In co‑packaged optics bring‑up, what design‑for‑test hook most improved your ability to localize coupling loss and BER issues quickly at wafer or module test? Which specific loopback, tap monitor, or PRBS/BERT setup proved decisive and how would you instrument it next time?

The most effective design-for-test hooks to localize coupling loss and BER are per-lane loopbacks at the optical interface, low-loss tap monitors feeding fast photodiodes, and integrated PRBS/BERT engines with real-time error counters and optical power monitors. Implement short per-lane optical or electrical loopbacks that isolate the fiber or module coupling from the transceiver front end so you can tell whether errors originate in the coupling interface or in the PHY. Place small taps at the module input, immediately after the coupling interface, and at the on-chip optical interface so you can compare power and detect wavelength or alignment dependent loss. Run continuous wide-pattern PRBS with live BERT counters to capture BER trends and error bursts rather than single snapshot measurements. Next time I would instrument switchable per-lane loopback switches, route monitor PD outputs to ADCs and the test controller, enable on-die PRBS generators and counters, and correlate BER with optical power and temperature logs for fast root cause isolation.