OPTICS  AND  CAVITIES

        Most dye lasers are used for their tunability over wide wavelength ranges, and as such, most lasers have integral wavelength selectors in the cavity. Depending on the application, a simple grating may be sufficient to render a spectral output width. Regular diffraction gratings diffract incident light into many orders, so reflectivity of this element as a cavity optic is generally poor. As such, the laser must be operated at a high gain, sometimes close to super radiance, resulting in a broad spectral output. To increase reflectivity, a special type of grating called an echelle grating may be used which is designed to have a reflecting surface as large as possible. Such gratings can reflect up to 70% of incident light into one order. Further enhancement of the laser may be accomplished by using a beam-expanding telescope in the cavity. Aside from allowing the use of a larger grating area than the normally tiny beam emitted from the dye cell, the telescope can also collimate the highly divergent beam exiting from the dye cell. Without collimation the angular spread of light striking the grating limits the resolution possible with the grating alone.

       In addition to a diffraction grating, an etalon is frequently included to reduce output bandwidth. A tilted Fabry Perot etalon between the telescope and the grating greatly reduces the spectral width of the output. The etalon is placed in this manner to ensure that the beam passing through it is collimated, a requirement for proper operation. Etalons may be fine-tuned by changing the angle within the cavity or by changing the pressure of the gas (usually, air or dry nitrogen) between the plates of the etalon. The change of pressure results in a change of refractive index and hence a shift in the resonant frequency of the device. As well, etalons may be fabricated from a solid piece of quartz with reflective coatings on both sides. In this case, angle or temperature tuning may be used. Since the gain of a dye laser is generally large, output couplers are usually plane reflectors with reflectivities below 50%. Another popular use of a dye laser is to produce ultrashort modelocked. Modelocking requires an intracavity switch such as a passive saturable absorber or an EO modulator, usually a Pockels cell.