Christian Mühlig

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.

Bulk absorption measurements of highly transparent DUV/VUV optical materials

The laser induced deflection technique (LID) is introduced for measuring small absorption coefficients of highly transparent DUV/VUV optical materials with high sensitivity and accuracy. The measuring principle, the calibration and the developed experimental realization are explained. At 193 nm in situ absorption and fluorescence measurements of fused silica give evidence that a commonly observed absorption decrease at the onset of laser irradiation is a bulk effect and due to a diminution of oxygen deficient centers ODC II. This decline is caused by a single photon absorption process and terminates after a dose of 4-5 kJ/cm2. Fluence dependent bulk absorption measurements of fused silica are presented which indicate the presence of a nonlinear dependence between the absorption coefficient α and the fluence H. For calcium fluoride a very good agreement between direct absorption and conventional transmission measurements is obtained. At 157 nm, a modified compact experimental setup is introduced which exhibits a significantly higher sensitivity than that applied for 193 nm experiments. First measurements of high quality calcium fluoride show that the obtained absorption is independent on the laser repetition rate. The investigation of equivalent CaF2 samples of different thickness (10 mm and 20 mm) indicates that the measured absorption coefficient is virtually free of contributions from the irradiated surfaces. Finally, a very good agreement is obtained by comparing LID data with transmission measurements of 100 mm long samples.

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.

Absorption and Fluorescence Measurements in Optical Coatings

Absorption characterization of optical thin films and coatings has become one central task for the manufacturers e.g. to ensure stability in the production process, to verify functionalities and to understand possible performance changes and limitations during their use in e.g. high power laser applications. Set by this trend, numerous direct absorption measurement techniques have been developed in the last two decades which all—despite particular pro and cons—feature a high sensitivity. However, the different techniques possess remarkable differences regarding a universal and efficient absolute calibration procedure. After a survey of different measurement techniques with their calibration procedures, this chapter will focus on the laser induced deflection (LID) technique, its independent absolute calibration, particular measurement concepts and experimental results.

Investigation of vacuum deposited hybrid coatings of protic organic UV absorbers embedded in a silica matrix used for the UV protection of Polycarbonate glazing

A study of vacuum-deposited organic–inorganic hybrid coatings for UV protection of polycarbonate is presented. UV-absorbing compounds, which are commonly used for polycarbonate, were embedded in a silica matrix by thermal co-evaporation under high vacuum. In addition to the optical properties of the coatings, the influence of the silica network on the organic UV absorber and the stability of the intramolecular hydrogen bond (IMHB) are discussed. A model is presented to show the interaction between the organic compound and the silica matrix. It could be shown with UV irradiation experiments that the hydroxyphenyltriazine compound exhibits higher UV stability in the hybrid coating than the hydroxybenzotriazoles.

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.