Recirculating Fiber Loop Research Articles

Space–time observation of the dynamics of soliton collisions in a recirculating optical fiber loop

We present experiments performed in a recirculating fiber loop in which we realize the single-shot observation of the space and time interaction of two and three bright solitons. The space–time evolutions observed in experiments provide clear evidence of a nearly-integrable nonlinear wave dynamics that can be easily interpreted within the framework of the inverse scattering transform (IST) method. In particular collisions between solitons are found to be almost perfectly elastic in the sense that they occur without velocity change and with only a position (time) shift quantitatively well described by numerical simulations of the integrable nonlinear Schrödinger equation. Additionally our experiments provide the evidence that the position (time) shifts arising from the interaction among three solitons are determined by elementary pairwise interactions, as it is well known in the IST theory.

Constant envelope OFDM transmission over metro-regional optical links

The transmission of constant envelope orthogonal frequency division multiplexing (CE-OFDM) signals is experimentally demonstrated in this paper to increase the reach of metro-regional optical links. A recirculating fiber loop was implemented for this aim. A reach extension of 100 km of standard single-mode fiber was achieved with the CE-OFDM-based system, considering 16 quadrature amplitude modulation (16-QAM) as subcarrier mapping and links without optical dispersion compensation. Link lengths as long as 700 km, with spectral efficiency of 1.89 b/s/Hz, were accomplished with performances below the forward error correction limit.

All-optical 2R regenerator based on similariton-induced spectral broadening

We numerically investigated an all-optical high-bit-rate 2R-regenerator based on similariton-induced spectral broadening of the optical signal spectrum. The proposed configuration incorporates two cascaded regeneration stages. Active and passive generation of similariton pulses in a highly nonlinear photonic crystal fiber and a comb-like dispersion decreasing fiber are used for the first and the second stages, respectively. The study of the static behavior of the regenerator linking the design to their nonlinear power transfer functions reveals the operating conditions under which the regenerator is able to reduce amplitude noise and improve the signal extinction ratio. Furthermore, the potential of the proposed configuration is evaluated to regenerate an optical signal at 40 Gbit / s, and the 2R-regenerator is then assessed in a long recirculating fiber loop to investigate its cascadibility. The ability of the 2R-regenerator to enable operating at high-bit rates up to 160 Gbit / s is also investigated. The results demonstrate the high potential of similariton pulses for all-optical 2R-regeneration in future high-speed and long-haul transmission systems.

Long-haul propagation analysis of dark pulses employing an optical recirculating fiber loop technique

Dark pulses have been the subject of intense research nowadays. Several investigations constantly report fascinating insights into the fundamental physics of these structures generally observed in all-optical fiber schemes. In this paper, we investigated the experimental propagation of dark pulses at different optical fiber lengths. Distances as long as 1000 km were obtained by applying an optical recirculating fiber loop technique. Our results reveal that the dark pulses after propagating 1000 km on a standard single-mode fiber had a broadening rate of 0.114 ps/km, and a pulse-to-pulse timing jitter of 160 ps. These results provide the first experimental evidence that dark pulses may be successfully employed in long-distance optical applications.

Sub-kHz linewidth, hybrid III-V/silicon wavelength-tunable laser diode operating at the application-rich 1647-1690 nm.

The wavelength region about of 1650 nm enables pervasive applications. Some instances include methane spectroscopy, free-space/fiber communications, LIDAR, gas sensing (i.e. C2H2, C2H4, C3H8), surgery and medical diagnostics. In this work, through the hybrid integration between an III-V optical amplifier and an extended, low-loss wavelength tunable silicon Vernier cavity, we report for the first time, a III-V/silicon hybrid wavelength-tunable laser covering the application-rich wavelength region of 1647-1690 nm. Room-temperature continuous wave operation is achieved with an output power of up to 31.1 mW, corresponding to a maximum side-mode suppression ratio of 46.01 dB. The laser is ultra-coherent, with an estimated linewidth of 0.7 kHz, characterized by integrating a 35 km-long recirculating fiber loop into the delayed self-heterodyne interferometer setup. The laser linewidth is amongst the lowest in hybrid/heterogeneous III-V/silicon lasers.

Experimental realization of Fermi-Pasta-Ulam-Tsingou recurrence in a long-haul optical fiber transmission system

The integrable nonlinear Schrödinger equation (NLSE) is a fundamental model of nonlinear science which also has important consequences in engineering. The powerful framework of the periodic inverse scattering transform (IST) provides a description of the nonlinear phenomena modulational instability and Fermi-Pasta-Ulam-Tsingou (FPUT) recurrence in terms of exact solutions. It associates the complex nonlinear dynamics with invariant nonlinear spectral degrees of freedom that may be used to encode information. While optical fiber is an ideal testing ground of its predictions, maintaining integrability over sufficiently long distances to observe recurrence, as well as synthesizing and measuring the field in both amplitude and phase on the picosecond timescales of typical experiments is challenging. Here we report on the experimental realization of FPUT recurrence in terms of an exact space-time-periodic solution of the integrable NLSE in a testbed for optical communication experiments. The complex-valued initial condition is constructed by means of the finite-gap integration method, modulated onto the optical carrier driven by an arbitrary waveform generator and launched into a recirculating fiber loop with periodic amplification. The measurement with an intradyne coherent receiver after a predetermined number of revolutions provides a non-invasive full-field characterization of the space-time dynamics. The recurrent space-time evolution is in close agreement with theoretical predictions over a distance of 9000 km. Nonlinear spectral analysis reveals an invariant nonlinear spectrum. The space-time scale exceeds that of previous experiments on FPUT recurrence in fiber by three orders of magnitude.

Statistical Properties of the Nonlinear Stage of Modulation Instability in Fiber Optics.

We present an optical fiber experiment in which we examine the space-time evolution of a modulationally unstable plane wave initially perturbed by a small noise. Using a recirculating fiber loop as an experimental platform, we report the single-shot observation of the noise-driven development of breather structures from the early stage to the long-term evolution of modulation instability. Performing single-point statistical analysis of optical power recorded in the experiments, we observe decaying oscillations of the second-order moment together with the exponential distribution in the long-term evolution, as predicted by Agafontsev and Zakharov [Nonlinearity 28, 2791 (2015).NONLE50951-771510.1088/0951-7715/28/8/2791]. Finally, we demonstrate experimentally and numerically that the autocorrelation of the optical power g^{(2)}(τ) exhibits some unique oscillatory features typifying the nonlinear stage of the noise-driven modulation instability and of integrable turbulence.

Nonlinear Evolution of the Locally Induced Modulational Instability in Fiber Optics.

We report an optical fiber experiment in which we study the nonlinear stage of modulational instability of a plane wave in the presence of a localized perturbation. Using a recirculating fiber loop as the experimental platform, we show that the initial perturbation evolves into an expanding nonlinear oscillatory structure exhibiting some universal characteristics that agree with theoretical predictions based on integrability properties of the focusing nonlinear Schrödinger equation. Our experimental results demonstrate the persistence of the universal evolution scenario, even in the presence of small dissipation and noise in an experimental system that is not rigorously of an integrable nature.

Chromatic dispersion measurement based on a high-Q optoelectronic oscillator incorporating cascaded FIR and IIR filters

An approach for chromatic dispersion measurement is proposed and experimentally demonstrated based on an optoelectronic oscillator (OEO) incorporating a single bandpass microwave photonic filter (MPF), which is implemented by cascading a finite impulse response (FIR) MPF and an infinite impulse response (IIR) MPF jointly. The key concept of the proposed design is mapping the chromatic dispersion of a device under test (DUT) to the oscillating frequency of the OEO by embedding the DUT into the OEO loop. The oscillating frequency is mainly determined by the central frequency of the MPF, which depends on the length difference of the two arms of the Mach-Zehnder interferometer (MZI) and the chromatic dispersion value of the fiber cooperatively. The bandwidth of the MPF is further narrowed by an IIR filter based on a recirculating fiber loop for stabilizing OEO oscillation. The chromatic dispersion of fibers with lengths ranging from 20 km to 100 km are measured in the experiment.

13.4km OAM state propagation by recirculating fiber loop.

Enabled by an enhanced effective index separation (Δneff = 1.7 × 10-4) and low transmission loss (0.8dB/km), OAM states are propagated over 13.4km in an air core fiber using a recirculating fiber loop. We observe that intermodal crosstalk decreases rapidly with increasing effective index separation, Δneff, and an order of magnitude lower crosstalk may be achieved just by doubling Δneff. We find that, in agreement with coupled power theory, our fiber has mode coupling properties analogous to elliptical core PM fibers, which yield ~10 × or more lower crosstalk than for conventional LP fiber mode orders with the same Δneff. This confirms that, for OAM modes, birefringent perturbations rather than shape perturbations matter most. In the process of performing the loop experiment, we demonstrate that OAM states in these fibers can be preserved with low loss (≤ 0.2dB) and low crosstalk (-15dB) while splicing distinct segments of the air-core fiber. For well-designed fibers, we demonstrate that OAM modes can travel distances relevant for large-scale data centers.

Highly sensitive and reconfigurable fiber optic current sensor by optical recirculating in a fiber loop.

An advanced fiber optic current sensor (FOCS) is proposed based on recirculating fiber loop architecture for significantly enhancing the current sensitivity. The recirculating loop is constructed by a 2X2 optical switch and the standard single mode fiber (SSMF) is used as the sensing head. The proposed FOCS is coupler-free with low insertion loss which results in a significantly improved current sensitivity. We experimentally obtained a sensitivity of 11.5 degrees/A for 1-Km SSMF FOCS and a sensitivity of 21.2 degrees/A for 500-m SSMF FOCS, both of which have been enhanced by more than ten times. The flexible switch control of recirculating can support the FOCS to work for different current scenarios with the same system and thus reconfigurable operation of the FOCS has been achieved. The significantly enhanced high sensitivity with reconfigurable operation capability makes the proposed FOCS a promising method for practical applications.

Spectrally Efficient WDM Nyquist Pulse-Shaped 16-QAM Subcarrier Modulation Transmission With Direct Detection

The ability to transmit signals with high information spectral density (ISD) using low-complexity and cost-effective transceivers is essential for short- and medium-haul optical communication systems. Consequently, spectrally efficient direct detection transceiver-based solutions are attractive for such applications. In this paper, we experimentally demonstrate the wavelength-division multiplexed (WDM) transmission of 7×12 GHz-spaced dispersion pre-compensated Nyquist pulse-shaped 16-QAM subcarrier modulated channels operating at a net bit rate of 24 Gb/s per channel, and achieving a net optical ISD of 2.0 b/s/Hz. The direct detection receiver used in our experiment consisted of a single-ended photodiode and a single analog-to-digital converter. The carrier-to-signal power ratio at different values of optical signal-to-noise ratio was optimized to maximize the receiver sensitivity performance. The transmission experiments were carried out using a recirculating fiber loop with uncompensated standard single-mode fiber and EDFA-only amplification. The maximum achieved transmission distances for single channel and WDM signals were 727 and 323 km below the bit-error ratio of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. To the best of our knowledge, this is the highest achieved ISD for WDM transmission in direct detection links over such distances.

A Regenerative Variable Optical Buffer for NRZ and RZ Packets

We demonstrate a regenerative variable optical buffer suitable for both non-return-to-zero and return-to-zero intensity-modulated packets. The buffer is based on a re-circulating fiber loop that exploits a regenerative amplification stage based on cross-gain-compression in a semiconductor optical amplifier. This buffer does not suffer from any transient effect at the packets' edges, and due to its working principle, can be used for packets of different length. We report experimental results showing the buffering operation of 1-s long 10-Gbps packets up to 100 circulations (making possible a maximum buffering time of 130 s) with a bit error rate showing 0.5 dB power penalty every 10 rounds. The case of the buffering of noisy packets is also investigated. We find that because of the regenerative feature of the amplification stage, packets with different input noise levels show a similar quality at the output. In particular, signal regeneration is possible up to certain buffering times and depends on the input noise values.

Regenerative Optical Buffer Based on SOA-Amplified Recirculating Loop

We introduce a novel scheme for all-optical buffering based on a recirculating fiber loop with regenerative amplification by cross-gain compression in a semiconductor optical amplifier. Fifty circulations of 10-Gb/s packets with 1.2-dB power penalty every 10 rounds are reported obtaining more than 150-μs buffering time. A large resilience of the scheme to noisy signals is also demonstrated.

Assessing the Noise Statistics in Common Optical Transmission Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We experimentally investigate the probability density function (pdf) of sampled bits in common intensity-modulated direct-detection systems in linear and nonlinear regime, both single-channel and wavelength-division-multiplexed. To this aim, we use a recirculating fiber loop and selectively obtain the sample pdf for the various bits in a given sequence, which allows separating the intersymbol interference (ISI) effects. Our measurements confirm a recent theoretical simplified model, suggesting that, under the above conditions, a generalized analytic form is a good approximation of the true pdf, provided that a pattern-dependent effective noise density and noiseless signal value are assumed: all the pdfs estimated from the experimental data are indeed well fitted by this analytic form. Moreover, when nonlinear effects become dominating, the analytic form is still fitting properly the data histograms, showing, however, that the effective noise density increases. In the above regime, we clearly observe that ISI also affects the increase of the effective noise density. </para>

Optical networks research at UCL

New dynamic optical switching technologies and adaptive transmission techniques will allow increased capacity and flexibility in future core and metro networks. The Optical Networks Group at UCL has carried out research focused on these areas, in collaboration with industry and other university research groups, for over a decade. In this paper, the technical challenges to be overcome in implementing dynamic, high-capacity networks are discussed, and recent results of the group's work are presented. New network architectures based on wavelength-routing and optical burst switching have been proposed and analysed, and the experimental demonstration of burst switching using fast wavelength-tunable lasers is described. High-capacity dense wavelength division multiplexed transmission systems, using advanced regeneration and equalisation techniques, have been studied, both theoretically, and experimentally using a dynamically reconfigurable recirculating fibre loop. Research results on advanced optical signal formats, all-optical 3R regeneration at 40 Gbit/s channel rates, adaptive analogue and digital electronic processing techniques and optical finite impulse response equalisation are presented. Finally, the development of optical performance monitoring technology, a key component of future all-optical networks, is described. The paper concludes with a prediction of future trends in optical communications research.

Add/drop applications in fiber ring networks based on a reconfigurable optical add/drop multiplexer in a re-circulating loop

A periodic add/drop system in a fiber ring network was investigated using a reconfigurable optical add/drop multiplexer (ROADM) in a re-circulating fiber loop. After seven cascaded add/drop nodes at every 150km along the transmission, at bit error ratio (BER) equals to 10−9 and data rate of 10Gbps, we observed a 2.5dB power penalty for the passing through channels with 1050km transmission distance, and 0.3dB sensitivity penalty variation for the periodic add/drop channels at every 150km, respectively.

Static and synchronized switching noise management of replicated optical pulse trains

Demonstration is provided for the generation of high quality replicated optical pulse trains, from a single pulse. This uses the storage capability of an electro-optically switched recirculating fibre loop. For better understanding, the first step involves the development of general purpose derivations related to the evolution of the noise with near-unity loop gains. This is a basic tool to evaluate the propagation of the noise and the temporal evolution of the signal to noise ratio during the replication process. In a second step, the comparison of theoretical and experimental results will be shown to evidence good agreement, up to more than 1000 replica. Finally, experiments are performed on synchronised gain-switching inside the loop to help optimise the dynamic range of replication at the output of the system.

Bit error rate analysis of an all-optical buffer employed in a soliton based optical time division multiplexed transmission system

In this paper, we compare the theoretical and simulated bit error rate (BER) performance of an all-optical active recirculating fibre loop buffer (RFLB) employed at the transmitter node of an optical time division multiplexed based (OTDM) system, for several buffer storage times (delays). To the best of our knowledge, we believe that this is the first time a theoretical model detailing BER performance based on the delay storage time has been presented in the literature. Moreover, the theoretical model is extended to forecast the power penalties incurred when the delay approaches the millisecond time scale. Thereafter, simulation results are used to compare the power penalty when the RFLB is employed at a receiver node within an optical network scenario. In this work, soliton profiled OTDM packets transmitted at a maximum aggregate data rate of 20 Gbit/s are used as test signals. The buffering architecture consists of a dispersion-shifted fibre (DSF) loop and a switching mechanism. The analysis presented here is intended to promote the understanding of the performance hindering mechanisms within a RFLB architecture and may prove invaluable to future optical buffer designers.

Evolution of localization in frequency for modulated light pulses in a recirculating fiber loop.

We present an experimental demonstration of the evolution of localization in frequency of light pulses that are repeatedly kicked by phase modulation and then propagated along equally spaced lengths of fiber with weak dispersion. The experiment was performed with a long fiber recirculating loop that allows us to follow the pulse's spectral changes after each cycle.