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Abstract
Development of high numerical aperture fiber bundles (FBs) requires use of thermally matched pair of glasses with a high difference of refractive indices. We have developed a pair of glasses with high refractive index contrast ΔnD>0.2, suitable for fabrication of optical fiber bundles with numerical aperture NA > 0.85. Core glass was synthetized in the lanthanum oxide system Nb2O5-Ta2O5-SiO2-ZrO2-B2O3-Al2O3-La2O3-BaO-SrO. Borosilicate glass synthetized in oxide system SiO2-Al2O3-B2O3-MgO-CaO-Na2O-K2O, thermally matched to the core glass, is used for the fiber cladding. The glasses also have high transmission from 350 to over 600 nm, which makes them ideal for fluorescence imaging applications. These thermally stable, crystallization-free lanthanum and borosilicate glasses have been successfully applied to development of proof-of-concept large diameter optical fiber.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rapid progress in the development of the ex vivo and in vivo fluorescence biomedical research requires use of optical fibers and fiber imaging bundles (FBs) [1–3]. These components can play here a double role. Firstly, they deliver excitation light from an external source to the area of interest and secondly, they collect and guide the fluorescent signal to the detector [4]. After excitation of genetically modified cells with markers, a fluorescent signal is emitted. Laser stimulated fluorescence is a spontaneous omnidirectional emission [5]. Therefore it is difficult to collect optical signals into standard optical fibers or into imaging bundles with small numerical aperture. Instead, fluorescent microscopes with ultra-high numerical aperture objectives are used to collect signal and image. This approach, however, cannot be applied in in vivo systems where direct access with free space optics is limited. For this reason, imaging bundles are required. So far, commercial FBs were fabricated with the use of a limited number of suitable and commercially available rod and cladding glasses, which limited the achievable NA values of FBs to around 0.56 [6,7]. Limited NA allows to collect only very small fraction of fluorescent light and low contrast fluorescent images are obtained. There are several strategies usually used to increase quality of fluorescent imaging.
Long-time of signal collection or using strong excitation signal are among the most straightforward solutions. However, thermal effects and cell dynamics limit the practical use of such approaches. In other approaches, large FB pixels are used. However, this reduces the resolution of the obtained image [8]. This can be improved by using micro-optics [9] or microscanning with a FB tip at a cost of increased system complication [10]. An alternative approach involves distal micro-optics and micromechanics in favor of the proximal spatial light modulation [11,12]. Techniques such as speckle scanning microscopy [13] or speckle correlation [14] and techniques based on active light shaping [15–17] can be distinguished in this group. The idea of active light shaping to obtain the desired intensity distribution at the distal end of the fiber is based on the use of coherent FB [18–22] and can be used, for instance, for endoscopic optical coherence tomography (OCT) [20]. Another challenge related to application of FBs to deliver high-resolution images is crosstalk [11,12]. For effective reducing of the optical crosstalk between the individual fiber cores in an integrated high-resolution FB, and hence improving the overall imaging performance, NA values should be considerably higher.
Furthermore, fluorescent dyes such as fluorescein, trypan blue, rhodamine and several others, normally used for cell marking, are excited in the UV range and emit a fluorescent signal in the visible range. Therefore imaging bundles should have high transmission in the range of 350–700 nm. Morova et al. [25] recently reported a development of imaging bundles with NA of 0.59 with use of lead silicate F2 glasses. However, the short wavelength transmission edge was limited by transmission properties of glass to 460 nm. This limits its practical use for UV excited fluorescent imaging. To meet the requirement of fluorescent in vivo imaging new imaging bunds with very high NA and high transmission starting at a wavelength of 350 nm are required. For this purpose, two thermally matched glasses should be developed to enable joined and multiple thermal processing at a fiber drawing tower. They should, on one hand, have high transmission in spectral range from the UV to visible. On the other hand, they should have significantly higher refractive index contrast between the fiber core and the cladding compared to commercially made FBs [6,7,23–25]. This would allow for enhancement of the fluorescent signal collection, for decrease of the core-to-core distance, which would result in reduction of pixelation artefacts [26], and increase of the imaging bundle resolution [25].
The analysis of an Abbe diagram [27] shows that a glass with a high refractive index, from which individual cores of the FBs could be fabricated and which does not contain lead oxide (PbO) or cadmium oxide (CdO) components, should be based on lanthanum [28,29]. On the other hand, a glass with a low refractive index, from which the cladding of the FBs could be formed, could be based on borosilicate glass [30]. Moreover, lanthanum-based glasses are characterized by high transmission above 380 nm [31–35]. However, most of the commercial lanthanum-based glasses cannot be used for drawing fibers due to the high susceptibility to crystallization, which occurs during multiple thermal processing on the fiber drawing tower.
The exception is a custom-made lanthanum-aluminium glass, which has been successfully used by Kobelke [36] to develop highly non-linear photonic crystal fibers and gallium lanthanum sulphide glass for optical fibers and devices [37]. It is because of the highly non-linear refractive index, that lanthanum glasses are also used as an efficient bulk medium for supercontinuum generation [32,36,38]. In addition, due to a high refractive index value, low dispersion and absorption properties, lanthanum glasses are used for fabrication of special optical glasses, such as infrared absorbing glasses [39], as well as for solar control filters [40], and camera and telescope lenses. Recently, Alekseyev et al. [41] reported on the development of new glass compositions with 27mol% lanthanum oxide. Lanthanum glasses also find applications in non-optical domains as a part of dental prosthetic restoration composites [42].
In this work we aim to develop a pair of glasses characterized by the refractive index difference (contrast) ΔnD>0.2, which permits to obtain imaging bundles with numerical aperture above 0.8. The glasses should have high transmittance in the visible region, especially for wavelengths of 450-600 nm, which are used typically in the fluorescence imaging. Additionally, conforming to regulations governing medical in-vivo diagnostics, use glasses containing lead oxide (PbO) or cadmium oxide (CdO) components should be strictly avoided. Lanthanum glasses are very good alternative to lead glasses [28,29] for fiber bundle cores, while borosilicate glass is a good choice for the fiber cladding material [30].
2. Glass synthesis
High refractive index glasses without lead and cadmium in their composition, are based on high molecular weight oxides such as La2O3, Nb2O5, Ta2O5, BaO, or ZrO2. These glasses can achieve very high refractive indexes. For example, Schott LaSF type glasses [27] can reach nD = 2.0 as LaSF 35. However, our aim is not to achieve glass with a record high refractive index, but to develop glass with as high a refractive index as possible and, most importantly, with resistance to multiple thermal processing and the possibility of thermal matching with a low index glass. As a result, such a pair of glasses gives the possibility to develop optical fibers and imaging bundles.
The new glasses, referred to as core glasses, were synthesized in Nb2O5-Ta2O5-SiO2-ZrO2-B2O3-Al2O3-La2O3-BaO-SrO oxide system. The main glass-forming components that give a chance of non-crystallization are silicon oxide (SiO2) and boron oxide (B2O3). Lanthanum, niobium, tantalum and zirconium oxides (La2O3, Nb2O5, Ta2O5 and ZrO2, accordingly) are responsible for a partial increase of the refractive index. Other components such as aluminum oxide (Al2O3), barium oxide (BaO) and strontium oxide (SrO) have the function of modifying the glass network to achieve the required coefficient of thermal expansion and rheological properties. Several chemical compositions of niobium-lanthanum-borosilicate (LBS) glasses are considered to achieve the refractive index nD of synthetized glasses in the range of 1.7–1.8 (Table 1). The reason for all designed glasses, the sum of the molar content of silicon, boron and lanthanum oxides (SiO2+B2O3+La2O3) is constant at 57.5% mol. The most important parameter to be acquired is a high thermal stability (lack of the susceptibility to crystallization), which enables the multiple thermal processing required in drawing of optical fibers.
Table 1. Chemical composition of niobium-lanthanum-borosilicate glasses (LBS-2X series) [mol%].
Table 2. Chemical composition of sodium-potassium-borosilicate glasses (CG-2X series) [mol%].
Glasses with low refractive index, named as cladding glasses, were synthetized in SiO2-Al2O3-B2O3-MgO-CaO-Na2O-K2O oxide system (Table 2). High concentration of the small molecular weight SiO2 and B2O3 oxides allows to achieve as low refractive index nD as 1.51-1.53. Variation in the content of MgO, CaO, Na2O and K2O oxides permits, in turn, modification of the refractive index and viscosity characteristics of the glasses. Glasses of the CG series were designed to achieve their characteristic temperatures (mainly in the range of plastic shaping of glass) similar to the characteristic temperatures of LBS glasses (Table 3). The composition of CG glasses was also optimized to achieve high thermal stability and thermal expansion coefficient (CTE) with an average value of 90×10−7K-1 as achieved for LBS glasses (Table 3).
Table 3. Basic properties of the LBS-2X series niobium-lanthanum-borosilicate glasses.
Both series of the glasses were prepared by the melt-quenching method using high purity raw materials: Nb2O5 (Merck Optipur 99.999+%), Ta2O5 (Alfa Aesar 99.85%), SiO2 (powdered quartz glass 99.9%), ZrO2 (MEL Chemicals LTD >99%), H3BO3 (CHEMPUR >99.5%), Al2O3 (ACROS Organics 99.99%), La2O3 (Sigma-Aldrich 99.99%), BaCO3 (CHEMPUR Poland >99%), Ba(NO3)2 (CHEMPUR Poland >99%), SrCO3 (Sigma-Aldrich 99.9%), MgO (Inframat Advanced Materials 99.9%), CaO (Merck 99.9%), Na2CO3 (CHEMPUR Poland >99%), K2CO3 (CHEMPUR Poland >99%), KNO3 (Fluka >99.5%). The glasses were melted in a cylindrical 250 cm3 capacity platinum crucible within an electrical furnace under an air atmosphere. In each melt run 300 g of LBS and CG series glasses were processed. For good homogenization, it is important to start with well-mixed, preferably fine particle-sized raw materials. The glass melt was stirred by means of a quartz glass rod several times during the melting process which is the standard procedure used to obtain a homogeneous distribution of the refractive index in the glass volume [43]. In the case of the LBS glasses, after batch charging at 1200-1225°C, the temperature was increased up to 1300°C and then to 1375°C at a rate of 4°C/min.
After one hour tempering at the highest temperature, performed for the glass clarifying, the melt was cooled to a temperature of 1150°C at the rate of 3°C/min and subsequently cast into a rectangular graphite mold preheated to 625°C and annealed under a nitrogen atmosphere. The glass placed in a graphite mold was tempered in an electrical muffle furnace for 1 hour at a temperature of 655-660°C, 10°C higher than the transition temperature (Tg) of the glass, and was then cooled to ambient temperature at the rate of 0.3°C/min. The melting processes of CG series glasses followed a similar temporal profile but at higher overall temperatures due to the chemical composition of the glasses: batch charging (1250-1300°C), melting and clarifying (1350–1390°C), casting (1340°C). Graphite mold before melt casting was preheated to 450°C, and annealing tempering was performed at 570-590°C.
3. Methodology of glass properties measurements
The refractive index of the lanthanum core glasses was measured by use of the well-known goniometric method. The glass samples were prepared in the shape of a triangular prism and the apex angle was measured. Spectral lamps Ne, H, He were used for the measurements. After determining the critical angle of the prism and the angles of the smallest deflection for individual wavelengths, the values of refractive index were calculated. The measured discrete refractive indices were fitted using the Sellmeier dispersion equation. The Sellmeier equation is an empirical equation describing wavelength dependence of the refractive index n and has a form n2(λ) = 1+Σi(Biλ2/(λ2–Ci2)), where Ci and Bi denote the Sellmeier coefficients. From this fitting the Sellmeier coefficients were derived and further used for calculation of refractive index for any wavelength in the visible range.
In addition, refractive index of the borosilicate cladding glasses was also measured in Abbe’s refractometer (model 4 T, ATAGO Japan, accuracy 2×10−4), since these glasses has relatively low refractive index fitting to the operation range of refractometer. The polished glass samples 10×15 mm with a thickness of 5 mm was used for the measurements.
Measurements of the glass transmittance and absorbance were performed with the VARIAN Cary 500 spectrophotometer in the range of 200-3300 nm. Double-side polished samples with 2 and 5 mm thicknesses were used for the measurements. The absorbance values of 2 and 5 mm samples were used for calculation of the absorption coefficient.
Baehr Thermoanalyse GmbH Dilatometer 801 was used for characterization of the linear thermal expansion coefficients (CTE). Rods with dimensions 4×4×30 mm were used for these measurements. The four characteristic glass temperatures related to the dynamic viscosity η = 1014.6; 1013.4; 1013 and 1011 P are determined based on the obtained thermal expansion curve. These include temperatures of lower annealing tl, transformation Tg, upper annealing tu and dilatometric softening point DSP, respectively. In addition, the four characteristic glass temperatures related to lower viscosity η = 109; 106; 104 and 102 P (temperatures of sample curvature Tc, sphere creation Tsph, hemisphere creation Ths and glass spreading Tspr, respectively) were determined from the shape of the sample observation in a Leitz II A-P heat microscope. A test cube with a 4 mm side length was used for the glass characteristic temperature measurements with a heat rate of 10°C/min applied. The final viscosity curves were determined based on the measurements in the dilatometer and in the heat microscope.
Preliminary information about the crystallization susceptibility of glass was obtained using the isothermal heating method (2 hours of tempering at the temperature, at which glass sample placed in heating microscope takes shape of a sphere; it corresponds with η = 106 P). The sample surface was observed after cooling down with an optical microscope to identify areas of excessive crystallization.
Differential Scanning Calorimetry (DSC) measurements, second method of crystallization susceptibility testing, were performed using a Netzsch 449 F1 Jupiter simultaneous thermal analyzer. The glass samples were heated up at a rate of 10°C/min in an air atmosphere from 30 to 1100°C for the both LBS and CG series glasses. The weight of the powder samples was 60 mg for each glass. The transformation temperature Tg, and, if it was possible, the crystallization beginning temperature Tx, the maximum crystallization temperature Tc and the melting temperature Tm were determined from the DSC curves using onset method.
The third method used to examine crystallization susceptibility was based on X-Ray diffraction (XRD). Rigaku diffractometer was used. The measurement was carried out in Bragg-Brentano geometry in the range of 2Θ = 10-90 degrees with a step of 0.02 degree.
The densities of the sample glasses were determined by the well-known method of hydrostatic weighing (Archimedes’s method). The measurements were performed using flat-polished parallel samples of glass with dimensions of 20×15×10 mm.
The microhardness defines the mechanical strength of the glasses and determines the durability of the manufactured optical products (small rods, fibers, image guides). A Zwick microhardness meter was used for measurements of the mechanical strength of the glass samples under Q = 0.2 kG (1.96 N) loading. Hardness is defined as HV = 1.8544×Q/d2, where d denotes the average diameter of the diamond prism indentation in glass.
The water durability of the glasses was qualified based on the glass sample mass decrease after six hours boiling in distilled water. The measurements were executed using mechanically polished samples of glasses with dimensions of 20×15×10 mm. The sample mass decrease (Δm) after a 6 hour-long exposure to boiling water, counted on 100 cm2 of the sample surface (mg/100cm2), is the measure of water durability of glass. The higher value of decrease of sample mass denotes lower resistance of the glass against water interaction.
4. Experimental results
Refractive index nD (for sodium line λ = 589 nm) of the developed LBS-2X series glasses is high and covers the range of 1.6890-1.7911 (Table 3). Increase of refractive index is caused by a content growth of lanthanum heavy oxide (La2O3) introduced instead of the small molecular weight SiO2 and B2O3 oxides, which are characterized by low partial refractive indexes. Increase of nD is almost linear in the whole range of La2O3 content (3.5-13.5 mol%) (Fig. 1).
Fig. 1. Refractive index of the LBS-2X series glasses in function of La2O3 concentration (measurement precision 2 × 10−4).
The measured values of the refractive index allowed to determine spectral characteristics of the LBS glasses (Fig. 2) and Sellmeier dispersion equation (Table 4). LBS glasses can be classified to the glass group named as lanthanum flints (LaF, accordingly to the standard optical glass classification [27]). They are highly dispersive glasses, characterized by relatively low values of Abbe number νd (Fig. 3). For example, LBS-23 glass has νd = 45.10. However, it is higher than in the case of lead silicate glasses with high content of PbO, e.g. Schott SF6 glass with νd = 24.43.
Table 4. Sellmeier dispersion coefficients for LBS-2X glasses.
The cladding glasses series CG-2X were designed to obtain a relatively low values of nD, and their coefficient of thermal expansion (CTE) should remain on the same level, about 90×10−7K-1. Measurement spectral characteristics of refractive index (Fig. 4, Table 6), in particular for nD and α clearly confirms achievement of the aim (Table 5). Refractive indices of CG-2X glasses stay on the low level nD = 1.5125-1.5202 and their CTEs are placed in the range of (88.7-89.6)×10−7K-1.
Table 5. Basic properties of the CG-2X series sodium-potassium-borosilicate glasses.
Table 6. Sellmeier dispersion coefficients for CG-2X glasses.
Among synthesized glasses a pair LBS-23 and CG-22 glasses allow to achieve the target contrast of refractive index ΔnD>0.2 (1.7295-1.5192 = 0.2103; Table 3 and Table 5) and high numerical aperture NA = 0.827. The numerical aperture is calculated using equation:
where n1 and n2 denote refractive index of the core and cladding glasses, accordingly.4.1 Glass transmittance
The transmittance of LBS-2X series glasses is high and at wavelengths λ>370 nm and exceeds 80%. In the wavelength range 450-600 nm, particularly important for fluorescent imaging, transmission is equal to 83.6-86.5% at λ = 450 nm and 84.8-87.3% at λ = 600 nm (Table 3). Transmittance decreases with increase of La2O3 content and simultaneous decrease of total content of SiO2+B2O3 (Fig. 3). Short-wave absorption edge λs located at λ<300 nm shifts from 288 nm to 283 nm when La2O3 content increases (Fig. 5, Table 3). Due to high transmittance in short-wave range of 350-450 nm, the glass is colorless with subtle yellowish tint.
Fig. 5. Transmittance of LBS-2X series glasses in the range of 250-600 nm (d – thickness of glass sample).
The transmittance of the CG-2X series remains practically on the constant high level above 90% and is equal to 91.1-91.7% at λ = 450 nm and T = 91.8-92.3% at λ = 600 nm (Fig. 6, Table 5). Short-wave absorption edge λs is far off shifted in the side of short waves and is located at λ = 236-238 nm. Due to high transmittance in the ultraviolet and all visible ranges CG glasses are perfectly colorless.
Fig. 6. Transmittance of CG-2X series glasses in the range of 200-600 nm (d – thickness of glass sample).
The selected core and cladding glasses, LBS-23 and CG-22, have very low absorption coefficient for the range of λ = 400-600 nm, crucial for imaging bundles dedicated to fluorescent imaging (Fig. 7).
Fig. 7. Absorption coefficient vs. wavelength for the LBS-23 (core) and CG-22 (cladding) selected pair of glasses.
4.2 Viscosity
The measured viscosity characteristics show that the LBS-2X glasses have limited temperature range, where their forming is possible (Fig. 8). Forming temperature range for the developed LBS-2X lanthanum glasses is similar to all compositions and limited to 75°C, if typical for commercial glasses viscosity range of η = 107÷104 P is considered. For typical soft glasses, the forming temperature range is usually above 100°C [39]. For this reason, such glasses as LBS are named “short glasses”. CG cladding glasses can be classified as “long glasses”, since their forming temperature range is broader and exceeds 135°C (Fig. 9).
A fiber drawing processes are performed in much narrower range of viscosity η = 107÷108 P [24]. Therefore forming temperature range for fiber drawing processes are 24°C and 46 °C for LBS-2X and CG glasses, respectively. Based on analysis of rheological properties of LBS and CG glasses we can select a pair of glasses LBS-23 and CG-22 well suited to joint thermal processing on fiber drawing tower by the rod-in-tube method. Their temperature difference for central drawing viscosity of logη = 7.5 is as low as 14°C (Fig. 9). In other pairs of LBS-23 core glass with CG-21 and CG-23 cladding glasses, this difference (Δt) is noticeably higher, 21°C and 18°C, accordingly.
4.3 Thermal expansion coefficient
We have measured that the thermal expansion coefficient increases monotonically with an increase of the La2O3 content in the glass (Fig. 10). Usually low values of the thermal expansion coefficient are preferred in case of fiber and image bundle development, since low thermal expansion coefficient denotes that glass is less sensitive to the rapid temperature changes. Glass with low thermal expansion coefficient can be easier cut and polished and thermally processed during preform and subpreform component development. Optimal value of CTE should be equal or smaller than α = 90×10−7K-1.
Fig. 10. Thermal expansion coefficient (CTE) of the LBS-2X series glasses in function of La2O3 concentration. We assume that total content of SiO2, B2O3 and La2O3 in LBS-2X glasses is constant and equal to 57.5 mol% (measurement precision 0.5 10−7K-1).
4.4 Crystallization properties
The DSC measurements for both series of glasses LBS and CG are presented in Fig. 11 and Fig. 12, respectively. The four characteristic glass temperatures are summarized in Table 7. Usually a temperature difference ΔT = Tx–Tg is a good indicator of glass crystallization abilities [44]. The glasses where ΔT > 100°C should be thermally stable and no crystallization is observed for long-term heating at softening temperatures (logη = 6-9). Glasses that offer a high thermal stability (ΔT = Tx–Tg) and a low temperature interval (Tm–Tx) are the best candidates for optical fibers fabrication due to their relatively small probability of crystallizations problems. The relevant parameter that represents both of these temperature intervals is the Hruby Number (HR) defined as (Tx–Tg)/(Tm–Tx) [45,46]. For thermal stable glasses HR should achieve values higher than 1.0 [44].
Table 7. Characteristic temperatures of the LBS and CG glasses determined based on DSC measurements.
The Hruby Number was calculated for LBS glasses with high content of L2O3 based on DSC characteristics. It was not possible to determine HR in the case of some measured LBS and CG glasses, since the Tm temperatures for LBS-21, LBS-22, LBS-23 and for all CG series glasses were not identified. There was lack of clearly marked egzo- and endo- effects in the region of the highest examined temperatures (Fig. 11, Fig. 12, Table 7). Therefore, all these glasses are good candidates for fiber fabrication, since crystallization processes should not occur.
The lack of crystallization of both glasses was also confirmed by X-Ray Diffraction Analysis (XRD) analysis. The curve pairs presented for LBS-23 (Fig. 13) and CG-22 (Fig. 14) show only Lorentz's broad scattering features, which proves the lack of long-range atomic order.
The lack of crystallization is also proved by tests under a heating microscope, where only correct shapes of samples were observed as temperature increases, and most of all sphere and half-sphere shapes.
After 2-hour isothermal treatment at temperature Tsph (Table 3) small, individual crystallites were observed on the sample surface of LBS-25 and LBS-26 glasses only. These observations confirm the conclusions drawn from the DSC curves for these glasses. LBS-25 and LBS-26 glasses are characterized by a low value of Hruby number 0.635 and 0.614, accordingly (Table 6).
4.5 Glass density, microhardness and water durability
Density of the LBS glasses increases in the range of 4.3-4.51 g/cm3 with increasing of La2O3 content from 3.5 to 13.5 mol% (Table 3). In the case of three glasses of CG series, their density remains practically on the same level (ρ = 2.45-2.48 g/cm3; Table 5).
Microhardness for both series of the glasses is at high levels: HV = 8.25-8.59 GPa for LBS glasses according to a grow of La2O3 content (Table 3) and HV > 7 GPa for CG glasses, as shown in Table 5.
Water durability of both series of the developed glasses is on a high level, as shown in Fig. 15. It is comparable with water durability measured for standard BK7 Schott glass (Table 3, Table 5). Achieved high water durability is advantageous property of the glasses as it facilitates their mechanical polishing.
Fig. 15. Water durability of example LBS and CG glasses. As a reference water durability optical borosilicate BK7 Schott glass. Smaller mass decrease denotes higher water durability.
4.6 Verification of thermal compatibility of selected lanthanum and silicate glasses
Thermal compatibility of the selected pair of LBS-23 and CG-22 glasses was verified experimentally. The LBS-23 glass plate was stacked between two plates of the CG-22 glass. The glass structure was aggregated at the temperature 20°C above the curvature temperature of the LBS-23 glass (Tc + 20°C = 730°C) for 30 minutes. After annealing (0.3°C/min), polished cross section was observed under polarized light microscope. As a reference, a similar ‘sandwich’ structure for another pair of glasses, LBS-24 and CG-22, was measured. Microscope images of the tested two pairs of glasses are shown in Fig. 16. Stress in the test structures induced by difference of the thermal expansion of composed glasses can be estimated based on difference in colors [47]. The better result is observed for the originally selected pair of glasses, LBS-23 and CG-22. There was very low internal stress between the two glasses in this case. Low internal stress confirms very small difference in the thermal expansion coefficients between component glasses at the level of Δα = 1.0×10−7K-1 (Fig. 16(a)). For a second pair of glasses, LBS-24 and CG-22, TEC difference is much higher, equal to Δα = 2.9×10−7K-1. It causes higher stress in the ‘sandwich’ structures, that we can observe as bright yellow areas in cladding plates, as shown in Fig. 16(b).
Fig. 16. Glass ‘sandwiches’ observed under polarized light microscope: a) LBS-23 core glass combined with CG-22 cladding glass (Δα = 1.0×10−7K-1); b) LBS-24 core glass combined with CG-22 cladding glass (Δα = 2.9×10−7K-1).
The obtained results confirm a selection of LBS-23 and CG-22 glasses, for further use in fiber bundles, since both glasses have very good crystallization resistance, relatively small difference of thermal expansion coefficients and similar temperatures for fiber drawing (Δt = 14°C at logη = 7.5).
5. Development of the test optical fiber
We have fabricated a large diameter optical fiber as a test-bad to verify the suitability of the selected pair of glasses LBS-23 and CG-22 for development of the image bundle. We have developed rods and tubes using the casting method followed by mechanical grinding and polishing. LBS-23 core glass and CG-22 cladding glass were melted in a portion of 500 g. The core glass was cast into graphite mold having a rectangular inner shape. The tubes were cast in a preheated (280°C) graphite casting form equipped with a polished, round stain steel rod, covered by a thin layer of graphite, placed in the center. Molten glass at a temperature of 1340°C is poured into mold through longitudinal mouth located in its upper part. After casting the mold attains a temperature higher than 650°C. After the annealing process, run in an electric muffle furnace under nitrogen atmosphere, the casting form is opened and the glass is further mechanically processed. The rod surface and both, the outer and inner surfaces, of the tubes are mechanically grounded and polished and then washed in ethanol and dried in a glove box under a nitrogen atmosphere.
Rod-in-tube type preform was assembled and drawn using of fiber optic drawing tower with argon protective atmosphere (Fig. 17(a)). A large core rigid optical fibers with 1 mm outer diameter and flexible fiber with outer diameter of 124 µm are drawn (Fig. 17(b)).
Fig. 17. Development of test fiber. Rod-in-tube preform after drawing process (a). Cross section of the drawn subpreform of the fiber observed with the polarized light microscope to identify internal stress in glass.
The numerical aperture of the fabricated rigid fiber was measured with the standard goniometric method [48]. As a source we used a single mode 532 nm CW laser (Coherent Verdi 5W). The input beam was coupled to the core using high NA microscope objective ×60/0.85 to ensure efficient excitation of guided modes. A 50 cm long fiber samples were used. Far field intensity distribution at the fiber output was scanned using silicon photodiodes (400–1100 nm) (Fig. 18). Distance between fiber surface and photodetector was set to 75 mm. Measured half-angle at which intensity decreases to 5% of the maximum intensity is equal to 59.8°. This angle corresponds to the numerical aperture of NA = 0.86, which is well agreed with the theoretical results where NA = 0.83.
Next, we measured the fiber’s confinement losses using the standard cutback method [49]. As a light source, we used an Ocean Optics tungsten halogen lamp HL-2000-LL, and a compact Czerny-Turner spectrometer from Thorlabs (sensitivity range of 200-1000 nm) was used. A 2.3 m long fibre sample was used for measurements, which was then shortened to 1.73 m, 1.24 m, 0.6 m. The spectrograms obtained for particular fiber lengths allowed to determine confinement losses. The results presented in Fig. 19 show that losses for short wavelengths are relatively high and reach about 6-7 dB/m in the range from 350 to 450 nm. Transmission is limited mainly by absorption of bulk LBS-23 glass of the fiber core in UV range. High noise observed for UV range is related to limited power of the halogen lamp and low sensitivity of the spectrometer. In the wavelength range above 520 nm losses are below 2 dB/m. The obtained results confirm that the fabrication procedure was correct, both glasses are thermally matched and fiber cladding is well integrated with the core. Bending of the fiber did not cause fiber breaking, therefore we can conclude that internal stress between glasses is mitigated. The recorded attenuation characteristic is also satisfactory for a imaging bundles since usually samples longer than 1 meter are not used in practice.
Fig. 19. Confinement losses measured for test fibers fabricated using LBS-23 and CG-22 glasses. Raw experimental data are included in the supplement “Dataset 1” [50].
6. Conclusions
We have developed a pair of glasses with high refractive contrast (ΔnD = 0.21) suitable for joint thermal processing in manufacturing of optical fibers with high numerical aperture (NA = 0.86). High refractive index glass is free from lead and cadmium undesirable in medical tools dedicated for in vivo use. The barium-lanthanum-borosilicate core glass (LBS-23) is composed of 3% Nb2O5, 1% Ta2O5, 27% SiO2, 3% ZrO2, 27% B2O3, 1% Al2O3, 3.5% La2O3, 25% BaO, 9.5% SrO. The LBS-23 glass is characterized by nD = 1.7296 and CTE = 90.6×10−7K-1 (for the range of 20-300°C).
The low refractive index glass is alkaline borosilicate glass (58% SiO2, 2.5% Al2O3, 14.2% B2O3, 10% MgO, 1.3% CaO, 4% Na2O, 10% K2O) with nD = 1.5190 and CTE = 89.6×10−7K-1.
Thermal properties of both glasses are very well matched. In particular their viscosities in the range of fiber drawing (logη = 7-8), are very similar. Their viscosity curves have the crossing point in this range and their temperature difference at logη = 7.5 is very small (Δt ≈ 14°C). Both developed glasses were also verified to be thermally stable and resistant to crystallization. These parameters and high transmission in the range 350-600 nm allows to use them for development of image bundles dedicated to fluorescence imaging using stack and draw technique. As a proof-of-concept, we fabricated a large core step index optical fiber. Successful drawing of the fiber verified thermal compatibility of the developed glasses, as well as their thermal and mechanical stability. The numerical aperture of 0.86, possible to obtain with the selected glasses, is experimentally confirmed. Relatively low measured loss for tested fiber samples, not exceeding 7 dB/m in the UV wavelength range, and below 2 dB/m for wavelengths above 520 nm, confirm good integration between the core and cladding glasses and their potential for development of imaging bundles for fluorescence imaging.
Funding
Fundacja na rzecz Nauki Polskiej (POIR.04.04.00-00-1C74/16); European Commission (H2020-MSCA-ITN-2016 722380); statutory subvention of Research Network Lukasiewicz – Institute of Electronic Materials Technology.
Disclosures
The authors declare no conflicts of interest.
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