1. Introduction

Adaptive mirrors based on thermo-optically induced modifications of the surface offer a promising alternative to conventional adaptive optics since they can combine simplicity with low cost. A thermal expansion based adaptive mirror with electrical heating was already presented in [1]. Other adaptive mirrors based on MEMS or piezo-crystals are relatively expensive and sometimes also lack in lateral resolution. A rather elegant way is the use of a thermo-optically driven adaptive mirror based on thermal expansion. Such a system has been described in [2]. A similar solution is found in using thermal dispersion [3]. Both might be promising tools to modify the output distributions of laser beams [4] or to correct slowly varying aberrations of optical systems.

The mirror described in [2] showed, however, an insufficient optical surface quality. Also its lateral and temporal resolution may be further enhanced with the use of thinner layers. Thin layers coated on a substrate with high thermal conductivity allow a more efficient axial heat removal. This will lead to higher resolution, both, spatially and temporally.

In our letter, we report on a thermo-optically driven adaptive mirror that is also based on Sylgard 184 as in [2]. The Sylgard layer has a thickness of 300 μm. Black dye is added to enhance optical absorption. The layer is coated onto a sapphire substrate for better heat removal. We present a preparation method that leads to a rather high-quality optical surface. The thin film is characterized in an interferometer with and without thermo-optical modification. The thermo-optical modifications induced by imaging a line pattern onto the rear side of the layer are analyzed. Both, spatial and temporal resolution is measured.

2. Materials

A selection of materials with a high coefficient of thermal expansion is shown in Tab. 1. Cellulose Acetate and low-density polyethylene (LDPE) are two examples with very high thermal expansion coefficients. Although very interesting from this point of view, they have a relatively high specific heat capacity and are not particularly suited for custom preparation of thin layers. Still in an acceptable range of the thermal expansion coefficient are Teflon films. With still a quite high coefficient of thermal expansion of 310 ppm, Sylgard 184 is a suitable choice. It is supplied as two-part liquid component kit and can be molded in the desired shape. It cures at room temperature in about 48 hours. Cured Sylgard 184 is insoluble in water, stable and elastic at temperatures from -50°C to 200°C.

Table 1. Selected materials for thin films

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3. Preparation of Sylgard 184 layers

In order to increase the absorption for optical heating, the Sylgard is dyed with black pigment (Elfenbeinschwarz 242). A concentration of 20 mg/ml was enough to increase the absorption while keeping the curing ability of Sylgard unaffected. As measured with a Perkin-Elmer Lambda 9 spectrometer, the transmittance of a 300 μm thick black dyed Sylgard layer is about 25 % between 500 nm and 1000 nm, whereas pure Sylgard is almost completely transparent.

 

Fig. 1. Preparation procedure for thin Sylgard films

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The preparation of the Sylgard layer is shown schematically in Fig. 1. Good surface quality is achieved if Sylgard is cast and then pressed between two glass plates with λ/20 surface quality. The layer thickness is preset with a spacer. Since the adhesion of glass to Sylgard is very strong, removing of the upper window leads to destruction of the film. It can, however, easily be removed, if the glass plate has previously been coated with a 100 nm layer of NaCl. This NaCl layer is applied by standard evaporation technique. After curing the Sylgard, the glass plates with the cast Sylgard layer are placed into a container with deionized water. After one day, the NaCl layer is dissolved and the upper window can be removed without damaging the Sylgard layer.

Some thin film mirrors were then gold sputtered to increase the reflection. Nevertheless, interferometric measurements have been performed without gold layer for better contrast in the interferometer.

4. Thin film mirror characterization

The adaptive mirrors are characterized in the arrangement shown in Fig. 2. The adaptive mirror is tested with an interferometer (Zygo Mark II). The mirror can be heated from the rear side with a slide projector (Leica Pradovit color 2) with a 250 W incandescent lamp. Light transmitted through the adaptive mirror is blocked with a band-pass filter for 632 nm. The pattern of the heated zone can be chosen with a suitable slide. The optics of the projector is modified with an additional lens of 160 mm focal length. The image on the adaptive mirror is 1.5 times the size of the slide. The intensity is 370 mW/cm².

 

Fig. 2. Experimental arrangement for thin film mirror characterization

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Figure 3 shows the interferogram of a 300 μm thick Sylgard layer on a sapphire substrate. Over the diameter of 24 mm of the Sylgard layer the flatness of the surface is about half a fringe (λ/4 @ 632 nm).

 

Fig. 3. Interferometric image of a 300 μm Sylgard layer on sapphire

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Fig. 4. Interference fringes due to disturbance of 1 lp/mm in 300 μm Sylgard layer

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Figure 4 shows the interferogram of the adaptive mirror when irradiated in the centre with the image of three horizontal lines of 0.5 mm width and 0.5 mm spacing (one line-pair per mm). The coarse distortion of the fringes is due to the band-pass filter. For a better visibility of the surface deformations the interferograms are evaluated using IDEA 1.7 software [6]. The resolution of the adaptive mirror is measured with a USAF 1951 test pattern (Fig. 5, left).

 

Fig. 5. Test pattern left; Response of the adaptive mirror irradiated with the test pattern right

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Figure 5 right shows the response of the active mirror when irradiated with the test pattern. The blurred image shows that the resolution is not sufficient to clearly reproduce the test pattern. Nevertheless, in the largest structures some contrast still can be measured. Profiles across the pattern are recorded to determine the line contrast. An example with contrast B/A = 60 % is shown in Fig. 6. Fig. 7 shows the contrast of several patterns with different number of line pairs.

 

Fig. 6. Contrast B/A = 60 % in a 300 μm Sylgard layer irradiated with 1.12 lp/mm

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Fig. 7. Contrast for 300 μm Sylgard layer. The trend line follows a power law

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Irradiation with 0.25 lp/mm at 370 mW/cm² intensity results in a surface modulation of 350 nm at a slope of 237 nm/mm. The resolution of 0.25 lp/mm gives a contrast of 100 %. As shown in Fig.7 the contrast falls to 30 % with a resolution of 1.6 lp/mm.

A cycle of heating on and off was recorded with a camera at 15 frames per second. The movie clip was analyzed frame per frame to detect the start of the heating and the beginning of static modification. The rise time (10 % to 90 %) and fall time was measured to be 12 and 11 frames, respectively, just less than one second.

5. Conclusion

Experiments are performed with the goal to enhance the spatial and temporal resolution of a thermo-optically driven adaptive mirror. A technique to prepare thin films of Sylgard is described. The 300 μm thick films have an optical surface quality better than λ/4 over the 24 mm diameter of the substrate. The resolution of the thin film mirror was analyzed in an interferometer. Modulation of 350 nm is achieved with an intensity of only 370 mW/cm². 100 % contrast is obtained with a pattern of 0.25 lp/mm. It drops to 30 % at a resolution of 1.6 lp/mm. As for the temporal resolution, rise and fall times are below one second.