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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 10 — May. 20, 2013
  • pp: 11819–11826
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Inner structure detection by optical tomography technology based on feedback of microchip Nd:YAG lasers

Chunxin Xu, Shulian Zhang, Yidong Tan, and Shijie Zhao  »View Author Affiliations


Optics Express, Vol. 21, Issue 10, pp. 11819-11826 (2013)
http://dx.doi.org/10.1364/OE.21.011819


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Abstract

We describe a new optical tomography technology based on feedback of microchip Nd:YAG lasers. In the case of feedback light frequency-shifted, light can be magnified by a fact of 106 in the Nd:YAG microchip lasers, which makes it possible to realize optical tomography with a greater depth than current optical tomography. The results of the measuring and imaging of kinds of samples are presented, which demonstrate the feasibility and potential of this approach in the inner structure detection. The system has a lateral resolution of ~1μm, a vertical resolution of 15μm and a longitudinal scanning range of over 10mm.

© 2013 OSA

1. Introduction

In 1991, Huang proposed the principle of OCT (optical coherence technology), and used it in retinal imaging [1

1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

]. Compared with the traditional medical imaging techniques, OCT has the advantages of non-contact, non-damaging, high-resolution, and real-time. In recent years, this technology has been developed rapidly and applied to many medical areas gradually, such as ophthalmology [1

1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

4

4. W. Drexler and J. G. Fujimoto, “State-of-the-art retinal optical coherence tomography,” Prog. Retin. Eye Res. 27(1), 45–88 (2008). [CrossRef] [PubMed]

], dermatology [5

5. T. Gambichler, V. Jaedicke, and S. Terras, “Optical coherence tomography in dermatology: technical and clinical aspects,” Arch. Dermatol. Res. 303(7), 457–473 (2011). [CrossRef] [PubMed]

8

8. A. F. Fercher, “Optical coherence tomography - development, principles, applications,” Z. Med. Phys. 20(4), 251–276 (2010). [CrossRef] [PubMed]

], cardiology [8

8. A. F. Fercher, “Optical coherence tomography - development, principles, applications,” Z. Med. Phys. 20(4), 251–276 (2010). [CrossRef] [PubMed]

11

11. F. Alfonso, M. Paulo, N. Gonzalo, J. Dutary, P. Jimenez-Quevedo, V. Lennie, J. Escaned, C. Bañuelos, R. Hernandez, and C. Macaya, “Diagnosis of spontaneous coronary artery dissection by optical coherence tomography,” J. Am. Coll. Cardiol. 59(12), 1073–1079 (2012). [CrossRef] [PubMed]

] and gastroenterology [12

12. E. Zagaynova, N. Gladkova, N. Shakhova, G. Gelikonov, and V. Gelikonov, “Endoscopic OCT with forward-looking probe: clinical studies in urology and gastroenterology,” J Biophotonics 1(2), 114–128 (2008). [CrossRef] [PubMed]

14

14. E. Osiac, A. Săftoiu, D. I. Gheonea, I. Mandrila, and R. Angelescu, “Optical coherence tomography and Doppler optical coherence tomography in the gastrointestinal tract,” World J. Gastroenterol. 17(1), 15–20 (2011). [CrossRef] [PubMed]

]. However, the depth of OCT is generally about 2~3mm. A greater depth will lead to further weakening of the intensity of the scattering light, which makes a lower signal to noise ratio. Therefore, OCT technology has difficulties in improving the imaging depth currently.

In the case of frequency-shifted feedback, the microchip laser shows a very high optical feedback sensitivity [15

15. E. Lacot, R. Day, and F. Stoeckel, “Coherent laser detection by frequency-shifted optical feedback,” Phys. Rev. A 64(4), 043815 (2001). [CrossRef]

] (106, at least two orders of magnitude lager than Laser Diode). This character has been used in a variety of physical measurements, such as Doppler velocimetry based on feedback of microchip laser [16

16. K. Otsuka, R. Kawai, Y. Asakawa, and T. Fukazawa, “Highly sensitive self-mixing measurement of Brillouin scattering with a laser-diode-pumped microchip LiNdP(4)O(12) laser,” Opt. Lett. 24(24), 1862–1864 (1999). [CrossRef] [PubMed]

], optical feedback vibration measurements [17

17. K. Otsuka, K. Abe, J. Y. Ko, and T. S. Lim, “Real-time nanometer-vibration measurement with a self-mixing microchip solid-state laser,” Opt. Lett. 27(15), 1339–1341 (2002). [CrossRef] [PubMed]

], optical feedback tomography technique [18

18. E. Lacot, R. Day, and F. Stoeckel, “Laser optical feedback tomography,” Opt. Lett. 24(11), 744–746 (1999). [CrossRef] [PubMed]

], and optical feedback displacement measurement [19

19. X. Wan, D. Li, and S. L. Zhang, “Quasi-common-path laser feedback interferometry based on frequency shifting and multiplexing,” Opt. Lett. 32(4), 367–369 (2007). [CrossRef] [PubMed]

]. Due to the high sensitivity, the feedback of the microchip laser has a unique advantage in the optical detection of weak signals. It is expected that the depth of measurement and imaging may exceed current OCT.

2. The principle of frequency-shifted feedback based on the microchip laser

In the microchip laser, G(ωr)can be up to 106 in magnitude, while it is only 103 for Laser Diode. Therefore, we can adjust the external frequency shift to close to the relaxation frequency (cannot be ωrsince it will bring large noise and the nonlinear effect) and take advantage of this amplification to the feedback light strength to improve the detection depth of the tomography technique. The magnification is an important characteristic in our system and different from the traditional laser interferometry technology (existing OCT principle-based); by using it, it's possible to get a better result than OCT.

Based on the principle of the microchip laser frequency-shifted feedback, we set up the experimental system of the laser feedback tomography, as shown in Fig. 2
Fig. 2 Configuration of the optical tomography technology based on feedback of microchip Nd:YAG laser. ML, microchip laser; BS, beam splitter; PD, photodiode; LIA, lock-in amplifier; L1~L3, lens; PIN, pinhole; AOM1, AOM2, Acousto-optical demodulator; BE, beam expander; B1, reflect mirror; Obj, objective; SA, sample; ST, stage; SG, signal generator; PC, computer.
. Firstly, the microchip Nd:YAG laser emits the single longitudinal mode laser, whose frequency isω. The light is converged by L1 to a pinhole, and collimated by L2. Then the light is divided into two beams by the splitter. The transmitted light passes through the AOMs to get the frequency (ω+Ω). After that, the laser beam goes through the beam expander lens, is reflected by B1 to change the direction and finally converged to the sample through the objective lens. The scattering light from the sample returns to the laser via the original path, which is frequency shifted again to achieve a frequency of (ω+2Ω). The pinhole makes the feedback light mostly be scattered by the area near the convergence spot of the sample in order to get the tomography effect. The laser optical power is modulated by the feedback light, so we can get the sample's information by detecting the output of the laser using PD and demodulating the measurement signal using LIA. It should be noted here that in our system the relaxation frequency of the laser is less than 1.5MHz, while the general amount of frequency shift caused by the AOM is dozens of megahertz. Consequently, we use two AOMs whose frequency shifts are 70MHz and 70MHz ± 1MHz respectively to generate the frequency shift needed.

To realize the light scanning in the inner structure of the sample, the objective lens is fixed in a vertical translation stage to scan in the longitudinal direction, while the sample is placed on a two-dimensional translation stage to get the horizontal movement. Then the scattering intensity in different points of the sample can be achieved, and converted to grayscale or pseudo-color images, which can reflect the inner structure of the sample. In the system, we have adopted an objective with long working distance up to 20.4mm to increase the scanning range in the direction of the depth.

3. Experiment results

By the experiment system described above, we have done a series of experiments to test the inner structure detection ability of the frequency-shifted feedback system.

3.1 Longitudinal scanning results of multi-layer interfaces

In this experiment, we use two samples to scan in the depth direction. Figure 3(a)
Fig. 3 Longitudinal scanning results of multi-layer interface: (a) seven-layer glasses; (b) multi-layer sample.
is the scanning result of the seven-layer glasses (thickness of about 8mm). It’s clearly that there are strong feedback signals from eight layered interfaces. Another sample is composed of the olive oil (2mm), honey solution (9mm), plastic containers (2mm) and aluminum. Thus, we get four interfaces (air - oil, oil - honey solution, honey solution - plastic and plastic - aluminum). The experimental result is shown in Fig. 3(b), and we can see that the feedback signals present the four interfaces mentioned above. Under the existing experimental conditions, the vertical resolution is about 15 microns. The results show that our system can realize the function of tomography and the scanning range can be over 10mm, which are the bases of our following experiments.

3.2 Two-dimensional / Three-dimensional scanning and image reconstruction

3.2.1 Scan result of 3 layer glass reactor

In this experiment, we add one-dimensional horizontal scanning on the basis of the vertical one. The demodulated signals are collected by the data card to the computer for image reconstruction, and then we can get the vertical-sectional image of the sample. A three layer (1mm each) glass reactor is stacked on a black surface to act as the sample. As is shown in Fig. 4(a)
Fig. 4 Scanning results of 3 layer glass reactor: (a) two-dimensional scanning image of 3 layer glass reactor; (b) single longitude scanning result of 3 layer glass reactor.
(4mm x 2.5mm, 2.5mm is for the longitudinal direction), except the top layer, there are two close lines in other interfaces, which is due to the air gap(tens of microns) between two adjacent surfaces. Besides, we take an image of a single line in the vertical direction. As Fig. 4(b) shows, each signal peak represents a surface and there is the air gap between every two adjacent peaks. In addition, while the sample is 3mm thick, the actual displacement of the vertical translation stage is less than 3mm due to the effect of the refractive index (see Section 4.2 for detail).

3.2.2 Scan imaging of foam structure

In this experiment, the foam is taken as the sample. The scanning range is 8mm x3.5mm (3.5mm is for the longitudinal direction). As shown in Fig. 5
Fig. 5 Two-dimensional scan image of foam.
, a typical hollow mesh structure of the foam is obtained.

In order to get more information, we add the third dimensional scanning and achieve a three-dimensional tomography for the foam. Thus, the scanning range is 8mm × 10mm × 3.5mm now. Figure 6
Fig. 6 Three-dimensional scan image of foam (Media 1).
shows one view of the reconstruction. From the picture, we can see the surface of the foam as well as the inner structure. If observing from other perspectives, we can obtain more information about the foam.

4. Discussion

4.1 The resolution of the system

The resolution is a key parameter, which is important in making a judgment on the performance of a measurement device. The lateral resolution of the system is determined by the optical parameter of the objective lens and the wavelength; it can be expressed as

Δx=0.61λ2NA
(2)

Wherein, λ represents the wavelength of incident light, and NA is the numerical aperture of the objective lens. In the experiments, we use the wavelength of 1.064μm while the numerical aperture of the objective lens is 0.42. Taking them into the Eq. (2), the lateral resolution can be calculated as 1.1μm. Using a standard grating (made by MikroMasch corporation with the period of 3.0μm) as a sample, the steps of the grating can be distinguished clearly. It means that the transverse resolution is at least better than 1.5μm and sufficient for the measurement.

Vertical resolution of this system is similar to the traditional confocal system whose vertical resolution is determined by the defocus response curve. The narrower the full width at half maximum of the defocus response curve is, the higher the longitudinal resolution can be as well as the ability of tomography. With the system we build, the vertical resolution is about 15μm while the working range can reach 20.4mm.

4.2 The physical distance reflected by the image

In the images we reconstruct, the coordinate data indicate the displacements of the translation stages. Therefore, the vertical one doesn’t mean the physical thickness of the sample. The effect of the refractive index of the sample should be considered when we want to get the physical depth.

Take the single-layer sample for example (as shown in Fig. 7
Fig. 7 The diagram of the single-layer sample.
). Assuming that the optical axis of the light beam is perpendicular with the surface of the sample, the numerical aperture of the objective lens is NA and the incident angle of the light isθ, we can get
NA=nsinθ=sinθ(n=1)
(3)
According to the principle of refraction, we have

nsinθ=n1sinθ1=NA
(4)

4.3 The detection range of the system

Since the system is a point to point scanning system, the horizontal scanning range has no practical limit, and is determined by the scanning range of the translation stage. As to the vertical range, the limit is the working distance of the objective lens. Furthermore, the actual depth the system can detect is influenced by several factors, such as the optical power, the absorption and scattering of the sample, the noise, etc., which make it difficult to give a certain value. We can get a large detection depth if the sample is good at the optical transparency, like the multilayered liquid we describe in Section 3.1 whose total depth is more than 10mm. However, it can only reach 2 ~3 mm in the foam as shown in Fig. 5.

4.4 The applications of the system

According to the results presented in Section 3, the system has the capability of penetration in the low reflecting and scattering medium, which makes it possible to achieve the tomographic imaging. Using this feature, we can measure the internal structure of the samples made of low-reflection or high-scattering materials. In addition to the foregoing mentioned samples (multilayered liquid, multi-layered glass, foam), it can also be used to measure the interior etching structure of the MEMS device to see whether it’s fabricated as demanded, or the biological sample as OCT. In view of the high sensitivity of this approach, it is possible to get better results than traditional methods.

5. Conclusion

In the case of frequency-shifted feedback, light can be magnified by a fact of 106 in the microchip Nd:YAG lasers, which is advantaged in the weak light detection and makes it possible to increase the imaging depth in tomography. As shown in the scanning results of different samples, our optical feedback system based on microchip Nd:YAG laser is capable of tomographic imaging of low reflecting and high scattering medium, so it can be applied to the internal fine structure measurements of the devices made of transparent or translucent materials as well as the biomedical detection. In the existing experimental conditions, the system has a lateral resolution of ~1μm, a vertical resolution of ~15μm and an imaging depth of up to 10mm.

Our future work will focus on the following three aspects: 1. the improvement of the resolution. In some cases, the vertical resolution needs to be in the order of nanometer, which can be achieved by using the phase portion of the signal in our system. And it's very tough work; 2. the subsequent image processing. Digital image processing is a very important step. Efficient algorithm is able to improve the quality of the image, enhance detailed information submerged by the noise, and thereby increase the detection range. For example, Fig. 6 is directly obtained from the original data without any image processing and seems to be a little crude. We can do a better job if applying the subsequent image processing; 3. applications in other measurement areas, such as biomedical imaging as OCT.

Acknowledgments

This project is supported by National Natural Science Foundation of China (Grant No. 30870662) and Natural Science Foundation of Beijing, China (Grant No. 3091002).

References and links

1.

D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

2.

M. L. Gabriele, G. Wollstein, H. Ishikawa, L. Kagemann, J. Xu, L. S. Folio, and J. S. Schuman, “Optical coherence tomography: history, current status, and laboratory work,” Invest. Ophthalmol. Vis. Sci. 52(5), 2425–2436 (2011). [CrossRef] [PubMed]

3.

D. F. Kiernan, W. F. Mieler, and S. M. Hariprasad, “Spectral-domain optical coherence tomography: a comparison of modern high-resolution retinal imaging systems,” Am. J. Ophthalmol. 149(1), 18–31, 31.e2 (2010). [CrossRef] [PubMed]

4.

W. Drexler and J. G. Fujimoto, “State-of-the-art retinal optical coherence tomography,” Prog. Retin. Eye Res. 27(1), 45–88 (2008). [CrossRef] [PubMed]

5.

T. Gambichler, V. Jaedicke, and S. Terras, “Optical coherence tomography in dermatology: technical and clinical aspects,” Arch. Dermatol. Res. 303(7), 457–473 (2011). [CrossRef] [PubMed]

6.

X. Liang and S. A. Boppart, “Biomechanical properties of in vivo human skin drom dynamic optical coherence elastography,” IEEE T. Biomed. Eng. (N.Y.) 57(4), 953–959 (2010). [CrossRef]

7.

A. Alex, B. Považay, B. Hofer, S. Popov, C. Glittenberg, S. Binder, and W. Drexler, “Multispectral in vivo three-dimensional optical coherence tomography of human skin,” J. Biomed. Opt. 15(2), 026025 (2010). [CrossRef] [PubMed]

8.

A. F. Fercher, “Optical coherence tomography - development, principles, applications,” Z. Med. Phys. 20(4), 251–276 (2010). [CrossRef] [PubMed]

9.

A. Tanaka, K. Shimada, G. J. Tearney, H. Kitabata, H. Taguchi, S. Fukuda, M. Kashiwagi, T. Kubo, S. Takarada, K. Hirata, M. Mizukoshi, J. Yoshikawa, B. E. Bouma, and T. Akasaka, “Conformational change in coronary artery structure assessed by optical coherence tomography in patients with vasospastic angina,” J. Am. Coll. Cardiol. 58(15), 1608–1613 (2011). [CrossRef] [PubMed]

10.

F. Abtahian and I. K. Jang, “Optical coherence tomography: basics, current application and future potential,” Curr. Opin. Pharmacol. 12(5), 583–591 (2012). [CrossRef] [PubMed]

11.

F. Alfonso, M. Paulo, N. Gonzalo, J. Dutary, P. Jimenez-Quevedo, V. Lennie, J. Escaned, C. Bañuelos, R. Hernandez, and C. Macaya, “Diagnosis of spontaneous coronary artery dissection by optical coherence tomography,” J. Am. Coll. Cardiol. 59(12), 1073–1079 (2012). [CrossRef] [PubMed]

12.

E. Zagaynova, N. Gladkova, N. Shakhova, G. Gelikonov, and V. Gelikonov, “Endoscopic OCT with forward-looking probe: clinical studies in urology and gastroenterology,” J Biophotonics 1(2), 114–128 (2008). [CrossRef] [PubMed]

13.

G. Zuccaro, N. Gladkova, J. Vargo, F. Feldchtein, E. Zagaynova, D. Conwell, G. Falk, J. Goldblum, J. Dumot, J. Ponsky, G. Gelikonov, B. Davros, E. Donchenko, and J. Richter, “Optical coherence tomography of the esophagus and proximal stomach in health and disease,” Am. J. Gastroenterol. 96(9), 2633–2639 (2001). [CrossRef] [PubMed]

14.

E. Osiac, A. Săftoiu, D. I. Gheonea, I. Mandrila, and R. Angelescu, “Optical coherence tomography and Doppler optical coherence tomography in the gastrointestinal tract,” World J. Gastroenterol. 17(1), 15–20 (2011). [CrossRef] [PubMed]

15.

E. Lacot, R. Day, and F. Stoeckel, “Coherent laser detection by frequency-shifted optical feedback,” Phys. Rev. A 64(4), 043815 (2001). [CrossRef]

16.

K. Otsuka, R. Kawai, Y. Asakawa, and T. Fukazawa, “Highly sensitive self-mixing measurement of Brillouin scattering with a laser-diode-pumped microchip LiNdP(4)O(12) laser,” Opt. Lett. 24(24), 1862–1864 (1999). [CrossRef] [PubMed]

17.

K. Otsuka, K. Abe, J. Y. Ko, and T. S. Lim, “Real-time nanometer-vibration measurement with a self-mixing microchip solid-state laser,” Opt. Lett. 27(15), 1339–1341 (2002). [CrossRef] [PubMed]

18.

E. Lacot, R. Day, and F. Stoeckel, “Laser optical feedback tomography,” Opt. Lett. 24(11), 744–746 (1999). [CrossRef] [PubMed]

19.

X. Wan, D. Li, and S. L. Zhang, “Quasi-common-path laser feedback interferometry based on frequency shifting and multiplexing,” Opt. Lett. 32(4), 367–369 (2007). [CrossRef] [PubMed]

OCIS Codes
(040.2840) Detectors : Heterodyne
(180.1790) Microscopy : Confocal microscopy
(180.5810) Microscopy : Scanning microscopy
(110.6955) Imaging systems : Tomographic imaging

ToC Category:
Microscopy

History
Original Manuscript: April 4, 2013
Revised Manuscript: April 27, 2013
Manuscript Accepted: April 27, 2013
Published: May 7, 2013

Virtual Issues
Vol. 8, Iss. 6 Virtual Journal for Biomedical Optics

Citation
Chunxin Xu, Shulian Zhang, Yidong Tan, and Shijie Zhao, "Inner structure detection by optical tomography technology based on feedback of microchip Nd:YAG lasers," Opt. Express 21, 11819-11826 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-10-11819


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References

  1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science254(5035), 1178–1181 (1991). [CrossRef] [PubMed]
  2. M. L. Gabriele, G. Wollstein, H. Ishikawa, L. Kagemann, J. Xu, L. S. Folio, and J. S. Schuman, “Optical coherence tomography: history, current status, and laboratory work,” Invest. Ophthalmol. Vis. Sci.52(5), 2425–2436 (2011). [CrossRef] [PubMed]
  3. D. F. Kiernan, W. F. Mieler, and S. M. Hariprasad, “Spectral-domain optical coherence tomography: a comparison of modern high-resolution retinal imaging systems,” Am. J. Ophthalmol.149(1), 18–31, 31.e2 (2010). [CrossRef] [PubMed]
  4. W. Drexler and J. G. Fujimoto, “State-of-the-art retinal optical coherence tomography,” Prog. Retin. Eye Res.27(1), 45–88 (2008). [CrossRef] [PubMed]
  5. T. Gambichler, V. Jaedicke, and S. Terras, “Optical coherence tomography in dermatology: technical and clinical aspects,” Arch. Dermatol. Res.303(7), 457–473 (2011). [CrossRef] [PubMed]
  6. X. Liang and S. A. Boppart, “Biomechanical properties of in vivo human skin drom dynamic optical coherence elastography,” IEEE T. Biomed. Eng. (N.Y.)57(4), 953–959 (2010). [CrossRef]
  7. A. Alex, B. Považay, B. Hofer, S. Popov, C. Glittenberg, S. Binder, and W. Drexler, “Multispectral in vivo three-dimensional optical coherence tomography of human skin,” J. Biomed. Opt.15(2), 026025 (2010). [CrossRef] [PubMed]
  8. A. F. Fercher, “Optical coherence tomography - development, principles, applications,” Z. Med. Phys.20(4), 251–276 (2010). [CrossRef] [PubMed]
  9. A. Tanaka, K. Shimada, G. J. Tearney, H. Kitabata, H. Taguchi, S. Fukuda, M. Kashiwagi, T. Kubo, S. Takarada, K. Hirata, M. Mizukoshi, J. Yoshikawa, B. E. Bouma, and T. Akasaka, “Conformational change in coronary artery structure assessed by optical coherence tomography in patients with vasospastic angina,” J. Am. Coll. Cardiol.58(15), 1608–1613 (2011). [CrossRef] [PubMed]
  10. F. Abtahian and I. K. Jang, “Optical coherence tomography: basics, current application and future potential,” Curr. Opin. Pharmacol.12(5), 583–591 (2012). [CrossRef] [PubMed]
  11. F. Alfonso, M. Paulo, N. Gonzalo, J. Dutary, P. Jimenez-Quevedo, V. Lennie, J. Escaned, C. Bañuelos, R. Hernandez, and C. Macaya, “Diagnosis of spontaneous coronary artery dissection by optical coherence tomography,” J. Am. Coll. Cardiol.59(12), 1073–1079 (2012). [CrossRef] [PubMed]
  12. E. Zagaynova, N. Gladkova, N. Shakhova, G. Gelikonov, and V. Gelikonov, “Endoscopic OCT with forward-looking probe: clinical studies in urology and gastroenterology,” J Biophotonics1(2), 114–128 (2008). [CrossRef] [PubMed]
  13. G. Zuccaro, N. Gladkova, J. Vargo, F. Feldchtein, E. Zagaynova, D. Conwell, G. Falk, J. Goldblum, J. Dumot, J. Ponsky, G. Gelikonov, B. Davros, E. Donchenko, and J. Richter, “Optical coherence tomography of the esophagus and proximal stomach in health and disease,” Am. J. Gastroenterol.96(9), 2633–2639 (2001). [CrossRef] [PubMed]
  14. E. Osiac, A. Săftoiu, D. I. Gheonea, I. Mandrila, and R. Angelescu, “Optical coherence tomography and Doppler optical coherence tomography in the gastrointestinal tract,” World J. Gastroenterol.17(1), 15–20 (2011). [CrossRef] [PubMed]
  15. E. Lacot, R. Day, and F. Stoeckel, “Coherent laser detection by frequency-shifted optical feedback,” Phys. Rev. A64(4), 043815 (2001). [CrossRef]
  16. K. Otsuka, R. Kawai, Y. Asakawa, and T. Fukazawa, “Highly sensitive self-mixing measurement of Brillouin scattering with a laser-diode-pumped microchip LiNdP(4)O(12) laser,” Opt. Lett.24(24), 1862–1864 (1999). [CrossRef] [PubMed]
  17. K. Otsuka, K. Abe, J. Y. Ko, and T. S. Lim, “Real-time nanometer-vibration measurement with a self-mixing microchip solid-state laser,” Opt. Lett.27(15), 1339–1341 (2002). [CrossRef] [PubMed]
  18. E. Lacot, R. Day, and F. Stoeckel, “Laser optical feedback tomography,” Opt. Lett.24(11), 744–746 (1999). [CrossRef] [PubMed]
  19. X. Wan, D. Li, and S. L. Zhang, “Quasi-common-path laser feedback interferometry based on frequency shifting and multiplexing,” Opt. Lett.32(4), 367–369 (2007). [CrossRef] [PubMed]

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