Every periodic spectral ripple traces back to a pair of reflecting surfaces forming a Fabry-Perot cavity — measure the period and you locate it.
| R₁ | R₂ | Depth |
|---|---|---|
| 1% | 1% | −14 dB |
| 0.5% | 0.5% | −17 dB |
| 0.1% | 0.1% | −24 dB |
| 0.05% | 0.05% | −27 dB |
| 0.01% | 0.01% | −34 dB |
| Air gap | FSR | Typical origin |
|---|---|---|
| 0.34 mm | 2522 pm | Thin-element (polarizer-class) gap |
| 1.00 mm | 858 pm | Memory anchor: 1 mm ↔ 858 pm |
| 1.57 mm | 547 pm | Lens–window gap scale |
| 6.00 mm | 143 pm | Long cavity (e.g. 1.7 mm InP chip × n3.5 equivalent) |
When periodic ripple appears on a device spectrum, measure its period (FSR) and back out the equivalent n·L cavity length, then check it against the thickness × index of every element in the package: the chip waveguide itself (InP, n≈3.5), collimating lens, window, polarizer, epoxy gaps… each candidate has a unique signature period. Example: 144 pm ripple on a 1310 nm device gives n·L ≈ 5.95 mm — exactly the round-trip of a 1.7 mm InP chip (×3.5), pointing to residual facet reflection rather than a packaging element.
For OCT / interferometric applications, spectral ripple appears in the coherence domain as a secondary coherence peak (ghost) at the equivalent path n·L, with amplitude tracking the ripple depth — which is why low ripple is a key SLD deliverable. Two engineering countermeasures: AR coating — every decade of reflectivity reduction on both surfaces cuts ripple depth by ~10 dB (table above); and tilting elements so reflections walk off the optical path — the reflected beam deviates by 2θ and walks off by L·tan(2θ), so thin (short-cavity) elements need larger tilts. Our SLD packaging uses both, and every unit is screened for ripple on an OSA.
Related tools: Coherence length · PM polarization beat · dB↔linear ratio
※ 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.