A diffractive-optic unstable resonator for low-gain lasers

Unstable optical resonators have been found to be very useful in applications where large fundamental mode volume and good higher-order mode discrimination are required. Gaussian-reflectivity mirrors (GRMs) are commonly used as output couplers to improve the far-field pattern of unstable resonators. We have shown that GRM unstable resonators can be designed to produce flat near-field output beams even when the central reflectivity of the GRM is higher than the conventional l/m² value. This is accomplished by altering the internal Gaussian mode using a custom-made diffractive optical element. This allows for maximally flat output beams from high-magnification and low-gain laser systems