Tx bias current and laser ageing

Of the five DDM monitors, laser bias current is the one that predicts the future. A laser wears out by needing more current for the same light; the module's control loop hides this from the Tx power reading until the very end, but the bias value shows it from the first month. This page explains the physics behind the number, what "normal" looks like per laser type, how to read a trend and when to plan a replacement.

How a laser is driven

A semiconductor laser starts emitting above its threshold current I_th; above it, output rises almost linearly with current (the slope efficiency, mW/mA). The transmitter sets a bias current above threshold and adds a modulation current for the ones and zeros. DDM reports the bias (SFF-8472 A2h 100–101, 2 µA per LSB), not the modulation current.

Most modules run an automatic power control (APC) loop: a monitor photodiode watches the output and the driver adjusts bias to keep average power constant. That is why Tx power in DDM stays flat for years while bias creeps up — and why a flat Tx power tells you nothing about ageing.

What moves the bias

Cause	Direction	Time scale	Reversible
Temperature — threshold current grows exponentially with temperature (characteristic temperature T₀ ≈ 50–70 K for DFB)	up ~1 %/°C for uncooled DFB; VCSEL less until roll-over	minutes	yes — normalise before judging
Ageing — defects in the active region reduce efficiency	up, slowly	months to years	no
Cooling (TEC) set point	none on bias; visible in TEC current	—	—
Coupling loss / facet contamination — APC compensates for less light reaching the fibre	up	sudden or slow	sometimes (cleaning of a pigtail-less module: no)
Fake or frozen value	constant	—	—

Typical ranges per laser type: Typical values.

Wear-out vs sudden death

Mode	Mechanism	DDM signature	Where
Gradual wear-out	dark-line defects, facet oxidation, dopant diffusion; Arrhenius — lifetime roughly halves per +10 °C	bias rising 20–50 % over years, then Tx power finally sagging, then Tx fault	all laser types; fastest on hot, high-power modules
Catastrophic optical damage (COD)	facet melts at high optical density	instant Tx loss; bias may jump to the driver's limit	high-power FP/DFB (ZX, ER, PON OLT)
ESD / surge	junction damage	Tx dead or power halved from one reading to the next	any; often handling-related
Driver or APC failure	electronics	bias 0 or at maximum, Tx flags	any
TEC failure	laser overheats or wavelength drifts	laser temperature alarm, TEC current at limit, DWDM channel drifts out of the filter	cooled DFB/EML

Failure statistics and batch effects: Failures.

End-of-life criteria

Manufacturers test lasers to standards such as Telcordia GR-468 and define end of life by a change from the initial value at constant output power:

Criterion	Typical EOL definition
Bias current	+20 % (strict, telecom) … +50 % (common) above the value recorded at installation
Threshold current	+50 %
Output power at fixed current	−2 … −3 dB

For monitoring purposes: a 15–20 % rise from baseline is the point to schedule a replacement; +50 % or a bias that has hit the driver's ceiling means the module is living on borrowed time (Monitoring).

Reading a trend correctly

Normalise for temperature — compare readings at similar module temperature, or plot bias against temperature and look at the residual. A +10 °C summer excursion raises DFB bias by ~10 % on its own.
Compare with the module's own baseline, not with another module: two identical part numbers can differ by 30 % in bias from day one.
Watch the slope, not the level — a bias that is high but flat is a laser that has always needed more current; a bias climbing 1 % per month is a laser dying.
Check Tx power last — when APC can no longer compensate, Tx power drops and the far end's Rx falls with it. By then the bias curve has been telling the story for months.
Multi-lane modules — compare the four or eight lanes; one lane ageing faster than its siblings is the common failure mode (Per-lane diagnostics).

Laser types and their ageing

Laser	Ageing behaviour	Notes
850 nm VCSEL	very long life at low current; sensitive to high temperature (thermal roll-over) and ESD	bias 4–9 mA; a VCSEL at 12 mA is suspicious
FP 1310 nm	robust, cheap; mode hops show as small power steps, not bias	1G LX, GPON ONU
DFB uncooled	steady creep; strong temperature dependence	10G LR/ER, CWDM, 25G LR
DFB/EML cooled	bias stable because TEC holds temperature; ageing shows as bias creep at constant laser temperature; TEC current reveals ambient problems	DWDM, ER/ZR, 100G LR4
Silicon-photonics CW laser	one high-power laser shared by lanes; its failure takes the whole module	400G DR4/FR4

Component details: Lasers, Laser evolution.

Bias as a fingerprint

Because bias differs from laser to laser, a constant bias across a batch of modules is as telling as a constant Tx power: the value is written into memory, not measured (DDM levels). A real module's bias moves with temperature within minutes of insertion.

In CodingBox

The DDM screen logs bias against time and temperature; a module read at incoming inspection and again when pulled shows its whole life in one chart. Storing the installation reading in the code database provides the baseline the end-of-life criteria above depend on.