Spatial hole burning is a phenomenon that occurs in laser systems when several light beams pass through a laser medium with inhomogeneous absorption. It refers to the process wherein some areas of the medium experience a loss in gain, resulting in holes in the output beam. The performance of laser systems may be constrained by this phenomenon, especially when high optical power is desired. This phenomenon occurs in certain types of optical materials.

There are a few requirements that must be met for spatial hole burning to be observed and controlled:

• Strong absorption line of the material - The material must have very strong absorption lines at the wavelength of the optical field. 
• Condition for population inversion - There must be more excited atoms or ions than ground-state atoms or ions in the material.
• Spatially confined optical field -  The hole in the intensity only occurs in the region where the field is intense enough to create a population inversion.
• Long fluorescence lifetime of the material - The long fluorescence lifetime allow the populations of the upper and lower level to build up enough to create a population inversion.

Spatial hole burning occurs in homogeneously broadened laser medium, when optical interference patterns are used to selectively remove spatial locations in the presence of a strongly absorbing material. When a system experiences spatial hole burning, some areas of the system absorb light more strongly compared to other regions, resulting in a hole or area of decreased light intensity. 

In spatial hole burning, when the standing wave  pattern of a single longitudinal mode develops within a homogeneously broadened medium, the laser intensity pattern along the length of the gain medium is regularly spaced. The spacing occurs at half-wavelength intervals, where the intensity is zero at the cavity mirrors and at each half-wavelength gap between the mirrors. Within the gain medium, the gain profile exhibits periodic spatial variations that are 90 degrees out of phase with the laser intensity profile. Higher order transverse modes might use this periodicity to their advantage to produce potent secondary modes. To produce additional gain in the primary mode, ring lasers were created in an effort to recycle the light back into the system with a small phase error.

Figure 1: Spatial Hole Burning

Factors responsible for Spatial Hole Burning

The occurrence and properties of spatial hole burning in a material are affected by several factors:

• Intensity of the absorbed light
• Duration of the absorption process
• Temperature of the material

Spatial hole burning is one of the effects of mode competition that have significant impact on the gain profile of the laser. Mode competition refers to a phenomenon where multiple optical modes compete for the same energy levels in a laser system. It results in several laser output modes, which drastically reduces the amplifier gain at each of the competing frequency bands. This gain reduction is shown in Doppler broadened emissions, where the laser's output spectrum density is determined by the width of the Doppler broadened beam.

Applications of Spatial Hole Burning

Despite being a limiting factor in the performance of a laser, there are a few research applications where the effects of spatial hole burning have been studied and exploited. These applications include gain-switching, wavelength division multiplexing in fiber-optic communication systems, and spectroscopy for high-resolution spectra in homogeneously broadened media. Although they have potential, these applications are still in the early stages of development and have not yet been widely used in real-world settings.