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Abstract
Fluorotellurite glasses based on TeO2-BaF2-Y2O3(TBY) with a broadband transmission window of 350–6000 nm, relatively low phonon energy, stable chemical and physical characteristics compared to fluoride glasses have been investigated to develop high-powered mid-infrared (MIR) fiber lasers. In this work, a series of xTeO2-(90-x)BaF2−10Y2O3 (TBYx) (x = 60,65,70,75 mol%) fluorotellurite glasses were prepared with the conventional melting-quenching method in an argon glove box. Then, laser damage characteristics of glass samples under 3000 and 4000 nm MIR femtosecond laser with different pulses was compared and studied. The TBY60 glass has the highest damage threshold, which reaches 1.08 J/cm2 and 0.852 J/cm2 at 4000 nm and 3000 nm, respectively. The femtosecond laser-induced damage threshold (LIDT) of TBY glasses decreased from 1.08 to 0.782 J/cm2 as TeO2 content increased from 60 to 75 mol% at the wavelength of 4000 nm. In addition, the effect of pulse numbers is consistent with exponential defect accumulation model.
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1. Introduction
MIR fiber lasers have gained lots of significant applications in the fields of basic scientific research, optical communication, biomedicine, environmental monitoring and national defense and security [1]. Specifically, the laser source with a spectral coverage of 3–5 µm has attracted extensive attention because the window contains a region of high atmospheric transmission, which makes the laser source have potential applications in optical detection and ranging or infrared countermeasure. Traditional silica fiber is extensively used in modern communication due to its low loss, high mechanical strength and stability. However, it is difficult to use in 3–5 µm MIR band due to the transmission range of silica fiber is less than 2.4 µm [2].
In recent years, multicomponent glass fibers represented by chalcogenide, fluoride, and fluorotellurite fibers had received much attention in the fields of supercontinuum light source [3,4], MIR laser generation [5] and biochemical detection [6,7]. Among these soft-glass fibers, chalcogenide fibers possess the highest nonlinear refractive index (1.5×10−17 m2/W for As2Se3 [8] and 3×10−18 m2/W for As2S3 [9]) and has the best transparency above 6 µm. However, because the glass transition temperature is relatively low, resulting in the low optical damage threshold of the glass, chalcogenide fiber is mainly used for ultra-broadband MIR-SC, and rarely used for the development of high-power laser source [10,11].
Fluoride fibers with wide MIR transmission window (0.3–7.5 µm), low theoretical loss (about 2 orders of magnitude lower than silica fiber), small phonon energy (the maximum phonon energy is approximately 600 cm−1) and small material dispersion [12]. In recent years, breakthroughs have been reported in various indexes of MIR light source based on ZBLAN fluoride fibers. In terms of rare earth ion-doped fiber laser, almost all the reported MIR lasers with wavelength beyond 2.2 µm are realized by using ZBLAN fiber doped with rare earth ions as gain medium. In generating the MIR-SC light source, ZBLAN fiber is widely used to obtain MIR supercontinuum laser light source with high power, high coherence and wide bandwidth. Tianyi Wu et al. [11] reported a 10-Watt-level MIR supercontinuum with the range of 0.8–4.7 µm in a section of InF3 fiber. Aydin et al. [13] did some experiments on the long-period stability of high powered MIR laser based on ZBLAN fiber. Since the protective end cap is an undoped ZBLAN end cap, the photodegradation or damage of the ZBLAN end cap is observed owing to the OH diffusion in the fiber tip after the output power of the MIR laser is approximately 20 W and continuous operation for 10 min. As the end cap was replaced by the AlF3 based glass end cap, the duration was extended to 7 hours [14]. The practical application of high-powered MIR laser source based on ZBLAN fiber is hindered due to the poor chemical and thermal stability of ZBLAN glass.
Tellurite fibers have good transparency in the range of 0.4–5 µm, stable chemical property and high nonlinearity. It is an ideal dielectric material for high-power MIR laser source. Thapa et al. [15] showed the generation of SC range from 1 to 4.4 µm with the power of 1.2 W in tellurite fiber. Shi et al. [16] gained 2.1 W MIR-SC with spectral range of 1920 –3080 nm when the core material was 70TeO2-20ZnO-10BaO tellurite glass fiber. However, the relatively low glass transition temperature (about 330 ° C) of tellurite fiber prevents the further improvement of the output power of MIR laser source [16]. To improve the performance of MIR laser sources with tellurite fiber, fluorotellurite fibers made of TeO2-BaF2-Y2O3 glasses with broad transmission window (350–6000 nm), good chemical and physical stabilities have been developed [17]. Important achievements have been made in the generation of high-powered supercontinuum spectrum. Jia [18] demonstrated 4.5 W MIR-SC generation with the coverage of 1017–3438 nm in TBY fluorotellurite fibers with a length of 60 cm, and the pump power was about 10.48 W. Li [19] reported that using a TBY fluorotellurite fiber as nonlinear medium and obtained 22.7 W MIR-SC generation in the range of 930–3950 nm at an emission pump power of about 39.7 W. Yao [20] utilized a TBY fluorotellurite fiber with core diameter of 6.8 µm and length of 0.6 m, 10.4 W SC (0.947 - 3.934 µm) was obtained when the pump power was approximately 15.9 W.
In the above report, the pump laser and these nonlinear fibers were usually coupled by lens coupling or mechanical connection (end butt joint) [16–19,21]. In this coupling mode, the end face of the optical fiber was in an unprotected state, thus the end face was easy to be damaged when working for a long time under high power conditions. Fluorotellurite glass based on TBY glasses have high nonlinearity and damage threshold. However, there is no systematic study on the relationship between laser damage threshold and the composition of TBY glass has been conducted. In this study, a series of TeO2-BaF2-Y2O3 glasses were prepared and the damage characteristics of femtosecond laser on the glass are studied in detail. The findings provide a reference for optical fiber dielectric materials to obtain a higher power output SC spectrum or Raman laser output in the near future.
2. Experimental
2.1 Glass and fiber preparation
Fluorotellurite glass samples with the composition of xTeO2-(90-x)BaF2-10Y2O3(TBY) (x = 60, 65, 70, 75 mol%), marked as TBY60, TBY65, TBY70, and TBY75 were melted using a conventional melting-quenching method in an argon glove box, and the starting chemical materials have high purity (5N). After annealing at a constant temperature for 180 min, the temperature naturally drops to room temperature. The glass samples were polished to the size of Ф9 mm ×2 mm for subsequent optical measurement.
2.2 Properties of the samples
All the measurements of thermal, optical and femtosecond laser were carried out at room temperature. The transmittance in 200–2500 nm and 2500–6700 nm was obtained by the ultraviolet spectrophotometer (Lambda 950 UV-VIS-NIR, PerkinElmer) and Fourier transform infrared spectroscopy (Nicolet 380, Thermo Scientific), respectively. The thermal properties of the TBY glasses were investigated by using differential scanning calorimetry (Differential Scanning Calorimetry, DSC, Q2000) at a heat rate of 10 K/min under the protection of a flowing nitrogen atmosphere. Raman spectra were obtained by confocal micro-Raman spectrometer (InVia, Renishaw) and 785 nm excited laser with a resolution of 0.5 cm−1 with the coverage of 100–700 cm−1.
2.3 Laser damage measurement and analysis
The femtosecond laser experimental configuration consisted of a Ti: sapphire femtosecond laser (Mira900D+, Coherent) and an optical parametric amplifier (OPA) (Legend Elite + OperA Solo, Coherent) to emit 150 fs pulses at different central wavelengths of 4000 and 3000 nm with a repetition of 1kHz. The laser beam emitted by the OPA system firstly passes through a pair of polarizers and uses a beam attenuator to adjust the power required for the experiment. The electronic shutter is used as the function of setting different pulse numbers. Under a given number of pulses, other experimental conditions remain unchanged except that the laser power can be adjusted. Then, the laser beam is focused on the sample surface through a CaF2 lens with a focal length of 30 mm, forming a nearly circular spot on the surface. To better guarantee the reliability of the experiment, the irradiation was repeated four times at different positions of the same sample under the same conditions. Schematic diagram of experimental device for femtosecond laser-induced damage testing is shown in Fig. 1.
The surface ablation morphology of the glasses after laser ablation was observed by ultra-long depth of field optical microscope (VHX-1000E, Keyence). The microscope can display the ablation in the sample surface from 3D perspective view, and can easily obtain the area of the damage craters generated by the laser. The morphology of the glasses before and after laser damage can be observed by optical microscope and scanning electron microscope (VEGA3 SB-EasyProbe, Tescan).
3. Results and discussion
3.1 Transmission and band gap characteristics of TBY glass samples
The transmission spectra of the four TBY samples are illustrated in Fig. 2(a). It is distinct that they have analogous transmittance characteristics with the coverage of 2.6–6 µm. With the addition of TeO2 content from 60 to 75mol%, the transmission ratio decreases from 86 to 75% at 3000 nm. Figure 2(b) and 2(c) show the MIR absorption edge is red shifted from 6180 nm to 6269 nm, and the ultraviolet absorption edge is blue shifted from 387 nm to 365 nm. The reason for this phenomenon is that under the same covalent conditions, the BaF2 can provide a lower skeleton oscillation frequency than TeO2. In other words, the Ba2+ and F- may reduce the field strength and loosen the basic unit structure of TeO2. Therefore, the infrared cut-off frequency can be reduced, resulting in the red shift of MIR absorption edge. In addition, because the electronegativity of fluorine atoms is higher than that of oxygen atoms, the band gap of fluorotellurite glass is larger than that of tellurite glass, which corresponds to the blue shift of UV absorption edge. The residual OH impurities in the glass form an absorption peak at 3000 nm, but Fig. 2(a) shows a weak absorption peak at 3000 nm, indicating that the hydroxyl group is effectively eliminated in the melting process of the glass [21]. During the melting process, the OH content may be reduced due to the following chemical reactions [22,23].
Fig. 2. (a) Transmission spectra versus wavelength. Insert: four images of bulk TBY glasses with 2 mm thickness. (b) UV absorption edge. (c) MIR absorption edge. (d) Relationship between TeO2 content and band gap of TBY glass samples with thickness of 2 mm. The inset of Fig. 2(d) is the relationship between (αhv)2 and photon energy.
The transmission curve can directly obtain the absorption curve of the glass sample in the band of 400–2500 nm. The optical band gap of the sample can be calculated according to the Tauc formula as follow [Eq. (4)] [24]:
where K is a constant, ${E_g}$ is the optical band gap, n is the index number and $hv$ is the photon energy. The illustration in Fig. 2(d) shows the Tauc diagram and fitting procedure of TBY glass, and the band gap values are obtained by extrapolating the intersection of the linear part curves and the x-axis. Figure 2(d) reveals the relationship between TeO2 content and band gap of TBY glass samples, the Eopt decreases from 3.847 to 3.732 eV as the TeO2 content increases from 60 to 75 mol%.3.2 Thermal properties of TBY glass samples
The thermal properties of TBY glass samples are shown in Fig. 3. According to Fig. 3, the thermal characteristics and glass forming ability of TBY glasses can be deduced. The influence of TeO2 content on the thermal stability of the aforementioned glasses was indagated. As the TeO2 content increased from 60 to 75 mol%, the transition temperature (Tg) of TBY glasses increased from 427 to 432 °C. It is discovered that the addition of BaF2 does not have a great effect on the transition temperature. According to reports, the fracture of Te-O bond and the resulting network will reduce the Tg value. However, the size of Ba2+ is larger, which is expected to fill the voids caused by fractures in the network, increasing the Tg value of these samples. In other words, fluorine ions may form non bridged ion metal fluoride bonds, resulting in the breakage of covalent Te-O-Te bonds and the decrease of Tg value. In this paper, the value of Tg remains almost constant, indicating that the effect of ion bond produced by F- is offset by larger Ba2+ [25,26]. The Tg of the TBY fluorotellurite glass is much higher than that previously reported typical 70TeO2-20Zn2O-10Na2O (∼302 °C) tellurite glass [27], ZBLAN (53ZrF4-20BaF2-4LaF3-3AlF3-20NaF) (∼271 °C) fluoride glass [27], and As2S3 (∼187 °C) chalcogenide glass [28]. Accordingly, the TBY fluorotellurite glass shows better physical and chemical characteristics and has extraordinary potential in the development of high-power MIR fiber lasers.
3.3 Raman spectra of TBY glass samples
In order to study the Raman gain characteristics after the change of TeO2 and BaF2 content in the fluorotellurite glass and understand the evaluation of the following physical properties, the Raman spectra of TBY glasses were measured as shown in Fig. 4(a). The TBY glasses typically are composed of TeO4 triangular bipyramid (TBP) structural units where each oxygen atom is shared between two TBP units. Two axial oxygen atoms and a lone pair electron are arranged in an equilateral triangle around the central Te atom and the top oxygen atom on the long axis. The TeO4 TBP units can be embellished by adding various modified atoms to convert to TeO3 triangular pyramids (TP) and TeO3+σ polyhedral units, thereby forming non bridged oxygen (NBO) atoms in the glass network system. The existence of bridging atoms and non-bridged atoms decides the network of these materials [29–31].
Fig. 4. (a) Raman spectra of xTeO2 -(90-x) BaF2-10Y2O3 glasses. (b-e) Deconvoluted Raman spectra of xTeO2-(90-x) BaF2-10Y2O3 glasses. (f) Intensity ratios of bands corresponding to non-bridging and bridging oxygen atoms.
The spectra of all potential Gaussian sub-bands are deconvoluted by Origin software are shown in Fig. 4(b-e). The major bands of TBY glasses are centered at around 350, 440, 680 and 770 cm−1. The intensity of the band at 440 cm−1 is caused by the symmetrical stretching vibration of Te-O-Te chain [32], which is directly proportional to the content of TeO2. This transition is mainly due to the transition of the glass network from the main TeO4 unit to TeO3 units [33]. The modification of TeO4 network is attributed to the electronegativity of barium that requires more oxygen atoms, which is realized by changing the structure of TeO2. F- tend to destroy the O-H bond, resulting in the increase of free O2- ions. This leads to the formation of more non-bridged oxygen atoms, resulting in an increase in band strength at 770 cm−1 with the TeO2 decrease. The band at 680 cm−1 is owing to the anti-symmetric stretching vibration of Te-O bond of oxygen bound to one Te ion on both sides, and the frequency band at 770 cm−1 is consistent with non-bridged oxygen ions that can bind to one Te ion and offset the charge by Ba2+ [34,35]. The ratio of intensities of 770 cm−1 band to 680 cm−1 shows a monotonic increase with the decrease of TeO2 (Fig. 4(f)) [32,36]. This band corresponds to the non-bridging oxygen atoms that arise from the Te-O- bonds in the TeO3+σ and TeO3 - trigonal pyramids.
3.4 Femtosecond laser-induced damage characteristics of TBY glass samples
When the femtosecond laser interacts with the surface of glass samples, several interaction modes including ablation and damage are studied. The ablation mechanism of femtosecond laser is largely caused by the accumulation of conduction band electrons (CBE) [37]. The damage caused by laser irradiation mainly includes three processes. First, the multiphoton ionization process excites the peripheral electrons of the material from the valence band to the conduction band. Then the stimulated electrons and free electrons in the material further absorb energy as seed electrons to form avalanche ionization. In this process, the energy of the stimulated electrons is continuously transmitted downward in the collision to form bountiful conduction band electrons. Finally, a large number of conduction band electrons are gathered to form a dense plasma. When the plasma energy exceeds the damage threshold of the material, the material exhibits optical damage [38].
In this paper, laser-induced damage was performed in the “S-on-1” regime according to ISO standard 11254-1.2 [38]. To further comprehend the effect of laser wavelength on LIDT, similar laser damage experiments were carried out at 3000 nm. All glass samples were irradiated under multiple pulses, and the average power was reduced from 32 µJ to 15 µJ. Obviously, compared with 3000 nm (photon energy approximately 0.41 eV), the photon energy at the wavelength of 4000 nm is about 0.31 eV, so more photons are required to transfer electrons from the valence band to the conduction band.
The damage threshold of the femtosecond laser with various parameters in glass samples are calculated by linear regression algorithm. The linear regression method measures the damage threshold energy (Eth) of the material according to the linear relationship between the damage crater diameter (D) and the incident laser energy (Ein). A relationship is observed between the incident laser energy (Ein), the damage threshold energy (Eth) and the damage diameter (D) as follows:
The laser energy ${E_{in}}$ can be calculated according to the equation
where Pavg is the laser average power and R is the repetition rate of the laser. Through the linear fitting curve between D2-LnE and the intercept x0 of the curve and the x-axis, the laser damage threshold flux Eth = exp (x0) corresponding to the optical material can be obtained. According to the aforementioned two formulas, the spot radius w0 and the laser damage threshold energy can be calculated, and then the LIDT can be further gained by the equationBased on the preceding formula, the ablation thresholds can be calculated.
3.4.1 Variation of LIDT with a different number of pulses
The damage threshold depends on pulse numbers, which is due to the “incubation effect”. When multiple pulses act on the glass surface, the former pulse will strengthen the energy absorption of the latter pulse. When defects generated in the glass, the energy will be deposited into the material, so that the defect energy level appears in the forbidden band, which will strengthen the absorption of the latter pulse and lead to the modification of mechanical or chemical damage. Therefore, it is necessary to study the effect of pulse numbers on material damage threshold [39]. The materials were irradiated with the femtosecond laser at different pulses at a central wavelength of 4000 nm and a repetition rate of 1 kHz, and the effect of pulses on LIDT of TBY60 glass was analyzed.
When the pulses increased from 100 to 10000, the energy thresholds are 4.74, 5.145, 5.936, 6.686, 7.486, 9.034 µJ respectively. As can be seen from Fig. 5(b), the corresponding LIDTs were 1.08, 0.89, 0.793, 0.705, 0.605, 0.546 J/cm2. The effect of the number of laser pulses on damage threshold follows the exponential defect accumulation model [39,40]. The illustration in Fig. 5(b) demonstrates the relationship between the diameter of the damage crater and the pulse numbers, when the pulse number is small, the diameter of the damage crater considerably decreases. With the further increase of the number of pulses, the change trend tends to be flat. The SEM images of TBY60 glass sample after exposure to increasing pulses at 4000 nm are presented in Fig. 5(c). In the interaction between TBY glasses and femtosecond laser, incubation effects in dielectric materials can be greatly influenced by the excitation and generation of CBEs, thereby eventually leading to an accumulation of defect sites. Therefore, when the number of pulses is large, the reduction of damage threshold is not obvious, which is also similar to the previously reported findings on Ge-S glass [41].
Fig. 5. (a) Linear fitting of logarithmic relationship between ablation areas of TBY60 glass and incident laser energy. (b) Change trend of LIDT with the different numbers of pulse. The inside figure reveals the relationship between the diameter and the number of pulses. (c) SEM images of TBY60 glass sample after exposure to different pulses at 4000 nm
3.4.2 LIDT changes with the different TeO2 contents
To study the effect of TeO2 content on LIDT, we analyzed the morphology of the four samples irradiated with the same power (20 µJ), as demonstrated in Fig. 6. In the experiment, the central wavelength of the femtosecond laser is 4000 nm, the repetition rate is 1 kHz and the number of pulses is 100. The laser power irradiated to the sample surface is controlled by adjusting the attenuation plate in the femtosecond laser system. Using the same method, the LIDTs of xTeO2-(90-x) BaF2-10Y2O3 (x = 60, 65, 70, 75 mol%) glasses are 1.08, 0.913, 0.846, and 0.782 J/cm2, respectively. The LIDTs of the glasses show a similar trend as the absorption boundary of the glass, which can be explained as follows: optical materials with smaller absorption boundary are more likely to accumulate conduction band electrons and quickly reach conduction band electron saturation state, resulting in the micro explosion. The material with a large absorption boundary slows down the conduction band electron accumulation of the material and increase LIDT. Figure 6(a) and 6(b) show the 2D and 3D modes of the glasses with different TeO2 contents. The degree of ablation depth becomes deeper from 11.75 to 14.91 µm as the TeO2 content increased from 60 to 75 mol%.
Fig. 6. (a) 2D and (b) 3D optical microscope images of damage under different content TBY glasses at 4000 nm
3.4.3 LIDT variation at wavelengths of 3 µm and 4 µm
To comprehend the laser wavelength on LIDT, similar laser damage experiments were carried out at 3000 nm. The laser output mode is linear polarization and the polarization direction is vertical. For the femtosecond pulse laser with wavelengths of 3000 nm and 4000 nm, the photon energy are 0.41 eV and 0.31 eV, respectively. In the photoionization process, materials need absorb different photons to transmit from valence band to conduction band. This absorption process leads to different degrees of multiphoton ionization (MPI), which influences the plasma accumulation velocity in the conduction band and surface damage in the material. LIDT results under irradiation at 3000 nm and 4000 nm wavelengths for TBY glass samples as presented in Fig. 7(a). The process of MPI requires 10 photons at 3000 nm and 13 photons at 4000 nm for TBY60 glass, and thus LIDT is 0.825 J/cm2 at 3000 nm and 1.08 J/cm2 at 4000 nm.
Fig. 7. (a) LIDT results under irradiation at 3000 nm and 4000 nm wavelengths for TBY glass samples. SEM images of TBY60 glass sample after exposure to different powers at (b) 3000 nm and (c) 4000 nm
The morphological characteristics of the damage craters in four glasses were analyzed by SEM to further comprehend the effects of power and wavelength on LIDT. Figures. 7(b) and 7(c) compare the damage craters under 3000 nm and 4000 nm irradiation at the same laser power. The absorption coefficients of TBY60 at 3000 nm and 4000 nm are 0.186 and 0.116 cm−1 respectively. The laser damage threshold at 4000 nm is higher, which is in part related to the lower absorption coefficient at 4000 nm. When the absorption coefficient is relatively low, it does not play an important role in damage. At this time, it can be explained by the laser spot curve, that is, the shorter the laser working wavelength is, the smaller the corresponding spot radius is, and the more concentrated the energy distribution is. In order to quantitatively compare LIDT at 3000 nm and 4000 nm, relevant parameters are listed in Table 1. It can be clearly seen that TBY60 glass has better resistance to laser damage than chalcogenide glass materials at the same wavelength [38,42–44].
Table 1. LDIT of TeO2-BaF2-Y2O3 glasses at different wavelength
4. Conclusion
The damage characteristics of TeO2-BaF2-Y2O3 glass samples were studied by using mid-infrared femtosecond laser. The LIDT of the glass was systematically studied using different laser parameters. The results show that the increase of TeO2 can effectively decrease the LIDT of TBY glass. All four samples have a similar decreasing tendency on LIDTs at wavelengths of 3000 nm and 4000 nm. When the content of TeO2 increased from 60 mol% to 75mol%, the damage threshold at 4000 nm and 3000 nm decreased from 1.08 to 0.782 J/cm2 and from 0.825 to 0.604 J/cm2, respectively. The effect of the number of laser pulse on LIDT follows the exponential defect accumulation model. The laser-induced damage threshold of TBY60 glass decreased from 1.08 to 0.546 J/cm2 with the pulse numbers increasing from 100 to10000. The results are helpful for utilizing the TBY fluorotellurite glasses in high-powered fiber laser operations.
Funding
National Natural Science Foundation of China (62090064, 62090063, 62090065).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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