tivity on the other side (AR/ HR). In this case the grating forms the distributed mirror on the antireflection-coated side, while the semiconductor facet on the high reflectivity side forms the other mirror. These lasers generally have higher output power since the light is taken from the AR side, and the HR side prevents power being lost from the back. Unfortunately, during the manufacturing of the laser and the cleaving of the facets, it is virtually impossible to control at which point in the buried grating the laser cleaves to form the facet. So sometimes the laser HR facet forms at the crest of the buried grat- ing, and sometimes on the slope. Depending on the phase of the grating and the optical mode, the laser output spectrum can vary. Frequently, the phase of the highly reflec- tive side occurs at a point where two longitudinal modes have the same cavity gain, and thus the laser operates at two modes simultaneously. Thus such AR/HR lasers have to be screened at manufacturing and parts that are multimode or have poor SMSR have to be scrapped. Additionally, the phase of the cleaving affects the wavelength, and thus controlling the output wavelength of a batch of lasers in manufacturing can be a challenge. An alternative approach is a phase-shifted DFB laser. In this case both facets are antireflection-coated and there is a phase shift in the cavity. This could be a single 1/4 wave shift at the center of the cavity, or multiple smaller shifts distributed in the cavity. Such devices have much better reproducibility in wavelength and theoretically they are all single mode, independent of the production process. A selection of characteristics of practical DFB lasers for use as sources in telecommunica- tion system is reported in Table 5.3. 5.2.2 High-Power Pump Lasers If the ability to produce laser diodes with a strictly monochromatic output field that are stable in wavelength and power is instrumental to design optical transmitters for DWDM Technology for Telecommunications: Integrated Optics and Microelectro