Understanding the effect of laser chirp on signal transmission is of high importance to system designers and laser manufacturers. Interacting with fiber chromatic dispersion, laser chirp causes a power penalty that limits distance over which signals can propagate. Time-resolved chirp (TRC), the time variation of the instantaneous optical frequency of a laser transmitter, along with the chromatic dispersion characteristics of the transmission system, may be used to predict system performance.
The International Electrotechnical Commission (IEC) in working group 1 of Subcommittee SC86C is addressing TRC measurement and the calculation of power penalty due to laser chirp and chromatic dispersion. International standard IEC 61280-2-10, “Time-resolved chirp and alpha-factor measurements of laser transmitters,” was published in July 2005. Applicable to directly modulated lasers and lasers modulated by electroabsorption and Mach-Zehnder devices, this international standard describes the apparatus and procedures required to obtain TRC data. The apparatus consists of a frequency discriminator or a monochromator, a high-frequency optical oscilloscope, and a pseudorandom binary sequence generator. The output is a table or graph of instantaneous optical frequency versus time.
Dispersion penalty calculation is currently under development in working group 1. The eighth in
a series of Fibre Optic Communication System Design Guides, it is entitled IEC 61282-8, “Calculating dispersion penalty from measured time-resolved chirp data.” The direct measurement of dispersion penalty requires two bit-error-ratio measurements typically from 10-4 to 10-6, which is a complex and time-consuming measurement. In comparison, the calculation of dispersion from TRC is faster and can easily be accomplished for a variety of fiber types and lengths. The calculations are valid for all types of single-longitudinal-mode lasers at data rates of 2.5 Gbits/sec and higher with NRZ modulation formats. Publication of this guideline is expected at the end of 2006.
Jack Dupre is an optical test and standards consultant and is secretary for IEC SC86C. He can be reached at email@example.com.