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PM Circulator: Cylindrical vs Boxed, by the Test Data

September 24, 2026

A circulator is a non-reciprocal three-port device: light entering Port 1 exits Port 2, light entering Port 2 exits Port 3, and nothing takes the path backward. Two real builds on this page do exactly that job in two very different housings — and the housing turns out to track a real difference in spec floor, not just size.

PM CirculatorPM Photonics

1550nm PM circulator, cylindrical package, with its test data sheet showing insertion loss, isolation and polarization dependent loss

A 1550 nm circulator in a Φ5.5×L38 mm cylindrical package, shown with its own factory test data sheet.

Send light in Port 1 and it comes out Port 2. Send light in Port 2 and it comes out Port 3. Try to send it back into Port 2 from Port 3's direction and it doesn't return to Port 1 — that one-way routing is what makes a circulator useful for separating an outgoing signal from its own reflections, without the loss a splitter would cost.

Two real builds, two housings

The photo above is a 3-port circulator built at 1550 nm: Φ5.5×L38 mm cylindrical package, SMF-28e fiber in 900 µm loose tube, 1.0 m pigtails, FC/APC connectors. The table below is the actual factory test data for that unit, next to a second real build at 1310 nm in a flat 100×80×10 mm box.

Parameter	1550 nm, cylindrical	1310 nm, boxed
Package	Φ5.5×L38 mm	100×80×10 mm
Fiber / cable	SMF-28e, 900 µm loose tube	Corning Ultra fiber, 2.0 mm cable
Insertion loss, P1→P2	≤ 1.2 dB (measured 1.12)	≤ 1.1 dB (measured 0.83)
Insertion loss, P2→P3	≤ 1.2 dB (measured 0.93)	≤ 1.1 dB (measured 1.00)
Polarization dependent loss	≤ 0.25 dB (measured 0.05–0.06)	≤ 0.15 dB (measured 0.02–0.07)
Isolation (minimum)	≥ 30 dB (measured 54–60)	≥ 40 dB (measured 47)
Return loss	≥ 45 dB (measured 54–70)	≥ 50 dB (measured 53–58)
Power handling	5 W max	≤ 500 mW
Operating temperature	0 to +70°C	−5 to +75°C

The boxed 1310 nm build carries a tighter spec floor across insertion loss, PDL and isolation than the cylindrical 1550 nm build — but it trades that for a much lower power handling ceiling, 500 mW against 5 W. Neither package is simply “the better one”: a fiber-laser feedback path that needs to survive watts of circulating power wants the cylindrical build's headroom; a low-power sensing or coherent-receiver path that needs the tightest possible PDL and isolation floor is better served by the boxed build.

Close-up of the same 1550nm circulator showing the FC/APC connectors and Port 3 fiber label — A closer look at the same 1550 nm unit — Port 3's pigtail is labeled directly on the fiber jacket.

Why polarization dependent loss gets its own line

Every circulator spec sheet on this page lists polarization dependent loss (PDL) as a headline parameter, not a footnote. A circulator with high PDL doesn't just cost extra dB — it costs a different amount of dB depending on the polarization state of the light passing through, which is exactly the kind of instability a coherent receiver or a polarization-multiplexed link can't tolerate. Holding PDL to 0.25 dB or tighter, as both builds here do, is what lets a circulator sit in a signal path that's already carrying polarization-encoded information without corrupting it.

Circulator test data sheet for a 1310nm boxed circulator, listing insertion loss, PDL, isolation, DIR, return loss and package dimensions — The full test data sheet for the 1310 nm boxed build.

What the boxed package looks like assembled

Boxed PM Circulator package, 100 by 80 by 10 millimeters, with FC/APC pigtails — A box-style circulator package sharing the 100×80×10 mm footprint of the boxed build above, with FC/APC pigtails brought out on flexible cable.

Specifying a PM Circulator

Wavelength band — sets the baseline insertion loss and isolation figures.
Package — cylindrical or boxed, which as shown here also tracks a real spec-floor and power-handling tradeoff, not just footprint.
Operating axis — single-axis where only one polarization state needs routing, dual-axis where both need to be preserved through the device.
Fiber, connector and pigtail length — set per leg as with the rest of the PM line.

Return loss and isolation floors are configuration-specific rather than one fixed number across the whole PM Circulator range — ask for the datasheet at your exact wavelength, package and axis configuration before locking in a design.