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Ring-Shaped Bifocal Lens Used for Fluorescent Self-Referenced Holographic Imaging Márton Zsolt Kiss1,2

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Ring-Shaped Bifocal Lens Used for Fluorescent Self-Referenced Holographic Imaging Márton Zsolt Kiss1,2 Kiss Journal of the European Optical Society-Rapid Publications (2016) 12:2 Journal of the European Optical DOI 10.1186/s41476-016-0002-z Society-Rapid Publications RESEARCH Open Access Ring-shaped bifocal lens used for fluorescent self-referenced holographic imaging Márton Zsolt Kiss1,2 Abstract We propose an alternative and simple solution to self-referenced digital holographic imaging based on a ring-shaped bifocal lens, without the need of any mirrors, polarizers or spatial light modulators. We discuss the imaging properties of the ring-shaped bifocal lens in self-referenced holography. The easy applicability of this bifocal lens is demonstrated on a realized microscope setup for volumetric observation of freely moving fluorescent objects, based on a conventional light microscope. Keywords: Digital holography, Fluorescence microscopy, Holographic optical elements, Vision in depth, Single-shot imaging Background position of the target object does not permit the proper The aim of our research is to develop a microscope that interference of the target and reference beams, or the can detect, localise and image freely moving fluorescent light emission of the targets themselves is the relevant objects within a thick volume in real time. The realization feature that is to be detected and reconstructed. In these of such an instrument would result in an immediate in- cases, Self-referenced Holographic Setups (SHS) can be dustrial benefit, for example, real-time water monitoring applied. In these setups, the target and reference beams systems [1] can be built, where the use of self-referenced are the differently modulated lights of the same emitted holography [2, 3] is undoubtedly very profitable. (or reflected) light of the measured object. The require- Holographic imaging is based on an interference ment of proper interference is that the optical path dif- phenomenon, in which wavefronts are captured to re- ference of the two beams has to be smaller than the construct the image of the measured objects [4]. Two of coherence length of the light. its main advantages are the possibility to increase the Self-referenced Holography (SH) is frequently applied depth of the observed volume without having a consid- to image fluorescent [13], distant and extended objects erable loss in the resolution [5], and the potential of [14], or stars [15]. An architecture of the SHS can be lens-less imaging [6]. In the traditional in-line [7] and based on either an interferometer (e.g. Hariharan-Sen off-axis [8] setups, one light source with an appropriate [16, 17]), bi- or multifocal lens [18, 19], or on a diffract- length of coherence is used to implement both the target ive optical element [20]. and the reference beams. Such a light source can be a Although the concept of bifocal lens based SHS was laser [9], a LED [10], an electron gun [11], or some other already presented in the early years of holography [4], sources [12] as well. and the application of birefringent bifocal lens, double However, in some cases, these traditional arrange- half lens, Fresnel zone plate, and their mixed solutions ments cannot be applied. Either the size and/or the were proposed, only some of them were tested experi- mentally [21]. Recently, the application of Spatial Light Correspondence: [email protected] Modulators (SLM) extended the list of bi- and multifocal 1Faculty of Information Technology and Bionics, Pázmány Péter Catholic University, Budapest H-1083, Hungary lens based SHS [22]. 2Hungarian Academy of Sciences, Institute for Computer Science and Usually, interferometer-based SHSs are quite large and Control, Computational Optical Sensing and Processing Laboratory, Budapest complex, due to the mirrors required, which also makes H-1111, Hungary © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kiss Journal of the European Optical Society-Rapid Publications (2016) 12:2 Page 2 of 9 them exceptionally sensitive to vibrations. Furthermore, curvatures (R1,R2) at the detector plane. Our calcula- the used beam splitter typically result in more than 50 % tions pointed out that the interaction of these waves gen- light intensity loss. Polariser based SHSs are also erates an interference intensity pattern, which is the same doomed to face the latter problem, which is also a par- as the intensity of the interference of a plane wave and a ticularly challenging limitation when the light emission spherical wave with radius of curvature Rd (Eq. (1)), where of the objects is little, as in the case of the above- mentioned fluorescent imaging. R à R All the SLM- and some of the interferometer-based R ¼Æ 1 2 : ð1Þ d R −R setups also offer the possibility of twin-image elimin- 1 2 ation; however this can usually be fulfilled only for static objects, as the method is based on phase-shifting inter- Rd is used to define the reconstruction distance of the ferometry and requires multiple exposures of the same corresponding object point. As the SIPs are incoherently object [23]. summed up, they do not disturb each other, and there- The above limitations of the earlier approaches di- fore, their reconstruction distances remain unbiased. rected our attention to the application of a special bi- This property is important when the reconstruction is focal lens in SHS. That is, to apply a Ring-shaped Bifocal used for object localization. However, the more incoher- Lens (RBL) [24] to implement SH. According to our best ent summation of the SIPs, the less relative dynamic knowledge, so far this approach was not proposed, used range of the detector, which results in considerable loss or tested by others. of contrast of the captured digital hologram. In this paper, our goal is to prove that the RBL is an Next, our novel optical solution is presented for creat- efficient tool for making holography based fluorescent ing double coherent waves from a single one for the pur- volume detection, localization and imaging. First, let us pose of self-referenced holography. overview the main details of the imaging method of the SH, and outline the hologram generating principle of the RBL hologram generation RBL in the next section. Second, the main advantages At the central point of our SHS stands the Ring-shaped and disadvantages of the RBL made holograms are dis- Bifocal Lens that realise the self-referenced hologram cussed. Finally, we demonstrate the applicability, and the with the required beam splitting. This splitting is the re- ability to fluorescent targets volume imaging of the RBL sult of the division of the aperture of the RBL. The two through the experimental study of the RBL based SHS. apertures are different; the central one is circular while the other is a ring around it. Both of them are symmetric Methods to the optical axis. These aperture areas have a focus dif- The self-referenced imaging and the RBL ference, which in general can be reached with the optical Let us briefly summarize the self-referenced hologram property of the material (e.g. grin lens), or with the generating method. An SHS generates two waves (a geometry. Here, we use an RBL where the geometry of wave pair) with different wavefront curvatures from a the lens generates the different focuses. To ensure light coming from a point of the object. exactly two focuses, only one of the two surfaces (the The interference of the pair of waves produces a Self- right one) is diversified, as it can be seen in Fig. 1. referenced Interference Pattern (SIP). The SIP captured In our experiments, we use a custom made RBL, by a digital camera is the digital intensity hologram. Sev- which consists of a central plano-convex lens (focal eral object points will have several SIPs by the SHS, and length 400 mm), and a “biplane ring-shaped lens” having the camera captured image of their sum also is the infinite focus. The outer diameter of the whole RBL is digital intensity hologram. 10 mm while the diameter of the inner lens is 6 mm. When the light coming from different points of the The scheme of the hologram generation of the actual object are coherent with each other, the complex ampli- RBL is shown in Fig. 2 when the source object is at an tude of their SHS generated SIPs is added. Otherwise, infinite distance. One can see that the RBL creates two only the intensities of their SIPs are summed. (Obviously beams, a central placed cone shaped one, and a hollow there is a case when the SIPs are partially coherent with one. These beams have a ring-shaped cross section at each other.) Thus, in the particular case of fluorescent the plane of detection. The self-referenced hologram of objects, only the intensities of the SIPs are summed, a single point created by the RBL is ring-shaped as it is irrespectively of the type of their excitation light. shown in Fig. 4. Because the middle of this hologram is Let us analyse the formation of a single SIP in the fol- missing, we call it gappy hologram. The shape of the lowing. A spherical wave emitted (or reflected) by a sin- hologram is determined by the shape of the outer aper- gle point is divided than modulated differently by the ture of the RBL, and only the divergences of the beams SHS into two spherical waves, with a radius of depend on the focal parameters (Fig.
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