Christian G. Schäffer

Broadband radio-over-fiber-based wireless access with 10 Gbits/s data rates

Radio over fiber is a very attractive technology for delivering microwave and millimeter-wave signals containing broadband multimedia services. The generation of an optical double-sideband with suppressed carrier signal is a straightforward method because only one microwave modulator driven at half the millimeter-wave frequency is required. Then only one sideband is modulated with 10 Gbits/s baseband data forming a millimeter-wave single-sideband signal. Transmission experiments prove this modulation scheme to be dispersion tolerant, and error-free transmission is demonstrated after 21 km of single-mode fiber and wireless links up to 2.5 m.

Field trial investigation of 16-states MLSE equalizer for simultaneous compensation of CD, PMD and SPM

We investigate 16-states MLSE equalization over buried fibers having CD up to 4480 ps/nm, PMD up to 92 ps and launch powers up to 12 dBm. Results show a superior performance for joint compensation of all distortion effects.

Heterodyne Radio over Fiber system with 10 Gbps data rates

A coherent Radio over Fiber (RoF) system using unlocked laser sources for frequency translation is investigated. Transmission experiments prove this simple scheme to provide broadband multimedia services with decent bit error rates.

Optical Microwave Signal Generation Using a Fiber Loop

The use of a fiber loop mirror for the generation of dispersion tolerant double-sideband with suppressed carrier signals is presented. An asymmetric phase difference within the loop is in charge of filtering out the desired harmonic components at the output. This phase difference is generated by different methods, such as a phase modulator or cross-phase modulation effects in semiconductor optical amplifiers. The loop offers high efficiency and stability within a bandwidth of more than 130 nm. This scheme is suitable for remote signal generation in wavelength-division multiplexing systems.

The evaluation of thin electroless plated metal layers by means of Surface Plasmon Resonance

The object of the presented work is the excitation and propagation of plasmons along thin metal layers. The main focus is on plasmon excitation along electroless plated metal layers by utilizing a Kretschmann-Rather coupler, whose concept is shown in detail. Therefore an impedance model is presented for calculating the reflectivity of multilayer systems. With the Kretschmann-Rather coupler experimental results for the reflectivity of the analysed metal layers were obtained. Comparing the theoretically calculated reflectivity values with the experimental results the samples are characterised in terms of thickness, complex permittivity and scattering on both sides of the metal layer. The information gained from the extrapolation of the layer parameters provides a basis for the further development of a fibre optic biosensor.

Flexible manufacturing method for long-period fibre gratings with arbitrary index modulation profiles

Special long-period fibre gratings (LPGs) are needed in multiple applications. For example, LPGs are employed as mode converters in the forward direction to utilise the waveguide dispersion of higher order modes. Other applications are gain flattening of optical amplifiers or filtering in general. In this paper, we propose a flexible manufacturing method for LPGs, which enables the generation of tailored arbitrary index modulation profiles. The results of apodized and chirped gratings are presented, which show the flexibility and reliability of the writing set-up.

Application of MLSE technology in optical communication systems and performance evaluation in 10Gbit/s field trials

In this paper we analyze the effort of realizing advanced MLSE digital equalizer technology in optical 10Gbit/s systems and evaluate its performance for ISI compensation in field applications by comparing 4-state vs. 16-state Viterbi decoders.

Coherent optical frequency comb generation using an amplified fibre loop

The paper describes the generation of a coherent optical frequency comb (OFC) with more than 500 GHz bandwidth and a comb spacing in the lower GHz range. The comb source is based on an amplified fibre loop where phase locking is achieved using an optical phase modulator. Different to Mach-Zehnder based OFC generators a low modulation index for phase modulation was found to be sufficient for broadband comb generation. The structure of the used fibre loop is discussed in detail and results obtained during lab measurements are presented. The carriers of the optical frequency comb are meant to form the basis of a multi channel Radio-over-fibre system.

Performance monitoring in high speed optical networks

The world wide growing bandwidth demand in optical networks leads to channel data rates of 40 Gbit/s and higher (e.g. 100 Gbit/s). At these data rates polarization mode dispersion (PMD) is one of the limiting factors even in metro networks. Due to stochastically changing environmental conditions, such as temperature drifts, vibrations and pressure, PMD parameters fluctuate with time. Therefore, adaptive compensation and fast measurement systems are required to overcome the PMD induced impairments. One possibility for the realization of a fast measurement system is the spectrally resolved measurement of the Stokes parameters of modulated optical signals. This offers the advantage to directly measure first and second order PMD in the channel during operation of the optical transmission system at various locations in the network. Additionally the optical signal to noise ratio (OSNR) can be determined. The obtained information can be used to steer polarization mode dispersion compensation modules (PMDCs), for long term measurements at different points in the network studying the statistical behavior of PMD in real networks and for network management purposes e.g. routing information. In this paper measurement results for two realized polarimeter setups, a coherent detection polarimeter and a stimulated Brillouin scattering based one, in a 10 Gbit/s NRZ system, are shown.

Volterra based nonlinear equalizer with reduced complexity

A model for a reduced complexity nonlinear electrical equalizer based on the Volterra theory is presented which can be utilized to mitigate dispersion and other distortions in optical communication systems. In recent years several electronic equalizers like the feed forward equalizer (FFE), the decision feedback equalizer (DFE) and the maximum likelihood sequence estimator (MLSE) were intensively investigated in optical communication systems. Also, nonlinear FFE/DFE structures based on the Volterra theory were proposed. These nonlinear equalizers can mitigate the effects of dispersion much better than the classical FFE/DFE but are more complicated to build. In a practical system the Volterra nonlinearity is therefore limited to second and third order. However, the number of filter coefficients for such a nonlinear FFE of appropriate order (e.g. 5 taps) is still very high for the nonlinear parts. This results in a very high effort in the equalizer control and optimization algorithm and makes a practical implementation questionable. In this Paper we present a reduced model for Volterra based nonlinear equalizers were the order of the nonlinear parts can be set separately from the linear order. This results in less complex filter structures as the number of coefficients is reduced drastically.