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In this work, a novel beam scanner design based on non-moving parts is introduced which will eliminate the phase and inaccuracy problems of the mechanical scanners while providing two times imaging speed improvement for optical coherence tomography systems. The design is comprised of electro-optically activated switches that are placed on the sample arm. For the example considered here, lateral resolution of 20 µm, and lateral scanning range of 1 mm are aimed at which resulted in a scanner size of 1 mm × 9 mm. Due to its compact size, proposed design can also be implemented in forward-looking endoscopic probes.
© 2016 Optical Society of America
1. Introduction
Optical coherence tomography (OCT) is a non-invasive optical technique for high-resolution three-dimensional imaging of biological tissues [1]. Current state-of-the-art OCT systems operate in the Fourier-domain, using either a broad-band light source and a spectrometer, known as “spectral-domain OCT (SD-OCT)”, or a rapidly tunable laser, known as “swept-source OCT (SS-OCT)”. Accurate image formation in OCT is obtained by synchronizing A-scan acquisition with the movement of the focused beam in the sample arm. Current OCT scanning mechanisms in both configurations are based on precise lateral scanning of the incident light beam using mechanical scanners such as galvanometer actuated mirrors. These mechanical scanners cause some major problems such as image distortion, phase errors, beam jitter, and inaccuracies due to non-uniform scan pattern, etc. Additionally, there is a trade-off between scanning speed and the imaging range of these mechanical scanners.
In the literature, parallel OCT imaging using multi-beams to illuminate the sample simultaneously is proposed with the aim of eliminating the mechanical scanning as well as the speed improvement [2–10]. This technique allows simultaneous imaging from multiple sample locations and therefore improves OCT axial scan rate by a factor equal to the number of beams used simultaneously. Lee et al. has introduced interleaved OCT idea in which the full spectral width of the source is divided into several sets of unique spectrally interleaved wavelength components [6]. Each point is illuminated by roughly the full spectral width of the source. However, this system uses virtually imaged phased arrays that lead to around ~10 dB extra losses in the sample arm. Spectral encoded endoscopy introduced by Tearney et al. spectrally spreads the light on to a sample, however it suffers from very poor axial resolution [7,8]. The existing multi-beam OCT systems have complicated system architecture with multiple interferometers which have prohibited wide utilization of this approach to date. Moreover, in this approach the high-speed data acquisition and high throughput data transfer became a roadblock for further investigation of the idea.
In this work, a novel beam scanner design based on integrated optics is presented in order to eliminate the mechanical scanning and improve the imaging speed of the OCT systems. The design is comprised of electro-optically activated wavelength-independent full couplers that act as voltage-controlled switches. By sequentially activating these switches, the propagating light is steered from one imaging location towards the next one within a few nanoseconds. The end of each arm is divided into two branches with a certain optical length difference between them for simultaneous imaging of two different points on the sample. The number of branches can be increased further in accordance with the desired speed improvement. This scanner design can be applied to other imaging techniques as well. Due to its very compact size, it can also be implemented in a forward-looking endoscopic probe which is in great demand in numerous medical applications.
2. Beam scanner design
2.1 Material system
The proposed sample arm configuration was simulated for the lithium niobate (LN)-on-silicon waveguide platform as it is being one of the most versatile and well-developed active optical materials [9]. The material system is 300-nm-thick ion-sliced lithium niobate film on oxidized silicon wafer. The oxide thickness is 3 μm. The refractive index of the LN layer is 2.22 at 1300 nm, and its electro-optic (EO) coefficient is (r33 ~30 pm/V) [9]. Single mode rib waveguides with 0.2 µm of slab height and 1 µm of waveguide width were designed. The effective refractive index of the rib waveguide was calculated to be1.85 by using beam propagation method (BPM) simulations. The three-dimensional illustration of the waveguide structure as well as the optical mode profile are given in Fig. 1. The minimum bending radius of the curved waveguides was calculated to be R = 100 µm with a bending loss of 0.01dB/cm. The propagation loss of the LN waveguides defined by the ion-implantation-assisted wet etching is around 0.23 dB/cm. Metallic electrodes can be defined using gold or chromium. A 500-nm-thick silicon dioxide (SiO2) top cladding will be used to prevent propagation losses induced by the electrodes. The fiber-to-chip coupling losses (~6 dB) can be reduced to < 0.5 dB by using a high numerical aperture fiber [11].
Fig. 1 Three-dimensional view of the waveguide stack with relevant design parameters b) Beam propagation method simulation of the optical mode. The blue outline shows the cross-sectional profile of the waveguide geometry.
2.2 Working principle of the electro-optic switch
One of the main components of the proposed scanner design is an electro-optic switch which is a wavelength-insensitive Mach-Zehnder-type interferometric coupler as illustrated in Fig. 2 [12,13].
Fig. 2 a) The schematic of the wavelength-insensitive electro-optic switch. An electrode is placed on top of the right arm of the coupler which is drawn in gray. b) Beam propagation method simulation of the switch; (Left) No phase difference between coupler arms, the input light will stay in the same arm (bar state). (Right) For a π phase difference between coupler arms, the input light will cross-couple to the other arm (cross state). L, D, Δx are the lengths of the straight sections of the directional couplers, and the delay section, and the separation between coupler arms, respectively.
It is comprised of a pair of directional couplers connected by a delay section in which a phase shift is introduced. The second directional coupler cancels deviations introduced by the former, if these deviations are similar in both couplers. An electrode is placed on the right arm of the electro-optic switch as shown in gray in Fig. 2 (a). When the voltage is off, the lights on both arms will be in phase and the input light will stay in the same arm, i.e. bar state as illustrated in the left part of Fig. 2 (b). With the applied voltage, the effective refractive index of that arm is locally increased due to the electro-optic effect which induces a phase difference between two arms. At a certain voltage value corresponding to a π phase difference between arms, the sample beam cross-couples to the other arm, i.e. cross state (Fig. 2 (c) right part). It is also possible to achieve switching operation using pressure-induced refractive index change by choosing the material technology accordingly.
2.3 Scanner layout and its working principle
The non-moving scanner design is comprised of two main components; namely electro-optic switches and two-mode interference based beam splitters/combiners. The design of each component is presented in more detail in the following subsections. Figure 3 (a) is the schematic of the OCT system with the non-moving beam scanner design. For simplicity, only 4 arms of the scanner are shown in the figure. Input light is divided into two arms with an integrated 3-dB beam splitter; half of it towards the reference arm which is integrated on the same chip for further size reduction, the other half towards the sample arms. Each sample arm consists of an electro-optic switch (Fig. 3 (b)) for beam steering and a beam splitter/combiner (Fig. 3 (c)). The electro-optic switch changes the propagation direction of the sample beam from bar state to cross state by an applied voltage corresponding to π phase difference between coupler arms. By activating each switch sequentially, the sample beam can be steered from one imaging location to the next until whole imaging range is scanned.
Fig. 3 a) Schematic of the OCT system with the proposed non-moving beam scanner design. Light coming from the input waveguide is divided into two; half towards on-chip reference arm, half towards the sample arm where several electro-optic switches are placed. The end of each sample arm is divided into two branches with a constant length difference, i.e. ΔL, between each for simultaneous imaging. b) Schematic of the electro-optic switch. c) Schematic of the two-mode interference based beam splitter/combiner. The splitting ratio is 50/50. d) Due to ΔL, signals from two different physical locations on the sample will be detected at two different depth locations which are separated by 2ΔL.
In order to double the imaging speed, each sample arm is divided into two branches with a certain length difference, i.e. ΔL, between them. In this way, two physical locations on the sample can be simultaneously illuminated. When images are formed, signals from two different locations will be detected at two different depths separated by 2ΔL as depicted in Fig. 3 (d). The number of branches can be increased further in accordance with the desired speed improvement. The separation between each branch is chosen to be d = 20 µm. For a scanning range of 1 mm, 24 electro-optic switches are used which results in a scanner size of around 1mm × 9 mm (9 mm2). Using the central part of a focusing lens, light can be successfully delivered into the tissue and collected back through same path. Returned signal from different sample locations are combined at the 3-dB coupler and interfered with light from the reference arm. A single detector and a high-speed data acquisition card is utilized to record interference signal from all beams simultaneously.
The switching time of an electro-optic coupler is only few nanoseconds, (< 10 ns), therefore scanning of a 1 mm wide area on the sample would take approximately 200 nanoseconds. However, in order to avoid data acquisition related problems and increase the integration time for higher signal to noise ratio, it is necessary to apply some time delay between each imaging point. Even for a long time delay, e.g. 1millisecond, a reasonably high scanning speed, i.e. ~1 kHz, can be achieved.
2.4 Electro-optic switch design
The design of the electro-optic switch was made in two steps. Firstly, lengths of the straight coupling sections of the directional coupler and the delay part were calculated to achieve full coupling using equations given in [12] as L = 95 µm and Δx = 0.28µm, respectively. The separation between coupler arms was chosen as D = 1 µm in order to reduce the overall device length to 0.65 mm. Secondly, the coupler designed in the first step was used to simulate the required mode effective refractive index increment for a π phase difference by scanning the refractive index difference (Δn) between coupler arms from 0 to 5 × 10−3 with 10−4 step size (Fig. 4 (a)). It was found to be Δn = 2 × 10−3. The required voltage value to induces such index difference was calculated to be 21 Volts by using below equation [14]
for an overlap factor of Г = 0.3, electro-optically active layer thickness of t = 0.3 μm, effective refractive index of ne = 1.85, and electro-optic coefficient of r33 = 30 pm/V. The coupling loss was simulated to be 0.04 dB. The simulated splitting ratio between two arms stays constant over 100 nm bandwidth, as shown in Fig. 4 (b). The coupling ratio remains the same even after a certain voltage applied on one arm of the coupler (Fig. 4 (b), bottom).Fig. 4 Simulation results of the electro-optic switch a) Refractive index difference between coupler arms versus power on the same arm (I2). For Δn = 2 × 10−3, 99% of the input light is cross-coupled to the other arm. b) The coupler is wavelength independent for a wavelength range of 100 nm, and its wavelength independency does not change after the voltage is turned on.
2.5 Tolerance analysis of the electro-optic switch
The change in coupling ratio of the electro-optic switch due to the process non-uniformity and limitations in reproducibility has been investigated. The refractive index of the cladding layer can have non-uniformities of up to ± 3 × 10−4, and the core layer can show thickness variations up to ± 1% over the wafer. The waveguide width can vary by ± 0.1 μm.
The simulation results of the effects of these process-dependent deviations are summarized in Table 1. The wavelength-independent couplers used in electro-switches are relatively fabrication tolerant devices as indicated in Table 1. Variations in the refractive index of the cladding layer has the minimum effect on coupling ratio whereas the maximum variation in coupling ratio was calculated to be 0.2% for ± 1% change in core thickness which is still insignificant.
Table 1. Effect of the Technological Tolerances on Electro-optic Switch Performance
2.6 Two-mode interference beam splitter/combiner design
The beam splitter/combiner used in the end of each arm is based on two-mode interference (TMI). It is wavelength independent and compared to an optical Y junction it is more fabrication tolerant, and reproducible. Figure 5 (b) and Fig. 5 (c) demonstrate the beam propagation simulation results of the TMI-based beam splitter and combiner, respectively. The separation between input waveguides, the width and the length of the slab region are h = 0.8 μm, w = 3 μm, and l = 9 μm, respectively. The splitting ratio is constant over 200 nm wavelength range as shown in Fig. 5 (d). The overall loss of the beam combiner/splitter was simulated to be 0.18 dB.
Fig. 5 a) Schematic of the TMI-based beam splitter (b) and combiner (c). The loss of the splitter and combiner was simulated to be 0.18 dB. d) The splitting ratio remains constant over 200 nm bandwidth range.
3. Discussions
The scanner design presented in this work can be applied to any imaging technique or other measurement methods in which speed improvement or a stable and accurate beam scanning method is desired. For example, confocal microscopy can benefit significantly from non-moving scanning approach. Additionally, real time OCT imaging of blood cells through blood flow can become feasible with the speed improvement of the proposed idea.
Due to its small size, the proposed scanner design can also be implemented in forward-looking endoscopic probes. The current probe designs are comprised of small-scale optical components which limits their minimum achievable device size and requires rather sophisticated engineering, especially for 3-D measurement. Moreover, the scanner units within these probes are quite expensive. Given these characteristics, the developed probe would be ideal for high-speed metrology applications, for instance monitoring the depth of a laser ablation crater or determining the distance to critical structures during laser or conventional surgery. Additionally, by combining this probe with vibrography [15, 16], middle ear disorders can be investigated in vivo. The proposed scanner size increases linearly with the imaging range. Therefore, for a reasonable imaging range, e.g. 2.5 mm, the scanner width will become 2.5 mm which is still small enough to be used in an endoscopic probe.
The proposed on-chip OCT system using non-moving scanner design has several advantageous compared to bulky SD-OCT and SS-OCT systems. Firstly, it will be a more compact, cost-efficient, and rugged system. Secondly, the non-moving scanner is a stand-alone device that can be implemented in commercial OCT systems as well. It can eliminate the aforementioned mechanical scanner related problems. Additionally, the speed and scanning range trade-off of galvanometer scanners will be overcome with this scanner as its imaging range and scanning speed are decoupled. Thirdly, the imaging speed of the system will be two times faster which can be increased further by using more branches in the sample arms.
The overall loss of the proposed scanner is estimated to be around 2 dB including ~0.5 dB of propagation loss, 0.5 dB of coupling loss by using a high numerical aperture input fiber [14], 0.04 dB/electro-optic switch loss for each pass (0.96 dB for the last channel), and 0.018 dB/beam combiner loss (0.036 dB for double pass).
4. Conclusions
In summary, a novel OCT beam scanner design was presented for eliminating the phase and inaccuracy problems of the mechanical scanners while providing two times imaging speed improvement with the multi-beam illumination. The proposed design can be applied to other imaging modalities as well as other measurement techniques. Different switching mechanisms (e.g. pressure-based) can be applied in different material platforms, depending on the power consumption, and switching speed requirements. Due to the lack of cleanroom facilities at the current institute, the experimental performance of the proposed scanner design could not be demonstrated in this work. However, it is expected that the layout described in here will evoke some experimental interest and will be followed up by several research groups and companies.
Funding
Technology Foundation STW, Innovational Research Incentives Scheme Veni (SH302031).
Acknowledgments
The author thanks Prof. Ton van Leeuwen for the fruitful discussions.
References and links
1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]
2. L. An, P. Li, T. T. Shen, and R. Wang, “High speed spectral domain optical coherence tomography for retinal imaging at 500,000 A‑lines per second,” Biomed. Opt. Express 2(10), 2770–2783 (2011). [CrossRef] [PubMed]
3. T. Schmoll and R. A. Leitgeb, “Heart-beat-phase-coherent Doppler optical coherence tomography for measuring pulsatile ocular blood flow,” J. Biophotonics 6(3), 275–282 (2013). [CrossRef] [PubMed]
4. B. Potsaid, B. Baumann, D. Huang, S. Barry, A. E. Cable, J. S. Schuman, J. S. Duker, and J. G. Fujimoto, “Ultrahigh speed 1050nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second,” Opt. Express 18(19), 20029–20048 (2010). [CrossRef] [PubMed]
5. S. Zotter, M. Pircher, T. Torzicky, M. Bonesi, E. Götzinger, R. A. Leitgeb, and C. K. Hitzenberger, “Visualization of microvasculature by dual-beam phase-resolved Doppler optical coherence tomography,” Opt. Express 19(2), 1217–1227 (2011). [CrossRef] [PubMed]
6. H. Y. Lee, H. Sudkamp, T. Marvdashti, and A. K. Ellerbee, “Interleaved optical coherence tomography,” Opt. Express 21(22), 26542–26556 (2013). [CrossRef] [PubMed]
7. G. J. Tearney, R. H. Webb, and B. E. Bouma, “Spectrally encoded confocal microscopy,” Opt. Lett. 23(15), 1152–1154 (1998). [CrossRef] [PubMed]
8. C. Boudoux, S. Yun, W. Oh, W. White, N. Iftimia, M. Shishkov, B. Bouma, and G. Tearney, “Rapid wavelength-swept spectrally encoded confocal microscopy,” Opt. Express 13(20), 8214–8221 (2005). [CrossRef] [PubMed]
9. E. L. Wooten, K. M. Kissa, A. Yi-Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. Maack, D. V. Attanasio, D. J. Fritz, G. J. McBrien, and D. E. Bossi, “A review of lithium niobate modulators for fiber-optic communications systems,” IEEE J. Sel. Top. Quantum Electron. 6(1), 69–82 (2000). [CrossRef]
10. P. DeNicola, S. Sugliani, G. B. Montanari, A. Menin, P. Vergani, A. Meroni, M. Astolfi, M. Borsetto, G. Consonni, R. Longone, A. Nubile, M. Chiarini, M. Bianconi, and G. G. Bentini, “Fabrication of smooth ridge optical waveguides in by ion implantation-assisted wet etching,” J. Lightwave Technol. 31(9), 1482–1487 (2013). [CrossRef]
11. O. D. Herrera, K.-J. Kim, R. Voorakaranam, R. Himmelhuber, S. Wang, V. Demir, Q. Zhan, L. Li, R. A. Norwood, R. L. Nelson, J. Luo, A. K.-Y. Jen, and N. Peyghambarian, “Silica/electro-optic polymer optical modulator with integrated antenna for microwave receiving,” J. Lightwave Technol. 32(20), 3861–3867 (2014). [CrossRef]
12. B. E. Little and T. Murphy, “Design rules for maximally flat wavelength-insensitive optical power dividers using Mach-Zehnder structures,” IEEE Photonics Technol. Lett. 9(12), 1607–1609 (1997). [CrossRef]
13. B. I. Akca, C. R. Doerr, G. Sengo, K. Wörhoff, M. Pollnau, and R. M. de Ridder, “Broad-spectral-range synchronized flat-top arrayed-waveguide grating applied in a 225-channel cascaded spectrometer,” Opt. Express 20(16), 18313–18318 (2012). [CrossRef] [PubMed]
14. I. B. Akca, A. Dana, A. Aydinli, M. Rossetti, L. Li, A. Fiore, and N. Dagli, “Electro-optic and electro-absorption characterization of InAs quantum dot waveguides,” Opt. Express 16(5), 3439–3444 (2008). [CrossRef] [PubMed]
15. S. Kling, I. B. Akca, E. W. Chang, G. Scarcelli, N. Bekesi, S. H. Yun, and S. Marcos, “Numerical model of optical coherence tomographic vibrography imaging to estimate corneal biomechanical properties,” J. R. Soc. Interface 11(101), 20140920 (2014). [CrossRef] [PubMed]
16. B. I. Akca, E. W. Chang, S. Kling, A. Ramier, G. Scarcelli, S. Marcos, and S. H. Yun, “Observation of sound-induced corneal vibrational modes by optical coherence tomography,” Biomed. Opt. Express 6(9), 3313–3319 (2015). [CrossRef] [PubMed]
