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.

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 of the
High Power Laser Energy Research Facility.

Inherently scalable designs

Disk lasers

conference.cite journal| url=http://bookstore.spie.org/index.cfm?fuseaction=DetailPaper&ProductId=143686&coden=PSISDG
author=K. Ueda| coauthors=N. Uehara| title=Laser-diode-pumped solid state lasers for gravitational wave antenna| journal= volume=1837| pages=336–345| year=1993| doi=10.1117/12.143686] One type of solid-state laser designed for good power scaling is the disk laser (or "active mirror"cite journal| url=http://bookstore.spie.org/index.cfm?fuseaction=DetailPaper&ProductId=143686&coden=PSISDG
author=K. Ueda| coauthors=N. Uehara| title=Laser-diode-pumped solid state lasers for gravitational wave antenna| journal=Proceedings of SPIE| volume=1837| pages=336–345| year=1993| doi=10.1117/12.143686] ). Such lasers are believed to be scalable to a power of several kilowattsfrom a single active element in continuous-wave operation. [cite journal| author=A. Giesen| coauthors=H. Hügel, A. Voss, K. Wittig, U. Brauch and H. Opower| title=Scalable concept for diode-pumped high-power solid-state lasers| year=1994
url=http://www.springerlink.com/content/n7350870q8q57324/?p=c874af0585094717b13bb41e3fc548da&pi=0
journal=Applied Physics B|volume=58| issue=5| pages=365–372| doi=10.1007/BF01081875] .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 examplecite journal
url=http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PSISDG005620000001000112000001&idtype=cvips&gifs=Yes&bproc=volrange&scode=5600%20-%205699
author= A.Giesen
title=Thin-disk solid-state lasers
journal=Proceedings of SPIE
volume=5620
pages=112–127
year=2004
doi=10.1117/12.578272] andcite journal
url=http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PSISDG005332000001000212000001&idtype=cvips&gifs=Yes&bproc=volrange&scode=5300%20-%205399
author= A.Giessen
coauthors=
title=Results and scaling laws of thin disk lasers
journal=Proceedings of SPIE
volume=5332
pages=212–227
year=2004
doi=10.1117/12.547973] ; 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 [cite journal| author=D. Kouznetsov|coauthors= J.-F.Bisson, J.Dong, and K.Ueda| title=Surface loss limit of the power scaling of a thin-disk laser| journal=JOSAB| volume=23| issue=6| pages=1074–1082| year=2006| url=http://josab.osa.org/abstract.cfm?id=90157| accessdate=2007-01-26| doi=10.1364/JOSAB.23.001074] . 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.cite journal|title=Highly efficient, high-gain, short-length, and power-scalable incoherent diode slab-pumped fiber amplifier/laser| author= Kouznetsov, D.| coauthors=Moloney, J.V.| journal=IEEE Journal of Quantum Electronics| url=http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=1242365&isnumber=27838| volume=39 | year=2003 | issue=11 | pages=1452–1461 | doi=10.1109/JQE.2003.818311] cite journal|title=Efficiency of pump absorption in double-clad fiber amplifiers. 2: Broken circular symmetry| author= Kouznetsov, D.| coauthors=Moloney, J.V.| journal=JOSAB| url=http://josab.osa.org/abstract.cfm?id=68991|volume=39 | year=2003 | issue=6 | pages=1259–1263|doi=10.1364/JOSAB.19.001259] cite journal|title=Modeling and optimization of double-clad fiber amplifiers using chaotic propagation of pump
author= Leproux, P.| coauthors=S. Fevrier, V. Doya, P. Roy, and D. Pagnoux| journal=Optical Fiber Technology| url=http://www.ingentaconnect.com/content/ap/of/2001/00000007/00000004/art00361
volume=7 | year=2003 | issue=4 | pages=324–339|doi=10.1006/ofte.2001.0361] cite journal
title=The absorption characteristics of circular, offset, and rectangular double-clad fibers
author=A. Liu| coauthors= K. Ueda| url= http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVF-497C4YV-BW&_user=10&_coverDate=12%2F15%2F1996&_alid=550869877&_rdoc=3&_fmt=summary&_orig=search&_cdi=5533&_sort=d&_docanchor=&view=c&_ct=3&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=688bbca25fdd98e29caadb676b003c1e
journal=Optics Communications| volume=132| year=1996| pages= 511–518
doi=10.1016/0030-4018(96)00368-9]

Fiber disk lasers

right|200px|thumb|Fiber_disk_laser,_proposed_bycite journal| url=http://www.maik.ru/cgi-bin/search.pl?type=abstract&name=lasphys&number=3&year=98&page=774
author=K. Ueda| coauthors=A. Liu
title=Future of High-Power Fiber Lasers
journal=Laser Physics| volume=8| pages=774–781| year=1998]

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 laserscite journal| url=http://www.maik.ru/cgi-bin/search.pl?type=abstract&name=lasphys&number=3&year=98&page=774
author=K. Ueda| coauthors=A. Liu
title=Future of High-Power Fiber Lasers
journal=Laser Physics| volume=8| pages=774–781| year=1998] cite journal
url=http://ieeexplore.ieee.org/iel5/6572/17547/00811381.pdf
author=K. Ueda
title=Scaling physics of disk-type fiber lasers for kW output
journal=Lasers and Electro-Optics Society
volume=2
pages=788-789
year=1999] cite journal
journal= Lasers and Electro-Optics Society 1999 12th Annual Meeting. LEOS '99. IEEE
url=http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=811381&isnumber=17547
author=Ueda
coauthors= Sekiguchi H., Matsuoka Y., Miyajima H. , H.Kan
title=Conceptual design of kW-class fiber-embedded disk and tube lasers
year=1999
volume=2
pages=217-218] cite journal
journal=Nature Photonics sample
url=http://www.nature.com/nphoton/journal/vsample/nsample/fig_tab/nphoton.2006.6_ft.html
author=Hamamatsu K.K.
title=The Fiber Disk Laser explained
pages=14–15
year=2006
doi=10.1038/nphoton.2006.6
volume=sample] .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.

Problem of heat sink

120px|right|thumb|Robust configuration for power scaling ">cite journal
journal=Research Letters in Physics
volume=2008
year=2008
pages=717414
doi=10.1155/2008/717414
url=http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/717414
title=Storage of Energy in Disk-Shaped Laser Materials
author=D.Kouznetsov] .The power scaling is limited by the ability to dissipate the heat. Usually, the thermal conductivity of materials designed for efficient laser action, is small compared to that of materials optimal for the hear transfer (metals, diamonds).For the efficient drain of heat from a compact device, the active medium should be a narrow slab; in order to give advantage to the amplification of light at wanted direction over the ASE, the energy and head whould be withdrawn in orthogonal directions, as it is shown in figure. At low background loss (typically, at the level of 0.01 or 0.001)the heat and the light can be withdrawn in the opposite directions, allowing active elements of wide apperture. In this case,combining of several active elements is used for the power scaling.

Coherent addition and combining beams

volume=10|pages=1167–1172| year=2002] cite journal| url=http://annex.jsap.or.jp/OSJ/opticalreview/TOC-Lists/vol12/12f0445tx.htm
author=D.Kouznetsov|coauthors=J.-F. Bisson. A. Shirakawa, K.Ueda
title=Limits of Coherent Addition of Lasers: Simple Estimate
journal= year=2005
doi=10.1007/s10043-005-0445-8]

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