Showing papers on "Mode scrambler published in 2015"
TL;DR: It is shown 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.
Abstract: 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 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 <1.5 dB power penalties at a bit-error-rate of 2 × 10(-3).
247 citations
TL;DR: In this paper, a two-mode fiber with a CO2 laser was used for converting the LP01 mode to any of the four higher order cylindrical vector modes or the LP11 modes.
Abstract: We fabricate mode converters by directly writing long-period gratings in a two-mode fiber with a CO2 laser. These mode converters allow the fundamental LP01 mode to be converted to any of the four higher order cylindrical vector modes or the LP11 modes. The transmission characteristics of these devices are insensitive to temperature variations and the mode conversion efficiency is insensitive to the polarization state of the input light. One of our typical gratings, which contains 15 grating periods, can provide a conversion efficiency >99% over a bandwidth of 34.0 nm in the C-band. These robust mode converters could find applications in mode-division-multiplexing systems and other applications that require cylindrical vector modes.
89 citations
TL;DR: The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described and the effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail.
Abstract: The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described. The free-space optics FMF-WSS dynamically steers spectral information-bearing beams containing three spatial modes from an input port to one of nine output ports using a phase spatial light modulator. Sources of mode dependent losses (MDL) are identified, analytically analyzed and experimentally confirmed on account of different modal sensitivities to fiber coupling in imperfect imaging and at spectral channel edges due to mode clipping. These performance impacting effects can be reduced by adhering to provided design guidelines, which scale in support of higher spatial mode counts. The effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail.
46 citations
TL;DR: In this paper, a few-mode fiber Bragg gratings (FM-FBGs) inscribed in two mode and four mode step-index fibers are presented, which can be used as reflective mode-converters for mode division multiplexed data transmission systems.
Abstract: We present the results on fabrication and the characterization of few-mode fiber Bragg gratings (FM-FBGs) inscribed in two mode and four mode step-index fibers. Under the conditions of selective input mode launching, coupling between specific modes of interest can be selectively excited and the self-coupling and cross-coupling properties at the associated resonant wavelengths can be clearly identified and verified by observing the reflected mode intensity profiles. Such FM-FBGs can potentially be used as reflective mode-converters for mode division multiplexed data transmission systems.
46 citations
TL;DR: An on-chip optical mode exchange between two multiplexed modes by using tapered directional couplers on silicon-on-insulator platform is presented and simulation results show efficient higher order mode excitation and mode exchange.
Abstract: 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.
45 citations
TL;DR: The mode property and light propagation in a tapered silicon-on-insulator (SOI) nanowire with angled sidewalls is analyzed and mode hybridization is observed and mode conversion between the TM fundamental mode and higher-order TE modes happens.
Abstract: 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.
45 citations
TL;DR: In this article, the authors investigated mode coupling in few-mode fibers induced by homogeneously and periodically applied mechanical stress, using computer-generated holograms to view the power transfer between individual modes.
Abstract: We investigate mode coupling in few-mode fibers induced by homogeneously and periodically applied mechanical stress. To view the power transfer between individual modes, a modal decomposition is performed at the end of the fiber using computer-generated holograms. Coupling between polarization and angular degenerated modes as well as between non-degenerated modes is confirmed experimentally and coupling parameters are inferred. The presented studies pave the way to detailed investigations of mode coupling in mode-multiplexed telecommunication systems and high-power high beam quality fiber lasers.
42 citations
TL;DR: In this paper, a ring-core FM erbium-doped fiber (RC-EDF) was employed to improve the differential modal gain (DMG) of a few-mode EDF amplifier.
Abstract: The differential modal gain (DMG) in a few-mode erbium-doped fibre amplifier (FM-EDFA) is successfully improved by employing a ring-core FM erbium-doped fibre (RC-EDF). The DMGs between LP01 and LP11 mode signals in the FM-EDFA with the RC-EDF are 1.8 and 1.6 dB for LP01 and LP11 mode pumping, respectively, whereas those in a FM-EDFA with a conventional step-index FM EDF are 9.2 and 6.1 dB, indicating 7.4 and 4.5 dB DMG improvement for LP01 and LP11 mode pumping, respectively.
37 citations
TL;DR: This work demonstrates 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).
Abstract: 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 6×6 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.
36 citations
TL;DR: A novel approach to adaptively control the beam profile in a few-mode fiber is experimentally demonstrated through an electric-controlled polarization controller, which achieves selective excitations of LP01 and LP11 modes, as well as significant improvement of the beam quality factor.
Abstract: A novel approach to adaptively control the beam profile in a few-mode fiber is experimentally demonstrated. We stress the fiber through an electric-controlled polarization controller, whose driven voltage depends on the current and target modal content difference obtained with the real-time mode decomposition. We have achieved selective excitations of LP01 and LP11 modes, as well as significant improvement of the beam quality factor, which may play crucial roles for high-power fiber lasers, fiber based telecommunication systems and other fundamental researches and applications.
35 citations
TL;DR: 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.
Abstract: We experimentally demonstrate mode conversion by exploiting optical reflection of tilted few-mode fiber Bragg grating (FM-FBG). Mode conversions from LP01 mode to higher symmetric and asymmetric modes are achieved, and more than 99.5% conversion efficiency from LP01 to LP11 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.
22 Mar 2015
TL;DR: A spatial 6-mode multiplexer with low loss and high mode selectivity with 100Gbit/s error-free transmission over 20km for 6 modes and over 40km bi-directional transmission with 3 modes in each direction is presented.
Abstract: We present a spatial 6-mode multiplexer with low loss and high mode selectivity. We demonstrate 100Gbit/s error-free transmission over 20km for 6 modes and over 40km bi-directional transmission with 3 modes in each direction.
TL;DR: This work analytically derive the coupling matrix of the multimode connector and numerically study the dependence of the matrix coefficients as a function of the butt-joint connection characteristics to assess the impact of the connector on the signal quality investigating different receiver digital signal processing (DSP) schemes.
Abstract: In mode-division multiplexed (MDM) transmission systems, mode coupling is responsible for inter-modal crosstalk. We consider the transmission of modulated signals over a few-mode fiber (FMF) having low mode coupling and large differential mode group delay in the presence of a non-ideal fiber connection responsible for extra mode coupling. In this context, we first analytically derive the coupling matrix of the multimode connector and we numerically study the dependence of the matrix coefficients as a function of the butt-joint connection characteristics. The numerical results are then validated through an experiment with a five-mode setup. Finally, through numerical simulations, we assess the impact of the connector on the signal quality investigating different receiver digital signal processing (DSP) schemes.
TL;DR: Using a mode transfer matrix formalism it is possible to understand and optimize mode effects in multimode fiber optic systems and demonstrate the accordance of the theory by two experiments.
Abstract: Fiber Bragg gratings (FBG) in multimode optical fibers provide a means for cost-effictive devices resulting in simplified and robust optic sensor systems. Parasitic mode effects in optical components of the entire measurement system strongly influence the measured multi-resonance reflection spectrum. Using a mode transfer matrix formalism we can describe these complex mode coupling effects in multimode optical systems in more detail. We demonstrate the accordance of the theory by two experiments. With this formalism it is possible to understand and optimize mode effects in multimode fiber optic systems.
TL;DR: In this article, the authors studied the effect of strong mode coupling via mode scrambler on the group delay scaling of a long-haul MIMO system and showed that the mode-dependent loss of the grating can be minimized while ensuring full intergroup mode coupling.
Abstract: In mode-division-multiplexing systems, multi-input multi-output (MIMO) equalization is used to compensate for linear impairments, including modal dispersion (MD) and modal crosstalk. The MIMO equalizer memory length depends on the group delay (GD) spread arising from MD. The GD spread arising from MD can be significantly reduced by introducing strong mode coupling via mode scramblers. We study the design of such mode scramblers implemented as long-period multimode fiber gratings for systems using $D = 12$ modes (six spatial modes). By optimizing the grating chirp function, we minimize the mode-dependent loss (MDL) of the grating while ensuring full intergroup mode coupling. We find a design yielding MDL and mode-averaged loss in the C band not exceeding 0.36 and 0.45 dB, respectively. We also verify the effect of such mode scramblers on the GD scaling of a long-haul system, demonstrating that the scramblers reduce the scaling of GD spread with length from a linear to a square-root dependence, as expected in the strong coupling regime.
TL;DR: In this article, the ability of modal decomposition based on the analysis of the near-field intensity profile using the stochastic parallel gradient descent algorithm is experimentally characterized in detail for the first time.
Abstract: The ability of modal decomposition based on the analysis of the near-field intensity profile using the stochastic parallel gradient descent algorithm is experimentally characterized in detail for the first time. The results show that the decomposition method proposed in this letter turns to be highly accurate and rapid. Further, this method is applied to analyze the mode behavior in dependence on the pump power in a high-power fiber amplifier with different initial mode compositions.
TL;DR: Numerical simulations based on the super-mode coupling approach were used to estimate fabrication parameters and to discuss the modal evolution in arbitrarily fused couplers.
Abstract: Fused-type mode-selective fiber couplers exciting the LP(11) mode are fabricated by well-defined fiber cladding reduction, pretapering and fusion. At a wavelength of 905 nm 80 % of the injected power in the single-mode fiber was transmitted in the few-mode fiber selectively exciting the LP(11) mode. The coupling behavior was experimentally investigated for the case of strong as well as weak fusion. Numerical simulations based on the super-mode coupling approach were used to estimate fabrication parameters and to discuss the modal evolution in arbitrarily fused couplers. The influence of changes in the coupler geometry on the super-modes and their modal weighting are analyzed by calculations of the effective refractive index and by modal decomposition.
TL;DR: In this article, the authors derived analytical expressions for the performance improvement of long-haul coherent optical communication systems using QSM fibers, taking into account the impact of excess loss and MPI on system performance.
Abstract: We use the power coupled-mode theory to study the interplay between multipath interference (MPI) and differential mode attenuation (DMA) in quasi-single mode (QSM) fibers. The analytical expressions derived assuming two mode propagation in QSM fibers show that MPI scales differently as a function of the span length for low and high DMA. Furthermore, we derive analytical expressions for the performance improvement of long-haul coherent optical communication systems using QSM fibers, taking into account the impact of excess loss and MPI on system performance. From these expressions, we calculate the maximum allowable coupling coefficient for different values of the DMA. We show, for example, that a QSM fiber with an effective area of 250 μm2, a coupling coefficient κ ≤ 6 × 10-4 km-1, and DMA equal to 4 dB/km offers a 1-dB performance advantage over a reference pure silica core single-mode fiber for spans of 100 km.
TL;DR: A design of low-loss THz Bragg fibers with a core size on the order of wavelength that operates near the cutoff frequency of its TE01 mode and a broadband Y-type mode converter based on branched rectangular metallic waveguides to facilitate coupling between the TE02 mode of the Bragg fiber and the TEM mode in free space with 60% efficiency are reported.
Abstract: We report a design of low-loss THz Bragg fibers with a core size on the order of wavelength that operates near the cutoff frequency of its TE01 mode. We also propose a broadband Y-type mode converter based on branched rectangular metallic waveguides to facilitate coupling between the TE01 mode of the Bragg fiber and the TEM mode in free space with 60% efficiency. Our fiber holds strong promise to facilitate beam-wave interaction in gyrotron for high-efficiency THz generation.
22 Mar 2015
TL;DR: A simple all-fiber spatial mode selective filter based on splicing a short segment of small-core SMF between two few-mode fibers is presented and an 8dB MDL equalization range with less than 2dB IL is demonstrated.
Abstract: We propose and demonstrate a simple all-fiber spatial mode selective filter based on splicing a short segment of small-core SMF between two few-mode fibers. An 8dB MDL equalization range with less than 2dB IL is demonstrated for a 2-mode fiber device.
TL;DR: This work proposes and demonstrate axicons for linear bulk-optic mode conversion at the output of higher order mode amplifiers, and proposes and demonstrates a route towards fiber integration of the axicon mode convertor by fabricating axicons directly on the tip of the fiber amplifier end-cap.
Abstract: Higher-order mode fiber amplifiers have demonstrated effective areas as large as 6000 μm2, allowing for high pulse energy and peak power amplification. Long-period gratings are used to convert the fundamental mode to the higher-order mode at the entrance to the amplifier, and reconvert back to the fundamental at the exit, to achieve a diffraction limited beam. However, long period gratings are susceptible to nonlinearity at high peak power. In this work, we propose and demonstrate axicons for linear bulk-optic mode conversion at the output of higher order mode amplifiers. We achieve an M2 of less than 1.25 for 80% mode conversion efficiency. Experiments with pulsed amplifiers confirm that the mode conversion is free from nonlinearity. Furthermore, chirp pulse amplifier experiments confirm that HOM amplifiers plus axicon mode convertors provide energy scalability in femtosecond pulses, compared to smaller effective area, fundamental mode fiber amplifiers. We also propose and demonstrate a route towards fiber integration of the axicon mode convertor by fabricating axicons directly on the tip of the fiber amplifier end-cap.
17 May 2015
TL;DR: The 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.
Abstract: 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.
TL;DR: In this article, a novel mode converter with broad bandwidth, high mode conversion efficiency, and large fabrication tolerance is proposed and fabricated on a silicon-on-insulator (SOI) platform.
Abstract: A novel mode converter with broad bandwidth, high mode conversion efficiency, and large fabrication tolerance is proposed and fabricated on a silicon-on-insulator (SOI) platform. By using a modified 3 $\times$ 3 multimode interference coupler and an enlarged butterfly-shaped phase shifter, mode conversion from the fundamental mode $(TE_0)$ to second-order mode $(TE_2) $ is achieved numerically and experimentally. Performance analysis of the mode converter is carried out numerically, and the results show high performance of the mode converter with mode conversion efficiency larger than 91% (i.e., insertion loss less than 0.40 dB) in the whole C-band. In addition, the mode converter has large fabrication tolerance. For example, the width and the length deviations of the multimode interference coupler can be as large as 50 and 250 nm, respectively, whereas the width and the length deviations of the phase shifter can be up to 25 and 400 nm, respectively.
TL;DR: The results show that PC-based mode MMUX/DEMMUX has the potential for high-capacity MDM optical communication systems with a low insertion loss, low mode crosstalk, and wide bandwidth.
Abstract: 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 (<0.36dB), low mode crosstalk (< -20.9 dB) and wide bandwidth (~100 nm).
10 Sep 2015
TL;DR: This work shows that a combination of ST coding at the transmitter and an optimal distribution of mode scramblers in the optical link can completely absorb the penalties induced by important levels of MDL in 6-mode SDM systems.
Abstract: Approaching the capacity limits of single-mode fiber based optical transmission systems, new fibers supporting the propagation of up to six orthogonal spatial modes, called few-mode fibers, stand as promising candidates for future high-capacity systems. Extensive research is being carried out to further increase the number of modes to multiplex more data. This technique is known as spatial division multiplexing (SDM). However, the co-existence of modes in the same space leads to inevitable modal crosstalk that may induce a loss of their orthogonality as well as power disparities. This phenomenon is called mode dependent loss (MDL) and mainly arises from optical components such as few-mode amplifiers. Although optical solutions were suggested to reduce MDL by inserting mode scramblers or using fibers with strong modal coupling, MDL was unfortunately not completely removed. In this work, we propose a DSP solution based on Space-Time (ST) coding along with OFDM, originally designed for multi-antenna channels, to mitigate MDL in SDM systems. We show that a combination of ST coding at the transmitter and an optimal distribution of mode scramblers in the optical link can completely absorb the penalties induced by important levels of MDL in 6-mode SDM systems. Later on, we address the complexity and scalability of the ST-coding solution and propose a sub-optimal decoding scheme that keeps the MDL-induced penalty low while considerably reducing the decoding complexity.
TL;DR: In this article, the external mode coupling in optical microresonators with a loss-dependent coupling coefficient is studied, and the authors show that the localized cavity loss can suppress the original whispering gallery modes and generate high-Q coupled modes, which illustrate a possible way to dynamically control the optical modes inside the micro-resonator.
Abstract: We study the external mode coupling in optical microresonators with a loss-dependent coupling coefficient. In the case of dielectric optical microresonators with localized absorption or radiation loss, the mode coupling between whispering gallery modes (WGMs) can result in a destructive interference of the mode fields in the lossy area and consequently reduce the mode loss. Once the reduction overcomes the introduced loss, the actual mode loss is reduced by increasing the loss factor. An anomalous relation between the actual mode loss and the introduced loss factor is then found, while the mode frequencies show parallel, crossing, or anticrossing behavior under different coupling conditions; such phenomena can be well explained by the analysis based on the coupling theory. The results show that the localized cavity loss can suppress the original WGMs and generate high-Q coupled modes, which illustrate a possible way to dynamically control the optical modes inside the microresonator.
TL;DR: In this paper, mode conversion at the output of a higher-order mode fiber amplifier was demonstrated with an axicon for the first time and achieved an M 2 of 1.25 for 82% conversion efficiency.
Abstract: Mode conversion at the output of a higher-order mode fiber amplifier is proposed and demonstrated with an axicon for the first time. M 2 of 1.25 is achieved for 82% conversion efficiency.
22 Mar 2015
TL;DR: Three-mode multiplexers were made by post-processing existing SMF-28 fibres by adiabatically tapered to form a multimode output core and converted to the LP01 and two LP11 with low loss.
Abstract: Three-mode multiplexers were made by post-processing existing SMF-28 fibres. The input was three identical SMFs which were fused together and adiabatically tapered to form a multimode output core. The light was converted to the LP 01 and two LP 11 with low loss (0.6–0.7 dB). The mode multiplexers were broadband.
Posted Content•
TL;DR: In this paper, a simple and efficient all-optical polarization scrambler based on the nonlinear interaction in an optical fiber between a signal beam and its backward replica is generated and amplified by a reflective loop.
Abstract: We report a simple and efficient all-optical polarization scrambler based on the nonlinear interaction in an optical fiber between a signal beam and its backward replica which is generated and amplified by a reflective loop. When the amplification factor exceeds a certain threshold, the system exhibits a chaotic regime in which the evolution of the output polarization state of the signal becomes temporally chaotic and scrambled all over the surface of the Poincare sphere. We derive some analytical estimations for the scrambling performances of our device which are well confirmed by the experimental results. The polarization scrambler has been successfully tested on a single channel 10-Gbit/s On/Off Keying Telecom signal, reaching scrambling speeds up to 250-krad/s, as well as in a wavelength division multiplexing configuration. A different configuration based on a sequent cascade of polarization scramblers is also discussed numerically, which leads to an increase of the scrambling performances.
23 Apr 2015
TL;DR: In this article, the authors investigated 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.
Abstract: 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.