External cavity diode and dye lasers can be spectrally tuned and made to operate in a single-mode. Light exiting from the laser cavity diverges, passes through a collimator, and is imaged on a grating. In Littrow and Littman configurations, the grating is positioned so that light of the correct wavelength is redirected back through the collimator and focused into the laser cavity.
Here are some examples of Grating Enhanced External Cavity Diode Laser (GEECLD) using gratings and grating systems produced by Wasatch Photonics.
This example uses a “retro” grating to redirect light back to the laser cavity. It is a low-efficiency grating that returns about 20% of the light back to the laser cavity while the remainder passes through the grating. The laser cavity can be fined tuned depending on the tilt of the grating. This design delivers the maximum dispersion for an “in air” grating. The line width of the laser is dependent on how close the grating is to the laser cavity.
This example uses a grating between two right angle prisms to form a “grating cube”. Since this grating is “in glass” it has a lot higher dispersion than the previous example. It also uses a high efficiency grating (97% +). The light is redirect light back to the laser cavity by a partially reflective surface on the output window of the grating cube. The amount of light that returns to the laser cavity is determined by the reflectivity of this surface. The returned light passes back through the grating and is dispersed a second time. While the remainder of the light passes through the partial reflective surface. The laser cavity can be fined tuned depending on the tilt of the grating cube. This design delivers the maximum dispersion and can be in close proximity to the laser cavity.
This example, like the last example, uses a grating between two right angle prisms. However, the output prism is larger and is positioned such that the light path is in a straight pass configuration.
A spatial filter can be placed on the output of the prism to work as a laser bandpass filter to eliminate any stray light. These cubes can also be made with an AR coating on the output window instead of the partial reflective coating and used solely as a laser bandpass filter.
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