Fabrication of a bistable distributed feedback laser amplifier with a completely linear chirped grating by electron beam lithography

The chirped grating is useful in distributed feedback (DFB) lasers as it removes threshold degeneracy, enhances wavelength tuning range, and reduces spatial hole burning. Moreover, it lowers the input power necessary for optical bistability in DFB laser amplifiers and enlarges the operation wavelength range as well. This optical bistability provides means of all optical flip-flop and wavelength conversion, and therefore is very important for optical networking. For the purpose of the optical bistability, the linearly chirped grating as illustrated is advantageous. However, making such gratings is difficult if the holographic exposure is used. Even with the electron beam exposure, patterning linear chirping is not straightforward as the grating pitch has to be changed typically by several ppm every grating line. One method of controlling the grating pitch beyond the maximum positioning resolution of electron beam writers is to incorporate the ability to change the field size. Gratings with slightly different pitches for WDM laser arrays have been successfully fabricated by this method. However, linearly chirped gratings require even finer controllability. We have therefore introduced the weighted exposure technique into the field size modulation, thus acquiring pitch controllability sufficient to write linear chirped gratings.