1. Introduction

There is currently a world-wide push towards optical application in the mid-infrared (mid-IR) wavelength range of 1.5-9 µm. Light sources in this wavelength range have numerous applications in green chemistry, where water is used as solvent, in medical diagnostics and spectroscopy, because most chemical compositions absorb within this range, as well as in the general food, pharmaceutical, and defense industries. With conventional silica glass fibers wavelengths longer than 2.4 µm cannot be accessed, simply because they exceed the near-infrared loss edge. These limitations can be overcome by moving to nonsilica glasses. Such glasses are often described as soft or compound in reference to their significantly lower melting temperatures than silica glass and they are often multi-components.

A number of soft glasses, such as tellurite, fluoride and chalcogenide glasses, transmit at wavelengths substantially beyond that of silica into mid-IR. The IR cutoff wavelength of a glass is principally determined by the overtone of fundamental network vibrations (referred to as multiphonon absorption). For low-phonon-energy glasses, the multiphonon edge is shifted to longer wavelengths, allowing high mid-IR transmission. Glass network former components containing heavy metals (such as TeO₂ and GeO₂) and/or having low bond strength (such as ZrF₄ and As₂S₃) exhibit lower phonon energies; that is, the maximum energy of the network vibrations within the glass is low compared with that of other compound glasses such as silicates, borates and phosphates. In addition to the multiphonon edge, effects such as scattering and impurity absorption reduce the glass transmission. In particular, the presence of OH leads to the formation of broad and intense absorption bands in the mid-IR owing to fundamental vibrations in this range. Thus, the water content must be minimized to achieve the high mid-IR transparency that is in principle possible in low-phonon-energy glasses [1].

Among non-silica glasses, tellurite glasses combine the attributes of a reasonably wide transmission region of 0.35–5 µm (up to 6.10 µm in this study), good glass stability, high rare-earth ion solubility and corrosion resistance, the lowest low phonon energy spectrum among oxide glass formers and high linear and nonlinear refractive indices [2,3]. First fabricated and reported by Wang et al. in 1994 [3], tellurite glass fibers have attracted increasing research interest for laser, amplifier and chemical sensor applications [4,5]. Among the already-reported tellurite glasses and fibers, the ternary tellurite glass with composition of 75TeO₂-20ZnO-5Na₂O has generated the greatest interest due to its excellent thermal stability and its refractive index compatibility with most ferroelectric oxides [4–7]. However, high transmission loss and heavy surface crystallization during fiber drawing must be overcome before this glass can be used for practical applications [8].

Conventional step-index soft fiber designs require the use of a pair of glasses for the core and cladding regions of the fiber respectively. These must be not only thermally and chemically compatible but they must also have the appropriate optical characteristics for guidance in the fiber core. Besides the loss from the present OH, the multistage fiber fabrication process to manufacture solid-core soft glass fibers often introduces dust, impurities, micro-bubbles, striations, and interface defects between the core and the cladding, resulting in higher transmission losses which have hampered the development of practical soft glass fibers. As a result, only a limited range of fibers are commercially available at present. In order to minimize transmission losses, the fabrication process requires meticulous dehydration to remove water, an ultra-clean fabrication environment to prevent dust and impurities, precise time and temperature controls during glass melting and pouring in order to prevent striations and micro-bubbles to form.

By optimizing glass composition and using a multistage dehydration process, we have developed a new ternary TeO₂-ZnO-Na₂O (TZN) glass providing excellent transparency from 0.38 µm up to 6.10 µm. By using suction casting, rotational casting, and rod-in-tube fiber drawing techniques [9,10], we report on the fabrication of a single-mode solid-core tellurite glass fiber with large mode area of 103 µm² and relatively low loss of 0.24~0.7 dB/m at 1550 nm. By using the continuous-wave self-phase modulation (cw-SPM) method [11–14], the non-resonant nonlinear refractive index n ₂ and the effective nonlinear parameter γ of the made TZN glass fiber were estimated to be 3.8×10^-19 m²/W and 10.6 W^-1·km^-1, respectively.

2. Experiments

We have optimized the glass composition so as to prevent surface crystallization during fiber drawing and therefore enhance the fiber strength; TZN glasses with compositions of 75TeO₂-20ZnO-5Na₂O (TZN-75) and 80TeO₂-10ZnO-10Na₂O (TZN-80) were prepared by conventional melting-quenching techniques described elsewhere [6,7]. Differential thermal analysis (DTA) was carried out on bulk samples using a NETZSCH STA 409C thermal analyzer in N₂ atmosphere with a scan rate of 10□/min. Based on the optimized TZN glass composition with 10 mol% Na₂O, a tellurite glass fiber with compositions of 78TeO₂-12ZnO-10Na₂O (Core, n=1.981) and 75TeO₂-15ZnO-10Na₂O (Clad, n=1.976) was fabricated by suction casting (Core/Clad structure), rotational casting (Clad tube), and rod-in-tube fiber drawing techniques [5,9,10].

The non-resonant nonlinear refractive index n ₂ and the effective nonlinear parameter γ of the made TZN fiber were measured using the cw-SPM method, of which the block diagram of the experimental setup was shown in [11,12]. The polarization state of the two input lights, λ ₁ and λ ₂, is made parallel by passing them through two separate polarization controllers (PCs). After passing through a band pass filter (BPF) with a 3 dB bandwidth of 30 nm to reduce the amplified spontaneous emission (ASE), the coupled signals are amplified by an Er-doped fiber amplifier (EDFA) and launched into a test optical fiber sample. The power ratio of the output signals I ₀ and I ₁ is measured on an optical spectrum analyzer (OSA) as a function of the EDFA output power. In order to protect the OSA from damage, an optical variable attenuator (ATT) is used in this experiment.

3. Results and discussion

3.1 Glass composition optimization for fiber fabrication

 

Fig. 1. DTA traces obtained for as-quenched glasses of different compositions.

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Table 1. Composition and physical parameters of different TZN glasses

Glass Composition Tg (°C) Tx (°C) ΔT(°C)

TZN-80 80TeO2 -10ZnO - 10Na2O 285 N/A ≥150

TZN-75 75TeO2 - 20ZnO - 5Na2O 302 407 105

To optimize the glass composition for fiber fabrication, more sodium monoxide Na₂O was introduced into the original formula, TZN-75, to make what is referred to as TZN-80. Acting as a glass modifier, Na₂O breaks off Te-O bonds and therefore reduces the tendency towards crystallization [5,8]. The results are summarized in Fig. 1 and Table 1. In order to efficiently decrease the OH content in the fabricated tellurite glasses and fibers, a multistage dehydration process was especially designed in this study. All the used compounds (purity ≥5N) were mixed in a glove box under N2 gas atmosphere; vacuum pumped at 100 °C for 4 hours; heated at 200 °C for 40 minutes, at 300 °C for 30 minutes, at 400 °C for 20 minutes, and finally melted at 800–850 °C for 3 hours in ultra-dry O2 atmosphere protected muffle furnace. This dehydration process is illustrated in Fig. 2, along with the resulting transmittance spectrum in the UV-VIS and mid-IR. An optical microscope was employed to examine the drawn fiber surfaces in Fig. 3.

 

Fig. 2. A multistage dehydration process (left) and the resulted UV-VIS+FTIR transmission spectrum (right) of TZN-80 glass (Thickness of the tested sample is 1.4 mm).

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Fig. 3. Surface microscopy graphs of TZN-75 and TZN-80 based fibers.

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It is easy to conclude that, compared to the original tellurite formula TZN-75, TZN-80 with 10 mol% Na₂O is a good candidate for fiber fabrication. It has a broader working temperature range, higher transmittance, wider IR transmission, and a better resistance against surface crystallization during fiber drawing, leading to low-loss and robust tellurite glass fibers.

3.2 Fiber losses depending on different fiber drawing conditions

By using the rod-in-tube fiber drawing technique, we chose a fiber cane (Core/Clad rod) of 1–3 mm thickness and inserted it into a clad tube with inner/outer diameters of 3 mm/12 mm. As shown in Fig. 4, fiber losses depend critically on fiber drawing conditions. When the diameter of the fiber cane was chosen to be 2.48 mm or above, the drawn TZN glass fiber presented the lowest loss of 0.24 dB/m at 1550nm. It is important to note that we drew the fiber in ambient atmosphere without using any gas atmosphere to protect the reheated fiber preform from the water contamination.

 

Fig. 4. Cross-section microscopy graphs of the TZN glass fibers with different losses at 1550 nm: (a) 0.24 dB/m loss with a regular core; (b) 0.7 dB/m loss with a distorted core and a small gap between core and clad; (c) 3 dB/m loss with a heavily distorted and diffused core; (d) 5 dB/m loss with a big gap between core and clad.

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3.3 Third-order non-resonant nonlinearity measured by the cw-SPM method

 

Fig. 5. Measured cw-SPM spectrum and simulated fundamental mode profile at 1550 nm.

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To determine the non-resonant nonlinear coefficients of the TZN glass fiber, the nonlinear phase shift φ _SPM and the corresponding power P _AVG were measured experimentally. P _AVG corresponds to the average power of the dual wave pump (wavelengths λ ₁=1549.807 nm and λ ₂=1550.045 nm in this experiment), measured in watts, and φ _SPM was calculated from the ratio I ₁/I ₀ of the intensity of the two pump beams to the intensity of the first order nonlinear sidebands using Eq. (1) [11–14]:

in which J_n is the n-th order Bessel function.

The non-resonant nonlinear refractive index n ₂ and the effective nonlinear parameter γ were then obtained from Eqs. (2) and (3) below [11–14]:

where λ=(λ ₁+λ ₂)/2 is the center wavelength of the two pumps, κ _ac is the slope coefficient determined from the linear region of the function φ _SPM/P _AVG, and L _eff and A _eff are the effective length and the effective area at 1550nm, respectively.

Figure 5 shows the fundamental mode profile simulated with RSOFT research software, along with the measured cw-SPM spectra of the TZN fiber with an effective length of 1.48 m and a large mode area of 103 µm² at a pump of 438 mW from an EDFA [12]. The non-resonant nonlinear refractive index n ₂ and the effective nonlinear parameter γ of the TZN glass fiber in this study were estimated to be 3.8×10^-19 m²/W and 10.6 W^-1·km^-1 respectively, i.e. more than 10 times those of the silica-based fiber, which intrinsically originate from highly distorted coordination polyhedra such as TeO₃ and TeO₄, non-bridging oxygens (NBOs) introduced by Na₂O, and large molar refraction from O^2- and the surrounding cations [11,15].

4. Conclusion

By optimizing glass composition and using a multistage dehydration process, a ternary 80TeO₂-10ZnO-10Na₂O glass shows excellent transparency from 0.38 µm to 6.10 µm. Based on this optimized glass composition with more Na2O than previously used, a hundred-meter long robust single-mode solid-core tellurite glass fiber was successfully fabricated in this study. At 1550 nm, the fiber loss is measured to be 0.24~0.7 dB/m, the mode area is as large as 103 µm², and the non-resonant nonlinear refractive index n ₂ and the effective nonlinear parameter γ were estimated to be 3.8×10^-19 m²/W and 10.6 W^-1·km^-1 respectively, which are more than 10 times those of the silica-based fiber. By modulating the core size and therefore changing the mode area, different applications in the infrared wavelength range can be explored in the future.