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Coherent Beam Combining

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Definition: a class of methods for beam combining, requiring mutual coherence of the combined beams

Categories: lasers, methods

How to cite the article; suggest additional literature

The term coherent beam combining (also called coherent beam addition) denotes one class of techniques within the more general technique of power scaling by beam combining. The goal is to combine several high-power laser beams so as to obtain a single beam not only with correspondingly higher power but also with more or less preserved beam quality and thus with increased brightness. Coherent combining also preserves the spectral bandwidth. An alternative class of techniques, discussed in a separate article, is spectral beam combining.

Side-by-side Combining Versus Filled-aperture Techniques

Techniques of coherent beam combining can be subdivided into

techniques for coherent beam combining

Figure 1: Techniques for coherent beam combining. a) Example of tiled-aperture combining, where the outputs from four fiber amplifiers are combined into one beam with a large area. Due to the synchronized optical phases, the overall beam has a reduced beam divergence. b) Example for filled-aperture combining, using a diffraction grating.

A special case of a filled-aperture technique is coherent polarization beam combining [21]. Here, one can have only two input beams. However, as the output polarization is linear, one can repeat the combination as required.

Techniques for side-by-side combining may have been inspired by the earlier implementation of phased-array antennas in radio frequency and microwave transmitters and receivers. In the optical domain, the realization is more difficult due to the much smaller wavelength, which introduces correspondingly tighter mechanical tolerances.

In any case, mutual coherence of the combined beams is essential; typically the relative phase deviations need to be well below 1 rad r.m.s. This is illustrated for both mentioned sub-classes with two examples, which may not be ideal in a practical sense but conceptually simple:

Apart from phase coherence, the beams involved must have a stable linear polarization, and the amplitude fluctuations should also not be excessive.

Methods for Obtaining Mutual Coherence

There are a variety of techniques to obtain the mutual coherence, which are briefly summarized in the following:

There are also techniques of passive beam combining where the input lasers automatically get into mutually coherent oscillation (via some feedback or nonlinear interactions) even though they not single-frequency lasers. However, single-frequency operation is typically required for actively stabilized laser arrays.

A special case is the non-collinear coherent superposition of ultrashort pulse beams [24]. Here, the term beam combining should actually not be used, since one only exploits the superposition of two clearly separate beams in the region around their foci. For some applications, that can be sufficient.

Coherent Beam Combining with Non-monochromatic Waves

Coherent beam combining is mostly done with monochromatic waves. However, it can also be applied to non-monochromatic input beams, as long as these are mutually coherent. For example, ultrashort pulses, having a broad optical spectrum, can be coherent combined [22]. It is then required that the path lengths are exactly matched such that the temporal peaks of the contributions of all input beams to the output occur at the same time. The broader the optical bandwidth, the more critical is that delay matching. For very broadband pulses (disregarding whether they are temporally stretched or not), arm length differences of only a few micrometers or even less are acceptable.

General Remarks

Overall, methods for coherent beam combining have not been very successfully applied, although many different approaches have been investigated. The main difficulty is to obtain phase coherence at high power levels in a sufficiently stable manner, working not only in a quiet laboratory environment but also in a mechanically more noisy industrial setting. Another challenge is the need to match precisely and stably wavefronts and polarization directions. Schemes using single-frequency signals and high-power fiber amplifiers may require additional measures to suppress problems with stimulated Brillouin scattering (SBS). In tiled-aperture systems, some degradation of beam quality is caused by a fill factor of less than unity, which leads to side lobes in the far-field beam pattern. In comparison, systems relying on spectral beam combining are more tolerant in various respects, but coherent combining may be used e.g. if a narrow emission spectrum is required.

Bibliography

[1]E. M. Philipp-Rutz, “Spatially coherent radiation from an array of GaAs lasers”, Appl. Phys. Lett. 26, 475 (1975)
[2]D. R. Scifres et al., “Phase-locked semiconductor laser array”, Appl. Phys. Lett. 33, 1015 (1978)
[3]D. G. Youmans, “Phase locking of adjacent channel leaky waveguide CO2 lasers”, Appl. Phys. Lett. 44, 365 (1984)
[4]M. Oka et al., “Laser-diode-pumped phase-locked Nd:YAG laser arrays”, IEEE J. Quantum Electron. 28 (4), 1142 (1992)
[5]K. H. No et al., “One dimensional scaling of 100 ridge waveguide amplifiers”, IEEE Photon. Technol. Lett. 6 (9), 1062 (1994)
[6]S. Saunders et al., “High power coherent two-dimensional semiconductor laser array”, Appl. Phys. Lett. 64, 1478 (1994)
[7]J. S. Osinski et al., “Phased array of high-power, coherent, monolithic flared amplifier master oscillator power amplifiers”, Appl. Phys. Lett. 66, 556 (1995)
[8]Y. Kono et al., “A coherent all-solid-state laser array using the Talbot effect in a three-mirror cavity”, IEEE J. Quantum Electron. 36 (5), 607 (2000)
[9]A. Shirakawa et al., “Coherent additional of fiber lasers by use of a fiber coupler”, Opt. Express 10 (21), 1167 (2002)
[10]D. Sabourdy et al., “Power scaling of fibre lasers with all-fibre interferometric cavity”, Electron. Lett. 38, 692 (2002)
[11]S. J. Augst et al., “Coherent beam combining and phase noise measurements of ytterbium fiber amplifiers”, Opt. Lett. 29 (5), 474 (2004)
[12]M. L. Minden et al., “Self-organized coherence in fiber laser arrays”, Proc. SPIE 5335, 89 (2004)
[13]L. Liu et al., “Phase locking in a fiber laser array with varying path lengths”, Appl. Phys. Lett. 85, 4837 (2004)
[14]T. Y. Fan, “Laser beam combining for high-power, high-radiance sources”, IEEE J. Quantum Electron. 11 (3), 567 (2005)
[15]H. Bruesselbach et al., “Self-organized coherence in fiber laser arrays”, Opt. Lett. 30 (11), 1339 (2005)
[16]L. Michaille et al., “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area”, Opt. Lett. 30 (13), 1668 (2005)
[17]T. M. Shay et al., “Self-synchronous and self-referenced coherent beam combination for large optical arrays”, IEEE J. Sel. Top. Quantum Electron. 13 (3), 480 (2007)
[18]W. Liang et al., “Coherent beam combining with multilevel optical phase-locked loops”, J. Opt. Soc. Am. B 24 (12), 2930 (2007)
[19]N. Satyan et al., “Coherent power combination of semiconductor lasers using optical phase-lock loops”, IEEE Sel. Top. Quantum Electron. 15 (2), 240 (2009)
[20]A. A. Ishaaya et al., “Passive laser beam combining with intracavity interferometric combiners”, IEEE Sel. Top. Quantum Electron. 15 (2), 301 (2009)
[21]R. Uberna et al., “Coherent polarization beam combining”, IEEE J. Quantum Electron. 46 (8), 1191 (2010)
[22]A. Klenke et al., “Basic considerations on coherent combining of ultrashort laser pulses”, Opt. Express 19 (25), 25379 (2011)
[23]A. Klenke et al., “530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system”, Opt. Lett. 38 (13), 2283 (2013)
[24]L. Ionel and D. Ursescu, “Non-collinear spectral coherent combination of ultrashort laser pulses”, Opt. Express 24 (7), 7046 (2016)

(Suggest additional literature!)

See also: beam combining, interference, spectral beam combining, power scaling of lasers, high-power lasers, coherence, synchronization of lasers
and other articles in the categories lasers, methods

In the RP Photonics Buyer's Guide, 4 suppliers for components for coherent beam combining are listed.

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