RP Photonics logo
RP Photonics
Encyclopedia
Technical consulting services on lasers, nonlinear optics, fiber optics etc.
Profit from the knowledge and experience of a top expert!
Powerful simulation and design software.
Make computer models in order to get a comprehensive understanding of your devices!
Success comes from understanding – be it in science or in industrial development.
The famous Encyclopedia of Laser Physics and Technology – available online for free!
The ideal place for finding suppliers for many photonics products.
Advertisers: Make sure to have your products displayed here!
… combined with a great Buyer's Guide!
VLib part of the
Virtual
Library

The Photonics Spotlight

Lossy Laser Cavities

Dr. Rüdiger Paschotta

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.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

Note that you can also receive the articles in the form of a newsletter or with an RSS feed.

If you like this article, share it with your friends and colleagues, e.g. via social media:

arrow