Dispersion-compensated Transmission Research Articles

Ultrahigh-speed Nyquist pulse transmission beyond 10 Tbit/s

This paper presents recent progress toward the realization of ultrahigh-speed optical time-division multiplexed transmission with a single-channel bit rate of more than 10 Tbit/s using a Nyquist pulse. We first describe non-coherent Nyquist pulse transmission with differential quadrature phase-shift keying (DQPSK). We present a single-channel 10.2 Tbit/s, DQPSK-300 km transmission, where the highest symbol rate of 2.56 Tbaud was demonstrated with an ultrashort Nyquist pulse train and an ultrafast demultiplexer using a nonlinear optical loop mirror. We then present a recent demonstration of a 1.28 Tbit/s/ch × 10 channel (total 12.8 Tbit/s) DQPSK transmission, where the transmission distance was successfully extended to 1500 km by employing a wavelength division multiplexing scheme and reducing the symbol rate to 320 Gbaud while retaining a total transmission capacity of more than 10 Tbit/s. Next, we describe coherent Nyquist pulse transmission with quadrature amplitude modulation (QAM). We present a single-channel polarization-multiplexed 1.28 Tbaud, 64 QAM-150 km transmission, where the highest transmission speed of 15.3 Tbit/s was achieved with a homodyne detection scheme using an optically phase-locked LO Nyquist pulse. Furthermore, we present recent results related to BER performance improvement in a 15.3 Tbit/s coherent Nyquist pulse transmission by newly constructing a low-nonlinearity, dispersion-compensated transmission line using a chirped fiber Bragg grating and a liquid crystal on silicon device. As a result, the required forward-error correction overhead was successfully reduced from 25.5% to 20%, and the spectral efficiency reached as high as 8.7 bit/s/Hz.

Optical Phase Conjugation in a Silicon Waveguide With Lateral p-i-n Diode for Nonlinearity Compensation

In-line optical phase conjugation is a well-known technique to enhance the received signal quality through nonlinearity compensation. Being able to implement the conjugation in cm-scale highly nonlinear devices, which can be integrated on a silicon chip, could potentially lead to several benefits in terms of small footprint and cointegration with linear signal processing functionalities, as well as lower power consumption. Here, we focus on silicon waveguides to implement the optical phase conjugation through four-wave mixing. The challenges in terms of conversion efficiency imposed by the presence of nonlinear loss are tackled by using a lateral p-i-n diode along the waveguide. When the diode is reverse biased, the conversion efficiency can be effectively enhanced by the decrease in free-carrier absorption. Low-penalty conversion can therefore be achieved for wavelength-division multiplexing (WDM) signals and the high quality of the generated idlers is critical in demonstrating a 1-dB Q-factor improvement through optical phase conjugation in a 5-WDM channel 16-QAM transmission system after 644 km of dispersion-compensated transmission. The performance improvement enables a performance better than the hard-decision forward error correction threshold for all the transmitted channels.

Ultra-High-Capacity Optical Packet Switching Networks with Coherent Polarization Division Multiplexing QPSK/16QAM Modulation Formats

Optical packet switching (OPS) networks and its subsystems, like the burst-mode receiver, are an essential technology currently used in passive optical networks (PONs). Moreover, OPS may play a fundamental role on future hybrid optical circuit switching (OCS)/OPS networks and datacenter networks. This paper focuses on two fundamental subsystems of packetized optical networks: the digital coherent burst-mode receiver and the electro-optical switch. We describe and experimentally characterize a novel digital coherent burst-mode receiver that makes uses of the Stokes parametrization to rapidly estimate the state of polarization (SOP) and optimize the equalizer convergence time. This burst-mode receiver is suitable for optical packetized networks that make use of advanced modulation formats such as quadrature amplitude modulation (QAM). We study the suitability of (Pb,La)(Zr,La)O3 (PLZT) optical switches for amplitude-variable coherent polarization division multiplexing (PDM) 16QAM modulation format and demonstrate a switching capacity of 10.24 Tb/s/port. We demonstrate a full 2 × 2 OPS node with a control plane capable of solving packet contention by means of packet dropping or buffering with a switching capacity of 10.24 Tb/s/port. Finally, we demonstrate the operation of the 2 × 2 OPS node with a record capacity of 12.8 Tb/s/port plus 100 km of dispersion-compensated fiber transmission.

Influence of self-phase modulation effect on waveform degradation and spectral broadening in optical BPSK-SSB fiber transmission

Self-phase modulation (SPM) effect is analyzed in a dispersion-compensated transmission using optical BPSK single sideband (SSB) modulation. The effect was evaluated numerically using both waveform degradation and spectral degradation, clarifying that waveform degradation is induced dominantly by peak power of the quadrature component of a Hilbert-transformed signal. Eye-opening degradation of BPSK-SSB is induced by lower fiber input power than the conventional double sideband (DSB) case because the SSB-homodyne system is sensitive to phase error resulting from SPM. Spectral degradation from SPM has two phases with increasing fiber input power. In the first phase, the sideband in the suppressive frequency region expands with increasing optical power. In the second phase, the spectral envelope in the non-suppressive frequency region becomes broad, and its shape is somewhat varied.

Dispersion compensation with photonic crystal line-defect waveguides

A concept for dispersion compensation in transmission is proposed, based on modes anti-crossing in photonic crystal (PC) line-defect waveguides. Quasi-constant positive and negative dispersion is possible in order of 100 ps/nm/mm on the bandwidth of 100 GHz. An adiabatic taper is proposed for efficient coupling into PC structure.

Performance of Dispersion-Compensated Optical Transmission Systems Based on Different Electrical Drivers

In this article, the simulation results have been presented for dispersive optical communication with different electrical drivers. The length of single-mode fiber has been put in a run that is compensated by dispersion-compensated fiber. The results have been presented and it is observed that of all the electrical drivers, the NRZ rectangular is best suited for dispersive optical communication systems. For this electrical driver, the parameters like BER remain constant and low. For other drivers, the BER decreases more for RZ super-Gaussian. Similarly, the Q factor increases with runs for all types of drivers except for NRZ rectangular, for which it decreases. For all the drivers, it is clear that as the runs are varied, the eye opening penalty increases, that is, there is less eye opening as the length of single-mode fiber is increased. It is also seen that the dispersion-compensated fiber compensates for the dispersion very effectively irrespective of the type of driver. Of the two formats, return to zero (RZ) and non-return to zero (NRZ), the earlier has more eye-opening penalty as the distortion is more and is clearly visible in all the rectangular, super-Gaussian, and raised cosine RZ formats. Of the three RZ formats, it is observed that RZ rectangular has the least eye-opening penalty. The non-return to zero format has less distortion and, of the two rectangular and raised cosine, the NRZ rectangular has less eye-opening penalty.

SPM effect on carrier-suppressed optical SSB transmission with NRZ and RZ formats

Tolerance to self-phase modulation (SPM) is evaluated in a dispersion-compensated transmission system using carrier-suppressed optical single side-band (SSB) modulation, which employed a four-branched LiNbO3 modulator and an ideal Hilbert transformer. Waveform degradation is dominated by the peak power of a Hilbert-transformed signal. RZ format is preferred to NRZ for optical SSB transmission.

Dispersion compensation in transmission using uniform long period fiber gratings

It is proposed that the high dispersion at the transmission band edges of uniform long period gratings (LPG) fabricated on relatively high Δ fibers can be used for efficient dispersion compensation. Since the transmission of LPG varies with length of the grating or refractive index modulation, we show that it is possible to tailor the transmission spectrum to obtain high transmission with constant dispersion and negligible delay ripple over a reasonable bandwidth. Since the proposed structure works in transmission it should be suitable for fiber optic communication links.

Performance evaluation of prechirped RZ and CS-RZ formats in high-speed transmission systems with dispersion management

This paper describes the transmission performance of prechirped return-to-zero (RZ) and prechirped carrier-suppressed return-to-zero (CS-RZ) signals over a periodically dispersion-compensated transmission line. We analyze the transmission characteristics of both formats, taking account the transmitter configuration expected, in which pulse chirping is generated by using both a phase modulator and a linear dispersion compensating device. We also discuss the dependence of the transmission characteristics on phase modulation, pre- and postcompensating dispersion, and receiver optical and electrical filter widths. We show that, in single-channel transmission, phase modulation effectively reduces the intrachannel nonlinear interaction and improves the transmission performance. Next, we discuss the transmission characteristics of chirped RZ and chirped CS-RZ signals in dense wavelength division multiplexed (DWDM) signal transmission. In 100-GHz spaced 40-Gb/s-per-channel systems, it is shown that the phase modulation must be carefully optimized in order to minimize the linear crosstalk and waveform distortion induced by the intra- and interchannel nonlinear interaction in the transmission fiber.

Long-range soliton interactions in dispersion-managed links

A numerical analysis of soliton propagation in a dispersion-compensated transmission system reveals the presence of a small dispersive-wave contribution to the propagating soliton that increases dramatically whenever the input soliton energy is detuned from its optimal value. A straightforward consequence of the dispersive-wave emission is an unexpectedly strong contribution to the long-range soliton-soliton interaction. We show that the combination of long- and short-range interactions induces soliton timing jitter that grows larger than that which arises from the Gordon-Haus effect.

Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5-km fiber link

We report for the first time proof-of-concept transmission experiments for femtosecond pulse code division multiple access (CDMA) operating over kilometer lengths of optical fiber in the 1.5-μm communication band. Our CDMA link consists of a femtosecond mode-locked Er-fiber laser and two chirped pulse fiber amplifiers, a pair of low-loss fiber-pigtailed pulse shapers for encoding and decoding, a 2.5-km dispersion-compensated transmission fiber, and an ultrafast nonlinear optical receiver. Our results demonstrate high fidelity transmission of spectrally encoded femtosecond pulses over 2.5 km of fiber, followed by the use of the nonlinear CDMA receiver to discriminate between correctly and incorrectly decoded signals with 20-dB contrast.

Fiber Bragg gratings for dispersion compensation in transmission: theoretical model and design criteria for nearly ideal pulse recompression

We propose a transmission-based dispersion compensator employing an apodized, unchirped fiber Bragg grating (FBG). A theoretical model for dispersion compensation in transmission based on the dispersive properties of the periodic structure is developed. A figure of merit is defined for optimization of the grating parameters for maximum recompression of dispersion-broadened optical pulses in long-haul communication systems. Numerical examples confirm that nearly perfect compensation with very low insertion losses can be achieved for many practical cases of interest.

10 Gbit/s dispersion-compensated transmission over 2245 km conventional fibres in a recirculating loop

The authors demonstrate 10 Gbit/s dispersion-compensated long distance transmission over conventional (non-dispersion-shifted) fibres in a recirculating loop incorporating dispersion compensation fibres (DCF). We obtain a bit error rate below 10/sup -9/ at 2245 km and show the feasibility of conventional fibre transmission over more than 2000 km long by using DCF with practical optical amplifier spacings of more than 80 km.