External-cavity diode lasers provide absolute references for WDM testing

he rapid growth in data traffic is driving development of highercapacity optical-fiber-transmission methods based on wavelength-division multiplexing (WDM). In WDM, several lasers at different wavelengths simultaneously transmit separate streams of data along a single fiber. At the receiving end, the different wavelengths are optically separated and individually detected. The transmission capacity of an existing fiber link can, therefore, be instantly increased by a factor equal to the number of individual wavelengths used. The rapid evolution of WDM technology has created the need for laser sources with output that is reliably locked to a known absolute wavelength (frequency) with high precision. Recently, the International Telecommunications Union (Arlington, VA) created a standard for 32-channel WDM transmission that consists of 40 wavelength channels, separated by only 100 GHz (0.8 nm), eight of which are for overhead—signal management. The close channel spacing is necessary to ensure that all the channels fit under the highgain portion of the erbiumdoped fiber-amplifier gain curve, centered around 1545 nm. Looking to the future, higher-density WDM formats with 64 channels separated by only 50 GHz (0.4 nm) are predicted. And some researchers are already experimenting with channel spacings of less than 2 GHz. Successful WDM operation depends on eliminating crosstalk between the different wavelength channels. This requires that all the active and passive system components operate within the spectral window they were designed for, without undue drift or chirp. At the manufacturing, installation, and servicing/repair stages, this requirement creates a need to check the wavelength performance of key components such as lasers, filters, and fiber gratings. Wavelength meters, spectrometers, and spectrum analyzers are important in this area. However, while these tools may suffice for field testing of the latest WDM systems, more-accurate, absolute methods are needed for use in quality control and testing laboratories, as well as for the research and development effort to produce future generations of WDM technology.