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Disk laser

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

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] and the Institute for Laser Science (Japan) [2]. Then, the concept was developed in various research groups, in particular, the University of Stuttgart (Germany)[4] and Tokyo Institute of Technology (Japan) [5] for Yb:doped glasses and semiconductor laser materials.

In the disk laser, the heat sink does not have to be transparent, so, it can be extremely efficient even at large transverse size $~L~$ of the device (Fig.1.). The increase in size may allow the power scaling to many kilowatts without significant modifications.

Limit of power scaling for disk lasers

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 $~L~$ should be increased at the power scaling. Then, to avoid strong losses due to the exponential growth of the ASE, the transverse-trip gain $~u=GL~$ cannot be large. This requires to reduce the gain $G~$; this gain is determined by the reflectivity of the output coupler and thickness $~h$. The round-trip gain $~g=2Gh~$ should remain larger than the round-trip loss $\beta~$ (the difference $g\!-\!\beta~$ determines the part of the energy of the optical field, which can be outputed from the laser cavity at each round-trip). The reduction of gain $G~$, at given round-trip loss $~\beta~$, 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 $Q~$ be the saturation intensity [7], [6] of the medum, $\eta_0=\omega_{\rm s}/\omega_{\rm p}~~$ be the ratio of frequencies, $R~$ be the thermal loading parameter. The key parameter $P_{\rm k}=\eta_0\frac{R^2}{Q\beta^3}~$ determines the maximal power of the disk laser. The correspnding optimal thickness can be estimated with $h \sim \frac{R}{Q \beta}$. The corresponding optimal size $L \sim \frac{R}{Q \beta^2}$. Roughly, the round-trip loss should scale inversely proportionally to 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 pump is also low. Therefore, the recycling of pump is required for the efficient operation. (See the additional mirror M at the left-hands 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

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[8]. Such a cap allows spontaneously emitted photons to escape from the active layer and prevents them from resonating in the cavity. This could allow an order of magnitude increase in the maximum power achievable by a disk laser[9].

References

1. ^ Thin disk lasers. Encyclopedia of Laser Physics and Technology.
2. ^ a b K. Ueda; N. Uehara (1993). "Laser-diode-pumped solid state lasers for gravitational wave antenna". Proceedings of SPIE 1837: 336–345.
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.
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.
5. ^ H.Soda; K.Iga, C.Kitahara and Y.Suematsu (1979). "GaInAsP/InP Surface Emitting Injection Lasers". Japanese Journal of Applied Physics 18 (12): 2329-2330.
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. Retrieved on 2007-01-26.; [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.
8. ^ J.R.Beach; E.C.Honea, C.Bibeau, S.A.Payne, H.Powell, W.F.Krupke, S.B.Sutton (2002). "High average power scaleable thin-disk laser". USA patent 6347109 (B1).
9. ^ D.Kouznetsov; J.F.Bisson, J.Dong, K.Ueda (2007). "Scaling laws of a thin disk lasers". Preprint ILS-UEC.