Editing Openai/69109937-2170-8005-9304-da49840bb5b9 (section)

==== From neutral sources (e.g. Physik Instrumente, EPIC/AEMtec): ====
* Passive alignment uses highly precise mechanical features (etched pockets, fiducials, stops). It is: - Simple and cheap - Great when tolerances are “moderate” - But can struggle when you need sub-0.2 µm precision, especially in some cutting-edge silicon photonics couplers. pi-usa.us<ref>{{cite web|title=pi-usa.us|url=https://www.pi-usa.us/en/tech-blog/photonics-alignment-techniques-active-passive|publisher=pi-usa.us|access-date=2025-11-10}}</ref>
* Active alignment: - Moves parts in x/y/z/θ/φ while a detector measures optical power in real time - Can optimize whatever coupling interface you have and get very close to the theoretical maximum - Is slow and expensive in high-volume production

State-of-the-art fiber–chip couplers in research:
* Typical commercial silicon-photonics grating couplers: often ~1–3 dB loss (50–80 % efficiency). opg.optica.org<ref>{{cite web|title=opg.optica.org|url=https://opg.optica.org/prj/abstract.cfm?uri=prj-7-2-201|publisher=opg.optica.org|access-date=2025-11-10}}</ref>
* Advanced multi-layer grating designs in foundry processes: ~4.7 dB loss for very broadband/dual-pol devices (≈34 % efficiency) – optimized for bandwidth and manufacturability, not just minimal loss. opg.optica.org<ref>{{cite web|title=opg.optica.org|url=https://opg.optica.org/abstract.cfm?uri=oe-30-17-31058|publisher=opg.optica.org|access-date=2025-11-10}}</ref>

So my earlier statement “active alignment generally results in better coupling efficiency” is only strictly true if:
* You use the same optical structure and
* Compare “free” active alignment vs passive placement using no specially designed features.

As soon as you specifically engineer an interposer with precision pedestals, mode-matching structures, mirrors, etc., passive alignment can get very close to what active alignment can do — and sometimes within ~0.2–0.5 dB.