Coherent Addition and Combining Beams
Scalability can also be achieved by combining separate laser beams. Completely independent beams cannot usually be combined to produce a beam with higher radiance than each beam has alone. Beams can only be combined if they are coherent with each other. Such beams can be combined actively or passively.
In the passive combining (or coherent addition) of lasers, only the few modes common to all of the combined lasers can be above the lasing threshold. Efficient passive combining of eight lasers has been reported. Further power scaling requires exponential growth of the gain bandwidth and/or length of the individual lasers.
Active combining implies the real-time measurement of the phase of individual lasers' output, and quick adjustment to keep them all in phase. Such adjustment can be done by adaptive optics, which is effective for suppression of phase noise at acoustic frequencies. Faster schemes based on all-optical switching are being researched.
Read more about this topic: Power Scaling
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