An asymmetric optical resonator comprises a waveguiding element forming a closed loop. A first circumference of the loop is different from a second circumference, the first circumference being measured at one end of the loop in a plane perpendicular to a cavity axis. The second circumference is measured at the opposite end of the loop in a plane perpendicular to the cavity axis. An effective refractive index of the waveguiding element varies along a circumferential direction of the loop.