Coherent beam combining

High-power fiber lasers have become central to both industrial and defense-oriented applications, thanks to their compact designs, relative efficiency, and low running costs. Yet, despite these strengths, scaling a single fiber laser to very high output powers remains challenging: thermal effects, nonlinear scattering (like stimulated Raman and Brillouin processes), and transverse mode instability (TMI) all impose upper limits on how much power one fiber can produce while maintaining good beam quality. Although single-mode fiber lasers have already reached output powers in the 6–10 kW range, and theoretical predictions go up to 40 kW, pushing beyond that using a single fiber amplifier becomes increasingly difficult and expensive. That is why beam-combining techniques have emerged as the most promising route to ultra-high-power lasers.

Beam combining techniques

Several strategies exist to merge multiple fiber lasers into one beam, each with its own advantages and prospects:

1. Side-by-side (incoherent) beam combining

The simplest method is to mount multiple lasers next to each other and point them at the same target. Often called “incoherent” or “side-by-side” combining, this achieves higher total power by brute force. This approach is straightforward—no phase locking is needed—and it is robust against channel failures because each beam can still function independently. However, combining all that light in one tight focal spot is difficult. The final “power on target” is limited by the larger effective beam size, which becomes a drawback when long-range precision is required. Systems of this sort have topped 30 kW but more recent versions, primarily for defense, have been trying to push to even higher powers.

2. Spectral beam combining (SBC)

A more sophisticated approach is to run each laser channel at a slightly different wavelength and then merge all outputs into a single beam via wavelength-selective optics such as diffraction gratings or dichroic mirrors. This technique, called spectral beam combining (SBC), keeps a high brightness in each channel and essentially stacks those distinct wavelengths into one combined output. SBC has already demonstrated outputs at the multi-kilowatt level—there are reports of 60 kW class fiber-laser systems realized by SBC of 96 individual fiber lasers, with beam quality that remains suitable for longer-range applications (better than side-by-side combining allows). Still, because each channel has its own wavelength, fine-tuning the system for certain optical applications—especially those requiring a single narrow line—can be more complicated.

3. Coherent beam combining (CBC)

Among all power-scaling methods, coherent beam combining (CBC) holds out the promise of the highest beam quality at high power. It is based on the idea that if multiple lasers operate at (nearly) the same wavelength and are locked in phase, their outputs will interfere constructively into a single near-diffraction-limited beam. In principle, this provides both a path to tens or even hundreds of kilowatts of power and the small, intense focal spots needed for applications such as materials processing and long-range directed energy.

In CBC, a seed laser is split into multiple channels and amplified in parallel fiber amplifiers. Each channel must remain phase-aligned with the others so that, when recombined, the beams undergo constructive interference. This phase alignment can be monitored and maintained with a feedback control system that detects any small phase error and corrects it in real time.

The more channels are in a CBC system, the more vital it is to have precise and adjustable delay in each beam path. A typical CBC system might use short fiber pigtails, piezo-mounted mirrors, or free-space delay arms to tweak path lengths. But accurate alignment often demands a dedicated fiber-based or free-space optical delay line. This is especially true for pulsed lasers, where poor temporal alignment can lead to complete loss of the pulse overlap. By tuning a motorized reflector inside the delay line, the system compensates for minute phase differences and ensures the necessary synchronization. Often the optical delay lines are combined with some kind of phase modulator which performs fine adjustments of the optical path by changing the refractive index of the propagating medium.

Because in CBC systems all beams share the same optical frequency and are “locked” to the same wavefront, theoretically nearly all the power can be concentrated into a single diffraction-limited spot. In practice, tens of kilowatts have been achieved in continuous-wave (CW) operation with potential to scale further. A notable milestone is a 100 kW-class CBC system created from slab lasers.

Recently, CBC systems operating at 2 µm wavelength have gained more attention, especially in defense applications. This is because at longer wavelengths (1.4 - 3 µm) the risk of permanent eye injury is reduced relative to 1 µm sources as well as the likelihood of collateral damage or ignition hazards. Modern Tm-based fibre lasers in this region can achieve multi-kilowatt outputs, and by employing coherent beam combining (CBC) the 2 µm systems can meet both the power requirements and stringent safety standards critical for modern directed energy weapon applications, including C-UAS (Counter-Umanned Aerial System) missions.

FODL MDX - a modular solution from OptiXs

19” rack compatible fiber optic delay lines

In OptiXs we have developed a modular system of fiber optic delay lines realized in the form of inserts compatible with the 19” rack system. The delay lines come with a controller which enables a computer based remote operation - for basic use we include an intuitive customized web interface, while ethernet and USB connections provide options for more advanced communication and automatization. Up to 8 delay lines can be driven by a single controller. Together with a state-of-the-art optical performance, our system of fiber optic delay lines is optimized for coherent beam combining applications.

Find out more about our delay lines here.