Power scaling

Power scaling of a laser is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design.

Usually, power scaling requires a more powerful pump source, stronger cooling, and an increase in size. It may also require reduction of the background loss in the laser resonator and, in particular, in the gain medium.

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MOPA

The most popular way of achieving power scalability is the "MOPA" (master oscillator/power amplifier) approach. The master oscillator produces a highly coherent beam, and an optical amplifier is used to increase the power of the beam while preserving its main properties. The master oscillator has no need to be powerful, and has no need to operate at high efficiency because the efficiency is determined mainly by the power amplifier. The combination of several laser amplifiers seeded by a common master oscillator is essential concept ot the High Power Laser Energy Research Facility.

Inherently scalable designs

Disk lasers

  One type of solid-state laser designed for good power scaling is the disk laser (or "active mirror"^[1]). Such lasers are believed to be scalable to a power of several kilowatts from a single active element in continuous-wave operation.^[2]. Perhaps, the expectations for power scalability of disk lasers is a little bit exaggerated: some of publications in favor of disk laser just repeat each other; compare, for example ^[3] and ^[4]; these articles differ with only titles.

Amplified spontaneous emission, overheating and round-trip loss seem to be the most important processes that limit the power of disk lasers ^[5]. For future power scaling, the reduction of the round-trip loss and/or combining of several active elements is required.

Fiber lasers

Fiber lasers are another type of solid-state laser with good power scaling. The power scaling of fiber lasers is limited by Raman scattering and Brillouin scattering, and by the fact that such lasers cannot be very long. The limited length of the double-clad fibers limits the usable power of the multi-mode pump, because the pump is not absorbed efficiently in the fiber's active core. Optimization of the shape of the cladding can extend the limit of power scaling.^[6][7][8][9]

Fiber disk lasers

The limit of power scaling of fiber lasers can be extended with lateral delivery of the pump. This is realized in so-called fiber disk lasers ^{[10] [11] [12] [13]} . The pump in such a laser is delivered from side of a disk, made of coiled fiber with doped core. Several such disks (with a coolant between them) can be combined into a stack.

Combining beams

Scalability can also be achieved by combining separate laser beams. Completely independent beams cannot usually be combined to produce a beam with higher radiance than each beam has alone. Beams can only be combined if they are coherent with each other. Such beams can be combined actively or passively.

In the passive combining (or coherent addition) of lasers, only the few modes common to all of the combined lasers can be above the lasing threshold. Efficient passive combining of eight lasers has been reported. Further power scaling requires exponential growth of the gain bandwidth and/or length of the individual lasers.

Active combining implies the real-time measurement of the phase of individual lasers' output, and quick adjustment to keep them all in phase. Such adjustment can be done by adaptive optics, which is effective for suppression of phase noise at acoustic frequencies. Faster schemes based on all-optical switching are being researched.

References

1. ^ ^{a b} K. Ueda; N. Uehara (1993). "Laser-diode-pumped solid state lasers for gravitational wave antenna". Proceedings of SPIE 1837: 336–345.
2. ^ A. Giesen; H. Hügel, A. Voss, K. Wittig, U. Brauch and H. Opower (1994). "Scalable concept for diode-pumped high-power solid-state lasers". Applied Physics B 58 (5): 365–372.
3. ^ A.Giesen (2004). "Thin-disk solid-state lasers". Proceedings of SPIE 5620: 112-127.
4. ^ A.Giessen (2004). "Results and scaling laws of thin disk lasers". Proceedings of SPIE 5332: 212-227.
5. ^ D. Kouznetsov; J.-F.Bisson, J.Dong, and K.Ueda (2006). "Surface loss limit of the power scaling of a thin-disk laser". JOSAB 23 (6): 1074–1082. Retrieved on 2007-01-26.
6. ^ Kouznetsov, D.; Moloney, J.V. (2003). "Highly efficient, high-gain, short-length, and power-scalable incoherent diode slab-pumped fiber amplifier/laser". IEEE Journal of Quantum Electronics 39 (11): 1452–1461. doi:10.1109/JQE.2003.818311.
7. ^ Kouznetsov, D.; Moloney, J.V. (2003). "Efficiency of pump absorption in double-clad fiber amplifiers. 2: Broken circular symmetry". JOSAB 39 (6): 1259-1263.
8. ^ Leproux, P.; S. Fevrier, V. Doya, P. Roy, and D. Pagnoux (2003). "Modeling and optimization of double-clad fiber amplifiers using chaotic propagation of pump". Optical Fiber Technology 7 (4): 324-339.
9. ^ A. Liu; K. Ueda (1996). "The absorption characteristics of circular, offset, and rectangular double-clad fibers". Optics Communications 132: 511–518.
10. ^ ^{a b} K. Ueda; A. Liu (1998). "Future of High-Power Fiber Lasers". Laser Physics 8: 774-781.
11. ^ K. Ueda (1999). "Scaling physics of disk-type fiber lasers for kW output". Lasers and Electro-Optics Society 2: 788-789.
12. ^ Ueda; Sekiguchi H., Matsuoka Y., Miyajima H. , H.Kan (1999). "Conceptual design of kW-class fiber-embedded disk and tube lasers". Lasers and Electro-Optics Society 1999 12th Annual Meeting. LEOS '99. IEEE 2: 217-218.
13. ^ Hamamatsu K.K. (2006). "The Fiber Disk Laser explained". Nature Photonics sample: 14-15.
14. ^ A.Shirakawa; T.Satou, T. Sekiguchi and K. Ueda (2002). "Coherent addition of fiber lasers by use of a fiber coupler". Optics Express 10: 1167-1172.
15. ^ D.Kouznetsov; J.-F. Bisson. A. Shirakawa, K.Ueda (2005). "Limits of Coherent Addition of Lasers: Simple Estimate". Optical Review 12 (6): 445-447.