Microfabricated optical fiber with microlens that produces large fieldof-view, video rate, optical beam scanning for microendoscopy applications

Transcription

1 Microfabricated optical fiber with microlens that produces large fieldof-view, video rate, optical beam scanning for microendoscopy applications Eric J. Seibel 123, Mark Fauver 1, Janet Crossman-Bosworth 1, Quinn Y. J. Smithwick 12, and Chris M. Brown 13 1 Human Interface Technology Laboratory, College of Engineering, University of Washington, Seattle, WA USA, (206) , 2 Department of Mechanical Engineering, University of Washington, Seattle, WA Department of Bioengineering, University of Washington, Seattle, WA ABSTRACT Our goal is to produce a micro-optical scanner at the tip of an ultrathin flexible endoscope with an overall diameter of 1 mm. Using a small diameter piezoelectric tube actuator, a cantilevered optical fiber can be driven in mechanical resonance to scan a beam of light in a space-filling, spiral scan pattern. By knowing and/or controlling the fiber position and acquiring backscattered intensity with a photodetector, an image is acquired. A microfabrication process of computer-controlled acid etching is used to reduce the mass along the fiber scanner shaft to allow for high scan amplitude and frequency. A microlens (<1 mm diameter) is fabricated on the end of the optical fiber to reduce divergence of the scanned optical beam. This added mass of the microlens at the free end of the fiber causes the location of the second vibratory node to shift to near the focal length of the microlens. The result is a microlens undergoing angular rotation along two axes with minimal lateral microlens displacement. Preliminary experimental results indicate that this method of optical beam scanning can deliver laser energy over wide fields of view (>50 degrees full angle), up to video scan rates (>10 KHz), while maintaining a scanner diameter of 1 mm. A comparison can be made to bi-axial mirror scanners being fabricated as a MEMS device (micro-electro-mechanical system). Based on the opto-mechanical performance of these microlensed fiber scanners, flexible catheter scopes are possible for new microendoscopies that combine imaging with laser diagnoses. Keywords: Microendoscopy, optical scanning, microlens, resonant waveguide, fiber, imaging INTRODUCTION Standard flexible endoscopes have a fundamental tradeoff when their overall diameter is reduced. Either field of view is restricted and/or spatial resolution of the acquired image is reduced (Niederer et al., 2001). The reason for this tradeoff is the use of a pixel-array detection system, such as a coherent bundle of optical fibers or a pixel-array camera that requires one fiber or sensor for each pixel within the display (Figure 1A). An alternative strategy is to produce a small optical beam scanner at the tip of a flexible catheter shaft, thereby eliminating the tradeoff of reducing image quality when reducing overall diameter of a microendoscope or catheter scope (Seibel et al., 2001). 46 Optical Fibers and Sensors for Medical Applications III, Israel Gannot, Editor, Proceedings of SPIE Vol (2003) 2003 SPIE /03/$15.00

2 The main advantage of using an optical scanner for microendoscopy applications is the ability to direct a beam of high-quality laser light onto tissue for integrated imaging, diagnosis, and therapy (Seibel & Smithwick, 2002). In this paper, initial results of the optical design and fabrication of a microlensed fiberoptic scanner are presented. This unique opto-mechanical design may be free from the standard tradeoff between image quality and overall diameter of a catheter scope, but also carries its own set of engineering tradeoffs. This is the first experimental investigation of the interdependence of the optical beam scanning design and resonant fiber dynamics, using our optical fiber microfabrication techniques that are both simple and low in cost. Figure 1. Top functional diagram depicts current technology that uses diffuse illumination, imaging lenses, and a pixel-array for image transmission and capture (coherent fiberoptic bundle or camera). Middle level depicts the distal end of an optical fiber that creates a point source that is scanned across an object plane and imaged out to the magnified illumination plane. The proposed opto-mechanical design is depicted in the bottom level with a microlensed optical fiber scanner creating a directed beam of illumination that is focused with a scan lens. In the two fiber-scanning embodiments, the backscattered light is detected with a single RGB photodetector, but light can be collected by the same optical fiber in a confocal arrangement. Proc. of SPIE Vol

3 The proposed fiber scanner is located at the distal tip of a flexible catheter for the insertion in the working channel of a flexible endoscope or bronchoscope. Laser light is combined at the proximal end into a single optical fiber with a small core diameter. At the distal end, the micromachined optical fiber is held as a short cantilever that is driven in mechanical resonance using a piezoelectric tube with a four-quadrant electrode. By scanning the fiberoptic cantilever in a spiral pattern, circular images can be generated with 10 to 20 micron resolution (Seibel et al., 2002). Recently, the addition of position sensors for the detection and control of fiber motion has been demonstrated to be able to reduce scanning distortions to nearly one pixel (Smithwick et al., 2002). Previously, the opto-mechanical system did not use a microlens at the tip of the fiberoptic scanner, thereby producing a point source imaging system (Figure 1 middle). Without a microlens, the light exits the fiber core at a divergence angle determined by the numerical aperture (NA) of the optical fiber, typically 0.11 NA for a singlemode optical fiber (FS-SN-3224, 3M Inc.). However, a microlens allows the divergence of light exiting the fiber scanner to be reduced, creating a more collimated beam or converging beam. By reducing the divergence of light exiting the fiber scanner, a different opto-mechanical system is produced using a scan lens to focus the scanned optical beam to the plane of illumination (Figure 1 bottom). Compared to point source imaging (Figure 1 middle), the tip of the fiber scanner does not have to physically trace the object plane, but can simply rotate about a fixed point in space. In this way, the proposed method of optical beam scanning with a microlensed fiber scanner is analogous to a dual-axis mirror scanner. The property of a microlensed fiber scanner to rotate about its vibratory node was predicted theoretically and demonstrated experimentally for the second mode of resonance by Seibel et al., (2001) and Fauver et al., (2002), respectively. THEORETICAL AND EXPERIMENTAL RESULTS Optical beam scanning theoretical optical designs The degree of optical beam collimation from the microlensed fiber scanner is investigated in the optical designs shown in Figure 2. In all cases the tip of the fiber scanner is depicted as a 0.8 mm diameter ball lens that is rotating about its center. As the ball lens rotates, the optical point source rotates the same amount with no lateral movement while small axial translations are ignored. The rays leaving the ideal point source are diverging from an air medium at 0.11 NA, before striking the ball lens (BK7 material). In Figure 2A, point source and ball lens are rotating together +/- 70 degrees and the exiting beam is converging at 4.8 degrees full cone angle. Whereas in Figure 2B, the point source and ball lens are rotating +/- 50 degrees and the emerging beam is approximately collimated (0.1 degree convergence) with diameter of 125 µm. In Figure 2C, the point source and ball lens are rotating +/- 50 degrees with an emerging beam that is diverging at 3.0 degrees full cone angle. Due to the restricted field of view of the fiber scanner with 3.0 degrees divergence, a beamsplitter and aperture are added to allow side-viewing as well as forward-viewing, as shown in Figure 2D. In all cases, the small beam diameter from the rotating ball lens creates optical performance that is diffraction limited, so the predicted spot size of the Airy radius is proportional to wavelength. For example in Figure 2A, the Airy disk diameter averages <15 to <25 microns for 405 nm to 635 nm 48 Proc. of SPIE Vol. 4957

4 laser diode sources. The resolution can be estimated to be the diameter of the illumination field or circumference of the curve divided by the Airy radius, which creates >965 (405 nm) and >575 (635 nm) resolvable pixels across the illumination field of 7.23 mm in Figure 2A. Of great importance to future catheter scope design is reducing the rigid length of the distal tip by keeping the axial length of the lenses as short as possible. By having the scanned optical beam more converging and not diverging, the rigid axial length is shown to be only 4.0 mm in Figure 2A versus 5.7 mm in Figure 2B. The largest scan lens in each design in Figure 2 is 3.0 mm overall diameter. A. Each beam is converging at 4.8 degrees. C. Each beam is diverging at 3.0 degrees (full angle). B. Each beam is collimated (0.1 degrees). D. Beamsplitter and aperture inserted in Figure 2C. Figure 2. The point source and ball lens rotated together to produce a theoretical approximation to optical beam scanning. The point source is modeled in air and the 0.8 mm diameter ball lens is BK7 glass in all cases. (A) The optical beam is rotated +/- 70 degrees and has 4.8 degrees of convergence (full cone angle) when leaving the ball lens. (B) The optical beam is rotated +/- 50 degrees with 0.1 degrees convergence. (C) The optical beam is rotated +/- 50 degrees with 3.0 degrees divergence (full cone angle). In all cases the beam diameter is approximately 125 microns when leaving the ball lens, making the optical designs diffraction-limited in visible wavelengths. The largest scan lens is 3.0 mm in overall diameter. At the forward illumination plane, the Airy diameters range from 15 to 30 microns in the visible spectrum, depending on wavelength. When comparing optical beam scanning designs, increasing the degree of beam convergence allows for increased field of view and reduced axial length of the optical system. Proc. of SPIE Vol

5 Optical beam scanning fiber microlens development Simple microlenses formed by heating the tip of a cylindrical optical fiber produces a ball lens with an overall diameter on the order of three times the diameter of the original optical fiber (Fauver, et al., 2002). The resulting microlens is fused onto the tip of the optical fiber and mechanical failure of the fiber scanner is rare. The microlens refractive index is a weighted average between the core and cladding of the original optical fiber material. Due to the limitation of both size and refractive index of the ball lens, the lowest angle of divergence is 4 to 5 degrees full cone angle. In addition, the diameter of the emerging beam is narrow due to the small size of the fused microlens and the low NA of the singlemode fiber. An alternative method is fusing a high refractive-index sapphire ball onto the tip of a singlemode optical fiber with optical epoxy. The resulting optical beam scanner produces a beam divergence of 1 to 2 degrees full cone angle, as shown in Figure 3. Since this microlens is not limited in size, larger microlenses of up to 1.0 mm diameter have been attached to mm diameter optical fibers. However, the fabrication method is tedious with a high probability of poor optical beam scanning performance due to lens decentration and eventual separation of the microlens from the fiber. Furthermore, the larger mass at the free end of the optical fiber reduces both the frequency and the angular deflection of the ball lens at the second mode of resonance. Figure 3. A 0.79 mm diameter sapphire ball lens (left) is vibrating at 4 KHz in second mode of resonance. The maximum angular deflection of the ball lens is 10 to 15 degrees full cone angle with the vibratory node at approximately the focal point of the fused microlens. All tip angular deflections are measured by the maximum diameter of the scan pattern onto a display surface at known distance. By measuring the optical beam diameter at two locations, the optical beam divergence was less than 2 degrees full cone angle. Optical beam scanning three-layer etch process A new fabrication method was developed that uses three liquid layers, with the central layer being hydrofluoric (HF) acid. This three-layer etching technique allows the shaft of the fiber scanner to be thinner and more flexible while protecting the distal tip microlensing. For example, the fiber scanner shown in Figure 4 was etched only along the shaft, reducing the overall diameter from 125 microns to 35 microns. The additional fiber mass at the distal tip can be used to form a ball lens of 50 Proc. of SPIE Vol. 4957

6 up to 400 microns diameter using the CO 2 laser heating method by Fauver et al., (2002). However, a barrel or GRadient INdex (GRIN) lens geometry is preferred over the more massive ball lens for higher resonance frequency scanning. A barrel lens can be formed by first fusing a short section of multimode optical fiber to the smaller core singlemode optical fiber and then form a refractive surface by CO 2 laser heating the distal tip. A goal in the fiber scanner fabrication process is to locate the distal vibratory node (second mode of resonance) to be at the desired source point of the optical beam scanner. By having the optical beam originate from a single point in space along the optical axis, the subsequent scan lens can be simplified. The alternative to the barrel lens is a GRIN lens, which can be fused to the distal end of the singlemode optical fiber. The GRIN lens can be protected during the 3-layer etching process, while the shaft of the optical fiber is etched to increase flexibility and angular deflection of the fiber scanner. Both methods are exemplified in Figure 4 where the 125 micron diameter distal section of the fiber scanner could be a microfabricated barrel lens or a fused GRIN lens. Figure 4. Three layer etching technique protects the 125 micron diameter distal tip (top), which can represent a microfabricated barrel lens or a fused GRIN lens. The fiber scanner (bottom) is shown vibrating at the second mode of resonance (10.9 khz) with approximately 25 degrees full angle. The vibratory node is located at the distal end of the 3-layer etched region, which corresponds to the effective source point for a microlensed fiber scanner that generates a scanned optical beam in two dimensions from this single point. Proc. of SPIE Vol

7 Optical beam scanning maximizing scan amplitude A working hypothesis is that increasing the shaft flexibility using the 3-layer etching process will increase the angular deflection of the fiber scanner, thus increasing field of view. This hypothesis was tested by etching the same axial length of six optical fibers for different amounts of time, resulting in shaft diameters that ranged from d 1 = 70.5 to d 6 = 12 microns, see Figure 5. The singlemode optical fiber was originally 125 microns in cladding diameter. All fibers were attached to the same piezoelectric actuator and driven at the same voltage amplitude. Since each fiber scanner was made the same total length, the resonant frequency shifted due to the difference in shaft thickness. Therefore, the frequency was found for the second mode of resonance for each fiber scanner and the corresponding maximum angular deflection at the tip was also recorded. The results are listed in Table 1 below. 125 microns Etch dia. d NOT DRAWN TO SCALE 1.68 mm 56 microns 4.7 mm 167 microns Figure 5. Three-layer etched fiber scanner is shown stationary (top) and vibrating (middle) at 3.7 khz in the second mode of resonance. A set of six identical optical fibers went through the same 3-layer etching process with the dimensions shown in the schematic at the bottom of Figure 5. The results of driving these six fiber scanners (same cantilever length) at approximately the same base amplitude are shown in Table Proc. of SPIE Vol. 4957

8 Table 1. Maximizing the tip angular deflection by varying the etching amount of the fiber scanner shaft. Etch diameter d # (microns) Second mode resonance Tip angular deflection (deg.) {maximum full cone angle} frequency (Hz) d d d d d d Figure 6 shows the frequency response of the piezoelectric actuator driving the fiber scanners listed in Table 1. Since the base amplitude is only rising slightly with increasing frequency from 1100 to 4400 Hz, the maximum fiber tip deflection of d 5 at 2027 Hz is authenticated. A photograph of the distal tip of d 5 is shown in Figure 7, exhibiting that a 35-degree angular tip deflection (full cone angle), will produce exactly 1.0 mm lateral displacement of its flexible shaft. Frequency response of piezoelectric actua Amplitude (microns) Fr equency ( H z Figure 6. The frequency response of the bimorph piezoelectric actuator (600/200/0.6-SA, APC Ltd.) that was used to drive the six fiber scanners was measured using a laser vibrometer (OFV-2600/OFV-302, Polytec, Germany). The frequency response of the actuator is shown to be relatively flat across Hz, indicating that fiber scanner with d 5 shank diameter produced the highest mechanical resonance gain. Proc. of SPIE Vol

9 1 mm Figure 7. A representative fiber scanner from Table 1 shows a maximum lateral displacement is 1.0 mm at 35 degrees (full cone angle) of angular deflection. Enclosures of high-amplitude fiber scanners are expected to be no greater than 2.0 mm inner diameter for future catheter scope prototypes. DISCUSSION & CONCLUSIONS Preliminary results have been presented on optical beam scanning using a resonant microlensed fiber scanner. This unique form of optical scanning can be made at low cost into ultrathin designs (< 2 mm diameter) for microendoscopic applications. Theoretical optical designs were used to understand the interdependencies (tradeoffs) with scanning a microlensed optical fiber. The degree of collimation of the optical beam dramatically affects the field of view and axial length of the fiber scan system. Ideally, a collimated or slightly converging beam will be generated at the scanning fiber tip, which has an optical source located at the second vibratory node of the resonant scanner. Experimental results show that increasing the size and refractive index of the ball lens produces low divergence beams while the large mass reduces both the amplitude and frequency of vibration. The fabrication of a barrel or GRIN lenses fused onto the distal tip of a singlemode fiber is expected to decrease the lens mass significantly. By selectively etching only the shaft of the fiber scanner while protecting the microlens tip, the amplitude of vibration can be increased greatly. Further optomechanical design, analysis, and optimization will be required to achieve concurrently, the desired combination of wide field of view (> 50 degrees), scanning at video scan rates (>10 khz), while being contained in a 2 mm enclosure. This report shows that optical beam scanning from a microfabricated fiber scanner can produce a wide field of view from an ultrathin size at resonant scanning frequencies lower than video rates (1 to 4 khz). This lower scan frequency will reduce the image acquisition frame rate to less than 30 Hz allowing some distortion due to motion artifact. Applications range from peripheral bronchoscopy (image guide biopsy and cytology) to imaging early neoplasia in the gastrointestinal ducts. 54 Proc. of SPIE Vol. 4957

10 ACKNOWLEDGEMENT Financial support has been provided by the National Science Foundation (grant # ) and The Washington Technology Center, Seattle, WA. The CO 2 laser for micromachining the optical fibers was donated by Synrad Incorporated, Mukilteo, WA. The custom 3-layer acid etching chamber was manufactured by Jeff Magula. REFERENCES Fauver, M.E., Crossman-Bosworth, J.L., Seibel, E.J. (2002) Microfabrication of fiber optic scanners. Optical Scanning 2002, Proc. SPIE, Vol. 4773: Niederer, P., Hafliger, J., Blessing, P., Lehareinger, Y., Doswald, D., and Felber, N. (2001) Image quality of endoscopes. Proc. SPIE Biomonitoring and Endoscopy Technologies. 4158:1-10. Seibel, E.J. and Smithwick, Q.Y.J. (2002) Unique features of optical scanning, single fiber endoscopy. Lasers in Surgery & Medicine. 30: Seibel, E.J., Smithwick, Q.Y.J., Brown, C.M., and Reinhall, P.G. (2001) Single fiber flexible endoscope: general design for small size, high resolution, and wide field of view. Proc. SPIE Biomonitoring and Endoscopy Technologies. 4158: Seibel, E.J., Smithwick, Q.Y.J., Crossman-Bosworth, J.L., Myers, J.A. (2002) Prototype scanning fiber endoscope. Optical Fibers and Sensors for Medical Applications II, Proc. SPIE, Vol. 4616: Smithwick, Q.Y.J., Vagners, J., Reinhall, P.G., and Seibel, E.J. (2002) Modeling and control of the resonant fiber scanner of a novel scanning scope. Proceedings of the IEEE EMBS/BMES joint meeting, Houston, TX, October 23, 2002, vol. 2: Proc. of SPIE Vol