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. 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). Then, the concept was developed in various research groups, in particular, the University of Stuttgart (Germany) for 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 $$~L~$$ of the device (Fig.1.). The increase in size allows the power scaling to many kilowatts without significant modification of the design.

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. 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 outputted 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 , of the medium, $$\eta_0=\omega_{\rm s}/\omega_{\rm p}$$ 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 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 .

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 .