October 15, 2008 / Vol. 33, No. 20 / OPTICS LETTERS
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Combined system of optical coherence tomography and fluorescence spectroscopy based on double-cladding fiber Seon Young Ryu,1 Hae Young Choi,1 Jihoon Na,1 Eun Seo Choi,2 and Byeong Ha Lee1,* 1
Department of Information and Communications, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea 2 Department of Physics, College of Natural Science, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501-759, South Korea *Corresponding author:
[email protected]
Received July 9, 2008; revised August 14, 2008; accepted September 3, 2008; posted September 16, 2008 (Doc. ID 98549); published October 13, 2008 We report the development of an all-fiber multimodal system, based on a double-cladding fiber (DCF) and related devices, suitable for simultaneous measurements of optical coherence tomography (OCT) and fluorescence spectroscopy (FS). The DCF together with a DCF coupler and a single-body DCF lens has assisted in the realization of a multimodal but single-unit probe for the combined system. The DCF lens allowed simultaneous focusing of input beams for OCT and FS and also the effective collection of both signal beams from a sample. The DCF coupler could extract the OCT signal via the core channel and the FS signal through the cladding channel. The OCT image and the fluorescence spectra of a plant tissue were then simultaneously measured to validate the performance of the proposed multimodal system. © 2008 Optical Society of America OCIS codes: 110.4190, 110.4500, 170.6280, 060.2280.
Optical imaging techniques are attractive in diverse biomedical fields owing to their noninvasiveness and high resolution. Of these techniques, optical coherence tomography (OCT) is well known as a modality that can visualize cross-sectional microstructures of a biological tissue, with a few micrometers depth resolution [1]. Fluorescence spectroscopy (FS) is also another useful technique; it images functional properties such as biochemical change or tissue morphology [2]. To date, several attempts have been made to combine different modalities to obtain complementary information. For instance, ex vivo breast tissue and wound healing of in vivo skin tissue were investigated by a combined Raman spectroscopy and OCT system [3], and identification and characterization of mouse colon tissues [4] and atherosclerotic plaques [5] were made by combining OCT and FS modalities. However, even though these combined systems were primarily implemented using fiber optic devices, the use of bulk optics such as dichroic mirrors, beam splitters, and/or focusing lenses was unavoidable [3,5]. By using multiple fibers, one for each modality for example, the signals of different modalities could be separated completely, though the simple mechanical assembly of multiple fibers was bulky and not cost effective. Moreover, achieving a common viewpoint with multiple fibers has also proven to be quite complex, thus increasing the difficulty in using a common focusing lens [4]. Therefore, development and implementation of an efficient all-fiber system that can achieve multiple modalities with a simple and small probe has been expected. Recently, a specialty fiber, a double-clad structured optical fiber [double-cladding fiber (DCF)], has been employed in endoscopy and nonlinear microscopy [6–8]. The double-cladding structure of a DCF provides another light-guiding channel in addition to the 0146-9592/08/202347-3/$15.00
conventional core channel. In our previous work [9], it was shown that a single DCF could be utilized as an efficient probe for FS. The single DCF probe carried the excitation beam through its core and at the same time effectively collected the fluorescence signal with its concentric but large-area inner cladding. To extract only the fluorescence signal in the inner cladding, a special DCF coupler was invented and utilized. In this Letter, we propose the use of a DCF and related devices as a means of implementing an all-fiber OCT-FS multimodal system. In this system, a single piece of DCF is used as the common probe for both OCT and FS modalities. Through the DCF core, the OCT incident beam and FS excitation beam are simultaneously delivered to a sample. The OCT signal coming from the sample is delivered to an IR detector through the core of the DCF probe as usual, whereas the FS signal is collected separately by the large-area inner cladding of the same probe via a DCF coupler and then directed to a spectrometer. Here, a common single-body DCF lens is formed to focus the OCT beam and increase the collecting efficiency of the fluorescence signal. Finally, the OCT image and the fluorescence spectra of a plant tissue concurrently measured with the proposed system are presented to validate its performance. Figure 1 is the schematic of the proposed all-fiber OCT-FS multimodal system. In this system a broadband light beam from a superluminescent diode (SLD; 1310 nm center wavelength, 45 nm spectral bandwidth, 7 mW fiber pigtailed output) for OCT and a monochromatic laser beam from an Ar-ion laser (488 nm, 20 mW fiber pigtailed output) for FS are combined by a 2 ⫻ 1 wavelength division multiplexed (WDM) coupler and launched into a single-mode fiber (SMF). Both beams are then individually split using © 2008 Optical Society of America
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Fig. 1. (Color online) Schematic of the combined OCT-FS system. SLD, superluminescent diode; WDM, wavelength division multiplexer; SMF, single-mode fiber; DCF, doublecladding fiber; DCF lens, fiber lens made on a DCF; NDF, neutral density filter; LPF, long-pass filter; PD, photodiode; and ⫻, fiber fusion spliced point.
a 50/ 50 SMF coupler, with one part going to the reference arm of the OCT modality. The other part of both input beams is then delivered to the common probe via the through port of the DCF coupler. The single fiber lens formed at the end of the probe subsequently focuses both beams onto a sample and collects the signal beams for OCT and FS, which occurs at the same time and from the same sampling point. The OCT signal from the sample is delivered back to the 50/ 50 SMF coupler via the DCF coupler along the core mode, and interference with the OCT reference signal is then detected using a photodiode (PD) and processed into a 256 grayscale OCT image. Meanwhile, the FS signal is collected by the DCF lens and coupled to the cross port of the DCF coupler through the inner cladding of the DCF and finally directed to a spectrometer (QE65000, Ocean Optics, USA) after being long-pass filtered (LPF at 500 nm) for elimination of the FS excitation beam. The OCT signal and the FS signal in the inner cladding of the DCF probe that are coupled to the SMF are decayed out owing to the high index coating on the SMF. Both the FS beam directed to the PD and the OCT signal directed to the FS spectrometer are not effective, since their wavelengths are much too far apart. The single-mode operation required for OCT measurement and the efficient collection for weak fluorescence signal are simultaneously satisfied using the single-mode core and the large-area inner cladding of a DCF, in conjunction with a common fiber lens. The DCF was fabricated by drawing a conventional SMF preform, but coating with a low-index polymer; the original silica cladding of the fiber acted as an inner cladding, and the polymer coating worked as an outer cladding as well as a jacket. Based on this design, the core of the DCF could operate as single mode over the cut-off wavelength of 1100 nm, and the inner cladding was multimode owing to its large diameter. Since the core size 共10 m兲 and the cladding size 共125 m兲 of the fabricated DCF were the same as that of a conventional SMF; therefore, the coupling loss between the DCF and SMF could be kept negligible. The DCF coupler was fabricated by mating two DCF pieces side by side using either a side polishing
method [10] or a fused biconical tapered method [11]. The coupler coupled only the inner cladding modes in both DCF pieces without affecting the core modes. Therefore, the FS signal collected through the inner cladding of the DCF probe could be selectively directed to the spectrometer. Here, the cladding mode coupling efficiency of the DCF coupler was ⬃30% at a spectral range of 600– 900 nm, as shown in Fig. 2(a). Note that since the theoretical limit of the coupling efficiency is 50%, we can still expect some improvements. To construct a compact and common single-body probe, a small segment of a coreless silica fiber (CSF) was fusion spliced at the end of the DCF, and a fiber lens was formed using a conventional fusion splicer [12]. To increase the focal length of the probe, for the OCT modality, a large diameter 共180 m兲 CSF was used. The length of the CSF segment was 450 m, and the radius of curvature was 90 m. The focal length of the fiber lens was measured by monitoring the optical power variation reflected from a mirror with respect to the mirror position. As shown in Fig. 2(b), the focal length was 740 m, and its 1 dB tolerance range was ⬃1000 m. In addition, the spot size of the DCF lens was measured using the sharp-edge method [12]; the spot size was 9 m at a wavelength of 1310 nm. However, a simple enlargement of the CSF diameter is not effective for the FS measurement, because the FS signal is ultimately delivered to a section of DCF having only a 125 m channel diameter. To overcome this problem, by adopting a DCF having an inner cladding diameter as large as that of the CSF, we could improve the FS detection efficiency. It should be noted, however, that the precise focusing properties for the FS could not be simply identified at this time, as the FS excitation beam was guided by the core, whereas the fluorescence signal from a sample was guided by the multimode inner cladding of the DCF. A more comprehensive investigation is subsequently under preparation. The coupling efficiency of the WDM coupler was 92% at 1310 nm but 54% at 488 nm; thus, further improvement can be expected. In addition, the coupling ratio of the SMF coupler was 50% at 1310 nm and 53% at 488 nm. Everything included, 34% of the SLD power and 23% of the Ar-ion laser power were ultimately delivered to the sample. The OCT axial resolution and the sensitivity were 18 m and −85 dB, respectively. The fluorescence spectrum was measured with a spectral resolution of 3.5 nm and cali-
Fig. 2. (Color online) (a) Measured coupling efficiency of the DCF coupler and (b) the measured focal length of the DCF lens at 1310 nm.
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brated with a tungsten halogen lamp (LS-1-CAL, Ocean Optics, USA). To demonstrate the performance of the proposed OCT-FS system, both an OCT image and a series of fluorescence spectra of a plant tissue were simultaneously taken. The FS measurements were repeatedly performed at every 10 m transverse OCT scanning (B scan). The sample was a sliced tomato, and the scan was made by moving the DCF probe with a translation stage. In the OCT image of Fig. 3(a) we can clearly distinguish the cell walls and also discern the seed region on the right-hand side; measured at 250⫻ 625 pixels (image size= 2.5 mm⫻ 2.0 mm in the transverse and the longitudinal directions, respectively). Figure 3(b) is the pseudocolored stack of fluorescence spectra, and Fig. 3(c) is the spectrum measured at the 1.0 mm position, where the peaks at 690 and 740 nm are the well-known red and far-red fluorescence peaks of chlorophyll a of green plants [13]. The spectrum measured at the 2.0 mm position is very weak, as shown in Fig. 3(d), but in a similar spectral shape as the 1.0 mm spectrum. Based on this measurement, we can infer that the population of fluorophores is small in the seed region. In conclusion, we have developed an all-fiber OCT-FS multimodal system using a specialty fiber
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and related fiber devices. The core and the inner cladding regions of a DCF provided two concentric channels suitable for simultaneous measurements of OCT and FS modalities. The concentric configuration of both channels allowed the use of a common lens, a simple single-body fiber lens. In addition, the specially designed DCF coupler allowed the selective access to only the FS signal. The feasibility of the proposed system was then confirmed by simultaneously measuring the OCT and the FS of a plant tissue, a tomato. Since the design parameters of the fiber and fiber optic devices are controllable over a wide range, they can be easily adopted for other modalities, including multiphoton microscopy. Owing to its simple and compact configuration, the proposed OCT-FS system has the potential to assist in the detection of diseases or/and cancers through the implementation of an ultrasmall multimodal endoscope. This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (R01-2007-00020821-0); the Ministry of Knowledge and Economy of Korea through the Ultrashort Quantum Bean Facility Program; and a grant from the institute of Medical System Engineering (iMSE) and the GIST Technology Initiative (GTI) at the Gwangju Institute of Science and Technology (GIST), Korea. References
Fig. 3. (Color online) (a) The OCT image and (b) a stack of fluorescence spectra of a sliced tomato, simultaneously taken with the proposed OCT-FS system. Both the spectra taken at position (c) 1.0 mm and (d) 2.0 mm had main fluorescence peaks at 690 and 740 nm, though the spectrum at 2.0 mm was very feeble.
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