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.

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.