Fiber lasers are lasers where the active gain medium consists of an optical fiber doped with a rare-earth element, such as erbium, ytterbium, neodymium, or others.
The optical fibers are flexible, so the laser can be thousands of meters long for greater gain, yet takes up very little space because it can be coiled like a rope, and the large surface to volume ratio means that it's easy to cool. In addition, fiber laser are very durable and project a high-quality beam.
