Compact all-fiber optical coherence tomography (OCT) probes allow insertion into small and tight areas of the body, enabling catheter-based endoscopic imaging. There have been many studies using OCT endoscopic systems for clinical applications, including vascular, digestive, respirator, and reproductive systems.1 Fiber-based endoscopic OCT probes have been developed for ophthalmic surgery and advanced microsurgical tools.2–8 An example includes hand-held automatic OCT-distal sensor-guided ophthalmic surgical devices that can compensate physiological tremors during surgery.3–4,6 With such systems, OCT-distal sensors should be disposable with a long sensing range. It has been shown that common-path (CP) swept-source OCT (SSOCT) allows the use of a disposable probe of an arbitrary length. It also can be made to be compact, simple, free from dispersion and polarization mismatches, and tolerable to system vibrations.
However, bare fiber probes, commonly used as a sensing probe for CP-OCT, have two main drawbacks: strong divergence of the sample beam and dependence of OCT sensitivity on the refractive index of the working environment. To overcome the strong divergence of bare fiber probes, various lensed probe systems have been proposed and demonstrated. These include fused coreless fiber,9,10 ball lens,11–14 GRIN lens,15 and conical-frustum tip.16 While most of them work well for standard OCT configurations, they do not work well for CP setups, especially in vitreous media (i.e., water). This is mainly caused by the inability to readily fabricate the reference surface with an optimized reflectivity. Those designed for CP-OCTs10,12,15,16 used lens/air interfaces as the reference, and their performance could degrade significantly in water because the lens effectiveness and/or the reference power level are significantly reduced.
For this paper, we designed and fabricated elliptical high-refractive-index epoxy-lensed fiber probes that exhibit long working distances and large depths of focus (DOF) in vitreous media (i.e., water) compared to standard bare or half-ball-lensed fiber probes. The refractive index difference between the high-refractive-index epoxy and fiber core keeps the reference beam power independent of the working environment. Additionally, the availability of a wide range of epoxies with different indices allows fine-tuning of the reference reflectivity by mixing them in proportion. To validate our approach, we fabricated multiple probes with a range of beam expansion lengths and lens curvatures. We then measured the signal-to-noise ratio (SNR) of the lensed OCT probes as a function of the target depth and compared them with theoretically calculated SNR. The endoscopic OCT-distal sensors were then tested for OCT B-mode imaging using a phantom target comprising several layers of tape to confirm the improved performance of the proposed lensed fiber probes.
Design of Lensed Fiber Probe
A key performance metrics of OCT-distal sensors is long sensing depth, and the region of DOF outside the probe is considered the effective sensing range. The working distance, DOF, and beam waist depend on the beam expansion length and the lens curvature, which can be calculated with the ABCD matrix method.17 For the lensed fiber, ABCD can be calculated as18
Figure 1 shows the calculated working distance, beam waist diameter, DOF, and effective sensing range as a function of beam expansion length and lens curvature. The working distance is a distance from lens surface to the beam waist, where a plus/minus sign indicates which side of the lens surface the beam waist lies. It is positive when the beam waist lies outside the probe, and, conversely, it is negative when the beam waist is virtual and occurs inside the fiber probe. To obtain effective sensing range longer than 500, 1000, 1500, or , beam expansion length must be longer than 157, 228, 282, and , respectively. Lens curvature must be around 1/5 of the expansion length. Because the fiber has a cladding, it is easier to make a lens of curvature. Therefore, we set the lens curvature to and targeted expansion region length around , because it provides the longest effective sensing range (1.6 mm) for the curvature. The beam waist diameter of the targeted lensed fiber was expected to be in water. The red stars indicate the geometry of fabricated lenses.
Fabrication of Lensed Fiber and Lens Geometry
Fabrication of Lensed Fiber
The lensed fiber OCT sensor was fabricated using UV curable epoxy (Norland Optical Adhesive, ). First, a semispherical lensed probe was fabricated by applying the epoxy to the fiber end, which naturally forms a semispherical shaped lens, owing to surface tension. Two fibers having semispherical epoxy ends were placed inline and brought together so that the epoxy lenses were in contact. We controlled axial and transverse positions of the fibers using linear stages while monitoring them using digital microscopes and our CP-SSOCT system to ensure they were perfectly aligned. A beam expansion rod was fabricated by curing the combined epoxy using ultraviolet light and breaking connection with one of the fibers. The length of the expansion rod was controlled by changing the distance between fiber ends. The fabricated expansion rod length was limited to around because of the limited amount of epoxy that could be applied to bare fibers. A beam expansion rod of was fabricated by applying more epoxy on the expansion rod via repeating the process. The end of the expansion rod was capped by applying a semispherical epoxy lens. This process can be highly reproducible when the direction and position of the fiber are controlled precisely. Another factor in the reproducibility is the amount of the epoxy picked up by the fiber. This is also related to the precise control of the fiber. An epoxy lens can be quickly distorted when even a small amount of epoxy touches the sidewall of the fiber or the expansion rod. However, once epoxy forms a spherical shape at fiber end and expansion rod end, the shape of the lens is highly repeatable and durable. Figure 2 schematically shows the fabrication steps from bare fiber to the designed elliptical-lensed fiber. For comparison, we also constructed lensed fibers having different expansion region lengths and lens curvatures. Lens 1, lens 2, and lens 3 were fabricated by applying and curing spherical epoxy on bare fiber, beam expansion rod, and beam expansion rod, respectively.
Figure 3 shows en-face OCT images of the fabricated lensed fiber probe with lens 1, lens 2, lens 3, and a expansion rod. The curvature of the spherical surface was measured using OCT images with customized Matlab code, which uses least squares to fit a sphere on a manually selected edge. The length of beam expansion region was measured by our OCT system. Beam expansion lengths and curvatures of fabricated lenses are listed in Table 1. Lens 3 is the one closest to our design. Only lens 3 has positive working distance because it has a sufficient beam expansion area that allows the focusing of the beam. Lens 1 and lens 2 simply reduce beam divergence.
Fabricated lens geometry and theoretically calculated working distance, beam waist diameter, and effective sensing range.
|Lens length (μm)||Surface radius (μm)||Working distance (μm)||Beam waist diameter (μm)||Effective sensing range (μm)|
We measured and verified the output-beam characteristics of the fabricated probes by comparing the beam spot diameter measured from knife-edge method19 to simulated beam diameter via the ABCD method. Figures 4(a) and 4(b) show the erf function result using the knife-edge measurement and resultant beam shapes of bare fiber and each lensed fiber at 1-mm distance. We performed knife-edge measurements of each lensed fiber at various distances. Figure 4(c) shows the measured and calculated beam diameter in air. The mode-field diameter was , and the lens geometry listed in Table 1 was used for calculation. Calculated and measured beam diameters match reasonably well throughout the distance ranges, confirming the measured geometry of our lens and calculation. Although it is difficult to measure beam diameter in water, we can use the calculated values shown in Fig. 4(d).
Performance of the Lensed Fiber Probes
The SNR versus depth was measured to show improved performance of the lensed fiber probes. Theoretical SNR was also calculated using the following equation to confirm the measured SNR:20 Sample power at each depth was estimated by returned sample beam diameter at the fiber end because the peak intensity of a Gaussian beam is inversely proportional to the square of the beam diameter. Coupling loss was considered when and were calculated. For reflected sample beam diameters, ABCD can be calculated as
A mirror was used as a sample, and the power at the fiber end was set to 0.15 mW to avoid saturation. Noise was limited by detector noise because of the low power. Detector noise has frequency dependence.20 Thus, we estimated NEP from the measurement of detector noise (i.e., A-scan image without an input signal) assuming NEP at 1 mm is .
Figures 5(a) and 5(b) show the SNR of each lensed fiber versus its depth in both air and water, respectively. Bare fiber showed relatively high SNR in air, despite its large divergence caused by higher reference power. However, it degraded significantly in water. Lens 3 showed the highest SNR in water. The 10-dB range of the bare fiber was , and it increased to with lens 3. The measured and calculated SNR fits reasonably well throughout the distance range, except for lens 3 in water. The difference is probably caused by the large mode mismatching caused by the tight beam focus.
Optical Coherence Tomography Imaging Performance
To confirm the improved SNR and resolution, B-scan images of a phantom target comprising several layers of tape were obtained at various distances using the lensed fibers. The fiber probe was attached to a motorized translation stage (Thorlabs MT1-Z8), and the probe was scanned horizontally across the phantom. Figures 6 and 7 show the OCT images working in water and air, respectively. The OCT images showed similar trends as with the previous beam diameters and SNR measurements. In water, the designed lensed fiber showed the best image quality at all distances. All probes showed similar image quality at distance except bare fiber, which has a lower SNR because of low reflection at the fiber end. However, the designed lensed fiber showed better quality than other lensed fibers when the probe was further away from the sample because of its long working distance and DOF. The quality improvement from the designed lensed fiber can be clearly seen at the distance. In air, fiber probes without a lens and with lens 2 showed better quality, as expected. A fiber probe with lens 3, optimized for working in wet environments, showed the worst image quality because of strong divergence and smaller reference than bare fiber.
We have demonstrated that CP-SSOCT imaging and sensing in water can be enhanced using a high refractive index elliptical epoxy-lensed fiber probe. We designed lensed fibers having long working distances and DOFs in water to obtain sufficiently long effective distal sensing ranges. Enhanced performance was confirmed by SNR and B-scan OCT images. The SNR of the proposed fiber probe, compared to a bare fiber at a distance of 1.5 mm, was increased by 25 dB and the resolution improved from 476 to . We can conclude that the improved performance in water indicates that such high refractive index epoxy-lensed fibers can work effectively in ophthalmic and vascular applications.
Soohyun Lee is a PhD student in the Electrical and Computer Engineering Department, Johns Hopkins University. She received her BS and MS degrees in electrical engineering from Korea Advanced Institute of Science and Technology in 2010 and 2012, respectively. She worked as a research engineer at the Electronics and Telecommunications Research Institute from 2012 to 2016.
Changho Lee is currently an assistant professor at Chonnam National University (CNU), Republic of Korea, since 2017. He received his PhD from Kyungpook National University in 2013. Before joining the CNU, he worked as an assistant research professor and a postdoctoral fellow at Pohang University of Science and Technology from 2013 to 2016 and Johns Hopkins University from 2016 to 2017.
Ruben Verkade is a graduate student in mechanical engineering from Eindhoven University of Technology (TU/e). He received his BSc degree in mechanical engineering from TU/e in 2016. As part of his master’s, he did his internship under Dr. Kang in the summer of 2018, where he researched optical coherence tomography for retinal detection. Currently, he is pursuing his graduation project at DEMCON Macawi Respiratory Systems.
Gyeong Woo Cheon received his BS degree in electrical engineering and his MS degree in biomedical engineering from Seoul National University in 2008 and in 2010, respectively. He worked at Korea Electrotechnology Research Institute as a researcher for two years and he received his PhD in electrical and computer engineering from Johns Hopkins University in 2016. Currently, he is a lead advanced computing engineer with General Electric Global Research.
Jin U. Kang is a Jacob Suter Jammer professor of electrical and computer engineering. He holds a joint appointment in the Department of Dermatology at the Johns Hopkins University School of Medicine and is a member of Johns Hopkins’ Kavli Neuroscience Discovery Institute and Laboratory for Computational Sensing and Robotics. He is a fellow of the Optical Society of America, the International Optics Society, and the American Institute for Medical and Biological Engineering.