9.6.4 Wavelength Stability and Other Issues

The design of modern WDM systems requires a careful consideration of many transmitter and receiver characteristics. An important issue concerns the stability of the carrier frequency (or wavelength) associated with each channel. Most WDM systems employ for each channel a narrow-bandwidth semiconductor laser designed to operate in a single mode using either a DFB or distributed Bragg reflector (DBR) structure (see Section 5.2 of LT1). The output wavelength of such sources is set by a built-in grating through the relation λ_c = 2Λ, where is the effective mode index and Λ is the period of the built-in grating.

The channel wavelength λ_c for DFB or DBR lasers can only change if the mode index changes. The refractive index of any material depends on temperature. In the case of semiconductors, λ_c can change with temperature at a rate of about 0.1 nm/°C [164]. Similar changes can also occur with the aging of lasers [165]. Such wavelength changes are generally not of concern for single-channel systems or coarse WDM systems for which channel spacing exceeds 2 nm. However, they become critically important for dense WDM systems in which channel spacing can be below 25 GHz when the bit rate is 10 Gb/s, and the situation is worse for ultradense WDM systems in which each channel is designed to operate at 2.5 Gb/s. For such systems, it is important that the carrier frequencies of all channels remain stable to within 1 GHz or so. In fact, the maximum allowed wavelength drift throughout the laser lifetime is 10 pm for 10-Gb/s channels with a spacing of 25 GHz (ITU recommendation G.692).

Most DFB and DBR lasers stabilize chip temperature by integrating a thermoelectric cooler within the laser package. Such a device can stabilize the temperature to within 1°C or so and provides sufficient wavelength stability for WDM systems with a channel spacing of 100 GHz or more. However, a thermoelectric cooler is not sufficient when channel spacing is 50 GHz or less. A number of techniques have been developed for stabilizing the laser wavelength to meet the requirement of dense WDM systems [166]-[175]. One technique employs electrical feedback provided by a frequency discriminator based on molecular gases to lock the laser frequency to a specific resonance frequency. For example, one can use ammonia, krypton, or acetylene for semiconductor lasers operating in the 1.55-µm region, as all three have resonances near that wavelength. Frequency stability to within 1 MHz can be achieved by this technique. Another technique makes use of the optogalvanic effect to lock the laser frequency to an atomic or molecular resonance [167].

The use of a molecular gas is not always practical, and its resonance frequencies do not coincide with the standard channel frequencies on the ITU grid. What one really needs for WDM systems is a comb of uniformly spaced and well-stabilized frequencies [168]. In one approach, a Michelson interferometer, calibrated with a frequency-stabilized master DFB laser, was used to provide a set of equally spaced reference frequencies [169]. A filter, an arrayed waveguide grating, or any other filter with a comb-like periodic transmission spectrum can also be used for this purpose [170]. A fiber grating is useful for frequency stabilization but a separate grating is needed for each channel since its reflection spectrum is not periodic [171]. A frequency-dithering technique in combination with an arrayed waveguide grating and an amplitude modulator can stabilize channel frequencies to within 0.3 GHz [172].

In a more practical design, a wavelength monitor is integrated within the DFB-laser package, resulting in a laser module whose wavelength can be stabilized to within 5 pm or so [173]. Figure 2.15 in Section 2.4 shows schematically the design of a DFB laser packaged with a wavelength monitoring unit. In this device, light from the rear facet of the DFB laser is collimated by a lens and is split by a prism into two beams, one of which is used for monitoring the output power. The other beam is used for monitoring the laser wavelength and is passed through a Fabry-Perot étalon with periodic transmission peaks (separated by channel spacing) before it falls on a photodetector. To ensure that the comb of frequencies does not drift because of temperature variations, two separate thermoelectric coolers are used for the laser and the Fabry-Perot etalon. As shown in Figure 9.34, laser wavelength changes by less than 2 pm over a temperature range from -5 to 70°C when two thermoelectric coolers are employed. Such a scheme allows one to lock the laser wavelength precisely on the ITU grid.

Figure 9.34: Measured wavelength drift as a function of module temperature at 20-mW output power for a laser module integrated with a wavelength monitor. (After Ref. [173]; ©2003 IEEE.)

An important issue for all-optical networks (see Chapter 10) is related to the loss of signal power occurring because of insertion losses associated with add-drop filters that may be used at multiple network modes. Optical amplifiers are used to compensate for such losses, but not all channels are amplified by the same factor, unless the gain spectrum is flat over the entire bandwidth of the WDM signal. Although gain-flattening filters are commonly employed, channel powers can still deviate by 10 dB or more depending on the route taken by a WDM signal. It may then become necessary to control the power of individual channels (through selective attenuation) at each node within a WDM network to make channel powers nearly uniform. The issue of power management in WDM networks is quite complex, and it requires attention to many details [176]—[180]. The buildup of amplifier noise can also become a limiting factor when the WDM signal passes through a large number of amplifiers. Another major issue concerns dispersion management [181]. In a reconfigurable network the exact path taken by a WDM channel can change in a dynamic fashion. Such networks will require compensation of residual dispersion at individual nodes. Network management is an active area of research and requires attention to many details [182].