Tunable hyperchromatic lens system for confocal hyperspectral sensing Phuong-Ha Cu-Nguyen,1,* Adrian Grewe,2 Matthias Hillenbrand,2 Stefan Sinzinger,2 Andreas Seifert,1 and Hans Zappe1 1
Department of Microsystems Engineering, University of Freiburg, Germany 2 Optical Engineering, Ilmenau University of Technology, Germany * [email protected]
Abstract: A new approach for confocal hyperspectral sensing based on the combination of a diffractive optical element and a tunable membrane fluidic lens is demonstrated. This highly compact lens system is designed to maximize the longitudinal chromatic aberration and select a narrow spectral band by spatial filtering. Changing the curvature of the fluidic lens allows the selected band to be scanned over the whole given spectrum. A hybrid prototype with an integrated electro-magnetic micro-actuator has been realized to demonstrate the functionality of the system. Experimental results show that the spectrum transmitted by the system can be tuned over the entire visible wavelength range, from 450 to 900 nm with a narrow and almost constant linewidth of less than 15 nm. Typical response time for scanning the spectrum by 310 nm is less than 40 ms and the lens system shows a highly linear relationship with the driving current. © 2013 Optical Society of America OCIS codes: (220.4000, 220.4830) Optical design and fabrication; (230.3990) Optical devices; (110.4234) Imaging systems
References and links 1. C.-I. Chang, Hyperspectral Imaging : Techniques for Spectral Detection and Classification (Kluwer Academic/Plenum Publishers, 2003). 2. G. K. Naganathan, L. M. Grimes, J. Subbiah, C. R. Calkins, A. Samal, and G. E. Meyer, “Visible/near-infrared hyperspectral imaging for beef tenderness prediction,” Comput. Electron. Agr. 64, 225 – 233 (2008). 3. S. J. Kim, F. Deng, and M. S. Brown, “Visual enhancement of old documents with hyperspectral imaging,” Pattern Recognit. 44, 1461 – 1469 (2011). 4. Y. Roggo, A. Edmond, P. Chalus, and M. Ulmschneider, “Infrared hyperspectral imaging for qualitative analysis of pharmaceutical solid forms,” Anal. Chim. Acta 535, 79 – 87 (2005). 5. F. Kruse, J. Boardman, and J. Huntington, “Comparison of airborne hyperspectral data and eo-1 hyperion for mineral mapping,” Geosci. Remote Sens. 41, 1388–1400 (2003). 6. F. D. van der Meer, H. M. van der Werff, F. J. van Ruitenbeek, C. A. Hecker, W. H. Bakker, M. F. Noomen, M. van der Meijde, E. J. M. Carranza, J. B. de Smeth, and T. Woldai, “Multi- and hyperspectral geologic remote sensing: A review,” Appl. Earth Obs. Geoinf. 14, 112 – 128 (2012). 7. P. Mouroulis and M. M. McKerns, “Pushbroom imaging spectrometer with high spectroscopic data fidelity: experimental demonstration,” Opt. Eng. 39, 808–816 (2000). 8. N. Gat, “Imaging spectroscopy using tunable filters: a review,” Proc. SPIE 4056, 50–64 (2000). 9. H. R. Morris, C. C. Hoyt, and P. J. Treado, “Imaging spectrometers for fluorescence and raman microscopy: Acousto-optic and liquid crystal tunable filters,” Appl. Spectrosc. 48, 857–866 (1994). 10. R. W. Slawson, Z. Ninkov, and E. P. Horch, “Hyperspectral imaging: Wide area spectrophotometry using a liquid crystal tunable filter,” Astr. Soc. P. 111, 621–626 (1999). 11. O. Aharon and I. Abdulhalim, “Liquid crystal Lyot tunable filter with extended free spectral range,” Opt. Express 17, 11426–11433 (2009).
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27611
12. K. Körner, Ch. Kohler, E. Papastathopoulos, A. Ruprecht, T. Wiesendanger, Ch. Pruss, and W. Osten, “Arrangement for rapid locally resolved flat surface spectroscopic analysis or imaging has flat raster array of pinholes turned about acute angle relative to spectral axis on detector matrix which fills up with elongated su-matrices,” Patent DE102006007172 (2007). 13. A. Grewe, M. Hillenbrand, and S. Sinzinger, “Bildgebende hyperspektrale Sensorik unter Einsatz verstimmbarer Optiken,” Photonik 1/2013, 38–41 (2013). 14. M. Hillenbrand, A. Grewe, and S. Sinzinger, “Parallelized chromatic confocal systems enable efficient spectral information coding,” Opt. Des. Eng., SPIE Newsroom (2013). 15. H. Zappe, Fundamentals of Micro-Optics (Cambridge University Press, 2010). 16. A. Werber and H. Zappe, “Tunable microfluidic microlenses,” Appl. Opt. 44, 3238–3245 (2005). 17. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (John Wiley & Sons, Inc., 2007). 18. H. P. Herzig, ed., Micro-optics: Elements, Systems and Applications (Taylor & Francis, 1997). 19. P. Liebetraut, P. Waibel, P. H. C. Nguyen, P. Reith, B. Aatz, and H. Zappe, “Optical properties of liquids for fluidic optics,” Appl. Opt. 52, 3203–3215 (2013). 20. W. Zhang, H. Zappe, and A. Seifert, “Polyacrylate tunable microlens with on-chip thermopneumatic actuation,” in “International Conference on Optical MEMS and Nanophotonics,” (2012), pp. 57–58. 21. W. Zhang, H. Zappe, and A. Seifert, “Polyacrylate membranes for tunable liquid-filled microlenses,” Opt. Eng. 52, 046601–046601 (2013). 22. A. R. Jha, “Narrowband solid state acousto-optic tunable filter,” in “Proceedings of Microwave and Optoelectronics Conference, SBMO/IEEE MTT-S International,” (1995), pp. 287–291. 23. J. P. Bentley, Principles of Measurement Systems (Pearson, 2005).
1.
Introduction
Hyperspectral imaging, implemented in applications as diverse as mineralogy, geology, ecology, agriculture, astronomy, and biology, plays an important role in many fields. The essence of this imaging technique is that each pixel includes spatial, spectral, and even temporal information [1]. Due to their versatility, hyperspectral imaging techniques have been successfully demonstrated for numerous of types of sensing, for example beef tenderness prediction [2], historical document conservation [3], chemical imaging [4], mineral mapping [5], and geologic remote sensing [6]. Numerous scanning approaches to obtain hyperspectral images have been demonstrated. One can do the line scanning in the spatial dimension by using a pushbroom approach to capture the spectral image of one line at a time [7]. A more convenient approach is scanning in the spectral dimension by using an electrically tunable filter in front of the camera lens [8]. Liquid crystal tunable filters are among those which are based on Lyot filters, employing tunable liquid crystal wave plates that allow the filter to transmit a narrow spectral band without any moving parts [9]. A liquid crystal tunable filter for wide-area spectrophotometry has been used in the 400 – 720 nm spectral range with a linewidth of 10 nm FWHM [10]. Alternatively, a liquid crystal tunable Lyot filter was demonstrated in [11] for continuous tuning in the range of 500 – 900 nm with a nominal linewidth in the range 50 – 100 nm. In contrast to these approaches, we employ here a tunable filter with a broad tuning range (450 – 900 nm) and constant spectral resolution based on the principle of chromatic confocal microscopy [12, 13]. The concept is implemented using a hyperchromatic lens (HCL) system with a large longitudinal chromatic aberration (LCA) [14]. The HCL is realized based on the combination of a diffractive optical element (DOE) and a tunable refractive lens. The spectral information of the object point is transmitted through the lens system but separated along the optical axis due to the LCA. A pinhole placed on the image plane allows a selected spectral band to reach the detector, providing the narrow transmitted linewidth. At present, there are several technologies for realizing tunable refractive lenses, mainly based on electrowetting lenses, electro-optical lenses, hydrogel microlenses, tunable pneumatic microlenses, and liquid crystal microlenses [15]. We rely here on pressure-tunable lenses with liquid displaced into or out of a micro-fluidic lens chamber, combined with an elastomer membrane which can expand to form either a concave or convex lens. #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27612
We will first introduce the concept and design of the hybrid HCL system with an integrated actuator. Subsequent sections will present the fabrication process of the lens system followed by a description of the measurement setup and a discussion of the experimental results. 2.
Concept and design
We begin by discussing the system concept, the design of the individual components, an estimate of the performance and relevant simulation results. 2.1.
Concept
The concept of the tunable HCL system, based on the principle of confocal microscopy, is presented in Fig. 1(a) and 1(b). The HCL system consists of a fixed diffractive optical element (DOE) and a tunable concave lens designed for optimization (i.e., maximization) of LCA. Due to the large LCA, different wavelengths are focused at different points on the optical axis. A pinhole placed in the image plane can thus filter out a narrow spectral band.
Fig. 1. Schematic diagram of the HCL system. The focal length of the concave lens can be tuned to adjust the focal point of, for example, (a) blue or (b) red on the pinhole, such that only one of these wavelengths reaches the detector.
The tunability of the concave lens is realized by use of a membrane-based fluid-filled lens, whose focal length can be tuned by variation of the pressure [16]. As shown in the example of Fig. 1(a) and 1(b), when the deformation of the membrane changes, either the blue or the red parts of the spectrum are consequently in focus on the pinhole plane. All the wavelengths between blue and red can thus be scanned on the detector, without any moving elements. This scanning is the basis for the hyperspectral detector, which can thus generate a complete spectrum for a single imaged pixel. 2.2.
Design of the tunable lens and pressure actuator
The design of the tunable lens and actuator are shown in Fig. 2. The refractive lens is created from a circular silicon aperture, an elastomer membrane and a lens chamber filled with optical liquid, H2 O in this case. The lens chamber is connected to the actuating chamber by a fluidic channel for actuation. The actuator consists of neodymium (NdFeB) magnets and a spiral coil of insulated copper wires. When a current is applied to the coil, it causes a magnetic force to
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27613
be applied to the magnet, resulting in its movement. The liquid is then pumped into and out of the lens chamber, thereby deforming the lens membrane, hence changing the focal length. As illustrated in Fig. 2, when the repulsive force is generated and applied to the magnet, a reduction in the pressure is introduced, causing a concave deformation of the optical membrane.
Fig. 2. View of the lens system with integrated actuator. The curvature of the concave membrane lens can be tuned by volume displacement, caused by the movement of Neodymium magnet, which results from the repulsive force between the magnet and current-driven spiral coil.
2.3.
Design of DOE
The second element of the lens system is the DOE, configured as a Fresnel zone plate consisting of radial circular rings with increasing radii and decreasing widths. The rings act as a grating structure, which has a phase function based on the radial distance and the grating period [17]. The DOE can be designed to serve as a spherical lens with focal length f0 by introducing a phase shift φ (r) = −πr2 /λ0 f0 (1) to the incoming light, where f0 is the design focal length corresponding to the design wavelength λ0 . Our DOE is designed here to have a focal length of 20 mm at the reference wavelength of 550 nm. The phase function can be translated into a surface-relief profile h(r) for a transmissive thin DOE given by λ0 φ (r)mod2π h(r) = , (2) n(λ0 ) − 1 2π where n is the refractive index of the DOE material and φ (r) mod 2π is the phase function wrapped to the interval (0 − 2π) [18]. Hence h(r) is a continuous sawtooth profile with a maximum height λ0 hmax = . (3) n(λ0 ) − 1 The continuous profile DOE is predicted to have ideally 100% first-order diffraction efficiency [18]. In practice, however, the DOE is fabricated on fused silica by sequence of multiple lithography and etch steps. The element therefore has a staircase-like profile and the diffraction efficiency is given by sin(π/M) 2 η =( ) , (4) π/M where M is number of phase levels [18]. To balance fabrication process costs and sufficient diffraction efficiency, the DOE here was realized with 4 phase levels using a 2-step lithography and etch process, which results in a predicted diffraction efficiency of 81%.
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27614
2.4.
Tuning range of HCL
The tuning range of the HCL is modeled using the commercial ray tracing software Zemax. The lens system consists of a DOE and an H2 O-based tunable refractive lens with lens diameter of 3 mm and thickness of each lens of 1 mm, as shown in Fig. 3.
Fig. 3. Layout of the tunable HCL system, consisting of diffractive and H2 O-based refractive lenses, which yield an image distance of 52 mm. For a wavelength of λ1 = 450 nm, the focal length of the refractive lens has to be adjusted to f1 = −200 mm to focus the object (point source) to the fixed image distance of 52 mm. For a wavelength of λ2 = 900 nm, the focal length of the tunable refractive lens has to be readjusted to f2 = −18 mm to focus the object to the same image plane.
The DOE is designed to have a focal length of 20 mm at the design wavelength of 550 nm. Without loss of generality, the object and image are assumed to be at conjugate distances of the HCL system; the spectral tuning range was chosen to be from 450 to 900 nm. Thus, the wavelength λ1 = 450 nm should be in focus at the first actuating setting of the refractive lens, which is defined as focal length f1 , while the wavelength λ2 = 900 nm corresponding to the last actuating state of the refractive lens, is specified as focal length f2 . f1 and f2 are both negative values since the tunable element is a concave lens; the corresponding sag heights of the membrane are h1 and h2 . The focal length f1 was designed to be -200 mm, with a sag height of the membrane h1 = −17 µm. The focus point of wavelength λ1 = 450 nm is then at a distance of 52 mm from the HCL system. Based on simulation results, in order to have the λ2 = 900 nm focus at the same distance, the focal length of the refractive lens has to be tuned to f2 = -18 mm, corresponding to sag height h2 = −200 µm. The design parameters are summarized in Table 1. The tuning range of the HCL system is strongly dependent on the tuning capability of the refractive lens. In this paper, the specification of spectral range is limited to 450 – 900 nm, but in principle a larger scanning range can be achieved. To improve the tuning capability, one can use liquid with a higher refractive index compared to H2 O, for example silicone oil with refractive index nd = 1.4 [19], as the medium for the fluidic lens. 2.5.
HCL system simulation
To evaluate the spectral filtering characteristics of our HCL system, illustrated in Fig. 3, we employed the Zemax Extended Source Encircled Energy feature. This ray tracing-based model realizes the extended object by tracing the rays from different points within a pinhole of 50 µm #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27615
Table 1. Design parameters of the HCL system
parameters fr of H2 O lens fd of DOE f of lens system lens diameter object distance image distance spectral scanning
values tuning from -200 to -18 mm 20 mm at wavelength λ0 = 550 nm 26 mm 3 mm 52 mm 52 mm 450 – 900 nm
diameter. The rays passing the HCL and hitting the circular detecting area of 50 µm diameter in the image plane of the HCL were integrated to determine the intensity of the spectral response.
Fig. 4. Simulated spectral responses of the HCL system at different wavelengths; the FWHM of the peak is about 8 nm for each spectral band.
Using this approach, the spectral signals of four wavelengths corresponding to four settings of the tunable lens were calculated, and are shown in Fig. 4. The spectral width FWHM (fullwidth at half-maximum) of the spectral bands are 8.2 nm, 7.9 nm, 7.9 nm, and 8.4 nm for wavelengths of 450 nm, 600 nm, 750 nm, and 900 nm. These results show that the HCL system can achieve constant and narrow bandpass filtering throughout the wide spectral range of 450 – 900 nm. 3. 3.1.
Fabrication Tunable lens and magnetic actuator
We discuss the fabrication process for the design structure in Fig. 2. The silicon aperture of the refractive lens is fabricated by standard micromachining, with KOH etching 400 µm deep from
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27616
the back side of the Si wafer and DRIE through etching from the front side to create a circular aperture with a diameter of 3 mm. The aperture is covered by a radially stretched polyacrylate membrane 50 µm thick [20, 21]. The lens chamber is filled with liquid (H2 O) to generate the optical lens. The fluidic structures of the lens system are made from high performance adhesive transfer tape 3M 467MP 200MP. The elements are fabricated by CO2 laser cutting and successively stacked with the aid of alignment marks. The magnetic actuator consists of a neodymium (NdFeB) magnet with residual magnetism of 1.17 – 1.21 T mounted inside the actuating chamber, which is sealed by a polyacrylate membrane. A spiral coil of insulated copper wires, 100 µm in diameter with an inductance of 3 mH, is placed on the opposite side, together with the magnet. 3.2.
DOE fabrication
The DOE was designed for diffraction at the reference wavelength of 550 nm with the focal length of 20 mm. The element is optimized for nearly perfect imaging of an object at infinity. According to the optimization, the surface-relief profile φ for the thin DOE is designed as φ = −3826.597ρ 2 + 30.979ρ 4 − 0.480ρ 6
(5)
with ρ = r/3.6 mm, where r is the radial distance to the center of the DOE. To fabricate the diffractive element in a fused silica wafer, contact lithography and inductively coupled plasma reactive ion etching (ICP-RIE) are employed. The element used for the experiment was realized with 4 phase levels resulting from a 2-step lithographic process. At the outer radius of 3.6 mm, the DOE has a minimum local period of 3.0 µm and a feature size of 0.75 µm. The profile at the central part of the diffractive lens was characterized by white light interferometer and illustrated in Fig. 5.
Fig. 5. Central part of the DOE, measured by a white light interferometer.
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27617
3.3.
Prototype
A prototype of the tunable HCL was fabricated in two parts: the tunable refractive lens including the magnetic actuator and the DOE. These two components were then assembled into the complete HCL system, as shown in Fig. 6.
Fig. 6. Photograph of the prototype HCL system after assembly. The front side of the combined lens with the optical aperture is seen on the left, while the DOE is seen on the right.
4.
Measurements and results
4.1.
Measurement setup
The measurement setup to characterize the HCL system is presented in Fig. 7. A white light source is coupled into a 1 mm diameter fiber. An imaging lens is used to form a homogeneous image on the aperture stop with a diameter of 50 µm which becomes the white light input for the tunable HCL system. The image is formed on the facet of a 50 µm diameter fiber, from where the signal is transmitted to the spectrometer for analysis. The object and image distances of the HCL system are those as given in Table 1.
Fig. 7. Schematic of the setup for characterization of the HCL system. The 50 µm aperture stop is in the object plane, the 50 µm fiber is in the image plane of the system.
To extend this setup for 2D imaging, the aperture stop in Fig. 7 will be replaced by a pinhole #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27618
array with, for example, pinhole diameters of 50 µm and spacings of 100 µm. By using the achromatic imaging lens between the 2D object and this pinhole array, the object will be sharply imaged onto the array such that it acts as a confocal filter. If the array is moved laterally (in 2D) to cover the entire image, the lens system will be able to generate a 2D image with the same high spatial resolution. 4.2.
Spectral response
The spectral signal measured at the output fiber may be tuned by increasing or decreasing the DC current applied to the actuating spiral coil. The continuous and reversible spectral tuning from 450 nm to 900 nm is clearly shown in two diagrams in Fig. 8(a) and 8(b). The spectral signals filtered out from the original white light source using the HCL are plotted in Fig. 8(a). It is seen that the spectral response deviates from the white light envelope more strongly as the wavelength increases. This deviation is due to the variation of DOE efficiency with wavelength; efficiency was maximum at the design wavelength of 550 nm and decreases for longer or shorter wavelengths. For that reason, a normalization with the DOE efficiency for each wavelength over the full measurement range is necessary; the measured efficiency of the DOE for different wavelengths is given in Table 2. Table 2. Measured light source-independent efficiency of the DOE at different wavelengths.
wavelength (nm) efficiency
450 0.57
500 0.61
550 0.75
600 0.74
650 0.70
700 0.65
750 0.59
800 0.56
850 0.53
900 0.39
After normalizing the spectral signal with the white light spectrum and DOE efficiency for each evaluated wavelength, the relative intensities are almost constant over the entire range as shown in Fig. 8(b). This implies that the HCL system has a uniform response for the design spectral range. It was furthermore seen that the correlation between magnet drive current (lens actuator) and wavelength shift was highly linear, with a correlation coefficient of R2 ≈ 0.9973. The demonstrated spectral range includes visible and some near-infrared wavelengths, which is a considerably larger range than shown in previous work on acousto-optic tunable filters, which were limited to 450 to 550 nm [22], or liquid-crystal tunable filters, with a tuning range 400 to 720 nm [10]. The linewidth of the spectral signals was also investigated and plotted in Fig. 8(b), with the magnitude varying from 12 nm to 14 nm. The magnitude and uniformity of the linewidth over the entire scan range is considerably improved when compared to, for example, a liquid crystal tunable Lyot filter with the linewidth in the range 50 – 100 nm [11]. The constant value of the linewidth can be explained by the fact that, during the tuning process, the focal lengths of the different wavelengths remain the same, since these are defined by the object and image distances. Hence the spot size of the image is almost the same for all wavelengths and therefore the narrow spectral peaks of equal widths are coupled into the fiber while the HCL is tuned. In comparison to the analytical prediction from Fig. 4 with a linewidth of 8 nm, the larger experimental values may be due to alignment errors during fabrication and measurement, as well as diffraction effects which are not considered in the simulation. The linewidth is dependent on the LCA; the image spot size in the image plane of the HCL; and the diameter of the fiber leading to the spectrometer. Linewidth could be improved by minimizing the diameter of the pinhole in the imaging plane or by replacing the 50 µm fiber with one with a smaller core, which would, however, decrease the signal-to-noise ratio (SNR). The signal-to-noise ratio of the spectral response from the HCL is best for the high peaks in the center of the spectrum at about 600 nm. The measured SNR for the peak at 600 nm is 9, #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27619
Fig. 8. Measured spectral responses of the HCL system (with H2 O lens) at different actuation settings, demonstrating a continuous spectral tuning and constant response of the HCL system in the range of 450 nm to 900 nm. (a) The white light spectrum and HCL spectral response from 450 nm to 900 nm before normalization and (b) the HCL spectral responses after normalization with the white light spectrum and DOE efficiency. The upper line shows the spectral width of the individual spectral signals; the FWHM varies between 12 – 14 nm.
whereas the SNR drops down to 1.5 for the small peaks at 450 nm and 900 nm. 4.3.
Tuning speed
As shown in Fig. 8, the spectral peak can be switched between different wavelengths by varying the DC current to the spiral coil. To evaluate the dynamic performance of the lens system, a high-speed video capturing spectral signal with resolution of 7 ms was recorded while the actuating current was subject to a step function input. The wavelengths with maximum intensity are plotted against time in Fig. 9 and summarized in Table 3. Table 3. Response time of the lens system subject to a step function of the magnet driving current. The rise and fall times correspond to tuning ranges of 500 – 600 nm and 500 – 810 nm.
wavelength range rise time (ms) fall time (ms)
500 – 600 nm
500 – 810 nm
32 ± 11 30 ± 8
34 ± 9 37 ± 6
The response time is defined as that needed for the lens system to shift from one wavelength to the next and to stabilize at the new state (the target wavelength ±3 nm). The rise times τ1 and τ2 , corresponding to tuning ranges of 500 – 600 nm and 500 – 810 nm, respectively, are similar despite the large difference in tuning range. In linear systems, the response time does not depend on the amplitude of the stimulating step function [23], the actuation current in our case. For higher actuation currents, and hence greater wavelength intervals, the system responds with higher speed due to the higher applied force.
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27620
Fig. 9. Recorded wavelength with maximum intensity as a function of time subject to a step rise of the actuation current. The rise times τ1 and τ2 correspond to the tuning ranges of 500 – 600 nm and 500 – 810 nm, respectively. The equipment-based resolution of the time axis is 7 ms, explaining the discrete nature of the data.
5.
Conclusion
The concept of a highly-compact tunable HCL system has been demonstrated and a flexible prototype with integrated actuator has been designed. The measurement results have shown the functionality of the lens as a continuously tunable filter, with a large dynamic range (450 – 900 nm), uniform response and almost constant narrow linewidth (12 – 14 nm) for the entire spectral range. The response time is less than 40 ms for different tuning ranges and tuning is highly linearly correlated with driving current. The same concept may be applied to implement two-dimensional hyperspectral confocal imaging. Acknowledgments This work was supported by the Federal Ministry of Education and Research in the project Optical Microsystems for Ultra-compact Hyperspectral Sensor (OpMihySen, BMBF Förderkennzeichen FZK 16SV5575K).
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27621
Department of Microsystems Engineering, University of Freiburg, Germany 2 Optical Engineering, Ilmenau University of Technology, Germany * [email protected]
Abstract: A new approach for confocal hyperspectral sensing based on the combination of a diffractive optical element and a tunable membrane fluidic lens is demonstrated. This highly compact lens system is designed to maximize the longitudinal chromatic aberration and select a narrow spectral band by spatial filtering. Changing the curvature of the fluidic lens allows the selected band to be scanned over the whole given spectrum. A hybrid prototype with an integrated electro-magnetic micro-actuator has been realized to demonstrate the functionality of the system. Experimental results show that the spectrum transmitted by the system can be tuned over the entire visible wavelength range, from 450 to 900 nm with a narrow and almost constant linewidth of less than 15 nm. Typical response time for scanning the spectrum by 310 nm is less than 40 ms and the lens system shows a highly linear relationship with the driving current. © 2013 Optical Society of America OCIS codes: (220.4000, 220.4830) Optical design and fabrication; (230.3990) Optical devices; (110.4234) Imaging systems
References and links 1. C.-I. Chang, Hyperspectral Imaging : Techniques for Spectral Detection and Classification (Kluwer Academic/Plenum Publishers, 2003). 2. G. K. Naganathan, L. M. Grimes, J. Subbiah, C. R. Calkins, A. Samal, and G. E. Meyer, “Visible/near-infrared hyperspectral imaging for beef tenderness prediction,” Comput. Electron. Agr. 64, 225 – 233 (2008). 3. S. J. Kim, F. Deng, and M. S. Brown, “Visual enhancement of old documents with hyperspectral imaging,” Pattern Recognit. 44, 1461 – 1469 (2011). 4. Y. Roggo, A. Edmond, P. Chalus, and M. Ulmschneider, “Infrared hyperspectral imaging for qualitative analysis of pharmaceutical solid forms,” Anal. Chim. Acta 535, 79 – 87 (2005). 5. F. Kruse, J. Boardman, and J. Huntington, “Comparison of airborne hyperspectral data and eo-1 hyperion for mineral mapping,” Geosci. Remote Sens. 41, 1388–1400 (2003). 6. F. D. van der Meer, H. M. van der Werff, F. J. van Ruitenbeek, C. A. Hecker, W. H. Bakker, M. F. Noomen, M. van der Meijde, E. J. M. Carranza, J. B. de Smeth, and T. Woldai, “Multi- and hyperspectral geologic remote sensing: A review,” Appl. Earth Obs. Geoinf. 14, 112 – 128 (2012). 7. P. Mouroulis and M. M. McKerns, “Pushbroom imaging spectrometer with high spectroscopic data fidelity: experimental demonstration,” Opt. Eng. 39, 808–816 (2000). 8. N. Gat, “Imaging spectroscopy using tunable filters: a review,” Proc. SPIE 4056, 50–64 (2000). 9. H. R. Morris, C. C. Hoyt, and P. J. Treado, “Imaging spectrometers for fluorescence and raman microscopy: Acousto-optic and liquid crystal tunable filters,” Appl. Spectrosc. 48, 857–866 (1994). 10. R. W. Slawson, Z. Ninkov, and E. P. Horch, “Hyperspectral imaging: Wide area spectrophotometry using a liquid crystal tunable filter,” Astr. Soc. P. 111, 621–626 (1999). 11. O. Aharon and I. Abdulhalim, “Liquid crystal Lyot tunable filter with extended free spectral range,” Opt. Express 17, 11426–11433 (2009).
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27611
12. K. Körner, Ch. Kohler, E. Papastathopoulos, A. Ruprecht, T. Wiesendanger, Ch. Pruss, and W. Osten, “Arrangement for rapid locally resolved flat surface spectroscopic analysis or imaging has flat raster array of pinholes turned about acute angle relative to spectral axis on detector matrix which fills up with elongated su-matrices,” Patent DE102006007172 (2007). 13. A. Grewe, M. Hillenbrand, and S. Sinzinger, “Bildgebende hyperspektrale Sensorik unter Einsatz verstimmbarer Optiken,” Photonik 1/2013, 38–41 (2013). 14. M. Hillenbrand, A. Grewe, and S. Sinzinger, “Parallelized chromatic confocal systems enable efficient spectral information coding,” Opt. Des. Eng., SPIE Newsroom (2013). 15. H. Zappe, Fundamentals of Micro-Optics (Cambridge University Press, 2010). 16. A. Werber and H. Zappe, “Tunable microfluidic microlenses,” Appl. Opt. 44, 3238–3245 (2005). 17. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (John Wiley & Sons, Inc., 2007). 18. H. P. Herzig, ed., Micro-optics: Elements, Systems and Applications (Taylor & Francis, 1997). 19. P. Liebetraut, P. Waibel, P. H. C. Nguyen, P. Reith, B. Aatz, and H. Zappe, “Optical properties of liquids for fluidic optics,” Appl. Opt. 52, 3203–3215 (2013). 20. W. Zhang, H. Zappe, and A. Seifert, “Polyacrylate tunable microlens with on-chip thermopneumatic actuation,” in “International Conference on Optical MEMS and Nanophotonics,” (2012), pp. 57–58. 21. W. Zhang, H. Zappe, and A. Seifert, “Polyacrylate membranes for tunable liquid-filled microlenses,” Opt. Eng. 52, 046601–046601 (2013). 22. A. R. Jha, “Narrowband solid state acousto-optic tunable filter,” in “Proceedings of Microwave and Optoelectronics Conference, SBMO/IEEE MTT-S International,” (1995), pp. 287–291. 23. J. P. Bentley, Principles of Measurement Systems (Pearson, 2005).
1.
Introduction
Hyperspectral imaging, implemented in applications as diverse as mineralogy, geology, ecology, agriculture, astronomy, and biology, plays an important role in many fields. The essence of this imaging technique is that each pixel includes spatial, spectral, and even temporal information [1]. Due to their versatility, hyperspectral imaging techniques have been successfully demonstrated for numerous of types of sensing, for example beef tenderness prediction [2], historical document conservation [3], chemical imaging [4], mineral mapping [5], and geologic remote sensing [6]. Numerous scanning approaches to obtain hyperspectral images have been demonstrated. One can do the line scanning in the spatial dimension by using a pushbroom approach to capture the spectral image of one line at a time [7]. A more convenient approach is scanning in the spectral dimension by using an electrically tunable filter in front of the camera lens [8]. Liquid crystal tunable filters are among those which are based on Lyot filters, employing tunable liquid crystal wave plates that allow the filter to transmit a narrow spectral band without any moving parts [9]. A liquid crystal tunable filter for wide-area spectrophotometry has been used in the 400 – 720 nm spectral range with a linewidth of 10 nm FWHM [10]. Alternatively, a liquid crystal tunable Lyot filter was demonstrated in [11] for continuous tuning in the range of 500 – 900 nm with a nominal linewidth in the range 50 – 100 nm. In contrast to these approaches, we employ here a tunable filter with a broad tuning range (450 – 900 nm) and constant spectral resolution based on the principle of chromatic confocal microscopy [12, 13]. The concept is implemented using a hyperchromatic lens (HCL) system with a large longitudinal chromatic aberration (LCA) [14]. The HCL is realized based on the combination of a diffractive optical element (DOE) and a tunable refractive lens. The spectral information of the object point is transmitted through the lens system but separated along the optical axis due to the LCA. A pinhole placed on the image plane allows a selected spectral band to reach the detector, providing the narrow transmitted linewidth. At present, there are several technologies for realizing tunable refractive lenses, mainly based on electrowetting lenses, electro-optical lenses, hydrogel microlenses, tunable pneumatic microlenses, and liquid crystal microlenses [15]. We rely here on pressure-tunable lenses with liquid displaced into or out of a micro-fluidic lens chamber, combined with an elastomer membrane which can expand to form either a concave or convex lens. #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27612
We will first introduce the concept and design of the hybrid HCL system with an integrated actuator. Subsequent sections will present the fabrication process of the lens system followed by a description of the measurement setup and a discussion of the experimental results. 2.
Concept and design
We begin by discussing the system concept, the design of the individual components, an estimate of the performance and relevant simulation results. 2.1.
Concept
The concept of the tunable HCL system, based on the principle of confocal microscopy, is presented in Fig. 1(a) and 1(b). The HCL system consists of a fixed diffractive optical element (DOE) and a tunable concave lens designed for optimization (i.e., maximization) of LCA. Due to the large LCA, different wavelengths are focused at different points on the optical axis. A pinhole placed in the image plane can thus filter out a narrow spectral band.
Fig. 1. Schematic diagram of the HCL system. The focal length of the concave lens can be tuned to adjust the focal point of, for example, (a) blue or (b) red on the pinhole, such that only one of these wavelengths reaches the detector.
The tunability of the concave lens is realized by use of a membrane-based fluid-filled lens, whose focal length can be tuned by variation of the pressure [16]. As shown in the example of Fig. 1(a) and 1(b), when the deformation of the membrane changes, either the blue or the red parts of the spectrum are consequently in focus on the pinhole plane. All the wavelengths between blue and red can thus be scanned on the detector, without any moving elements. This scanning is the basis for the hyperspectral detector, which can thus generate a complete spectrum for a single imaged pixel. 2.2.
Design of the tunable lens and pressure actuator
The design of the tunable lens and actuator are shown in Fig. 2. The refractive lens is created from a circular silicon aperture, an elastomer membrane and a lens chamber filled with optical liquid, H2 O in this case. The lens chamber is connected to the actuating chamber by a fluidic channel for actuation. The actuator consists of neodymium (NdFeB) magnets and a spiral coil of insulated copper wires. When a current is applied to the coil, it causes a magnetic force to
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27613
be applied to the magnet, resulting in its movement. The liquid is then pumped into and out of the lens chamber, thereby deforming the lens membrane, hence changing the focal length. As illustrated in Fig. 2, when the repulsive force is generated and applied to the magnet, a reduction in the pressure is introduced, causing a concave deformation of the optical membrane.
Fig. 2. View of the lens system with integrated actuator. The curvature of the concave membrane lens can be tuned by volume displacement, caused by the movement of Neodymium magnet, which results from the repulsive force between the magnet and current-driven spiral coil.
2.3.
Design of DOE
The second element of the lens system is the DOE, configured as a Fresnel zone plate consisting of radial circular rings with increasing radii and decreasing widths. The rings act as a grating structure, which has a phase function based on the radial distance and the grating period [17]. The DOE can be designed to serve as a spherical lens with focal length f0 by introducing a phase shift φ (r) = −πr2 /λ0 f0 (1) to the incoming light, where f0 is the design focal length corresponding to the design wavelength λ0 . Our DOE is designed here to have a focal length of 20 mm at the reference wavelength of 550 nm. The phase function can be translated into a surface-relief profile h(r) for a transmissive thin DOE given by λ0 φ (r)mod2π h(r) = , (2) n(λ0 ) − 1 2π where n is the refractive index of the DOE material and φ (r) mod 2π is the phase function wrapped to the interval (0 − 2π) [18]. Hence h(r) is a continuous sawtooth profile with a maximum height λ0 hmax = . (3) n(λ0 ) − 1 The continuous profile DOE is predicted to have ideally 100% first-order diffraction efficiency [18]. In practice, however, the DOE is fabricated on fused silica by sequence of multiple lithography and etch steps. The element therefore has a staircase-like profile and the diffraction efficiency is given by sin(π/M) 2 η =( ) , (4) π/M where M is number of phase levels [18]. To balance fabrication process costs and sufficient diffraction efficiency, the DOE here was realized with 4 phase levels using a 2-step lithography and etch process, which results in a predicted diffraction efficiency of 81%.
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27614
2.4.
Tuning range of HCL
The tuning range of the HCL is modeled using the commercial ray tracing software Zemax. The lens system consists of a DOE and an H2 O-based tunable refractive lens with lens diameter of 3 mm and thickness of each lens of 1 mm, as shown in Fig. 3.
Fig. 3. Layout of the tunable HCL system, consisting of diffractive and H2 O-based refractive lenses, which yield an image distance of 52 mm. For a wavelength of λ1 = 450 nm, the focal length of the refractive lens has to be adjusted to f1 = −200 mm to focus the object (point source) to the fixed image distance of 52 mm. For a wavelength of λ2 = 900 nm, the focal length of the tunable refractive lens has to be readjusted to f2 = −18 mm to focus the object to the same image plane.
The DOE is designed to have a focal length of 20 mm at the design wavelength of 550 nm. Without loss of generality, the object and image are assumed to be at conjugate distances of the HCL system; the spectral tuning range was chosen to be from 450 to 900 nm. Thus, the wavelength λ1 = 450 nm should be in focus at the first actuating setting of the refractive lens, which is defined as focal length f1 , while the wavelength λ2 = 900 nm corresponding to the last actuating state of the refractive lens, is specified as focal length f2 . f1 and f2 are both negative values since the tunable element is a concave lens; the corresponding sag heights of the membrane are h1 and h2 . The focal length f1 was designed to be -200 mm, with a sag height of the membrane h1 = −17 µm. The focus point of wavelength λ1 = 450 nm is then at a distance of 52 mm from the HCL system. Based on simulation results, in order to have the λ2 = 900 nm focus at the same distance, the focal length of the refractive lens has to be tuned to f2 = -18 mm, corresponding to sag height h2 = −200 µm. The design parameters are summarized in Table 1. The tuning range of the HCL system is strongly dependent on the tuning capability of the refractive lens. In this paper, the specification of spectral range is limited to 450 – 900 nm, but in principle a larger scanning range can be achieved. To improve the tuning capability, one can use liquid with a higher refractive index compared to H2 O, for example silicone oil with refractive index nd = 1.4 [19], as the medium for the fluidic lens. 2.5.
HCL system simulation
To evaluate the spectral filtering characteristics of our HCL system, illustrated in Fig. 3, we employed the Zemax Extended Source Encircled Energy feature. This ray tracing-based model realizes the extended object by tracing the rays from different points within a pinhole of 50 µm #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27615
Table 1. Design parameters of the HCL system
parameters fr of H2 O lens fd of DOE f of lens system lens diameter object distance image distance spectral scanning
values tuning from -200 to -18 mm 20 mm at wavelength λ0 = 550 nm 26 mm 3 mm 52 mm 52 mm 450 – 900 nm
diameter. The rays passing the HCL and hitting the circular detecting area of 50 µm diameter in the image plane of the HCL were integrated to determine the intensity of the spectral response.
Fig. 4. Simulated spectral responses of the HCL system at different wavelengths; the FWHM of the peak is about 8 nm for each spectral band.
Using this approach, the spectral signals of four wavelengths corresponding to four settings of the tunable lens were calculated, and are shown in Fig. 4. The spectral width FWHM (fullwidth at half-maximum) of the spectral bands are 8.2 nm, 7.9 nm, 7.9 nm, and 8.4 nm for wavelengths of 450 nm, 600 nm, 750 nm, and 900 nm. These results show that the HCL system can achieve constant and narrow bandpass filtering throughout the wide spectral range of 450 – 900 nm. 3. 3.1.
Fabrication Tunable lens and magnetic actuator
We discuss the fabrication process for the design structure in Fig. 2. The silicon aperture of the refractive lens is fabricated by standard micromachining, with KOH etching 400 µm deep from
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27616
the back side of the Si wafer and DRIE through etching from the front side to create a circular aperture with a diameter of 3 mm. The aperture is covered by a radially stretched polyacrylate membrane 50 µm thick [20, 21]. The lens chamber is filled with liquid (H2 O) to generate the optical lens. The fluidic structures of the lens system are made from high performance adhesive transfer tape 3M 467MP 200MP. The elements are fabricated by CO2 laser cutting and successively stacked with the aid of alignment marks. The magnetic actuator consists of a neodymium (NdFeB) magnet with residual magnetism of 1.17 – 1.21 T mounted inside the actuating chamber, which is sealed by a polyacrylate membrane. A spiral coil of insulated copper wires, 100 µm in diameter with an inductance of 3 mH, is placed on the opposite side, together with the magnet. 3.2.
DOE fabrication
The DOE was designed for diffraction at the reference wavelength of 550 nm with the focal length of 20 mm. The element is optimized for nearly perfect imaging of an object at infinity. According to the optimization, the surface-relief profile φ for the thin DOE is designed as φ = −3826.597ρ 2 + 30.979ρ 4 − 0.480ρ 6
(5)
with ρ = r/3.6 mm, where r is the radial distance to the center of the DOE. To fabricate the diffractive element in a fused silica wafer, contact lithography and inductively coupled plasma reactive ion etching (ICP-RIE) are employed. The element used for the experiment was realized with 4 phase levels resulting from a 2-step lithographic process. At the outer radius of 3.6 mm, the DOE has a minimum local period of 3.0 µm and a feature size of 0.75 µm. The profile at the central part of the diffractive lens was characterized by white light interferometer and illustrated in Fig. 5.
Fig. 5. Central part of the DOE, measured by a white light interferometer.
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27617
3.3.
Prototype
A prototype of the tunable HCL was fabricated in two parts: the tunable refractive lens including the magnetic actuator and the DOE. These two components were then assembled into the complete HCL system, as shown in Fig. 6.
Fig. 6. Photograph of the prototype HCL system after assembly. The front side of the combined lens with the optical aperture is seen on the left, while the DOE is seen on the right.
4.
Measurements and results
4.1.
Measurement setup
The measurement setup to characterize the HCL system is presented in Fig. 7. A white light source is coupled into a 1 mm diameter fiber. An imaging lens is used to form a homogeneous image on the aperture stop with a diameter of 50 µm which becomes the white light input for the tunable HCL system. The image is formed on the facet of a 50 µm diameter fiber, from where the signal is transmitted to the spectrometer for analysis. The object and image distances of the HCL system are those as given in Table 1.
Fig. 7. Schematic of the setup for characterization of the HCL system. The 50 µm aperture stop is in the object plane, the 50 µm fiber is in the image plane of the system.
To extend this setup for 2D imaging, the aperture stop in Fig. 7 will be replaced by a pinhole #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27618
array with, for example, pinhole diameters of 50 µm and spacings of 100 µm. By using the achromatic imaging lens between the 2D object and this pinhole array, the object will be sharply imaged onto the array such that it acts as a confocal filter. If the array is moved laterally (in 2D) to cover the entire image, the lens system will be able to generate a 2D image with the same high spatial resolution. 4.2.
Spectral response
The spectral signal measured at the output fiber may be tuned by increasing or decreasing the DC current applied to the actuating spiral coil. The continuous and reversible spectral tuning from 450 nm to 900 nm is clearly shown in two diagrams in Fig. 8(a) and 8(b). The spectral signals filtered out from the original white light source using the HCL are plotted in Fig. 8(a). It is seen that the spectral response deviates from the white light envelope more strongly as the wavelength increases. This deviation is due to the variation of DOE efficiency with wavelength; efficiency was maximum at the design wavelength of 550 nm and decreases for longer or shorter wavelengths. For that reason, a normalization with the DOE efficiency for each wavelength over the full measurement range is necessary; the measured efficiency of the DOE for different wavelengths is given in Table 2. Table 2. Measured light source-independent efficiency of the DOE at different wavelengths.
wavelength (nm) efficiency
450 0.57
500 0.61
550 0.75
600 0.74
650 0.70
700 0.65
750 0.59
800 0.56
850 0.53
900 0.39
After normalizing the spectral signal with the white light spectrum and DOE efficiency for each evaluated wavelength, the relative intensities are almost constant over the entire range as shown in Fig. 8(b). This implies that the HCL system has a uniform response for the design spectral range. It was furthermore seen that the correlation between magnet drive current (lens actuator) and wavelength shift was highly linear, with a correlation coefficient of R2 ≈ 0.9973. The demonstrated spectral range includes visible and some near-infrared wavelengths, which is a considerably larger range than shown in previous work on acousto-optic tunable filters, which were limited to 450 to 550 nm [22], or liquid-crystal tunable filters, with a tuning range 400 to 720 nm [10]. The linewidth of the spectral signals was also investigated and plotted in Fig. 8(b), with the magnitude varying from 12 nm to 14 nm. The magnitude and uniformity of the linewidth over the entire scan range is considerably improved when compared to, for example, a liquid crystal tunable Lyot filter with the linewidth in the range 50 – 100 nm [11]. The constant value of the linewidth can be explained by the fact that, during the tuning process, the focal lengths of the different wavelengths remain the same, since these are defined by the object and image distances. Hence the spot size of the image is almost the same for all wavelengths and therefore the narrow spectral peaks of equal widths are coupled into the fiber while the HCL is tuned. In comparison to the analytical prediction from Fig. 4 with a linewidth of 8 nm, the larger experimental values may be due to alignment errors during fabrication and measurement, as well as diffraction effects which are not considered in the simulation. The linewidth is dependent on the LCA; the image spot size in the image plane of the HCL; and the diameter of the fiber leading to the spectrometer. Linewidth could be improved by minimizing the diameter of the pinhole in the imaging plane or by replacing the 50 µm fiber with one with a smaller core, which would, however, decrease the signal-to-noise ratio (SNR). The signal-to-noise ratio of the spectral response from the HCL is best for the high peaks in the center of the spectrum at about 600 nm. The measured SNR for the peak at 600 nm is 9, #195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27619
Fig. 8. Measured spectral responses of the HCL system (with H2 O lens) at different actuation settings, demonstrating a continuous spectral tuning and constant response of the HCL system in the range of 450 nm to 900 nm. (a) The white light spectrum and HCL spectral response from 450 nm to 900 nm before normalization and (b) the HCL spectral responses after normalization with the white light spectrum and DOE efficiency. The upper line shows the spectral width of the individual spectral signals; the FWHM varies between 12 – 14 nm.
whereas the SNR drops down to 1.5 for the small peaks at 450 nm and 900 nm. 4.3.
Tuning speed
As shown in Fig. 8, the spectral peak can be switched between different wavelengths by varying the DC current to the spiral coil. To evaluate the dynamic performance of the lens system, a high-speed video capturing spectral signal with resolution of 7 ms was recorded while the actuating current was subject to a step function input. The wavelengths with maximum intensity are plotted against time in Fig. 9 and summarized in Table 3. Table 3. Response time of the lens system subject to a step function of the magnet driving current. The rise and fall times correspond to tuning ranges of 500 – 600 nm and 500 – 810 nm.
wavelength range rise time (ms) fall time (ms)
500 – 600 nm
500 – 810 nm
32 ± 11 30 ± 8
34 ± 9 37 ± 6
The response time is defined as that needed for the lens system to shift from one wavelength to the next and to stabilize at the new state (the target wavelength ±3 nm). The rise times τ1 and τ2 , corresponding to tuning ranges of 500 – 600 nm and 500 – 810 nm, respectively, are similar despite the large difference in tuning range. In linear systems, the response time does not depend on the amplitude of the stimulating step function [23], the actuation current in our case. For higher actuation currents, and hence greater wavelength intervals, the system responds with higher speed due to the higher applied force.
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27620
Fig. 9. Recorded wavelength with maximum intensity as a function of time subject to a step rise of the actuation current. The rise times τ1 and τ2 correspond to the tuning ranges of 500 – 600 nm and 500 – 810 nm, respectively. The equipment-based resolution of the time axis is 7 ms, explaining the discrete nature of the data.
5.
Conclusion
The concept of a highly-compact tunable HCL system has been demonstrated and a flexible prototype with integrated actuator has been designed. The measurement results have shown the functionality of the lens as a continuously tunable filter, with a large dynamic range (450 – 900 nm), uniform response and almost constant narrow linewidth (12 – 14 nm) for the entire spectral range. The response time is less than 40 ms for different tuning ranges and tuning is highly linearly correlated with driving current. The same concept may be applied to implement two-dimensional hyperspectral confocal imaging. Acknowledgments This work was supported by the Federal Ministry of Education and Research in the project Optical Microsystems for Ultra-compact Hyperspectral Sensor (OpMihySen, BMBF Förderkennzeichen FZK 16SV5575K).
#195009 - $15.00 USD Received 31 Jul 2013; revised 19 Oct 2013; accepted 20 Oct 2013; published 4 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.027611 | OPTICS EXPRESS 27621
