Figure 2. Experimental setup and data showing nonlinearity-induced symmetry breaking between counter-propagating light.

(a) Schematic of the setup. The light of an external cavity diode laser (ECDL) is split into two parts and sent into a fused silica whispering gallery resonator from opposite directions. Two optical circulators C1, 2 split off the light coming from the microresonator and enable the measurement of the clockwise and counterclockwise transmission using photodiodes PD1 and PD2. The ratio between the counter-propagating pump powers can be adjusted using an amplitude modulator (AM). (b) Transmission vs. laser frequency measured on two consecutive laser sweeps across the resonance with equal powers of 80 mW launched in each counter-propagating direction. The system spontaneously breaks the symmetry and “picks” one of two possible states allowing light to couple in from just one direction, i.e. CW (top) or CCW (bottom). (c) Measurement of optical switching between the CW and CCW states, showing a hysteresis loop. The graph is obtained by tuning the laser into resonance and modulating the power in one direction up and down ~30 times. Inset: illustrations of the power flow in the two states.