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

Sairam Iyer*, Norman Lippok, Stéphane Coen, Poul Nielsena and Frédérique Vanholsbeeck

Physics Department, The University of Auckland, Private Bag 92019, Auckland, New Zealand ªBioengineering Institute, The University of Auckland, Private Bag 92019, Auckland, New Zealand

ABSTRACT

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 2nd-order dispersion can be tuned independently. Their abilities are demonstrated in biological tissues with the TD-OCT system reaching a significant sensitivity of 86 dB.

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 [2-4]. 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]. Also, 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. Another approach in realizing an all-fiber OCT system was to create a variable delay line based on stretching a chirped fiber Bragg grating (CFBG) [7]. This was particularly attractive as only a small expansion of the CFBG resulted in an optical group delay of about 100 times the actual physical stretch [8]. However, this CFBG introduced a large amount of chromatic dispersion, which they compensated by using another identical CFBG in the opposite direction. By 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. Besides 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 [11,12], 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 dB, 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. Andersen,Brett E. Bouma, Proc. of SPIE-OSA Biomedical Optics, SPIE Vol. 7372, 73721V

© 2009 SPIE-OSA · CCC code: 1605-7422/09/$18 · doi: 10.1117/12.831784

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[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 [2,3,5,7]. 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 zero-dispersion 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.

2. PRINCIPLE BEHIND THE FIBER-BASED VARIABLE DISPERSION COMPENSATOR 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 2β changes upon elastic stretching. As reported in [10,13], lengthening the fiber by ΔL increases the fiber length-integrated dispersion L22 βφ = by LC Δ2β 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 A and B, exhibiting different dispersion coefficients A

2β and B2β , respectively. Stretching (or

unstretching) both fibers by the same extra length ΔL, 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 ( A

AC 2β - BBC 2β ) ΔL. 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 BA LLL −=Δ and dispersion imbalance BA

222 φφφ −=Δ , the amounts ALΔ and BLΔ by which one has to stretch the two arms of the interferometer to balance both the group delay [Eq. (1)] and the dispersion [Eq. (2)] are such that:

BA LLL Δ=Δ+Δ (1)

BB

BAA

A LCLC Δ=Δ+Δ 222 ββφ (2)

Solving this linear system of equations gives:

LC

LB

BA Δ

−+

Δ−

−=Δ1

11

1

2

2

κβφ

κ (3)

LC

L BB

B Δ−

+Δ

−−=Δ

111

2

2

κκ

βφ

κ (4)

Where ( ) ( )B

BA

A CC 22 ββκ = 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

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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 ΔL without changing the dispersion. Note that it is advantageous to use fibers with dispersion coefficients as widely different as possible ( 0,1 ≈>> κκ or 0<κ ) to maximize the amount of dispersion that can be compensated. Also, a large value of κ essentially decouples the two stretchers: most of the dispersion adjustment is obtained by stretching the highly dispersive fiber A by an amount << ΔL while stretching fiber B mainly tunes the relative group delay.

3. ALL-FIBER TIME DOMAIN OCT As 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. At 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 PDB150A). 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 382 ≈Aβ ps²/km at 845 nm by white light interferometry. In contrast, we used some of Crystal Fibre LMA-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, 232 ≈Bβ 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 1-2% [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

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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 dB 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 20 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 two-dimensional cross-sectional OCT imaging of an onion as shown in Fig. 2, by using a stepper motor to perform the lateral scan. For this measurement the resolution of our OCT system was 20 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 200 horizontal pixels. Apart from global contrast variation, no image processing was performed and the SNR was improved by averaging each depth scan 40 times.

Fig. 2. Acquired OCT images revealing polygonal cellular structure of an onion sample.

4. ALL-FIBER SPECTRAL DOMAIN OCT SD-OCT acquires all depth information with a single measurement without the need of a longitudinal scanning mechanism [16]. A 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 806.3 nm center wavelength. A commercial spectrometer (HR2000, Ocean Optics) provided 14 bit modulation depth of the spectrum, a spectral resolution of δλ = 0.22 nm on 2048 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. A finite number of 200 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 δλλ nz 4/2

0= 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 dB stronger

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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. As 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 A/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 dB 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 52 dB at a fastest scanning rate of 1 kHz per A-scan.

Fig. 3: Experimental setup of the all-fiber dispersion compensated Spectral domain OCT system.

After 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 -20 dB. 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 = 0.014 and is easily seen in the linear plot. The dimensions are in good agreement assuming an average refractive index of 1.35.

Fig. 4: B-scan of a gradient index plastic optical fiber. Right plot shows a cross section as one A-scan of the fiber.

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5. CONCLUSION By 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. ACKNOWLEDGEMENTS 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.

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