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Fig.1. An optically-pumped disk laser (active mirror).

A disk laser or active mirror (Fig.1.) is a type of solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium.¹ Despite their name, disk lasers do not have to be circular; other shapes have also been tried.

Disk lasers should not be confused with Laserdiscs, which are a disk-shaped optical storage medium.

Disk lasers should not be confused with Fiber laser disks, which are a disk-shaped coils of a fiber lasers, pumped from side.

Active mirrors and disk lasers

Fig.2. A disk laser (active mirror) configuration presented in 1992 at the SPIE conference.²

Initially, disk lasers were called active mirrors, because the gain medium of a disk laser is essentially an optical mirror with reflection coefficient greater than unity. An active mirror is a thin disk-shaped double-pass optical amplifier.

The first active mirrors were developed in the Laboratory for Laser Energetics (USA) ³. Then, the concept was developed in various research groups, in particular, the University of Stuttgart (Germany)⁴for Yb:doped glasses.

In the disk laser, the heat sink does not have to be transparent, so, it can be extremely efficient even at large transverse size of the device (Fig.1.). The increase in size allows the power scaling to many kilowatts without significant modification of the design ⁵.

Limit of power scaling for disk lasers

Fig.3. Bouncing ray of ASE in a disk laser

The power of such lasers is limited not only by the power of pump available, but also by overheating, amplified spontaneous emission (ASE) and the background round-trip loss.⁶ To avoid overheating, the size should be increased at the power scaling. Then, to avoid strong losses due to the exponential growth of the ASE, the transverse-trip gain cannot be large. This requires to reduce the gain ; this gain is determined by the reflectivity of the output coupler and thickness . The round-trip gain should remain larger than the round-trip loss (the difference determines the part of the energy of the optical field, which can be output from the laser cavity at each round-trip). The reduction of gain , at given round-trip loss , requires to increase the thickness h. Then, at some critical size, the disk becomes too thick and cannot be pumped above the threshold without overheating.

Some features of the power scaling can revealed from a simple model. Let be the saturation intensity ⁷, ⁶ of the medium, be the ratio of frequencies, be the thermal loading parameter. The key parameter determines the maximal power of the disk laser. The correspnding optimal thickness can be estimated with . The corresponding optimal size . Roughly, the round-trip loss should scale inversely proportionally to the cubic root of the power required.

An additional issue is the efficient delivery of pump. At the low round-trip gain, the single-pass absorption of the pump is also low. Therefore, the recycling of pump is required for the efficient operation. (See the additional mirror M at the left-hand side of figure 2.) For the power scaling, the medium should be optically thin, and many passes of pump required; the lateral delivery of pump ⁷ also might be a possible solution.

Anti-ASE cap

Fig.4. Uncovered disk laser and that with undoped cap ⁸.

In order to reduce the impact of ASE, an anti-ASE cap consisting of undoped material on the surface of a disk laser has been suggested ^{9 10}. Such a cap allows spontaneously emitted photons to escape from the active layer and prevents them from resonating in the cavity. Rays cannot bounce (Fig.3) as in uncovered disk. This could allow an order of magnitude increase in the maximum power achievable by a disk laser ⁸. In both cases, the back reflection of the ASE from the ecges of the disk should be suppressed. This can be done with absorbing layers, shown with green in FIgure 4. At the operation close to the maximal power, the siginificant part of the energy goes to the ASE; therefore, the absorbing layers also shoul be supplied with heat sinks, which are not shown in the figure.

Fig5. Upper bound of loss β at which the ouptut power P_s of a single disk laser is still achievable. Dashed line corresponds to uncovered disk; thick solid curve represents the case with undoped cap ⁸.

The estimate of the maximal power, acheivable at given loss β is very sensitive to β . The estimate of the uper bound of β, at which the desired output power P_s is acievable is robust. This estimate is plotted versus normalized power s = P_s / P_d in figure 5. Here, P_s is the output power of the laser, and P_d = R² / Q is dimensional scale of power; it is related with the key parameter P_k = P_d / β³. The fick dashed lline represents the estimate for theuncovered disk. The thick solid line shows the same for the disk with undoped cap. Thin solid line represents the qualitative estimate β = s^{1 / 3} without coefficients. Circles corresponds to the experimental data for the power achieved and corresponding estimates for the background loss β. All future experiments and numerical simulations and estimates are expected to give values of (β,s), that are below the red dashed line in Fig.5 for the uncovered disks and below the blue curve for the disks with anti-ASE cap. This can be interpreted as scaling law for disk lasers ¹¹.

In vicinity of the curves mentioned, the efficiency of the disk laser is low; the most of power of pump goes to the ASE and is absorbed at the edges of the device. In this cases, the distribution of the pump available among several disks may significantly improve the performance of lasers. Indeed, some lasers reported use several elements combined in the same cavity.

Pulsed operation

Similar scaling laws take place for the pulsed operation. In quasi continuous wave regime, the maximal mean power can be estimated scaling the saturation intensity with the fill factor of the pump, product of the duration of pump to the repetition rate. At short duration of pulses, the more detailed analysis is required ¹². In both cases, the maximal energy of the output pulses is roughtly inversely proportional to the cube of the background loss β; the undoped cap may procides an additional order of magnitude of the mean output power, under condition, that this cap does not contribute to the background loss.

See also

• VCSEL
• VECSEL
• Thermal shock
• round-trip gain
• power scaling
• gain medium

References

1. ^ "Thin disk lasers". Encyclopedia of Laser Physics and Technology. http://www.rp-photonics.com/thin_disk_lasers.html. 
2. ^ K. Ueda; N. Uehara (1993). "Laser-diode-pumped solid state lasers for gravitational wave antenna". Proceedings of SPIE 1837: 336–345. doi:10.1117/12.143686. http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PSISDG001837000001000336000001&idtype=cvips&gifs=yes. 
3. ^ A.Abate; L.Lund, D.Brown, S.Jacobs, S.Refermat, J.Kelly, M.Gavin, J.Waldbillig, and O.Lewis (1981). "Active mirror: a large-aperture medium-repetition rate Nd:glass amplifier". Applied Optics 1837: 351–361. http://ao.osa.org/abstract.cfm?id=24598. 
4. ^ 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. doi:10.1007/BF01081875. http://www.springerlink.com/content/n7350870q8q57324/?p=c874af0585094717b13bb41e3fc548da&pi=0. 
5. ^ C.Stewen; K.Contag, M.Larionov, A.Giesen, H.Hugel (2000). "A 1-kW CW thin disc laser". IEEE J. of Selected Topics in QE 6: 650–657. doi:10.1109/2944.883380. ISSN http://worldcat.org/issn/1077-260X I NSPEC Accession Number= 6779337 1077-260X I NSPEC Accession Number= 6779337]. 
6. ^ ^{a b} 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. doi:10.1364/JOSAB.23.001074. http://josab.osa.org/abstract.cfm?id=90157. Retrieved on 26 January 2007. ; [1]
7. ^ ^{a b} D.Kouznetsov; J.F.Bisson, K.Takaichi, K.Ueda (2005). "Single-mode solid-state laser with short wide unstable cavity". JOSAB 22 (8): 1605–1619. doi:10.1364/JOSAB.22.001605. http://josab.osa.org/abstract.cfm?id=84730. 
8. ^ ^{a b c} D.Kouznetsov; J.F.Bisson (2008). "Role of undoped cap in the scaling of thin-disk lasers". JOSAB 25: 338–345. doi:10.1364/JOSAB.25.000338. http://www.opticsinfobase.org/abstract.cfm?URI=josab-25-3-338. 
9. ^ Stephen A. Payne; William F. Krupke, Raymond J. Beach, Steven B. Sutton, Eric C. Honea, Camille Bibeau, Howard Powel (2002). "High average power scaleable thin-disk laser". US patent 6347109. http://www.patentstorm.us/patents/6347109.html. 
10. ^ Beach; Raymond J. (Livermore, CA),; Honea; Eric C. (Sunol, CA), Bibeau; Camille (Dublin, CA), Payne; Stephen A. (Castro Valley, CA), Powell; Howard (Livermore, CA), Krupke; William F. (Pleasanton, CA), Sutton; Steven B. (Manteca, CA) (2002). "High average power scaleable thin-disk laser". USA patent 6347109. http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=13&f=G&l=50&co1=AND&d=PTXT&s1=6347109&OS=6347109&RS=6347109. 
11. ^ D.Kouznetsov; J.-F.Bisson, K.Ueda (2008). "Scaling laws of disk lasers". Optical Materials. doi:10.1016/j.optmat.2008.03.017. 
12. ^ D.Kouznetsov. (2008). "Storage of energy in disk-shaped laser materials". Research Letters in Physics 2008: 717414. http://www.hindawi.com/journals/rlp/aip.717414.html. 

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