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

Partial reflectors in interferometers and polarization-sensitive devices (beam splitters used in reverse) such as beam-splitting cubes are common examples of systems that combine two beams (adding beams so that they are co-linear). While these components perform beam combining, they typically are inefficient and/or limited in the number of beams that can be combined. Polarization beam combining, for instance, only works with two beams because the light has only two distinguishable states.

Figure 1. Simple comparison between coherent and spectral beam combination. (Top Panel): Coherent beam combining by matching the phase of each emitter. (Bottom Panel): Spectral beam-combining using a prism.
Coherent Beam Combination (CBC) and Spectral Beam Combination (SBC) are both capable of combining large numbers of optical beams. These methods were developed to increase overall source “brightness.” Both techniques increase the output power as the number of beams, N, and increase the far-field peak intensity as N2. For the same final aperture size, they can both provide equivalent far-field peak intensity.

Spectral Beam Combining

Figure 2. The arrangement used at Aculight to spectrally beam-combine 1,400 diode laser emitters.
Spectral Beam Combining (SBC) is a technique that spatially overlays the outputs of several laser emitters operating at specific wavelengths into a single beam. Combination is possible because each beam is distinguishable via its unique wavelength. Early forms of SBC have been used in a number of industries. In optical telecommunications, for example, the technique called wavelength division multiplexing (WDM) uses the same basic principles. Optical data channels are made co-linear on a dispersive element such as a grating. Over 80 channels have been combined in this manner and subsequently propagated in a single mode fiber. A good way to see how this works is to imagine many optical beams directed at different angles, but made to overlap spatially on a simple prism (see Figure 1).

By picking the wavelengths of each beam correctly, the beams emerge on the opposite side of the prism in a single beam where all of the input beams have been made co-linear. There are no fill factor losses (each beam is perfectly overlapped spatially). If each input beam is diffraction limited and high-quality optics are used, the combined output beam also will be diffraction limited. The spectral content of the combined beams will cover the bandwidth range of the input beams. So SBC provides diffraction-limited output, but the combined beam contains a spread in bandwidths. In order to achieve high-density combining of many individual lasers, a combining element with high resolving power, such as a diffraction grating, is required. Using such a device Aculight Corp. has combined 1,400 individual lasers into a single beam (see Figure 2).

Coherent Beam Combination

With Coherent Beam Combination (CBC), the outputs of the laser emitters to be combined are positioned side by side so that they form a single, spatially coherent larger aperture. Normally, this is accomplished by operating each laser emitter at the same wavelength and adjusting the phase of each emitter to match the others. This requires phase adjustment of each laser to a fraction of a wave.

Ideally, the result is a combined beam with narrow-band output compared to SBC and diffraction-limited beam quality. Another arrangement for CBC is called self-organizing, where appropriate feedback is provided that automatically sets the wavelength and phase of each emitter to achieve near-diffraction-limited output. In this case, the output wavelength changes in time to satisfy the cavity resonant conditions.


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