​​​​​​​Chyeguide

​​Single-Mode DFB Laser Diodes 

​(a) Asymmetric facet coatings 

​Asymmetric facet reflections such as one cleaved facet and the other facet AR coated (Fig. 5(a)) can break the mode degeneracy (compared to Fig. 4(c)). Simulated ASE spectrum just below threshold is shown in Fig. 6(a). This approach is relatively easy to implement, but the lasing wavelength varies around the Bragg wavelength, depending on the grating phase at the reflection facet, which cannot be controlled. 

(b) Phase-shift grating

If a phase shift (usually quarter wave) is introduced in the center of the grating region (Fig. 5(b)), lasing is obtained at the Bragg wavelength (Fig. 6b)). In this case, the grating behaves as band-pass filter (since the center region consists  of 2 x quoter-wave (half-wave resonance), while in the uniform grating case, the grating behaves as a band-rejection filter (since it consists of quoter-wave stack).  

​

(c) Gain (loss) grating

​The conventional grating has a variation of the mode index along the propagation direction, which is induced by waveguide corrugation (grating layer thickness modulation). This is called, "index coupled" DFB laser. If a periodic gain (or loss) modulation is introduced in the grating by periodic etching of the active layer or adding a periodic loss layer, single-mode operation is obtained. This is called, "gain coupled", or "loss coupled" DFB laser.       

​​Tutorials 3

Fig. 5 Schematic laser diode structures, (a) uniform DFB laser diode with asymmetric facet coatings, and (b) phase-shifted DFB laser diode.

Fig. 6 Simulated ASE spectra of (a) uniform grating DFB laser diode with one AR coated facet and the other facet reflection, and (b) phase-shifted DFB laser diode.