Coherent Beam Combining

High power lasers at the multi-kiloWatt output power level are a critical requirement for several DEW applications. Fiber lasers are an attractive choice for such laser sources as they have near-perfect beam quality (M^2< 1.1), high wall-plug efficiency, and relative ease of thermal management. However, their output power is limited to around 1 kW due to nonlinear effects as well as thermally-induced mode instabilities. Therefore, the pathway to achieving multi-kW power levels while preserving the beam quality is through either (i) coherent beam combining or (ii) spectral beam combining. The former technique is preferred due its scalability and a relatively simple Master Oscillator Power Amplifier (MOPA) configuration. However, there are technological challenges in this approach due to nonlinear and thermal effects on the longitudinal phase of the output radiation. On the other hand, spectral beam combining has its own challenges in terms of generation of spectrally locked laser sources, precisely tuned gratings, and maintaining stable characteristics with respect to temperature over time. While spectral beam combining is being investigated at LASTEC, this proposal aims to investigate coherent beam combining with active collaboration between IITM, IISc, and LASTEC researchers.

The specific objectives of the work are three-fold:

  • Study the influence of nonlinearities and thermal effects on the temporal phase of the output radiation in high power fiber amplifiers
  • Design and development of nonlinearity managed, narrow-linewidth 500 W fiber laser modules necessary for power combining.
    • Coherent beam combining of 2 commercial fiber amplifiers with 80% efficiency, M^2< 1.2