In this work, we study the pulsed laser crystallization of hydrogen-free amorphous germanium-nitrogen alloys (a-GeN). We discuss the role of nitrogen during phase transitions and the possible application of the resulting structure as an optical diffraction grating. The crystallized region results of pure microcrystalline germanium (μc-Ge). An indication that Ge-N bonds have broken and nitrogen outdiffused of the film is obtained from infrared spectroscopy and confirmed by Raman spectra. A pattern of alternating a-GeN and μc-Ge lines with a period of about 4 μm acts as an optical diffraction grating due to the difference in optical properties between the two materials, and the three dimensional surface profile, caused by N2 effusion, that is formed on the sample.