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We describe the use of a synchronously pumped ringdown cavity for measuring total optical losses, absorption and scattering, in thin optical films of arbitrary thickness on transparent substrates. This technique is compared with a single-pulse ringdown cavity regime and is shown to have a superior signal-to-noise ratio and resolution. We also provide an analysis of the factors affecting the resolution of the technique. Using this ringdown cavity pumped by a conventional mode-locked Ti:sapphire laser, we experimentally detect losses of only 58 [+ or -] 9 and 112 [+ or -] 9 parts per million in [Ta.sub.2][O.sub.5] and Si[O.sub.2] films, respectively. To our knowledge, these are so far the lowest losses measured in thin films on stand-alone transparent substrates. OCIS codes: 300.1030, 300.6250, 310.6860.

A sensitive N[O.sub.2] analyzer, that uses a commercial LED emitting near 450 nm, has been developed. The prototype can measure N[O.sub.2] concentrations as low as 1 part in [10.sup.9] as defined by the Environmental Protection Agency. The response time of the system is 1 min. In addition, a demonstration of the use of external-cavity frequency-doubled diode laser radiation to accomplish similar measurements is reported. However, because of low nonlinear optical conversion efficiencies, comparable detection limits were not possible. Nonetheless, frequency-doubled diode-laser wavelengths have the potential to reach farther into the UV than LED technology, allowing the detection and measurement of numerous other gaseous species of interest.

High mechanical stress can affect the performance of multilayer thin film optical coatings, causing wavefront aberrations. This is particularly important if the multilayer stack is deposited onto thin substrates, such as those used in adaptive optics. Stress in thin film coatings is dependent on the deposition process, and ion beam sputtering (IBS) thin films are known to have high compressive stress. In the present work, we show that stress in IBS $ {{\rm SiO}_2} $SiO 2 thin films can be reduced from 490 MPa to 48 MPa using high-energy $ {{\rm O}_2} $O 2 assist ion bombardment during deposition while maintaining high optical quality. A comparison of the reduction of stress in $ {{\rm SiO}_2} $SiO 2 deposited from oxide and metal targets is provided.

We present the optical and structural characterization of films of T a 2 O 5 , S c 2 O 3 , and S c 2 O 3 doped T a 2 O 5 with a cation ratio around 0.1 grown by reactive sputtering. The addition of S c 2 O 3 as a dopant induces the formation of tantalum suboxide due to the "oxygen getter" property of scandium. The presence of tantalum suboxide greatly affects the optical properties of the coating, resulting in higher absorption loss at λ =1064 n m . The refractive index and optical band gap of the mixed film do not correspond to those of a mixture of T a 2 O 5 and S c 2 O 3 , given the profound structural modifications induced by the dopant.

Ta 2 O 5 /SiO 2 high reflection (HR) interference coatings for λ∼1  μm offer superior performance at high irradiance conditions. However, these coatings are not good candidates for high peak power conditions in comparison to HfO 2 /SiO 2 multilayer stacks. Here we show that the modification of the top layers design of a quarter wave Ta 2 O 5 /SiO 2 high reflector leads to 4-5 fold increase in the laser damage fluence compared to a quarter wave (Ta 2 O 5 /SiO 2 ) 15 when tested at λ=1.03  μm using pulse durations of 0.19 and 4 ns and peak power densities of 43.5 and 216  GW/cm 2 . One of the designs achieved a laser damage threshold fluence of 174  J/cm 2 at 4 ns, which is 10% higher than that of a HfO 2 /SiO 2 quarter wave design.