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
In 1991, Huang proposed the principle of OCT (optical coherence technology), and used it in retinal imaging . 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 [1–4], dermatology [5–8], cardiology [8–11] and gastroenterology [12–14]. 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  (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 , optical feedback vibration measurements , optical feedback tomography technique , and optical feedback displacement measurement . 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
A simplified model of frequency-shifted feedback based on the microchip laser is shown in Fig. 1 . The two surfaces of the microchip laser form the laser cavity, while the output surface of the laser and the reflector constitute the external cavity. In the case of optical feedback, the light emitted by the laser is reflected by the reflector in the external cavity and returns to the laser cavity through the same optical path. The feedback light carrying the information of the external cavity is modulating the inner light field of the laser, resulting in the changes of characteristics (power, phase, polarization, etc.) of the laser. If detecting and demodulating the output of the laser, we can get these varieties, from which the information of the external cavity can be achieved. Moreover, with the frequency shifting elements (as the Acousto-optical modulator in Fig. 1) in the external cavity, the feedback light is frequency shifted by an amount of in a round trip, which can provide an additional magnification in the laser's output power. By using the rate equation model, the optical power modulation of frequency shifted feedback in microchip laser is derived as follows [15,18,19]
In the microchip laser, 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 since 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 . Firstly, the microchip Nd:YAG laser emits the single longitudinal mode laser, whose frequency is. 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 (). 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 (). 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) 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) (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 , 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 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.
4.1 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
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 ). 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 is, we can get
If we suppose that when the vertical scanning distance of the translation stage is s, the focus spot of the laser beam moves from the upper surface of the sample to the lower surface, the following formula can be derived:Eq. (4) and Eq. (5) simultaneously, we haveEq. (6), the relationship between the thickness of the sample d and the longitudinal displacement of the translation stage s is related to the refractive index of the layer. In regular situation, the amount of the movement of the stage is smaller than the physical depth in the sample. However, if the medium of a layer is air, the thickness of this layer is the same as the movement amount of translation stage.
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.
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.
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]
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]
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]
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]