Time and Spectral domain all-fiber Optical Coherence Tomography systems with variable dispersion compensators

Transcription

1 Time and Spectral domain all-fiber Optical Coherence Tomography systems with variable dispersion compensators Sairam Iyer *, Norman Lippok, Stéphane Coen, Poul Nielsen a and Frédérique Vanholsbeeck Physics Department, The University of uckland, Private ag 9019, uckland, New Zealand ªioengineering Institute, The University of uckland, Private ag 9019, uckland, New Zealand STRCT We use variable dispersion compensators to build time (TD-OCT) and spectral (SD-OCT) domain all-fiber optical coherence tomography systems operating in the 800 nm wavelength range. The all-fiber tunable dispersion compensator is based on a pair of fiber stretchers made with different fiber types in which the group delay and the nd-order dispersion can be tuned independently. Their abilities are demonstrated in biological tissues with the TD-OCT system reaching a significant sensitivity of 86 d. Keywords: OCT, Optical fibers, Chromatic dispersion, Dispersion compensation, Fiber stretching. 1. INTRODUCTION Optical Coherence Tomography (OCT) is an interferometric real-time non-invasive optical imaging technique that can be used to perform high-resolution imaging of biological tissues [1]. The image slicing ability of OCT can be achieved in the time domain (TD-OCT) by moving the coherence gate within the depths of the tissue via a scanning optical time delay line. In the spectral domain or Fourier OCT (SD-OCT) this is achieved by dispersing the light exiting the interferometer onto an array detector and examining its spectral distribution. The axial resolution is inversely proportional to the bandwidth of the white light source. Over the past decade, OCT has been implemented with widely different geometries. In particular, various authors have used optical fibers and broadband fiber couplers to construct the interferometer [-4]. Fiber-based OCT systems present several advantages in terms of compactness, flexibility, and easiness of light distribution to the sample. Most of the demonstrated fiber systems still rely, however, on free-space components, most notably in the optical delay line [4-6]. lso, the use of optical fibers pose special problems as it often introduces a significant chromatic dispersion mismatch between the two arms of the interferometer, which in turn broadens the point-spread-function (PSF) and degrades the axial resolution of the OCT system [5,6]. These issues can be mitigated by using a common-path interferometer combined with an autocorrelator [3]. However, this method only shifts the problem as a fiber-based autocorrelator will exhibit its own fiber-induced dispersion imbalance. nother approach in realizing an all-fiber OCT system was to create a variable delay line based on stretching a chirped fiber ragg grating (CFG) [7]. This was particularly attractive as only a small expansion of the CFG resulted in an optical group delay of about 100 times the actual physical stretch [8]. However, this CFG introduced a large amount of chromatic dispersion, which they compensated by using another identical CFG in the opposite direction. y doing this, the dispersion wasn t perfectly compensated, resulting in a PSF about 6 times wider than the theoretical value. Dispersion can in principle be cancelled by cutting the fiber lengths with sub-mm precision, but this is very impractical. esides numerical compensation techniques [9], dispersion compensators either use free-space elements [6], only provide a static amount of dispersion [5], or lead to an uncontrollable change in group delay [10]. Clearly, a fiber-based variable dispersion compensator is required for the design of an all-fiber OCT system. In previous publications [11,1], we successfully constructed a fiber-based variable dispersion compensator that can be used for OCT applications. The focus of this paper is to demonstrate our technique in all-fiber time and spectral domain OCT systems operating at 840 nm. It is worth noting that the TD-OCT system s signal-to-noise ratio (SNR) was measured to be 86 d, which to our knowledge is the highest measured SNR for any all-fiber TD-OCT system to date * Corresponding author: s.iyer@auckland.ac.nz Optical Coherence Tomography and Coherence Techniques IV, edited by Peter E. ndersen, rett E. ouma, Proc. of SPIE-OS iomedical Optics, SPIE Vol. 737, 7371V 009 SPIE-OS CCC code: /09/$18 doi: / SPIE-OS/ Vol V-1

2 [3,7]. Further, we demonstrate the systems abilities in transparent and turbid media by obtaining images of plastic optical fiber (POF) and onion cells, respectively. It is important to note that we are doing our experiments in the 800 nm wavelength range (where tissue absorption is minimal) in contrast to a lot of other fiber-based OCT systems which operate around 1.3 μm [,3,5,7]. This difference is significant. Firstly, 1.3 μm is a wavelength used in some telecommunication systems, and there are therefore many standard high-quality components and fibers available for that particular wavelength. In contrast, it is only recently that biomedical research has created a demand for 800 nm fiber components, and only very few are actually available. Secondly, the dispersion of standard fibers is actually very low in the 1.3 μm region since it is close to the zerodispersion wavelength of silica. We believe that researchers working on 1.3 μm fiber-based OCT systems do not suffer too much from dispersion imbalance as: the dispersion of their fibers is low, and their fibers are of high-quality because they were designed for telecommunication systems (i.e. very uniform along their length). Moreover, most of these previous experiments use sources with bandwidth smaller than ours (~50 nm instead of our bandwidth of 85 nm) which significantly relaxes dispersion issues as they are proportional to the square of the source bandwidth. Clearly, the fact that our all-fiber OCT systems are operating around 800 nm on such a large bandwidth puts new constraints on dispersion compensation and emphasizes the effectiveness of our technique.. PRINCIPLE EHIND THE FIER-SED VRILE DISPERSION COMPENSTOR To understand the operation of our dual fiber-stretcher dispersion compensator, consider how the dispersion of a fiber of length L and second-order dispersion coefficient β changes upon elastic stretching. s reported in [10,13], lengthening the fiber by increases the fiber length-integrated dispersion φ = β L by Cβ where C is a coefficient that accounts for the strain-induced optogeometrical changes of the fiber and of its dispersion characteristics. Note that C is typically lower than 1 (about 0.85 for the fiber used in [13]) which means that the dispersion of a stretched fiber is lower than that of an unstretched fiber of the same length. Now, consider a Mach-Zehnder interferometer with two arms made of different fiber types and, exhibiting different dispersion coefficients β and β, respectively. Stretching (or unstretching) both fibers by the same extra length, the optical delay between the two arms of the interferometer is left unchanged (without loss of generality, we assume here for simplicity that the two fibers have the same group veolocity) but the difference in integrated dispersion is modified by ( Cβ - Cβ ). Clearly, stretching allows for a continuous change in the relative dispersion between the two arms of the interferometer and this is the crux of our all-fiber dispersion compensator. More specifically, if the Mach-Zehnder interferometer presents an initial path length imbalance = L L and dispersion imbalance Δ φ = φ φ, the amounts and by which one has to stretch the two arms of the interferometer to balance both the group delay [Eq. (1)] and the dispersion [Eq. ()] are such that: Δ L + = (1) Δ + Cβ = Cβ φ () Solving this linear system of equations gives: 1 Δφ = C β 1 Δφ = C β 1 + κ + (3) (4) Where ( β ) ( C β ) κ = C is essentially the ratio between the dispersion coefficients of the two fibers. In the above equations, the first term represents the amount of stretching needed to cancel the original dispersion imbalance while SPIE-OS/ Vol V-

3 keeping the relative group delay constant (same term in both equations) while the second term represents what is required to vary the path difference by without changing the dispersion. Note that it is advantageous to use fibers with dispersion coefficients as widely different as possible ( κ >> 1, κ 0 or κ < 0 ) to maximize the amount of dispersion that can be compensated. lso, a large value of κ essentially decouples the two stretchers: most of the dispersion adjustment is obtained by stretching the highly dispersive fiber by an amount << while stretching fiber mainly tunes the relative group delay. 3. LL-FIER TIME DOMIN OCT s shown in Fig. 1, our OCT system is based on a Mach-Zehnder configuration in which the light from a 85 nm wide 840 nm SLED source (T840-HP from Superlum) is split and recombined with two single-mode broadband fiber couplers. The sample arm incorporates a third identical coupler to direct the light towards the sample and collect the reflected signal. The light is focused onto the sample through an interchangeable 1.6 m lens-tipped probe fiber. The probe has a focal length of 105µm, yielding a confocal parameter of 750 µm. t the output of the interferometer, the OCT signal is detected by two photodiodes in a balanced detector configuration and has 5 adjustable gain settings (Thorlabs PD150). The variable optical delay required to scan the depth of the sample is obtained by stretching part of the fiber in the reference arm with a piezo-electric actuator. Fig. 1. Experimental setup of the all-fiber dispersion compensated OCT system Each arm of the interferometer has an additional fiber stretcher for dispersion compensation. These stretchers are made by wrapping 4 m of fiber around a cylindrical rubber piece sandwiched in between two metallic plates attached together by a screw. Tightening the screw compresses and radially expands the rubber, which stretches the fiber. The stretcher in the sample arm and most of the interferometer, including the couplers, is made up of FiberCore SM800 fiber. For that fiber, we measured a dispersion coefficient β 38 ps²/km at 845 nm by white light interferometry. In contrast, we used some of Crystal Fibre LM-5 photonic crystal fiber (PCF) for the stretcher in the reference arm. This fiber has properties similar to that of the SM800 fiber in terms of its single-mode guidance, high transparency in our wavelength range, and similar core diameter (5 μm versus 5.6 μm for the SM800 fiber) while simultaneously exhibiting a significantly different dispersion parameter, β 3 ps²/km at 845 nm. Note that the use of two different fiber types introduces per se a dispersion imbalance, and as silica fibers can only be stretched by about 1-% [13], that imbalance cannot be canceled by the dual stretcher system. The bulk of that imbalance can easily be compensated by replacing some of the SM800 fiber in the sample arm by a corresponding (unstretched) length of PCF. I.e. both fiber types are present in each arm of the interferometer. The fibers must only be cut with coarse precision since our stretcher can fine tune the fiber lengths over several cm with sub-mm accuracy. In our work we were actually able to measure the relative delays between different wavelength components and adjust the fiber lengths accordingly by switching on and off independently the 3 multiplexed SLEDs of our light source. Note that using two arms of identical length of the same fiber is no guarantee of a perfect dispersion balance [13,14]. This was particularly striking in our preliminary experiments entirely based on SM800 fibers and in which our depth resolution was as high as SPIE-OS/ Vol V-3

4 400 μm instead of the expected 5.1 μm because of differences of the order of 1% (consistent with the manufacturer specification for these non-telecom-grade fibers) in the dispersion of different batches of fiber as well as longitudinal fluctuations along the fiber lengths [11]. The signal-to-noise ratio (SNR) of our system was measured to be 86 d at the balanced detector s highest gain setting. The piezoelectric fiber stretcher incorporated in the reference arm implements the longitudinal scanning function with a scan range of 490 µm (in air), at a scanning frequency of 0 Hz, resulting in the interferometer having a Doppler frequency of 40 khz. In order to demonstrate imaging of a turbid sample with a known thickness and structure [15], we performed twodimensional cross-sectional OCT imaging of an onion as shown in Fig., by using a stepper motor to perform the lateral scan. For this measurement the resolution of our OCT system was 0 µm (lateral) x 5.6 µm (longitudinal) and about 0.5 mw of optical power was delivered to the sample. Despite the fact that our resolution is moderate due to the limited bandwidth of our source and the focusing limitations of the lens-tipped fiber, the polygonal onion cellular structure can still be identified. The image size is 6100 vertical pixels by 00 horizontal pixels. part from global contrast variation, no image processing was performed and the SNR was improved by averaging each depth scan 40 times. Fig.. cquired OCT images revealing polygonal cellular structure of an onion sample. 4. LL-FIER SPECTRL DOMIN OCT SD-OCT acquires all depth information with a single measurement without the need of a longitudinal scanning mechanism [16]. spectral domain OCT setup was implemented by removing the piezoelectric fiber stretcher and replacing the balanced detector with a spectrometer (Fig. 3). For the spectral domain OCT setup a superluminescent diode delivered ~1mW of optical power to the sample, offering a spectral bandwidth Δλ = 39.8 nm at nm center wavelength. commercial spectrometer (HR000, Ocean Optics) provided 14 bit modulation depth of the spectrum, a spectral resolution of δλ = 0. nm on 048 imaged pixels, and thus, a spectral bandwidth of 450 nm. The spectrometer bandwidth was not designed to match the bandwidth of the light source in order to support the axial resolution given by the coherence length. The spectrometer used for the demonstration of the dispersion compensation technique offered no degree of freedom for adjustment of spectral properties, required for the light source. finite number of 00 pixels were illuminated by the full width of half maximum of the spectrum. Thus, the spectrometer bandwidth was too large for our light source, reducing the pixel spacing and depth range without improving the axial resolution [17]. The depth range corresponds to the spectral resolution as given by z = λ0 / 4nδλ and was limited to ~500 μm, enough to demonstrate our compensation technique. For efficient usage of detector dynamic range some considerations had to be made, owned to the Mach-Zehnder configuration of our all-fiber OCT setup. The input 50/50 coupler of the time domain setup was replaced with a 99/1 coupler, where 99% of the light was directed to the sample. This prevents the pixel array from saturating due to the strong reference signal. Using a mirror as sample, the sample arm signal at the interferometer output was ~7 d stronger SPIE-OS/ Vol V-4

5 than the reference signal in this configuration. Currently, the sample arm suffers from losses due to the 50/50 fiber coupler (75%) and single mode fiber coupling losses of the back scattering sample electromagnetic field. s a result the intensity ratio between the two arms at the interferometer output is approximately the same as that of a 50/50 input coupler in typical OCT designs. This will be improved in the near future by using an optical circulator so that sample arm losses are reduced to the single-mode fiber coupling only. This problem does not affect the time domain setup, where the system dynamic range approaches the system s sensitivity, using 16 bit /D conversion after DC removal. The spectral domain setup, however, uses a 14 bit, non-cooled CCD linescan device. In order to optimize dynamic range (typically ~50 d for non-cooled CCD devices), the fringe contrast, corresponding to the sum of all internal reflections of all sample depths, should approach pixel saturation level. The available detector dynamic range was measured to be 5 d at a fastest scanning rate of 1 khz per -scan. Fig. 3: Experimental setup of the all-fiber dispersion compensated Spectral domain OCT system. fter acquiring each spectrum a reference background spectrum was subtracted in order to remove the autocorrelation function and reduce the DC term [18]. The spectrum was then interpolated from the evenly acquired lambda space to evenly spaced frequency components in order to avoid frequency chirp after Fourier transforming [19]. Finally, the signal was inversely Fourier transformed to reveal the depth information of the sample. The function of the dispersion compensated spectral domain OCT system could be demonstrated by imaging a transparent medium, which offered a small change in the index of refraction in the order of less than -0 d. Fig. 4 shows the image of a multimode gradient index plastic optical fiber with a cladding diameter of 490 μm. The refractive index mismatch between cladding and core was Δn = and is easily seen in the linear plot. The dimensions are in good agreement assuming an average refractive index of Fig. 4: -scan of a gradient index plastic optical fiber. Right plot shows a cross section as one -scan of the fiber. SPIE-OS/ Vol V-5

6 5. CONCLUSION y using two fiber stretchers made up of different fiber types, we have implemented an all-fiber tunable dispersion compensator in an OCT system in which the delay and the dispersion in the two arms of the interferometer can be adjusted independently. Further, by implementing these variable dispersion compensators we have demonstrated all-fiber time and spectral domain systems operating at 840nm. The systems abilities were successfully proven in transparent and turbid media. Our setup is completely made up of fiber elements and does not require any critical alignment which in turn makes it compact and versatile for use in in vivo experiments. 6. CKNOWLEDGEMENTS This work has been supported by the Marsden Fund of The Royal Society of New Zealand, the Fluorescence applications in biotechnology and life sciences network and the Foundation for Research, Science and Technology of the New Zealand Government. [1] [] [3] [4] [5] [6] [7] [8] [9] [10] [11] [1] [13] [14] [15] [16] [17] 7. REFERENCES Huang D., Swanson E.., Lin C. P., Schuman J. S., Stinson W. G., Chang W., Hee M. R., Flotte T., Gregory K., Puliafito C.. and Fujimoto J. G., Optical coherence tomography, Science 54, (1991). Tearney G. J., ouma. E., oppart S.., Golubovic., Swanson E.. and Fujimoto J. G., Rapid acquisition of in vivo biological images by use of optical coherence tomography, Opt. Lett. 1, (1996). Sharma U., Fried N. M. and Kang J. U., ll-fiber common-path optical coherence tomography: Sensitivity optimization and system analysis, IEEE J. Sel. Top. Quantum Electron. 11, 799 (005). Ryu S. Y., H. Choi Y., Na J., Choi E., Yang G.-H. and Lee. H., Optical coherence tomography implemented by photonic crystal fiber, Opt. Quant. Elec. 37, (005). Chen Y. and Li X., Dispersion management up to the third order for real-time optical coherence tomography involving a phase or frequency modulator, Opt. Exp. 1, (004). Tsai T., Hsu I-J., Lu C.-W., Wang Y.-M., Sun C.-W., Kiang Y.-W. and Yang C. C., Dispersion compensation in optical coherence tomography with a prism in a rapid-scanning optical delay line, Opt. Quant. Elec. 37, (005). Choi E., Na J., Ryu S., Mudhana G. and Lee. H., ll-fiber variable optical delay line for applications in optical coherence tomography: feasibility study for a novel delay line, Opt. Exp. 13, 1334 (005). Yang C., Yazdanfar S. and Izatt,J. mplification of optical delay by use of matched linearly chirped fiber ragg gratings, Opt. Lett. 9, 685 (004). anazek K., Radnunsky. S. and Walmsley I.., lind dispersion compensation for optical coherence tomography, Opt. Commun (007). Fortier C., Fatome J., Pitois S., Couvercelle J., Leonard M., Pincemin E. and Reynaud F., Stretched fibre based dispersion compensation module for ultra high-speed telecommunication systems, Electron. Lett. 44, 105 (008). Iyer S., Coen S. and Vanholsbeeck F., ll-fiber optical coherence tomography system incorporating a dual fiber stretcher dispersion compensator, in Optical Fiber Sensors, OFS 19, Proceedings of SPIE, 7004, 37 (008). Iyer S., Coen S. and Vanholsbeeck F., ll-fiber optical coherence tomography system incorporating a dual fiber stretcher dispersion compensator, submitted to Opt. Lett. (009). Simohamed L. M. and Reynaud F., Characterisation of the dispersion evolution versus stretching in a large stroke optical fibre delay line, Opt. Comm. 159, (1999). Delage L. and Reynaud F., Kilometric optical fiber interferometer, Opt. Express 9, 67 (001). ouma. E., Tearney G. J., oppart S.., Hee M. R., rezinski M. E. and Fujimoto J. G., High-resolution optical coherence tomographic imaging using a mode-locked Ti:lO3 laser source, Opt. Lett. 0, (1995). Haeusler G. and Lindner M. W., Coherence Radar and Spectral Radar New Tools for Dermatological Diagnosis, Journal of iomedical Optics 3, 1-31 (1998). Wojtkowski M., Srinivasan V. J., Ko T. H., Fujimoto J. G., Kowalczyk. and Duker J. S., Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation, Optics Express 17 (11), (004). SPIE-OS/ Vol V-6

7 [18] [19] Wang R. K. and Ma Z., practical approach to eliminate autocorrelation artefacts for volume-rate spectral domain optical coherence tomography, Phys. Med. iol. 51, (006). Dorrer C., elabs N., Likforman J. P. and Joffre M., Spectral resolution and sampling issues in Fourier-transform spectral interferometry, J. Opt. Soc. m. 17, No. 10, (000). SPIE-OS/ Vol V-7