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A simple all-solid tellurite microstructured optical fiber which has only one layer of high-index rods in the cladding is proposed and fabricated in the paper. The core and the cladding with the low index are made from the TeO2–ZnO–Na2O–La2O3 glass, and the high-index rods are made from the TeO2–Li2O–WO3–MoO3–Nb2O5 glass. The guiding regime in this fiber can be explained by ARROW model. The fiber can support the near- and mid-infrared light transmitting in the core within the transmission bands while the all-solid silica microstructured optical fiber cannot. When the pump light is outside the transmission bands, the light will transmit in six TLWMN rods.
©2013 Optical Society of America
1. Introduction
All-solid microstructured optical fibers (MOFs) with an array of isolated high-index rods in a low index cladding have already been widely studied during the last decade [1–6]. And they have been applied in optical fields, such as amplifiers and lasers, tunable bandpass filterings, the supercontinuum generation and dispersion compensation components, etc [7–11]. In all-solid MOFs, light is confined in the low-index core due to the existence of the photonic bandgap (PBG). However, the guiding regime can also be described analytically by use of the antiresonant reflecting optical waveguide (ARROW) model [12–14]. According to this guiding mechanism, the optical properties of the all-solid MOFs are governed largely by the thickness and the refractive-index contrast of the first high-index layer rather than by the periodic arrangement of the alternating layers [15]. So the structure of a simple all-solid MOF with only one layer of high-index rods can be easily controlled compared with that with several layers in the cladding by the stack-and-draw technique [16–19]. On the other hand, tellurite glass presents dispersive, high nonlinear properties and can support the mid-infrared light transmission [20–24]. If these properties are combined with the simple all-solid MOF structure, it could lead to the development of optical components of great interest. Therefore, we have fabricated a simple all-solid tellurite MOF.
In this paper, we present a simple all-solid tellurite MOF with only one layer of high-index rods in the cladding. The core and the cladding with the low index are made from the TeO2–ZnO–Na2O–La2O3 (TZNL) glass, while the high-index rods are made from the TeO2–Li2O–WO3–MoO3–Nb2O5 (TLWMN) glass. The transmission bands and the modes of the MOF are calculated by the multipole method. According to the ARROW guiding mechanism, when the pump light is within the transmission bands, the fiber can support the near- and mid-infrared light transmitting in the core and be used as a filter. When outside the transmission bands, the pump light will transmit in six TLWMN rods.
2. Fabrication
The simple all-solid tellurite MOF is fabricated by two kinds of tellurite glass, and the fabrication process is illuminated in Fig. 1 . Tellurite glass composition is very important for the MOF fabrication because during the fiber drawing process, the core, the cladding and the high-index rods should be thermally stable and compatible, and thermal expansion and softening temperature of two tellurite glass must be similar. In this work, the core and the cladding with low index are made from TZNL glass while the high-index rods from TLWMN glass. These two kinds of glass have similar thermal and mechanical properties, which is necessary for successful preparation of MOF. Additionally, the difference in the refractive indices (Δn) is about 0.096 at λ = 1.55 μm. The fiber drawing temperature of both tellurite glass is the same, about 370 °C. First a TLWMN glass rod and a hexagonal TZNL glass rod are prepared by the casting method, and a TZNL glass tube is prepared by the rotational casting method. Then the hexagonal TZNL and the TLWMN glass rods are elongated to prepare the capillaries shown in Figs. 1(b) and 1(d), and the images of cross sections are shown in Figs. 2(a) and 2(b), respectively. The diameters of the hexagonal TZNL glass capillary and the TLWMN glass capillary are 4.37 mm and 1.24 mm. After that, the elongated hexagonal TZNL glass rod and six TLWMN glass capillaries are inserted into the TZNL glass tube (a) to produce the preform (e). Then the preform is elongated to prepare the cane (f) at 370 °C. Finally, the cane is inserted into another jacket TZNL glass tube (g) and drawn into fiber at the same temperature. The jacket tube (g) is utilized to decrease the ratio of the core to the cladding size. Also, the pressure in the preform is reduced in order to avoid the interstitial hole formation in the fiber. By this process, we can obtain the simple all-solid tellurite MOF.
Fig. 1 Schematic diagram for the fabrication of the simple all-solid tellurite MOF: a. TZNL glass tube, b. the hexagonal TZNL glass rod, c. TZNL tube with the elongated hexagonal TZNL glass rod inside, d. the elongated TLWMN glass rods, e. TZNL glass tube to be stacked with the elongated rods, f. cane, g. TZNL glass tube.
Fig. 2 The images of cross sections of the hexagonal TZNL glass capillary (a), and the TLWMN glass capillary (b).
Figure 3 shows the cross-sectional structure of the simple all-solid tellurite MOF. We can see that the diameters of the TLWMN rods are slightly different, with the largest one 3.24 μm and the smallest 2.97 μm. In order for calculation,the average value D1 = 3.10 μm is selected for the diameter of the six rods, and calculation results by multipole method prove that the optical transmission properties have not been greatly affected. The diameter of the TZNL glass core is D0 = 8.34 μm, and The pitch Λ = 5.73 μm. The refractive indexes of the TZNL glass core (n0) and the TLWMN glass rods (n1) are given by the Sellmeier equation
Fig. 3 Scanning optical microscope images of the cross section of the simple all-solid tellurite MOF. (a). The optical microscope image of the reflected light. (b) The optical microscope image of the transmitted light.
At λ = 1.55 μm, n0 = 1.962 and n1 = 2.058. Figures 3(a) and 3(b) show the optical microscope images of the reflected light and the transmitted light, respectively.
3. Characterization
The real (R(neff)) and the imaginary parts (η(neff)) of the effective refractive index, and the chromatic dispersion (D) which are calculated by the multipole method are shown in Fig. 4 . The confinement loss and the material loss are two component losses for the simple all-solid tellurite MOF. However, once the material of the fiber is selected, the material loss is determined. Here we only consider the confinement loss which is decided by the structure. The confinement loss (α) can be obtained from η(neff) by the equation α = 8.686k0•η(neff) [25]. We can see that there are three transmission bands from 1.00 μm to 2.80 μm. The wavelength regions are: 1.10 μm−1.18 μm (3rd band), 1.30 μm−1.64 μm (2nd band) and 1.78 μm−2.66 μm (1st band). From Fig. 4(b) we can see that the dispersion shows great change at the 3rd band, but little at the 1st band and the 2nd band. Three zero-dispersion wavelengths (1.109μm, 1.409 μm and 1.936 μm) are within three bands, respectively. This characteristic is very interesting and will be of wide applications. In order to show how the confinement loss increases for less layers, the η(neff) of two and three layers all-solid tellurite MOF are calculated and shown in Fig. 4(c). We can see that the confinement loss increases with the layer decreasing. The simple all-solid tellurite MOF can support the near-infrared light transmission and display much better performance than the silica MOF. Furthermore, it can support the mid-infrared light transmitting in the core within the transmission bands, and this is what the silica MOF cannot achieve.
Fig. 4 (a), The real part (R(neff)) of effective refractive index of the simple all-solid tellurite MOF. (b), The chromatic dispersion of the simple all-solid tellurite MOF. (c), The imaginary parts (η (neff)) of effective refractive index of the all-solid tellurite MOF with one, two and three layers.
The optical field intensity of the simple all-solid tellurite MOF is calculated at different wavelengths, as shown in Fig. 5 . The light at λ = 1.55 μm is within the 2nd band, and there is the fundamental mode confined in the core, as shown in Fig. 5(a). The light at λ = 1.70 μm is outside the transmission bands and is confined in the TLWMN rods and transmits in them, as shown in Fig. 5(b). Under this condition, the simple all-solid tellurite MOF can be used as a multi-core fiber.
Fig. 5 The optical field intensity of the simple all-solid tellurite MOF at different wavelengths. (a) λ = 1.55 μm, (b) λ = 1.70 μm.
In order to measure the transmission bands of the simple all-solid tellurite MOF, the tunable supercontinuum source (SC450) is used as a pump light. The output spectrum is about from 500nm to 1800nm. A 50 cm length of the fiber is used and the pump light is coupled in the core by an aspheric lens with NA = 0.47. The output end of the MOF is butt-coupled to a single mode fiber which connects with the OSA. The 8 μm core SMF optical fiber is used in order to make sure collected the beam only from the core of the simple all-solid tellurite MOF. Figure 6 shows the transmission spectrum of the simple all-solid tellurite MOF from 800 nm to 1800 nm, and there are two transmission bands (2nd and 3rd band). Because there is only one layer in the cladding, the confinement effect is not strong. A part of the light is lost in the cladding and the transmission bands approximately correspond to Fig. 4(c) (2nd and 3rd band). Strong water absorption at wavelengths around 1400 nm is introduced during the fiber fabrication at the 2nd band.
Figure 7 shows the measured mode profiles of the simple all-solid tellurite MOF at λ = 1.55 μm and λ = 1.70 μm. The 1.55 μm laser pulse with 400 fs and the repetition rate of 16.75 MHz is generated from the homemade fiber laser, and the 1.70 μm laser pulse with 200 fs and the repetition rate of 80 MHz is generated from an Optical Parametric Oscillator (OPO). The output end of the MOF is butt-coupled to a Laser Beam Profiler (LEPAS-11) and the mode fields are measured. During the experiment process, we must make sure the laser beam is only coupled in the core. The light at λ = 1.55 μm is within the 2nd band and confined in the core, as shown in Fig. 7(a), which is consistent with Fig. 5(a). There is only one layer and the confinement effect is not very strong, so part of the light is lost in the cladding. The light at λ = 1.70 μm is outside the transmission bands and confined in the six TLWMN rods in the cladding, as shown in Fig. 7(b), which is consistent with Fig. 5(b). Light transmits in the six TLWMN rods and the fiber can be considered as a multi-core fiber. These experimental results can be explained by the ARROW guiding mechanism [13]. When wavelengths do not satisfy the resonant reflection condition (within transmission bands), antiresonant reflection takes place, and light is confined in the core. For other wavelengths that satisfy the resonant reflection condition (outside transmission bands), the light is confined in the high-index TLWMN rods.
Fig. 7 The measured mode profiles of the simple all-solid tellurite MOF at λ = 1.55 μm (a), and λ = 1.70 μm (b).
4. Summary
A simple all-solid tellurite MOF with only one layer of TLWMN rods in the cladding is proposed in the paper. When the pump light is within the transmission bands, the fiber can support the near- and mid-infrared light transmitting in the core and be used as a filter. When outside the transmission bands, the light transmits in six TLWMN rods and under this condition, the fiber can be considered as a multi-core tellurite fiber. Apart from these, the simple all-solid tellurite MOF can be considered as a step-index tellurite fiber when the laser beam is coupled to every TLWMN rod in the cladding. It can be widely applied in the photon device fields, such as wavelength conversion, switching and mid-infrared highly nonlinear devices, etc.
Acknowledgment
This work is supported by MEXT, the Support Program for Forming Strategic Research Infrastructure (2011-2015).
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