Pump diode lifetime, not fiber, dominates source economics

Ask what limits a fiber laser's service life and the intuitive answer is the fiber. In practice, doped gain fiber in a properly sealed source is remarkably stable. The wear item is the bank of pump diodes that supply optical energy to it.

How diodes age

Semiconductor laser diodes degrade gradually rather than failing outright. Output at a given drive current declines over time, so the control system raises current to hold power constant, which increases junction temperature, which accelerates degradation. End of life is usually defined as the point where rated power can no longer be reached within safe current limits.

The degradation rate is strongly temperature dependent. A commonly cited rule of thumb is that every ten degrees of junction temperature reduction roughly doubles expected lifetime — which makes thermal design the single largest determinant of how long a source lasts.

What this means operationally

Rated lifetimes of 100,000 hours assume the diodes are held at their design temperature. Real installations frequently do not achieve that. A chiller with fouled heat exchangers, a coolant loop with degraded flow, an ambient temperature above specification in an unventilated enclosure, or a filter nobody changed — each raises junction temperature and each shortens life in a way that shows up years later as a source that no longer reaches full power.