K. Obermann

Four-wave mixing in semiconductor optical amplifiers for frequency conversion and fast optical switching

Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is an important tool for frequency conversion and fast optical switching in all-optical communication networks. We review the main applications of SOAs as nonlinear optical components. Concentrating on FWM, we define general parameters that are of relevance for signal processing applications. We show, how basic experiments and general simulation procedures can be used to determine optimum operating conditions for the intended applications. Besides a comprehensive investigation of FWM among continuous waves, we present new experimental results on FWM with picosecond optical pulses. A comparison of both reveals a different behavior and demonstrates that new optimization criteria and advanced theoretical models have to be applied for the case of short optical pulses. Moreover, we discuss the possibility to extract the dynamical SOA parameters from our experiments.

Performance analysis of wavelength converters based on cross-gain modulation in semiconductor-optical amplifiers

An analytical theory describing all-optical wavelength converters based on cross-gain modulation (XGM) in semiconductor-optical amplifiers is derived. Our theory consists of two parts: a large-signal analysis yielding the transmission function for the signal, and a small-signal analysis in order to describe the transformation of the signal and probe intensity noise. Both the large-signal as well as the small-signal theory reveal similar performance for the co- and the counterpropagating injection scheme for bit rates up to 2.5 Gb/s. This is confirmed by computer simulations. Consequently, the counterpropagating configuration is preferable because the implementation is simpler and conversion to the same wavelength is possible. In order to increase the conversion efficiency it is better to reduce the average signal power than to increase the probe power, which additionally reduces the output power range. However, there is a tradeoff between conversion efficiency and output extinction ratio. According to the small-signal analysis, the relative-intensity noise (RIN) due to the probe and due to the amplified spontaneous emission is negligible. Moreover, the converted signal has a lower RIN than the input signal.

Noise analysis of frequency converters utilizing semiconductor-laser amplifiers

This paper deals with a general problem concerning semiconductor-laser amplifiers used for frequency conversion. The amplified spontaneous emission (ASE) of a saturated amplifier is investigated experimentally and theoretically. An analytical solution accounting for the spatial dependence of the inversion parameter as well as the spectral dependence of the ASE is derived. Hence, the results can be applied to arbitrary saturation conditions and frequency shifts. Our theory is applied to frequency converters based on four-wave mixing and is found to be in good agreement with both the numerical results and the experimental data. In order to quantify the performance of a frequency converter, a noise figure is defined and shown to be strongly dependent on the frequency detuning and the power of the input waves.

Turn-on jitter in zero-biased single-mode semiconductor lasers

An analytical expression for the turn-on delay probability density function (pdf) of single-mode lasers biased below threshold is derived, which is in good agreement with results based on computer simulations and measurements. It is shown, that the pdf obtained only depends on the on-state yielding simple requirements for the transmitter in order to achieve a given BER. The results are of particular interest for designing optical interconnects, where zero-biased lasers are used.

Turn-on jitter of zero-biased nearly single-mode VCSEL's

The turn-on jitter of zero-biased nearly single-mode vertical-cavity surface-emitting lasers (VCSELs) is experimentally investigated. Since during the turn-on event both the dominant and the suppressed polarization are exited, an analytical expression describing the probability density function of the turn-on delay for a single-mode VCSEL is derived, which accounts for both polarizations. The measurement results and the theory are in good agreement as long as the VCSEL is transversal single-mode.

Noise characteristics of semiconductor-optical amplifiers used for wavelength conversion via cross-gain and cross-phase modulation

The amplified spontaneous emission of a semiconductor-optical amplifier saturated by two input waves is investigated theoretically and experimentally for both co- and counterpropagating injection. For the first time, an analytical expression for counterpropagating input waves is presented and compared with the copropagating case. It is shown, that copropagating injection can significantly reduce the noise level. The theoretical results are also compared with the experimental data and found to be in good agreement.

Comparison of N x 40 Gb/s and 4N x 10 Gb/s WDM transmission over standard single-mode fiber at 1.55 μm

We compare the performance of dispersion-managed 4/spl times/40, 16/spl times/10, and 16/spl times/40 Gb/s wavelength-division-multiplexed transmission over 1000-km standard single-mode fiber operating at 1.55 /spl mu/m. It is shown, that for N/spl times/40 Gb/s, the system performance is mainly limited by the degradations of each single channel. No significant additional degradation due to cross-phase modulation or four-wave mixing could be observed. Moreover, it is shown that focusing on the total fiber-input power, both transmission schemes, 4/spl times/40 Gb/s at 100-GHz channel spacing and 16/spl times/10 Gb/s at 25-GHz channel spacing, yielding the same total bandwidth show almost identical performance.

Saturation of semiconductor optical amplifiers due to amplified spontaneous emission

We solved the nonlinear integral equation for the carrier density by the method of fixed-point iteration for different SOA lengths. We also investigated the reduction in the gain recovery time due to the saturation caused by ASE, which is important for both linear and nonlinear applications. The spontaneous carrier lifetime can be reduced by approximately a factor of 10 when sufficiently long SOAs are used.

Turn-on jitter of zero-biased polarization controlled single-mode VCSELs

The turn-on jitter of zero-biased, polarization controlled single-mode VSCELs is investigated experimentally. It is found that during the turn-on event, both the suppressed polarization and transverse higher modes are existent yielding a reduction of the turn-on jitter. On the other hand, when the transmission link contains polarizations sensitive elements or spatial filtering this means that the turn-on jitter is significantly enhanced.

Modeling of semiconductor optical amplifiers for interferometric switching applications

Summary form only given. Various applications of all-optical signal processing, e.g, demultiplexing in optical time-division multiplexing (OTDM) systems or optical sampling, can be accomplished by all-optical switches based on semiconductor optical amplifiers (SOAs) in interferometric configurations. To optimize such switching devices, it is useful to have a model which describes the temporal characteristics of the switches accurately. The SOA model, which we present, takes the pulse propagation and the gain dynamics into account. The gain dynamics is determined by carrier density modulation (also called carrier density pulsation, CDP) and carrier heating (CH), calculated in separate rate equations.