Optical circulator
A three- or four-port non-reciprocal device that routes light sequentially: port 1 → port 2, port 2 → port 3, port 3 → port 4 (or back to port 1 for three-port). Used in bidirectional optical systems and Bragg grating filters.
An optical circulator routes light from port to port in a fixed cyclic order. The standard three-port circulator transmits:
- Light entering port 1 → exits port 2
- Light entering port 2 → exits port 3
- Light entering port 3 → exits port 1 (or is absorbed in some designs)
Like the optical isolator, the circulator relies on the Faraday effect for its non-reciprocal behavior. It is essentially an isolator with the rejected reverse light routed to a third port rather than being dumped.
Typical specifications at 1550 nm:
| Parameter | Telecom three-port circulator |
|---|---|
| Forward insertion loss (1→2, 2→3) | 0.6 – 1.0 dB |
| Reverse isolation (2→1, 3→2) | 40 – 50 dB |
| Directivity (1→3) | 45 – 55 dB |
| Polarization-dependent loss | 0.15 dB |
| Return loss | 50 dB |
| Operating wavelength | C-band (1530 – 1565 nm typical) |
Standard applications:
| Use case | Configuration |
|---|---|
| Bragg grating add/drop filter | Input → port 1 → port 2 → FBG → reflected back → port 3 (drop) |
| Double-pass fiber amplifier | Signal → port 1 → port 2 → fiber amplifier → reflective mirror → returned light → port 3 |
| OCT and FBG sensing | Source on port 1, sensor/sample on port 2, detector on port 3 |
| Bidirectional fiber link separation | Distinguish counter-propagating signals on a single fiber |
Four-port circulators extend the cyclic routing: port 1 → 2 → 3 → 4 → (terminated). Used when the third output port also requires non-cyclic dumping.
Circulators eliminate the need for splitters in many configurations. Where a 50:50 splitter would impose a 3 dB loss on each pass, a circulator routes signals with low loss (insertion loss of 1 dB or less per pass) and provides high isolation between forward and reverse paths.
A common architectural use is the Bragg grating add/drop filter: input light enters port 1, exits port 2 into the FBG which reflects the target wavelength back to port 2, and the reflected light is routed to port 3 as the dropped channel. All other wavelengths transmit through the FBG and exit from its far end.
References: B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019). E. Hecht, Optics, 5th ed. (Pearson, 2017).