The real world homogenizing performance of the fabricated linear diffusers was evaluated with an Excimer laser at the Bayrische Laser Zentrum in Erlangen, Germany. 10 pulses of 28 ns pulse width were applied to a PMMA sample slide. The traces were then inspected with a polarizing optical microscope equipped with Nomarski prism and used in differential interference contrast (Figure 5.8). This gives a good qualitative impression of the homogeneity of the beam profile. The laser was a Coherent-Laser ExciStar M-100 ArF Excimer laser at 193 nm
5.5. Performance evaluation with an Excimer laser
with a mean power of up to 12 W, a pulse energy of 120 mJ and a pulse width of 28 ns. The beam dimensions at the exit were 4 mm × 15 mm and the beam divergence 1 mrad × 3 mrad. The setup was composed of a fly’s eye condenser based flat top element (FT5 Microlens Array Nr. 18-00254 from SUSS MicroOptics), the random linear diffuser, a field lens and the PMMA slide which served as target. See Figure 5.7 for a schematic illustration of the setup.
A B C D
Figure 5.7: Schematics of the test setup with an Excimer laser source. From left to right: raw beam, fly’s eye condenser flat top element (A), random linear diffuser (B), field lens (C) and PMMA target slide (D) at the focus of the field lens.
Figure 5.8a shows the trace left in the PMMA sample if only the flat top element is used without any linear diffuser. A regular diffraction pattern generated by this micro optical element is clearly visible. If we add the linear diffuser after the flat top element (Figure 5.8b) we see that everything gets smeared out resulting in a smooth profile. Even after several runs the linear diffuser element did not exhibit any signs of laser induced damage showing the suitability for high power laser applications.
(a) (b)
Figure 5.8: Traces left in a PMMA slides by an Excimer laser after passing through different beam shaping elements. (a) Flat top element from SUSS MicroOptics. The periodic intensity peaks are clearly visible.(b) Fly’s eye condenser flat top element combined with a linear diffuser with a diffusion angle of 1° FWHM. Compared to (a) the intensity is very smooth without any periodic intensity peaks.
Chapter 5. Results
no big surprise since the coherence length of the monomode HeNe laser that was used as a source for the goniophotometer measurements is considerably longer than the coherence length of an Excimer laser. The Excimer laser is actually a source that is only partially coherent [53]. In order to model such a source correctly it would be necessary to overlay the far field intensity obtained by the diffraction model (Section 3.3) with a convolution of the source intensity profile with the diffuser [54].
6
Conclusion
A true one dimensional diffuser suitable for high power laser beam shaping application has been realized based on concave structures preventing focusing hot spots. The FWHM angle of the diffuser can be selected within the range of 0.1° to 20° by adapting a single fabrication parameter (etch time) which influences the ROC of the lens. An analytical model for the estimation of the diffusion angle was proposed and its precision to a few 10th of degrees experimentally verified. A diffraction based model of the linear diffuser showed good agree- ment with the measured data. Simulation based on Gaussian beam decomposition algorithm was also done, showing that this approach gives somewhat questionable information on the intensity profile but results in comparable prediction of the FWHM diffusion angle. The per- formance of the fabricated devices inside a complete beam shaping system was verified and it was shown that the use of a linear diffuser can improve the homogeneity of a line generated with a fly’s eye beam shaping element. The fabricated diffuser showed absolutely no central transmission peak (zero order) as it is required by many beam homogenizing applications. The measured transmission efficiency is very good. The Fresnel losses for the shallow structures are comparable to an unstructured fused silica slide and could be reduced even further by the use of apropriate anti reflection coatings.
Part II
7
Introduction
Spreading the angular intensity distribution of diverging sources such as (uncollimated) LEDs or laser diodes can not be done with the technology proposed in the first part of this thesis on small angle diffusers. Since the microlens array based technology allows a maximal diffusion angle of 20° FWHM it is not efficiently spreading the angular spectrum for sources with a divergence of the same order as the maximum diffusion angle or higher. For sources with a large divergence the small angle diffusers only work as homogenizers.
Instead of the microlens based diffuser that is basically made for collimated sources and small angles, an angle transformer of some sort is needed that transforms an input half angle θi into
an output half angle θo. See Figure 7.1 for a schematic illustration of an angular transformer
with acceptance half angle θi, output angle θoand the input and output aperture with diameter
2a and 2a�. The output angle θocan be up to π/2 thus creating a Lambertian source out of a source with less angular extend.
θ
iθ
iθ
o2a
2a'
Figure 7.1: Schematic illustration of an angle transformer with acceptance half angle θi, output
angle θoand the input and output aperture with diameter 2a and 2a�.
Such angular transformers exist in the form of non imaging concentrators like the compound conic concentrators (e.g. the compound parabolic concentrator CPC), the light cone con- centrator or a combination thereof. They have been invented at the beginning of the second half of the last century almost simultaneously by Baranov, Hinterberger and Winston [55, 56].
Chapter 7. Introduction
Primary applications for those concentrators is the field of thermal solar energy [57, 58]. The novelty of the approach presented in this thesis lies in the compactness of the design and the use of the concentrator not as such but rather as an angle transformer with very high efficiency. When the dimensions of the classical solar concentrators are usually of the order of a few 10 cm the design developed in this thesis has dimensions of a few mm or less. Compound parabolic concentrators (CPC) in the µm range have been realized by Atwater et al. [59] for potential use as LED collimator. Figure 7.2 shows an SEM micro graph of a micro CPC array fabricated by two photon absorption lithography and focused ion beam drilling reported in [59].
Figure 7.2: SEM micro graph of a densely packed micro photonic CPC array reported by Atwater (http://e3scenter.org/pubs/102.html).
For the scope of this thesis the lower limit of the miniaturization is given by the limit when diffraction induces larger angle deviations than refraction. Typically at dimensions of < 50 µm the effects of diffraction start to dominate refraction for light in the visible range and we call this limit the limit of refraction.
Chapter 8 is giving an introduction to non imaging optics and its basic design principles such as the edge ray principle or the string method. The design of the compound parabolic concentrator and its variations in the form of the dielectric filled CPC and the θi/θotransformer
are described in more details in Section 8.4.
Chapter 9 treats possible applications for compact CPC arrays other than large angle diffusers for diverging sources. The possible applications discussed in this thesis are LED collimation at chip level, fiber coupler with large NA and improved light management for photovoltaic solar cells. The implementation and fabrication of a compact dielectric CPC array is discussed in Chapter 10.