Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

Advanced optical modulation formats have become a key ingredient to the design of modern wavelength-division-multiplexed (WDM) optically routed networks. In this paper, we review the generation and detection of multigigabit/second intensity- and phase-modulated formats and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Modulation Formats for High-Capacity Optical Transport Networks

Polybinary, optical amplitude modulated phase shift keying (AM-PSK) polybinary, M-ary amplitude shift keying (ASK), and polyquaternary signaling schemes operating at 10 Gb/s are investigated in 1550-nm lightwave systems operating over standard, single-mode fiber. The premise for exploring these signal types is that they concentrate power at frequencies closer to the optical carrier where phase distortion of the optical field from chromatic dispersion is less severe. Issues such as modulator chirp, optimal level spacing in a 4-ary ASK signal, and phase modulated to amplitude modulated (PM-AM) noise conversion from a nonzero laser linewidth are studied. It is found that higher order polybinary signals do not offer an improvement in dispersion tolerance over a duobinary signal. 4-ary ASK is demonstrated to increase the dispersion-limited distance to 225 km experimentally and 350 km through simulation, but at the expense of a /spl sim/8 dB degradation in receiver sensitivity due to the increased number of levels and the signal dependence of signal-spontaneous beat noise. Furthermore, the linewidth requirement for a 4-ary ASK signal is less than 1 MHz in order to minimize the impact of PM-AM relative intensity noise (RIN) when transmitting over 200-300 km.

Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems

Aircraft operators are faced with increasing requirements to extend the service life of air platforms beyond their designed life cycles, resulting in heavy maintenance and inspection burdens as well as economic pressure. Structural health monitoring (SHM) based on advanced sensor technology is potentially a cost-effective approach to meet operational requirements, and to reduce maintenance costs. Fiber optic sensor technology is being developed to provide existing and future aircrafts with SHM capability due to its unique superior characteristics. This review paper covers the aerospace SHM requirements and an overview of the fiber optic sensor technologies. In particular, fiber Bragg grating (FBG) sensor technology is evaluated as the most promising tool for load monitoring and damage detection, the two critical SHM aspects of air platforms. At last, recommendations on the implementation and integration of FBG sensors into an SHM system are provided.

Fiber optic sensors for structural health monitoring of air platforms.

We propose a dispersion-tolerant optical duobinary transmission system which uses a binary intensity modulation direct detection (IM-DD) receiver. The proposed system also relaxes the fiber input power limitation due to stimulated Brillouin scattering (SBS). Tolerance to chromatic dispersion in the proposed system is estimated by a simulation. A 10 Gb/s optical duobinary signal can be transmitted over 200 km of standard single-mode fiber (SMF) with 1 dB dispersion penalty. The advantages of the proposed system are confirmed experimentally. The optical duobinary signal was successfully received by the binary IM-DD receiver at the same sensitivity as a binary IM signal. Power penalty due to chromatic dispersion over a 164-km standard SMF was about 1.8 dB at 10 Gb/s. The SBS threshold of the 10 Gb/s optical duobinary modulated light was more than +20 dBm.

Dispersion-tolerant optical transmission system using duobinary transmitter and binary receiver

The use of duobinary optic signalling is proposed for extending the dispersion-limited propagation distances at 10 Gbit/s without the use of finely tuned dispersion compensation techniques. Experimental transmission system performance evaluation over 100 km of standard step-index fibre at l.55 μm has been achieved with a penalty of only 0.5 dB.

10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fibre

The use of optical time-domain reflectometry (OTDR) on long-span fiber transmission systems containing in-line optical amplifiers is discussed. Having identified the specific requirements for OTDR equipment, measurements were carried out on systems of up to 300 km in length, containing three semiconductor laser amplifiers. The results demonstrate that OTDR can be used not only for fault location on fiber links several hundred kilometers in length but also as an alternative to standard system supervisory techniques, thus providing the potential for minimizing the hardware in the optical amplifier stages of future long-span fiber optic transmission systems. >

Optical time domain reflectometry on optical amplifier systems

A 20 Gbit/s optical time division multiplexed transmission system based on components and subsystems operating at not more than 5 GHz repetition rate is reported. Full BER measurements indicate a penalty of only 0.8 dB after transmission through 205 km of dispersion-shifted fibre. The penalty was mostly attributed to the accumulation of noise from erbium-doped fibre repeater amplifiers in the system. >

20 Gbit/s, 205 km optical time division multiplexed transmission system

By employing erbium fibre amplification of both the outgoing transmitter pulses and the returning backscatter signals of an optical time domain reflectometer (OTDR), a substantial increase is achieved in the one-way dynamic measurement range. First experimental results are presented showing almost 10dB net improvement in OTDR range, with the potential established for performance increases of up to 20dB one-way. The prospects are discussed for such performance improvements both in high-resolution OTDR applications and for long-distance fault location.

OTDR performance enhancement through erbium fibre amplification

The measurement range of long-haul optical time domain reflectometry (OTDR) may be extended substantially by introducing distributed Raman gain in the fibre under test. With semiconductor pump lasers an increase in the fault location range of at least 10dB one-way is achievable, without requiring the OTDR to have an increased processing dynamic range.

L.C. Blank

Papers

10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fibre

Optical time domain reflectometry on optical amplifier systems

20 Gbit/s, 205 km optical time division multiplexed transmission system

OTDR performance enhancement through erbium fibre amplification

Raman-assisted long-distance optical time domain reflectometry