For the circulator mode (see diagram), signal light incident from the ports 1, 2, 3 and 4, exits from the ports 2, 3, 4 and 1, respectively, constituting a 1-2-3-4-1 circular path.
For this to work, a control field is launched into port 1 (see diagram), and this excites the coupling between mechanical motion in the sphere and a clockwise optical field. This happens when the frequency difference between control signal and the controlled signal is equal to the mechanical frequency of a certain vibration mode in the sphere.
Re-tuning the control frequency to the other side of the controlled signal, again by a difference equal to the mechanical resonance, changes the set-up into an amplifier, causing a boosted version of the signal entering port 1 to leave port 2.
“When only focusing on ports 1 and 2 [in the circulator case], it is also an efficient optical isolator. For directional amplifiers, signal light incident from port 1 is amplified and exits from port 2, not the other way around,” said the University. “Thus in the direction of 1-2 has directional amplification.”
Exactly how it works is described in the paper ‘Reconfigurable optomechanical circulator and directional amplifier‘ in Nature Communications, which is freely available in full.
According to the paper , the microsphere resonator is evanescently coupled with the two-tapered microfibres.
Without the control pump, the system has a passive configuration, where it acts as a four-port add-drop filter, which can filter the signal between fibres via the resonance of the cavity.
“Other promising applications along this direction include non-reciprocal frequency conversion, a narrow-band reflector and creation of a synthetic magnetic field for light by exploiting multiple optical modes in a single cavity,” according to the paper.
The demonstrated device, according to the University, can be generalised to single-photon, microwave and acoustic circuits.