Periodic ripple on the OSA again? Before tearing the device apart — if the period matches the PM-fiber beat, it's a measurement artifact.
Light traveling through polarization-maintaining (PM) fiber accumulates a phase difference between the TE and TM modes because of the core's intrinsic birefringence (Δn ≈ 10⁻⁴ class). The OSA's grating has polarization-dependent loss, weighting the two modes differently — and periodic "ripple" appears on the spectrum. It is not an FP cavity in the package but a measurement artifact, routinely misdiagnosed as a device defect.
① Match the period: use the reverse calculator to get Δn·L from the measured period; if it agrees with "known Δn × pigtail length" (e.g. 1.5 m of standard PANDA at 1310 nm gives ≈2.5 nm), suspect the beat. ② Move the fiber: gently twist or bend the fiber, or add a polarizer before the OSA — a beat artifact keeps its period constant while the amplitude changes visibly; a real FP ripple changes neither. Once confirmed, fix the fiber layout during measurement or fuse out stress points. Use together with FP ripple diagnostics as mutually exclusive hypotheses.
Related tools: Spectral ripple & FSR · Δλ↔Δν converter
※ Formulas on this page assume ideal models; all device parameters shown are typical values — refer to the datasheet and the serialized factory test report shipped with each unit. For selection support, contact sales@lncetek.com.