Simon Bublitz

Characterization of low losses in optical thin films and materials

Residual absorption in optical coatings and materials is directly measured by means of the laser-induced deflection (LID) technique. For transmissive coatings a measurement strategy is introduced that allows for the separation of different absorptions of the investigated sample (bulk, coating, surface) by use of only one sample. Laser irradiation yields absorption values between 2 x 10(-3) and 2.9 x 10(-2) for antireflecting and highly reflecting (HR) coatings at 193 nm and 30.6 x 10(-6) for a HR mirror at 527 nm. Use of laser-induced fluorescence at 193 nm excitation reveals trivalent cerium and prasodymium and hydrocarbons in different single layers and coatings. In addition to correlation with absorption data, the influence of a high fluorescence quantum yield on the absorption measurement is discussed.

Sensitive and absolute absorption measurements in optical materials and coatings by laser-induced deflection technique

Abstract. The laser-induced deflection (LID) technique, a photo-thermal deflection setup with transversal pump-probe-beam arrangement, is applied for sensitive and absolute absorption measurements of optical materials and coatings. Different LID concepts for bulk and transparent coating absorption measurements, respectively, are explained, focusing on providing accurate absorption data with only one measurement and one sample. Furthermore, a new sandwich concept is introduced that allows transferring the LID technique to very small sample geometries and to significantly increase the sensitivity for materials with weak photo-thermal responses. For each of the different concepts, a representative application example is given. Particular emphasis is placed on the importance of the calibration procedure for providing absolute absorption data. The validity of an electrical calibration procedure for the LID setup is proven using specially engineered surface absorbing samples. The electrical calibration procedure is then applied to evaluate two other approaches that use either doped samples or highly absorptive reference samples.

Laser induced deflection technique for absolute thin film absorption measurement: optimized concepts and experimental results

Using experimental results and numerical simulations, two measuring concepts of the laser induced deflection (LID) technique are introduced and optimized for absolute thin film absorption measurements from deep ultraviolet to IR wavelengths. For transparent optical coatings, a particular probe beam deflection direction allows the absorption measurement with virtually no influence of the substrate absorption, yielding improved accuracy compared to the common techniques of separating bulk and coating absorption. For high-reflection coatings, where substrate absorption contributions are negligible, a different probe beam deflection is chosen to achieve a better signal-to-noise ratio. Various experimental results for the two different measurement concepts are presented.

Nonlinear absorption in single LaF 3 and MgF 2 layers at 193 nm measured by surface sensitive laser induced deflection technique

We report nonlinear absorption data of LaF(3) and MgF(2) single layers at 193 nm. A highly surface sensitive measurement strategy of the laser induced deflection technique is introduced and applied to measure the absorption of highly transparent thin films independently of the substrate absorption. Linear absorptions k=(alphaxlambda)/4pi of 2x10(-4) and 8.5x10(-4) (LaF(3)) and 1.8x10(-4) and 6.9x10(-4) (MgF(2)) are found. Measured two photon absorption (TPA) coefficients are beta=1x10(-4) cm/W (LaF(3)), 1.8x10(-5), and 5.8x10(-5) cm/W (MgF(2)). The TPA coefficients are several orders of magnitude higher than typical values for fluoride single crystals, which is likely to result from sequential two step absorption processes.

Enhanced laser-induced deflection measurements for low absorbing highly reflecting mirrors

A new concept enhances the capability of photo-thermal absorption measurements with transversal probe beam guiding by overcoming drawbacks such as a lack of sensitivity for materials with low photo-thermal response and/or round substrate geometry. The sandwich concept using the laser-induced deflection technique is introduced and tested for the investigation of highly reflecting (HR) coatings. The idea behind the sandwich concept is based on the decoupling of the optical materials for the pump and probe beams. This is realized by either placing a HR coated rectangular substrate in between two optical (sandwich) plates or attaching a HR coated thin round substrate onto one optical plate. For both configurations, the sandwich concept results in a strong increase in sensitivity for the measurement of HR coatings deposited onto photo-thermally insensitive substrates. Experiments reveal that for a CaF2 substrate, up to two orders of magnitude enhancement in sensitivity can be achieved.

Laser induced deflection (LID) method for absolute absorption measurements of optical materials and thin films

We use optimized concepts to measure directly low absorption in optical materials and thin films at various laser wavelengths by the laser induced deflection (LID) technique. An independent absolute calibration, using electrical heaters, is applied to obtain absolute absorption data without the actual knowledge of the photo-thermal material properties. Verification of the absolute calibration is obtained by measuring different silicon samples at 633 nm where all laser light, apart from the measured reflection/scattering, is absorbed. Various experimental results for bulk materials and thin films are presented including measurements of fused silica and CaF2 at 193 nm, nonlinear crystals (LBO) for frequency conversion and AR coated fused silica for high power material processing at 1030 nm and Yb-doped silica raw materials for high power fiber lasers at 1550 nm. In particular for LBO the need of an independent calibration is demonstrated since thermal lens generation is dominated by stress-induced refractive index change which is in contrast to most of the common optical materials. The measured results are proven by numerical simulations and their influence on the measurement strategy and the obtained accuracy are shown.

Bulk absorption properties of LBO crystals

We present the results of comprehensive bulk absorption studies on numerous LBO crystals at 1070nm, 532nm and 355nm using the sandwich-LID measurement concept. One aim of the studies is answering the question whether there is a correlation between bulk absorption coefficients at the different applied wavelengths. Further, intensity dependent absorption measurements at 355nm are performed to investigate the nonlinear absorption in the LBO crystals. From the results it is verified, that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e. to sequential three photon absorption in addition to a potential intrinsic 3-photon absorption process.

Effect of Ion Beam Figuring and Subsequent AR Coating Deposition on the Surface Absorption of CaF 2 at 193nm

The CaF2 surface absorption at 193nm is investigated as function of surface treatment by different ion beam figuring (IBF) parameters and susequent AR coating deposition using laser induced deflection (LID) technique.

Measuring Low Absorption Losses in Coatings and Substrates by Laser Induced Deflection

An overview is given on the different concepts to absolutely measure very low absorption in optical materials and coating by using the laser induced deflection (LID) technique.

Direct absorption measurements in thin rods and optical fibers

Abstract. We report on the first realization of direct absorption measurements in thin rods and optical fibers using the laser-induced deflection (LID) technique. Typically, along the fiber processing chain, more or less technology steps are able to introduce additional losses to the starting material. After the final processing, the fibers are commonly characterized regarding losses using the so-called cut-back technique in combination with spectrometers. However, this serves only for a total loss determination. For optimization of the fiber processing, it would be of great interest not only to distinguish between different loss mechanisms, but also have a better understanding of possible causes. For measuring the absorption losses along the fiber processing, a particular concept for the LID technique is introduced and requirements, calibration procedure, as well as first results are presented. It allows us to measure thin rods, e.g., during preform manufacturing, as well as optical fibers. In addition, the results show the prospects also to apply the new concept to topics like characterizing unwanted absorption after fiber splicing or Bragg grating inscription.