It is common to optically pump a laser with light from another laser; for example, fiber lasers are usually pumped with laser diodes. A relatively unusual way of pumping, however, is to place its gain medium inside the laser resonator of its pump laser, rather than using the output beam of the pump laser. Typically, the laser gain medium absorbs only a quite limited fraction of the received pump light, allowing the pump laser still to work with a moderate amount of gain. Such an intracavity pumping arrangement can be a solution for cases where only a very limited amount of pump absorption is achieved, so that single-pass pumping would be inefficient. In some cases, it is also relevant to achieve a particularly high pump intensity in that away. Also, intracavity pumping allows for a rather compact setup.
Bulk lasers, made from discrete elements like a laser crystal and some mirrors, are best suited for intracavity pumping, as one can relatively easily place another laser crystal in the resonator (possibly combining them to a single crystal by diffusion bonding), as shown in Figure 1. The intracavity power is in most cases substantially higher than the power which could be coupled out with some output coupler mirror. One may then use laser mirrors which are highly reflecting at the wavelength of the pump laser, while one of them serves as the output coupler of the other laser. The pump absorption in the second crystal is usually the dominant loss for the pump laser.
Typically, the pump laser itself is pumped with one or several laser diodes, the radiation of which must be coupled in through some dichroic mirror (for end pumping) or from the side (for side pumping).
Vertical external-cavity surface-emitting lasers could be similarly suitable for intracavity pumping of other lasers, except that the reflector which is integrated into the semiconductor gain chip may not be sufficiently broadband, or may then need to be specially optimized. Alternatively, one may use another intracavity mirror which does not allow the radiation of the laser to get to the semiconductor gain chip.
Intracavity pumping is also applied to some optical parametric oscillators (OPOs)  and Raman lasers, particularly to those for continuous-wave operation, where it is otherwise often challenging to reach the required high pump power.
Example: Yb Laser in a Nd Laser Resonator
In some cases, an ytterbium-doped laser gain medium is used for laser emission on its zero-phonon line around 0.98 μm. As there is strong reabsorption at such a wavelength, such a laser requires operation with a rather high degree of excitation of the Yb3+ ions. One then requires pumping with a high intensity at a somewhat shorter wavelength, and typically has only quite limited pump absorption there. It is not possible to solve that problem by using more ytterbium in the system, as that would lead to strong amplified spontaneous emission (ASE) at longer wavelengths.
An elegant solution for efficient operation is then intracavity pumping, for example at 914 nm with a Nd:YVO4 laser .
Example: Ho Laser in Tm Laser Resonator
Holmium lasers are often used for generating laser light in the 2.1-μm region (somewhat longer than reachable with thulium-doped laser gain media). It is possible to do in-band pumping of holmium with 2-μm radiation from a thulium laser, for example a thulium-doped fiber laser. However, efficient pump absorption is not easy to achieve. A good solution can be to place the holmium-doped laser crystal within the laser resonator of a Tm:YLF laser or Tm:YAG laser [2, 12, 16, 17], which also results in a compact setup.
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