Most conventional lasers use a single amplification material to produce a single output beam. To increase the output power, more energy has to be pumped into the amplification material and this can lead to non-linearity and thermal distortion that make the output beam quality degrade. These laser types are not easily scalable to higher powers.
A more scalable way to reach high powers is to combine more than one radiation source. In its simplest form, the light from several lasers is combined incoherently. In this case, the output power is simply the sum of the powers of the individual lasers. However, more intense beams are created by coherently combining the laser beams.
Coherent addition needs the beams to interfere with each other and a simple way to do this is to generate all the beams from the same seed source. The light from the seed is divided into separate beams which are individually amplified to higher powers. Additionally, the phase of each beam has to be controlled to ensure that resulting beam intensity is maximised. This type of laser arrangement is particularly suited to fibre lasers where all of the components for the seed laser, amplifiers, phase controllers and splitters are already available commercially. Fibre lasers also have the advantages of high efficiency, good beam quality and simple thermal management.
Having created the array of sources, the beams have to be very accurately collimated and aligned to ensure that the light combines as efficiently as possible. Below is an example of a multi-lens assembly that was used to collimate and align fibre lasers in a laboratory demonstration. This all-silica head is stable thermally and particularly suitable for high optical powers. The position of the core of each fibre had to be positioned to within one or two microns in the array of holes at the back to ensure that beam combination efficiency is kept high.
Image from “A multi-channel phase locked fibre bundle laser”, David C. Jones ; Andrew J. Turner ; Andrew M. Scott ; Steven M. Stone ; Roy G. Clark ; Christopher Stace ; Craig D. Stacey, Proc. SPIE 7580, Fiber Lasers VII: Technology, Systems, and Applications (February 17, 2010).