Long-haul Optical Links Research Articles

Progress of Transmission Technologies for UDTV

This paper describes the recent progress of research and development on transmission technologies for Super Hi-Vision (SHV), which is the form of ultrahigh-definition television (UHDTV) with the highest resolution class. The paper covers the transmission systems for SHV program production and satellite broadcasting. For short-haul transmission, an optical digital interface for video equipment was developed. It wavelength-multiplexes eight 10-Gb/s optical signals. It was experimentally confirmed that the transmission distance was at least 2 km, and that a series connection of the interfaces did not severely deteriorate jitter performance. We are also developing a 120-GHz-band wireless contribution short-haul link consisting of three transmitter/receiver pairs. All three pairs transmit and receive a 10.3-Gb/s signal at the same frequency. Experiments showed that transmission with little crosstalk is possible over a distance of 1.25 km when the receiver spacing is 8 m. A long-haul optical contribution link that we developed can transmit a 24-Gb/s SHV signal over a distance of 260 km by using optical amplifiers and no electronic repeaters. An IP network would be a good choice for long-distance links such as an international live contribution link. Several live IP transmissions of compressed video were conducted. Satellite relay transmission and broadcasting technology is also being developed. In particular, we have developed a QPSK/8PSK wide-band modem that uses LDPC and BCH codes. The modem could transmit 251-Mb/s (QPSK) and 677-Mb/s (8PSK) signals and was used in a live SHV satellite-transmission experiment.

Optimized transmitter module for NRZ-duobinary in long-haul optical transmission link

In this work we present a comparative analysis of three different NRZ-duobinary transmitter modules made of either push–pull configuration based on dual arm MZIM or delay-and-add circuit based single arm MZIM or a duobinary filter followed by single arm MZIM in an ASE-noise-limited 10Gbps optical link. The performance of all three transmitter modules has been optimized for modulator chirp and extinction ratio. These optimized transmitter modules have been further investigated to observe which transmitter-generated pulse offers comparatively better tolerance to impairments induced by group velocity dispersion (GVD) and self phase modulation (SPM). Simulative analysis reports that delay-and-add circuit based single arm MZIM transmitter module outperforms the other two transmitter modules and hence can be treated as the optimized transmitter module for NRZ-duobinary in long-haul optical transmission link.

Broadband Dispersion-Induced Power Fading Compensation in Long-Haul Analog Optical Link Based on 2-Ch Phase Modulator

A broadband dispersion compensation scheme based on a 2-Ch phase modulator (2-Ch PM) is proposed and experimentally demonstrated in the long-haul analog optical link. Benefiting from the complementary power fading function for the phase and intensity modulated link, which are simultaneously obtained by using the 2-Ch PM, the broadband and long-reach dispersion compensation is implemented. The experimental results show that the dispersion-induced power fading is greatly mitigated for the radio frequency (RF) signal with a bandwidth up to 18 GHz over 24.45-km single-mode fiber (SMF) transmission. Meanwhile, the system spurious-free dynamic range (SFDR) is improved with a maximum value of 24 at the frequency of 17.5 GHz; the vector signal transmission performance has also been improved.

Faster-than-Nyquist and beyond: how to improve spectral efficiency by accepting interference

We investigate the application of time and frequency packing techniques, an extension of the classical faster-than-Nyquist signaling, to long-haul optical links. These techniques provide a significant spectral efficiency increase and represent a viable alternative to overcome the theoretical and technological issues related to the use of high-order modulation formats. Adopting these techniques, we successfully demonstrate through simulations the transmission of 1 Tbps over 200 GHz bandwidth in a realistic (nonlinear) long-haul optical link.

Optical Comb Sources for High Dynamic-Range Single-Span Long-Haul Analog Optical Links

We present a new technique for achieving high spurious-free dynamic range in long-haul analog optical links. Our technique utilizes an optical comb as the optical carrier in an externally intensity-modulated direct-detection link architecture. By distributing the optical carrier power over the comb (in contrast to concentrating it in a single continuous-wave laser, as in a conventional link), this technique circumvents the optical power limitations imposed by stimulated Brillouin scattering allowing a substantial increase in optical launch power. Increased launch power translates directly to decreased optical amplification requirements at the link terminus and results in a significantly lower noise floor than may be achieved in a conventional link architecture. To our knowledge, the dynamic range of SFDR=105.5 dB (1-Hz bandwidth, L=50 km link) is the highest reported to date for a single-span long-haul analog optical link.

OFDM versus Single-Carrier Transmission for 100 Gbps Optical Communication

We analyze the orthogonal frequency division multiplexing (OFDM) technique in long-haul next generation optical communication links and compare it with the well-established single-carrier (SC) data transmission using high-level modulation formats and coherent detection. The analysis of the two alternative solutions is carried out in the 100 Gbps scenario, which is commonly considered to be the next upgrade of existing optical links, with special emphasis on quaternary phase-shift keying (QPSK) modulations. The comparison between OFDM and SC takes into account the main linear and nonlinear impairments of the optical channel, e.g., group velocity dispersion (GVD), polarization mode dispersion (PMD), self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), as well as the phase noise due to transmit and receive lasers, their relative frequency offset, other synchronization aspects, the overall complexity, the power and spectral efficiency, and the technological constraints.

Nonlinear signal–noise interactions in dispersion-managed links with various modulation formats

Purpose of this paper is to highlight the principles of the nonlinear signal–noise interaction (NSNI) in dispersion-managed long-haul optical links and provide a quantitative understanding of the system parameters for which NSNI sets the nonlinear performance of the most popular intensity and phase modulation formats, namely on–off keying, differential binary and quadrature phase-shift keying and coherent quadrature phase-shift keying.

Full-duplex ROF transport systems based on a broadband ASE light source and nonlinear distortion suppression scheme

A full-duplex radio-over-fiber (ROF) transport system employing a broadband amplified spontaneous emission (ASE) light source and nonlinear distortion suppression scheme is proposed and demonstrated. A 10 GHz/70 Mbps data rate (compatible WiMAX format) signal is externally modulated and transmitted over a 120 km single-mode fiber (SMF) transmission both for down/up-link. Good bit error rate (BER) performance and a clear eye diagram were achieved in our proposed systems. Such a proposed full-duplex ROF transport system is suitable for long-haul microwave optical links.

TO EMPLOY SOA-BASED OPTICAL SSB MODULATION TECHNIQUE IN FULL-DUPLEX ROF TRANSPORT SYSTEMS

A full-duplex radio-over-fiber (ROF) transport system employing semiconductor optical amplifier (SOA)-based optical single sideband (SSB) modulation technique is proposed and demonstrated. For our proposed approach, it is relatively simple to implement as it requires only one SOA to generate optical SSB signal. Over an80 km single-mode fiber (SMF) transmission, low bit error rate (BER), clear eye diagram, and low third-order intermodulation distortion to carrier ratio (IMD3/C) were achieved. Our proposed full-duplex ROF transport systems are suitable for the long-haul microwave optical links. Corresponding author: H.-H. Lu (hhlu@ntut.edu.tw).

Characterisation of a parallel optical all pass filter for chromatic dispersion equalisation in 10 Gb/s system

The chromatic dispersion (CD) that occurs in single mode fibres (SMFs) is an important issue that needs to be addressed in long-haul optical communication links. The effect of CD is pulse spreading which in turn leads to inter-symbol interference, thus resulting in the deterioration of the bit error rate (BER) performance of the system. An alternative CD compensation technique that utilises a parallel optical all pass filter (p-OAPF) is presented, where the p-OAPF design is based on the inverse phase response of the SMF. The p-OAPF is based on a class of all-pass filter. Simulation results of the proposed technique show an increase in the repeater-less length of a point-to-point optical communication system by up to three and four times using a non-return-to-zero data format with a rectangular and Gaussian pulse shape, respectively, at error-free condition (BER < 10 -9 ). The results also show that the p-OAPF is robust in performing dispersion equalisation at a wide range of SMF lengths to attain error-free communication.

Experimental Demonstrations of Electronic Dispersion Compensation for Long-Haul Transmission Using Direct-Detection Optical OFDM

We present experimental demonstrations using direct-detection and optical-orthogonal frequency division multiplexing (DD-OOFDM) for the compensation of chromatic dispersion in long-haul optical fiber links. Three transmitter designs of varying electrical and optical complexity are used for optical single sideband (OSSB) transmission and the theory behind each design is discussed. The data rates achieved for the three systems are 10, 12, and 20 Gbit/s for fiber distances between 320 and 400 km. A discussion of system overheads is provided together with simulations of the required optical signal-to-noise ratio (OSNR).

A Full-Duplex Radio-on-Photonic Crystal Fiber Transport System

A full-duplex radio-on-fiber transport system based on wavelength-division-multiplexing and optical add-drop multiplexing techniques, as well as proper length of photonic crystal fiber (PCF) is proposed and demonstrated. Over a combination of single-mode fiber and PCF transmission, low third-order intermodulation-distortion-to-carrier ratio (IMD3/C) and bit-error-rate values were obtained in our proposed systems. Such a proposed full-duplex radio-on-PCF system is suitable for the long-haul microwave optical links

A Fully Integrated 10-Gb/s Receiver With Adaptive Optical Dispersion Equalizer in 0.13-$\mu{\hbox {m}}$ CMOS

A 10 Gb/s receiver, containing an adaptive equalizer, a clock and data recovery, and a de-multiplexer, is implemented in 0.13-mum CMOS. The chip is intended for long-haul optical fiber links where chromatic and polarization mode dispersions are reach-limiting factors. The equalization is performed by a continuous time filter and a two-tap decision feedback equalizer while automatic threshold and phase adjustments are embedded in the CDR. Use of an analog equalizer with digital adaptation garners total power dissipation of 950 mW. Error-free operation over 200 km of single mode fiber is demonstrated. With 140 km of single mode fiber, optical signal to noise ratio penalty is only 2dB. Differential group delay of 100 ps can also be tolerated

Fiber nonlinearity pre- and post-compensation for long-haul optical links using OFDM

The nonlinear power limit of optical links using optical Orthogonal Frequency Division Multiplexing (OFDM) for dispersion compensation can be significantly improved using an optimum combination of nonlinearity precompensation and postcompensation. The compensation is implemented at the transmitter and at the receiver as computationally-efficient power-dependent phase shifts with a single tuning parameter. The system is robust against the exact details of the fiber plant's dispersion and power levels. Using an optimum combination of pre and post compensation allows a 2-dB increase in launch power for 2000-km standard single-mode fiber (S-SMF) systems and 5-dB when 6 ps/nm/km fibers are used. Using pre or post compensation alone approximately halves these values.

Power-weighted dispersion distribution function for characterizing nonlinear properties of long-haul optical transmission links

Based on the first-order perturbation theory, we introduce the power-weighted dispersion distribution (PWDD) function to characterize nonlinear properties of fiber-optic transmission links. This technique offers a new perspective on dispersion management for pseudolinear transmission. A rectangular PWDD model is used to describe the formation of ghost pulses analytically in the time domain without relying on the assumption that the pulse shape is Gaussian.

Improving full-duplex radio-on-fiber transport system performance by employing an optical single sideband/vestigial sideband filter

A full-duplex radio-on-fiber (ROF) transport system based on wavelength division multiplexing (WDM) and optical add-drop multiplexing techniques, as well as employing a simple fiber Bragg grating (FBG) filter at the receiving site, is proposed and demonstrated. Over a long-haul fiber link, good transmission performance is obtained in our proposed systems. Since our proposed system does not use a sophisticated optical single sideband (SSB) transmitter, it reveals a prominent alternative with advantages in simplicity and cost. Such a proposed full-duplex ROF transport system is suitable for the long-haul microwave optical link.

Employing double external light injection techniques to improve radio-on-fiber systems performance

A radio-on-fiber (ROF) transport system employing double external light injection techniques to improve systems’ performance is proposed and demonstrated for the first time. Simultaneously using external light injection techniques to increase the frequency response of the laser diode and the saturation output power of the semiconductor optical amplifier (SOA), leading to decrease third order intermodulation distortion (IMD 3) and SOA gain-saturation-induced distortion. Such a proposed ROF transport system is suitable for the long-haul microwave optical links.

Study of long-haul optical links using 1.55μm direct modulation for return path in CATV networks

Study of long-haul optical links using 1.55μm direct modulation for return path in CATV networks

Enhancement of amplifier spacing in long-haul optical links through the use of large-effective-area transmission fiber

The use of large effective area (LEA) dispersion-shifted fibers is investigated for enhancing the amplifier spacing in long-haul optical links. Our analysis leads to the definition of a novel figure of merit for amplified links, which is valid for both soliton and NRZ systems. It is shown that exploiting fibers with effective areas between 70 and 90 /spl mu/m/sup 2/ enables a dramatic increase of the amplifier spacing (i.e., in excess of 100%) by comparison to systems using classical 50-/spl mu/m/sup 2/ fibers, The maximum attenuation that can be tolerated for LEA fibers is also determined as a function of the effective area. >