Distributed Forward Coupled (DFC) Laser

The device structure and the operation principle of the distributed forward coupled (DFC) laser are presented. In this laser, periodic absorption losses induce the codirectional coupling of the two forward propagating modes in a twin-waveguide structure. Exploiting the strong dependence of the coupling wavelength on the effective index difference of the two modes enables the wide wavelength tuning of these devices by only one wavelength control current. In close analogy with the contradirectionally coupled DFB and DBR laser structures, the DFC laser corresponds to the DFB laser, while the previous types of codirectionally coupled tunable lasers more resemble the DBR type devices. Accordingly, the homogeneous distribution of the forward coupling along the entire cavity length yields optimum wavelength selectivity and side mode suppression, as compared with the previous approaches. Theoretically, well-designed 600-μm long InGaAsP–InP DFC lasers, operating at 1.5-μm wavelength, yield tuning ranges up to 100 nm and side mode suppression ratios up to 30 dB.