One obstacle in the scaling of high-power fiber lasers arises because of nonlinear effects (e.g., stimulated Brillouin scattering [SBS]) due to the large intensity times length product. Efforts to raise the power threshold include:

  1. reducing the Brillouin gain by combining materials with positive and negative elasto-optic coefficients or tailoring the acoustic index to avoid guiding the acoustic wave,
  2. reducing the effective Brillouin gain by using a seed linewidth much wider than the Brillouin bandwidth,
  3. enlarging the Brillouin bandwidth relative to the seed linewidth,
  4. lowering the laser intensity by enlarging the fiber core, and
  5. minimizing the required active fiber length by pumping at the wavelength of maximum absorption and doping as heavily as possible.
Experimental setup with chirped diode laser (ChDL), distributed feedback Bragg (DFB) laser, electro-optic modulator (EOM), photodiodes (PDs), cladding mode stripper (CMS), and end cap

Conventional approaches to broadening the seed linewidth reduce the coherence length, making it difficult to coherently combine multiple fiber amplifiers. For example, a seed bandwidth of 40 GHz (coherence length in fiber = 5 mm) will require path-length matching of much less than 1 mm to maintain high coherence.

It has been shown that chirped seed amplification (CSA) in conjunction with acousto-optic frequency shifters and feedback circuitry can maintain coherence between 2 fiber amplifiers, despite path-length differences of 10–50 cm. The maximum path-length difference is limited to cΔ v maxn, where Δv max is the maximum available frequency shift, β is the chirp, and and c/n is the speed of light in the fiber. It is also limited by the intrinsic coherence length of the unchirped laser. The laser described in this report has an unchirped bandwidth of 40 MHz (full width at half maxixum [FWHM]), therefore a coherence length of 5 m.

In a tiled geometry, this ability to control the phase electronically can be used to predistort an emitted wavefront to compensate for atmospheric turbulence. CSA also leads to an SBS threshold that is fiber length independent in the long fiber limit. This approach is compatible with the other techniques for suppressing SBS, except those that increase the Brillouin linewidth. The present work describes the use of CSA with a micro-electromechanical system (MEMS)-vertical-cavity, surface-emitting laser (VCSEL) seed to scale the output power of a ytterbium (Yb) fiber amplifier to 1.6 kW, while at the same time allowing a delivery fiber of 19 m. A numerical simulation of the experiment shows that the SBS threshold could be raised to 2 kW by optimizing the pump attenuation.

This work was done by by Jeffrey O. White, Sensors and Electron Devices Directorate, ARL; Mark Harfouche and Amnon Yariv, Department of Electrical Engineering, California Institute of Technology; John Edgecumbe, Nufern; Naresh Satyan and George Rakuljic, Telaris, Inc. for the Army Research Laboratory. ARL-0204


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Stimulated Brillouin Scattering (SBS) Suppression and Long Delivery Fibers at the Multikilowatt Level with Chirped Seed Lasers

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This article first appeared in the September, 2017 issue of Aerospace & Defense Technology Magazine.

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