Receivers: PIN, APD, TIA & coherent detection

The receiver decides how weak a signal a link can tolerate. Two kinds of photodiode do almost all the work, a pair of amplifiers turns their current into bits, and for the longest links a fundamentally different scheme — coherent detection — replaces them.

Photodiodes

A PIN diode converts each photon to at most one electron — simple, linear, wide dynamic range, low bias voltage. An APD is run near breakdown (tens of volts) so each photon triggers an avalanche of electrons, buying 5–10 dB of sensitivity at the price of temperature-dependent bias, more noise and a lower overload point. That extra sensitivity is what turns a 40 km module into an 80 km one, or lets a PON OLT hear an ONU through a 1:64 splitter.

Key parameters: responsivity (A/W, ~0.8–0.9 for InGaAs), bandwidth (must cover the symbol rate — ~7 GHz for 10G, ~25 GHz for 25G NRZ / 50G PAM4), dark current (noise floor), overload (saturation power, typically −1…+3 dBm; too much light also causes errors).

From current to bits

photodiode ──► TIA ──► LA (NRZ)  or  linear amp ──► ADC ──► DSP (PAM4) ──► CDR ──► host

• TIA (trans-impedance amplifier) converts the microamp photocurrent to a voltage with as little added noise as possible — it largely sets receiver sensitivity.
• LA (limiting amplifier) slices the waveform into clean 1/0 levels for NRZ links.
• Linear TIA + ADC + DSP — PAM4 has four levels, so the signal must stay linear until a DSP equalises and slices it; this is why PAM4 modules cost and consume more.
• Burst-mode receivers (PON OLT side) add fast gain and threshold reset between time slots from different ONUs (PON transceivers).
• Multi-lane ROSAs use 4- or 8-element PIN arrays behind a lens array or a demultiplexer, each lane with its own TIA channel — hence per-lane Rx power in DDM (Per-lane diagnostics).

Sensitivity, OMA and FEC

Sensitivity is quoted for a target bit-error ratio: 10⁻¹² for classic NRZ links, but for 25G+ and PAM4 the spec point is the pre-FEC BER (e.g. 2.4×10⁻⁴) that RS-FEC can correct — so a "−10 dBm sensitivity" 100G module is only usable with FEC on (FEC & link training). Multi-mode specs often state sensitivity in OMA rather than average power; the difference can be ~3 dB (Calibration).

Coherent detection

Beyond ~80–120 km, or at 100G+ per wavelength, direct detection runs out of budget. A coherent receiver mixes the incoming light with a local-oscillator laser in a 90° optical hybrid and detects both polarisations on balanced photodiode pairs; ADCs digitise the result and a DSP recovers amplitude, phase and polarisation — and compensates chromatic dispersion and PMD digitally. Sensitivity improves by 15–20 dB and the modulation can be QPSK or 16/64-QAM. This is the receiver inside 400ZR/ZR+ and transport modules (Coherent optics).

SOA-assisted receivers

Some long-reach PAM4 modules (400G ER, some 100G) put a semiconductor optical amplifier ahead of the photodiode to pre-amplify the light — a middle path between an APD and coherent, at extra power and cost.

What the receiver tells you in DDM

• Rx power is the photodiode's own measurement — the one DDM value that is nearly always real (DDM levels).
• The low Rx alarm threshold is roughly the receiver's sensitivity; the high Rx alarm is its overload (Thresholds & alarms).
• LOS asserts when Rx falls below the loss-of-signal level — a hardware comparator on the TIA output, faster than any DDM read.
• A receiver degrading (APD gain drift, TIA ageing) shows as rising errors at normal Rx power — DDM alone will not catch it; pair it with error counters.

In CodingBox

The DDM screen shows Rx power per lane against the module's sensitivity and overload thresholds, and the Check transceiver screen decodes the reach and rate codes that imply a PIN or APD receiver. When recoding a module, keep those codes honest to the receiver it actually carries.

Sensitivity and overload windows per interface, and how to size an attenuator from DDM readings: Rx power & link budget, Typical values.