An overview is given of the direct modulation performance of high-speed semiconductor lasers. The high-speed response characteristics are described using a cascaded two-port model of the laser. This model separates the electrical parasitics from the intrinsic laser and enables these subsections to be considered separately. The presentation concentrates on the small-signal intensity modulation and frequency modulation responses, and the large-signal switching transients and chirping. Device-dependent limitations on high-speed performance are explored and circuit modeling techniques are briefly reviewed.

High-speed modulation of semiconductor lasers

The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate. >

Electrical signal processing techniques in long-haul fiber-optic systems

Chirp-induced dispersion penalties in high-bit-rate optical fiber transmission are assessed using numerical integration of laser rate equations and a Fourier transform fiber dispersion routine. The roles of the imposed modulation waveform and laser design parameters are evaluated from computer generated eye diagrams and simple analytical observations. Consistent with experiment, we find device dependent optimum laser extinction ratios. In addition, we address the delicate balance between nonlinear chirp-induced dispersion penalties and the speed limitations imposed by linear current filtering on both the laser transmitter and the receiver. These considerations become increasingly important at higher bit rates such as 8 Gbit/s.

Computer simulation of high-bit-rate optical fiber transmission using single-frequency lasers

This paper presents an introduction to the principles of lightwave system engineering. The treatment is historical rather than categorical-lightwave systems are described in terms of their evolution through four generations of technology, from a first generation operating at 0.85 μm wavelength over multimode fiber to a fourth generation employing coherent techniques at 1.55 μm. Basic engineering considerations such as fiber dispersion and receiver sensitivity are introduced early, then refined as the discussion progresses toward higher-performance, more sophisticated systems. The fundamental mechanisms that limit the performance of a given technology are quantified, and a figure of merit, the product of bit rate times maximum repeater spacing, is estimated. Values of this product range from about 2 Gbits/s . km for first-generation technology to roughly 900 Gbits/s . km for coherent systems.

http://ieeexplore.ieee.org/iel5/3/23078/01072601.pdf

Lightwave primer

The 25 years since the founding of the Journal of Lightwave Technology have seen more than three orders of magnitude increase in the capacity of optical transmission systems. This dramatic increase was brought about by the deployment of WDM and advances in high-speed transmission technologies.

High-Capacity Optical Transmission Systems

Using a multiplexer and demultiplexer built from discrete GaAs FET's, a directly modulated distributed feedback laser, and a low capacitance avalanche photodiode receiver, we have achieved transmission at 4 Gbit/s over 103 km of single-mode optical fiber at 1.54 μm.

4-Gbit/s transmission over 103 km of optical fiber using a novel electronic multiplexer/demultiplexer

Frequent power dropouts are observed in the single dominant line of a number of dc-biased 1.3-μm injection lasers, each of which exhibits a nearly single-longitudinal-mode CW spectrum. During the dropout events, some of which essentially extinguish the dominant line for 1 to 8 ns, total laser output power is observed to remain constant: the power lost from the dominant mode appears in a side mode of very low average power. These events are capable of causing bit errors in a communications system. It was found that the frequency of events greater than a given threshold value falls exponentially with an increasing ratio of main-to-side-mode powers and that this ratio must exceed 100:1 in order to avoid a significant contribution to system error rates when the laser is operated continuously and an external modulator is used. These observations are in agreement with recently published calculations. The requirements on mode power ratio for the case of directly modulated lasers are much more severe. We present a calculation of the optical power penalty resulting from these mode power fluctuations.

B. Kasper

Papers

4-Gbit/s transmission over 103 km of optical fiber using a novel electronic multiplexer/demultiplexer

Mode power partition events in nearly single-frequency lasers