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Transceivers do wear out — lasers age, and heat accelerates it. Knowing the signature of a dying module lets you replace it before the link does it for you.

How a laser dies

Laser degradation shows up as a drop in optical output of 20–30% from its original value. The tell-tale signs, in order of appearance:

• Rising Tx bias current — the module drives the laser harder to hold its power.
• Unstable output power — Tx wanders instead of holding steady.
• Rising pre-FEC BER on links that use forward error correction.
• Finally, a sudden current spike and Tx dropping to zero.

What failures correlate with

Field experience is consistent: failures correlate far more with production batch and operating temperature than with brand.

A widely discussed case: 1000BASE-SX modules dying at one or two a month — working yesterday, today a current spike and zero Tx — while 20-year-old modules in the same network kept running. The culprits identified were specific batches, a room that ran +4 °C hotter after an air-conditioning fault, and the dense thermal environment of 48-port switches.

The hidden failure in quad modules

In 40G/100G modules built from four lanes, the classic silent failure is Rx degradation on a single lane. A LACP bundle or the module's own aggregation masks it: the link stays up, throughput quietly drops, and nothing alarms. Check per-lane DDM, not just the module total.

Counterfeits

Counterfeit modules are discussed less than "bad batches", but they exist. A clone copies the OEM's EEPROM contents and usually passes an initial test — then fails after a switch firmware upgrade brings stricter authentication, or degrades quickly because of a cheap laser.

Practical detection:

• DDM trends — a clone's laser ages fast; watch bias current and Tx over weeks.
• Serial numbers — duplicates across "different" modules are a red flag.
• Compare with a known-good read of the same part in your code database.

What to do

1. Trend DDM over time, not just at installation — see DDM levels.
2. Keep spares by batch; when one module from a batch fails early, watch its siblings.
3. Manage temperature — a few degrees matter in a full 48-port chassis.
4. Read per-lane values on quad modules.

In CodingBox

The DDM screen shows bias current and Tx/Rx per lane with the module's thresholds, logs measurements over time and exports CSV — the trend data that reveals a degrading laser. Every read lands in the code database, so a module can be compared with an earlier state of itself or with a known-good unit of the same part.

How bias current announces a wear-out failure months ahead, and the end-of-life criteria manufacturers use: Tx bias & laser ageing.

Which factory steps prevent these failures — chip screening, alignment, three-temperature calibration, burn-in — and what low-cost producers skip: Manufacturing & testing.

Label fields, latch colours and regulatory marks to compare against the EEPROM when a counterfeit is suspected: Labels & colour codes.