Showing papers on "Polarization mode dispersion published in 2012"

Mode-Division Multiplexing Over 96 km of Few-Mode Fiber Using Coherent 6 $\,\times\,$ 6 MIMO Processing

Abstract: We report simultaneous transmission of six spatial and polarization modes, each carrying 40 Gb/s quadrature-phase-shift-keyed channels over 96 km of a low-differential group delay few-mode fiber. The channels are successfully recovered by offline DSP based on coherent detection and multiple-input multiple-output processing. A penalty of ;28 dB.

Stokes-space analysis of modal dispersion in fibers with multiple mode transmission

Abstract: Modal dispersion (MD) in a multimode fiber may be considered as a generalized form of polarization mode dispersion (PMD) in single mode fibers. Using this analogy, we extend the formalism developed for PMD to characterize MD in fibers with multiple spatial modes. We introduce a MD vector defined in a D-dimensional extended Stokes space whose square length is the sum of the square group delays of the generalized principal states. For strong mode coupling, the MD vector undertakes a D-dimensional isotropic random walk, so that the distribution of its length is a chi distribution with D degrees of freedom. We also characterize the largest differential group delay, that is the difference between the delays of the fastest and the slowest principal states, and show that it too is very well approximated by a chi distribution, although in general with a smaller number of degrees of freedom. Finally, we study the spectral properties of MD in terms of the frequency autocorrelation functions of the MD vector, of the square modulus of the MD vector, and of the largest differential group delay. The analytical results are supported by extensive numerical simulations.

On nonlinear distortions of highly dispersive optical coherent systems

Abstract: We investigate via experiments and simulations the statistical properties and the accumulation of nonlinear transmission impairments in coherent systems without optical dispersion compensation. We experimentally show that signal distortion due to Kerr nonlinearity can be modeled as additive Gaussian noise, and we demonstrate that its variance has a supra-linear dependence on propagation distance for 100 Gb/s transmissions over both low dispersion and standard single mode fiber. We propose a simple empirical model to account for linear and nonlinear noise accumulation, and to predict system performance for a wide range of distances, signal powers and optical noise levels.

Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica

Abstract: Femtosecond laser exposure produces form and stress birefringence in glasses, mainly controlled by laser polarization and pulse energy, which leads to challenges in certain applications where polarization mode dispersion or birefringence splitting is critical for the desired responses from optical devices. In this paper, parallel laser modification tracks with different geometries were applied to preferentially stress the laser-written waveguides and explore the possibility of tuning the waveguide birefringence in devices fabricated in bulk fused silica glass. Polarization splitting in Bragg grating waveguides showed the laser modification tracks to controllably add or subtract stress to the pre-existing waveguide birefringence, demonstrating independence from the nanograting induced form birefringence and the contributions from material stress. Stressing bars are shown that offer tunable birefringence in the range from ∼0 up to 4.35 × 10−4, possibly enabling great flexibility in designing polarization dependent devices, as well as making polarization independent devices.

A PAPR reduction technique based on a constant envelope OFDM approach for fiber nonlinearity mitigation in optical direct-detection systems

Abstract: In this paper, we propose a new peak-to-average power ratio reduction technique based on a constant envelope orthogonal frequency division multiplexing (CE-OFDM) approach to mitigate fiber induced nonlinearities in direct-detection optical OFDM (DDO-OFDM) systems. Simulation results show that the proposed 10 Gbps DDO-CE-OFDM system using 16-quadrature amplitude modulation (16-QAM), 2.66 GHz signal bandwidth, and different values of electrical phase modulation index outperforms DDO-OFDM systems as it increases the fiber nonlinearity tolerance in fiber links without optical dispersion compensation. The bit error rate of the proposed transmission scheme is decreased by a factor of 1000 if compared to conventional DDO-OFDM systems, for 10 dBm of optical input power and considering a span of 960 km of standard single-mode fiber.

Widely Tunable Burst Mode Digital Coherent Receiver With Fast Reconfiguration Time for 112 Gb/s DP-QPSK WDM Networks

Abstract: A widely tunable burst mode digital coherent receiver is implemented in a 112 Gb/s DP-QPSK WDM system. The receiver performance is validated in a 24-channel WDM test-bed using a commercially available DS-DBR laser as the local oscillator. It is demonstrated that the wavelength tunable laser can switch to any one of the 24-channels in less than 130 ns, thus enabling the dynamic reception of 5 μ s optical bursts. The performance of the DS-DBR local oscillator laser is commensurate with burst mode coherent reception when differential decoding is employed and the parallel DSP implementation does not impair the polarization and frequency tracking performance of a digital coherent receiver under burst mode operation. The worst case reconfiguration time of the burst mode receiver, which is a combination of the laser switching time and the CMA convergence time, is less than 410 ns when switching from a single channel to any other channel in the WDM grid. It is shown that the variation in reconfiguration time is dependent on the convergence time of the CMA equalizer, which is adversely affected by certain input states of polarization.

Statistical moments-based OSNR monitoring for coherent optical systems

Abstract: We present a novel statistical moments-based method for optical signal-to-noise ratio (OSNR) monitoring in polarization-multiplexed (pol-mux) coherent optical systems. This technique only requires the knowledge of the envelope of the equalized signal before phase correction, which can be achieved by using any two arbitrary statistical moments, and it is suitable for both constant and non-constant modulus modulation formats. The proposed estimation method is experimentally demonstrated for 10-Gbaud pol-mux coherent systems using QPSK and 16-QAM. Additionally, numerical simulations are carried out to demonstrate 20-Gbaud systems using 16-QAM and 64-QAM. The results show that the OSNR can be estimated accurately over a wide range of values for QPSK, 16-QAM or 64-QAM systems up to 1920-km long and with up to 50-ps all-order polarization mode dispersion. By setting a proper reference value for calibration, the proposed algorithm also shows good tolerance when the received signal is not well compensated.

A robust real-time 100G transceiver With soft-decision forward error correction [Invited]

Abstract: We demonstrate a 120 Gb/s coherent polarization-multiplexed quadrature-phase-shift-keyed transceiver with soft-decision forward-error-correction (SD-FEC) coding based on Turbo Product Code. This industry-first transceiver module utilizes a 40 nm complementary metal-oxide semiconductor (CMOS) application-specific integrated circuit with integrated analog-to-digital conversion, digital signal processing and SD-FEC, and is packaged according to a multi-source agreement from the Optical Internetworking Forum. Through several long-haul and ultra-long-haul system experiments (over 1000 km to 3760 km), we validate the robustness of the transceiver and demonstrate its high tolerance to various system impairments, including fiber nonlinearity, chromatic dispersion up to 60,000 ps/nm, polarization mode dispersion, polarization-dependent loss, polarization transients and multiple-path interference.

Time-domain multimode dispersion measurement in a higher-order-mode fiber.

Abstract: We present a new multimode dispersion measurement technique based on the time-of-flight method. The modal delay and group velocity dispersion of all excited modes in a few-mode fiber can be measured simultaneously by a tunable pulsed laser and a high speed sampling oscilloscope. A newly designed higher-order-mode fiber with large anomalous dispersion in the LP02 mode has been characterized using this method, and experimental results are in good agreement with the designed dispersion values. The demonstrated technique is significantly simpler to implement than the existing frequency-domain or interferometry-based methods.

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.

On the nonlinear threshold versus distance in long-haul highly-dispersive coherent systems.

Abstract: We show that the accumulation rate of nonlinearity in highly-dispersive long-haul coherent links can also be measured from the nonlinear threshold decrease rate, and provide simulations of such rates for both single- and cross-channel effects We then show how the estimated rate can be used for the overall system design

Mode-evolution-based polarization rotator-splitter design via simple fabrication process.

Abstract: A mode-evolution-based polarization rotator-splitter built on InP substrate is proposed by combining a mode converter and an adiabatic asymmetric Y-coupler. The mode converter, consisting of a bi-level taper and a width taper, effectively converts the fundamental TM mode into the second order TE mode without changing the polarization of the fundamental TE mode. The following adiabatic asymmetric Y-coupler splits the fundamental and the second order TE modes and also converts the second order TE mode into the fundamental TE mode. A shallow etched structure is proposed for the width taper to enhance the polarization conversion efficiency. The device has a total length of 1350 µm, a polarization extinction ratio over 25 dB and an insertion loss below 0.5 dB both for TE and TM modes, over the wavelength range from 1528 to 1612 nm covering all C + L band. Because the device is designed based on mode evolution principle, it has a large fabrication tolerance. The insertion loss remains below 1 dB and the polarization extinction ratio remains over 17 dB with respect to a width variation of +/− 0.12 µm at the wavelength of 1570 nm, or +/− 0.08 µm over the entire C + L band.

Orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings.

Abstract: Spectral characteristics of orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings (PM-FBG) are proposed and analyzed experimentally and theoretically. Different from the polarization mode coupling in PM-FBG due to side pressure, a resonant peak at the middle of two orthogonal polarization modes is found when the PM-FBG is twisted purely which is attributed to the cross coupling of polarization modes. Its intensity increases with the twisting rate. A new coupled mode equation is built to describe the pure twist polarization mode coupling, in which both the normal strain induced by strain-applied parts and the tangential strain induced by twisting are taken into consideration and expressed in a unified coordinate. The novel phenomenon and its explanation are believed to be helpful for PM-FBG applications in fiber sensor and laser technologies.

Adaptive frequency-domain equalization for the transmission of the fundamental mode in a few-mode fiber.

Abstract: We propose and experimentally demonstrate single-carrier adaptive frequency-domain equalization (SC-FDE) to mitigate multipath interference (MPI) for the transmission of the fundamental mode in a few-mode fiber. The FDE approach reduces computational complexity significantly compared to the time-domain equalization (TDE) approach while maintaining the same performance. Both FDE and TDE methods are evaluated by simulating long-haul fundamental-mode transmission using a few-mode fiber. For the fundamental mode operation, the required tap length of the equalizer depends on the differential mode group delay (DMGD) of a single span rather than DMGD of the entire link.

Extensive comparisons of optical fast-OFDM and conventional optical OFDM for local and access networks

Abstract: A performance comparison between a recently proposed novel technique known as fast orthogonal frequency-division multiplexing (FOFDM) and conventional orthogonal frequency-division multiplexing (OFDM) is undertaken over unamplified, intensity-modulated, and direct-detected directly modulated laser-based optical signals. Key transceiver parameters, such as the maximum achievable transmission capacity and the digital-to-analog/analog-to-digital converter (DAC/ADC) effects are explored thoroughly. It is shown that, similarly to conventional OFDM, the least complex and bandwidth efficient FOFDM can support up to ~20 Gb/s over 500 m worst-case multimode fiber (MMF) links having 3 dB effective bandwidths of ~200 MHz × km. For compensation of the DAC/ADC roll-off, a power-loading (PL) algorithm is adopted, leading to an FOFDM system improvement of ~4 dB. FOFDM and conventional OFDM give similar optimum DAC/ADC parameters over 500 m worst-case MMF, while over 50 km single-mode fiber a maximum deviation of only ~1 dB in clipping ratio is observed due to the imperfect chromatic dispersion compensation caused by one-tap equalizers.

Hamiltonian tools for the analysis of optical polarization control

Abstract: The study of the polarization dynamics of two counterpropagating beams in optical fibers has recently been the subject of a growing renewed interest, from both the theoretical and experimental points of view. This system exhibits a phenomenon of polarization attraction, which can be used to achieve a complete polarization of an initially unpolarized signal beam, almost without any loss of energy. Along the same way, an arbitrary polarization state of the signal beam can be controlled and converted into any other desired state of polarization, by adjusting the polarization state of the counterpropagating pump beam. These properties have been demonstrated in various different types of optical fibers, i.e., isotropic fibers, spun fibers, and telecommunication optical fibers. This article is aimed at providing a rather complete understanding of this phenomenon of polarization attraction on the basis of new mathematical techniques recently developed for the study of Hamiltonian singularities. In particular, we show the essential role that play the peculiar topological properties of singular tori in the process of polarization attraction. We provide here a pedagogical introduction to this geometric approach of Hamiltonian singularities and give a unified description of the polarization attraction phenomenon in various types of optical fiber systems.

Experimental demonstration of 1.08 Tb/s PDM CO-SCFDM transmission over 3170 km SSMF.

Abstract: Coherent optical single-carrier frequency-division-multiplexing (CO-SCFDM) is a promising candidate for future high-speed long-haul optical fiber transmission system. Being a modified form of coherent optical orthogonal frequency division multiplexing (CO-OFDM), the CO-SCFDM can inherit the advantages such as low computation complexity and high flexibility, while suffers less nonlinear impairment due to much lower peak-to-average power ratio (PAPR). In this paper, we experimentally demonstrate 1.08 Tb/s polarization-division multiplexing (PDM) CO-SCFDM transmission over 3170 km standard single-mode fiber (SSMF) with Erbium-doped fiber amplifier (EDFA) only. The back-to-back and nonlinear transmission performances for CO-OFDM and CO-SCFDM are also compared.

Efficient sweep buffering in swept source optical coherence tomography using a fast optical switch

Abstract: We describe a novel buffering technique for increasing the A-scan rate of swept source optical coherence tomography (SSOCT) systems employing low duty cycle swept source lasers. This technique differs from previously reported buffering techniques in that it employs a fast optical switch, capable of switching in 60 ns, instead of a fused fiber coupler at the end of the buffering stage, and is therefore appreciably more power efficient. The use of the switch also eliminates patient exposure to light that is not used for imaging that occurs at the end of the laser sweep, thereby increasing the system sensitivity. We also describe how careful management of polarization can remove undesirable artifacts due to polarization mode dispersion. In addition, we demonstrate how numerical compensation techniques can be used to modify the signal from a Mach-Zehnder interferometer (MZI) clock obtained from the original sweep to recalibrate the buffered sweep, thereby reducing the complexity of systems employing lasers with integrated MZI clocks. Combining these methods, we constructed an SSOCT system employing an Axsun technologies laser with a sweep rate of 100kHz and 6dB imaging range of 5.5mm. The sweep rate was doubled with sweep buffering to 200 kHz, and the imaging depth was extended to 9 mm using coherence revival. We demonstrated the feasibility of this system by acquiring images of the anterior segments and retinas of healthy human volunteers.

Modified annular photonic crystals with enhanced dispersion relations: polarization insensitive self-collimation and nanophotonic wire waveguide designs

Abstract: A novel (to our best knowledge) type of photonic crystal (PC) structure called modified annular PC (MAPC) that is composed of dielectric rods with off-centered air holes is thoroughly studied. The plane wave expansion method is applied for spectral analysis. A complete photonic bandgap region with a considerable value of gap width Δω/ω=7.06% is achieved by optimizing the structural parameters of the proposed periodic medium. By introducing geometrical asymmetry to the primitive cell of PC, we engineer the dispersion properties of the proposed photonic structure such that conventional equifrequency contours for the second band can be transformed into tilted rectangular shapes. This feature enables us to demonstrate the polarization insensitive tilted self-collimation effect. A hybrid structure composed of dielectric nanowire and MAPCs is offered to obtain a high degree of polarization independent guiding of light. The two-dimensional finite-difference time-domain method is carried out to verify the light guiding efficiencies. Polarization insensitive optical functionalities achieved by MAPC structure can be deployed in integrated optical circuits.

Method for improving the resolution and accuracy against birefringence dispersion in distributed polarization cross-talk measurements

Abstract: We present a novel method for improving the spatial resolution and amplitude accuracy of distributed polarization cross-talk measurements in a polarization maintaining (PM) fiber against its birefringence dispersion. We show that the broadening of measured polarization cross-talk peaks caused by birefringence dispersion can be restored by simply multiplying the measurement data with a compensation function. The birefringence dispersion variable in the function can be obtained by finding the widths of measured cross-talk envelopes at known distances along the fiber. We demonstrate that this method can effectively improve spatial resolution and amplitude accuracy of the space-resolved polarization cross-talk measurements of long PM fibers.

Theory of polarization attraction in parametric amplifiers based on telecommunication fibers

Abstract: We develop from first principles the coupled wave equations that describe polarization-sensitive parametric amplification based on four-wave mixing (FWM) in standard (randomly birefringent) optical fibers. We show that in the small-signal case these equations can be solved analytically, and permit us to predict the gain experienced by the signal beam as well as its state of polarization (SOP) at the fiber output. We find that, independently of its initial value, the output SOP of a signal within the parametric gain bandwidth is solely determined by the pump SOP. We call this effect of pulling the polarization of the signal towards a reference SOP the polarization attraction, and we call the parametric amplifier the FWM polarizer (which can equivalently be called the fiber-optic parametric amplifier polarizer). Our theory is valid beyond the zero polarization mode dispersion (PMD) limit, and it takes into account moderate deviations of the PMD from zero. In particular, our theory is capable of analytically predicting the rate of degradation of the efficiency of the parametric amplifier, which is caused by the detrimental PMD effect.

Polarization entanglement generation at 1.5 μm based on walk-off effect due to fiber birefringence

Abstract: In this Letter, a linear scheme to generate polarization entanglement at 1.5 μm based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four-wave mixing. A 90 deg offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single-side and two-photon interference in two nonorthogonal polarization bases are demonstrated. A two-photon interference fringe visibility of 89±3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly a efficient and stable fiber based polarization-entangled quantum light source at the optical communication band.

High-fidelity, 160 fs, 5 μJ pulses from an integrated Yb-fiber laser system with a fiber stretcher matching a simple grating compressor.

Abstract: Although femtosecond microjoule Yb-fiber systems are attractive because of a straightforward power scalability, they inherently suffer from a lowered pulse fidelity as a result of complex dispersion and nonlinearity management. Here, we present an integrated Yb-fiber system delivering high-fidelity microjoule pulses compressible down to 160 fs. The system uses a dispersion compensating fiber stretcher that is specially designed to match the dispersion of a 1480 lines/mm grating compressor. Performance analysis suggests the further possibility of scaling the pulse energy to tens of microjoules without pulse quality deterioration using this dispersion management scheme.

Chromatic dispersion estimation in 40 Gb/s coherent polarization-multiplexed single carrier system using complementary Golay sequences

Abstract: We experimentally demonstrate full-transmission chromatic dispersion estimation with high accuracy in 40-Gb/s coherent polarization-multiplexed single-carrier system using binary complementary Golay training sequences. The technique is verified to be robust against polarization-mode-dispersion and amplified-spontaneous-emission noise.

Digital demodulator architecture

Abstract: Methods, systems, and devices are described for a digital demodulator device for processing received optical signals. The device may include a quadrature error filter that receives a digitized version of an optical signal, and removes quadrature errors to generate a filtered series of data samples. The device may also include a frequency offset removal module for performing frequency rotation on the filtered series of data samples. The device may include a chromatic dispersion compensation module which removes chromatic dispersion from horizontal and vertical polarization channels. The device may include a polarization mode dispersion (PMD)/polarization dependent loss (PDL) compensation module which compensates for interference caused by PMD and PDL. The device may also include a phase recovery module configured to track and correct phase.

Decision directed least radius distance algorithm for blind equalization in a dual-polarization 16-QAM system

Abstract: Robust PMD tolerance and fast pre-convergence speed of decision directed least radius distance algorithm is demonstrated via a 224Gb/s dual-polarization 16-QAM system simulation.

A multi-objective approach to design all-optical and translucent optical networks considering CapEx and QoT

Abstract: In this paper we propose a methodology based on an evolutionary multi-objective optimization algorithm, called NSGA-II to design the topology and define the devices for both all-optical and translucent optical network. We aim to define the topology layout and the specification of the optical devices that should be deployed in the network in order to minimize simultaneously the total installation cost of a communication network (CapEx) and the total network blocking probability (performance criterion). To accomplish that, we propose a capital cost model for the network. We considered the following physical layer impairments: losses in optical devices, amplified spontaneous emission in optical amplifier and homodyne crosstalk in optical cross connect, polarization mode dispersion and residual dispersion. Our proposed methodology can solve the network topology design problem taking into account the physical layer impairments and the capital costs simultaneously. We also present a case study to show the effectiveness of our methodology to define the degree of transparency of the network.

Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers

Abstract: We report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to −1.6 ps2 is experimentally studied. Strongly positively chirped pulses from 4.3 ps (0.01 ps2) to 39 ps (2.5 ps2) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8 ps (−0.07 ps2) to 3.9 ps (−1.6 ps2). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped-pulse regimes are discussed.

Experimental characterization of the counter-propagating Raman polarization attraction

Abstract: Recently, fiber Raman amplifiers have proven to be effective in the all-optical control of the state of polarization of signals in single-mode telecommunications optical fibers. Previous works predicted the existence of a quantitative relationship between the achieved degree of polarization and the mean Raman gain. Here, we experimentally validate such a relationship in the case of counter-propagating Raman-based polarization attractors for different pump and signal powers and for different fiber link lengths.