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## Beam combinable, kilowatt all-fiber amplifiers for directed energy

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In this chapter, several beam combining architectures suitable for DE applications have been reported, and are divided into two distinct categories: incoherent beam combination (IBC) and coherent beam combination (CBC). The purpose of each method is to increase the far field intensity when brightness and power of a single fiber amplifier is limited. In IBC, an array of fiber lasers is superimposed in the far field without control of the relative spectra or phases of the different fibers. No brightness enhancement is obtained from this method, limiting the maximum intensity in the far field. Although such beam combining has been successfully demonstrated by the Navy [5], this approach is limited to propagation ranges on the order of a few kilometers. In spectral beam combining (SBC), a separate class of IBC, incoherent beams of different wavelengths are spatially overlapped to create a single output beam of multiple colors. The brightness of the propagating beam can be increased at the cost of increased spectral bandwidth. Similarly, SBC has the advantage of not requiring active phase control or mutual temporal coherence of the individual beams. Alternatively, coherent beam combining (CBC) schemes require proper phase, frequency, and polarization relationships for efficient beam combination. Overall, due to their brightness enhancements, for longer range and higher power/ intensity DE applications, SBC and CBC are potentially more appealing.

Chapter Contents:

• 9.1 Introduction
• 9.2 Time-dependent nonlinear SBS theory and model
• 9.3 Phase modulation in kW class all-fiber amplifiers
• 9.3.1 WNS and PRBS SBS suppression comparison
• 9.3.2 WNS and PRBS coherent beam combining analysis
• 9.3.3 Filtered PRBS phase modulation
• 9.3.3.1 Filtered PRBS: coherent combining
• 9.3.3.2 Filtered PRBS: SBS suppression
• 9.3.4 PRBS re-coherence
• 9.4 Multi-kW coherent beam combining of PRBS modulated fiber amplifiers
• 9.5 Laser gain competition of all-fiber amplifiers
• 9.5.1 Laser gain competition (two-tone): power scaling
• 9.5.2 Laser gain competition (two-tone): beam combining
• 9.6 Conclusion
• Acknowledgments
• References

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