Direct answer
PDL describes transmission-loss variation with input polarization; PER describes output polarization extinction. They measure different properties. For PM assemblies or polarization-sensitive devices, separately specify wavelength, input state, axis datums and test method. Low PDL alone does not establish high PER.
Three dB values refer to different measurements
IL, PDL and PER may all appear in dB on a purchase specification. Establish their definitions before comparing values. EXFO's Application Note 061 discusses transmission under changing input polarization. OZ Optics' DTS0142 describes output extinction and axis orientation relative to connector keying.
This guide serves PM multifiber assemblies, photonic packaging and laboratory optical paths. It explains reports without assigning unverified limits to ARRANEX configurations.
| Metric | Question | Conditions to record |
|---|---|---|
| Insertion loss, IL | How much power is lost under the specified reference and input state? | Wavelength, reference, ports and input state |
| Polarization-dependent loss, PDL | How much does loss vary with input polarization? | State coverage, wavelength and method |
| Polarization extinction ratio, PER | What is the output contrast on specified analysis axes? | Input polarization, axes, keying and method |
Low PDL indicates less transmission sensitivity to input polarization. PER acceptance depends on the PM application. One dB field should not represent all three requirements.
Worked readings, not product results
Assume one wavelength, one reference and adequate input-state coverage. If loss varies from 0.80 to 1.20 dB, PDL is 0.40 dB.
In a separate linear-polarization analysis, assume a power ratio of 100 between the specified dominant and orthogonal axes. Under that power-ratio definition, PER = 10 × log10(100) = 20 dB. These calculations do not share a measurement object.
| Example | Result | Not established |
|---|---|---|
| Loss varies from 0.80 to 1.20 dB | PDL 0.40 dB | Output PER or axis angle |
| Axis power ratio 100 | PER 20 dB | Loss over all input states |
| IL 0.80 dB at one input state | That state's loss | Worst-case IL and PDL |
Confirm the instrument analysis, polarization assumptions and uncertainty in the test plan. A single input-state loss reading does not cover every polarization state.
Specify the measurement method
EXFO distinguishes polarization scanning and matrix calculation. Scanning depends on state coverage; the test system's own polarization sensitivity also needs treatment. Align methods and reference configurations before comparing report summaries.
OZ distinguishes real-time ER from worst-case ER over a recording interval. If the requirement is a minimum during operation, specify duration, placement and approved environmental conditions. An instantaneous result and a logged minimum have different meanings.
| RFQ field | Suggested definition | Acceptance purpose |
|---|---|---|
| Channels and direction | Each input-output pair | Avoid hiding a channel in an assembly average |
| Wavelength | Single point or sweep, valid passband | Bound the specification |
| Polarization | Input state, dominant axis and keying datum | Compare PER and PM configurations |
| Method | PDL scanning or matrix method; ER analysis | Establish coverage and algorithm |
| Time and state | Instantaneous or worst case, placement and temperature | Match the acceptance requirement |
Let the function determine the requirement
A component intended to preserve a polarization axis needs PER and axis review. A device intended to transmit consistently despite changing input polarization needs appropriate PDL evidence. A polarization-selective device needs limits consistent with its intended function.
Avoid assigning identical PDL and PER limits to every assembly. Have the equipment designer identify the required signal behavior, then approve per-channel tests.
Procurement FAQ
- Does low PDL prove good polarization preservation? It addresses input-state sensitivity; review output PER and axes separately.
- Is PER the transceiver modulation extinction ratio? Here PER means polarization extinction ratio. Spell it out to avoid confusion with digital high/low optical levels.
- Is PDL required after IL passes? Follow the application; one IL reading does not establish a polarization sweep.
- Should every PM fiber be recorded? Define per-channel records rather than treating one fiber as the entire assembly.
- How should differing laboratory results be compared? Align wavelength, references, polarization, axes, method and duration before reviewing uncertainty and specimen state.
Related products and reading
Product pages describe configuration scopes. Actual supply, drawings and performance require project confirmation.
Contact an engineer
Prepare interface drawings, port requirements or existing test records and contact an engineer to discuss fiber-assembly configuration and verification.
Sources and review date
Sources checked on 2026-10-14. Third-party references explain technology and specified configurations, not ARRANEX performance or certification. Purchasing tables are review recommendations; worked examples use stated assumptions.
Sources checked: 2026-10-14