Isolators & circulators

Isolators & circulatorsSubmitted By: Tanvi Nara ( 14ECP017)

IntroductionCouplers and other such optical devices : Reciprocal devicesIsolators and circulators : Non-reciprocal devicesNon-reciprocal optics means that changes in the properties of light passing through the device are not reversed when light passes through opposite direction. Circulator is a passive on-reciprocal 3 or 4-port device.

CirculatorIsolatorWhen one port of a 3-port circulator is terminated, it can be used as an isolator, since signal can travel in only one direction b/w remaining ports.

Isolators are used at output of optical amplifiers to prevent reflections from entering the devices, which would otherwise degrade the performance.

2 key parameters of an isolator: Insertion loss (~1dB) Isolation(~40-50dB)

Isolator from a circulatorCirculatorA circulator is similar to an isolator, except that it has multiple ports, typically (a)three or (b)four.Circulators work on the same principles as isolators.

Principle of OperationState of polarization (SOP) of light refers to the orientation of its electric field vector(E) on a plane that is orthogonal to its direction of propagation.E=Ex +Eywhere Ex and Ey are the horizontal and vertical modes respectively.

Assume that the input light signal has the vertical SOP as shown in the figure. It is passed through a polarizer which passes only light energy in the vertical SOP.The polarizer is followed by a Faraday rotator i.e. a non-reciprocal device that rotates the SOP by 45deg, regardless of the direction of propagation.The faraday rotator is followed by another polarizer that passes SOPs with this 45deg orientation.Thus light signal from left to right is passed without any loss.But the light signal from going from right to left due to reflection, with the same 45deg SOP orientation, is rotated another 45deg by rotator, and thus blocked by the first polarizer.

Note: In the above example, a particular SOP was assumed for input light signal but practically we cant control the SOP of a light signal.Practical polarization-independent isolator The SWP (spatial walk-off polarizer) splits the signal into two orthogonally polarized components.SWP are realized using birefringent crystals whose refractive index is different for two components.Faraday rotator rotates the SOP by 45deg

Propagation from L to RHalf wave plate ( a reciprocal device) rotates the SOPs by 45deg in clockwise direction for signals propagating from L to R and by 45deg in anticlockwise direction for signal going from R to LTherefore, combination of rotator and half wave plate converts horizontal polarization into vertical and vice versa and the two signals are combined by another SWP at the output.

Propagation from R to LFor reflected signals in the reverse direction, the half wave plate and Faraday rotator cancel each others effects, and SOPs remain unchanged as they pass through these two devices and are thus not recombined by the SWP at the input.

Advantages of circulatorCirculators can be used to achieve bi-directional transmission over single fibre.Because of its high isolation of the input and reflected optical powers and its low insertion loss, optical circulators are widely used in advanced communication systems and fibre optic sensor applications.ReferencesOptical networks by Rajiv Ramaswami