Lossy Laser Cavities

Posted on 2007-02-22 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2007_02_22.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on output coupling efficiency, wall-plug efficiency

What kind of efficiency would you expect from a laser where you have 50% parasitic losses for the light circulating in the laser resonator? Probably not a very good one, right?

Well, things are not necessarily that bad in such a case. Imagine a simple fiber laser, where the output coupler is just a bare (and perpendicularly cleaved) fiber end, providing some 4% reflection, while the other end of the rare-earth doped fiber has a reflector with 50% losses. Assuming no other parasitic losses, the round-trip gain under steady-state lasing conditions must be ≈17 dB, so the single-pass gain is 8.5 dB. Compared with the power hitting the output coupling end, the power hitting the lossy reflector end is down by some (14 - 17/2) dB = 5.5 dB, and half of that power is lost. So for every watt hitting the output coupling end, we get 0.96 W coupled out and 0.14 W lost at the other end. The resulting output coupling efficiency is 87% – actually not that bad!

The key conclusion is that the importance of intracavity losses for the power efficiency (see also: wall-plug efficiency) strongly depends on the available laser gain, which determines how high the output coupler transmission can be.

As an interesting detail, note that 50% losses for the circulating light do not necessarily mean that 50% of all generated light power is lost. In a high-gain laser resonator, the 50% loss may apply to some power which is far lower than that hitting the output coupler mirror.

These aspects are important for high-gain lasers, such as many fiber lasers: these can be very tolerant to intracavity losses. On the other hand, one then often also requires quite massive influences e.g. to achieve wavelength tuning, single-frequency operation, or mode locking.