Showing papers on "Mode scrambler published in 2012"

Few Mode Transmission Fiber With Low DGD, Low Mode Coupling, and Low Loss

Abstract: A transmission fiber for mode division multiplexing supporting LP01 and LP11 modes, with low differential group delay, low mode coupling, and low loss for both modes is presented. Spatially and spectrally resolved mode imaging (S2 imaging) is used for characterization.

Few mode transmission fiber with low DGD, low mode coupling and low loss

Abstract: A transmission fiber for mode division multiplexing supporting LP 01 and LP 11 modes, with low differential group delay, low mode coupling and low loss for both modes is presented. S2 imaging is used for characterization.

Thermally induced mode coupling in rare-earth doped fiber amplifiers.

Abstract: We present a simple semianalytical model of thermally induced mode coupling in multimode rare-earth doped fiber amplifiers. The model predicts that power can be transferred from the fundamental mode to a higher-order mode when the operating power exceeds a certain threshold, and thus provides an explanation of recently reported mode instability in such fiber amplifiers under high average-power operation. We apply our model to a simple step-index fiber design, and investigate how the power threshold depends on various design parameters of the fiber.

“Photonic lantern” spectral filters in multi-core fibre

Abstract: Fiber Bragg gratings are written across all 120 single-mode cores of a multi-core optical Fiber. The Fiber is interfaced to multimode ports by tapering it within a depressed-index glass jacket. The result is a compact multimode "photonic lantern" filter with astrophotonic applications. The tapered structure is also an effective mode scrambler.

Fiber LPG Mode Converters and Mode Selection Technique for Multimode SDM

Abstract: The use of multimode fibers in mode division multiplexed space-division multiplexing systems offers one solution to the capacity limitations of single-mode fiber transmission. Passive components to control the individual modes in few-mode fiber (FMF) are key elements to build more complex modules and components necessary for a high performance system. Fiber-based mode converters are important elements in the FMF mux/demux, and long-period gratings have been investigated to provide mode conversion in two- and four-mode fibers. A method to separate and monitor the modes in real time is described as the basis of a method to measure the individual modal performance of a component during fabrication.

Temperature-induced index gratings and their impact on mode instabilities in high-power fiber laser systems.

Abstract: Mode-interference along an active fiber in high-power operation gives rise to a longitudinally oscillating temperature profile which, in turn, is converted into a strong index grating via the thermo-optic effect. In the case of mode beating between the fundamental mode and a radially anti-symmetric mode such a grating exhibits two periodic features: a main one which is radially symmetric and has half the period of the modal beating, and a second one that closely follows the mode interference pattern and has its same period. In the case of modal beating between two radially symmetric modes the thermally induced grating only has radially symmetric features and exhibits the same period of the mode interference. The relevance of such gratings in the context of the recently observed mode instabilities of high-power fiber laser systems is discussed.

Modeling and characterization of a few-mode EDFA supporting four mode groups for mode division multiplexing.

Abstract: Numerical and experimental study of a Few-Mode (FM) Erbium Doped Fiber Amplifier (EDFA) suitable for mode division multiplexing (MDM) is reported. Based on numerical simulations, a Few-Mode Erbium Doped Fiber (FM-EDF) has been designed to amplify four mode groups and to equally amplify LP11 and LP21 mode groups with gains greater than 20 dB and with a differential modal gain of less than 1 dB. Experimental results confirmed the simulations with a good concordance. This modal gain equalization is obtained by tailoring the erbium spatial distribution in the fiber core with a ring-shaped profile.

Characterization of Multimode Fiber by Selective Mode Excitation

Abstract: Each mode of a 2 km 50 μm OM2 grade multimode fiber is precisely excited at multiple orientations using a binary phase spatial light modulator (SLM) to generate a detailed modal description of the fiber and minimize modal dispersion over 4.5 THz of optical bandwidth.

Spot-Based Mode Couplers for Mode-Multiplexed Transmission in Few-Mode Fiber

Abstract: We present the theory for a novel low-loss mode coupler based on multiple Gaussian spots, which can selectively address all spatial and polarization modes of a few-mode fiber. Specifically, we show spot arrangements and how to construct them for few-mode fiber with a large number of modes, and analyze in detail designs for few-mode fibers supporting six and 12 spatial and polarization modes. For six-mode few-mode fibers, couplers with no mode-dependent loss and a coupler insertion loss <; 2 dB are possible, whereas for few-mode fiber supporting 12 modes, a mode-dependent loss <; 1 dB and coupler insertion loss <; 3 dB can be simultaneously achieved.

Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers.

Abstract: We demonstrate scaling of the effective area of higher-order mode, Er-doped fiber amplifiers Two Er-doped higher-order mode fibers, one with 3800 μm2 Aeff in the LP0,11 mode, and one with 6000 μm2 effective area in the LP0,14 mode, are demonstrated Output beam profiles show clean higher order modes, and S2 imaging measurements show low extraneous higher order mode content CW and pulsed amplifier experiments are reported Nanosecond pulses are amplified to 05 mJ pulse energy with 05 MW peak power

Optical Vortices in a Fiber: Mode Division Multiplexing and Multimode Self-Imaging

Abstract: The optical vortices (Dennis et al., 2009; Desyatnikov et al., 2005; Soskin & Vasnetsov, 2001) or angular harmonics exp(imφ) describe a wavefront peculiarity, or helical dislocation, when in passing around the origin of coordinates the light field phase acquires a phase shift of 2πm, where m is the optical vortex's order. The generation and propagation of the laser vortices in free space has been studied fairly well, meanwhile, the excitation of individual vortex modes and obtaining desired superpositions thereof in optical fibers present a greater challenge (Berdague & Facq, 1982; Bolshtyansky et al., 1999; Dubois et al., 1994; Karpeev & Khonina, 2007; Mikaelian, 1990; Soifer & Golub, 1994; Thornburg et al., 1994; Volyar & Fadeeva, 2002). Note that the most interesting is the excitation and propagation of pure optical vortices that are not stepor graded-index fiber modes. However decomposition of the light fields in terms of angular harmonics has a number of advantages over other bases, including modal ones, when dealing with problems of laser beam generation and analysis and mode division multiplexing. As distinct from the classical LP-modes, the angular harmonics are scaleinvariant when coupled into the fiber and selected at the fiber's output using diffractive optical elements (DOEs) (Dubois et al., 1994; Karpeev & Khonina, 2007; Soifer & Golub, 1994; Thornburg et al., 1994). This gives much freedom in choosing parameters of an optical scheme, allowing one to effectively counteract noises, as it will be demonstrated below. A term "mode division multiplexing" (MDM) is used for multimodal optical fibers when describing methods for data transmission channel multiplexing, with each spatial fiber mode being treated as a separate channel that carries its own signal (Berdague & Facq, 1982; Soifer & Golub, 1994). The essence of mode division multiplexing is as follows: laser beams as a linear superposition of fiber modes can be used to generate signals that will effectively transmit data in a physical carrier a multimodal fiber. The data transmitted can be contained both in the modal composition and in the energy portion associated with each laser mode. The MDM concept has not yet been turned to practical use because a definite mode superposition with desired between-mode energy distribution is difficult to excite. Another reason is that there is energy redistribution between modes when transmitting data in real

Mode coupling effects in multi-mode fibers

Abstract: In mode-division-multiplexed systems using coherent detection, strong mode coupling is beneficial. Mode coupling reduces modal dispersion, minimizing signal processing complexity. In combination with modal dispersion, mode coupling creates frequency diversity, mitigating the mode-dependent gain of optical amplifiers.

Weakly-Guiding Mode-Selective Fiber Couplers

Abstract: This paper presents analytical descriptions of the behavior of weakly-guiding/weakly-coupled mode-selective fiber couplers, based on coupled-mode theory. Mode-selective couplers rely on the phase-matching of a higher-order mode in one fiber with the fundamental mode of a second, closely-positioned fiber. Their behavior is shown to be highly-dependent on the principle and azimuthal mode numbers of the higher-order mode, as well as its spatial-orientation. Zero coupling is shown to be possible for an asymmetric higher-order mode even when there is perfect phase-matching. The theory is also numerically simulated, and could assist in the future design of efficient mode-selective couplers for a wide range of optical communications and sensor systems.

Perspectives of principal mode transmission in mode-division-multiplex operation

Abstract: We investigate the concept of principal modes and its application for mode division multiplexing in multimode fibers We start by generalizing the formalism of the principal modes as to include mode dependent loss and show that principal modes overcome modal dispersion induced by modal coupling in mode division multiplexing operation, even for multi-mode-fibers guiding a large number of modes, if the product of modulation bandwidth, fiber length and differential group delay is equal or less than one in each transmission channel If this condition is not sustained, modal dispersion and crosstalk at the receiver limit the transmission performance, setting very high constraints towards modal coupling

Mode evolution in long tapered fibers with high tapering ratio

Abstract: We have experimentally studied fundamental mode propagation in few meters long, adiabatically tapered step-index fibers with high numerical aperture, core diameter up to 117 μm (V = 38) and tapering ratio up to 18. The single fundamental mode propagation was confirmed by several techniques that reveal no signature of higher-order mode excitation. It can be, therefore, concluded that adiabatic tapering is a powerful method for selective excitation of the fundamental mode in highly multimode large-mode-area fibers. Annular near field distortion observed for large output core diameters was attributed to built-in stress due to thermal expansion mismatch between core and cladding materials. The mechanical stress could be avoided by an appropriate technique of fiber preform fabrication and drawing, which would prevent the mode field deformation and lead to reliable diffraction-limited fundamental mode guiding for very large core diameters.

Reception of Dual-Spatial-Mode CO-OFDM Signal Over a Two-Mode Fiber

Abstract: We demonstrate the use of mechanical grating based mode converters to achieve two forms of dual-spatial-mode transmission: LP01 and LP11, and dual LP01 modes. High modal extinction ratio (>; 20 dB) is shown for the mode converter within a 10-nm wavelength range. We first present 107-Gb/s coherent optical OFDM (CO-OFDM) transmission over a 4.5-km two-mode fiber using LP01 and LP11 modes where the mode separation is performed optically. We then show 58.8-Gb/s CO-OFDM transmission using dual LP11 modes where the mode separation is achieved via 4 × 4 electronic MIMO processing.

Mode conversion in rectangular-core optical fibers.

Abstract: Mode conversion from the fundamental to a higher-order mode in a rectangular-core optical fiber is accomplished by applying pressure with the edge of a flat plate. Modal analysis of the near and far field images of the fiber’s transmitted beam determines the purity of the converted mode. Mode conversion reaching 75% of the targeted higher-order mode is achieved using this technique. Conversion from a higher-order mode back to the fundamental mode is also demonstrated with comparable efficiency. Propagation of a higher-order mode in a rectangular-core fiber allows for better thermal management and bend-loss immunity than conventional circular-core fibers, extending the power-handling capabilities of optical fibers.

850-nm VCSEL Transmission Over Standard Single-Mode Fiber Using Fiber Mode Filter

Abstract: We report on a novel fiber mode filter consisting of an LP11 spatial mode rotator and an LP11/LP01 mode coupler to mitigate 850-nm modal dispersion over standard single-mode fibers (SMF-28). We demonstrate 10-Gb/s error-free transmission over 1 km of SMF-28 fiber using directly modulated vertical-cavity surface-emitting lasers and we measure a 3.3-dB reduction in the received optical power required to achieve a bit-error-ratio of 10-12 when compared to transmission over 800 m of OM3 multimode fiber.

Impact of modal crosstalk and multi-path interference on few-mode fiber transmission

Abstract: Crosstalk caused by random mode coupling can make mode-multiplexing infeasible. However, transmission of single mode over few-mode fibers is relatively insensitive to mode mixing and therefore they can be used as large-effective area fibers.

Optimization of Spatial Aperture-Sampled Mode Multiplexer for a Three-Mode Fiber

Abstract: An optimization procedure for spatial mode multiplexing from individual single-mode fibers into a three-mode fiber based on a spatial aperture sampling concept has been developed. By placing space-variant imaging elements between the single-mode and few-mode fibers, each beam aperture can be shaped for lower loss coupling and low mode-dependent losses. The optimization achieves a record theoretical -1.5-dB insertion loss, improving on the previous theoretical -2-dB record.

Dynamic Behavior of Mode Partition Noise in Multimode Fiber Links

Abstract: In this paper we investigate and quantify the dynamic behavior of the mode partition noise variance. We found that the mode partition noise variance is largest at the bit boundaries and depends on the slope (edge transition times) of the signal in links with one propagating fiber mode. In multimode fibers we show similar dependence, although the mode partition noise is reduced because fiber mode groups mix the chromatically dispersed laser modes responsible for mode partition noise.

Very long range pulse coding distribution type Fiber Brillouin sensor combined with Fiber Brillouin frequency shift device

Abstract: The invention discloses a very long range pulse coding distribution type Fiber Brillouin sensor combined with a Fiber Brillouin frequency shift device, and the sensor is manufactured by the pulse coding principle, the fiber Brillouin scattering frequency shift effect, the fiber stimulated Raman scatting and Raman light amplification effect, the coherent amplification Brillouin scattering light strain, temperature effect and the optical time analysis principle. The very long range pulse coding distribution type Fiber Brillouin sensor comprises a waveform generator, a narrow-linewidth single-frequency fiber laser, two fiber shunts, a pulse coding photomodulator, a fiber Brillouin frequency shift device, two fiber circulators, an Er-doped fiber amplifier, a polarization mode scrambler, a fiber narrow-band reflection filter, a fiber pump-signal coupler, a fiber Raman pump laser, a single-mode sensing fiber, a fiber filter, a photoelectric receiver, an amplifier module, two digital signal processors, a photoelectric heterodyning and receiving amplifier module and a computer. According to the sensor, the laser pulse photon number of an incident sensing fiber can be effectively improved, the signal to noise ratio of a sensor system is improved, and the measurement distance, the measurement precision and the spatial resolution are improved.

Method and system for ultrashort pulse fiber delivery using higher order mode fiber

Abstract: Embodiments of the present invention describe systems and methods for delivering ultrashort laser pulses through an optical fiber system with higher order mode fiber output and without pre-chirping. In one embodiment of the present invention, an all-fiber delivery system comprises a mode-locked solid-state or fiber laser for generating laser pulses in the 0.2 μm to 1.3 μm wavelength range, a single mode fiber with normal dispersion, and a long-period-grating mode converter, and a higher order mode fiber with anomalous dispersion, wherein the all-fiber delivery system is free of bulk optics, and propagates laser pulses without pulse pre-chirping, and wherein higher order mode output beam from the all-fiber delivery system comprises pulses at less than about 200 femtoseconds.

7-core 2-mode fibers with large Aeff to simultaneously realize “3M”

Abstract: 7-core 2-mode fibers with large Aeff of 170µm2 (LP01 mode), and 250µm2 (LP11 mode) were successfully designed and fabricated. We measured optical properties, and confirmed large Aeff and low cross-talks.

Method and system for compensating mode conversion over a communications channel

Abstract: A circuit can compensate for intra pair skew or mode conversion in a channel by applying a second or corrective mode conversion effect that counters the channel's mode conversion. The circuit can process the common mode signal with a frequency dependent filter prior to injection back into the differential mode. The circuit can implement the reverse mode conversion with passive circuits using integrated resistors and metal oxide semiconductor (MOS) switches. In certain embodiments, such actions can proceed effectively without necessarily consuming active power.

Mode coupling effects in mode-division-multiplexed systems

Abstract: Mode coupling is a key to overcoming major challenges in mode-division-multiplexed (MDM) long-haul systems using coherent detection. Strong mode coupling reduces the modal group delay spread, minimizing the complexity of digital multi-input multi-output (MIMO) signal processing. Likewise, strong mode coupling mitigates the mode-dependent gain of optical amplifiers, maximizing average channel capacity. When combined with modal dispersion, strong mode coupling creates frequency diversity, dramatically reducing outage probability. Remarkably, the statistical distributions of strongly coupled modal group delays or gains depend only on the number of modes and the variances of accumulated delay or gain, and can be derived from the eigenvalue distributions of certain random matrices. Insight into mode coupling suggests alternative approaches for MDM based on optical MIMO signal processing, which may offer reduced power consumption in short-reach direct-detection systems.

Stabilized Large Mode Area in Tapered Photonic Crystal Fiber for Stable Coupling

Abstract: A rigorous modal solution approach based on the numerically efficient finite element method (FEM) has been used to design a tapered photonic crystal fiber with a large mode area that could be efficiently coupled to an optical fiber. Here, for the first time, we report that the expanded mode area can be stabilized against possible fabrication tolerances by introducing a secondary surrounding waveguide with larger air holes in the outer ring. A full-vectorial -field approach is employed to obtain mode field areas along the tapered section, and the Least Squares Boundary Residual (LSBR) method is used to obtain the coupling coefficients to a butt-coupled fiber.

LP01 and LP11 mode division multiplexing link with mode crossbar switch

Abstract: A mode crossbar switch scheme between LP 01 and LP 11 modes is proposed and experimentally demonstrated. A spatial light modulator is used as a mode crossbar switch controller to solve the issue of LP 11 mode random rotation. It is demonstrated that lower than -10dB mode crosstalk ratios can be achieved over a 10km few-mode fibre transmission for both bar and cross switch cases.

On the thermal origin of mode instabilities in high power fiber lasers

Abstract: Detailed simulations on the temperature profile inside of a large mode area fiber in high power operation with mode interference are presented. These simulations show that the pump power is not homogeneously absorbed along the fiber, which in turn gives rise to an oscillating temperature profile along the fiber. This longitudinal temperature profile creates an index grating with the right period to transfer energy between the interfering modes. Two cases are analyzed: mode beating between the fundamental mode and a radially anti-symmetric mode and mode beating between the fundamental mode and a radially symmetric mode.