was kept unchanged at ±2°C.

International audience; We propose a new technique to selectively excite the fundamental mode in a few mode fiber (FMF). This method of excitation is made from a single mode fiber (SMF) which is inserted facing the FMF into an air-silica microstructured cane before the assembly is adiabatically tapered. We study theoretically and numerically this method by calculating the effective indices of the propagated modes, their amplitudes along the taper and the adiabaticity criteria, showing the ability to achieve an excellent selective excitation of the fundamental mode in the FMF with negligible loss. We experimentally demonstrate that the proposed solution provides a successful mode conversion and allows an almost excellent fundamental mode excitation in the FMF (representing 99.8% of the total power).

We investigate the effect of asymmetric propagation of light wave in two-mode fiber by breaking symmetry of propagation vectors with an off-axis excitation. Spatial mode rotation is observed for an azimuthal variation in launching conditions.

We demonstrate that the mode field diameter of a two-mode fiber can be measured within a relative error of less than 5% using a conventional OTDR when the mode coupling loss between modes is not considered.

In this work, we present a mode selective converter based in a dual core photonic crystal fiber with non-identical cores. Mode conversion between the LP01 and LP11 modes is obtained and the operating wavelength can be continuously tuned by varying the refractive index of the liquid in the holes.

Mode coupling in a step-index 2-LP-mode fiber has been experimentally studied using advanced S2 analysis. A wire-mesh is used to locally induce micro-bending and it is demonstrated that coupling coefficients can be extracted.

A novel kind of microwave mode converter, transforming the large-aspect-ratio TE10 mode into the standardized TE10 mode in a rectangular waveguide, is designed in this paper. It’s designed with syntonic slotted waveguide antenna arrays theory and optimized by software via finite element method. A case, transforming TE10 mode with across Section 160 mm * 10 mm into the TE10 mode in the standardized waveguide BJ120 (19.050 mm * 9.525 mm) at 12.5 GHz, has been presented. The conversion efficiency reaches 99.5 % at 12.5 GHz, and exceeds 90 % over 12.19–12.81 GHz. The band of interest can be expanded to 11.67–13.34 GHz, almost 2.7 times, by embedding three different coupling plates. It can be used to connect the high-power microwave (HPM) generator with sheet electron beams (SEB) and a traditional horn antenna. With one more coaxial-to-waveguide transducer, it can also be used to cool-test some instruments correlative to HPM generator driven by SEB.

Intermediate mode interchange using low-crosstalk mode multiplexer/demultiplexer in a 6 mode-division multiplexed transmission line has been demonstrated. We experimentally confirmed the reduction of differential mode group delay and tap size in 12×12 MIMO equalizer.

Large mode 3C® optical fiber operating at 300kW of peak power and 25W of average power is shown to maintain truly single transverse mode functionality even after more than 1.5 years of continuous operation.

In the paper, a feasible mode control model based on Fourier Optical analysis is proposed to increase the transmission capacity of optical fiber communication. Our model combines fresnel diffraction and 4f system to implement mode control, where the phase-only spatial light modulator is employed as a spatial filter and the loss of amplitude of phase-only modulation is compensated by simulated annealing algorithm. Numerical results show that our model is feasible, and the original modes can be converted to the desired modes accurately.

We demonstrate 3-mode mode-division-multiplexing passive optical network (MDM-PON) based on low-mode-crosstalk few-mode fiber and all-fiber mode multiplexer/demultiplexer. Error-free performance is achieved for 7.4-km low-mode-crosstalk three-individual-spatial-mode transmission with 10-Gb/s optical on-off keying signal and direct detection.

We report a 10 spatial mode multiplexer based on the technique of Multi-Plane Light Conversion. The device shows average 4.4 dB insertion loss and 21 dB mode selectivity across the full C+L-band.

We calculate mode coupling and higher-order mode suppression in chalcogenide negative curvature fibers. The loss ratio of the higher-order mode to the fundamental mode is 58, while loss of the fundamental mode is under 0.4 dB/m.

Twist-induced mode coupling is experimentally analyzed in a 2-mode fiber by means of the S2 imaging technique. Results confirm a rotation of the transmitted field pattern proportional to the fiber rotation in agreement with the theoretical prediction.

A polarization selective weakly-fused fiber coupler composed of a few mode fiber and a single mode fiber is demonstrated with high efficiency higher than 90%. The coupler profile optimization has allowed to remove polarization and spatial degeneracy in the first and second high order mode.

We demonstrate phase sensitive and insensitive amplification and nonlinear mode conversion in few mode fibers by studying the four wave mixing effect.

All fiber broadband mode converter based on semicircular-distribution long period fiber gratings is proposed. Simulation results show that the mode conversion efficiency from LP 01 mode to LP 11 mode is more than 85% over wavelength range of 200nm.

We experimentally demonstrate 20 Gbit/s quadrature phase-shift keying (QPSK) signal transmission in a 2.6 km conventional graded-index OM3 multimode fiber (MMF) by selective exciting 4 different OAM mode groups with fiber based mode exciter and mode filter, and achieve bit-error rate (BER) below 2e-3.

Hybrid square-rectangular lasers capable of mode control are proposed and fabricated. Single mode and unidirectional emission operation are obtained with a side-mode suppression-ratio up to 46 dB and single-mode-fiber coupling loss of 3.2 dB.

A mode converter with multi-waveguide output for millimeter wave gyro-device applications is proposed in this article, which is used to convert the TE01 circular waveguide mode into the TE10 rectangular waveguide mode. Computer simulations with Ansoft HFSS code show that the energy transmission coefficient of larger than 95.96% may be reached in the frequency range from 34.094 GHz to 35.8 GHz (a bandwidth of 1.706 GHz) at a VSWR of lower than 1.5 for the input operating mode.

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 mode property and light propagation in a tapered silicon-on-insulator (SOI) nanowire with angled sidewalls is analyzed. Mode hybridization is observed and mode conversion between the TM fundamental mode and higher-order TE modes happens when light propagates in a waveguide taper which is used very often in the design of photonic integrated devices. This mode conversion ratio is possible to be very high (even close to 100%) when the taper is long enough to be adiabatic, which might be useful for some applications of multimode photonics. When the mode conversion is undesired to avoid any excess loss as well as crosstalk for photonic integrated circuits, one can depress the mode conversion by compensating the vertical asymmetry in the way of reducing the sidewall angle or introducing an optimal refractive index for the upper-cladding. It is also possible to eliminate the undesired mode conversion almost and improve the desired mode conversion greatly by introducing an abrupt junction connecting two sections with different widths to jump over the mode hybridization region.

Mode Division Multiplexing (MDM) over Few Mode Fiber (FMF) enhances the capacity of an optical fiber system by providing additional degrees of freedom in the form of modes which are selectively excited at the transmitter and selectively detected at the receiver. The key to capitalize on such a system is implementation of low loss mode multiplexer (mux) and de-multiplexer (de-mux) with low modal crosstalk. In this paper we dwell upon design and software implementation of such a low loss mode mux and de-mux for Two Mode Fiber (TMF).

We present an on-chip optical mode exchange between two multiplexed modes by using tapered directional couplers on silicon-on-insulator platform. The device consisting of mode multiplexing and mode exchange is compact with relatively large fabrication error tolerance. The simulation results show efficient higher order mode excitation and mode exchange. A low excess loss less than 0.5 dB and high extinction ratio larger than 15 dB over 10 nm wavelength range from 1535 to 1545 nm are achieved.

Reduction of the group delay (GD) spread is crucial for minimizing multi-input multi-output (MIMO) signal processing complexity in mode-division multiplexing (MDM) systems using multi-mode fibers (MMFs). In this paper, we focus on GD statistics and management techniques. First, we review the generalized Jones and Stokes representations for modeling propagation in MMF, and describe key GD properties derived using the two representations. Then, we describe three approaches for GD management: (a) optimized fiber design, (b) mode coupling and (c) GD compensation. For approach (a), we explain design principles for minimizing the GD spread. We review experimental results to date, showing that fabrication non-idealities significantly increase the GD spread, and this approach alone may not achieve sufficiently low GD spread. For approach (b), we describe mechanisms for inducing intragroup and intergroup coupling. We describe mode scrambler designs based on photonic lanterns or long-period fiber gratings, both of which can ensure strong intergroup coupling. For approach (c), we review GD-compensated system design principles and show that GD compensation is only partially effective in the presence of random intragroup or intergroup coupling. We provide design guidelines for minimizing MIMO signal processing complexity in MDM systems.

Mode division multiplexing (MDM)– using a multimode optical fiber’s N spatial modes as data channels to transmit N independent data streams – has received interest as it can potentially increase optical fiber data transmission capacity N-times with respect to single mode optical fibers. Two challenges of MDM are (1) designing mode (de)multiplexers with high mode selectivity (2) designing mode (de)multiplexers without cascaded beam splitting’s 1/N insertion loss. One spatial mode basis that has received interest is that of orbital angular momentum (OAM) modes. In this paper, using a device referred to as an OAM mode sorter, we show that OAM modes can be (de)multiplexed over a multimode optical fiber with higher than −15 dB mode selectivity and without cascaded beam splitting’s 1/N insertion loss. As a proof of concept, the OAM modes of the LP11 mode group (OAM−1,0"Equation missing" and OAM+1,0), each carrying 20-Gbit/s polarization division multiplexed and quadrature phase shift keyed data streams, are transmitted 5km over a graded-index, few-mode optical fibre. Channel crosstalk is mitigated using 4 × 4 multiple-input-multiple-output digital-signal-processing with −3.

A channel modeling method is developed and proposed to practically model the various effects of mode couplings for optical communication systems with multi-core fiber (MCF) and/or few-mode fiber (FMF). The method is similar to existing channel modeling methods in which strong coupling is exclusively considered but extends them to cover both strong and weak couplings. With the method, the entire channel is divided into K statistically independent sections where each section may represent an optical device or the span of a few-mode fiber transmission line that maintains coherence. Each section is modeled by a channel matrix model comprising full random unitary matrices and a differential mode delay (DMD) matrix. To further enhance the modeling of weak coupling, the matrix is modified by segregating it into partial block matrices and inserting correlation terms among blocks. The method is evaluated with data obtained in two representative cases: midrange (40 km) and long-haul (527 km) transmissions. Its effectiveness and practicality are validated from agreement obtained between simulation and measured results for the two cases.

A novel mode multiplexer and demultiplexer (MMUX/DEMMUX) based on 2-D photonic crystal (PC) at 1550 nm is proposed. The PC-based mode MMUX/DEMMUX including mode conversion function with a single-mode and multi-mode waveguides can be realized by quasi phase-matching TE(0) & TE(1) modes of two waveguides. 2DFinite-Difference-Time-Domain and beam propagation methods are used for simulation. The results show that PC-based mode MMUX/DEMMUX has the potential for high-capacity MDM optical communication systems with a low insertion loss (

To measure the focal ratio degradation of astronomical fibers, we use laser as the light source which can do with bringing the light to a focal point and we can control the injection position. The laser has a high coherency and there are a number of modes in the large-core fiber. Consequently, the output spot will be a speckle. The presence of speckle makes the energy distribution of optical field uneven, so that it's hard to calculate the spot radius. To reduce the influence of the speckle, the contact-type mode scrambler is developed in our research. It's used to make the fiber vibrate at a suitable frequency of 0-80 Hz, and the vibration amplitude is 0.5 cm. The phase of different propagation modes is changed randomly in order to vibrate the modes and reduce the influence of the speckle. Different vibration result is observed by setting different vibration frequency and different CCD exposure time. The estimate of the vibration result can be given by using the method of contrast and Fourier Transform. Three different core diameters (10µm, 125µm, 320µm) are contrast with each other for convenience. The result shows that the vibration frequency between 65-70 Hz is best, and the optimum exposure time is 30 ms for all three diameters. The vibration of fiber using mode scrambler can reduce the speckle obviously under coherent illumination.

We propose a simple design of LP 11 spatial mode converter by splicing an all-fiber binary phase plate segment between SMF and FMF. More than 13dB modal extinction ratio was successfully achieved with 4.5dB insertion loss over a wide wavelength range.

A novel type of interferometer with an ultra-small footprint is presented. The functional principle is based on the excitation of the first and the second order modes in an integrated dual-mode strip waveguide. The light is coupled from a single mode fiber to the dual-mode waveguide by a conventional grating coupler followed by a taper. The difference in the mode propagation velocity leads to a phase difference between the fundamental mode and the second order mode. A proof of concept in the silicon on insulator technology shows promising extinction ratios of around 30 dB.

We propose an InP-based monolithically integrated few mode transmitter structure based on multimode interference coupler (MMI), and demonstrate an integrated mode converter and multiplexer (converter-multiplexer). This converter-multiplexer can convert the TE 0 mode to either TE 0 or TE 1 mode and multiplex the two modes into a common output port. The device can be potentially used in mode division multiplexing system.

We present a novel ring resonator structure for optical mode conversion for mode division multiplexing. The proposed micro structure (

In this paper, we investigate the impact of inter-modal four-wave mixing on mode- and wavelength-division-multiplexing systems. A set of coupled nonlinear Schrödinger equations, including linear mode coupling, is derived allowing to isolate the inter-modal four-wave mixing terms. The efficiency of inter-modal four-wave mixing between degenerate LP modes is found to be significantly higher than the intra-modal four-wave mixing efficiency. However, it is shown that the inter-modal four-wave mixing efficiency between degenerate modes is significantly reduced by the linear mode coupling.

Few-mode fiber (FMF) has become very popular for use in multiplexing telecommunications data over fiber optics. The simplicity of producing FMF and the relative robustness of the optical modes, coupled with the simplicity of reading out the information make this fiber a natural choice for communications. However, little work has been done to take advantage of this type of fiber for sensors. Here, we demonstrate the feasibility of using FMF properties as a mechanism for detecting flexure by exploiting mode coupling between modes when the cylindrical symmetry of the fiber is perturbed. The theoretical calculations shown here are used to understand the coupling between the lowest order linearly polarized mode (LP01) and the next higher mode (LP11x or LP11y) under the action of bending. Twisting is also evaluated as a means to detect flexure and was determined to be the most reliable and effective method when observing the LP21 mode. Experimental results of twisted fiber and observations of the LP21 mode are presented here. These types of fiber flexure sensors are practical in high voltage, high magnetic field, or high temperature medical or industrial environments where typical electronic flexure sensors would normally fail. Other types of flexure measurement systems that utilize fiber, such as Rayleigh back-scattering [1], are complicated and expensive and often provide a higher-than necessary sensitivity for the task at hand.

A trench assisted, 0.15-NA, effective single mode leaky Erbium Doped Fiber Amplifier is proposed for high power applications. The segmented-core of the fiber enables to achieve 783 μm 2 mode area which reduces nonlinearity and increases power handling capacity of the fiber. The leakage loss of the modes is controlled by annular core segments and low index trench in the cladding. The leakage loss of the LP 11 mode is 10.49 dB/m and that of the fundamental mode is 0.017 dB/m at 1530 nm wavelength. We have injected 20 mW power into each mode and have calculated the gain for 26 m length of the fiber using 250 W pump at 980 nm wavelength. The signal power in the LP01 mode is amplified to 122 W and that in the LP 11 mode drops down to 5.78×10 -27 W. The low output signal power in LP 11 mode ensures selective single mode amplification of fundamental mode. Selective single mode amplification confirms good beam quality at the output end.

We experimentally investigate the mode excitation ratio of a two-mode fiber (TMF) based on the bending method. TMFs with three different kinds of mode excitation ratio of LP11 mode to LP01 are prepared and successfully measured by our present method.

An all-fiber based tapered mode selective coupler (AFTMSC) is proposed and it can convert LP01 mode to higher-order modes (LP11, LP21 and LP02 mode) with coupling efficiency better than 70% in wavelength range of 1450nm to 1650nm.

We design and fabricate a sidewall-corrugated polymer waveguide grating to achieve conversion between the LP01 and LP11a modes. A typical fabricated device offers a conversion efficiency >99% with a bandwidth of 20 nm and a thermal-tuning sensitivity of −4.1 nm/°C over the S+C+L band. This mode converter could be integrated with other polymer waveguide devices to achieve advanced functions for applications in mode-division-multiplexing transmission systems.

We experimentally demonstrate mode conversion by exploiting optical reflection of tilted few-mode fiber Bragg grating (FM-FBG). Mode conversions from LP(01) mode to higher symmetric and asymmetric modes are achieved, and more than 99.5% conversion efficiency from LP(01) to LP(11) mode is obtained using a 1.6°-tilted FM-FBG. Influences of the weakly tilted FM-FBG parameters on the property of mode conversion is analyzed and discussed. A simultaneous mode conversion and demultiplexing scheme for 4-mode × 3-wavelength multiplexing transmission is proposed and the modal crosstalk is analyzed based on the transmission spectra of the tilted FM-FBGs. The proposed approach shows potential applications in mode and wavelength division multiplexing communication systems.

Next generation few mode fibers (FMF) promise to substantially increase the spectral efficiency of existing state-of-the-art optical communication networks by an order of magnitude [1]. In FMF, individual propagating modes are considered as independent optical communication channels that carry separate streams of data. The performance of these communication streams however, suffers from inter channel interference (ICI) that depends on the physical characteristics of the optical fiber. The ICI mainly results of two impairments, namely the mode coupling and the differential mode delay. It is known that step index (SI) FMF is the less expensive and the easiest to fabricate in addition to having a limited number of physical design parameters, i.e., step refractive index and core diameter. Our objective here is first to investigate the design trade-offs of SI-FMF and then identify the parameters intervals that minimize the inter channel interference by reducing: the mode coupling and the differential mode delay. Our numerical simulation identifies the desired design regions that minimize these impairments separately. Our analysis also illustrates the challenge to minimize both impairments simultaneously and get compromising design solutions.

In this paper, application of branch line coupler to design a differential mode wideband bandpass filter and to suppress common mode noise is proposed. The schematic contains one modified branch line coupler with centrally loaded open circuited stubs in the symmetrical plane to achieve high attenuation under common mode excitation. First differential mode and common mode circuits are analyzed separately for the proposed schematic and then quantitative approach, based on distributed transmission line method is proposed to explain the simulated frequency response of differential filter. No via hole is required to design differential filter, is other advantage for fabrication of the filter. A 65 % fractional bandwidth (FBW) is calculated at 5.7 GHz Centre frequency and Common mode attenuation (CMRR), greater than 13 dB is calculated under the differential passband bandwidth.

Fiber-chip edge couplers are extensively used in integrated optics as one of the key structures for coupling of light between planar waveguide circuits and optical fibers. In this work, a new fiber-chip edge coupler concept with large mode size for coupling to submicrometer silicon photonic wire waveguides is presented. The coupler allows direct coupling to conventional SMF-28 optical fiber and circumvents the need for lensed fibers. We demonstrate by simulations a 95% mode overlap between the mode at the chip facet and a high numerical aperture single mode optical fiber with 6 μm mode field diameter (MFD). We also demonstrate a modified design with 89% overlap between the mode at the chip facet and a standard SMF-28 fiber with 10.4 μm MFD. The coupler is designed for 220 nm silicon-oninsulator (SOI) platform. An important advantage of our coupler is that large mode size is obtained without the need to increase buried oxide (BOX) thickness, which in our design is set to 3 μm. This remarkable feature is achieved by implementing in the SiO2 upper cladding two thin high-index Si3N4 layers. The high-index layers increase the effective refractive index of the upper cladding layer near the facet and are gradually tapered out along the coupler to provide adiabatic mode transformation to the silicon wire waveguide. Simultaneously, the Si-wire waveguide is inversely tapered along the coupler. The mode overlap at the chip facet is studied using a vectorial 2D mode solver and the mode transformation along the coupler is studied by 3D Finite-Difference Time-Domain simulations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 μm., SPIE Optics + Optoelectronics, April 13-15, 2015, Prague, Czech Republic, Series: Proceedings of SPIE; no. 9516

The differential mode (DM) and common mode (CM) in the generalized four-conductor lines are analysed with the current division factors. Novel extended mixed-mode S-parameters, in which the cross-mode S-parameters not exist, is proposed based on the independent mode analysis. In addition, a new mode conversion between the standard S-parameters and the extended mixed-mode S-parameters is also presented with the new conversion matrices which contain the current division factors. The validity of the proposed extended mixed-mode S-parameters and mode conversion are verified by simulations.

We propose all-fiber mode multiplexer composed of two consecutive LP11 mode selective couplers that allows for the multiplexing of LP01 mode and two-fold degenerate LP11 modes. We demonstrate WDM transmission of 32 wavelength channels with 100 GHz spacing, each carrying 3 modes of 120 Gb/s polarization division multiplexed quadrature phase shifted keying (PDM-QPSK) signal, over 560 km of few-mode fiber (FMF). Long distance transmission is achieved by 6 x 6 multiple-input multiple-output digital signal processing and modal differential group delay compensated link of FMF. The all-fiber mode multiplexer has considerable potential to be used in mode- and wavelength-division multiplexed transmission. (C) 2015 Optical Society of America

We show the mode crosstalk between the fundamental mode and the 1st order mode based on the optical mode switch to investigate the performance experimentally for the first time. In order to evaluate the mode crosstalk, we fabricated and integrated multimode interference (MMI) mode filter with the optical mode switch. The crosstalk of approximately −10 dB between modes has been evaluated by using this mode filter.

This work presents modification of Gaussian approximation. Here we propose to describe mode field distribution in fiber core by Laguerre-Gaussian functions and by modified Bessel function of the second kind in the fiber cladding. Results of error estimation are presented for equivalent mode field radius calculation occurring due to mode field distribution approximation by Laguerre-Gaussian functions in the cladding region. According to obtained results the error is negligible for low-order modes. However it increases up to 1% and even more for modes with (m + l) > 4, where m is radial and l is azimuthal mode order.