Optical coherence tomography (OCT) is a widely used optical imaging technology with an increasing number of applications [1]. It has particular importance in the medical field since it can provide non-invasive, sub-micrometer resolution diagnostic images. Current OCT systems contain fiber-based or free-space bulky components which make these instruments costly. A significant decrease in the size and cost of an OCT system is possible through integrated optics. With a suitable material technology and optimum design, one can fabricate extremely compact and low-cost OCT systems, especially suited for applications in which instrument size may play an important role. In the literature there is only limited data on the implementation of OCT systems on a chip [2-3]. In this work, we present the successful attempt toward miniaturization of a spectral-domain OCT (SD-OCT) system by designing an arrayed waveguide grating (AWG) spectrometer in silicon-oxynitride (SiON) for the 1300-nm spectral range. The AWG spectrometer, with its excellent performance and compactness, has a high potential for various spectroscopic applications. The operation principle of an AWG is explained in detail in Ref. [4]

Akça, B.I.

de Ridder, R.M.

Kalkman, J.

Nguyen, V.D.

Pollnau, Markus

van Leeuwen, Ton

Worhoff, Kerstin

We present experimental results of a spectral-domain optical coherence tomography system that includes an integrated spectrometer. A depth range of 1 mm and axial resolution of 22 μm was measured. A Scotch tape was imaged

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Toward miniaturized optical coherence tomography

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Toward Miniaturized Optical Coherence Tomography  B. I. Akca,1 V. D. Nguyen,2 J. Kalkman,2 T. G. van Leeuwen,2,3 K. Wörhoff, 1  R.M. de Ridder,1 and M. Pollnau1 1Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands 2University of Amsterdam, Academic Medical Center, Biomedical Engineering & Physics, Amsterdam, 1100 DE, The Netherlands 3University of Twente, Biomedical Technology Institute, Biophysical Engineering, Enschede, 7500 AE, The Netherlands e-mail: b.i.akca@ewi.utwente.nl 1. Introduction Optical coherence tomography (OCT) is a widely used optical imaging technology with an increasing number of applications [1]. It has particular importance in the medical field since it can provide non-invasive, sub-micrometer resolution diagnostic images. Current OCT systems contain fiber-based or free-space bulky components which make these instruments costly. A significant decrease in the size and cost of an OCT system is possible through integrated optics. With a suitable material technology and optimum design, one can fabricate extremely compact and low-cost OCT systems, especially suited for applications in which instrument size may play an important role. In the literature there is only limited data on the implementation of OCT systems on a chip [2-3].  In this work, we present the successful attempt toward miniaturization of a spectral-domain OCT (SD-OCT) system by designing an arrayed waveguide grating (AWG) spectrometer in silicon-oxynitride (SiON) for the 1300-nm spectral r nge. The AWG spectrometer, with its excellent performance and compactness, has a high potential for various spectroscopic applications. The operation principle of an AWG is explained in detail in Ref. [4].  2. Design The spectral range of the SD-OCT system near 1300 nm was specifically selected for skin imaging. For this device, we aimed at 18.5-µm depth resolution (determined by the full width at half maximum values of the transmission spectrum of the AWG) and 1-mm depth range (determined by the wavelength spacing per output waveguide). The free spectral range (FSR = 78 nm) and wavelength resolution (∆λ = 0.4 nm) of the AWG were determined to meet requirements. The remaining design parameters were calculated using the standard equations for AWGs [4]. Single-mode silicon oxynitride (SiON) channel waveguides with 2 µm width and 0.8 µm height were used for the AWG spectrometer. For the upper cladding 4-µm-thick silicon dioxide was used. The core and cladding refractive indices w re 1.55 and 1.4485 at 1.3 µm, respectively. The minimum bending radius of curved waveguides was calculated to be 500 µm. The minimum spacings between the arrayed waveguides and between the 195 output waveguides were optimized using the beam propagation method in order to reduce loss and crosstalk values.  3. Measurements and Results A schematic of the fiber-based SD-OCT system with AWG spectrometer is shown in Fig. 1a. The Michelson interferometer was illuminated using a superluminescent diode which had a partially-polarized Gaussian-like spectrum with a bandwidth of 40 nm and a central wavelength of 1300 nm. Via a circulator the light was coupled into a 90/10 beamsplitter. Polarization controllers were placed into both, sample and reference arm. The backreflected light was redirected through the optical circulator to the spectrometer. The diffraction grating was replaced with the integrated AWG. The collimated beam was imaged with a high numerical aperture camera lens onto a 46 kHz CCD linescan camera. The acquired spectra were processed by first subtracting the reference arm spectrum, then compensating the dispers on, and finally resampling to k-space. We achieved a depth range of 1mm, as shown in Fig.1b. The measured signal-to-noise ratio (SNR) was 75 dB. The axial resolution (FWHM) determined from a Gaussian fit to the point spread function in amplitude at various depths. A slight decrease in depth resolution was observed at higher depth ranges, which we attribute to misalignment and noise. As an example, an image of a Scotch tape was obtained with a depth range of 600 µm which is quite close to the theoretical value of 700 µm (the refractive index of Scotch tape is ~1.4-1.5). A better image could be obtained with a higher SNR by reducing the sources of noise in the set-up. MirrorMirrorSuper luminescentdiodeAWG SpectrometerDiffractiongratingCCD cameraXY scannerPolarizationcontrollersCirculatorReference arm10 %90 %0 200 400 600 800 10000102030 OCT signal (a.u.)Depth range (µm)0 500 1000 1500 2000 25002004006008001000Scanning range in X (µm)Depth range (µm)      Fig. 1. a) Optical measurement set-up of fiber-based Michelson interferometer with integrated arrayed waveguide grating (AWG) spectrometer. b) Point spread function of the 1300 nm-SD-OCT system at different depths. The maximum depth range is1 mm. c) Image of a Scotch tape.  4. Conclusions We have demonstrated the first partially integrated SD-OCT system by designing a SiON-based AWG spectrometer for the 1300-nm spectral region. The measurement results are in good agreement with the theoretical calculations. An imaging depth of 1 mm and an axial resolution of 22 µm (at 100 µm depth) were measured. An image of a Scotch tape was obtained with a SNR ratio of 75 dB. In future work, we will include an i tegrated Michelson interferometer as well as integrated focusing systems on the same chip with the AWG spectrometer.  This work is supported by the Dutch Senter-Novem MEMPHIS Project. 5. References [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, 1178-1181 (1991). [2] D. Culemann, A. Knuettel, and E. Voges, “Integrated optical sensor in glass for optical coherence tomography,” IEEE J. Select. Topics Quantum Electron. 5, 730-734 (2000). [3] D. Choi, H. Hiro-Oka, H. Furukawa, R. Yoshimura, M.Nakanishi, K. Shimizu, and K. Ohbayashi, “Fourier domain optical coherence tomography using optical demultiplexers imaging at 60,000,000 lines/s,” Opt. Lett. 33, 1318-1320 (2008). [4] M. K. Smit and C. van Dam, “PHASAR-based WDM-devices: Principles, design and applications,” IEEE J. Select. Topics Quantum Electron. 2, 236–250 (1996).  Keywords: Arrayed waveguide grating (AWG), optical coherence tomography (OCT) 

English

Toward Miniaturized Optical Coherence Tomography

Towards a Miniaturized Optical Coherence Tomography System B. I. Akca,1 V. D. Nguyen,2 J. Kalkman,2 T. G. van Leeuwen,2,3 K. Wörhoff,1  R.M. de Ridder,1 and M. Pollnau1 1Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands 2 Biomedical Engineering & Physics University of Amsterdam, Academic Medical Center, , Amsterdam, 1100 DE, The Netherlands 3 Biomedical Photonic Imaging, MIRA Institute for Biomedical Technology & Technical Medicine, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands e-mail: b.i.akca@ewi.utwente.nl  Abstract: We present experimental results of a spectral-domain optical coherence tomography system that includes an integrated spectrometer. A depth range of 1 mm and axial resolution of 22 µm was measured. A Scotch tape was imaged. OCIS codes: (170.4500) Optical coherence tomography; (230.3120) Integrated optics devices  1. Introduction Optical coherence tomography (OCT) is a widely used optical imaging technology  [1]. It has particular importance in the medical field since it can provide non-invasive, micrometer resolution diagnostic images of biological tissues. Current OCT systems contain fiber-based or free-space bulky optical components which make these instruments costly. A significant decrease in the size and cost of an OCT system is possible through the use of integrated optics. With a suitable material technology and optimum design, one can fabricate extremely compact and low-cost OCT systems, especially suited for applications in which instrument size may play an important role. In the literature there is only limited data on the implementation of OCT systems on a chip [2-5].  The AWG spectrometer, with its excellent performance and compactness, has a high potential for various spectroscopic applications. In this work, we present the first successful step towards miniaturization of a spectral-domain OCT (SD-OCT) system by designing an arrayed waveguide grating (AWG) spectrometer in silicon-oxynitride (SiON) for the 1300-nm spectral range. 2. Design The spectral range of the SD-OCT system was specifically selected at 1300 nm center wavelength for skin imaging. For this device, we aimed at 21-µm depth resolution (determined by the full width at half maximum values of the transmission spectrum of the AWG) and 1-mm depth range (determined by the wavelength spacing per output waveguide). These requirement necessitate a large free spectral range (FSR = 78 nm) and high wavelength resolution (∆λ = 0.4 nm) of the AWG. The operation principle of an AWG is explained in detail in Ref. [6]. The remaining design parameters [7] were calculated using the standard equations for AWGs [6]. Single-mode silicon oxynitride (SiON) channel waveguides with 2 µm width and 0.8 µm height were used for the AWG spectrometer. For the upper cladding 4-µm-thick silicon dioxide was used. The core and cladding refractive indices were 1.55 and 1.4485 at 1.3 µm, respectively. The minimum bending radius of curved waveguides was calculated to be 500 µm. The minimum spacings between the arrayed waveguides and between the 195 output waveguides were optimized using the beam propagation method in order to reduce loss and crosstalk values.  3. Measurements and Results A schematic of the fiber-based SD-OCT system with AWG spectrometer is shown in Fig. 1a. The Michelson interferometer was illuminated using a superluminescent diode which had a partially-polarized Gaussian-like spectrum with a bandwidth of 40 nm and a central wavelength of 1300 nm. Via a circulator the light was coupled into a 90/10 beamsplitter. Polarization controllers were placed into both, sample and reference arm. The backreflected light was redirected through the optical circulator to the spectrometer. The diffraction grating, which is commonly used in this SD-OCT system [8] was replaced with the integrated AWG. The collimated beam was imaged with a high numerical aperture camera lens onto a 46 kHz CCD linescan camera. The acquired spectra were processed by first subtracting the reference arm spectrum, then compensating the dispersion, and finally resampling to k-space. We achieved a OCT imaging up until the maximum depth range of 1 mm, as shown in Fig.1b. The measured signal-to-noise ratio (SNR) was 70 dB at 100 micrometer depth. Figure 2a shows the axial resolution (FWHM) determined from a Gaussian fit to the point spread function in amplitude at various depths. A slight decrease in OSA/ CLEO 2011       CWB3.pdf  depth resolution was observed at larger depths, which we attribute to misalignment and noise. As an example of tomographic imaging, an image of a Scotch tape was obtained by scanning the sample arm beam over the sample. We can observe the layered structure up to the maximum imaging depth. MirrorMirrorSuper luminescentdiodeAWG SpectrometerDiffractiongratingCCD cameraXY scannerPolarizationcontrollersCirculatorReference arm10 %90 % 0 200 400 600 800 10000102030  OCT signal (a.u.)Depth range (µm)  Fig. 1. a) Optical measurement set-up of fiber-based Michelson interferometer with integrated arrayed waveguide grating (AWG) spectrometer. b) Point spread function of the 1300 nm-SD-OCT system at different depths. The maximum depth range is1 mm.  0 200 400 600 800 10001820222426 FWHM (µm)Depth (µm) 0 500 1000 1500 2000 25002004006008001000Scanning range in X (µm)Depth range (µm) Fig. 2. a) Axial resolution (FWHM) values of the 1300-nm SD-OCT system for all depth ranges. b) Image of a Scotch tape.  4. Conclusions We have demonstrated the first partially integrated SD-OCT system by designing a SiON-based AWG spectrometer for the 1300-nm spectral region. The measurement results are in good agreement with the theoretical calculations. An imaging depth of 1 mm and an axial resolution of 22 µm (at 100 µm depth) were measured. An image of a Scotch tape was obtained with a SNR ratio of 70 dB. In future work, we will include an integrated Michelson interferometer as well as integrated focusing systems on the same chip with the AWG spectrometer.  This work was supported by the Smart Mix Program of the Netherlands Ministry of Economic Affairs and the Netherlands Ministry of Education, Culture and Science. 5. References [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, 1178-1181 (1991). [2] D. Culemann, A. Knuettel, and E. Voges , “Integrated optical sensor in glass for optical coherence tomography,” IEEE J. Select. Topics Quantum Electron. 5, 730-734 (2000). [3] E. Margallo-Balbas, M. Geljon, G. Pandraud, and P. J. French, “Miniature 10 kHz thermo-optic delay line in silicon,” Opt. Lett. 35, 4027-4029 (2010). [4] G. Yurtsever and R. Baets, “Towards integrated optical coherence tomography system on silicon on insulator,” Proceedings of the Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XIV, Proc. SPIE 7554, 1605-7422 (2010). [5] D. V. Nguyen, N. Ismail, F. Sun, K. Wörhoff, T. G. van Leeuwen, and J. Kalkman, "SiON integrated optics ellipti ccouplers for Fizeau-based optical coherence tomography," J. of Lightw. Technol.  28, 2836 (2010). [6] M. K. Smit and C. van Dam, “PHASAR-based WDM-devices: Principles, design and applications,” IEEE J. Select. Topics Quantum Electron. 2, 236–250 (1996). [7] B. I. Akca, V. D. Nguyen, J. Kalkman, T. G. van Leeuwen, K. Wörhoff, R.de Ridder, and M. Pollnau, “Towards on-chip optical coherence tomography,” submitted. [8] J. Kalkman, A. V. Bykov, D. J. Faber, and T. G. van Leeuwen "Multiple and dependent scattering effects in Doppler optical coherence tomography"; Opt. Exp. 18, 3883 (2010).  OSA/ CLEO 2011       CWB3.pdf  

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