An extendable four-port circulator includes a middle birefringent crystal, a
first
birefringent crystal, a first non-reciprocal device, a second birefringent crystal,
and a second non-reciprocal device. The first non-reciprocal device is coupled
to the first birefringent crystal. The second non-reciprocal device is coupled
to the second birefringent crystal. The middle birefringent crystal includes a
first surface, a second surface, a third surface, and a fourth surface. The first
surface is coupled to the first non-reciprocal device. The second surface is coupled
to the second non-reciprocal device. The third surface defines first and second
extension interfaces. The fourth surface defines third and fourth extension interfaces.
A multi-port circulator includes at least one extendable four-port circulator.
