1. Field of the Invention
This invention relates generally to the transmission of data over optical fibers and, more particularly, to reducing the amount of pulse narrowing in the electrical signal that drives a limiting E/O converter in the transmitter in order to improve the performance of the transmission.
2. Description of the Related Art
Optical fiber is widely used as a communications medium in high speed digital networks, including local area networks (LANs), storage area networks (SANs), and wide area networks (WANs). There has been a trend in optical networking towards ever-increasing data rates. While 100 Mbps was once considered extremely fast for enterprise networking, attention has recently shifted to 10 Gbps, 100 times faster. As used in this disclosure, 10 Gigabit (abbreviated as 10G or 10 Gbps) systems are understood to include optical fiber communication systems that have data rates or line rates (i.e., bit rates including overhead) of approximately 10 Gigabits per second.
Regardless of the specific data rate, application or architecture, communications links (including optical fiber communications links) invariably include a transmitter, a channel and a receiver. In an optical fiber communications link, the transmitter typically converts the digital data to be sent to an optical form suitable for transmission over the channel (i.e., the optical fiber). The optical signal is transported from the transmitter to the receiver over the channel, possibly suffering channel impairments along the way, and the receiver then recovers the digital data from the received optical signal.
In an optical fiber communications system, the optical power output by a laser is commonly modulated in a binary fashion to send data over an optical fiber. Nominally, the optical power is high for the duration of a bit period to send a logical “1,” and low to send a logical “0.” This is commonly referred to as on-off-keying (OOK) and, more specifically, as non-return-to-zero (NRZ) on-off-keying, where “on” means high laser power and “off” means low laser power. When on-off keying is used, the transmitter usually includes a laser driver that acts as a limiter, clipping (i.e., limiting) the electrical signal driving the laser, in accordance with the on-off mode of operation of the laser. The transmitter may also include circuitry that conditions the signal entering the limiter. Conventional wisdom is that this circuitry should be optimized to minimize the data-dependent jitter DDJ (i.e., maximize the horizontal eye opening or horizontal eye width) of the optical signal output by the laser. Furthermore, conventional wisdom suggests that in many cases this is best achieved by minimizing DDJ of the electrical signal entering the limiter.
However, as shown below, metrics other than DDJ may be just as important, or even more important, in designing transmitters.