Since the first realization of the 2.0 μm laser output in rare earth (RE) ion Ho3+-doped crystals^[1], the 2.0 μm region laser emission is of great interest for its numerous potential applications, including eye-safe LIDAR, medicine, spectroscopy, remote sensing, and mid-infrared (IR) generation^{[2–6" target="_self" style="display: inline;">–6]}. Tm3+ and Ho3+ have been studied as active ions for a laser in this wavelength region in various hosts. Tm3+ can create two excited states at the F43 state by the cross relaxation (CR) (H63+H43→F43+F43) energy transfer (ET) process with Tm3+, which is demonstrated to increase quantum efficiency^[7]. Compared to Tm3+, Ho3+ possesses higher gain cross sections, longer radiative lifetime, and longer-operating laser wavelength. However, the low efficiency of laser action on the Ho3+:I75→I85 emission limits further applications^[8,9]. Otherwise, the lack of a pumping band in the 800 or 980 nm region corresponding to commercially available high-power laser diodes (LDs) was a drawback in Ho3+ singly doped systems. In pursuit of efficiency, Ho3+-doped glasses have been sensitized with Tm3+, Yb3+, or Er3+^{[1012" target="_self" style="display: inline;">–12]}. In codoping Tm3+ as a sensitizer system, the F43 absorption band of Tm3+ permitted LD pumping, particularly, combines the effective CR among Tm3+, leading to an increase in quantum efficiency and the effective ET from Tm3+ to Ho3+ to achieve a Ho3+:I75→I85 laser, which can be applied to a wider range of applications requiring continuous wave (CW) 2.1 μm laser radiation^[13].

In order to obtain a strong IR emission from Ho3+, the host glass matrix is as important as the sensitizer. The early demonstrations of Tm3+/Ho3+ codoped fiber lasers involved fluoride glass as the host material^[14]. The low-phonon energy, high doping level, low viscosity, and wide transparency from the ultraviolet (UV) to IR of fluoride glasses, allowing for the observation of RE ion-doped laser emissions in a large optical range, make the materials good candidates for applications in laser technology^[15]. Fluoroaluminate (AYF) as representatives of fluoride glass are known to show several properties, such as a small refractive index and dispersion, and a high chemical durability implicated for fiber laser practical use, when compared with the properties in other fluoride glass systems. According to previous reports, the addition of some oxides, especially the addition of P2O5, is effective in stabilizing the glass state^[16]. Moreover, some articles describe properties of fluoride systems in which phosphates were introduced in a form of NaPO3^[17], Ba(PO3)2^[18,19], and Ba(H2PO4)2^[20]. However, fluoride systems with high-doped phosphates are highly similar to fluorophosphate systems, and most studies of fluoride systems focus on structural properties. Therefore, we investigate the fluoride system modified by phosphates in small amounts, especially focusing on the spectral properties of the glass, which has so far been rarely reported.

Based on previous investigations, we studied the physical, chemical, and typical properties, which include the stimulated emission cross section, upper state lifetime for the transition, and absorption spectra for diode laser pumping of Tm3+/Ho3+ codoped fluoroaluminate glasses with the introduction of Al(PO3)3. In addition, the ET processes between Tm3+ and Ho3+ are analyzed. We look forward to the results of better optical properties along with the advantages of 2.0 μm laser output.

In this work, the AYF glasses had the composition of 95(AlF3−YF3−MgF2−CaF2−SrF2−BaF2)−5Al(PO3)3−xTmF3−yHoF3 (x=0.2, y=0, 0.1, 0.2, 0.3, 0.4, singed as TH0, TH1, TH2, TH3, TH4; y=0.2, x=0, 0.1, 0.2, 0.3, 0.4, singed as HT0, HT1, HT2, HT3, HT4, respectively). Commercial grade chemicals of fluoride were used as starting materials. Approximately 15 g doped glass batches of different compositions were melted in an alumina crucible at 950°C for 20 min with a closed lid for each batch, and then the melts were poured onto a preheated copper plate and annealed near the glass transition temperature for several hours. The glass samples were fabricated and polished to the size of 20mm×10mm×1mm for the optical property measurements after it naturally cooled down to room temperature, and a part of the milled glass sample was used for the differential scanning calorimetry (DSC). Through the Archimedes principle, the densities were tested using distilled water as the immersion liquid. The density was measured to be 3.73±0.01g/cm3, and the refractive index of the glass was calculated to be 1.64.

The DSC was measured using a NETZSCH DTA 404 PC at the heating rate of 15 K/min. The absorption spectra were recorded on a JASCOV 570UV/VIS spectrophotometer in the range of 400-2200 nm at room temperature. The fluorescence spectra were obtained with a computer-controlled Triax 320 spectrofluorimeter with a 1.5 W 808 nm LD using a PbSe detector. The fluorescence lifetime was determined by a combined fluorescence lifetime and a steady state spectrometer (FLSP 920) (Edingburg Co.). The Raman spectrum of the glass was measured with a Renishaw in Via Raman microscope in the 100–1000cm−1 spectrum range using a 532 nm excitation line.

Glass stability versus devitrification may be estimated from the characteristic temperatures, including glass transition temperature (Tg), crystallization onset temperature (Tx), and the peak temperature of crystallization (Tp) measured by the DSC, and the results are shown in the Fig. 1. In some cases, the assessments may be not always accurate, and the ΔT difference between the Tg and the Tx is widely believed to have the value is strongly correlated to the crystallization tendency. A large ΔT means strong inhibitions to the processes of nucleation and crystallization^[21], which reflects greater thermal stability of the glass. In the Fig. 1, the Tg and Tx are noted as a value of 456°C and 560°C, respectively, and ΔT is calculated as 104°C, which is higher than the value of 81°C of the reported AYF glass^[22]. Moreover, the value calculated by the formula kgl=(Tx−Tg)/(Tm−Tp) is developed as a new parameter to judge glass formation^[21], and the higher one reflects the greater thermal stability of the glass, where Tm is the melting temperature of the glass. The kgl is figured out to be 0.297 in this Al(PO3)3-doped AYF glass system, and larger than the previous value of 0.171 in the AYF system without Al(PO3)3^[22], which reveals a better glass-forming ability and chemical durability.

Fig. 1. DSC curve of the AYF glass with 5 mol% Al(PO3)3 introduction. Raman spectrum in the 100–1200cm−1 range of undoped RE ions AYF glass with Al(PO3)3 introduction.

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In order to investigate the structural properties of the AYF glass modified by Al(PO3)3, the Raman spectrum of the prepared sample of undoped RE ion glass is shown in Fig. 1. A peak in the low-frequency Raman band around 560cm−1 is observed clearly, which is related to [AlF4] vibration^[23,24]. A band at 1090cm−1 is due to the stretching vibration of O-P-O after introducing the metaphosphate into the AYF glass^[23].

The IR absorption spectra of the Tm3+ and Ho3+ singly doped, respectively, and Tm3+/Ho3+ codoped AYF glasses modified by Al(PO3)3 at room temperature between 400–2200 nm are shown in Fig. 2. The spectral absorption peak shapes of all of the samples are similar, and the absorption intensity is proportional to the doping concentrations, indicating that Tm3+ and Ho3+ are uniformly incorporated into the glassy network and do not cause aggregation or local ligand field variations^[25]. The absorption bands of Ho3+ including 449, 544, 641, 1149, and 1944 nm correspond to the transitions from the I85 ground state to the higher levels F35, (F45, S25), F55, I65, and I75, as is labeled, respectively, in the Fig. 2. It can be seen from Fig. 2 that no absorption peak exists in the Ho3+ single-doped AYF glass with an Al(PO3)3 introduction in the range of 750–850 nm, while the absorption of Tm3+, corresponding to the optical transition H63→H43, can be observed. It indicates that Tm3+ can be effectively excited by the 808 nm pumping source, and Ho3+ can be sensitized effectively through an ET.

Fig. 2. Absorption spectra of Tm3+ and Ho3+ singly doped and codoped samples. The inset is the transmittance spectrum of the sample.

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The inset of Fig. 2 presents the mid-IR transmittance spectrum of Tm3+/Ho3+ codoped AYF modified by the Al(PO3)3 sample. As it can be seen, the maximum transmittance reaches as high as 92%. As we know, the stretching vibration of free OH− groups will participate in the ET of RE ions and reduce the intensity of the emission^[26]. The content of the OH− groups in the glass can be expressed by the OH− absorption coefficient, which can be given by αOH−=In(T/T0)/l,(1)where l is the thickness of the sample (1 mm), and T0 and T are the transmitted and incident intensities, respectively. The absorption coefficient at 3 μm is 0.067cm−1, which is much lower than 0.113cm−1 of the AYF original glass^[22]. The good mid-IR transmission property proves that AYF glass modified by Al(PO3)3 is a potential candidate for mid-IR laser materials.

The Judd-Ofelt (J-O) parameter Ωλ(λ=2,4,6) based on the J-O theory is used to analyze the local structure and bonding in the vicinity of RE ions^[27,28]. According to the absorption spectrum (Fig. 2), the Ωλ(λ=2,4,6) of Ho3+ within various glasses are calculated (shown in Table 1)^[29,30]. The parameter, Ω2, is hypersensitive to structure, which is related to the covalency parameter through the nephelauxetic effect and polarizability of the ligands around RE ions^[31]. It is seen that Ω2 of the AYF glass modified by Al(PO3)3 is slightly larger than fluoride, but much smaller than oxide glasses, which indicates that the present sample glasses possess a lower covalency and a higher symmetry of the ligand, owing to the O2− ions introduction owning a higher polarizability than F− ions. Ω6 is a vibronic dependent parameter, which is related to viscosity, rigidity, and the dielectric. Ω4/Ω6 is an important parameter for predicting the stimulated emission in a laser active host^[32]; in this work, the value of Ω4/Ω6 is 2.04. The root-mean-square error deviation δrms is 0.18×10−6 for Tm3+/Ho3+ codoped AYF glass modified by Al(PO3)3.

The radiative parameters, radiative transition probability (A), branching ratio (β), total radiative transition probability (ΣA), and radiative lifetime (τrad) of Ho3+ in the 0.3 mol% Tm3+/0.2mol%Ho3+ (HT3) sample are calculated according to Ωλ (presented in Table 2). From Table 2, A of the Ho3+: I75→I85, I55→I85, and I55→I85 transitions in the AYF glass modified by Al(PO3)3, are as high as 118.74±0.1, 254.46±0.1, and 3490.34±0.1s−1, respectively, which are higher than the values of 61.44, 135.59, and 2242.06s−1 in other kinds of fluoride glass^[33]. Thus, this Tm3+/Ho3+ codoped AYF glass modified by Al(PO3)3 can be selected as an appropriate host material to achieve a stronger 2.0 μm fluorescence on account of its high spontaneous emission probability, which means a better opportunity of obtaining laser actions^[34].

Figure 3 presents the fluorescence spectra of the prepared Tm3+/Ho3+ codoped glass samples with various Tm3+ or Ho3+ molar concentrations in the wavelength region of 1350–2200 nm. The inset of Fig. 3 shows the simplified energy level diagram of the Tm3+/Ho3+ codoped system. The 808 nm pump source excites the Tm3+ from the H63 ground state to the higher H43 level. On the one hand, a portion of the Tm3+ ions at the H43 level decay to the F43 meta-stable level by radiative transition emitting 1.47 μm photons, and then returns to the ground state with a strong 1.8 μm emission via the F43→H63 transition. On the other hand, another part of the Tm3+ ion at the H43 level can also transfer energy to the H63 ground state and decay rapidly to the F43 level via the CR process Tm3+(H43)+Tm3+(H63)→Tm3+(F43)+Tm3+(F43). Then, the Tm3+ located at the F43 level transfers energy to the neighboring ground state of Ho3+ by the ET process Tm3+(F43)+Ho3+(I85)→Tm3+(H63)+Ho3+(I75), so that a mid-IR fluorescence emission of 2.0 μm associated with the Ho3+:I75→I85 transition takes place. In the Fig. 3(a), the concentration of Ho3+ is increased using values 0.1, 0.2, 0.3, and 0.4 mol%, while the concentration of Tm3+ remains fixed at 0.2 mol%. It can be seen that the fluorescence intensity at 1.47 μm is almost consistent, which is caused by two ET processes that may counteract each other to a certain extent. For one thing, the increase of Ho3+ facilitates the ET of Tm3+:H43→Ho3+:I55, which leads to the decrease of 1.47 μm fluorescence. For another, the CR among Tm3+ is weakened by the enlargement of the distance among Tm3+ due to the addition of Ho3+, which results in a 1.47 μm fluorescence enhancement. The two processes reach equilibrium so that the fluorescence intensity at 1.47 μm remains constant. In contrast, the intensity of 1.8 μm is limited to the numbers of Tm3+ staying on the F43 level. As the Ho3+ concentration increases, most of the Tm3+ are transported to the I75 level, making for a 1.8 μm fluorescence reduction and a 2.0 μm fluorescence enhancement, which proves the effective ET of Tm3+:F43→Ho3+:I75 and without concentration quenching happening. From Fig. 3(b), it is worth noting that the emission intensities at 1.47, 1.8, and 2.0 μm show increases of various degrees with the increment of Tm3+ concentration, and the intensity reaches the maximum when Tm3+ increases to 0.4 mol%, which can be attributed to the CR mechanism. According to the above results, a conclusion can be drawn that the sensitization effect of Tm3+ on Ho3+ is significant and simultaneous, and the strong fluorescence for the H43→H63 transition of Tm3+ can still be observed at lower Ho3+ concentrations.

Fig. 3. Fluorescence spectra of Tm3+/Ho3+ codoped AYF glass modified by Al(PO3)3 with (a) different Ho3+ concentrations and (b) different Tm3+ concentrations. (c) The inset shows the energy level scheme of the Tm3+/Ho3+ system. ETU, ET upconversion.

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Figure 4(a) shows the decay curve of the Ho3+ in the HT series. The experimental lifetimes of the 2.0 μm emission turn out to be 1.72, 2.02, 2.53, and 2.86 ms, respectively. The result indicates that the lifetime of Ho3+ increases after, meanwhile, the ET efficiency η from Tm3+ to Ho3+ can be determined from the lifetime value by using the following formula^[29]: η=1−ττ0,(2)where τ0 and τ are the lifetimes of Tm3+:F43 level in Tm3+ singly doped and Tm3+/Ho3+ codoped samples, respectively. The ET efficiency of the TH1-4 samples, where the Tm3+ concentration was fixed at 0.2 mol%, are shown in the Fig. 4(b). It can be seen that the ET efficiency increases with an increasing Ho3+ concentration and reaches a maximum of 88% when the Ho3+ concentration is up to 0.4 mol%. Tm3+ is codoped with a constant increase concentration, which concurs with the result of the fluorescence spectra. Thus, efficient ET from Tm3+ to Ho3+ can be obtained efficiently in the present glass, which is helpful in 2.0 μm emission.

According to the measured absorption spectra shown in Fig. 3, the absorption cross section (σabs) can be calculated via the following equation^[35]: σabs(λ)=2.303log[I0(λ)I(λ)]Nl,(3)where I0(λ) and I(λ) are the incident optical intensity and optical intensity throughout the sample, respectively. N is the concentration of Ho3+, and l is the thickness of the sample. Moreover, on the basis of the obtained absorption cross section, the stimulated emission cross section (σemi), which is an extremely useful parameter to determine the possibility of achieving the laser effect is further calculated by using the McCumber formula^[36], σemi(λ)=σabs(λ)ZlZuexp[hckT(1λZL−1λ)],(4)where h, c, k, and T are the Planck constant, the photon frequency, the Boltzmann constant, and the temperature (the room temperature in this case). Zl and Zu are the partition functions of the lower and upper levels, respectively. Zl/Zu simply becomes the degeneracy weighting of the I75, I85 states in the high-temperature limit, hence, the value is equal to 1.13. λZL is the wavelength for the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets (zero phonon line). As shown in Fig. 5, both the absorption and emission cross sections of the Ho3+:I85→I75 transition are shown, and the maximum values of them at 2.0 μm are 5.9×10−21 and 7.6×10−21cm−2, respectively. The result is preferable to that of various rare-earth-ion-doped glasses, as shown in Table 3^{[20,31,37–40" target="_self" style="display: inline;">–40]}, thus this AYF glass modified by Al(PO3)3 is considered to be suitable for IR emission. The products of full width at half maximum (FM) and emission cross section (FM×σemi) and emission cross section and measured decay lifetime (σemi×τ) are important parameters used in optical amplifiers to evaluate bandwidth properties and gain properties, respectively. The FM×σemi and σemi×τ are, respectively, calculated values of 145.35×10−20cm2nm and 2.17×10−20cm2s of the HT4 sample. Both of the values are higher than those of 124.80×10−20cm2nm and 1.84×10−20cm2s in fluorophosphate glasses^[3].

Fig. 4. (a) decay curves of Ho3+ in the HT1-4 samples, where the Ho3+ concentration is fixed at 0.2 mol% and (b) ET efficiencies from Tm3+ to Ho3+ in the TH1-4 samples.

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Fig. 5. Absorption cross section and emission cross section of the Ho3+:I85→I75 transition in the HT4 sample at the 2.0 μm region. The inset shows the products of the emission cross section and measured decay lifetime (σemi×τ) in the HT1-4 samples.

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In conclusion, Tm3+/Ho3+ codoped AYF glass with an Al(PO3)3 introduction, possessing higher thermal stability against crystallization, is investigated, and the introduced PO3− plays an effective role in improving the glass-forming ability. We investigate the fluorescence at 1.47, 1.8, and 2.0 μm with emission performances and ET characteristics by a series of doping concentrations of RE ions. The conclusion can be drawn that Ho3+ produces a strong 2.0 μm fluorescence with the help of the sensitization of Tm3+. Besides, the fluorescence intensity of 1.8 μm decreases, while at the same time the 2.0 μm fluorescence intensity adds up with the increase of Ho3+ with no concentration quenching occuring in this case. It can be speculated that intensity of the 2.0 μm emission can be significantly increased by highly doping RE ions. The higher products of FM×σemi and σemi×τ prove that the new AYF glass can achieve high gains when used in the laser amplifier. The results indicate that Tm3+/Ho3+ codoped AYF glass modified by Al(PO3)3 could be a promising material for a widely tunable laser or broadband amplifier applications.