The optocirculator is the circulator used for optical communication. It is similar to the isolator. Optical Isolator inserts resistance to the reverse signal by way of insertion loss, optical circulator takes the light on one port and guides it out to the non reciprocal port.
While in isolator there are only 2 ports, circulator has 3 ports or 4 ports. Both the 3 port and 4 port circulators are depicted in the figure-1. As shown in the figure, signal incident on port 1 is passed out from port 2, similarly signal incident on port 2 is passed out from port 3 and input to port 3 is passed out from port 1. The same is applicable for 4-port circulator. There are circulators available either for clockwise direction or for anticlockwise direction. They are basically used to create bidirectional optical link as shown in the figure-2. It also provides add and drop functionality in optical multiplexer and optical de-multiplexer.
As shown in the figure-2, signal (marked in pink) from left to right is transmitted through two circulators having 3 ports and signal from right to left marked in blue is transmitted in the other direction over the same fiber optic cable. This forms the bidirectional link using circulators. From the figure it is derived that port-1 acts as INPUT port, port-3 as OUTPUT port and port-2 as both INPUT/OUTPUT port. Hence transmitter-1 and 2 can transmit signal with wavelength 1550 nm on port-1 simultaneously and the same can be received on the respective other sides on port-3.
Optical circulators are widely used in fiber amplification systems, WDM networks, OTDRs (optical time-domain reflectometers), covering unidirectional optical link to the bidirectional link and test and measurement instruments.
Typical Specifications for Optical Circulator:
• Number of Ports( 3 or 4 ports)
• Grade type ( P or A)
• Central Wavelength (1310nm or 1550nm)
• Insertion loss (0.6/0.8) over +/- 20 nm bandwidth
• Isolation (min.40dB)
• Directivity (60 dB)
• Return Loss (60 dB)
• Power handling limit (300 mW)
• Max. WDL (0.2 dB)
• Max. PDL (0.1 dB)
• Max. PMD (0.1 ps)