Optimization of multiplexed holographic gratings in ... - OSA Publishing
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OPTICS LETTERS / Vol. 33, No. 6 / March 15, 2008
Optimization of multiplexed holographic gratings in PQ-PMMA for spectral–spatial imaging filters Yuan Luo,1,2,* Paul J. Gelsinger,2 Jennifer K. Barton,1,2,3 George Barbastathis,4 and Raymond K. Kostuk1,2 1
Department of Electrical and Computing Engineering, University of Arizona, Tucson, Arizona 85721, USA 2 College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA 3 Division of Biomedical Engineering, University of Arizona, Tucson, Arizona 85721, USA 4 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA *Corresponding author: [email protected]
Received November 19, 2007; revised January 26, 2008; accepted February 6, 2008; posted February 12, 2008 (Doc. ID 89889); published March 11, 2008 Holographic gratings formed in thick phenanthrenquinone- (PQ-) doped poly(methyl methacrylate) (PMMA) can be made to have narrowband spectral and spatial transmittance filtering properties. We present the design and performance of angle-multiplexed holographic filters formed in PQ-PMMA at 488 nm and reconstructed with a LED operated at ⬃630 nm. The dark delay time between exposure and the preillumination exposure of the polymer prior to exposure of the holographic area are varied to optimize the diffraction efficiency of multiplexed holographic filters. The resultant holographic filters can enhance the performance of four-dimensional spatial–spectral imaging systems. The optimized filters are used to simultaneously sample spatial and spectral information at five different depths separated by 50 m within biological tissue samples. © 2008 Optical Society of America OCIS codes: 090.4220, 090.7330, 110.0110, 090.2890, 160.0160.
Spectral–spatial volume holographic imaging systems [1,2] require highly selective filters to discriminate wavefronts originating from different depths within a 3D object. The filter should also operate at longer wavelengths allowing greater imaging depths within tissue samples. The holographic filters formed in the volume recording material are phase gratings and offer high angular–spectral selectivity and the ability to multiplex multiple gratings in the same area [3]. In this Letter we describe the design of holographic transmission filters formed in the phenanthrenquinone-doped poly(methyl methacrylate) (PQ-PMMA) material. Experimental results demonstrate the performance of the volume holographic filters in a spectral–spatial imaging system to reconstruct images of 3D objects. The holographic recording material is formed by using solutions of methyl methacrylate (MMA), 2,2-azobis(2-methlpropionitrile) (AIBN), and phenanthrenquinone (PQ) that are mixed in a respective weight ratio of 100:0.5:0.7. PQ-doped PMMA is an attractive material for making holographic Bragg filters, since the material can be formed with minimal shrinkage and refractive index change after processing [4]. In addition, the PQ-PMMA holographic filters provide high spectral and angular Bragg selectivity. The mixture is poured into a mold similar to that shown in Fig. 1, consisting of two glass plates separated by a flexible spacer. The mixed solution solidifies after curing at 50°C for 120 h. After the thermal polymerization process is complete, the solid sample is approximately 1.8 mm thick. For our experimental system configuration, the resultant material is cut into 5 cm⫻ 5 cm squares and fitted onto a sample holder for holographic exposure. 0146-9592/08/060566-3/$15.00
The holograms are recorded with the setup shown in Fig. 2 by using an argon-ion laser operating at a wavelength of 488 nm. The reference beam is a collimated wave, and the signal beam is a spherical wave originating from a point corresponding to the depth position within the sample. A different z position is recorded for each depth that will be imaged within the sample. The positions of the point source in the signal arm are controlled by moving a microscope objective lens with numerical aperture (NA) of 0.65 along the axial direction. A second microscope objective lens with 0.55 NA remains in a fixed position in the signal arm, forming the point source. A relay system is used in the signal arm to maintain constant irradiance at the hologram plane as the 0.65 NA microscope object is moved. The nominal angle between two arms is ⬃68° and is changed by ⌬ with each exposure to record a hologram with a different reference beam angle and point source location. The angle settings, point source locations, and exposure time settings are automated by using a LabView control system. The hologram exposures are varied to in-
Fig. 1. Mold for the sample mixture preparation. © 2008 Optical Society of America
March 15, 2008 / Vol. 33, No. 6 / OPTICS LETTERS
crease the efficiency of gratings that select positions deeper within the tissue sample. Figure 3 is a rigorous coupled wave simulation [5] of the angular selectivity for a single grating formed with material parameters similar to our PQ-PMMA samples (thickness ⬃1.8 mm, n ⬃ 1.49, absorption coefficient ⬃0.009/ mm). The FWHM of the efficiency versus angle curve is ⬃0.03° and is equal to the measured selectivity of the experimental gratings. This implies that to avoid cross talk between multiplexed gratings, the angular difference 共⌬兲 between reference beams angles should be greater than 0.03°. In the experiments ⌬ was set at ⬃1° to avoid image overlap during reconstruction. The dark delay time 共tdd兲 between each exposure affects the diffraction efficiency 共兲 of multiplexed
567
Table 1. Single Gratings with Constant Exposure of 610 mJ/ cm2
(%)
Preillumination 共mJ/ cm2兲 0 120 360 460 760
20 38 44 40 48
Table 2. Two Multiplexed Gratings with Constant Exposure of 610 mJ/ cm2 Preillumination 共mJ/ cm2兲
Cumulative Grating Strength M=2冑 共兺i=1 i兲
0 220 290 360
0.78 1.06 1.04 1.31
Table 3. Five Multiplexed Gratings with tdd = 10 s and Preillumination of 360 mJ/ cm2 Grating Order 1 2 3 4 5 Fig. 2. Construction setup of the multiplexed holographic filters. M, mirrors; BS, beam splitter.
Fig. 3. grating.
Rigorous coupled wave simulation for single
Exposure Energy 共mJ/ cm2兲
(%)
579 670 762 853 944
17 26 33 40 46
gratings formed in optical polymers [6]. To quantify this effect in PQ-PMMA holograms, five multiplexed gratings were made with two collimated waves (⌬z = 0, and ⌬ = 1°). Five multiplexed gratings were made with dark delay times varied from 10 s to 5 min with constant exposure energy 共⬃760 mJ/ cm2兲 per grating and no preexposure energy. Figure 4 shows for the two different values of the dark delay time. The tdd was 10 s for sample F29A, and 5 min for sample F31. Number 1 indicates the first grating formed in the sequence, and number 5 is the last grating. The results show that with a longer delay time the cumulative grating strength, defined as C M 冑 i [7], is lower. Therefore, the overall efficiency = 兺i=1 is lower with a longer delay time and the efficiency of the first grating formed in the sequence is enhanced at the expense of gratings formed with later expo-
Fig. 4. Measured multiplexed grating diffraction efficiency with different dark delay times between exposures.
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OPTICS LETTERS / Vol. 33, No. 6 / March 15, 2008
Fig. 5. Measured multiplexed grating diffraction efficiency of five multiplexed gratings with tdd = 10 s and a preillumination of 360 mJ/ cm2.
Fig. 6. Experimental imaging geometry.
crease in the diffraction efficiency of single and multiple gratings formed in PQ-PMMA is possible with PIE values of a few hundred milli joules per square centimeter. After the exposure energy is adjusted as shown in Table 3 and a preexposure of 360 mJ/ cm2 with constant tdd = 10 s is added, the efficiencies are significantly improved. The gratings shown in Table 3 and Fig. 5 were formed with a spherical and planar wave with the point source moved by increments of ⌬z = 50 m and separation angle between reference waves of ⌬ = 1°. The maximum is 46%, and the minimum is 17%. This indicates that the sample of five multiplexed PQ-PMMA gratings with ⬃1.8 mm thickness has an M-number 共M / # 兲 of 2.81. The normalized M / #, M / # divided by sample thickness, is 1.6 and close to the values reported in [7,10], which have normalized M / # of 1.6 and 1.75, respectively. The diffraction efficiency of grating 5 was made higher, since it is used to image at a deeper point within the sample. Figure 6 shows the hologram in the experimental imaging system. Each multiplexed grating acts as a spatial–spectral filter to simultaneously project multiple 2D images onto a CCD camera. Figure 7 shows multiple images simultaneously displayed by using this system. Both images were reconstructed by using a red LED with a peak wavelength of 630 nm and a spectral bandwidth of 30 nm. Figure 7(a) is the image of onion skin reconstructed by a hologram of two multiplexed gratings with diffraction efficiencies of 49.3% and 43.0%, respectively. Figure 7(b) is also an image of an onion reconstructed by using a multiplexed hologram with five superimposed gratings and diffraction efficiencies as shown in the Table 3. The optical sections are separated by ⬃50 m in depth. The width of each image increases with the spectral width of the source. The authors thank the National Institutes of Health for providing financial support (grant 5R21CA118167-02) for this research.
Fig. 7. Multiple 2D sections reconstructed by the volume holographic multiplexed spatial–spectral filters.
sures. When tdd is shorter 共10 s兲 the efficiency of the last grating formed in the sequence and the average efficiency for all gratings increases. Preillumination exposure (PIE) also has an important influence on the multiplexed grating efficiency. The PIE provides sufficient energy to reduce the concentration of the inhibitor that initially suppresses the creation of free radicals during the polymerization process [6,8,9]. To simplify the measurement of the PIE effect, holograms were made with constant tdd and exposure energy 共⬃610 mJ/ cm2兲 and with different value of PIE ranging from 0 to ⬃760 mJ/ cm2. The resulting performance with different values of PIE is shown in Table 1. Two multiplexed gratings were also made with different values of PIE and constant tdd and exposure energy 共⬃610 mJ/ cm2兲. The resultant cumulative grating strengths are shown in Table 2. The results indicate that a significant in-
References 1. W. Liu, D. Psaltis, and G. Barbastathis, Opt. Lett. 27, 854 (2002). 2. Z. Li, D. Psaltis, W. Liu, W. R. Johson, and G. Bearman, Proc. SPIE 5694, 33 (2005). 3. H. Coufal, L. Hesselink, and D. Psaltis, Holographic Data Storage (Springer-Verlag, 2002). 4. R. K. Kostuk, W. Maeda, C.-H. Chen, I. Djordjevic, and B. Vasic, Appl. Opt. 44, 7581 (2005). 5. M. G. Moharam and T. K. Gaylord, J. Opt. Soc. Am. 71, 811 (1981). 6. J. V. Kelly, M. R. Glesson, C. E. Close, F. T. O. Neil, and J. T. Sheridan, Opt. Express 13, 6990 (2005). 7. G. J. Steckman, I. Solomatine, G. Zhou, and D. Psaltis, Opt. Lett. 23, 1310 (1998). 8. A. V. Galstyan, R. S. Hakobyan, S. Harbour, and T. Galstian, Electron. Liq. Cryst. Commun. 2004/May/05 11:13:17 (2004). 9. M. R. Glesson, J. V. Kelly, C. E. Close, F. T. O. Neil, and J. T. Sheridan, Proc. SPIE 5827, 232 (2005). 10. S. H. Lin, K. Y. Hsu, W. Chen, and W. T. Whang, Opt. Lett. 25, 451 (2000).
OPTICS LETTERS / Vol. 33, No. 6 / March 15, 2008
Optimization of multiplexed holographic gratings in PQ-PMMA for spectral–spatial imaging filters Yuan Luo,1,2,* Paul J. Gelsinger,2 Jennifer K. Barton,1,2,3 George Barbastathis,4 and Raymond K. Kostuk1,2 1
Department of Electrical and Computing Engineering, University of Arizona, Tucson, Arizona 85721, USA 2 College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA 3 Division of Biomedical Engineering, University of Arizona, Tucson, Arizona 85721, USA 4 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA *Corresponding author: [email protected]
Received November 19, 2007; revised January 26, 2008; accepted February 6, 2008; posted February 12, 2008 (Doc. ID 89889); published March 11, 2008 Holographic gratings formed in thick phenanthrenquinone- (PQ-) doped poly(methyl methacrylate) (PMMA) can be made to have narrowband spectral and spatial transmittance filtering properties. We present the design and performance of angle-multiplexed holographic filters formed in PQ-PMMA at 488 nm and reconstructed with a LED operated at ⬃630 nm. The dark delay time between exposure and the preillumination exposure of the polymer prior to exposure of the holographic area are varied to optimize the diffraction efficiency of multiplexed holographic filters. The resultant holographic filters can enhance the performance of four-dimensional spatial–spectral imaging systems. The optimized filters are used to simultaneously sample spatial and spectral information at five different depths separated by 50 m within biological tissue samples. © 2008 Optical Society of America OCIS codes: 090.4220, 090.7330, 110.0110, 090.2890, 160.0160.
Spectral–spatial volume holographic imaging systems [1,2] require highly selective filters to discriminate wavefronts originating from different depths within a 3D object. The filter should also operate at longer wavelengths allowing greater imaging depths within tissue samples. The holographic filters formed in the volume recording material are phase gratings and offer high angular–spectral selectivity and the ability to multiplex multiple gratings in the same area [3]. In this Letter we describe the design of holographic transmission filters formed in the phenanthrenquinone-doped poly(methyl methacrylate) (PQ-PMMA) material. Experimental results demonstrate the performance of the volume holographic filters in a spectral–spatial imaging system to reconstruct images of 3D objects. The holographic recording material is formed by using solutions of methyl methacrylate (MMA), 2,2-azobis(2-methlpropionitrile) (AIBN), and phenanthrenquinone (PQ) that are mixed in a respective weight ratio of 100:0.5:0.7. PQ-doped PMMA is an attractive material for making holographic Bragg filters, since the material can be formed with minimal shrinkage and refractive index change after processing [4]. In addition, the PQ-PMMA holographic filters provide high spectral and angular Bragg selectivity. The mixture is poured into a mold similar to that shown in Fig. 1, consisting of two glass plates separated by a flexible spacer. The mixed solution solidifies after curing at 50°C for 120 h. After the thermal polymerization process is complete, the solid sample is approximately 1.8 mm thick. For our experimental system configuration, the resultant material is cut into 5 cm⫻ 5 cm squares and fitted onto a sample holder for holographic exposure. 0146-9592/08/060566-3/$15.00
The holograms are recorded with the setup shown in Fig. 2 by using an argon-ion laser operating at a wavelength of 488 nm. The reference beam is a collimated wave, and the signal beam is a spherical wave originating from a point corresponding to the depth position within the sample. A different z position is recorded for each depth that will be imaged within the sample. The positions of the point source in the signal arm are controlled by moving a microscope objective lens with numerical aperture (NA) of 0.65 along the axial direction. A second microscope objective lens with 0.55 NA remains in a fixed position in the signal arm, forming the point source. A relay system is used in the signal arm to maintain constant irradiance at the hologram plane as the 0.65 NA microscope object is moved. The nominal angle between two arms is ⬃68° and is changed by ⌬ with each exposure to record a hologram with a different reference beam angle and point source location. The angle settings, point source locations, and exposure time settings are automated by using a LabView control system. The hologram exposures are varied to in-
Fig. 1. Mold for the sample mixture preparation. © 2008 Optical Society of America
March 15, 2008 / Vol. 33, No. 6 / OPTICS LETTERS
crease the efficiency of gratings that select positions deeper within the tissue sample. Figure 3 is a rigorous coupled wave simulation [5] of the angular selectivity for a single grating formed with material parameters similar to our PQ-PMMA samples (thickness ⬃1.8 mm, n ⬃ 1.49, absorption coefficient ⬃0.009/ mm). The FWHM of the efficiency versus angle curve is ⬃0.03° and is equal to the measured selectivity of the experimental gratings. This implies that to avoid cross talk between multiplexed gratings, the angular difference 共⌬兲 between reference beams angles should be greater than 0.03°. In the experiments ⌬ was set at ⬃1° to avoid image overlap during reconstruction. The dark delay time 共tdd兲 between each exposure affects the diffraction efficiency 共兲 of multiplexed
567
Table 1. Single Gratings with Constant Exposure of 610 mJ/ cm2
(%)
Preillumination 共mJ/ cm2兲 0 120 360 460 760
20 38 44 40 48
Table 2. Two Multiplexed Gratings with Constant Exposure of 610 mJ/ cm2 Preillumination 共mJ/ cm2兲
Cumulative Grating Strength M=2冑 共兺i=1 i兲
0 220 290 360
0.78 1.06 1.04 1.31
Table 3. Five Multiplexed Gratings with tdd = 10 s and Preillumination of 360 mJ/ cm2 Grating Order 1 2 3 4 5 Fig. 2. Construction setup of the multiplexed holographic filters. M, mirrors; BS, beam splitter.
Fig. 3. grating.
Rigorous coupled wave simulation for single
Exposure Energy 共mJ/ cm2兲
(%)
579 670 762 853 944
17 26 33 40 46
gratings formed in optical polymers [6]. To quantify this effect in PQ-PMMA holograms, five multiplexed gratings were made with two collimated waves (⌬z = 0, and ⌬ = 1°). Five multiplexed gratings were made with dark delay times varied from 10 s to 5 min with constant exposure energy 共⬃760 mJ/ cm2兲 per grating and no preexposure energy. Figure 4 shows for the two different values of the dark delay time. The tdd was 10 s for sample F29A, and 5 min for sample F31. Number 1 indicates the first grating formed in the sequence, and number 5 is the last grating. The results show that with a longer delay time the cumulative grating strength, defined as C M 冑 i [7], is lower. Therefore, the overall efficiency = 兺i=1 is lower with a longer delay time and the efficiency of the first grating formed in the sequence is enhanced at the expense of gratings formed with later expo-
Fig. 4. Measured multiplexed grating diffraction efficiency with different dark delay times between exposures.
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OPTICS LETTERS / Vol. 33, No. 6 / March 15, 2008
Fig. 5. Measured multiplexed grating diffraction efficiency of five multiplexed gratings with tdd = 10 s and a preillumination of 360 mJ/ cm2.
Fig. 6. Experimental imaging geometry.
crease in the diffraction efficiency of single and multiple gratings formed in PQ-PMMA is possible with PIE values of a few hundred milli joules per square centimeter. After the exposure energy is adjusted as shown in Table 3 and a preexposure of 360 mJ/ cm2 with constant tdd = 10 s is added, the efficiencies are significantly improved. The gratings shown in Table 3 and Fig. 5 were formed with a spherical and planar wave with the point source moved by increments of ⌬z = 50 m and separation angle between reference waves of ⌬ = 1°. The maximum is 46%, and the minimum is 17%. This indicates that the sample of five multiplexed PQ-PMMA gratings with ⬃1.8 mm thickness has an M-number 共M / # 兲 of 2.81. The normalized M / #, M / # divided by sample thickness, is 1.6 and close to the values reported in [7,10], which have normalized M / # of 1.6 and 1.75, respectively. The diffraction efficiency of grating 5 was made higher, since it is used to image at a deeper point within the sample. Figure 6 shows the hologram in the experimental imaging system. Each multiplexed grating acts as a spatial–spectral filter to simultaneously project multiple 2D images onto a CCD camera. Figure 7 shows multiple images simultaneously displayed by using this system. Both images were reconstructed by using a red LED with a peak wavelength of 630 nm and a spectral bandwidth of 30 nm. Figure 7(a) is the image of onion skin reconstructed by a hologram of two multiplexed gratings with diffraction efficiencies of 49.3% and 43.0%, respectively. Figure 7(b) is also an image of an onion reconstructed by using a multiplexed hologram with five superimposed gratings and diffraction efficiencies as shown in the Table 3. The optical sections are separated by ⬃50 m in depth. The width of each image increases with the spectral width of the source. The authors thank the National Institutes of Health for providing financial support (grant 5R21CA118167-02) for this research.
Fig. 7. Multiple 2D sections reconstructed by the volume holographic multiplexed spatial–spectral filters.
sures. When tdd is shorter 共10 s兲 the efficiency of the last grating formed in the sequence and the average efficiency for all gratings increases. Preillumination exposure (PIE) also has an important influence on the multiplexed grating efficiency. The PIE provides sufficient energy to reduce the concentration of the inhibitor that initially suppresses the creation of free radicals during the polymerization process [6,8,9]. To simplify the measurement of the PIE effect, holograms were made with constant tdd and exposure energy 共⬃610 mJ/ cm2兲 and with different value of PIE ranging from 0 to ⬃760 mJ/ cm2. The resulting performance with different values of PIE is shown in Table 1. Two multiplexed gratings were also made with different values of PIE and constant tdd and exposure energy 共⬃610 mJ/ cm2兲. The resultant cumulative grating strengths are shown in Table 2. The results indicate that a significant in-
References 1. W. Liu, D. Psaltis, and G. Barbastathis, Opt. Lett. 27, 854 (2002). 2. Z. Li, D. Psaltis, W. Liu, W. R. Johson, and G. Bearman, Proc. SPIE 5694, 33 (2005). 3. H. Coufal, L. Hesselink, and D. Psaltis, Holographic Data Storage (Springer-Verlag, 2002). 4. R. K. Kostuk, W. Maeda, C.-H. Chen, I. Djordjevic, and B. Vasic, Appl. Opt. 44, 7581 (2005). 5. M. G. Moharam and T. K. Gaylord, J. Opt. Soc. Am. 71, 811 (1981). 6. J. V. Kelly, M. R. Glesson, C. E. Close, F. T. O. Neil, and J. T. Sheridan, Opt. Express 13, 6990 (2005). 7. G. J. Steckman, I. Solomatine, G. Zhou, and D. Psaltis, Opt. Lett. 23, 1310 (1998). 8. A. V. Galstyan, R. S. Hakobyan, S. Harbour, and T. Galstian, Electron. Liq. Cryst. Commun. 2004/May/05 11:13:17 (2004). 9. M. R. Glesson, J. V. Kelly, C. E. Close, F. T. O. Neil, and J. T. Sheridan, Proc. SPIE 5827, 232 (2005). 10. S. H. Lin, K. Y. Hsu, W. Chen, and W. T. Whang, Opt. Lett. 25, 451 (2000).
