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Optoelectronics & Communications

Gain, bandwidth improve for fiber-optic parametric amplifiers

Optical parametric amplifiers (OPAs) have demonstrated impressive performance but must overcome several challenges before practical application is possible.
26 November 2007, SPIE Newsroom. DOI: 10.1117/2.1200711.0886

Optical fiber communication has enjoyed tremendous growth since the 1990s because of the ever-increasing demand for transmission capacity, which has been driven by the widespread use of the Internet and the progress of multimedia technologies. Optical fibers feature low loss (< 0.25dB/km) over a very wide frequency range, ∼1460-1660nm in a standard single-mode fiber (SMF) and 300nm in a recently developed type of fiber. This low loss window can support high-speed data transmission over hundreds of kilometers without regeneration. Therefore, fiber optics appear to be the most promising solution both for high-capacity, long-haul systems and for local and metropolitan area networks.

To fully utilize the available bandwidth of optical fiber, one of the most promising techniques is to multiplex many wavelengths and transmit them in a single fiber, a process called wavelength division multiplexing (WDM). WDM would be impossible to deploy without the the erbium-doped fiber amplifier (EDFA) developed in the early 1990s, which amplifies signals satisfactorily for WDM systems. However, the conventional EDFA (C-band) has a limited amplification window (1530-1562nm) and bandwidth (32 nm). Therefore, researchers have explored alternative amplification windows outside the C-band EDFA, including L-band EDFA (1570-1605nm), S-band (1450-1520nm), Raman amplifiers, and semiconductor optical amplifiers (SOA). However, each of these has its own limitations and constraints. Thus, we have investigated alternative types of amplifiers, and the optical parametric amplifier (OPA) is one of the most promising technologies.

A fiber OPA relies on the third-order nonlinear susceptibility χ(3) of glass, where a signal frequency at ωs is amplified by a strong co-propagating pump at ωp in a fiber. Therefore, OPAs may find applications as optical amplifiers in WDM transmission. Another frequency, called idler, is also generated at ωi = 2ωp − ωs. Idler contains essentially the same modulation information as the input signal, but with an inverted spectrum. Signal and idler can grow together if pump power is high enough and phase matching occurs. Therefore, the pump is usually operated slightly above the zero-dispersion wavelength (λ0) of the fiber.

In recent years, impressive performance has been demonstrated in several areas. For instance, fiber OPAs can exhibit gains in excess of 60dB1,2 with a large variety of gain spectra. Gain bandwidths of 400nm and tunable narrow-band gain regions have been demonstrated.3 In addition, noise figures of 3.7dB have been achieved,4 limited by other third-order nonlinear processes.5 Polarization-insensitive operation in both one-pump6 and two-pump configurations has occurred.7 Finally, due to the inverted spectrum of the idler with respect to that of the signal, placing an OPA in the middle of a fiber span can cause mid-span spectral inversion, which counteracts the effect of fiber dispersion and some nonlinear effects.8

Although an OPA is used in a continuous-wave regime in typical systems, a pulsed-pump regime has demonstrated larger bandwidth and higher peak gain through combination with an optical filtering technique.9 Furthermore, by modulating the pump, it is possible to modulate the signal and/or idler at the output. This can be used to implement a variety of signal processing functions, including fast signal switching, demultiplexing of time-division-multiplexed signals,10 retiming and reshaping of waveforms,11 and optical sampling.12

Many challenges must be overcome for fiber OPAs to be useful in communication applications. In multi-wavelength systems, these are four-wave mixing (FWM), cross-phase modulation (XPM), and cross-gain modulation (XGM) between signals.13 The pump-to-signal relative intensity noise transfer and frequency/phase to signal intensity conversion are also potential challenges for practical fiber OPAs. In particular, the signal crosstalk due to FWM and XGM effects has been shown to be a constraint on using fiber OPAs in WDM systems.13 We have proposed several schemes to suppress such crosstalk, namely the two-orthogonal-pump OPA,14 WDM signals with polarization-interleaving,15 and recently a return-to-zero differential phase-shift-keying (RZ-DPSK) signal format. The latter suppresses the power penalty due to crosstalk up to 3dB, as shown in Figure 1, which compares eye diagrams, and in Figure 2, which compares the bit-error-rates (BER) of different formats.16

Figure 1. Eye diagrams of de-multiplexed wavelength division multiplexing (WDM) signals at 1567.6nm before and after an optical parametric amplifier (OPA). Left column: A return-to-zero differential phase-shift-keying (RZ-DPSK) signal. Right column: A conventional on-off-keying (OOK) signal. Timescales are 50ps/div for return-to-zero signals and 20ps/div for no return-to-zero signals.16

Figure 2. Bit-error-rate (BER) plots for RZ-DPSK (left) and OOK (right) signals at 1567.6nm. Dashed lines and empty diamonds: Back-to-back. Solid lines and solid diamonds: After OPA.16

Substantial progress has been made in recent years in the development of fiber OPAs, including high gain, gain bandwidth of several hundred nanometers, polarization-independent configurations, and low noise figures. We anticipate that with further progress on high-power pumps, highly-nonlinear fibers with tailored dispersion properties, and suppression techniques for stimulated Brillouin scattering, fiber OPAs and related devices will find practical applications in areas such as high-power wavelength conversion and optical communication.

Kenneth Kin-Yip Wong, Bill P. P. Kuo
The University of Hong Kong
Pokfulam Road, Hong Kong

Kenneth Kin-Yip Wong received his PhD degree in electrical engineering at Stanford University in 2003. He is currently an assistant professor in the Department of Electrical and Electronic Engineering at The University of Hong Kong. His research focuses on dense wavelength division multiplexing (DWDM) systems, fiber nonlinearity, and fiber optical parametric amplifiers.

Michel E. Marhic
Institute of Advanced Telecommunications
University of Wales
Swansea, UK
Georgios Kalogerakis, Leonid G. Kazovsky
Stanford University
Stanford, CA