Temperature and supply voltage

Temperature and Vcc are the two DDM monitors people skip — until a module that "was fine" flaps every afternoon, or a batch dies a year early. Temperature drives laser ageing, wavelength, bias and receiver sensitivity; voltage decides whether the module's electronics work at all. This page explains what the two numbers mean, what changes them, what they change in turn, and what to do with them.

Temperature

What is measured

The sensor sits on the module PCB or inside the controller — not on the laser die. It reads a few degrees above the case and well below the laser junction. Case-temperature ratings in datasheets refer to the housing; the DDM value tracks it with an offset.

Grade	Case temperature	Where
Commercial (C-temp)	0 … +70 °C	most data-centre and enterprise optics
Extended (E-temp)	−5 … +70 or −10 … +80 °C	many long-reach and PON modules
Industrial (I-temp)	−40 … +85 °C	outdoor, cell-site, industrial switches

Typical readings: 20–35 °C on a bench, 25–55 °C in a populated switch, 60–70 °C in a dense 1U faceplate with poor airflow or with a 10GBASE-T copper neighbour.

What temperature changes

Effect	Magnitude	Consequence
Laser wavelength	DFB ≈ 0.08–0.1 nm/°C; VCSEL ≈ 0.06 nm/°C; FP ≈ 0.3–0.5 nm/°C with mode hops	CWDM (20 nm grid, ±6.5 nm filter) tolerates an uncooled DFB over 0–70 °C; DWDM (0.8 nm grid) does not — hence TEC-cooled lasers (Wavelength bands)
Threshold and bias current	≈ +1 %/°C for DFB	APC raises bias; see Tx bias & laser ageing
Output power at fixed current	falls with temperature	hidden by APC until the driver limit
Receiver sensitivity	degrades 0.5–1 dB from 25 °C to 70 °C; APD gain is strongly temperature-dependent (compensated internally)	marginal links fail on hot days
Laser lifetime	roughly halves per +10 °C (Arrhenius)	a module at 65 °C ages about four times faster than one at 45 °C
DDM accuracy	calibration holds within spec over the operating range; outside it, readings drift	trust less near the limits
Module behaviour at the high alarm	flags, then vendor-dependent: throttling, Tx squelch, or nothing; hosts may disable the port	Power & thermal

Where the heat comes from

The module's own dissipation: 0.8 W (SFP+ SR) to 15–20 W (800G, coherent ZR).
Neighbours in a dense faceplate — the hottest cage is usually the middle one.
Airflow direction mismatch (front-to-back chassis with back-to-front modules), blocked filters, missing blanking panels and empty cages without dust caps.
Copper SFP+ (10GBASE-T) at 2.5–4 W in a cage designed for 1.5 W heats itself and both neighbours.
Enclosure: a PON stick inside a home router or a module in a sealed outdoor box runs 15–25 °C above ambient.

Thresholds

Typical factory thresholds for a commercial module: high alarm 75–80 °C, high warning 70 °C, low warning −5 … 0 °C, low alarm −10 … −5 °C. A module living at 68 °C is not "fine because no alarm" — it is at the edge of its rating with a quartered lifetime (Thresholds & alarms).

Cooled lasers: laser temperature and TEC current

DWDM, many ER/ZR and 100G LR4 modules carry a thermo-electric cooler. Two optional monitors describe it: laser temperature (the set point, typically 25–45 °C, held to ±0.1–0.5 °C) and TEC current (signed: positive = cooling, negative = heating, in percent of maximum or mA). A TEC near 100 % means the module is fighting its environment and the wavelength is about to drift out of the filter; a laser temperature that no longer holds the set point means the TEC has failed (Tunable transceivers).

Supply voltage

What is measured

Vcc is measured inside the module, after the connector, the inrush limiter and the filter — typically 50–150 mV below the host rail. Specification: 3.3 V ± 5 % (3.135–3.465 V). Typical thresholds: low alarm 2.9–3.0 V, low warning 3.1 V, high warning 3.5 V, high alarm 3.6 V.

What voltage tells you

Reading	Meaning
3.20–3.40 V, stable	normal
3.10–3.20 V	host rail is weak or the port is loaded (high-power module, many neighbours); watch under full traffic
< 3.1 V or dropping under load	brown-out: resets, LOS bursts, Tx fault, "module not detected" cycles; a host power problem, not an optics problem
> 3.47 V	host regulator fault; may shorten module life
0.00 V or 6.55 V	calibration or read error, not physics (Accuracy & limits)
Several modules on one card all low	the card's 3.3 V rail or its connector; typical after adding high-power modules to an old line card

Vcc rarely explains a slow degradation but often explains intermittent faults that follow traffic load or module insertion elsewhere on the same card. It is also the first thing to check on the bench when a module is "dead": a programmer or cradle that cannot supply the module's power class shows it as low Vcc or a module that resets during reads (EEPROM read & write errors).

Checklist

Read temperature and Vcc together with the optical values; log them.
Compare temperature with neighbouring modules — one hot module in a cool row is its own problem (high-power class, fault); a hot row is airflow.
Correlate flaps with the time of day and with temperature trends.
For cooled modules, watch TEC current and laser temperature, not just case temperature.
Correlate Vcc dips with traffic peaks and with insertion of other modules.
Keep modules under 60 °C where you can — it is the cheapest life-extension there is.

In CodingBox

DDM shows temperature and Vcc against the module's own thresholds and logs them; the optional laser-temperature and TEC-current monitors appear for modules that implement them. Thresholds are editable in the EEPROM editor — tightening a temperature warning to the level your environment should never reach is a legitimate use.