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Spectral vs. Coherent Beam Combining: How Do They Compare?

Intensity and Efficiency

Both SBC and CBC will provide near-diffraction-limited output and equivalent brightness. Take SBC first. Here, N diffraction-limited beams are made spatially overlapping and co-linear. The resulting output beam has the beam quality of any one of the input beams - diffraction-limited. Limitations are set by the ability to maintain the diffraction-limited quality of each laser emitter throughout the SBC optics, and the efficiency of the dispersive element (usually about 95 percent).

In CBC, the combined beams look like they come from one extended, diffraction-limited aperture; however, the physical separation of each of the individual emitters can lead to degradation of the beam quality, specifically the fill factor. Also, additional wave front distortion can result from errors in matching the phase from one aperture to the next.

Ideally, both CBC and SBC will provide the same far-field peak intensity for the same output aperture size and the same total power. Take a linear array as an example: for CBC, the power is increased by increasing the number of laser emitters placed side by side, N. Far-field spot diameter decreases as the number of emitters grows because the aperture is increasing with each additional laser emitter combined. The result is the well-known N2 increase in far-field intensity. With SBC, the N beams are made to overlap spatially, so the aperture size does not grow as power is added. But one must compare the far-field peak intensity for these techniques with the same output aperture. Accordingly, expanding the SBC aperture to match the CBC aperture width increases the far-field intensity by another factor of N — leading to the same N2 scaling.

Both types of beam combining exhibit comparable efficiency. Specifically, the efficiency is driven by the single-pass transmission of the optics used in the beam-combining process, and both CBC and SBC use similar numbers of optical surfaces in each beam line. In SBC devices, the largest loss element is the grating, with about 95 percent efficiency into first order. In CBC devices, there is a comparable or larger loss in far-field peak power imposed by the fill factor between adjacent beam lines.

A useful feature of SBC for some applications is that its output by definition contains a spread in spectral content, typically 5 to 20 nm, although this depends on system design. This requires that the lasers to be combined are capable of operation over this bandwidth. In practice, only a few optical elements can provide the control that sets the wavelength of each laser emitter, so complex electronic/optical controls are not required.

CBC systems have to control the phase of each beam to a fraction of a wavelength. This generally requires additional controls to maintain the correct wavelength and relative phase of each beam line. CBC beams can be steered by creating a “tilted” phase front over the full beam-combined aperture. This provides a steerable beam through electronic controls - no moving parts. (By mechanically turning the dispersive element in an SBC device, one can achieve beam-steering in one dimension.) Another significant characteristic of CBC is its generally narrow bandwidth, which may be essential to access spectroscopic features, but can be a problem in terms of non-linearities and target speckle.

Future Development

Both CBC and SBC are developing rapidly, and a complete picture of the capability of each is still evolving. There are two major areas where development will strongly influence the outcome. The first area is development of the laser emitters. Fiber laser emitters now offer large output powers with excellent beam quality and a large emitter diameter (20 to 50 μm) with a large operating bandwidth. Semi-conductor lasers also are being developed with improved specifications.

The second area involves the specific application: some require narrow bandwidth and favor CBC, while others have an advantage with broader bandwidths, so SBC is more appropriate. Just some of the applications that could benefit from these techniques include long-range illumination, sensing, high-power fiber laser pumping, fiber-delivered therapeutics, welding, cutting, and machining.

This article was written by Dennis Lowenthal, co-founder of Aculight Corporation and vice president of research and development, and Andrew Brown, Aculight’s director of business development.