Mats Sköld

High-Speed, Low-Current-Density 850 nm VCSELs

We report on the design, fabrication, and evaluation of large-aperture, oxide-confined 850 nm vertical cavity surface emitting lasers (VCSELs) with high modulation bandwidth at low current densities. We also compare the use of InGaAs and GaAs quantum wells (QWs) in the active region. Both VCSELs reach an output power of 9 mW at room temperature, with a thermal resistance of 1.9deg C/mW. The use of InGaAs QWs improves the high-speed performance and enables a small-signal modulation bandwidth of 20 GHz at 25degC and 15 GHz at 85degC. At a constant bias current density of only 11 kA/cm2, we generate open eyes under large-signal modulation at bit rates up to 25 Gbit/s at 85degC and 30 Gbit/s at 55degC.

Constellation diagram analysis of DPSK signal regeneration in a saturated parametric amplifier

The regeneration of 10 Gbit/s differential phase shift keying (DPSK) signals in a saturated fiber optic parametric amplifier (FOPA) acting as an optical intensity regenerator is investigated. The regeneration properties are investigated in amplitude and phase separately by means of coherent detection and constellation diagram analysis. A significant reduction of the amplitude fluctuations is observed and the measured signal-to-noise ratio (SNR) after DPSK demodulation is increased up to 10 dB. However, from the constellation analysis it is found that the FOPA introduces phase noise to the signal. The FOPA induced phase noise originates from pump amplitude fluctuations that are transferred to the signal through cross-phase modulation.

40-Gbaud 16-QAM transmitter using tandem IQ modulators with binary driving electronic signals

We propose a novel 16-quadrature amplitude modulation (QAM) transmitter based on two cascaded IQ modulators driven by four separate binary electrical signals. The proposed 16-QAM transmitter features scalable configuration and stable performance with simple bias-control. Generation of 16-QAM signals at 40 Gbaud is experimentally demonstrated for the first time and visualized with a high speed constellation analyzer. The proposed modulator is also compared to two other schemes. We investigate the modulator bandwidth requirements and tolerance to accumulated chromatic dispersion through numerical simulations, and the minimum theoretical insertion attenuation is calculated analytically.

All-Optical Waveform Sampling in High-Speed Optical Communication Systems Using Advanced Modulation Formats

We review techniques to characterize optical data with very high fidelity using the principle of optical sampling based on four-wave mixing in a highly nonlinear fiber. This approach results in sub-ps time resolution without artifacts in the impulse response and also excellent sensitivity allowing statistical analysis. In particular we describe implementations aimed at differentially phase encoded data (such as DQPSK) with all-optical balanced detection, formats requiring the complete signal field recovery (such as QAM) with phase-sensitive sampling, and signals requiring real-time recovery with all-optical real-time sampling. The all-optical approach to characterize optical waveform is shown to be very attractive and practical way to capture the details of various kinds of data and is, in principle, also scalable to extremely high baud rates.

QPSK phase and amplitude regeneration at 56 Gbaud in a novel idler-free non-degenerate phase sensitive amplifier

We introduce a novel input-idler-free non-degenerate phase sensitive amplifier (PSA) configuration and use it for simultaneous phase and amplitude regeneration of quadrature phase shift keyed (QPSK) signals demonstrated at symbol rates up to 56 Gbaud.

PMD-insensitive DOP-based OSNR monitoring by spectral SOP measurements

We present a DOP (degree of polarization) based OSNR (optical signal-to-noise ratio) monitoring method with spectral SOP (state of polarization) measurement to perform OSNR measurements insensitive to PMD (polarization-mode dispersion). Measurements at OSNR = 25 dB and DGD (differential group delay) = 32% of bitslot are performed with a standard deviation of 0.67 dB.

Characterization of Complex Optical Modulation Formats at 100 Gb/s and Beyond by Coherent Optical Sampling

Optical fiber communication systems are migrating toward the use of sophisticated, nonbinary, modulation formats. It is thus important to develop measurement techniques and solutions capable of unambiguously quantifying such signals and associated hardware. Although one can adapt certain techniques developed for wireless signal characterization, significant differences exist between optical and RF wireless implementations of advanced modulation formats. To meet this measurement demand we present a method for characterizing high symbol rate multilevel modulation optical signals with high fidelity, based on coherent optical sampling, as a complement to the standard real-time electrical sampling approach. The technique enables constellation- and waveform monitoring, bit-error rate and error-vector magnitude analysis, which makes it suitable for transmitter characterization.

Transmission of 240 Gb/s PM-RZ-D8PSK over 320 km in 10 Gb/s NRZ-OOK WDM system

We present the first demonstration of 40 Gbaud PM-RZ-D8PSK and its successful transmission in a 100 GHz-spaced 10 Gb/s NRZ-OOK WDM system over a 320-km transmission link along with the studies of WDM nonlinear crosstalk.

Fiber communications using convolutional coding and bandwidth-efficient modulation

In this paper we evaluate numerically the advantage of combining convolutional coding and bandwidth-efficient modulation. We compare different multilevel modulation formats, line codes, and hard/soft decision decoding. Compared with DPSK modulation (with the same bandwidth and information transmission rate), an improvement of almost 5 dB is observed for bit error rates around 10-8. We also study the robustness to intersymbol interference in the form of chromatic dispersion, and find that the improvement of the coded transmission lines improves over the uncoded even in presence of chromatic dispersion.

Robust design of all-optical PSK regenerator based on phase sensitive amplification

More compact, stable, and efficient configuration of a recently-developed regenerator is presented. The regenerator is assessed at data rates up to 56 Gbit/s using white phase noise for the first time.