Optocirculator Basics: Functionality and Applications

The optocirculator is a circulator designed specifically for optical communication. Think of it as an optical isolator but with a clever twist. While an optical isolator simply blocks signals traveling in the reverse direction by introducing insertion loss, an optical circulator redirects the light from one port to the next non-reciprocal port.

opto circulator-3 port and 4 port optical circulator

Unlike isolators which only have two ports, circulators come with either three or four ports.

As depicted above, a signal entering port 1 exits from port 2. Similarly, a signal entering port 2 exits from port 3, and a signal entering port 3 exits from port 1. This principle applies to both 3-port and 4-port circulators. These circulators are available in both clockwise and counter-clockwise configurations.

Their primary use is to create bidirectional optical links. They also enable “add and drop” functionality within optical multiplexers and de-multiplexers.

Bidirectional optical link using circulators

In the above diagram, a signal (marked in pink) travels from left to right through two 3-port circulators. Simultaneously, a signal (marked in blue) travels from right to left over the same fiber optic cable. That’s how a bidirectional link is established using circulators!

From this setup, we can identify:

• Port 1: INPUT port
• Port 3: OUTPUT port
• Port 2: INPUT/OUTPUT port

Therefore, transmitter-1 and transmitter-2 can simultaneously transmit signals with a wavelength of 1550 nm on port 1, and these signals can be received on the corresponding output sides on port 3.

Optical circulators find extensive use in:

• Fiber amplification systems
• WDM (Wavelength Division Multiplexing) networks
• OTDRs (Optical Time-Domain Reflectometers)
• Bridging unidirectional and bidirectional optical links
• Test and measurement instruments

Typical Specifications for Optical Circulators

• Number of Ports: (3 or 4)
• Grade Type: (P or A)
• Central Wavelength: (1310nm or 1550nm)
• Insertion Loss: (0.6/0.8 dB) over +/- 20 nm bandwidth
• Isolation: (min. 40dB)
• Directivity: (60 dB)
• Return Loss: (60 dB)
• Power Handling Limit: (300 mW)
• Max. WDL (Wavelength Dependent Loss): (0.2 dB)
• Max. PDL (Polarization Dependent Loss): (0.1 dB)
• Max. PMD (Polarization Mode Dispersion): (0.1 ps)