368 Fiber Grating Lasers and Amplifiers distributed over 1 cm of the same laser, the optimum value acquires a slightly larger value of 1.3p radians [86]. The position of the phase shift changes as a direct result of spatial hole burn- ing, since the intensity is highest at the center of the laser for a symmetrically positioned phase shift. The distributed phase-shift laser is as efficient as the discrete off-center phase-shifted laser. 8.5.1 Multifrequency Sources Dual-frequency and multifrequency sources can be built by combining tech- niques presented in the preceding sections. A novel and particularly simple arrangement is the four-grating coupled cavity arrangement based on the sin- gle-frequency laser shown in Fig. 8.16. By adding an extra grating matched to the external grating in Fig. 8.16, a dual-frequency laser is formed. In order to ensure the coupling between the cavities, the grating bandwidths are chosen to overlap slightly. This configuration produces a robust laser that performs as a single entity, and a schematic is shown in Fig. 8.21. The emission spectra consist of two single-frequency lasing modes at defined by the grating pairs, and tune without mode hops. However, the difference frequency tunes with temperature, strain, or pump power (thermally induced). The coupled cavity reported by Chernikov [52] operated at a difference frequency of