Glass diffractive optical beam shaper for laser applications Steffen Reichel*, Uwe Petzold**, Helge Vogt*, Ralf Biertümpfel* *SCHOTT AG, Advanced Optics, 55122 Mainz, Germany **Technical University of Darmstadt, Department of Physics, 64298 Darmstadt, Germany mailto:steffen.reichel@schott.com Applications, such as pico-projectors need a rectangular beam distribution – so called flat hat distribution. SCHOTT presents a diffractive optical element (DOE) made out of glass that shapes the Gaussian light distribution of the laser source into a flat hat distribution. Design and measurements for different designs are compared showing the good performance of the SCHOTT DOEs. 1 Introduction Applications, such as pico-projectors with laser sources or medical laser skin treatment, require a rectangular homogeneous beam distribution – so called flat hat distribution. However, many lasers emit only Gaussian shaped light (due to the fun- damental - TEM00 - mode). SCHOTT presents a glass diffractive optical ele- ment (DOE) that shapes the Gaussian light distri- bution into a flat hat. Optical glasses have the sig- nificant advantage of high laser durability, low scat- tering losses, high resistance to moisture, chemi- cals, and temperature compared to polymer DOEs. The glass DOEs were manufactured by using SCHOTT’s precise molding technology. The design as well as measurement results of manufacturing SCHOTT DOEs are presented in comparison. 2 Introduction into DOEs Design In order to understand how a phase DOE works we repeat the result of scalar diffraction theory for the case of Fraunhofer diffraction, where the dif- fracted electric field E dif on a screen (with coordi- nates x, y, z) can be calculated from [1], [2]: ( ) ( ) ∫∫ +∞ ∞ − +∞ ∞ − − + − ⋅ = 0 0 0 0 0 0 0 , , , dy dx e y x P C z y x E z z yy xx k i dif , (1) where C = C(x,y,z) is a factor that we ignore here for simplicity, k = 2π/λ, and P(x 0 ,y 0 ) is the pupil function at the position x 0 and y 0 . Eq. 1 is the two dimensional Fourier transform of the pupil function. The diffracted electric field E dif corresponds to the required light distribution on the screen (= flat hat). The pupil function is the required phase DOE. This means, in the case of Fraunhofer diffraction, the DOE can be calculated by inverse Fourier trans- form of the diffracted electric field on a screen. -In most practical applications the far-field approxima- tion (Fraunhofer diffraction) cannot be used and thus Fresnel approximation must be used instead. In addition, Eq. (1) assumes a homogeneous light distribution, which is not the case when using laser illumination with Gaussian distribution. Thus the calculation of the DOE (pupil function) is a complex mathematical problem, which is iteratively solved and implemented in commercial software as illus- trated in example [3]. DOEs are designed to use the 1 st diffraction order and the continuous phase is approximated by a staircase function - typically an 8 or 16 level quan- tization – resulting in a theoretical diffraction effi- ciency of 95% or 98%, respectively [1]. This maxi- mum efficiency can be archived (should this be “achieved”) using Blaze technique [5] where the height h of the DOE structure (corresponding to 2π phase difference) must be chosen to [4] () 1 − = λ λ n h , (2) with λ the vacuum wavelength and n the refractive index of the material at wavelength λ. 3 Glass DOE manufacturing by precise molding SCHOTT uses a fast replication technique for the DOE manufacturing which is capable of producing mass quantities. In this precise molding process a replication master is first manufactured by lithogra- phy. Then the DOE is manufactured in a clean room facility as follows, see Fig. 1: A piece of glass is heated to a temperature where the glass is de- formable. Then the glass is pressed into the final shape. Eventually, the glass is cooled down. The correct thermal management of the cooling step is essential for the accuracy of the DOE which can be coated (with anti-reflective coating) and sold to the customer. SCHOTT offers the following glasses that are especially suited for our DOE manufacturing process: P-LaSF47 (n d = 1.8016), P-SK57 (n d = 1.5843), N-LaF33 (n d = 1.7821), and P-SF67 (n d = 1.8998). These glasses show a very high performance concerning repeatability and accuracy of the pressed DOE structures. DGaO Proceedings 2009 – http://www.dgao-proceedings.de – ISSN: 1614-8436