# A Lunar-Orbiting Satellite Constellation for Wireless Energy Supply
## 3 System Architecture and Key Technologies
### Power Scaling by Laser Beam Combination  
There are numerous ways of combining laser beams [Figure 5], and some are better suited to the application than others. In the case of spectral-beam-combining methods, beams of different wavelengths are combined through simple but scalable optical set-ups. The use of diffractive elements (gratings) rather than refractive elements (prisms) is more suited for high-power applications, as they offer more flexibility in design and allow the substrate to be cooled, providing a means to combine closely separated wavelengths while also facilitating thermal control. Another type of system that is important in long-range applications are tiled-aperture systems, in which the beams are co-aligned but are not co-axial. The assumption is that at long range the natural diffraction spreading of the individual beams will ensure that they overlap and form a far-range beam that can be more or less uniform. The tiled approach provides more flexibility than a full aperture system with co-axial beams, and facilitates scalability. It is also not restricted to spectral combining, a factor that is not strictly necessary or required in power transmission systems. The approach to tiled aperture combining systems depends on whether or not the beams are coherent. If the beams are coherent (meaning they require mutual coherence of the combined beams), this means that their relative phase can be controlled such that the wavefronts effectively form a continuous single larger wavefront, and the far-field intensity pattern is set by the diffraction of this single larger wavefront. The consequence is that the intensity of the beam increases as \(n^{2}\), with \(n\) being the number of combined beams. For the incoherent case, the far-field intensity pattern is simply the sum of the individual intensity patterns, and the intensity scales only as \(n\). The beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Ref. [16] presents a solution to an all-fiber coherent-beam-combining optical system towards a higher output for the fiber laser. Clearly the advantages of coherent beam combining can be profound, although in reality the wavefront has discontinuities which have the effect of superimposing \"ring\" structures on the far-field pattern, smearing some of the power into side-lobes of the main beam. This effect may be significant in the application of power transmission over very long distances, in the case that coherent combining methods prove to be suitable for the application. For any tiled-aperture method, it can be demonstrated that there is an optimal spacing between the beams which results in a maximum intensity in the far field, and this will have to be included in future works. To provide the constructive interference that will yield the \(n^{2}\) intensity increase, the beams must be brought into phase alignment, and typically the phase deviations need to be well below 1 rad root mean square. In a laser-beam-combining system on a spacecraft in orbit around the Moon, the stability  
Figure 5: Coherent and incoherent combination. requirements for the satellite platform would likely be too strict to be feasible. If this proves to be true, the system would have to be designed to work with an incoherent combination of laser beams that preserves the quality and diffraction limit of the single beams as much as possible. In [34], a practical example of a tiled-aperture high-power coherent beam combiner is given, in which the fibers to be combined are individually positioned to an accuracy of 1 micron. There are some general rules to be considered, but there are likely to be various ways of achieving the final aim, and the optimal solution will depend upon many driving parameters such as cost, performance, power, waveband, etc. For this particular application, new challenges also arise from the environment in which the system has to operate.