For the first time, an all-solid tellurite optical glass rod with a transversely disordered refractive index profile was fabricated successfully as a medium to study the transverse localization of light and near-infrared (NIR) optical image transport. Two tellurite glass compositions of 70TeO2-8Li2O-17WO3-3MoO3-2Nb2O5 (TLWMN) and 75TeO2-15ZnO-5Na2O-5La2O3 (TZNL) which have a small difference in softening temperature (about 0.5 °C), compatible thermal expansions from room to 400 °C and broad transmission range from about 0.4 up to 6.0 µm were developed for a successful fabrication process. The tellurite transversely disordered optical rod (TDOR) consists of high and low-index units (TLWMN and TZNL, respectively). The diameter of each unit is 1.0 μm and their refractive index difference was about 0.095 at 1.55 µm. Experimental results showed that after a CW probe beam at 1.55 μm propagated in a 10-cm-long tellurite TDOR, the beam became localized. In addition, NIR optical images at 1.55 μm of numbers on a test target were transported. The captured images at the output facet of the tellurite TDOR are visually clear with high contrast and high brightness. The quality of our transported optical images can be comparable or higher than the results which were obtained by a polymer Anderson localized fiber and by a commercially available multicore imaging optical fiber.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
From the 1990s, infrared (IR) laser sources far from the visible range have been widely used for the study of biological systems including lipids, proteins, peptides, bio-membranes, nucleic acids, animal tissues, microbial cells, plants and clinical samples [1–3]. For medical applications, IR light sources have been extensively used for breast cancer detection, brain imaging, vascular disorder diagnosis, heart treatment and so on . One of the great advantages of using IR laser sources is the harmless interaction of small doses of IR radiations with biomedical objects . In addition, the near infrared (NIR) region from 700 to 2500 nm has been known as the most efficient region for lights to be transmitted through biological tissues such as skin and blood [5, 6]. Another important milestone was marked in the 2000s when a large number of publications and patents concerning IR spectroscopy and IR imaging were emerged in biological and medical communities [1, 5]. For instance, S. Mil’shtein et al. has developed a low-cost IR imaging system which allows one to scan biological objects like human arms, legs, palms, and fingers to visualize different tissues, tendons, muscles, ligaments, blood vessels, bones, and cartilages . This IR imaging technology can allow the visualization of moving biomedical objects and allow the doctor to touch or examine them physically while imaging is proceeding. This novel feature is not possible for the conventional x-ray examination which requires the object to be still . Due to the desire to perform deep tissue imaging for in vivo biomedical applications, the importance of long wavelength and NIR imaging has dramatically increased . Up to the present, the developments of NIR image generation and NIR image transport are still active research fields.
In practice, multicore optical fibers have been employed to transport optical images [9–15]. However, the quality of the image transport is limited by the pixelation effect and the inter-core coupling results in low-contrast and blurred images [9, 10]. Recently, optical image transport using transverse localization of light has been demonstrated in a polymer disordered optical fiber made of poly methyl methacrylate (PMMA) and poly styrene (PS) whose refractive index difference is 0.1 [16, 17]. The transported visual image quality was even better or comparable with those obtained by using commercial multicore image fiber. The numerical and experimental results showed that highly disordered optical fibers with large refractive index difference can transport high quality optical images. Moreover, as compared with other advanced fiber-based imaging methods, no additional pre- and post-processing is required to obtain the image and it can be easily operated in a fully flexible endoscopic system . Those properties are very advantageous to optical imaging applications in biological and medical fields. On the other hand, the image resolution in polymer disordered optical fibers is currently limited by the optical attenuation as well as the quality of cleaving and polishing surfaces of the polymer fibers . A disordered silica glass optical fiber with random air hole, high refractive index difference and high filling fraction has been proposed as an alternative medium but the image transport properties have not been demonstrated yet .
Based on the shortcomings of previous works and the knowledge of highly nonlinear glasses, we propose an all-solid tellurite optical glass rod with a transversely disordered refractive index profile as a new medium to study the transverse localization of light and NIR image transport. Compared to the glass-air random structure, the all-solid structure is advantageous because it is easier to be fabricated, easier to control the filling fraction and it has higher mechanical stability due to the absence of air-hole structures. From the view point of glass materials, among non-silica glasses, tellurite glasses exhibit not only wide transmission ranges from the visible region up to 6.0 µm in the IR region but also low glass transition temperatures , high thermal and chemical stabilities . These thermal and mechanical properties are beneficial to the fiber fabrication process. In our previous works, tellurite glasses were developed such that their refractive index difference can be as large as 0.49 at 1.55 μm [21, 22]. This large value of refractive index difference can be favorable to improve the beam localization effect by reducing the localized beam radius and its variation as discussed by S. Karbasi et al. [23, 24]. In addition, high nonlinearity of tellurite glasses (about tens of times higher than that of pure silica glass ) can play an important role in the performances of transverse localization of light and optical image transport because it was mentioned that the localization process can be enhanced under the nonlinear effect of self-focusing in presence of Kerr nonlinearity . Consequently, not only does the beam radius become narrower, but also the localization of light appears at a lower disorder level [12, 26]. To the best of our knowledge, this is the first time that the transverse localization of light and NIR image transport at 1.55 μm have been realized by using an all-solid tellurite transversely disordered optical rod (TDOR).
2. Materials and fabrication process
2.1. Glass material properties
Two tellurite glasses composed of TeO2, Li2O, WO3, MoO3, Nb2O5 (TLWMN) and TeO2, ZnO, Na2O, La2O3 (TZNL) were developed by our group aiming at their high compatibility of thermal and mechanical properties. These properties are important to avoid residual stresses and cracks which can damage the glass rod during fabrication process. A thermal mechanical analysis (TMA) system (Rigaku, Thermo Plus TMA 8310) was employed to measure the glass thermal expansion and softening temperatures (Ts). Rectangular cylinder samples in the shape of 4 x 4 x 15 mm were prepared and their thermal expansion behaviors were analyzed from room temperature to around 400°C. During the measurement, a 100 mN force was applied to the top of the sample. The sample length expanded when the temperature increased and shrank after Ts as illustrated in Fig. 1. The thermal expansion properties of these TLWMN and TZNL glasses are similar when the temperature is raised from 200 to 400 °C and the difference between their Ts is as small as 0.5 °C as shown in Fig. 1. The compatible thermal properties of the TLWMN and TZNL tellurite glasses are very advantageous to the stability of the following fabrication process.
An UV/VIS/NIR spectrometer (Perkin Elmer, Lambda 900) and an FT-IR spectrometer (Perkin Elmer, Spectrum 100) were used to measure transmission spectra of the TLWMN and TZNL tellurite glasses. The thickness of glass samples was about 1 mm. The surfaces of glass samples were polished carefully to satisfy the requirements of the transmission measurements. As can be seen in Fig. 2, both of TLWMN and TZNL glasses have broad transmission ranges covering not only the telecommunication window from 1.3 to 1.6 μm but also the wavelength range up to 6.0 µm. In order to minimize the OH-absorption, the powders to make glass samples were prepared in a glove box purged by nitrogen gas and were melted in an electric furnace with dry argon and oxygen gases (dry melting process). The red dashed line in Fig. 2 shows the transmission of the TLWMN glass obtained by conventional melting process. A large spectral dip from 3.0 to about 5.5 μm was attributed to the OH-absorption which makes the transmittance as small as 55%. Contrarily, the red solid line shows the transmittance of the TLWMN glass obtained by using the glove box and dry melting process. Consequently, the effect of OH-absorption becomes less and the transmittance from 3.0 to 5.5 μm is larger than 75%.
The refractive indices of the TLWMN and TZNL glasses were measured at different wavelengths from 0.5 to 4.6 μm by using glass prisms and the minimum deviation method . The uncertainness of the measurement is as low as ± 10−4. The measured refractive indices were fitted to the Sellmeier equation  as given by Eq. (1) and were plotted in Fig. 3. The Sellmeier coefficients of TLWMN and TZNL glasses were shown in Table. 1. At 1.55 μm, the refractive index difference between the TLWMN and TZNL glasses is Δn = 0.095.
2.2. Fabrication process
TLWMN and TZNL glass rods were prepared by using dry melting method. The purity of the raw materials is 99.99%. These two rods were drawn down to fibers whose diameters were 150 µm by using our home-designed fiber drawing tower at a temperature of 440°C. In total, 4500 fiber segments which were 15 cm long were randomly stacked together. By this way, a bundle of fibers composed of both TLWMN and TZNL tellurite glasses was obtained with a disordered refractive index profile in the transverse dimension and the outer diameter was about 12 mm. The ratio between the number of TLWMN and TZNL fibers was 1:1 so that the filling fraction (f) was 0.5 as given by Eq. (2)Figs. 4 and 5 by schematic and experimental images, respectively. The image of a random square region in the cross-section of the all-solid tellurite TDOR was shown in Fig. 5. The dark random dots represent the high index units (TLWMN glass) and the bright background consists of low index units (TZNL glass). The diameter of each unit was approximately 1.0 μm.
3. Results and discussions
3.1 Measurement of near-field intensity profile
In order to investigate the light beam guidance and localization properties in the fabricated all-solid tellurite TDOR, an experimental setup was constructed as shown in Fig. 6. A CW light source at 1.55 μm (Agilent-8164B laser source) guided by a single mode optical fiber (Thorlabs 980 HP) was launched into a 10-cm long all-solid tellurite TDOR by using butt-coupling method. The near-field intensity at the output facet of the tellurite TDOF was recorded by a CCD camera of a beam profiler (Hamamatsu C5840) when the 980HP fiber was scanned across the input facet. The measurement was repeated more than 10 times for each of 3 different fabricated tellurite TDORs.
As a result, the output beam remained localized after its propagation and its near-field profile was observed with a visually localized spot. Figure 7 shows a typical near-field intensity profile of a localized beam which was experimentally obtained from our measurement. In Fig. 8, the corresponding cross section of the intensity profile was analyzed and plotted with exponentially decaying tails. It has been reported by Karbasi et al. that when the disorder of transverse refractive index profile and localized beam are absent, the beam profile would fill the entire cross section of the fiber after a few millimeters . Contrarily, the exponentially decaying tails of the near-field intensity profile which were shown in Figs. 7 and 8 provide clear evidence of the beam localization.
3.2 Measurement of the near-infrared optical image transport
To investigate the NIR optical image transport capability of the fabricated tellurite TDOR, a test target (Thorlabs, 1951 U.S. Air Force) was installed in front of a 10-cm-long tellurite TDOR in the experimental setup as shown in Fig. 9. Optical images of the number from group 6 on the test target were launched into of the tellurite TDOR by using the same laser source at 1.55 μm in section 3A. Figure 10 shows the image of the Thorlabs test target and the right inset shows images of the numbers 3, 4 and 5 of group 6 on the test target. The inset images were taken by an optical microscope (Nikon, Eclipse-ME600). The size of each number from group 6 on the test target is the same and their heights are about 300 μm. The Hamamatsu-C2741-03 beam profiler was used in this measurement due to its higher sensitivity as compared to that of the Hamamatsu-C5840 in Fig. 6.
Figure 11 shows NIR optical images of the numbers 3, 4 and 5 which were captured at the output facet of the tellurite TDOR. The captured images are visually clear with high contrast and high brightness. It is remarkable that the quality of our transported optical images can be comparable or higher than the results which were obtained by using a polymer Anderson localized fiber and by a commercially available multicore imaging optical fiber . Based on the scale bar in Fig. 11, the estimated height of each number is approximately 1.3 times larger than the real size on the test target. The increasing in the height of the number can be caused by the diffusive broadening occurred when the light propagated in the all-solid tellurite TDOR. But due to the localization of the light beam, the broadening effect is less such that localized beam and clear transported optical images can be observed. This result is also consistent with the concept of the transverse localization of light.
In addition, it has been reported that shorter wavelengths result in a stronger localization effect and a smaller localization beam radius [18, 23]. Consequently, laser light sources at 0.405 μm and 0.633 μm were used to study the properties of localization of light in case of polymer Anderson localized fibers [16, 17, 23] and commercially available multicore imaging optical fibers . However, it is very interesting that optical images at the wavelength of 1.55 μm which is far from the aforementioned visible lights can be transported by the transverse localization effect using our all-solid tellurite TDOF.
Another factor that can strengthen the transverse localization effect is the refractive index difference in the transverse profile. It has been demonstrated that larger refractive index difference between the host medium and the disorder sites results in strong localization and smaller value of the beam radius [23, 24]. In this work, transverse localization effect and NIR optical image transport were obtained experimentally by using our all-solid tellurite TDOR although the refractive index difference is about 0.095 at 1.55 μm. But, tellurite glasses with refractive index difference up to 0.49 have been demonstrated in practice by our group [21, 22]. Therefore it is reasonable to expect that by using tellurite transversely disordered optical fibers with larger refractive index difference, higher performances of transverse localization of light and NIR optical image transport can be obtained in the near future.
To the best of our knowledge, this is the first time that an all-solid tellurite glass rod with a transversely disordered refractive index profile was fabricated as a medium to study the transverse localization of light and NIR image transport. The tellurite transversely disordered optical rod consists of high and low-index units which were TLWMN and TZNL glasses, respectively). Those glasses have small difference in softening temperature (about 0.5 °C), compatible thermal expansion from room to 400 °C, broad transmission range from about 0.4 up to 6.0 µm and their refractive index difference was about 0.095 at 1.55 µm. Experimental results showed that after a CW probe beam at 1.55 μm propagated in a 10-cm-long tellurite TDOR, the beam became localized. In addition, NIR optical images at 1.55 μm of numbers on the test target were transported. The captured images at the output facet are visually clear with high contrast and high brightness. The quality of our transported optical images can be comparable or higher than the results which were obtained by a polymer Anderson localized fiber and by a commercially available multicore imaging optical fiber. With the advantages of using tellurite glasses and all-solid structure, all-solid tellurite transversely disordered optical fibers are promising candidates for the transverse localization of light and NIR optical image transport applications.
The Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number (15H02250); The JSPS-CERN joint research program (17K18891).
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