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Lasers & Sources
Coherent beam combination of thulium-doped fiber lasers
Coherent combination of laser beams can increase the output power while simultaneously offering improved fringe-contrast stability.
2 March 2011, SPIE Newsroom. DOI: 10.1117/2.1201101.003488
Thulium (Tm)-doped fiber lasers (TFLs) have become the latest revolution in high-power fiber-laser technology.1–6 Operating at 1.9–2.1μm, they fall into the ‘eye-safe’ laser category, thus offering potential advantages over 1μm devices. Power scaling of TFLs has rapidly gained momentum in recent years. A single-mode, single-frequency TFL amplifier yielding more than 600W output power was recently realized.5 Similarly, a two-stage TFL power amplifier producing more than 1kW was demonstrated in 2010.6Nevertheless, attempts to further increase TFL output power face great challenges, induced by nonlinear effects such as stimulated Raman and stimulated Brillouin scattering (SBS).
Coherent combination of laser beams can increase the output power while simultaneously maintaining beam quality. Several approaches have been proposed, and the robust, coherent combination of a small number of fiber lasers and/or amplifiers has been demonstrated.7–11 These demonstrations all focused on ytterbium (Yb)- or erbium-doped fiber lasers. Since many applications increasingly require high-brightness TFLs, coherent TFL beam combination is a key technology that must be further explored and developed.
First, high-power 2μm fiber-laser systems can benefit from operating at eye-safe wavelengths. This implies that the permissible power transmission in free space can be several orders of magnitude greater than at an operating wavelength of 1μm. In addition, the excellent transmission properties of TFLs in a turbulent atmosphere have been validated.12 Second, because of their longer operating wavelength, a higher SBS threshold has been predicted compared with Yb-ion-stimulated 1μm fiber lasers,6 thus potentially improving the power scaling of single-fiber light sources. Therefore, designing high-average-power fiber-laser systems requires many fewer laser channels if Tm- instead of Yb-doped fiber lasers are used. This could make the systems less complex and more compact.
Several approaches for coherent beam combination of 1μm-band fiber lasers (such as using interferometric arrays, mutual-injection locking, and active phasing based on multi-dithering techniques) can be applied directly to TFLs.13 Our experimental setup was based on using multi-dithering techniques (see Figure 1). The beam from the TFL is split into two sub-beams, one of which is coupled to the phase modulator and then sent into free space via the collimator. The other is directly coupled to the collimator. Then, part of the beam is sent to a focusing lens with a focal length of 1m that images the central lobe of the far field onto a pinhole. A photodetector is located immediately behind the pinhole. The electric signal transformed by the photodetector is used to produce the phase-control signal in the signal-processing circuit through a field-programmable gate array. The latter is programmed with a multi-dithering control algorithm. Another part of the beam is focused by a lens. An IR camera is located in the focal plane and can be used to diagnose the far-field profile of the combined beam.
Figure 1. Coherent beam combination of two thulium-doped lasers using multi-dithering.
When the control system is in open-loop configuration, we do not perform multi-dithering, and the beam phases fluctuate randomly because of fluctuations induced by the air-cooling machine, mechanical quivering, etc. The encircled power in the target pinhole fluctuates, and the intensity pattern in the observing plane keeps shifting. Figure 2(a) shows the long-exposure far-field intensity distribution. Its fringe contrast is less than 15%. When the closed-loop control system is switched on, we implement a phase-control algorithm and add phase-modulation and control signals to the modulator, so phase noise is compensated for efficiently. As a result, the intensity pattern in the observing plane is clear and steady. The fringe contrast of the long-exposure, far-field intensity distribution—see Figure 2(b)—is greater than 75%.
Figure 2. Long-exposure far-field intensity pattern of the combined laser beam. (a) Open loop. (b) Closed loop.
Since coherent beam combination can be used to increase the brightness of TFLs, it follows immediately that this technique can also be used to increase the brightness of lasers operating at different wavelengths. In fact, two laser beams with a central wavelength of 4.5μm have been successfully coherently combined using an interferometric cavity.14 A 50W continuous-wave visible laser source emitting at 589nm has also been obtained through frequency doubling of three coherently combined narrow-band Raman-fiber amplifiers.15 We believe that coherent beam combination will provide versatile solutions to increasing the brightness of laser beams of arbitrary wavelength. We plan to further investigate scaling the power of single TFLs and explore techniques to coherently combine a large array of TFLs.
Pu Zhou, Yanxing Ma, Xiaolin Wang, Hu Xiao, Jinyong Leng, Xiaojun Xu, Zejin Liu
National University of Defense Technology
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12. G. Bloom, C. Larat, E. Lallier, M. Carras, X. Marcadet, Coherent combining of two quantum-cascade lasers in a Michelson cavity, Opt. Lett. 35, no. 11, pp. 1917-1919, 2010.
13. P. Zhou, X. Wang, Y. Ma, H. Ma, K. Han, X. Xu, Z. Liu, Active and passive coherent beam combining of thulium-doped fiber lasers, Proc. SPIE
7843, pp. 784307, 2010. doi:10.1117/12.870646
14. Y. Feng, L. R. Taylor, D. B. Calia, 150W highly-efficient Raman fiber laser, Opt. Express 17, no. 26, pp. 23678-23683, 2009.
15. L. R. Taylor, Y. Feng, D. B. Calia, 50W CW visible laser source at 589nm obtained via frequency doubling of three coherently combined narrow-band Raman fibre amplifiers, Opt. Express 18, no. 8, pp. 8540-8555, 2010.