Optical Networking Best Practices Handbook

3.2. MULTIWAVELENGTH LASERS

Mode-locked lasers are common tools for producing short pulses in the time domain, including telecommunications applications at multigiga-Hertz repetition frequencies that require tunability in the C-band. Now they also can work as multiwavelength sources in WDM applications [3].

Both cost-effectiveness and performance are fundamental requirements of today's WDM systems, which are built using multiple wavelengths at precise locations on the International Telecommunications Union (ITU) standards grid. Because mode-locked lasers produce a comb of high-quality channels separated precisely by the pulse repetition frequency, one source can replace many of the distributed feedback lasers currently used. Channel spacing can range from >100 to 3.125 GHz [3].

This single-source solution for WDM system architectures can reduce costs and enable applications in metro and access networks, test and measurement instrumentation, and portable field-test equipment. New applications, such as supercontinuum generation, frequency metrology, and hyperfine distributed WDM, can also benefit from the laser's spectral and temporal properties [3].

3.2.1. Mode-locking

The output of mode-locked lasers in the time domain is a continuous train of quality pulses, which in this example exhibits a 25-GHz repetition rate, a 40-ps period, and a pulse width of approximately 4 ps. In general, a laser supports modes at frequencies separated by a free spectral range of c/2L, where L is the ...