The innovation engine for new materials

Damage Resistant High Transmission Optical Thin Film Coating for Diode Pumped Alkali Lasers (DPALs)

Seminar Group: 


Lindsay O'Brien Quarrie


Principal Research Engineer,
L-3 Telemetry West, Space Division


Friday, October 12, 2012 - 4:00pm


ESB 1001

The lifetime of Diode Pumped Alkali Lasers (DPALs) is limited by the reaction of rubidium vapor with the glass envelope of the gain medium. A high optical transmission coating was developed that resists alkali damage and protects the glass of the gain cell in a DPAL. The application of this coating to the optical windows of a DPAL will enhance transmission of both its input and output laser beams. Since the application of optical coatings to DPAL optical windows is relatively new, the methodology developed here can be readily modified for simulation of expected optical performance at input pump and output laser power wavelengths using different combination of thin film materials in a DPAL. Thin films have been used on optical materials, like glasses, the best-known examples being the use of the anti-reflective properties of thin films on optical lenses to improve light transmission through telescopes, binoculars and other optical systems. In this example, an optical coating reduces the effect of the large "air –glass" difference in the index of refraction, reducing the amount of light reflected from the glass surface. In the case of a DPAL optical window, which is typically made of fused silica, the opposite side of the optical window needs to have high optical transmission and also be resistant to alkali vapor. DPALs operate by converting a wavelength of light from a diode laser to an output beam at a longer wavelength using a heated gas cell containing a partial pressure of alkali metal. High coupling efficiency of the light through the gas cell is accomplished by matching the air-glass and glass-gas interfaces at the appropriate wavelengths using a dielectric stack of high and low index of refraction materials selected to work at the laser energies and protected from the alkali metal in the gas cell. The advantage of DPALs is that several pumping lasers can be combined in a single gain cell, allowing multiple lasers to be used to generate a single higher power output beam. This makes DPALs of great interest for medical and military applications that require lasers with high energy and excellent quality. The key to having a reliable and highly effective laser system is the optical thin film coating being developed here. Using a combination of theory and experiment, thin films of oxides of aluminum, zirconium, tantalum, titanium and silicon were down selected as potential alkali resistant layers for implementation as high optical transmission thin film Fabry-Perot filters on DPAL optical windows.