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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 27 — Dec. 17, 2012
  • pp: 28418–28430
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Two-reference swept-source optical coherence tomography of high operation flexibility

Ting-Ta Chi, Chiung-Ting Wu, Chen-Chin Liao, Yi-Chou Tu, Yean-Woei Kiang, and C. C. Yang  »View Author Affiliations


Optics Express, Vol. 20, Issue 27, pp. 28418-28430 (2012)
http://dx.doi.org/10.1364/OE.20.028418


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Abstract

The significantly less stringent operation of a two-reference swept-source optical coherence tomography (OCT) system for suppressing the mirror image is demonstrated based on the spatially localized image processing method. With this method, the phase difference between the two reference signals is not limited to 90 degrees. Based on the current experimental operation, the mirror image can be effectively suppressed as long as the phase difference is larger than 20 degrees. In other words, the adjustment of the beam splitter orientation for controlling the phase difference becomes much more flexible. Also, based on a phantom experiment, the combination the spatially localized mirror image suppression method with the two-reference OCT operation leads to the implementation of full-range optical Doppler tomography.

© 2012 OSA

1. Introduction

In Fourier-domain optical coherence tomography (OCT), for achieving higher operation sensitivity, the interference between the sample and reference signals is usually formed around the zero-delay point. In this situation, the OCT result includes the overlapping real and mirror images after the inverse-Fourier transform of the acquired real-number interfered spectral intensity. The mirror image must be suppressed before biomedical information can be clearly extracted from the OCT scanning. Theoretically, if the quadratic components of the OCT signal can be obtained, an inverse Fourier transform can lead to mirror image suppression [1

1. R. A. Leitgeb, C. K. Hitzenberger, A. F. Fercher, and T. Bajraszewski, “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography,” Opt. Lett. 28(22), 2201–2203 (2003). [CrossRef] [PubMed]

]. The quadratic components can be obtained with a multiple B-mode scan operation of OCT [1

1. R. A. Leitgeb, C. K. Hitzenberger, A. F. Fercher, and T. Bajraszewski, “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography,” Opt. Lett. 28(22), 2201–2203 (2003). [CrossRef] [PubMed]

6

6. E. Götzinger, M. Pircher, R. Leitgeb, and C. Hitzenberger, “High speed full range complex spectral domain optical coherence tomography,” Opt. Express 13(2), 583–594 (2005). [CrossRef] [PubMed]

]. However, such an operation slows down the imaging speed and may become unstable due to motion artifacts. Such problems can be solved by using a two-beam system [7

7. S. Yun, G. Tearney, J. de Boer, and B. Bouma, “Removing the depth-degeneracy in optical frequency domain imaging with frequency shifting,” Opt. Express 12(20), 4822–4828 (2004). [CrossRef] [PubMed]

10

10. A. H. Dhalla and J. A. Izatt, “Complete complex conjugate resolved heterodyne swept-source optical coherence tomography using a dispersive optical delay line,” Biomed. Opt. Express 2(5), 1218–1232 (2011). [CrossRef] [PubMed]

]. The key point is to obtain two signals with a phase difference of 90 degrees from the same lateral position on the sample. The use of a 3 x 3 fiber coupler can produce a phase shift for obtaining the quadratic signal and hence suppressing the mirror image [11

11. M. A. Choma, C. Yang, and J. A. Izatt, “Instantaneous quadrature low-coherence interferometry with 3 x 3 fiber-optic couplers,” Opt. Lett. 28(22), 2162–2164 (2003). [CrossRef] [PubMed]

]. However, such a complicated OCT system may suffer from the spectral limitation in broadband operation and the problem of temperature-caused instability such that the mirror image suppression capability is limited. To improve the mirror image suppression capability, a two-reference-arm scheme was proposed [12

12. K. S. Lee, P. Meemon, K. Hsu, W. Dallas, and J. P. Rolland, “Dual-reference full-range frequency domain optical coherence tomography,” Proc. SPIE 7170, 717004, 717004-7 (2009). [CrossRef]

]. However, strong phase noise can be generated from the two independent reference arms. Although a proportional-integral-derivative controller was added to one of the reference arms for real-time compensating the phase variation between the two arms [12

12. K. S. Lee, P. Meemon, K. Hsu, W. Dallas, and J. P. Rolland, “Dual-reference full-range frequency domain optical coherence tomography,” Proc. SPIE 7170, 717004, 717004-7 (2009). [CrossRef]

, 13

13. S. Zotter, M. Pircher, E. Götzinger, T. Torzicky, M. Bonesi, and C. K. Hitzenberger, “Sample motion-insensitive, full-range, complex, spectral-domain optical-coherence tomography,” Opt. Lett. 35(23), 3913–3915 (2010). [CrossRef] [PubMed]

], it is still difficult to exactly maintain the required 90-degree phase difference between the two reference arms. To improve the system stability, a common-path two-reference system was implemented by rotating a beam-splitting prism to obtain the 90-degree phase difference between the two reference signals [14

14. K. S. Lee, P. Meemon, W. Dallas, K. Hsu, and J. P. Rolland, “Dual detection full range frequency domain optical coherence tomography,” Opt. Lett. 35(7), 1058–1060 (2010). [CrossRef] [PubMed]

]. By using such a two-reference scheme, full-range (from –π to π) optical Doppler tomography (ODT) has been demonstrated based on a phantom experiment [15

15. P. Meemon, K.-S. Lee, and J. P. Rolland, “Doppler imaging with dual-detection full-range frequency domain optical coherence tomography,” Biomed. Opt. Express 1(2), 537–552 (2010). [CrossRef] [PubMed]

]. However, to guarantee the 90-degree phase difference between the two reference signals, the orientation of the beam-splitting prism needs to be precisely controlled such that the operational flexibility and stability are limited.

Besides the hardware approaches, software methods are useful for mirror image suppression. The BM-scan method based on a certain phase shift mechanism along the B-mode scan has been widely used [16

16. Y. Yasuno, S. Makita, T. Endo, G. Aoki, M. Itoh, and T. Yatagai, “Simultaneous B-M-mode scanning method for real-time full-range Fourier domain optical coherence tomography,” Appl. Opt. 45(8), 1861–1865 (2006). [CrossRef] [PubMed]

21

21. F. Jaillon, S. Makita, M. Yabusaki, and Y. Yasuno, “Parabolic BM-scan technique for full range Doppler spectral domain optical coherence tomography,” Opt. Express 18(2), 1358–1372 (2010). [CrossRef] [PubMed]

]. In this method, the real-number spectral signals of a 2-D image are first Hilbert transformed along the B-mode scan direction to give the complex spectral signals, which can lead to mirror image suppression after a Fourier transform. The phase shift along the B-mode scan in an OCT system can be obtained by using a scanning galvanometer for lateral scanning when the optical beam is aligned away from the galvanometer axis [22

22. B. Baumann, M. Pircher, E. Götzinger, and C. K. Hitzenberger, “Full range complex spectral domain optical coherence tomography without additional phase shifters,” Opt. Express 15(20), 13375–13387 (2007). [CrossRef] [PubMed]

24

24. R. A. Leitgeb, R. Michaely, T. Lasser, and S. C. Sekhar, “Complex ambiguity-free Fourier domain optical coherence tomography through transverse scanning,” Opt. Lett. 32(23), 3453–3455 (2007). [CrossRef] [PubMed]

]. On the other hand, recently, this research group demonstrated an alternative method for mirror image suppression, which is also based on a system phase shift along the B-mode scan [25

25. C. T. Wu, T. T. Chi, C. K. Lee, Y. W. Kiang, C. C. Yang, and C. P. Chiang, “Method for suppressing the mirror image in Fourier-domain optical coherence tomography,” Opt. Lett. 36(15), 2889–2891 (2011). [CrossRef] [PubMed]

, 26

26. C. T. Wu, T. T. Chi, Y. W. Kiang, and C. C. Yang, “Computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography,” Opt. Express 20(8), 8270–8283 (2012), doi:. [CrossRef] [PubMed]

]. In this method, it is first noted that the phase shifts between two neighboring A-mode scans of the real- and mirror-image signals are mutually reversed. We can use this property of reversed phase shift for differentiating the real image from the mirror image by solving two linked equations based on the assumptions of equal OCT signal intensities and phases (except the system phase shift along the B-mode scan) in the two neighboring A-mode scans. This method has the advantage of a significantly shorter computation time, when compared with the aforementioned BM-scan method. Also, because this mirror image suppression process is a spatially localized (SL) operation of OCT signals, we can select any concerned A- and B-mode scan ranges for processing to further save computation time. This method will be hereafter called the spatially localized method and abbreviated by the SL method. However, a good mirror-image suppression result based on this method requires a dense B-mode scan such that the assumptions of equal signal intensity and phase in the neighboring A-mode scans can be approximately correct. Although an iteration procedure in the SL method can lead to effective mirror image suppression for a structure OCT image when the B-mode scan is not dense enough, the iteration procedure results in an average effect among the signals of the involved neighboring A-mode scans. In this situation, the phase information induced by the Doppler effect in those A-mode scans is mixed, leading to the difficulty of implementing full-range ODT. To solve this problem, the two-reference OCT scheme for simultaneously generating two quadratic signals has been used to demonstrate its capability of mirror image suppression and full-range ODT operation, as discussed in the last paragraph. However, the conventional operation of a two-reference OCT system requires a stringent condition of 90-degree phase difference between the signals of the two references.

In this paper, we demonstrate the application of the SL method to the operation of a two-reference OCT system for improving the operational flexibility. As mentioned earlier, in the past two-reference OCT operation, the beam-splitting prism for combining the two reference beams must be precisely controlled to produce a 90-degree phase difference such that the interfered signals of the two reference beams can be used as the real and imaginary parts for inverse Fourier transform and mirror image suppression. In the following discussions, this procedure will be referred to as the quadratic method. By using the SL method for processing the two-reference signals, the operation of a two-reference OCT system becomes more flexible. In other words, the orientation range of the aforementioned beam-splitting prism can be significantly increased. The allowed deviation range of the two-reference phase difference from 90 degrees for effective mirror image suppression can be increased from almost zero to ~70 degrees. Also, a full-range ODT function can be implemented based on the combination of the SL method and the two-reference OCT operation. With the two-reference OCT operation, in applying the SL method, the previously used iteration procedure is unnecessary. In section 2 of this paper, the two-reference OCT setup is described. Also, the SL method is briefly reviewed. Then, in section 3, the mirror image suppression results based on the combination of the SL method and the two-reference OCT operation are discussed. Next, the full-range ODT results are demonstrated by performing a phantom experiment in section 4. Finally, conclusions are drawn in section 5.

2. Two-reference OCT setup and the SL method

Figure 1
Fig. 1 Setup of the two-reference OCT system. BS1-BS3: beam splitters. FBG: fiber Bragg grating.
shows the setup of the two-reference OCT system. A sweeping-frequency laser with the sweeping rate at 100 kHz and the central wavelength around 1060 nm (Axsun, 1060nm OCT engine) is used as the light source. The sweeping range of ~100 nm with ~70 nm in spectral full-width at half-maximum leads to an axial resolution of ~9.8 microns in the air. From the swept source, 5% light power is directed to a fiber Bragg grating (FBG) for synchronizing the frequency sweeping with data acquisition in each A-mode scan. Then, 10% of the rest light power is guided to the reference arm. The rest power is used for sample scanning through a galvanometer scanner. Although we also use a beam splitter (BS1) for generating a desired phase difference between the two reference signals through controlling its orientation, the optical layout before signal detection is different from previously reported [15

15. P. Meemon, K.-S. Lee, and J. P. Rolland, “Doppler imaging with dual-detection full-range frequency domain optical coherence tomography,” Biomed. Opt. Express 1(2), 537–552 (2010). [CrossRef] [PubMed]

]. In our design, another branch of the reference signal is produced through beam splitter BS2. It is further split into two through beam splitter BS3 for directing them to one of the ports of individual balanced detectors. The interfered signals corresponding to the two references are directed to other ports of individual balanced detectors. Such a layout has the advantage of the deletions of the dc component and autocorrelation signals [6

6. E. Götzinger, M. Pircher, R. Leitgeb, and C. Hitzenberger, “High speed full range complex spectral domain optical coherence tomography,” Opt. Express 13(2), 583–594 (2005). [CrossRef] [PubMed]

]. It also has the advantage of easy installation. The scanning depth range and system sensitivity of the OCT system are 3.7 mm and 102 dB, respectively.

The SL method was originally proposed to use the signals of two neighboring A-mode scans for differentiating the real image from the mirror image. Suppose rn and mn represent the complex OCT signals of the real and mirror images, respectively, in a certain A-mode scan pixel of the nth A-mode scan. Also, rn+1exp() and mn + lexp(-) represent the complex OCT signals of the real and mirror images, respectively, in the same A-mode scan pixel of the (n + 1)th A-mode scan. Here, θ denotes the system phase shift between the two neighboring A-mode scans. The summation of the complex signals of the real and mirror images is equal to the measured OCT signal after a k-space Fourier transform of the acquired spectral signal. Suppose that Sn and Sn+1 stand for measured OCT signals in the designated depth pixel of the nth and (n + 1)th A-mode scans, respectively. We have the following two equations:
Sn=rn+mn
(1)
and
Sn+1=rn+1exp(iθ)+mn+1exp(iθ).
(2)
In the SL method, we assume that except the system phase shift θ, the complex OCT signals in the same depth pixel of two neighboring A-mode scans are the same, i.e., rn = rn+1 and mn = mn+1 for any integer n. Under this assumption, for a given θ value, the two complex unknowns in Eqs. (1) and (2) can be solved to give r˜n and m˜n, which represent the solved real- and mirror-image signals, respectively, as
r˜n=SnSn+1exp(iθ)1exp(i2θ)
(3)
and
m˜n=SnSn+1exp(iθ)1exp(i2θ).
(4)
Therefore, the real-image signal can be differentiated from that of the mirror image.

For the evaluation in the SL method, two OCT signals of a fixed relative phase from the same or almost the same lateral position of the sample are needed. Here, the relative phase can be any non-zero value. In the current study, the OCT signals of the two references come from the same lateral position and have a fixed relative phase θ. When the SL method is applied for mirror image suppression by using the two interfered signals from the two references as Sn and Sn + 1 in Eqs. (1) and (2), the phase difference, θ, is not limited to 90 degrees. Compared to the case of the conventional quadratic method, in which the phase difference between the two references must be fixed at 90 degrees, the operation of the two-reference OCT system based on the SL method becomes more flexible.

3. Mirror image suppression with two-reference OCT

Figure 2(a)
Fig. 2 (a) Un-processed OCT scanning image acquired from balanced detector 1 when a mirror surface is used as the scanning sample and the phase difference, θ, between the two reference signals is around 80 degrees. (b)-(e): Mirror image suppression results based on the quadratic method when θ is 90, 80, 70, and 60 degrees, respectively. (f)-(j): Mirror image suppression results based on the SL method when θ is 90, 40, 30, 20 and 10 degrees, respectively.
shows the un-processed OCT scanning image (including the real image in the lower portion and the mirror image in the upper portion) acquired from balanced detector 1 when a mirror surface is used as the scanning sample and the phase difference, θ, between the two references is around 80 degrees. When the beam splitter (BS1) is rotated to obtain various θ values, all the un-processed OCT images look just like the one shown in Fig. 2(a). The θ value is obtained by evaluating the phase difference between the signals acquired from balanced detectors 1 and 2. The θ image corresponding to the OCT image in Fig. 2(a) is shown in Fig. 3(a)
Fig. 3 (a): θ image corresponding to the OCT image in Fig. 2(a). (b): B-mode line-scan profile of θ of the real image (the lower portion in part (a)). (c): B-mode line-scan intensity ratio between the OCT signals from balanced detectors 1 and 2.
. Here, the red and blue colors show that the θ values of the real and mirror images are opposite in sign and have the same magnitude. The line-scan profile of θ along the B-mode scan in the real image (the lower portion in Fig. 3(a)) is shown in Fig. 3(b). Here, one can see the small fluctuation of θ value in a range between 79.4 and 80 degrees. Figure 3(c) shows the intensity ratio between the OCT signals from balanced detectors 1 and 2 scanning along the B-mode direction. Here, one can see a small intensity ratio fluctuation within 1%.

Figure 5
Fig. 5 B-mode averages of the mirror image suppression ratios as functions of the phase difference θ for various experimental (Exp.) and simulation (Sim.) cases.
shows the mirror image suppression ratios as functions of the phase difference θ for various experimental and simulation cases. The suppression ratio is defined as the average of the maximum real-image intensity over all the scanned A modes divided by the average of the maximum mirror-image intensity over all the scanned A modes after the mirror image suppression process. The curves labeled by Exp.-SL and Exp.-Qd show the experimental results based on the SL and quadratic methods for mirror image suppression, respectively. Here, one can see that when θ = 90 degrees, both SL and quadratic methods can lead to a suppression ratio of ~24 dB. Actually, at θ = 90 degrees, these two methods are mathematically equivalent. As the θ value decreases to 80 degrees, the suppression ratio is reduced to ~12 dB when the conventional quadratic method is used. However, based on the SL method, the suppression ratio can still be as large as ~17.5 dB when θ is decreased to 20 degrees. In the simulation study, we consider a sinusoid spectral signal from either balanced detector. Such a signal represents the reflection from a sharp interface in the sample arm. The sinusoidal spectral signals from the two balanced detectors shift from each other by the phase difference θ. The curves labeled by Sim.-SL-0% and Sim.-Qd-0% show the simulations results based on the SL and quadratic methods, respectively, when the signal intensities from the two balanced detectors are assumed to be the same, i.e., they differ by 0%. In this situation, based on the SL method, the mirror image suppression ratio can be maintained at ~28 dB over the θ range from 10 through 90 degrees. However, by assuming that the signal intensities from the two balanced detectors differ by 1%, the variation of the simulation result essentially follows the decreasing trend of the experimental data with decreasing θ value. At θ = 90 degrees, the experimental result is slightly higher than that from the simulation. On the other hand, based on the quadratic method, at θ = 90 degrees, the suppression ratio can also reach ~28 dB in simulation by assuming the same signal intensity from the two balanced detectors. As θ decreases, its variation closely follows that of the experimental result. Based on the quadratic method, the assumption of 1% intensity difference between the signals from the two balanced detectors does not significantly change the simulation results except at θ = 90 degrees. From the simulation results shown in Fig. 5, it is believed that based on the SL method, if the optical setup can be further improved for making the signal intensities from the two balanced detectors exactly the same and stable, the mirror image suppression ratio can be further enhanced and phase difference range of high suppression ratio can be further extended to a value close to zero (see the curve labeled by Sim.-SL-0% in Fig. 5).

To see the mirror image suppression effects in scanning bio-tissue, in Figs. 6(a)
Fig. 6 OCT images by scanning the skin on a finger of a volunteer. The images in columns 1-4 show the un-processed results obtained from balanced detectors 1 and 2, and the processed results based on the quadratic and SL methods, respectively. Meanwhile, the images in rows 1-4 correspond to the conditions of θ = 90, 80, 30, and 20 degrees, respectively.
-6(p), we show the OCT images by scanning the skin on a finger of a volunteer. Among the 16 images, the four images in the first (second) column show the un-processed results obtained from balanced detector 1 (2). The four images in the third (fourth) column show the processed results based on the quadratic (SL) method. Meanwhile, the images in rows 1-4 correspond to the conditions of θ = 90, 80, 30, and 20 degrees, respectively. Here, again we can see the good mirror image suppression results based on both quadratic and SL methods when θ = 90 degrees. In either Fig. 6(c) or 6(d), a sweat gland in the sample can be clearly seen. When θ is decreased to 80 degrees, as shown in Fig. 6(h), the SL method leads to a result of almost complete mirror image suppression for showing two sweat glands at this scanning location. As shown in Fig. 6(g), although the mirror image is essentially suppressed based on the quadratic method, certain residual tissue features of the mirror image can still be seen. The mirror image suppression result is still quite good when θ = 30 degrees (see Fig. 6(l)) and can be acceptable when θ = 20 degrees (see Fig. 6(p)) based on the SL method. However, the results are quite poor based on the quadratic method when θ is smaller than 80 degrees (see Figs. 6(k) and 6(o)). It is noted that in obtaining the result of each θ value in Fig. 6 and other similar figures to be shown below, we need to rotate the beam splitter BS1 and re-align the optics setup of the OCT system. Therefore, the noise levels in different cases of various θ values can be different. Such a difference in noise level does not affect the demonstrations for the proposed technique.

4. ODT results with two-reference OCT

To demonstrate the ODT results based on the application of the SL method to process the images from the two-reference OCT operation, we prepare a setup for phantom experiment, as depicted in Fig. 7
Fig. 7 Setup for the phantom experiment of ODT.
. Here a capillary tube of ~400 μm in inner diameter is connected to a syringe, which is placed on a motorized stage. Bean milk is used as the moving material in the capillary tube. The light incidence in OCT operation and the portion of the capillary tube under illumination (the flow direction of bean milk) form an angle of about 80 degrees. The flow speed of bean milk can be controlled by the syringe to give various Doppler phase shifts, ϕ. It is noted that multiple scattering may occur in the used moving material (bean milk) that can affect the calibration of the Doppler shift. However, this effect is expected to be small [27

27. J. Kalkman, A. V. Bykov, G. J. Streekstra, and T. G. van Leeuwen, “Multiple scattering effects in Doppler optical coherence tomography of flowing blood,” Phys. Med. Biol. 57(7), 1907–1917 (2012). [CrossRef] [PubMed]

]. Figures 8(a)
Fig. 8 OCT images by scanning the capillary tube with motionless bean milk shown in Fig. 7. The images in columns 1-4 show the un-processed results obtained from balanced detectors 1 and 2, and the processed results based on the quadratic and SL methods, respectively. Meanwhile, the images in rows 1-4 correspond to the conditions of θ = 90, 80, 30, and 20 degrees, respectively.
-8(p) show the similar OCT intensity images to those in Figs. 6(a)-6(p) except for the scans on the capillary tube containing motionless bean milk as shown in Fig. 7. Here, as depicted in Fig. 8(a), the bright OCT features of the capillary tube structure are labeled by A-C for the real image and by A’-C’ for the mirror image. The features of A and A’ (B and B’) correspond to the proximate (distal) inner wall of the tube in the real and mirror images, respectively. Also, the features of C and C’ correspond to the proximate outer wall of the tube in the real and mirror images, respectively. The distal outer wall of the tube cannot be seen in the scanning images. It is noted that the feature of the proximate outer wall of the real image accidentally coincides with that of the distal inner wall of the mirror image except the portions around the left and right ends of the image in Fig. 8(a). They can be differentiated after the mirror image is suppressed, as shown in Figs. 8(c) and 8(d). Therefore, the region between A and B (A’ and B’) contains moving bean milk in the real (mirror) image. Also, the region between A and C (A’ and C’) correspond to the tube material in the real (mirror) image. Again, based on the SL method, the mirror image suppression is quite effective when θ is larger than 20 degrees (see Figs. 8(d), 8(h), 8(l), and 8(p)). However, based on the quadratic method, mirror image suppression becomes ineffective when θ is equal to or smaller than 80 degrees (see Figs. 8(c), 8(g), 8(k), and 8(o)).

Figures 9-12 show the ODT phase shift images under various conditions when θ is 90, 80, 30, and 20 degrees, respectively. In each figure set, the images in rows 1-3 show the un-processed data, the processed results based on the quadratic method, and the processed results based on the SL method, respectively. Meanwhile, the images in columns 1-7 show the results of 0, π/6, π/3, π/2, 2π/3, 5π/6, and π, respectively, in maximum Doppler phase shift, ϕmax, around the center of the capillary tube. In Fig. 9
Fig. 9 ODT phase shift images under various conditions when θ is 90 degrees. The images in rows 1-3 show the un-processed data, the processed results based on the quadratic method, and the processed results based on the SL method, respectively. Meanwhile, the images in columns 1-7 show the results of 0, π/6, π/3, π/2, 2π/3, 5π/6, and π, respectively, in maximum Doppler phase shift around the center of the capillary tube. Arrows are drawn to indicate two dashed lines in the three images of column 7 for line-scan demonstrations later.
for θ = 90 degrees, all the Doppler phase shift images with various ϕmax values are quite clear when either the quadratic or SL method is used. The decreasing ϕ value from the center to the rim in the capillary tube can be clearly seen. When θ is decreased to 80 degrees, as shown in the second row of Fig. 10
Fig. 10 ODT phase shift images similar to Fig. 9 when θ is 80 degrees.
, based on the quadratic method, certain residual phase shift features from the mirror image can be seen around the center of the capillary tube, particularly when ϕmax is large. Such residual phase shift features become stronger when θ = 30 and 20 degrees, as shown in the second rows of Fig. 11
Fig. 11 ODT phase shift images similar to Fig. 9 when θ is 30 degrees.
and Fig. 12
Fig. 12 ODT phase shift images similar to Fig. 9 when θ is 20 degrees
. On the other hand, the residual phase shift features are always quite weak for any ϕmax value, even when θ is as small as 20 degrees (see the images in the third row of Fig. 12), if the SL method is used for mirror image suppression.

The line-scan profiles of Doppler phase shift along the lower dashed lines (indicated by the arrows at the right end) in the individual images of the last columns in Figs. 9-12 for the cases of θ = 90, 80, 30, and 20 degrees are shown in Figs. 13(a)
Fig. 13 Line-scan profiles of Doppler phase shift along the lower dashed lines in the individual images of the last columns in Figs. 9-12 (indicated by the arrows at the right end) for the cases of θ = 90 (a), 80 (b), 30 (c), and 20 (d) degrees.
-13(d), respectively. In each part of Fig. 13, the curve of the un-processed result shows a portion of nearly zero phase-shift level around the center of the capillary tube. Outside this portion, the phase shift reasonably increases from the rim of the tube. The nearly zero phase-shift around the tube center is due to the dominating mirror image signal (strong scattering for high intensity) of the proximate inner-wall feature of the mirror image in this line-scan portion. Because the tube wall is motionless, the Doppler phase shift is about zero in this portion. Outside this portion, the motion region in the real image overlaps with the motionless tube material of weak OCT signal in the mirror image such that the calibration of Doppler phase shift is not degraded. In Fig. 13(a) for θ = 90 degrees, both the quadratic and SL methods lead to effective mirror image suppression and retrieve the Doppler phase shift variation. Here, the two curves obtained from the use of the quadratic and SL methods are completely coincident with each other. In Fig. 13(b) for θ = 80 degrees, the quadratic method results in a large-scale fluctuation of Doppler phase shift. This method can hardly help in retrieving the Doppler phase shift when θ is 20 or 30 degrees, as shown in Figs. 13(c) and 13(d). The SL method can retrieve the Doppler phase shifts quite well when θ is 80 and 30 degrees (see Figs. 13(b) and 13(c)). However, when θ is reduced to 20 degrees, the SL method cannot help either in retrieving the Doppler phase shift (see Fig. 13(d)).

Figures 14(a)
Fig. 14 Line-scan profiles of Doppler phase shift along the upper dashed lines in the individual images of the last columns in Figs. 9-12 (indicated by the arrows at the right end) for the cases of θ = 90 (a), 80 (b), 30 (c), and 20 (d) degrees.
-14(d) show the similar results to Figs. 13(a)-13(d) except for the line-scan profiles along the upper scanning dashed lines (indicated by the arrows at the right end) in the individual images of the last columns in Figs. 9-12. In this situation, the scanning line does not coincide with the feature of the proximate inner-wall of the mirror image (just crossing this feature). Therefore, the central portion of the line scan before mirror image suppression shows a Doppler phase-shift variation of strong random fluctuation. With effective mirror image suppression based on the quadratic method when θ is 90 degrees and the SL method when θ is larger than 20 degrees, the correct Doppler phase shift variations can be retrieved. It is noted that because both scanning lines in Figs. 13 and 14 do not exactly pass the center of the tube, the observed maximum Doppler phase shift is smaller than π.

It is noted that with 100 kHz in A-mode scan rate and 1060 nm in central wavelength of the OCT system, the Doppler phase shift of π corresponds to a flow speed of ~19 mm/sec along the direction of light incidence. Because of the 80-degree angle between the light incidence and flow directions, the flow speed of bean milk along the capillary tube is v = ~109 mm/sec.

5. Conclusions

In summary, we have demonstrated the significantly less stringent operation condition of a two-reference OCT system for suppressing the mirror image based on the SL image processing method. With this method, the phase difference between the two reference signals was not limited to 90 degrees. Based on the current experimental operation, the mirror image could be effectively suppressed as long as the phase difference between the two reference signals was larger than 20 degrees. In other words, the adjustment of the beam splitter orientation for controlling the phase difference became much more flexible. Also, based on a phantom experiment, the combination the SL mirror image suppression method with the two-reference OCT operation led to the implementation of full-range ODT.

Acknowledgment

References and links

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P. Targowski, M. Wojtkowski, A. Kowalczyk, T. Bajraszewski, M. Szkulmowski, and I. Gorczynska, “Complex spectral OCT in human eye imaging in vivo,” Opt. Commun. 229(1-6), 79–84 (2004). [CrossRef]

4.

Y. Yasuno, S. Makita, T. Endo, G. Aoki, H. Sumimura, M. Itoh, and T. Yatagai, “One-shot-phase-shifting Fourier domain optical coherence tomography by reference wavefront tilting,” Opt. Express 12(25), 6184–6191 (2004). [CrossRef] [PubMed]

5.

J. Zhang, W. Jung, J. Nelson, and Z. Chen, “Full range polarization-sensitive Fourier domain optical coherence tomography,” Opt. Express 12(24), 6033–6039 (2004). [CrossRef] [PubMed]

6.

E. Götzinger, M. Pircher, R. Leitgeb, and C. Hitzenberger, “High speed full range complex spectral domain optical coherence tomography,” Opt. Express 13(2), 583–594 (2005). [CrossRef] [PubMed]

7.

S. Yun, G. Tearney, J. de Boer, and B. Bouma, “Removing the depth-degeneracy in optical frequency domain imaging with frequency shifting,” Opt. Express 12(20), 4822–4828 (2004). [CrossRef] [PubMed]

8.

J. Zhang, J. S. Nelson, and Z. Chen, “Removal of a mirror image and enhancement of the signal-to-noise ratio in Fourier-domain optical coherence tomography using an electro-optic phase modulator,” Opt. Lett. 30(2), 147–149 (2005). [CrossRef] [PubMed]

9.

A. M. Davis, M. A. Choma, and J. A. Izatt, “Heterodyne swept-source optical coherence tomography for complete complex conjugate ambiguity removal,” J. Biomed. Opt. 10(6), 064005 (2005). [CrossRef] [PubMed]

10.

A. H. Dhalla and J. A. Izatt, “Complete complex conjugate resolved heterodyne swept-source optical coherence tomography using a dispersive optical delay line,” Biomed. Opt. Express 2(5), 1218–1232 (2011). [CrossRef] [PubMed]

11.

M. A. Choma, C. Yang, and J. A. Izatt, “Instantaneous quadrature low-coherence interferometry with 3 x 3 fiber-optic couplers,” Opt. Lett. 28(22), 2162–2164 (2003). [CrossRef] [PubMed]

12.

K. S. Lee, P. Meemon, K. Hsu, W. Dallas, and J. P. Rolland, “Dual-reference full-range frequency domain optical coherence tomography,” Proc. SPIE 7170, 717004, 717004-7 (2009). [CrossRef]

13.

S. Zotter, M. Pircher, E. Götzinger, T. Torzicky, M. Bonesi, and C. K. Hitzenberger, “Sample motion-insensitive, full-range, complex, spectral-domain optical-coherence tomography,” Opt. Lett. 35(23), 3913–3915 (2010). [CrossRef] [PubMed]

14.

K. S. Lee, P. Meemon, W. Dallas, K. Hsu, and J. P. Rolland, “Dual detection full range frequency domain optical coherence tomography,” Opt. Lett. 35(7), 1058–1060 (2010). [CrossRef] [PubMed]

15.

P. Meemon, K.-S. Lee, and J. P. Rolland, “Doppler imaging with dual-detection full-range frequency domain optical coherence tomography,” Biomed. Opt. Express 1(2), 537–552 (2010). [CrossRef] [PubMed]

16.

Y. Yasuno, S. Makita, T. Endo, G. Aoki, M. Itoh, and T. Yatagai, “Simultaneous B-M-mode scanning method for real-time full-range Fourier domain optical coherence tomography,” Appl. Opt. 45(8), 1861–1865 (2006). [CrossRef] [PubMed]

17.

R. K. Wang, “In vivo full range complex Fourier domain optical coherence tomography,” Appl. Phys. Lett. 90(5), 054103 (2007). [CrossRef]

18.

S. Makita, T. Fabritius, and Y. Yasuno, “Full-range, high-speed, high-resolution 1 microm spectral-domain optical coherence tomography using BM-scan for volumetric imaging of the human posterior eye,” Opt. Express 16(12), 8406–8420 (2008). [CrossRef] [PubMed]

19.

S. Vergnole, G. Lamouche, and M. L. Dufour, “Artifact removal in Fourier-domain optical coherence tomography with a piezoelectric fiber stretcher,” Opt. Lett. 33(7), 732–734 (2008). [CrossRef] [PubMed]

20.

K. Wang, Z. Ding, Y. Zeng, J. Meng, and M. Chen, “Sinusoidal B-M method based spectral domain optical coherence tomography for the elimination of complex-conjugate artifact,” Opt. Express 17(19), 16820–16833 (2009). [CrossRef] [PubMed]

21.

F. Jaillon, S. Makita, M. Yabusaki, and Y. Yasuno, “Parabolic BM-scan technique for full range Doppler spectral domain optical coherence tomography,” Opt. Express 18(2), 1358–1372 (2010). [CrossRef] [PubMed]

22.

B. Baumann, M. Pircher, E. Götzinger, and C. K. Hitzenberger, “Full range complex spectral domain optical coherence tomography without additional phase shifters,” Opt. Express 15(20), 13375–13387 (2007). [CrossRef] [PubMed]

23.

L. An and R. K. Wang, “Use of a scanner to modulate spatial interferograms for in vivo full-range Fourier-domain optical coherence tomography,” Opt. Lett. 32(23), 3423–3425 (2007). [CrossRef] [PubMed]

24.

R. A. Leitgeb, R. Michaely, T. Lasser, and S. C. Sekhar, “Complex ambiguity-free Fourier domain optical coherence tomography through transverse scanning,” Opt. Lett. 32(23), 3453–3455 (2007). [CrossRef] [PubMed]

25.

C. T. Wu, T. T. Chi, C. K. Lee, Y. W. Kiang, C. C. Yang, and C. P. Chiang, “Method for suppressing the mirror image in Fourier-domain optical coherence tomography,” Opt. Lett. 36(15), 2889–2891 (2011). [CrossRef] [PubMed]

26.

C. T. Wu, T. T. Chi, Y. W. Kiang, and C. C. Yang, “Computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography,” Opt. Express 20(8), 8270–8283 (2012), doi:. [CrossRef] [PubMed]

27.

J. Kalkman, A. V. Bykov, G. J. Streekstra, and T. G. van Leeuwen, “Multiple scattering effects in Doppler optical coherence tomography of flowing blood,” Phys. Med. Biol. 57(7), 1907–1917 (2012). [CrossRef] [PubMed]

OCIS Codes
(110.4500) Imaging systems : Optical coherence tomography
(170.3880) Medical optics and biotechnology : Medical and biological imaging

ToC Category:
Imaging Systems

History
Original Manuscript: September 18, 2012
Revised Manuscript: November 3, 2012
Manuscript Accepted: November 4, 2012
Published: December 7, 2012

Virtual Issues
Vol. 8, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Ting-Ta Chi, Chiung-Ting Wu, Chen-Chin Liao, Yi-Chou Tu, Yean-Woei Kiang, and C. C. Yang, "Two-reference swept-source optical coherence tomography of high operation flexibility," Opt. Express 20, 28418-28430 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-27-28418


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References

  1. R. A. Leitgeb, C. K. Hitzenberger, A. F. Fercher, and T. Bajraszewski, “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography,” Opt. Lett.28(22), 2201–2203 (2003). [CrossRef] [PubMed]
  2. M. Wojtkowski, A. Kowalczyk, R. Leitgeb, and A. F. Fercher, “Full range complex spectral optical coherence tomography technique in eye imaging,” Opt. Lett.27(16), 1415–1417 (2002). [CrossRef] [PubMed]
  3. P. Targowski, M. Wojtkowski, A. Kowalczyk, T. Bajraszewski, M. Szkulmowski, and I. Gorczynska, “Complex spectral OCT in human eye imaging in vivo,” Opt. Commun.229(1-6), 79–84 (2004). [CrossRef]
  4. Y. Yasuno, S. Makita, T. Endo, G. Aoki, H. Sumimura, M. Itoh, and T. Yatagai, “One-shot-phase-shifting Fourier domain optical coherence tomography by reference wavefront tilting,” Opt. Express12(25), 6184–6191 (2004). [CrossRef] [PubMed]
  5. J. Zhang, W. Jung, J. Nelson, and Z. Chen, “Full range polarization-sensitive Fourier domain optical coherence tomography,” Opt. Express12(24), 6033–6039 (2004). [CrossRef] [PubMed]
  6. E. Götzinger, M. Pircher, R. Leitgeb, and C. Hitzenberger, “High speed full range complex spectral domain optical coherence tomography,” Opt. Express13(2), 583–594 (2005). [CrossRef] [PubMed]
  7. S. Yun, G. Tearney, J. de Boer, and B. Bouma, “Removing the depth-degeneracy in optical frequency domain imaging with frequency shifting,” Opt. Express12(20), 4822–4828 (2004). [CrossRef] [PubMed]
  8. J. Zhang, J. S. Nelson, and Z. Chen, “Removal of a mirror image and enhancement of the signal-to-noise ratio in Fourier-domain optical coherence tomography using an electro-optic phase modulator,” Opt. Lett.30(2), 147–149 (2005). [CrossRef] [PubMed]
  9. A. M. Davis, M. A. Choma, and J. A. Izatt, “Heterodyne swept-source optical coherence tomography for complete complex conjugate ambiguity removal,” J. Biomed. Opt.10(6), 064005 (2005). [CrossRef] [PubMed]
  10. A. H. Dhalla and J. A. Izatt, “Complete complex conjugate resolved heterodyne swept-source optical coherence tomography using a dispersive optical delay line,” Biomed. Opt. Express2(5), 1218–1232 (2011). [CrossRef] [PubMed]
  11. M. A. Choma, C. Yang, and J. A. Izatt, “Instantaneous quadrature low-coherence interferometry with 3 x 3 fiber-optic couplers,” Opt. Lett.28(22), 2162–2164 (2003). [CrossRef] [PubMed]
  12. K. S. Lee, P. Meemon, K. Hsu, W. Dallas, and J. P. Rolland, “Dual-reference full-range frequency domain optical coherence tomography,” Proc. SPIE7170, 717004, 717004-7 (2009). [CrossRef]
  13. S. Zotter, M. Pircher, E. Götzinger, T. Torzicky, M. Bonesi, and C. K. Hitzenberger, “Sample motion-insensitive, full-range, complex, spectral-domain optical-coherence tomography,” Opt. Lett.35(23), 3913–3915 (2010). [CrossRef] [PubMed]
  14. K. S. Lee, P. Meemon, W. Dallas, K. Hsu, and J. P. Rolland, “Dual detection full range frequency domain optical coherence tomography,” Opt. Lett.35(7), 1058–1060 (2010). [CrossRef] [PubMed]
  15. P. Meemon, K.-S. Lee, and J. P. Rolland, “Doppler imaging with dual-detection full-range frequency domain optical coherence tomography,” Biomed. Opt. Express1(2), 537–552 (2010). [CrossRef] [PubMed]
  16. Y. Yasuno, S. Makita, T. Endo, G. Aoki, M. Itoh, and T. Yatagai, “Simultaneous B-M-mode scanning method for real-time full-range Fourier domain optical coherence tomography,” Appl. Opt.45(8), 1861–1865 (2006). [CrossRef] [PubMed]
  17. R. K. Wang, “In vivo full range complex Fourier domain optical coherence tomography,” Appl. Phys. Lett.90(5), 054103 (2007). [CrossRef]
  18. S. Makita, T. Fabritius, and Y. Yasuno, “Full-range, high-speed, high-resolution 1 microm spectral-domain optical coherence tomography using BM-scan for volumetric imaging of the human posterior eye,” Opt. Express16(12), 8406–8420 (2008). [CrossRef] [PubMed]
  19. S. Vergnole, G. Lamouche, and M. L. Dufour, “Artifact removal in Fourier-domain optical coherence tomography with a piezoelectric fiber stretcher,” Opt. Lett.33(7), 732–734 (2008). [CrossRef] [PubMed]
  20. K. Wang, Z. Ding, Y. Zeng, J. Meng, and M. Chen, “Sinusoidal B-M method based spectral domain optical coherence tomography for the elimination of complex-conjugate artifact,” Opt. Express17(19), 16820–16833 (2009). [CrossRef] [PubMed]
  21. F. Jaillon, S. Makita, M. Yabusaki, and Y. Yasuno, “Parabolic BM-scan technique for full range Doppler spectral domain optical coherence tomography,” Opt. Express18(2), 1358–1372 (2010). [CrossRef] [PubMed]
  22. B. Baumann, M. Pircher, E. Götzinger, and C. K. Hitzenberger, “Full range complex spectral domain optical coherence tomography without additional phase shifters,” Opt. Express15(20), 13375–13387 (2007). [CrossRef] [PubMed]
  23. L. An and R. K. Wang, “Use of a scanner to modulate spatial interferograms for in vivo full-range Fourier-domain optical coherence tomography,” Opt. Lett.32(23), 3423–3425 (2007). [CrossRef] [PubMed]
  24. R. A. Leitgeb, R. Michaely, T. Lasser, and S. C. Sekhar, “Complex ambiguity-free Fourier domain optical coherence tomography through transverse scanning,” Opt. Lett.32(23), 3453–3455 (2007). [CrossRef] [PubMed]
  25. C. T. Wu, T. T. Chi, C. K. Lee, Y. W. Kiang, C. C. Yang, and C. P. Chiang, “Method for suppressing the mirror image in Fourier-domain optical coherence tomography,” Opt. Lett.36(15), 2889–2891 (2011). [CrossRef] [PubMed]
  26. C. T. Wu, T. T. Chi, Y. W. Kiang, and C. C. Yang, “Computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography,” Opt. Express20(8), 8270–8283 (2012), doi:. [CrossRef] [PubMed]
  27. J. Kalkman, A. V. Bykov, G. J. Streekstra, and T. G. van Leeuwen, “Multiple scattering effects in Doppler optical coherence tomography of flowing blood,” Phys. Med. Biol.57(7), 1907–1917 (2012). [CrossRef] [PubMed]

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