FULL FIELD SWEPT-SOURCE OPTICAL COHERENCE TOMOGRAPHIC SYSTEM FOR SURFACE PROFILOMETRY OF MICROLENS ARRAYS

Transcription

1 FULL FIELD SWEPT-SOURCE OPTICAL COHERENCE TOMOGRAPHIC SYSTEM FOR SURFACE PROFILOMETRY OF MICROLENS ARRAYS Tulsi Anna, Dalip Singh Mehta a) and Chandra Shakher Laser Applications and Holography Laboratory, Instrument Design Development Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi , India. Abstract: A full field swept-source optical coherence tomography (FF-SS-OCT) system for surface profilometry of cylindrical microlens arrays has been proposed. In the optical arrangement, a swept-source system combined with a compact Michelson interferometer and an area detector was used. 3D surface profile of cylindrical micro-lenses i.e., both optically sectioned images and the phase map has been reconstructed. The proposed system does not require any mechanical scanning. In addition a common optical path enables the proposed technique to be highly stable. 1. INTRODUCTION The micro-lens arrays have been widely used in a variety of applications such as collimation, focusing, imaging, optical computing, display devices, TV projection systems, micro-lithography and telecommunication [1-3].Various fabrication and manufacturing techniques for micro-lens arrays, such as, photolithography, stuffing process, ion exchange, resist-melting technique and laser beam shaping have been developed [4-7]. Precise knowledge of imaging properties like surface geometry and refractive index is important to ensure high accuracy and lens-to-lens uniformity for its fabrication process. A number of different optical methods have been proposed for testing and characterization of micro-lens arrays [2,3,8-13]. Among these methods, the interferometric testing methods are more promising in its applications such that Twyman Green, Michelson, Mach Zehnder, digital holographic interferometer and phase shifting shearing interferometer [9-13]. These techniques have their own benefits but they are somewhat complicated and also they have low efficiency. To improve the yield efficiency, an effective technique based on optical interferometry has also been introduced for surface profile and refractive index distribution of a planar micro-lens array [1]. Recently a tomographic method and low coherence interferometry has also been introduced for the measurement of graded refractive index profile and thickness measurement of contact lenses [14, 15] respectively. More recently, a differential phase-contrast optical coherence tomography (DPC- OCT) in the time domain OCT with full-field OCT has been realized for a quantitative phase reconstruction of micro-lens arrays which has immense applications in industrial metrology [16]. Optical coherence tomography (OCT) has become a well-established technique in biomedical diagnostic and engineering fields [17,18]. It performs depth resolved optical slicing in the sample, making threedimensional (3D) reconstructions of internal structures possible. Based on low coherence interferometric technique, the OCT can be divided as time-domain OCT (TD-OCT) and frequency-domain (FD-OCT). Further, FD-OCT has two types- spectral domain (SD-OCT) and swept-source (SS-OCT) [17-19]. These Fourier methods offer faster imaging speed and also increase its sensitivity in comparison to TD-OCT. In swept-source (SS-OCT), the broad bandwidth optical source is tuned rapidly using suitable filter having narrow line width and the depth information retrieved by computing the fast Fourier transform of the recorded interference data. Most of the swept-source systems require rigorous mechanical scanning to sweep the frequency of the broad band light source and reference there in [20]. Recently we have introduced a full-field sweptsource optical coherence tomographic (FF-SS-OCT) system using a unique combination of superluminescent diode (SLD) and an acousto-optic tunable filter (AOTF) which is completely electronically controllable device [20]. By means of sweeping the frequency of the light source, the multiple interferograms generated by a compact Michelson interferometer were recorded by a chargecoupled device (CCD) camera. Hence both amplitude and phase map of the interference fringe signal were reconstructed simultaneously. We have applied the same system for the characterization and surface profile measurement of microlens arrays. Optically sectioned images of the cylindrical micro-lens arrays were obtained by selective Fourier filtering and the topography was retrieved from the phase map. The

2 main advantages of the proposed system are completely non-mechanical scanning, easy for alignment, high-stability and compactness. 2. PRINCIPLE AND EXPERIMENTAL DETAILS OF FF-SS-OCT SYSTEM The schematic diagram of a FF-SS-OCT system using the compact Michelson interferometer is illustrated in Fig. 1. Broadband light emitting from SLD (Model No. SLD-371-HP1-DIL-PM-PD, SUPERLUM Diodes Ltd.,) was coupled into the input of AOTF (NEOS Technologies, Inc., USA) through a polarization maintaining single mode optical fiber using a Fiber connector (FC). The light emerging from the swept source is then made incident into the compact Michelson interferometer. To make a compact and avoid tedious timeconsuming adjustment of Michelson interferometer the one arm of beam splitter BS) is coated with aluminum oxide, which acts as a reference mirror. SLD SLD driver FC connector AOTF R F Generator Collimating Lens L 1 Micro-lens Array Imaging lens L 2 Fig. 1. Schematic of Full-Field SS-OCT Light beam is then split into two different paths after passing through the interferometer and is directed towards the reference and the sample arms. The reflected beams from the reference mirror and scattering sample go back to the interferometer and generate a cross-correlation signal which is sensed by a detector i.e., CCD camera. The detected correlated intensity signal is usually expressed in k- space as [19]; I( x, S( x, R R RS 2 RRRS.cos(2 zk ( x, (1) where k is the optical wave number, S (x,y, is the source spectral density, R R and R S represent the reflectivity of the reference mirror and sample respectively, z is optical path difference between the sample and reference arm. The (x, is the interferometric phase-shift associated with the detector signal and can be expressed as; x, k 2k n x, k 1 (2) Once we get the phase value, a Two- dimensional CCD M2 B S M1 Computer refractive index distribution can be calculated using Eq. (2). Depth information of the sample can be retrieved by taking Fourier transformation. The axial resolution δz is equal to half of the coherence length l c by assuming that the full spectrum of the light 2 source is imaged onto l the 2ln CCD, 2 0 i.e. [17], z c 2 (3) where Δλ is the full width half maximum (FWHM) of the light source s spectrum and the lateral resolution of an OCT system depends upon the focusing condition and pixel size of CCD. The spectral characterization of SLD and AOTF has been achieved by a high resolution spectrometer (HR 4000 Ocean Optics Ltd). Spectral full-width half maximum (FWHM) of SLD is calculated as nm and at the input current of 174 ma (temperature 25 0 C), the peak power of SLD is 7.5 mw. AOTF has high speed, large tunability range nm with radio frequency (RF) of range MHz. These are solid-state electronically tunable optical filters that select precise wavelengths by applying appropriate RF and hence mechanically moving parts are not required. Application of RF to AOTF transducer controls the transmitted wavelength. The RF was changed linearly with a constant step of 0.1 MHz from 87 to 95 MHz and the tuned spectrum was recorded using spectrometer. It was observed that a linear relationship between RF and peak wavelength and for every 0.1 MHz shift in the input frequency. Observation reveals that the spectral line width (δλ) of the frequency-tuned AOTF spectrum remains the same throughout the sweeping width and is found to be ~1.5 nm. Assuming the SLD and AOTF tuned spectrum nearly Gaussian, the calculated coherence length l c of SLD, by substituting the λ = nm and λ 0 = nm in Eq. (3) turns out to be μm. Similarly l c of tuned spectrum from AOTF was calculated by substituting spectral line-width δλ = 1.5 nm and λ 0 = in Eq. (3) comes out to be mm. We obtain a fine-tuned spatially coherent but temporally low coherent light at the output of the AOTF i.e., a low-coherence interferometry. The output beam from the AOTF was collimated towards the object using collimating lens L 1. The sample i.e., cylindrical micro-lens arrays were obtained from ISUZU GLASS, JAPAN. The sample was mounted on a fixed object stage in front of the collimating lens L 1 and imaged by the imaging lens L 2 as shown in Fig.1. In the present case the area of illumination is quite large so that no lateral scanning is needed and with this above mentioned arrangement the entire object area remains focused. The refracted light from the sample was then made to incident on

3 Intensity (A.U.) the compact modified Michelson interferometer. Coated surface of the BS works as a fixed reference mirror and uncoated side is used for another mirror. An external mirror was placed very close to the uncoated side of the BS as shown in Fig. 1. This arrangement makes sure that the optical path difference between the two mirrors is nearly equal and which remains always within the coherence length of the light source. Hence the interference fringes can easily be obtained by simply placing the mirror in contact on the other side of BS. The source spectrum was then tuned in a constant step of about 0.75 nm by sweeping the input RF to AOTF in the step of 0.1 MHz from 87 MHz to 95 MHz. A total of 81 interferograms were recorded using a CCD detector (Roper Scientific, Inc.) having 1392 x 1024 pixels with each pixel size 6.5 μm x 6.5 μm. 3. RESULT AND DISCUSSION An algorithm written in MATLAB software is used for analysis of interferograms which were recorded by CCD camera. Figure 3 (a) shows the example of an interferogram recorded at RF of 91 MHz for the cylindrical micro-lens arrays. In this Figure only the fringes of different curvature are visible due to the height variation over the surface. Interferograms for the entire tuned spectrum were stacked together along the wavelength axis and corresponding variation of intensity was computed for a fixed lateral position of the interferograms. Fast Fourier transform (FFT) of the interference fringe signal was then computed which provides multiple peaks as shown in Fig. 3 (b) each corresponding to different depth layers of the object. Optically sectioned (amplitude) images of the micro-lens arrays were obtained by selective Fourier filtering of each peak. Figures 3 (c) (e) show the OCT images at different depth positions. Figure 3 (c) corresponds to a particular depth position which is near to the dc component. It can be seen from Fig. 3 (c) the cylindrical lens arrays and character written in glass are very blur. In this case also the best optically sectioned image was obtained by means of filtering the first-order peak (prominent) and the results of the OCT image of the cylindrical micro-lens arrays are shown in Fig. 3(d). In Fig. 3(d) the entire area of cylindrical lens arrays and some number written at the right corner of the glass i.e., 4.5 are clearly visible with sharp edges. Fig. 3(e) was obtained by filtering the higher order peak and this image have less information about the object. Hence as go to the higher depth detailed features of the object and quality of the image is degraded due to the presence of noise at higher frequency. Since the Fourier-transform technique gives information about both the amplitude and the phase of spectral interference fringe signals. Optically sectioned images of the object have been obtained from the amplitude information as can be seen in Figs. 3(c)-(e). However, we have also reconstructed the phase information of the interference fringe signal. The analysis was done by selecting the small area of the object and the 3D profilometry i.e., topography of the cylindrical lens arrays was obtained from the phase map which is shown in Fig. 3 (f). A uniform height variation of cylindrical lens arrays can be easily seen in the Fig. 3 (f). (a) (c) (e) Freq. c/ sec (b) (d) Fig.3. (a) an interferogram at RF of 91.0 MHz (b) Fourier transform of detected signal (c)-(e) optically sectioned images (f) Phase profile of cylindrical micro-lens arrays. The axial resolution of the present system depends upon the FWHM of source spectrum, of SLD i.e., 6.5 μm calculated from Eq. (3) and the maximum range of axial direction (imaging depth) depends upon the wavelength sampling interval and instantaneous line-width of the AOTF. For the central wavelength λ 0 = nm and wavelength sampling interval of 0.75 nm, the maximum imaging depth in present case comes out to be 236 µm. Therefore, by optimizing the sampling interval and the instantaneous line-width of the source, crosssectional tomographic images of the sample at a particular depth position could be obtained with better axial resolution. Lateral resolution of the system depends upon the focusing condition and pixel size of CCD. Only with a few microwatts of power system provide the 3D phase profile of the cylindrical micro-lens arrays so this system can be (f)

4 applied for the imaging, testing, subsurface uniformity and surface profiling of Micro-optical elements from the amplitude images. Phase profiles gives the quantitative information of refractive index distribution in microlens arrays which is the fundamental parameter for check the accuracy, quality of the micro-lens arrays. Low frame rate CCD camera limits the response time of the present system. Resolution of the present system can be further improved by using broad-band thermal sources or supercontinuum light sources with very short coherence length. 4. CONCLUSION We have demonstrated the 3D-surface profilometry of refracting samples using full field swept-source optical coherence tomography (FF-SS-OCT). Simultaneous measurement of the optically sectioned imaging and corresponding phase map of the microlens arrays were experimentally demonstrated. The present system is very compact because of its optical arrangement i.e., a SLD as low coherence light source, an AOTF as electronically controlled frequency tuning device and a Modified Michelson interferometer. This technique can play an important role in imaging and surface profiling of microlens arrays for inspecting the surface defects and its quality. 5. REFERENCES 1. C. Quan, S.H. Wang, C.J. Tay, I. Reading and Z.P. Fang, Integrated optical inspection on surface geometry and refractive index distribution of a microlens array, Opt. Commun. 225, 223 (2003). 2. T. Miyashita, Standardization for Microlenses and Microlens Arrays, J. Appl. Phy.; 46, 5391 (2007). 3. P. Nussbaum, R.Volkel, H.P. Herzig. M. Eisner and S. Haselbeck, Design, fabrication and testing of microlens arrays for sensors and Microsystems, J. Pure. Appl. Opt. 6, 617 (1997). 4. H. Ottevaere, B.Volckaerts, J. Lamprecht, J. Schwider, A. Hermanne, I. Veretennicoff and H. Thienpont, Two-dimensional plastic microlens arrays by deep lithography with protons: fabrication and characterization, J. Opt. A: Pure App.l Opt. 4, S22 (2002). 5. J. Bahr, K.H. Brenner, T. Singer, S. Sinzinger and M. Testorf, Index-distributed planar microlenses for three-dimensional micro-optics fabricated by silver-sodium ion exchange in BGG35 substrates, Appl. Opt. 33, 5919 (1994). 6. S. Haselbeck, H. Schreiber, J. Schwider, N. Streibl, Microlenses fabricated by melting photoresist, Opt. Eng. 6, 1322 (1993). 7. V. Sturm, H. G. Treusch and P. Loosen, Cylindrical microlenses for collimating highpower diode lasers, Proc. SPIE. 3097, 717 (1997). 8. L. Yulin, L. Tonghai, J. Guohua, H. Baowen, H. Junmin, W. Lili, Research on micro-optical lenses fabrication technology, Optik; 118, 395 (2007). 9. H.J. Tiziani, T. Haist and S. Reuter, Optical inspection and characterization of micro-optics using confocal microscopy, Opt. Lasers. Eng.36, 403 (2001). 10. S. Reichelt and H. Zappe, Combined Twyman Green and Mach Zehnder interferometer for microlens testing, App. Opt. 44, 5786 (2005). 11. S.H. Wang and I. Reading, Evaluation of a Microlens Array using Optical Interferometry, STR/03/046/PM1www.simtech.astar.edu.sg/Resear ch/technicalreports/tr0366.pdf. 12. F. Charriere, J. Kuhn, T. Colomb, F. Montfort, E. Cuche, Y. Emery, K. Weible, P. Marquet and C. Depeursinge, Characterization of microlenses by digital holographic microscopy, App. Opt. 45, 829 (2006). 13. H. Sickinger, O. Falkenstorfer, N. Lindlein and J. Schwide, Characterization of microlenses using a phase-shifting shearing interferometer, Opt. Egg. 33, 2680 (1994). 14. E. Acosta, D. Vazquez, L. Garner and G. Smith, Tomographic method for measurement of the gradient refractive index of the crystalline lens. I. The spherical fish lens, J. Opt. Soc. Am. (A 22), 424 (2005). 15. I. Verrier, C. Veillas and T. Lepine, Low coherence interferometry for central thickness measurement of rigid and soft contact lenses, Opt. Exp. 17, 9157 (2009). 16. B. Heise and D. Stifter, Quantitative phase reconstruction for orthogonal-scanning differential phase-contrast optical coherence tomography, Opt. Lett. 34, 1306 (2009). 17. B. E. Bouma and G. J. Tearney, Handbook of optical coherence tomography New York: Marcel Dekker. (2002). 18. D. Stifter, Beyond biomedicine: a review of alternative applications and developments for optical coherence tomography, Appl. Phys. B 88, 337 (2007). 19. M.V. Sarunic, M.A. Choma, C. Yang and J.A. Izatt, Instantaneous complex conjugate resolved spectral and swept-source OCT using 3x3 fiber couplers, Opt. Exp. 13, 957 (2005).

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