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We report for the first time multi-watt mid-infrared supercontinuum (SC) output from a dehydrated tellurite step-index fiber. The tellurite glass preform is melted in a dry-atmosphere filled glovebox and the water impurity in the glass has been reduced to 1ppm level. The fiber has a large core diameter of 12 µm and a numerical aperture (NA) of 0.21. Using the amplified Raman-shifted soliton pulses from a 1.55 µm erbium-doped fiber laser as the pump, a broadband 2-3 µm supercontinuum with a 3dB bandwidth of 985 nm has been generated from an 80cm-long dehydrated tellurite glass fiber. The power spectral density of the SC is above 3.5 dBm/nm in the whole range of 2-3 µm. The net output power of the supercontinuum from the tellurite fiber is measured to be 2.1 W. To the best of our knowledge, this is the highest average power of mid-IR SC generated from a tellurite fiber. The spectral density of above 3 dBm/nm is also one of the highest spectral densities of a SC achieved in 2.5-3 μm range from an oxide glass fiber.
© 2016 Optical Society of America
1. Introduction
Mid-infrared (mid-IR) fiber laser sources beyond 2 µm are highly demanded for many applications from military, security, environmental monitoring, to medical surgery etc. For example, because the mid-infrared spectral region of 2–20 μm is a domain, in which a large number of molecules undergo strong characteristic vibrational transitions, this makes mid-infrared spectroscopy a univocal way to identify and quantify molecular species [1], a mid-infrared LIDAR (LIght Detection And Ranging) is thus capable of combining both functions of ranging the distance and remotely monitoring the atmosphere compositions, e.g., explosive gases. Moreover, the mid-IR wavelengths of 2.1, 2.9 and 10.6 μm are important for medical laser surgery because the interaction between the laser energy and the tissues at different depths can be flexibly utilized at one optimum wavelength based on the water absorption coefficient at the different wavelength [2–4]. All these applications require a mid-IR multiwavelength laser source with watt-level output power or mW/nm-level power spectral density.
So far, (i) rare-earth-doped fiber lasers, (ii) rare-earth- or transition-metal-doped solid-state lasers, (iii) χ(2)-based nonlinear frequency convertors, and (iv) semiconductor quantum cascaded lasers (QCL), are the four major families of mid-IR solid-state laser sources [5].
In the case of rare-earth-doped fiber lasers, it is seen that both the output power and the slope efficiency drop dramatically when the lasing wavelength extends beyond 2 µm [6], due to the combination of the phonon energy of the host glass materials and the fiber propagation attenuation at the corresponding lasing wavelength. As a result, the reported lasing longwave limit of active silica glass fibers, which have a phonon energy of 1100 cm−1, is at ~2.2 μm [7], while the reported lasing longwave limit of active fluoride (ZBLAN, ZrF4-BaF2-LaF3-AlF3-NaF) glass fibers, which have with a phonon energy of 550 cm−1, is at 3.95 μm in a holmium-doped fiber with a slope efficiency of only 3.7% [8]. One of the major drawbacks of rare-earth-doped fiber lasers for realizing multiwavelength lasing in the mid-IR region is that the tunability of lasing wavelengths is small, since the emission bands of rare-earth ions are narrow and nearly wavelength-independent regardless of the host materials.
Amongst other rare-earth- or transition-metal-doped solid-state lasers, transition metal (TM2+, e.g., Cr2+ or Fe2+) doped II-VI chalcogenide crystals, e.g., ZnSe, ZnS, CdSe, CdS, ZnTe, are excellent multiwavelength mid-IR lasers with wide tuning range. For example, Cr2+-doped ZnS(Se) crystal lasers can be tuned between 2.0 and 3.0 μm, while Fe2+-doped CdSe crystal lasers can be tuned from 4 to 5 μm [9].
Semiconductor-based quantum cascade lasers (QCLs) currently form the basis of mid-IR photonics, because the electro-optical conversion is always the most convenient approach to obtain the light source. With the recently achieved tuning range from 3 μm to beyond 100 μm, QCLs are rapidly becoming practical mid-IR and far-IR sources for a wide variety of applications, such as trace-chemical sensing, health monitoring and IR countermeasures [10, 11]. The major drawbacks of QCLs as multiwavelength mid-IR laser sources are that (i) it so far cannot cover the wavelengths below 3 μm; (ii) the cost of a QCL with a fixed lasing wavelength is still high and it will be even more expensive for constructing a wavelength-tunable QCL; and (iii) the output average power of commercial QCLs operating at room temperature are typically less than 500 mW, which is somehow insufficient for many applications, e.g., air-borne LIDARs.
The family of nonlinear frequency convertors is extremely versatile as multiwavelength laser sources. Typically a χ(2)-based nonlinear-crystal optical parametric oscillator (OPO) or optical parametric amplifier (OPA) or difference frequency generator (DFG) are capable of generating laser from visible region to mid-IR region beyond 10 μm [5]. But in order to achieve watt-level output power and wide wavelength tunability, such a χ(2)-based nonlinear laser source is normally large in size and requires complex optical configurations. Instead, the relatively recent progress in developing dispersion-tailored highly nonlinear optical fibers [12] has shown that fiber-based χ(3) nonlinear laser sources, such as the supercontinuum (SC) [13], fiber OPOs [14], or frequency combs [15] can also fulfill this task. What is more, fiber lasers show significant advantages over other solid state lasers as an effective approach to provide economic, compact and flexible optical components. Finally, needless to say, excellent beam quality can be obtained from a single-mode nonlinear fiber.
A mid-IR transparent nonlinear glass is required for constructing a mid-IR nonlinear fiber laser source. The position of the IR absorption edge (i.e., the infrared longwave transmission limit) of an optical glass is intrinsically limited by the multiphonon absorption edge of the glass. Silica shows inferior transparence beyond 2 μm, due to (i) the strong fundamental vibration hydroxyl absorption at 2.7 μm and (ii) high loss (>50 dB/m) starting from 3 μm due to the tail of the multi-phonon absorption of Si-O network. In general, non-silica glasses, such as tellurite (TeO2 based), fluoride (typically ZrF4, AlF3, or InF3 based), and chalcogenide (chalcogen S, Se, Te based) glasses [16–18], possess excellent optical transparence in the wavelength range of 0.5-5 μm, 0.4-6 μm and 1-16 μm respectively and thus are attractive candidates as fiber materials for mid-infrared applications over the conventional silica glass.
Chalcogenide glass shows excellent IR transmission up to 16 μm and possesses high n2 of 100-1000 x 10−20 m2/W [19]. This makes it the best candidate as mid-infrared nonlinear medium. However, the zero dispersion wavelength of a chalcogenide glass bulk is beyond 5 μm. In order to make the zero dispersion wavelength of a chalcogenide glass fiber close to 1.5 or 2 μm, i.e., the lasing wavelength of the conventional erbium or thulium doped fiber lasers, very large waveguide dispersions need to be introduced, requiring the final fiber core diameter to be near 1 micron [20]. This is disadvantageous for power scaling. Indeed, the recent activities have shown that broadband mid-IR supercontinuum can be generated from a step-index chalcogenide glass fiber with a 16µm-diameter As2Se3 glass core. The fiber was excited by a femtosecond pulsed OPA source at 6.3 µm, which is close to the zero dispersion wavelength of the large-core fiber. The generated supercontinuum covers from 1.4 to 13.3 µm (30 dB bandwidth) with an average output power of 0.15 mW [21]. Taking the core diameter as the size of the emittance aperture, the spectral radiant exitance is then estimated to be no more than 1 mW/m2/nm. Because the blackbody at 300 K provides the similar value of the spectral radiant exitance and the similar bandwidth, the spectral intensity of the broadband mid-IR SC generated from such a chalcogenenide glass fiber [21] is actually very weak, far away from the requirement for practical usages.
Fluoride glass fibers have been proven to be a suitable nonlinear medium for generating high-power supercontinuum [22–25]. Although ZBLAN glass itself possesses a low nonlinear refractive index n2 (2 x 10−20 m2/W) like that of a silica glass, the low fiber attenuation of the fluoride fiber, 0.01-1 dB/m, in 2-4 μm region allows the accumulation of nonlinearity along the long fiber. In addition, the zero dispersion wavelength of the fluoride material is located at ~1.7 μm [26], which is located between the conventional high-power Er-doped fiber lasers and Tm-doped fiber lasers. This leads to the broad and flat 2-4 μm supercontinuum with a recorded average output power up to 21.8 W [24], corresponding to the spectral intensity of 10 mW/nm level. A very recent work reported that the supercontinuum extended to 5.4 μm with the spectral intensity more than 1 µW/nm from an InF3-based fiber, which has an extended transparent window up to 5.5 µm in comparison with the 4.5 µm limit of the ZBLAN fiber [27]. It should be noted that the configuration of the all-fiber device including a pulsed fiber pump source, erbium-doped fiber laser at ~1.55 μm or thulium-doped fiber laser at ~2 μm [22–25] is so-far the most promising approach to realize the compact practical mid-IR nonlinear fiber laser devices. The major shortcoming of a fluoride glass fiber as the high-power laser medium is that the glass is highly hygroscopic. Therefore, when the operation power is high and the generated heat will speed up the degradation of the fiber facets [28].
Alternatively, for a long time tellurite glasses have been viewed as a high-index, highly nonlinear version of the fluoride glasses. In terms of the thermal properties, both of them possess steep viscosity curves around their fiber drawing temperatures [29]. In terms of the optical properties, both bulk materials have the zero dispersion wavelengths around 2 μm [30] and the longwave transmission limit around 7 μm [29]. But tellurite glasses have high refractive index n (2.0-2.2) and nonlinear refractive index n2 (20-50 x 10−20 m2/W), while fluoride glasses typically have n of ~1.5 and n2 of ~2 x 10−20 m2/W, similar to that of a silica glass. Therefore, tellurite glasses are an ideal host material as a fiber nonlinear medium for 2-5 μm range. In principle, high-power broadband mid-IR processes such as four wave mixing can be realized using a single-mode large-mode-area tellurite fiber pumped by a high power 1.55 or 2 µm fiber laser [31, 32]. Since the tail of multiphonon absorption of tellurite glasses starts ~5 μm, the strong water absorption ranging from 3 to 4 μm in tellurite glass has been proven to the last barrier preventing the realization of a practical 2-5 μm tellurite-fiber-based mid-IR nonlinear laser source with watt-level output power. Without the dehydration process, the typical absorption coefficient of water impurity in a tellurite glass is ~1500 dB/m at the peak of 3.4 µm [33]. The recent dehydration works on mid-IR tellurite glass fibers have reduced the water impurity down to 1ppm level, by introducing halides into the starting chemicals and melting the glass in a dry-atmosphere-filled glovebox [34].
In this work, we report the fabrication of a dehydrated tellurite glass step-index fiber for mid-IR nonlinear applications. The fabricated tellurite fiber is with a core diameter of 12 µm and an NA of 0.21. Using the conventional Raman-shifted soliton pulses from a 1.55 µm erbium-doped fiber laser as the seed following with a thulium doped fiber amplifier as the pump, broadband mid-infrared supercontinuum has been generated from an 80cm-long dehydrated tellurite glass fiber. The net output power of the supercontinuum from the tellurite fiber itself is measured to be 2.1 W, with a 3 dB bandwidth of 985 nm ranging from 1985 nm to 2970 nm. To the best of our knowledge, this is the highest average power of mid-IR supercontinuum generated from a tellurite fiber and also the highest spectral density beyond 3 μm of the SC from an oxide glass fiber.
2. Fabrication of dehydrated tellurite glasses and fiber
The tellurite glass fiber is based on the composition of 70TeO2-20ZnO-10BaO (mol.%) [16]. The glass has a refractive index of 2.0 and nonlinear coefficients n2 of 2.6 x10−19 m2/W at 1.55 µm and 2.0 x10−19 m2/W at 2.0 µm respectively [35, 36].
The powders of TeO2, ZnO, BaO and BaCl2 with 99.999% purity are employed as the starting chemicals for glass melting. The compositions of the core and the cladding glasses are slightly different from each other so that the difference of their refractive indices could allow a sufficient NA for guidance and their thermal properties are still be compatible during the fiber drawing. Their index difference is estimated to be 0.012 according to their differential chemical compositions [37] and the NA is estimated to be 0.21. Both core and cladding glasses are melted inside a dry-atmosphere filled glovebox with ~1 ppm water level. The barium chloride powder is mixed into the starting chemicals for actively dehydration [34]. The glass melting has been kept under the temperature of 900 °C for 4 hours. The residual water impurity in the glass sample has been monitored by measuring the transmission spectrum of each bulk by a Fourier transform infrared (FTIR) spectrometer. In order to avoid the possible contamination from the polishing procedure, each glass melt has been casted into a preheated brass mould with a slot. The two parallel faces of the brass plates constructing the slot have been well polished and the surface roughness of the obtained glass slab is estimated to be less than 1/10 of the wavelengths in the FTIR measurement. Figure 1 shows the measured water-induced absorption spectra of the core bulk glass with a thickness of 6.66 mm. Under the same melting conditions, i.e., the melting temperature, the melting time, the gas flow rate and so on, it is seen that the peak water absorption in three selected samples varies between 5.8 dB/m and 7.1 dB/m in the range of 3.2-3.3 µm. Such fluctuation indicates the uncertainty of estimating the water-induced loss based on a cm-thick bulk sample. Ref [33]. provides an estimated OH impurity content of 3.7 x 1019 ions/cm3 in the tellurite glass prepared in the open atmosphere with a corresponding absorption coefficient of 3.02 cm−1 at 3.3 µm peak. The result in this work shows a reduction factor of 200 in terms of the OH absorption coefficient at 3.3 µm peak. Thus the OH content in our dehydrated tellurite glass is calculated to be 1.8 ± 0.2 x 1017 ions/cm3. Note that the two small peaks at 3.43 μm and 3.51 μm in Fig. 1 are artifacts, arising from the protective polymer thin film coated on the optic elements inside the FTIR instrument.
Figure 2 illustrates the measured DTA (differential thermal analysis) curve of the used core glass. It is seen that the glass has a glass transition temperature Tg at 330 °C. 40 mg of glass cullet is used in the measurement with a ramp rate of 10 °C/min. No crystallization peak is seen on the DTA curve, indicating that the glass has good thermal stability for fiber drawing.
Figure 3 illustrates the schematic procedure of using modified built-in casting technique [38] to make a core/cladding glass preform. First, the melt of the cladding glass is casted into a preheated brass mould (see Fig. 3(a)). Then the melt of the core glass is topped onto the cladding glass melt (see Fig. 3(b)). The mould is then moved to sit above the hole on the bottom plate to let the central unsolidified cladding melt leak out. The core glass melt then fills into the central void inside the cladding cylinder (see Fig. 3(c)). The final solidified preform has an outer dimension (OD) of 12 mm and a length of 70 mm. Note that the glass for making the core/cladding preform and the above bulk sample for FTIR measurement are from the same batch and their OH content should be the same.
Fig. 3 Schematic of making core/cladding preform by modified built-in casting method. (a) Casting cladding glass into preheated mould; (b) topping core glass melt onto the cladding glass; (c) core glass melt filling into center of the cladding glass in the preform.
The preform is then directly drawn into a step-index fiber with the yield of ~100 meters. Figure 4 shows the cross-sectional view of the fiber with an OD of 165 µm. The core shows a little ellipticity, with diameters of 11.8 x 11.1 µm.
It has been well understood that the relation between the glass composition and a specific property, such as the thermal expansion coefficient, the refractive index, the density and so on, are linear to all glass component concentrations, assuming an ideal mixture. This additivity principle, proposed by Otto Schott in the late 19th century for designing optical glasses, has been proven to be a powerful tool to precisely predict and design the glass properties [37]. Based on the data of the refractive index of TeO2-ZnO-BaO glasses provided by References [30, 39], the material dispersion curve of 70TeO2-20ZnO-10BaO (mol.%) glass is calculated from 1.0 µm to 4.5 µm (see Fig. 5). It is seen that the zero dispersion wavelength of the bulk material is located at 2.22 µm, which is slightly longer than the 2.15 µm of the commonly used tellurite glass composition: 75TeO2-20ZnO-5Na2O (mol.%) [31, 34].
Fig. 5 Material dispersion curve of fabricated fiber based on 70TeO2-20ZnO-10BaO (mol.%). Inset: detailed curves of material dispersion and total dispersion of fabricated fiber near 2.2 µm.
Due to the large core size of the fabricated fiber, the total dispersion (Dtotal = Dm + Dw) of the fabricated fiber should be dominated by the material dispersion (Dm). The waveguide dispersion (Dw) of the fundamental LP01 mode is calculated to be 1.5 ± 0.5 ps/nm/km in the range of 2.0-2.4 µm, much weaker than the material dispersion even around the zero dispersion wavelength of the bulk material. The zero dispersion wavelength (ZDW) of the fabricated large-core tellurite fiber is located at 2.20 µm, showing slightly blue-shifted in comparison with the material dispersion (see the inset in Fig. 5).
Figure 6 shows the calculated relative reduced Raman intensities of the studied tellurite glasses, 70TeO2-20ZnO-10BaO (TZB) (mol.%) and 75TeO2-20ZnO-5Na2O (TZN) (mol.%), in comparison with a referenced pure silica glass, normalized to the peak intensity of silica at 440 cm−1. The calculation procedure of relative reduced Raman intensity was described in detail in [32] by Shi et al. The TZB glass has a calculated peak Raman gain coefficient gR of 17 x 10−11 cm/W at 2.0 μm, ~35 times higher than that of silica (0.49 x 10−11 cm/W), indicating its potential as a mid-IR fiber-based Raman gain medium.
3. Generation of mid-infrared supercontinuum
The experimental setup for generating mid-IR SC from the fabricated tellurite fiber is shown in Fig. 7. The pump system consists of a 1.55μm erbium-doped fiber laser, a 25m-long SMF28 single-mode fiber, a thulium-doped fiber amplifier (TDFA) and a 0.8m-length of tellurite fiber. The 1.55μm erbium doped fiber laser possesses a pulse duration of 6 ns, a repetition of 150 kHz and an maximum average power of 4.5 W, corresponding to a peak power of 5 kW. During propagating along the SMF28 fiber, the spectrum of the 1.55μm pulses broadens while its wavelengths keeps shifting towards 2 μm, primarily due to the breaking up of the nanosecond pulses through modulation instability (MI) followed by soliton self-frequency shifting [22]. In such a procedure, the nanosecond pulses break up into picosecond and sub-picosecond pulses and consequently some of the generated pulses possess enhanced peak power [22]. After the 25m-long SMF28 fiber, these picosecond and sub-picosecond soliton pulses broaden ranging from 1.55 μm to 2.4 μm, and the average power of 1.55 μm pulses decreases from 4.5 W to 3 W mainly due to the quantum defects from the pump photons to the generated red-shifted photons.
The generated soliton pulses are then coupled into the TDFA. The short-wave components are absorbed by thulium dopants in the TDFA. Six fiber-pigtailed multimoded laser diodes at 793 nm with the total maximum pump power of 70.5 W are employed as the pump of the TDFA. The gain fiber of TDFA has a double-cladding geometry. The diameters of the core and the pump cladding are 10 μm and 130 μm, with the corresponding NA of 0.15 and 0.46, respectively. The gain fiber only supports single-mode operation at 2 μm. The absorption coefficient of the pump cladding is 4.7 dB/m at 793 nm. To ensure sufficient absorption of the pump light, the length of the gain fiber is chosen to be 3 m. Without the excitation of the 793-nm pump, the residual average power of the longwave part beyond 1.95 μm is measured to be 1.2 W.
A 0.5m-long SMF-28 fiber is spliced to the end of the TDFA to strip the residual pump power at 793 nm. The output end of this SMF-28 fiber is butt-coupled into the fabricated tellurite fiber with a length of 0.8 m. An angle of 8° is cut on both fibers to minimize the backward reflection. The coupling efficiency is measured to be ~60% per point at 2 μm, when the launched power is controlled at low level. Although the tellurite fiber supports 7 modes at 2 μm, it has a similar mode diameter, ~12 μm, for the fundamental LP01 mode to that of the SMF28 fiber. Figure 8(a) shows the far-field pattern observed from the output end of tellurite fiber, when the TDFA output is only 10mW. It is seen that for both orthogonal directions, the mode shows good Gaussian profiles. Effectively single-moded guidance is therefore ensured in the tellurite fiber during the coupling. In order to effectively remove the heat generated in the tellurite fiber under high pump power, water-cooling procedure has been applied onto the input end of the tellurite fiber.
Fig. 8 (a) far-field pattern observed from output end of tellurite fiber with TDFA output of 10mW. (b) Spectral traces from TDFA output and corresponding SC generated from 0.8-m tellurite fiber at different pump power of 5.2 W, 7.1 W, 9.8 W. The pink arrow indicates the position of the zero dispersion wavelength (ZDW) of the fabricated tellurite fiber.
Figure 8(b) illustrates the output traces of the TDFA at the average pump power of 5.2 W, 7.1 W, and 9.8 W respectively, and the corresponding SC traces generated from the tellurite fiber with the average output power of 3.2 W, 4.2 W and 5.1 W. The spectral traces are recorded by a PbSe detector with a single-grating monochromator (iHR320, Horiba). With the increase of the pump power from 5.2 W to 9.8 W, the spectrum of TDFA output crosses 2200 nm, which is the ZDW of the fabricated tellurite fiber, and the majority of the pump power is distributed beyond the ZDW of the fiber. The mid-IR SC generated from the tellurite fiber is then seen extending towards the longwave direction and shows fair flatness. At the maximum output power of 5.1 W, the generated SC possesses a high-spectral flatness with a 3 dB spectral bandwidth of 985 nm, spanning from 1985 nm to 2970 nm, and a 10 dB bandwidth of 1155 nm, spanning from 1925 nm to 3080 nm. Even at 3.05 μm, the spectral density is still above 0 dBm/nm, i.e., 1 mW/nm. No damage and degradation are observed on the input facet of the tellurite fiber under the maximum output power for 1 hour. Taking the 60% coupling efficiency into account, the total power conversion efficiency from the TDFA output to the total SC generated from the tellurite fiber is calculated to be as high as 87%, at the maximum pump power of 9.8 W. The net SC output generated from the tellurite fiber itself is calculated to be 2.1 W, by integrating the spectrum beyond 2530 nm, at the maximum pump power of 9.8 W. To the best of our knowledge, this is the reported highest average power of SC generated from a tellurite glass fiber. In addition, the spectral density of the SC output in 2.5-3 µm range in this work is comparable with the results in the very recent work on SC generated from a Germania (GeO2) glass core fiber [40]. It is believed to be one of the so-far recorded highest spectral density of SC generated from oxide glass fibers in the range of 2.5-3 µm.
All in all, because the oxide glass naturally possesses better chemical durability and less toxicity than its counterparts, chalcogenide and fluoride glass fibers, the performance of the fabricated dehydrated tellurite glass fiber shows good potential as a practical fiber-based mid-IR supercontinuum source with high power spectral density. To further extend the generated SC beyond 3 μm, a pulsed pump with higher peak power and/or shorter pulse duration is required. Such a work is currently ongoing.
4. Summary
We report the fabrication of a dehydrated tellurite glass step-index fiber for mid-IR nonlinear applications. The water impurity in the tellurite glass has been reduced to 1ppm level. The fiber has a large core diameter of 12 µm. Using the amplified Raman-shifted soliton pulses from a 1.55 µm erbium-doped fiber laser as the pump, broadband mid-infrared supercontinuum with the power spectral density above 3.5 dBm/nm has been generated from an 80cm-long dehydrated tellurite glass fiber. The 3 dB bandwidth of the SC is 1025 nm, ranging from 1975 nm to 3000 nm. The net output power of the supercontinuum from the tellurite fiber itself is calculated to be 2.1 W. To the best of our knowledge, this is the highest average power of a mid-IR supercontinuum generated from a tellurite fiber and also one of the highest spectral density of a SC in 2.5-3 μm range achieved from an oxide glass fiber.
Funding
National Natural Science Foundation of China (NSFC, Nos. 61527822, 61235010 and 61307054); and Beijing University of Technology, China.; Beijing Overseas Talents Center.
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