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Abstract
We report an ultra-compact optical zoom endoscope containing two tunable Alvarez lenses. The two tunable lenses are controlled synchronously by piezoelectric benders to move in directions perpendicular to the optical axis to achieve optical zoom while keeping images in clear focus without moving the scope. The piezoelectric benders are arranged circumferentially surrounding the endoscope optics with a diameter about 2 mm, which results in an ultra-compact form. The demonstrated endoscope is capable of optical zoom close to 3 × from field of view (FOV) 50° to 18° continuously with the required movements for its constituent optical elements less than 110 μm. Such optical zoom endoscopes may find their potential uses in healthcare and industrial inspection systems.
© 2017 Optical Society of America
1. Introduction
Endoscopes are widely used in medical applications for inspection of duct-shaped hollows and/or cavities inside human bodies. Optical zoom endoscopes can be highly advantageous, because they allow a convenient switching from a wide-field view to a close-up view for high resolution imaging once a region of interest is identified. This significantly minimizes the movement of the scope during operation to reduce the complexity of the procedure and risk of trauma. More importantly, optical zoom endoscopes also enable doctors to observe magnified images for further inspection while maintaining high image qualities. This is distinguished from standard endoscopes with electronic or digital magnification, which simply enlarge the images on the displays with degraded image qualities/resolutions. Reports further highlighted that high-magnification optical zoom endoscopy may potentially allow specific characterization of some abnormal tissues that is sufficiently accurate to defer biopsy to save costs and improve efficiency [1–4].
Commercially available zoom endoscopes utilize mobile lenses/lens groups moveable along the scope’s optical axis and involve complex large–stroke actuation mechanisms, which leads to bulky dimensions hence limits their applications [5]. Meanwhile, recent years have also witnessed significant technological advancements in the field of miniature varifocal lenses [6–8]. These ultra-compact lenses are capable of varying their focal lengths through changing their geometric shapes or refractive indices / index distributions. The former approach is mainly based on liquid materials and varies the lens shapes through various effects including altering pressure [9, 10] or electrowetting [11]. The latter approach utilizes liquid crystals [12, 13]. The application of such miniature varifocal lenses in endoscopes is attractive, because they promise to maintain small overall sizes of the endoscope probes and at the same time provide them with the dynamic functionalities such as adjustable focus and optical zoom [14–19].
Alternatively, miniature tunable lenses constructed using pure solid materials have been emerged in recent years, which are based on either stain-induced shape-changing polymer lenses [20, 21] or Alvarez lenses with free-form surfaces [22–27]. The latter is more attractive due to its advantages of large optical power variation with small lateral displacement, small required driving force, and enhanced design degrees of freedom offered by the free-form surfaces. Without liquid materials, these lenses are inherently robust and ease of packaging and handling processes. In recent years, tunable Alvarez lenses have been applied to miniature cameras, adjustable focus endoscopes [28], and further extended to multi-element [29] and dual-focus configurations [30]. It should also be noted that there exist some other interesting concepts of miniature solid tunable optics, for example curved deformable mirrors and micro lenses integrated on MEMS-driven movable stages [31–33]. These devices are compact and may be useful in zoom endoscopes, although their reported tuning ranges so far are relatively small.
In this paper, we report a novel ultra-compact optical zoom endoscope that consists of a pair of solid tunable lenses with freeform surfaces, and demonstrate experimentally an endoscope zoom optics with its constituent optical elements’ diameters less than 2 mm and capable of optical zooming close to 3 × from field of view (FOV) 50° (wide-angle configuration) to FOV 18° (telephoto configuration). Such an ultra-miniature zoom endoscope demonstrates outstanding advantages over its counterparts including minimum component movement, fast response time, and small form-factor and hence is believed to be useful in future endoscopic systems.
2. Optical zoom endoscope design
Figure 1 shows a schematic of the proposed optical zoom endoscope. The optics consists of two aspherical lenses with fixed focal lengths, a pair of miniature tunable Alvarez lenses, and an optical aperture. The positions of the optical elements along the optical axis are fixed. Optical zooming is achieved through varying the optical powers of the two Alvarez lenses. As shown in the figure, each Alvarez lens further consists of two closely placed paired optical elements with free-form surfaces, whose small lateral movements transverse to the optical axis provide a significant optical power variation of the Alvarez lens. The imaging device is a commercially available fiber bundle from Fujikura having 100,000 pixels and an outer diameter of about 1.5 mm, which transmits the image formed by the endoscope to an external CCD camera. The driving mechanism consists of four slim piezoelectric benders, whose working principle is highlighted in an inset of the figure. The mechanism is designed as follows. The fixed ends of the piezo benders are secured to the imaging fiber bundle, while the movable ends are secured to their respective optical elements through connecting to their driving slots. The two aspherical lenses and the optical aperture are secured to stationary connection bars (not shown in the figure for clarity) through the slots designed on their mounting platforms as indicated in the figure. When applied with driving voltages, these piezoelectric benders provide small lateral displacements at their tips to drive free-form optical elements to move transverse to the optical axis of the endoscope, thereby varying the optical powers of the two tunable Alvarez lenses. These tunable lenses are adjusted synchronously to change the optical magnification of the system while keep the image in sharp focus without varying the image plane. As shown in the figure, two benders are placed horizontally providing horizontal displacements for a pair of elements in one Alvarez lens, whilst the other two benders are arranged in a perpendicular direction and drive the other Alvarez lens vertically. We noticed that due to a perfect match of such a driving mechanism with the optical varifocal principle, the size of the proposed zoom endoscope could be potentially made very small without sacrificing its optical performance.
Fig. 1 Schematic illustration of the proposed zoom endoscope. The left inset shows a detailed zoom-in view of the endoscope optics and the right inset illustrates the operation mode of the piezoelectric bender.
The optical design is implemented through a series of ZEMAXTM ray-tracing optimization steps. We start with a paraxial optical design with the targeted system performances (such as optical zoom ratio and FOV) and constraints (such as optical element diameter and spacing limitations) taken into consideration. Conceptually, we start with two varifocal lenses fixed on the optical axis and separated with a gap d for zooming from wide-angle to telephoto configurations as shown in Fig. 2(a) [34]. For wide-angle configuration, the first lens (tunable lens A) is negative and thus the marginal ray height on the second tunable lens (tunable lens B) increases drastically. In addition, the expected FOV at this configuration leads to a large chief ray angle, thus making the required diameter of the second tunable lens unacceptably large for biomedical endoscope applications. To solve this issue, an aperture is inserted and the tunable lens B is replaced with two lenses, with one positive lens (lens 3, focal length fixed) placed immediately after the aperture and one tunable lens (lens 4) positioned at a distance behind the lens 3 as shown in Fig. 2(b). To reduce the required optical power tuning range of the tunable lens A, it is further replaced by a negative lens (lens 1, focal length fixed) and a tunable lens (lens 2) separated with a gap as shown in Fig. 2(c) [35]. The final paraxial ray-tracing design is highlighted in Fig. 2(d), where the maximum optical element diameter is about 2 mm, total length of the scope is 18.4 mm, optical zoom ratio is about 3 × , and each tunable lens’s tuning range is within [-135, 135] diopters.
Fig. 2 (a) Starting configuration of zoom optics with two varifocal lenses with positions fixed on the optical axis. (b) Tunable lens B is replaced with an aperture, lens 3 and lens 4 to reduce the overall optics diameter. (c) Tunable lens A is replaced with lens1 and lens 2 to increase the endoscope FOV or to reduce the focal length tuning range requirement for the same FOV. (d) Ray-tracing results of the paraxial design.
Next, we optimize each of the fixed and tunable Alvarez lenses individually using a thick lens model to achieve its required paraxial performance obtained in the previous step. To simplify the fabrication process, each lens element here is modelled as having one planar surface and one free-form surface. The free-forms surfaces of the fixed lenses are modelled using the Zernike Standard Sags Surface in ZEMAXTM, which can be described by the following equation.
where c is the curvature, k is the conic constant, r is the radial ray coordinate in lens units, Zi is the ith Zernike standard polynomial with ρ and φ indicating the normalized radial and angular ray coordinates, and αi and Ai are the coefficients on the aspherical and Zernike polynomial terms respectively. The freeform surfaces of the tunable Alvarez lenses are governed by extended polynomials.where x and y are coordinates describing the freeform surfaces and Ai is the coefficient of the ith extended polynomial term Ei(x,y). The polynomial terms are a power series of x and y starting from the first-order terms x and y followed by the second-order terms x2, xy, y2, etc. arranged in increasing order. In this zoom endoscope design, we used all the extended polynomial terms up to the 7th order. The lens material used here is a typical ultraviolet (UV) curable polymer (NOA 83H from Norland Products) with a refractive index of 1.56 in the visible spectrum.Once the initial surfaces of fixed and tunable Alvarez lenses are determined, we then replace the thin lenses in the paraxial design with these thick lenses and conduct further optimizations. We start with a ray-tracing optimization to enhance the system’s on-axis imaging performance, and then move on to the off-axis imaging performances by gradually increasing the FOV. Since the optical system is not rotationally symmetric, we set eight field values uniformly distributed around a cone surrounding the optical axis to evaluate the off-axis performances. It is noted that the higher-order aspherical, Zernike, and extended polynomial terms in the free-form surfaces’ governing equations are gradually added to the variable list step by step to avoid the local optimal solution. The final optimal design result is plotted in Fig. 3 with Fig. 3(a) and Fig. 3(b) showing the YZ and XZ views of the endoscope at three different zoom configurations. It is noted that the free-form elements in the 1st Alvarez lens are tuned in the y direction, whereas those in the 2nd Alvarez lens are tuned in the x direction. The coefficients of each free form surface are listed in Appendix A. The endoscope is capable of optical zooming about a total of 2.8 × from FOV 49.8° to 18° with maximum movements of its constituent elements less than 110 μm as shown in Fig. 3.
Fig. 3 (a) YZ view and (b) XZ view of the endoscope optics with three zoom configurations form top (telephoto) to bottom (wide-angle).
Figure 4 further provides the ZEMAXTM simulated imaging performances of the three zoom configurations of the designed endoscope, with Fig. 4(a), Fig. 4(b), and Fig. 4(c) respectively showing for optical zoom 1 × , 1.9 × , and 2.8 × . For each zoom configuration, the moduli of the optical transfer function (OTF) at the center, horizontal maximum, and vertical maximum of the FOV are plotted and their respective ray-tracing spot diagrams are plotted under the OTF plot with the circles indicating the sizes of the diffraction-limited Airy spots. Simulated imaging results using a standard USAF target at different zoom configurations are also obtained and provided in the insets of their respective OTF plots. It is noticed that optical aberrations in this optical zoom endoscope design is well controlled at the minimum level and the geometrical ray tracing spots are mostly smaller than the diffraction-limited Airy spots. Further details of the optical performances of the designed optical zoom endoscope at three different zoom configurations are summarized in Table 1 below.
Fig. 4 (a) OTF, simulated image, and ray-tracing spot diagrams of endoscope at configuration one (optical zoom: 1 × , FOV: 49.8° degrees). (b) OTF, simulated image, and ray-tracing spot diagrams of endoscope at configuration two (optical zoom: 1.9 × , FOV: 26° degrees). (c) OTF, simulated image, and ray-tracing spot diagrams of endoscope at configuration three (optical zoom: 2.8 × , FOV: 18° degrees). Circles in spot diagrams indicate the sizes of the diffraction-limited Airy spots.
Table 1. Summary of the performance of the final zoom system
As we can see from the above optimal design result, the largest FOV the endoscope can achieve is as high as 49.8°, and the maximum actuation displacement required for the entire optical zoom range (close to 3 × ) is only 110 μm. At 20% MTF, the spatial resolution of the designed system is greater than 120 cycles/mm at all three zoom configurations. The distortion is about 15% at the largest FOV at wide-angle configuration and falls down to about 3.2% at telephoto configuration. It should be noted that the maximum displacement of 110 μm also leads to a maximum tilt angle less than 0.3°at the tip of the piezoelectric bender. However, this tiny tilt angle theoretically has little effect on the image quality [36].
3. Fabrication, assembly, and testing
The fabrication and assembly process for the proposed endoscope is highlighted in Fig. 5. All optical elements were fabricated using a technique of precision diamond turning followed by a polymer molding replication process [37]. To facilitate the endoscope assembly process, silicon-on-insulator (SOI) chips fabricated using standard SOI MUMPS process are employed [38]. Each chip has a released silicon platform suspended by silicon micron springs. The platform has a through-hole opening to mount its designated optical element. The relative position of the opening with respect to the two selected neighboring edges of the chip is carefully designed and its precision is ensured by the photolithography process. These edges will be used as alignment references in the later endoscope assembly process.
Fig. 5 Endoscope assembly process. (a) Free-form optical elements are aligned, inserted, and fixed on their respective platforms on SOI chips. (b) two paired SOI chips are aligned and secured to form a tunable Alvarez lens. (c) Precision machined assembly jig. (d) SOI chips having the optical elements of the endoscope are aligned and fixed with the assembly jig. (e) Imaging fiber bundle is aligned to the optical elements, and piezoelectric benders are inserted into their respective driving slots on the SOI platform and secured to the fiber bundle. (f) After the assembly process, the silicon springs on the SOI chips are broken to detach the endoscope from the assembly jig.
As shown in the Fig. 5(a), the aspherical lenses and free-form elements of the Alvarez lenses are first aligned to their respective mounting platforms on SOI chips with the help of an optical microscope, and then they are inserted into the through-hole openings in the platforms and fixed with UV adhesive. The alignment of the optical elements with their respective SOI chips is critical. The UV adhesive used here plays a significant role in this step since it allows us to slightly adjust the position of the element under the microscope before it is completely fixed by hardening the adhesive. In Fig. 5(b), the two SOI chips having two paired free-form surfaces forming the tunable Alvarez lens are then lined up and secured to each other with their two perpendicular reference edge surfaces aligned with the help of a precision right angle block. Next, a mechanical jig as shown in Fig. 5(c) designed to further guide the endoscope assembly process is fabricated using precision machining. Next in Fig. 5(d), the SOI chips holding the aspherical lenses, aperture, and tunable Alvarez lenses are temporarily fixed to the jig with their reference edges firmly contacting with the reference planes 1 and 2 of the jig as indicated in the figure. This procedure ensures that all optical components’ initial transverse positions relative to the optical axis of the endoscope are aligned. In addition, the designated top or bottom surfaces of the SOI chips are also in firm contact with their respective reference surfaces offered by the blocks 1 to 3 as shown in the figure, thereby setting the gaps between these optical components to their optimal values in the ZEMAXTM final design. Subsequently, the imaging fiber bundle is inserted and secured to a support block as shown in Fig. 5(e). Two precision machined blocks 4 and 5 are then temporarily attached to the support block, and the fiber bundle are placed on the mechanical jig for alignment. The respective dimensions of the jig and blocks are designed such that when the left surface of the block 4 and the left surface of the jig (indicated as the alignment reference planes in the figure) coincide, the center of the imaging fiber bundle and the optical axis of the endoscope are aligned. In addition, when the back surfaces of the block 4 and the jig coincide, the end face of the fiber bundle is set to the image plane of the scope. In what follows, the mechanical jig and the piezo bender to be assembled are held separately by two precision alignment stages. The piezoelectric benders are commercially available modules SMBS1515T6P750WL (Dimension: 15 × 1.5 × 0.6 mm, resonant frequency: 4KHz ± 5%) from Steminc-Piezo. The movement of the bender with respect to the endoscope optics is monitored by two optical microscopes, and the movable end of the bender is inserted into its respective driving slot on the silicon platform holding the Alvarez lens element and secured with UV adhesive. The stationary end of the bender is then fixed to the fiber bundle, as shown in Fig. 5(e). In this step, care should be taken to align and insert the moveable end of the bender into the driving slot on the SOI chip as the width of the slot is only 20 μm larger than the thickness of the bender. The suspended SOI platform integrated with the optical elements may be destroyed if the relative position of the bender with respect to the driving slot is not precisely controlled during assembly. That is why we use two optical microscopes with CCD cameras to simultaneously monitor the front and back faces of the driving slot, and use two 3D alignment stages to precisely control the positions of the bender and mechanical jig. After the assembly of the benders, a stainless-steel connection bar is then inserted, using the same method, into the designated connection slots on the platforms holding the aspherical lenses and aperture to secure all these stationary optical components. The connection bar is then further secured to the support block on the fiber bundle. Finally, the micron sized silicon springs holding the optical component platforms are then broken manually and the endoscope is detached from the entire assembly setup, as shown in Fig. 5(f).
The device pictures and the final assembled scope are provided in Fig. 6. One aspherical lens and one free-form optical element from a tunable Alvarez lens assembled on their corresponding platforms on SOI chips are shown, respectively, in Fig. 6(a) and Fig. 6(b). Figure 6 (c) shows two paired SOI chips forming a tunable Alvarez lens. Figure 6(d) and Fig. 6(e) further show the front and size views of the assembled miniature optical zoom endoscope. In the last step of the assembly process, the silicon micron springs are manually cut to detach the scope from SOI chips. However, it was found that the force of cutting was difficult to control and might slightly offset the optical components from their optimal positions thus degrading the scope’s imaging performances. It was then decided that the proof-of-concept working principle was demonstrated without removing the silicon springs. It was found that the temporary supporting silicon beams connecting the mounting platforms to the substrates of the SOI chips have little effect on the movements of the suspended lenses.
Fig. 6 (a) The negative lens fitted on its mounting SOI chip. (b) Free-form optical element secured on a SOI chip. (c) Two paired free-form optical elements forming a tunable Alvarez lens. (d) and (e) show respectively the front and side views of the optical zoom endoscope.
4. Results
The piezoelectric bender is first characterized by monitoring its tip under an optical microscope when applied with driving voltages. The movement of the tip as a function of the driving voltage is shown in Fig. 7(a), where it is found that a maximum stroke of 135 μm in both positive and negative directions can be obtained with a driving voltage value of 140 V, which is sufficient for driving the two Alvarez lenses to achieve the desired optical zoom ratio. In our work, the four piezo benders are characterized separately, and results are similar to each other. Only a typical one is shown in the figure to keep it concise. An individual Alvarez lens is also tested for its response speed. In this experiment, a laser beam focuses through the lens to a 100 μm pinhole. The light passing through is then received by a photodetector. The Alvarez lens is driven by a square waveform voltage with a 1-Hz frequency to modulate its focal length. The photodetector’s response as a function of time is shown in Fig. 7(b), where the detailed device responses at the rising and falling edges of the square waveform is further highlighted in the insets. It can be observed that the response time of such a lens is about 15 ms (mainly due to underdamped oscillations), indicating potential high-speed operation of the proposed endoscope. The piezoelectric bender here is operated in open-loop mode. The oscillations shown in the device response might be suppressed with a closed-loop feedback thus further enhancing the lens tuning speed.
Fig. 7 (a) Displacement-voltage relationship of a typical piezoelectric bender used in the endoscope. (b) Measured response time of a tunable Alvarez lens. (c) to (e) Experimental demonstration of optical zooming (about 1 × , 2 × , and 3 × from left to right) with a target placed at about 20 mm away from the endoscope (Visualization 1).
Finally, the optical zooming functionality is demonstrated with a target object (a “NUS” text) fixed at a distance 20 mm away from the first lens of the endoscope. A high-resolution CCD camera is attached to an optical microscope that is focused on the output end face of the fiber bundle to capture the endoscope image transmitted through the fiber bundle. By adjusting the driving voltages of the two Alvarez lenses synchronously to vary their respective focal lengths, the optical magnification of the endoscope can be continuously changed with an optical zoom ratio about 3 × while keeping the object in focus without moving the end face of the imaging fiber bundle as shown in Fig. 6(c)~Fig. 6(e) (see the video in Visualization 1).
It is noted that in this experiment, the input voltages of the two piezo benders driving the paired two elements in one Alvarez lens are kept the same to reduce the number of voltage controls and hence simplify the driving system. For example, to capture the images in Fig. 7(c) to Fig. 7(e), the two driving voltages applied to the piezo benders in the first and second Alvarez lenses are set at [84V, −42V], [-50V, 68V], [-80V, 120V], respectively. Driving the four piezo benders separately may help to improve the position accuracy of each lens element (by taking the slight differences among these benders into consideration) and hence further enhance the image quality. In our experiments, we find that the image quality is generally superior to the reported experimental results of liquid lens or liquid crystal lens based endoscopes in reference [13] and [16] in terms of image resolution and distortion. Clearly, this result demonstrates the advantages of the proposed zoom endoscope design in achieving dynamic functionalities while keeping an ultra-compact probe size and a good imaging performance. However, we also note that the maximum driving voltage of this zoom endoscope can reach as high as 120V in our current demonstration. This might be a problem for in-vivo biomedical endoscope applications. Sealing the zoom optics and driving mechanisms with a good insulating tube might help to solve this problem. Another solution might be replacing the high-voltage piezo benders with low-voltage micro actuators, for example MEMS based electrothermal actuators [39]. This is highly possible since the required displacement to achieve a reasonable optical zoom ratio is not large.
5. Conclusion
We have reported the design, fabrication, and demonstration of an ultra-compact zoom endoscope, which utilizes two adjustable Alvarez lenses. We employed several unique design considerations in the reported endoscope. The first one is the tuning mechanism of the Alvarez lenses, which utilizes minute lateral displacements of optical components perpendicular to the optical axis to induce significant optical power variations of these lenses. The second is the mechanical driving mechanism using piezoelectric benders arranged circumferentially surrounding the endoscope optics with their length direction aligned parallel to the optical axis. The third one is the incorporation of a set of free-form surfaces for optical aberration correction and optimization. These collectively make the endoscope extremely compact but still having relatively large optical zoom ratio and satisfactory imaging performance. Such endoscopes may be useful in medical and industrial inspection systems.
Appendix A – Surface Coefficients in the Final Design
The coefficients determining the aspherical / free-form surfaces in the final optimal design is listed below. The aspherical / free-form surfaces are arranged in order from object space to image space (see Table 2).
Table 2. Surface coefficients of the final zoom endoscope design
Funding
Singapore Ministry of Education (MOE) (R-265-000-557-112).
Acknowledgment
Financial support from Singapore MOE research grant is gratefully acknowledged. The authors also thank the anonymous reviewers for their constructive comments that helped to improve the paper.
References and links
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35. Note: assuming that a lens with fixed optical power ϕ0 and a tunable lens with tunable optical power range [ϕ1, ϕ2] are separated with a gap t, the overall optical power tuning range of the pair is (Φ2 - Φ1) = (ϕ2 - ϕ1)(1-tϕ0), which is increased when ϕ0 < 0.
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