Er:Yb Glass vs Nd:YAG for Eye-Safe Laser Sources

Both are diode-pumped, Q-switched pulsed sources. The difference that matters is the level structure, and everything else follows from it.

Three levels against four

Nd:YAG is a four-level system. The lower laser level is effectively empty, so population inversion starts as soon as you pump. Er:Yb glass is three-level: the lower level is thermally populated, so you have to pump past a threshold before gain appears, and that threshold rises with temperature.

That single fact explains most of the practical differences. Er:Yb glass has lower gain per unit volume, so a given energy needs more glass, which means more mass and more pump current. It also limits repetition rate, because the medium has to recover between pulses.

Wavelength and eye safety

Nd:YAG emits at 1064 nm. The retina focuses it, which is why class limits bite quickly. Er:Yb emits around 1535 nm, which is absorbed before it reaches the retina, so a source can carry more energy inside the same class. That's the reason the band exists in this market.

It isn't a free upgrade. Detectors, seekers and coatings all have to work at the wavelength, and a number of legacy systems don't.

What you see in the data

A 100 µJ compact unit weighs 7 g and runs 1–10 Hz. The 2 mJ block weighs 105 g and runs 1–5 Hz. Three-level gain is why that curve is as steep as it is.

The high-repetition exception is worth knowing: at 5 kHz the energy is tens of microjoules, because you can't have both.

Choosing

Take Nd:YAG if the system already exists at 1064 nm and the safety case is covered. Take Er:Yb if you're building new and eye safety, or a lower hazard class, is a requirement in itself.

Talk through the wavelength decision

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