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Study of Mid-Infrared Emission and Structural Properties of Heavy Metal Oxide Glass and Optical Fibre Co-Doped with Ho3+/Yb3+ Ions

Bismuth-germanate glasses with low hydroxide content co-doped with Ho3+/Yb3+ ions have been investigated in terms of structural and spectroscopic properties. To reduce OH- ions content and improve transmittance value at the wavelength of 3.1 µm, the glass synthesis has been carried out in low vacuum conditions (45–65 mBar). The composition of the host glass based on heavy metal oxides affects the maximum phonon energy (hωmax = 724 cm−1), which low value has a positive impact on the mid-infrared emission parameters. Emission band at the wavelength of 2.87 µm was observed in glass co-doped with mol% 0.25 Ho2O3/0.75 Yb2O3 under 980 nm high power laser diode wavelength excitation. Lifetime measurements of the Yb3+:2F5/2 quantum level indicate efficient Yb3+ → Ho3+ energy transfer (η = 61%). The developed active bismuth-germanate glass was used as the core of optical fibre operating in the mid-infrared region.


Introduction
Research on new optical materials for the mid-infrared region has been conducted for many years due to their potential use in applications such as atmosphere pollution monitoring, eye-safe laser radar, remote sensing, and microsurgery [1][2][3][4]. The spectral transmittance in the mid-IR region of most common optical glasses based on oxides such as phosphate (1200 cm −1 ), silicate (1100 cm −1 ), or borate (1400 cm −1 ) is limited due to their high maximum phonon energies [5,6]. On the other hand, non-oxide glasses with low phonon energy and high transparency in mid-IR are characterized by poor thermal stability which complicates processing them into photonic structures [7][8][9][10][11]. Thus, glasses based on heavy metal oxides (HMO) are still investigated in optical material laboratories around the world [12][13][14]. Especially, bismuth-oxide based glasses characterized by relatively low maximum phonon energy, wide transparency windows, high thermal stability, and good mechanical and chemical properties are considered for use in optical fibre technology [15][16][17][18]. However, in the wavelength range near 3 µm exists an absorption band, which originates from hydroxide ions. In order to minimalize its adverse impact on the transmittance properties, the glass synthesis process has to be conducted in low vacuum conditions [19].
Among rare earth ions, holmium ions are characterized by the luminescence band at the wavelength of 2.87 µm corresponding to 5 I 6 → 5 I 7 transition. Unfortunately, due to its energy level scheme and no GSA (Ground State Absorption) transitions that overlap with low-cost laser diodes, Ho 3+ ions cannot be directly excited using 980 nm wavelength radiation. Therefore, ytterbium ions characterized by large absorption cross-section at 980 nm are commonly used as a sensitizer in excitation energy transfer to Ho 3+ ions [20][21][22][23].
In this paper, emission properties in the mid-infrared region of Ho 3+ /Yb 3+ co-doped heavy metal oxide glasses based on bismuth-germanate oxides and optical fibre have been investigated and compared. The optimal concentration of co-dopants has been estimated based on the luminescence intensity at 2.87 µm (Ho 3+ : 5 I 6 → 5 I 7 radiative transition). The energy transfer (ET) mechanism between lanthanide ions has been discussed in terms of Yb 3+ → Ho 3+ ET efficiency. Due to the high thermal stability (∆T = 124 • C), wide transparency window (0.51-5.5 µm) and low maximum phonon energy (hω max = 724 cm −1 ) bismuth-germanate glass has been selected as the core glass. The results determine that HMO glass in optical fibre is a promising material for construction of new lasers and fibre amplifiers operating in the mid-infrared range.

Experiment
Series of glasses with the composition of mol% (60 − x − y) (Bi 2 O 3 -GeO 2 )-40 (Na 2 O-Ga 2 O 3 )-x Ho 2 O 3 -yYb 2 O 3 were synthesized with the standard melt-quenching method from spectrally pure materials (99.99%, Sigma-Aldrich, St. Louis, MO, USA). Labels and specified lanthanide co-dopants composition have been included in Table 1. The homogenized powder was placed into a platinum crucible and melted at 1050 • C for 60 min. Modified synthesis method was conducted in electric furnace maintaining vacuum conditions  in order to minimalize OH − ions concentration. The glass melt was poured into a brass mold and then subjected to the annealing process at 400 • C for 12 h. Transparent and homogenous glasses without any visible crystallization effects have been obtained. Finally, samples were polished in order to provide a high optical quality surface for spectroscopic measurements. Differential Scanning Calorimetry (DSC) measurement was done using the SETARAM Labsys thermal analyzer (Setaran Instrumentation, Caluire, France) at the heating rate of 10 • C/min. The Fourier Transform Infrared Spectroscopy spectrum was recorded with a Bruker Company (Billerica, MA, USA) Vertex 70v spectrometer. Spectra were collected in 3000-450 cm −1 infrared region after 128 scans at 4 cm −1 resolution. Emission (using high power semiconductor laser diode λ exc = 980 nm as a pump source) and absorption spectra were collected by Stellarnet GreenWave Spectrometer (Stellarnet Inc., Tampa, FL, USA) (0.44-1.1) µm as well as Acton Spectra Pro 2300i monochromator (Princeton Instruments, Trenton, NJ, USA) with PbS and PbSe detectors (1.1-2.3 and 2.75-3.05, respectively) µm with 0.5 nm spectral resolution. For luminescence decay measurements a PTI QuantaMaster QM40 system (Horiba, Kyoto, Japan) coupled with a tuneable pulsed optical parametric oscillator (OPO) was used, it was pumped by the third harmonic of an Nd:YAG laser (OpotekOpolette 355 LD, Carlsbad, CA, USA).

Thermal and Structural Analysis
Differential Scanning Calorimetry experimental results of produced host glass have been presented in Figure 1. First endothermic and exothermic peaks, determining the transition temperature T g and crystallization temperature T x , have been estimated at 408 • C and 532 • C, respectively, and are comparable to other bismuth (T g = 418 • C; T x = 520 • C) and bismuth-germanate (T g = 442 • C; T x = 550 • C) glasses [24,25]. The high value of thermal stability parameter (∆T = T x − T g = 124 • C) confirms that investigated glass could be used in the optical fibre drawing process. Low melting temperature T m = 773 • C is an additional advantage, which further simplifies the production process and forming the material into new photonic structures.

Thermal and Structural Analysis
Differential Scanning Calorimetry experimental results of produced host glass have been presented in Figure 1. First endothermic and exothermic peaks, determining the transition temperature Tg and crystallization temperature Tx, have been estimated at 408 °C and 532 °C, respectively, and are comparable to other bismuth (Tg = 418 °C; Tx = 520 °C) and bismuth-germanate (Tg = 442 °C; Tx = 550 °C) glasses [24,25]. The high value of thermal stability parameter (ΔT = Tx − Tg = 124 °C) confirms that investigated glass could be used in the optical fibre drawing process. Low melting temperature Tm = 773 °C is an additional advantage, which further simplifies the production process and forming the material into new photonic structures. Besides high thermal stability essential in optical fibre technology, the composition of bismuthgermanate glass has been developed in terms of low phonon energy. Fourier Transform Infrared Spectroscopy measurements were used to evaluate the structural information of developed amorphous material ( Figure 2). In the Fourier-Transform Infrared Spectroscopy spectrum observed in 450-800 cm −1 region, which has been presented in the Figure 2 inset, three strong and broad bands have been identified after the decomposition process. The peak at around 460 cm −1 originates from bending Bi-O bonds vibrations in BiO6 hexahedral units [26,27], while the peak at the 638 cm −1 is attributed to stretching vibrations of Bi-O bonds (non-bridged oxygen) in BiO6 pyramidal units [28]. The third peak at the wavenumber of 724 cm −1 is a result of the motion of Ge-O two non-bridged oxygen atoms in GeO4 tetrahedral units [29,30], thereby this absorption band indicates the maximum phonon energy (MPE) of produced bismuth-germanate host glass. The value of MPE is smaller in comparison to silicate (963 cm −1 ) [31], germanate (1002 cm −1 ) [32], phosphate (1345 cm −1 ) [33], borate (1602 cm −1 ) [34], and comparable to other bismuth glasses (926-1183 cm −1 ) [35,36]. Besides high thermal stability essential in optical fibre technology, the composition of bismuth-germanate glass has been developed in terms of low phonon energy. Fourier Transform Infrared Spectroscopy measurements were used to evaluate the structural information of developed amorphous material ( Figure 2). In the Fourier-Transform Infrared Spectroscopy spectrum observed in 450-800 cm −1 region, which has been presented in the Figure 2 inset, three strong and broad bands have been identified after the decomposition process. The peak at around 460 cm −1 originates from bending Bi-O bonds vibrations in BiO 6 hexahedral units [26,27], while the peak at the 638 cm −1 is attributed to stretching vibrations of Bi-O bonds (non-bridged oxygen) in BiO 6 pyramidal units [28]. The third peak at the wavenumber of 724 cm −1 is a result of the motion of Ge-O two non-bridged oxygen atoms in GeO 4 tetrahedral units [29,30], thereby this absorption band indicates the maximum phonon energy (MPE) of produced bismuth-germanate host glass. The value of MPE is smaller in comparison to silicate (963 cm −1 ) [31], germanate (1002 cm −1 ) [32], phosphate (1345 cm −1 ) [33], borate (1602 cm −1 ) [34], and comparable to other bismuth glasses (926-1183 cm −1 ) [35,36].

Transmission, Absorbance Spectra, and Optical Bandgap Analysis
Synthesized host glass has been investigated in respect of mid-infrared transmission parameters. The mid-infrared transmission spectra shown in Figure 3 provide information about hydroxide group content within the structure of produced bismuth-germanate glass, which has been synthesized in a low vacuum environment (45-65 mBar) during the whole thermal process.
where l-sample thickness (2.0 cm), T-the value of the transmittance in the absorption peak (63%), and Tb-the value of the transmittance before absorption peak (79%). The estimated value of the hydroxide groups content and absorption coefficient has been calculated to be 47 ppm and 0.11 cm −1 , respectively, which are lower than in any known oxide glasses [30,[37][38][39]. Hence, the low value of OH − concentration, which acts in a glass matrix as fluorescence-quenching centers, indicates better mid-IR parameters and make the synthesized glass a promising material for mid-infrared applications.

Transmission, Absorbance Spectra, and Optical Bandgap Analysis
Synthesized host glass has been investigated in respect of mid-infrared transmission parameters. The mid-infrared transmission spectra shown in Figure 3 provide information about hydroxide group content within the structure of produced bismuth-germanate glass, which has been synthesized in a low vacuum environment (45-65 mBar) during the whole thermal process.

Transmission, Absorbance Spectra, and Optical Bandgap Analysis
Synthesized host glass has been investigated in respect of mid-infrared transmission parameters. The mid-infrared transmission spectra shown in Figure 3 provide information about hydroxide group content within the structure of produced bismuth-germanate glass, which has been synthesized in a low vacuum environment (45-65 mBar) during the whole thermal process.
where l-sample thickness (2.0 cm), T-the value of the transmittance in the absorption peak (63%), and Tb-the value of the transmittance before absorption peak (79%). The estimated value of the hydroxide groups content and absorption coefficient has been calculated to be 47 ppm and 0.11 cm −1 , respectively, which are lower than in any known oxide glasses [30,[37][38][39]. Hence, the low value of OH − concentration, which acts in a glass matrix as fluorescence-quenching centers, indicates better mid-IR parameters and make the synthesized glass a promising material for mid-infrared applications. The hydroxide ions concentration and the absorption coefficient at the wavelength of 3.1 µm and can be estimated in accordance with the following equations [10]: where l-sample thickness (2.0 cm), T-the value of the transmittance in the absorption peak (63%), and T b -the value of the transmittance before absorption peak (79%). The estimated value of the hydroxide groups content and absorption coefficient has been calculated to be 47 ppm and 0.11 cm −1 , respectively, which are lower than in any known oxide glasses [30,[37][38][39]. Hence, the low value of OH − concentration, which acts in a glass matrix as fluorescence-quenching centers, indicates better mid-IR parameters and make the synthesized glass a promising material for mid-infrared applications. Figure 4 shows the optical absorption spectra of developed glasses with different rare earth ions concentrations. The main absorption bands centered at 540, 645, 890, 1150, and 1972 nm associated to the ground state absorption transitions in holmium ions from 5 I 8 level to the higher energy levels ( 5 F 4 + 5 S 2 ), 5 F 5 , 5 I 5 , 5 I 6 , and 5 I 7 , respectively. The absorption band at 979 nm (Yb 3+ : 2 F 7/2 → 2 F 5/2 transition) was used to pump ytterbium ions (λ exc = 980 nm) which act as a donor in Yb 3+ /Ho 3+ system.  Figure 4 shows the optical absorption spectra of developed glasses with different rare earth ions concentrations. The main absorption bands centered at 540, 645, 890, 1150, and 1972 nm associated to the ground state absorption transitions in holmium ions from 5 I8 level to the higher energy levels ( 5 F4 + 5 S2), 5 F5, 5 I5, 5 I6, and 5 I7, respectively. The absorption band at 979 nm (Yb 3+ : 2 F7/2 → 2 F5/2 transition) was used to pump ytterbium ions (λexc = 980 nm) which act as a donor in Yb 3+ /Ho 3+ system. Regarding transmittance and absorbance spectra, the transmission window of produced glasses starts from 0.51 µm and reaches approximately 5.5 µm. Due to the large molar content of Bi2O3 within the glass matrix, UV absorbance band is shifted to the longer wavelengths in comparison to other oxide glasses [31,40,41], due to the reduced interatomic bond strength between the oxygen and metal [42].
The transmittance spectra absorption edges are used to investigate the electronic band structure and the optical transition of the non-crystalline and crystalline materials. The optical bandgap of amorphous material can be obtained by plotting (αhν) 1/γ against incident photon energy hν according to the Tauc's law [42] (Figure 5): where α is the linear absorption coefficient, A determines band edge parameter, Eg gives optical band energy, γ is index number denotes the type of electronic transition which cause the absorption. In the analysed case for allowed indirect transitions (γ = 1/2) the value of optical bandgap Eg, obtained from the (αhν) 1/γ vs hν variation and the linear extrapolation of the plotted curve, are found to be 2.38 eV, which is typical for the heavy metal oxide glasses [43,44].  Regarding transmittance and absorbance spectra, the transmission window of produced glasses starts from 0.51 µm and reaches approximately 5.5 µm. Due to the large molar content of Bi 2 O 3 within the glass matrix, UV absorbance band is shifted to the longer wavelengths in comparison to other oxide glasses [31,40,41], due to the reduced interatomic bond strength between the oxygen and metal [42].
The transmittance spectra absorption edges are used to investigate the electronic band structure and the optical transition of the non-crystalline and crystalline materials. The optical bandgap of amorphous material can be obtained by plotting (αhν) 1/γ against incident photon energy hν according to the Tauc's law [42] (Figure 5) where α is the linear absorption coefficient, A determines band edge parameter, E g gives optical band energy, γ is index number denotes the type of electronic transition which cause the absorption. In the analysed case for allowed indirect transitions (γ = 1/2) the value of optical bandgap E g , obtained from the (αhν) 1/γ vs hν variation and the linear extrapolation of the plotted curve, are found to be 2.38 eV, which is typical for the heavy metal oxide glasses [43,44].
Materials 2019, 12, x FOR PEER REVIEW 5 of 10 Figure 4 shows the optical absorption spectra of developed glasses with different rare earth ions concentrations. The main absorption bands centered at 540, 645, 890, 1150, and 1972 nm associated to the ground state absorption transitions in holmium ions from 5 I8 level to the higher energy levels ( 5 F4 + 5 S2), 5 F5, 5 I5, 5 I6, and 5 I7, respectively. The absorption band at 979 nm (Yb 3+ : 2 F7/2 → 2 F5/2 transition) was used to pump ytterbium ions (λexc = 980 nm) which act as a donor in Yb 3+ /Ho 3+ system. Regarding transmittance and absorbance spectra, the transmission window of produced glasses starts from 0.51 µm and reaches approximately 5.5 µm. Due to the large molar content of Bi2O3 within the glass matrix, UV absorbance band is shifted to the longer wavelengths in comparison to other oxide glasses [31,40,41], due to the reduced interatomic bond strength between the oxygen and metal [42].
The transmittance spectra absorption edges are used to investigate the electronic band structure and the optical transition of the non-crystalline and crystalline materials. The optical bandgap of amorphous material can be obtained by plotting (αhν) 1/γ against incident photon energy hν according to the Tauc's law [42] (Figure 5): where α is the linear absorption coefficient, A determines band edge parameter, Eg gives optical band energy, γ is index number denotes the type of electronic transition which cause the absorption. In the analysed case for allowed indirect transitions (γ = 1/2) the value of optical bandgap Eg, obtained from the (αhν) 1/γ vs hν variation and the linear extrapolation of the plotted curve, are found to be 2.38 eV, which is typical for the heavy metal oxide glasses [43,44]. Figure 5. The optical bandgap energy evaluation of (αhν) 2 vs photon energy plot. Figure 5. The optical bandgap energy evaluation of (αhν) 2 vs photon energy plot.

Luminescent Properties
As a result of pumping at 980 nm (P opt = 1 W), four luminescence bands in the mid-infrared region were observed. Figure 6a presents the emission spectra of lanthanide-doped glasses with a luminescence band centered at 2.87 µm, which could be attributed to the 5 I 6 → 5 I 7 radiative transition in holmium ions [45]. In samples with a constant concentration of holmium ions (0.25 mol%) and increased content of ytterbium ions (samples 025H025Y → 025H075Y), we observed a significant enhancement of emission intensity to a maximum value. This phenomenon is related to a smaller distance between donor and acceptor ions, which leads to efficient energy transfer from the excited 2 F 5/2 energy level of ytterbium. Simultaneously, increased concentration of Yb 3+ ions results in higher absorption coefficient at the 980 nm wavelength (Yb 3+ : 2 F 7/2 → 2 F 5/2 ). Reducing Ho 3+ ions content and constant Yb 3+ ions concentration of 0.75 mol% (samples 025H075Y → 0125H075Y) caused mid-infrared luminescence intensity decrease due to the lower concentration of acceptor ions and lower energy transfer efficiency. Moreover, lower holmium concentration in the glass matrix results in the higher distance between acceptor and donor ions. Analogous changes in the emission intensity at 1.2 µm and 2.03 µm bands were observed (Figure 6b,c). It is worth to note that luminescence in 2.87 µm and 1.2 µm infrared bands results from the depopulation of the same 5 I 6 excited level via radiative transitions (2.87 µm: 5 I 6 → 5 I 7 and 1.2 µm: 5 I 6 → 5 I 8 ), therefore the changes of maximum emission intensity are similar in the investigated co-doped samples (Figure 6a,b insets). Due to the dynamics of the 5 I 6 → 5 I 7 transition, preceding transition from the higher excited state 5 I 6 , which populates the 5 I 7 level (Figure 6c inset) luminescence intensity changes in the 2.0 µm band ( 5 I 7 → 5 I 8 ) are compatible.

Luminescent Properties
As a result of pumping at 980 nm (Popt = 1 W), four luminescence bands in the mid-infrared region were observed. Figure 6a presents the emission spectra of lanthanide-doped glasses with a luminescence band centered at 2.87 µm, which could be attributed to the 5 I6 → 5 I7 radiative transition in holmium ions [45]. In samples with a constant concentration of holmium ions (0.25 mol%) and increased content of ytterbium ions (samples 025H025Y → 025H075Y), we observed a significant enhancement of emission intensity to a maximum value. This phenomenon is related to a smaller distance between donor and acceptor ions, which leads to efficient energy transfer from the excited 2 F5/2 energy level of ytterbium. Simultaneously, increased concentration of Yb 3+ ions results in higher absorption coefficient at the 980 nm wavelength (Yb 3+ : 2 F7/2 → 2 F5/2). Reducing Ho 3+ ions content and constant Yb 3+ ions concentration of 0.75 mol% (samples 025H075Y → 0125H075Y) caused midinfrared luminescence intensity decrease due to the lower concentration of acceptor ions and lower energy transfer efficiency. Moreover, lower holmium concentration in the glass matrix results in the higher distance between acceptor and donor ions. Analogous changes in the emission intensity at 1.2 µm and 2.03 µm bands were observed (Figure 6b,c). It is worth to note that luminescence in 2.87 µm and 1.2 µm infrared bands results from the depopulation of the same 5 I6 excited level via radiative transitions (2.87 µm: 5 I6 → 5 I7 and 1.2 µm: 5 I6 → 5 I8), therefore the changes of maximum emission intensity are similar in the investigated co-doped samples (Figure 6a,b insets). Due to the dynamics of the 5 I6 → 5 I7 transition, preceding transition from the higher excited state 5 I6, which populates the 5 I7 level (Figure 6c inset) luminescence intensity changes in the 2.0 µm band ( 5 I7 → 5 I8) are compatible. Obtained results indicate that ET: 2 F5/2 (Yb 3+ ) + 5 I8 (Ho 3+ ) → 2 F7/2 (Yb 3+ ) + 5 I6 (Ho 3+ ) energy transfer efficiency (Figure 6d) reaches the highest value in glass co-doped with optimal molar composition of 0.25 Ho2O3 and 0.75 Yb2O3 in terms of mid-infrared luminescence. A significant role in the enhancement of mid-IR emission has a distance between lanthanide ions in the glass matrix. Reducing the distance between active centers by increasing the content of lanthanide ions, the probability of the ET: Yb 3+ → Ho 3+ and further emission intensity are increased [46]. Obtained results indicate that ET: 2 F 5/2 (Yb 3+ ) + 5 I 8 (Ho 3+ ) → 2 F 7/2 (Yb 3+ ) + 5 I 6 (Ho 3+ ) energy transfer efficiency (Figure 6d) reaches the highest value in glass co-doped with optimal molar composition of 0.25 Ho 2 O 3 and 0.75 Yb 2 O 3 in terms of mid-infrared luminescence. A significant role in the enhancement of mid-IR emission has a distance between lanthanide ions in the glass matrix. Reducing the distance between active centers by increasing the content of lanthanide ions, the probability of the ET: Yb 3+ → Ho 3+ and further emission intensity are increased [46].
To further validate the energy transfer process between ytterbium and holmium ions, emission spectra, and fluorescence lifetimes of Yb 3+ : 2 F 5/2 state at the wavelength of 1010 nm, which corresponds to 2 F 5/2 → 2 F 7/2 radiative transition, under 980 nm excitation has been observed and presented in Figure 7. Luminescence intensity at 1.01 µm band increasing from 025H025Y to 0125H075Y which is the result of: 1) higher amount of Yb 3+ ions in material and higher absorption cross-section at the 980 nm wavelength and 2) lower concentration of Ho 3+ ions which decreases the donor → acceptor energy transfer efficiency. In Figure 7b, it can be seen that the lifetime in Ho 3+ /Yb 3+ co-doped sample (τ HY ) of 266 µs is significantly shorter in comparison to the lifetime of Yb 3+ : 2 F 5/2 level in the single-doped glass where τ Y amounts 684 µs. The decrease of the Yb 3+ : 2 F 5/2 lifetime indicates the presence of an energy transfer process between holmium and ytterbium ions. Based on the measured lifetime values, ET efficiency has been calculated by the following equation [47]: To further validate the energy transfer process between ytterbium and holmium ions, emission spectra, and fluorescence lifetimes of Yb 3+ : 2 F5/2 state at the wavelength of 1010 nm, which corresponds to 2 F5/2 → 2 F7/2 radiative transition, under 980 nm excitation has been observed and presented in Figure  7. Luminescence intensity at 1.01 µm band increasing from 025H025Y to 0125H075Y which is the result of: 1) higher amount of Yb 3+ ions in material and higher absorption cross-section at the 980 nm wavelength and 2) lower concentration of Ho 3+ ions which decreases the donor → acceptor energy transfer efficiency. In Figure 7b, it can be seen that the lifetime in Ho 3+ /Yb 3+ co-doped sample (τHY) of 266 µs is significantly shorter in comparison to the lifetime of Yb 3+ : 2 F5/2 level in the single-doped glass where τY amounts 684 µs. The decrease of the Yb 3+ : 2 F5/2 lifetime indicates the presence of an energy transfer process between holmium and ytterbium ions. Based on the measured lifetime values, ET efficiency has been calculated by the following equation [47]: Estimated energy transfer efficiency equals to 61% which is higher than of other bismuthgermanate (40%) and comparable to germanate glasses (69-71%) [20,48,49]. This indicates that produced Ho 3+ /Yb 3+ heavy metal oxide glass is a potential material for an active optical fibre core operating in the wavelength range of 2.87 µm.

Active Optical Fibre Co-Doped with Ho 3+ /Yb 3+ Ions
In accordance with the glass emission measurement results, 025H075Y glass was used as an active core in optical fibre, while the silicate glass (NA = 1.61) was used as a cladding. The refractive index of the active core is equal to 2.19, which establish the numerical aperture NA close to 1. The manufactured optical fibre is characterized by 120 µm core diameter and 250 µm clad diameter. High numerical aperture and large core/clad diameter ratio allow efficient excitation with pumping radiation due to a high absorption coefficient. Figure 8 shows the emission band obtained in the produced optical fibre (l = 7 cm) at the wavelength of 2.87 µm under 980 nm pump radiation was presented. As a result of luminescence characterization, the value of Full Width at Half Maximum decreases from 80 nm in the glass to 71 nm in the optical fibre. Narrowing the emission spectrum band, as well as a redshift of the luminescence spectrum, occurs due to the reabsorption of the generated signal which propagates along the fibre core [50]. Estimated energy transfer efficiency equals to 61% which is higher than of other bismuth-germanate (40%) and comparable to germanate glasses (69-71%) [20,48,49]. This indicates that produced Ho 3+ /Yb 3+ heavy metal oxide glass is a potential material for an active optical fibre core operating in the wavelength range of 2.87 µm.

Active Optical Fibre Co-Doped with Ho 3+ /Yb 3+ Ions
In accordance with the glass emission measurement results, 025H075Y glass was used as an active core in optical fibre, while the silicate glass (NA = 1.61) was used as a cladding. The refractive index of the active core is equal to 2.19, which establish the numerical aperture NA close to 1. The manufactured optical fibre is characterized by 120 µm core diameter and 250 µm clad diameter. High numerical aperture and large core/clad diameter ratio allow efficient excitation with pumping radiation due to a high absorption coefficient. Figure 8 shows the emission band obtained in the produced optical fibre (l = 7 cm) at the wavelength of 2.87 µm under 980 nm pump radiation was presented. As a result of luminescence characterization, the value of Full Width at Half Maximum decreases from 80 nm in the glass to 71 nm in the optical fibre. Narrowing the emission spectrum band, as well as a redshift of the luminescence spectrum, occurs due to the reabsorption of the generated signal which propagates along the fibre core [50].

Conclusions
In the article, thermal stability, structural, and spectroscopic properties of a low phonon (hωmax = 724 cm −1 ) heavy metal oxide glass and optical fibre with core co-doped with Ho 3+ /Yb 3+ have been investigated. High thermal stability (ΔT = 124 °C) enables forming the fabricated glass into optical fibres, while low hydroxide content in the glassy matrix (47 ppm) provided high transparency in the mid-IR region. Based on the measurements of luminescence parameters the optimal content of codopants is 0.25 Ho2O3/0.75 Yb2O3 molar percentage in developed heavy metal oxide glass for which high emission intensity in the 2.87 µm band has been observed. To further examine Yb 3+ → Ho 3+ energy transfer mechanism decay, lifetimes measurements were carried out demonstrating ET efficiency equal to 61% in the 025H075Y glass sample which has been used as an active core in the optical fibre. Comparison of the luminescence spectra in bulk glass and optical fibre showed a reduction of FWHM from 80 to 71 nm and a redshift of 6 nm towards longer wavelengths in the optical fibre due to the reabsorption of the generated signal phenomenon. The obtained experimental results indicate the possibility of use developed optical fibre with Ho 3+ /Yb 3+ co-doped core in the midinfrared applications such as lasers or optical amplifiers.

Conclusions
In the article, thermal stability, structural, and spectroscopic properties of a low phonon (hω max = 724 cm −1 ) heavy metal oxide glass and optical fibre with core co-doped with Ho 3+ /Yb 3+ have been investigated. High thermal stability (∆T = 124 • C) enables forming the fabricated glass into optical fibres, while low hydroxide content in the glassy matrix (47 ppm) provided high transparency in the mid-IR region. Based on the measurements of luminescence parameters the optimal content of co-dopants is 0.25 Ho 2 O 3 /0.75 Yb 2 O 3 molar percentage in developed heavy metal oxide glass for which high emission intensity in the 2.87 µm band has been observed. To further examine Yb 3+ → Ho 3+ energy transfer mechanism decay, lifetimes measurements were carried out demonstrating ET efficiency equal to 61% in the 025H075Y glass sample which has been used as an active core in the optical fibre. Comparison of the luminescence spectra in bulk glass and optical fibre showed a reduction of FWHM from 80 to 71 nm and a redshift of 6 nm towards longer wavelengths in the optical fibre due to the reabsorption of the generated signal phenomenon. The obtained experimental results indicate the possibility of use developed optical fibre with Ho 3+ /Yb 3+ co-doped core in the mid-infrared applications such as lasers or optical amplifiers.

Conflicts of Interest:
The authors declare no conflict of interest.