Volume Lasers

The term bulk laser refers to a solid-state laser that uses a doped crystal or glass as the gain medium, and does not contain a waveguide device. In addition to the laser crystal (or glass), the laser resonator also contains some laser mirrors and possibly some other optical components such as polarizers, birefringence tuners, prisms, Q switches, or saturable absorbers for pattern locking.

The term volume lasers is used to distinguish this type of laser from waveguide lasers, especially fiber lasers (fiber-based lasers). In most cases, the laser gain medium is doped with rare earth ions or transition metal ions, but bulk lasers also include color center lasers.

Beam propagation

Since there is no waveguide structure, the beam propagates in free space between the optical elements, and the beam radius in the gain medium is basically not determined by the gain medium, but by the design of the laser resonator, which is of great significance.

• Resonators can be designed to have a large effective mode area in the crystal to allow, for example, Q-switching operation with very high pulse energies.
• Alternatively, the small mode area allows for a low threshold pump power, however, it means that the beam divergence is strong. It cannot be maintained on longer materials.
• The beam radius is affected by, for example, a thermal lensing, and may change when the pump power changes. The optimized resonator design minimizes the effects of thermal lensing and alignment sensitivity.

Air space

In most cases, the laser resonator of a bulk laser is formed by discrete laser mirrors placed around the crystal (or glass), with air space in between. It is also important to use air space between mirrors, crystals, and possibly other elements:

• It allows for the simple insertion of additional optics such as birefringence tuners or other types of optical filters, Q switches for nanosecond pulse generation, or nonlinear crystals for frequency doubling in the cavity.
• By optimizing the length of this air gap, the mode characteristics of the resonator can be adjusted, in addition to selecting a mirror with a specific radius of curvature.
• Optical components, especially laser mirrors, must be precisely aligned, and this alignment can be lost due to thermal drift or mechanical vibration.
• Airborne dust particles or organic matter can be deposited on the laser mirror and other components, especially when it comes to high light intensity and/or short wavelengths. Therefore, in order to operate reliably for a long time, the laser housing may therefore have to be sealed.

In many cases, mirrors and other optical components are attached to adjustable mounts with two or three micron screws for precise angle adjustments. In many industrial lasers and some mass-produced OEM laser modules, these adjustable mounts are replaced by fixed mounts, which can be more robust (less sensitive to temperature drift) and more cost-effective. The optics can be soldered to their mounts, resulting in a very stable setup.

It is also possible to use a laser crystal coated with a highly reflective electron mirror on one side as the end mirror of the resonator. There are also monolithic solid-state lasers where the beam path is entirely inside the crystal.

Comparison with other laser types

For the time being, bulk lasers and amplifiers are more suitable for devices with high peak power, while waveguide lasers and waveguide amplifiers are easier to operate with low thresholds and high gains. In addition, body lasers are more flexible, for example in a laboratory setting, because it is relatively easier to add or swap optical components, while waveguide lasers may be less expensive to manufacture.