The family of Sn-perovskite has recently emerged as a nontoxic compound for a future ecofriendly photonic technology. With an extraordinary quantum yield of emission at room temperature, bandgap tunability with the composition, and straightforward incorporation in optical architectures by chemistry methods, the ASnX₃ (A is an organic/inorganic anion and X is a halide) perovskite represents a suitable nontoxic material to implement lasers, optical amplifiers or light emitting diodes, among other devices. However, since Sn²⁺ is easily oxidized into Sn⁴⁺ losing its optoelectronic properties, there are few reports where a stable ASnX₃ device is provided. In this work, high-quality FASnI₃ (FA, formamidinium) polycrystalline thin films are successfully fabricated to demonstrate optical amplification and lasing. With an appropriate low temperature treatment and a polymethylmethacrylate(PMMA) clapping, the FASnI₃ films exhibit a remarkable stability and an efficient generation of amplified spontaneous emission (ASE) under nanosecond optical pumping. First, these films are deposited on a SiO₂ substrate to conform an optical waveguide whose geometrical parameters (i.e. thicknesses of the films) are properly designed to optimize the excitation and to enhance the generation of the photoluminescence. As a result, ASE is demonstrated with an extremely low threshold, about 100 nJ/cm², and a strong polarization anisotropy preferable to the transverse electric (TE) polarization. Moreover, the waveguide exhibits narrow lasing lines (< 1 nm) caused by the formation of random cavity loops in the polycrystalline grains. Then, the same structure is implemented in a flexible device using polyethylene terephthalate (PET) substrates. Here, despite that the formation of films formation of films on a flexible platform is always challenging, the flexible waveguide also shows ASE and RL lines with only one fold higher threshold (1 µJ/cm²). Finally, the FASnI₃ films are also incorporated as active medium in a Distributed Feedback Laser properly designed to achieve optical resonance at the ASE wavelength. This architecture  conforms an optical cavity able to demonstrate the generation of lasing action with peaks narrower than 1 nm. The proposed devices represent an important step towards the development of future cheap and green photonic technology based on Sn perovskites.