Yb3+ doped germanate-tellurite glass

Optical Materials 60 (2016) 252e257

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Enhanced effect of Er3þ ions on 2.0 and 2.85 mm emission of Ho3þ/ Yb3þ doped germanate-tellurite glass Yu Lu, Muzhi Cai, Ruijie Cao, Ying Tian, Feifei Huang, Shiqing Xu, Junjie Zhang* College of Materials Science and Engineering, China Jiliang University, Hangzhou, 310018, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 April 2016 Received in revised form 8 July 2016 Accepted 23 July 2016

We report what we believe is the first demonstration of the enhanced Ho3þ 2.0 mm (2.1 times) and 2.85 mm (2.6 times) emission by addition of Er3þ in the Ho3þ/Yb3þ co-doped germanate-tellurite (GT) glass with optimized extra-low hydroxyl content vOH (0.118 cm1). Meanwhile, enhanced 2.0 and 2.85 mm emission cross-sections (5.78  1021 and 17.9  1021 cm2) can be approximately increased 1.5 times higher, respectively. The enhanced 2.0 and 2.85 mm emission upon excitation of a conventional 980 nm laser diode are investigated by the analysis of energy transfer. In addition, the energy transfer mechanism and coefficient (CDA) from Yb3þ to Ho3þ and Er3þ to Ho3þ have been investigated in detail. Our results indicate that Ho3þ/Er3þ/Yb3þ triply-doped GT glass is a promising material for 2.0 mm and 3 mm fiber laser. © 2016 Elsevier B.V. All rights reserved.

Keywords: Germanate-tellurite glasses Ho3þ/Er3þ/Yb3þ triply-doped 2.0 and 2.85 mm emission

1. Introduction Infrared (IR) emission from rare-earth ions (RE3þ/Yb3þ, RE ¼ Er3þ, Tm3þ, Ho3þ) co-doped or triply-doped glasses has stimulated increasing interest for their promising applications in many areas including medical surgery, medical imaging, remote sensing and eye-safe laser radar, eat [1e4]. Ho3þ with 5I7/5I8 and 5I6/5I7 transitions can generate 2 mm and 3 mm emission, Unfortunately, Ho3þ can not be pumped directly by readily available commercial high-power 808 nm or 980 nm laser diodes owing to the lack of an appropriate absorption band [5]. Hence, Tm3þ is most common utilized as the sensitizer to solve this problem because they possess a strong absorption band near 808 nm wavelength and can effectively transfer energy to Ho ions. So far, Ho3þ/Tm3þ codoped glasses [5e7] have been investigated by researchers. Ho3þ/Yb3þ co-doped glass has also been extensively investigated not only due to their significant 2.0 and 2.85 mm emission from 5I7/5I8 and 5I6/5I7 transitions [8,9], respectively, but also due to Yb3þ ions play an important role in assisting the Ho3þ relevant emission because of its efficient absorption at 980 nm LD. Nevertheless, the energy transfer process from Yb3þ to Ho3þ is proven to be inefficient, in particular in the high maximum phonon energy of silica glass (1500 cm1) [10]. But

* Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.optmat.2016.07.031 0925-3467/© 2016 Elsevier B.V. All rights reserved.

Yb3þ to Ho3þ energy transfer efficiency is still low in tellurite glass; a recent study measured it to be 17% [11] although when they are amalgamated in a host matrix with relatively low phonon energies, which because the maximum phonon energy of glass system is smaller to reduce the loss of nonradiative energy due to multiphonon relaxation [12]. Therefore, development of an effective approach to improve indirectly energy transfer efficiency from Yb3þ to Ho3þ would be beneficial in enhancing the Ho3þ 2.0 and 2.85 mm emission. Trivalent erbium (Er3þ) has been confirmed that it, serve as acceptor ion, can be efficient transferred energy by Yb3þ ion in the Er3þ/Yb3þ co-doped glass [13] at the same time as the efficient energy transmitted to Ho3þ as donor ions in the Ho3þ/Er3þ codoped glass [14] pumped by 980 nm laser diode (LD), respectively. Hence, it is worth expected that the introduction of Er3þ can improve indirectly energy transfer efficiency from Yb3þ to Er3þ then to Ho3þ, at the same time transfer its energy to Ho3þ ion solely pumped by 980 nm LD. Previously, 2.0 mm emission of Ho3þ/Er3þ/ Yb3þ triply-doped fluorophosphate glasses has only been investigated by Li et al. [15], but the lack of research on the effect of Er3þ ions on 2.0 and 2.85 mm emission of Ho3þ/Yb3þ co-doped glass. In order to get powerful mid-infrared emissions from Ho3þ, the host glass is another factor to be considered as important as the sensitizer. It is well known that most of the studies in glass host mainly focused on chalcogenide, fluoride glasses and oxide glasses (silicate, germanate, tellurite). Although chalcogenide glass have quite low phonon energy and larger refractive index, it is difficult to

Y. Lu et al. / Optical Materials 60 (2016) 252e257

draw into fiber due to its relatively low recrystallisation temperature which is close to the fiber drawing temperature. Oxide glasses are more appropriate for practical applications as its high chemical durability and thermal stability though fluoride glasses have been extensively studied due to their low phonon energies [16]. Among oxides, germanate and tellurite glasses are instead very interesting because of their lower phonon energy than silicate glass. Therefore, germanate-tellurite glasses are regarded as promising materials for fiber lasers due to they combine the advantages from both germanate and tellurite glasses, i.e., good thermal stability, lower phonon energy, high rare earth solubility [17,18]. In this letter, we report what we believe to be the first demonstration of the enhanced Ho3þ 2.0 and 2.85 mm emission by addition of Er3þ in the Yb3þ/Ho3þ co-doped germanate-tellurite glass. Meanwhile, the enhanced 2.0 and 2.85 mm emission cross sections and the enhanced 2.0 mm fluorescence lifetime were obtained. The energy transfer (ET) processes between Yb3þ, Er3þ and Ho3þ were also discussed. In parallel, the energy transfer coefficient from Yb3þ to Ho3þ and Er3þ to Ho3þ are calculated and examined using the extended overlap integral method proposed by Forster and Dexter. 2. Experimental 2.1. Experimental procedure Ho3þ/Yb3þ co-doped and Ho3þ/Er3þ/Yb3þ triply-doped samples were prepared for comparison with high-purity (99%e99.99%) by traditional melt-quenching method. The investigated glasses have the following molar compositions: 75(GeO2þTeO2)(23.5x)(Nb2O5þYF3)0.5HoF3xErF31YbF3 (x ¼ 0, 0.25, 0.5, 0.75) denoted as HY, HYE0.25, HYE0.5 and HYE0.75, respectively. Raw materials (15 g) were mixed homogeneously and melted in a highpurity Al2O3 crucible with a SiC-resistance electric furnace at the temperature of 1050  C for 18 min. The melts were then thermally quenched by casting the melt into a copper mold preheated to 420  C and then annealed at this temperature for 2 h. Finally, the annealed samples were fabricated and optically polished to the size of 20  10  1.5 mm for the optical property measurement. 2.2. Measurements The infrared transmission spectra were measured by using a Thermo Nicolet (Nexus FT-IR Spectrometer) spectrophotometer in the range of 2.5e3.6 mm with resolution of 4 cm1. The visible upconversion (470e770 nm) fluorescence spectra were obtained with a TRIAX550 spectrofluorimeter upon the excitation of 980 nm LD with a maximum power of 2 W. Similar, fluorescence spectra (1800e3100 nm) were measured with a computer-controlled Triax 320 type spectrometer upon excitation by 980 nm laser diode with the maximum power of 2 W. Fluorescence lifetimes of 5I7 level (2 mm) were recorded with light pulses of the 980 nm LD and HP546800B 100-MHz oscilloscope. The lifetimes were calculated by fitting a single exponential function to the decay curve with normalized initial intensity. All samples were conducted at room temperature.

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sensitization of the Ho3þ ions using Er3þ ions can increase the pumping efficiency compare to HY sample. Fig. 1 presents the IR transmission spectrum of HYE0.75 (Ho3þ: 0.92  1020 ions/cm3, Er3þ:1.36  1020 ions/cm3) sample with 1.5 mm thickness. The concave existed around 3000 nm is ascribed to typical H2O absorption band, but as it can be declared that the simultaneous utilization of the fluoride (15 mol% mol YF3) and the shielding gas (O2) could bring about a better dehydration result, which can be associated with the depressed incorporation of environmental H2O and the facilitated evaporation of OH from the melt into outside environment. The absorption coefficient was calculated through the reported equation

vOH ¼

lnðT=T0 Þ l

(1)

where l (cm) is the thickness of the sample, T, To are the maximum transmittance and the transmittance around 3 mm, respectively. It is obvious that value vOH (0.118 cm1) after the introduction the fluoride (15 mol% YF3) and the shielding gas (O2) is lower than those of HYE0.75 (1.338 cm1) and HYE0.75 þ fluoride (0.634 cm1) glass. Moreover, the minimal value vOH (0.118 cm1) is also much lower than those of germanate [19] and tellurite glass [20], even lower than the reported fluoride glass (0.41 cm1) [21], demonstrating that the simultaneous utilization of fluoride and shielding gas (O2) is a more effective method to extract OH out of the mid-infrared laser glass during the fabrication process. It is widely recognized that the residual OH group is in charge of the absorption loss and affect the emission efficiency by participating in the energy transfer of rare earth ions so that quench the emission intensity. Therefore, the excellent infrared transmission property reveals the germanate-tellurite glass after removing hydroxyl measures is a potential candidate for IR laser materials.

3.2. Emission spectra and cross section Fig. 2 (a) and (b) show the Ho3þ 2.0 mm and 2.85 mm emission spectra of the Ho3þ/Yb3þ co-doped and Ho3þ/Er3þ/Yb3þ triplydoped samples in the wavelength region of 1800e2200 and 2600e3100 nm under excitation of 980 nm LD laser. The 2.0 mm and 2.85 mm emission intensity were obviously enhanced with the presence of Er3þ ions. Moreover, the 2.0 mm and 2.85 mm emission intensities both increase with Er3þ concentration. Compared to the HY sample, the 2.0 mm and 2.85 mm emission intensities of HYE0.75

3. Results and analysis 3.1. Absorption spectrum and fourier transform infrared spectra On the basis of previous reports [10e12], the absorption peak of Yb3þ (2F5/2) and Er3þ (4I11/2) in the wavelength region of 900e1100 nm appear the overlap, which indicate the sample HY and HYE can all be pumped by 980 nm LD. Then, the other

Fig. 1. The mid-IR transmission spectrum of HYE0.75 sample with and without the fluoride and shielding gas (O2). All samples are 1.5 mm thickness.

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Fig. 2. (a) 2.0 mm and (b) 2.85 mm fluorescence spectra of Ho3þ/Yb3þ co-doped and Ho3þ/Er3þ/Yb3þ triply-doped germanate-tellurite samples.

are increased by 2.1 and 2.6 times, respectively. It can be observed that the 2.85 mm emission is enhanced with obvious change on the emission line-shape although 2.0 mm emission peak is unchanged. The 2725 nm emission was observed due to the 4I11/2 / 4I13/2 transition with the import of Er3þ. Important parameter that used to estimate the emission ability of luminescent center for 2.0 mm and 2.85 mm such as emission cross section (sem). In comparison to the results emission cross section (sem) under the simplified model in the determination of the partition function, FüchtbauereLadenburg equation [22] was subsequently applied to scale the 2.0 mm and 2.85 mm emission cross-section (sem), respectively.

sem ðlÞ ¼

l4 Arad Z lIðlÞ  8pcn2 lIðlÞdl

(2)

where l is the emission wavelength, Arad is the spontaneous radiative transition probability of Ho3þ: 5I7/5I8 (69.33 s1) and 5 I6/5I7 (49.15 s1) transitions, which were estimated from the absorption spectrum and J-O intensity parameters and can be calculated by the formula provided in Ref. [23]. c is the velocity of light in vacuum, n is the refractive index of glass host, I(l) is the R 2 mm and 2.85 mm fluorescence intensity, and lIðlÞdl is 2 mm and 2.85 mm the integrated fluorescence intensity, respectively. Compared to the result, Fig. 3 (a) displays 2.0 mm emission crosssection (sem) of HYE0.75 (peak value is 5.78  1021 cm2), which is approximately 1.5 time higher than the value of HY (peak value is 4.03  1021 cm2), thereby indicating that Er3þ ions have a positive impact on the Ho3þ: 5I7 population. Meanwhile, the calculated sem peak values of HYE0.75 at 2 mm is also higher than those of the other germanate glasses (4.0  1021 cm2) [24] and tellurite glass (4.52  1021 cm2) [25] system. The calculated peak value of emission cross section (HYE0.75 glass) at 2.85 mm is reach up to

Fig. 3. (a) 2.0 mm and (b) 2.85 mm emission cross-section of the HY and HYE0.75 samples, respectively.

Y. Lu et al. / Optical Materials 60 (2016) 252e257

17.9  1021 cm2, which is also approximately 1.5 time higher than HY (12.4  1021 cm2) as shown in Fig. 3 (b). Similar, the calculated peak value of emission cross section (HYE0.75 glass) at 2.85 mm is also higher than those of the other germanate glasses (9.2  1021 cm2) [26] and tellurite glass (15.1  1021 cm2) [27] system. It is reported that high emission cross section is extremely useful to determine the possibility to achieve laser effect [28]. Therefore, it is expected that the developed germanatetellurite glass, especially the sample HYE0.75 has promising potential for 2.0 and 2.85 mm laser applications. 3.3. Energy transfer mechanism and microscopic parameters To further understand the enhancement of Ho3þ 2.0 and 2.85 mm emission, the energy transfer process between Ho3þ, Er3þ and Yb3þ ions has been analyzed based on previous investigation [11,15,25] and the energy level diagram involved in the Ho3þ/Er3þ/ Yb3þ triply-doped system was presented in Fig. 4. The energy transfer mechanism as follows: (1) The Yb3þ ions in ground state (2F7/2) are pumped to the 2F5/2 state (2F7/2þ a photon /2F5/2). The ions in the Er3þ:4I15/2 level are pumped to a higher 4I11/2 level via ground state absorption (GSA: Er3þ: 4I15/2 þ a photon / 4I11/2) when same excited by commercial 980 nm LD. (2) The energy (2F5/2) can be transferred to a neighboring Ho3þ (5I6 level) or Er3þ (4I11/2) by energy transfer process (ET1 and ET2) under the assistance of host phonons. With that the energy in the level (Er3þ: 4I11/2) can also be transferred to a neighboring Ho3þ (5I6 level) via an ET3 (Er3þ: 4I11/2 þ Ho3þ: 5 I8 / Er3þ: 4I15/2 þ Ho3þ: 5I6) process. Then, 2.85 mm emission takes place via radiative transition of Ho3þ: 5I6 / 5I7. (3) On the one hand, the ions in the 5I6 level decay to the lower 5 I7 level because of 2.85 mm emission or a multi-phonon relaxation process. On the other hand, the ions in the Er3þ: 4 I11/2 level can relax to the lower 4I13/2 level by a non-

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radiative process or decay radiatively to the next lower level 4I13/2 with 2.7 mm emission (4I11/2 / 4I13/2 þ 2.7 mm), then the energy in the 4I13/2 level is accumulated and then transfers to the adjacent Ho3þ: 5I7 level (ET4: Er3þ: 4I13/ 3þ 5 3þ 4 3þ 5 2 þ Ho : I8 / Er : I15/2 þ Ho : I7). Then, 2 mm emission takes place via radiative transition of Ho3þ:5I7 / 5I8. By incorporation of Er3þ, the upconverted green and red emissions shown in inset Fig. 4 increased but exhibited different increase tendency in the luminescence intensity, implying that the upconversion mechanisms are different for the green and red emission in 980 nm LD pumped Yb3þ/Ho3þ ions. Hence, to further understand and explain the enhancement of Ho3þ 2.0 and 2.85 mm emission, it is a feasible method to investigate enhancement of the upconversion process in the Ho3þ/Er3þ/Yb3þ triply-doped GT systems. From the inset Fig. 4, three enhanced emission bands centered at 546 nm, 657 nm and 751 nm and new 525 nm emission band (Er3þ: 4H11/2 / 4I15/2) can be observed. Although enhanced 546 nm and 657 nm emissions may be also attributed to Er3þ: 4S3/ 4 4 4 2 / I15/2 (546 nm) and F9/2 / I15/2 (657 nm) by the introduction 3þ of Er ions, enhanced 751 nm emission can only be attributed to the increase on the population behavior of upper lasing level (Ho3þ: 5 S2) or the decrease on the population behavior of the lower level (Ho3þ: 5I7). But the decrease on the population behavior of the lower level (Ho3þ: 5I7) can be excluded because of the enhanced 2.0 mm emission. Therefore, the increasing upper lasing level (Ho3þ: 5 S2) can be attributed to strengthen ESA1 process and increase on the population behavior of Ho3þ: 5I6 level. Then, this is confirmed by the upper lasing level (Ho3þ: 5I6) is further populated owing to this ET3 process after addition of Er3þ ions, by which enhanced 2.85 mm emission are obtained. Similarly, the further populated of the Ho3þ: 5I7 after addition of Er3þ can be also ascribed to the ET4 process. But to further verify that, the decay curves of Ho3þ: 5I7 level monitored at 2.0 mm of HY and HYE0.75 samples were determined and revealed in Fig. 5. It can be seen that the lifetime of 5 I7 about HYE0.75 sample is up to 8.65 ms, which was 53% longer

Fig. 4. Energy level diagram and energy transfer mechanism among Ho3þ, Er3þ and Yb3þ ions. The inset is the visible upconversion emission spectra of HY and HYE0.75 samples in the wavelength region of 470e770 nm.

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SN DA z

X

eðS0 þS0 Þ  D

A

 D N S þ SA SDA ð0; 0; EÞdðN; DE=Zw0 Þ; N!

(4)

A where SD O and SO are the Huang-Rhys factors, SDA (0,0,E) is the overlap between the zero phonon line shape of Yb3þ, Er3þ emission and Ho3þ absorption. Then the integral overlap in the case of mphonon emission by the donor and no phonon involvement by the acceptor can be expressed as

Z

Sm D A gemðMphononÞ ðEÞgabs ðEÞdE ¼ o eSo SDA ð0; 0; EÞ m!  Z Z  m So so D A e gem ¼ ðE  DEÞ gabs ðEÞdE m!

SDA ðm; 0; EÞ ¼

(5) Fig. 5. Decay curves of Ho3þ:5I7 level monitored at 2.0 mm region of HY and HYE0.75.

than that of HY sample (5.67 ms), thereby indicating that Er3þ ions have a positive influence on the 5I7 population. Furthermore, the measured lifetime (8.65ms) of the 2 mm emission in the samples (HYE0.75), is significantly larger than that of germanate (0.36 ms) [24] and tellurite (1.6 ms) glass [29]. A relatively long radiation lifetime generally reduced the laser oscillation threshold [30]. Therefore, the emission lifetime indicate that Yb3þ/Er3þ/Ho3þ triply-doped germanate-tellurite glass is a promising candidate material for 2.0 mm fiber laser. Based on the above analysis, Er3þ ions have a positive effect on 2 mm and 2.85 mm emission properties. Although the energy transfer process between Ho3þ,Yb3þ and Er3þ ions has been analyzed qualitatively, a quantitative understanding is required in order to explain the sensitization and optimize the corresponding laser system. According to Forster [31] and Dexter [32], The probability rate of energy transfer between donor (Yb3þ, Er3þ) and acceptor (Ho3þ) can be estimated as [33,34]:

where DE ¼ mZwo . The multi-phonon probability must be included because the measurements are carried out at some finite temperature T. and the emission cross section (sem) with m phonon emission and absorption cross section (sabs) with k phonon absorption can be proposed as





so Sm o e ðn þ 1Þm sD em ðE  E1 Þ m!

(6)





Sko eso ðnÞk sD abs ðE þ E2 Þ k!

(7)

sDemðmphononÞ ¼ sDem lþ z sDabsðkphononÞ ¼ sDabs l z

where E1 ¼ mZwo, E2 ¼ kZwo and DE ¼ E1þ E2. lþ denotes the translation of Yb3þ and Er3þ emission cross section systems wavelength by m-phonon emission lþ ¼ 1/(1/l-mhu0). Meanwhile, l denotes the translation of Ho3þ absorption cross section systems wavelength due to k-phonon absorption l ¼ 1/(1/l þ mhu0). Finally, the energy transfer coefficient is expressed by

CDA ¼

D 6cglow

∞ X

D ð2pÞ4 n2 gup m¼0

eð2nþ1ÞS0

Sm 0 ðn þ 1Þm m!

Z





A sDemis lþ m sabs ðlÞdl;

(8) WDA

2p ¼ jH j2 SN Z DA DA

(3)

where SN DA is the integral overlap between the m-phonon emission sideband of donor ions and k-phonon absorption line shapes of acceptor ions. jHDA j is the matrix element of the perturbation Hamiltonian between initial and final states in energy transfer process. N is the total phonons in the transfer process (m þ k ¼ N). In our case, the donor and acceptor in ET1 process are Yb3þ ions in 2 F5/2 level and Ho3þ ions in 5I6 level. At the same time, the donor and acceptor in ET3 process are Er3þ ions in 4I11/2 level and Ho3þ ions in 5I6 level. Furthermore, the donor and acceptor in ET4 process are Er3þ ions in 4I13/2 level and Ho3þ ions in 5I7 level. In the case of weak electron-phonon coupling which is suitable for rare earth ions. SN DA can be approximated by

where CDA is the energy transfer coefficient (cm6/s), n ¼ 1=ðeZwo =kT  1Þ is the average occupancy of phonon mode at temperature T. In this work, the donor are Yb3þ ions and acceptor are Ho3þ ions for HY glass, however, the donor are Yb3þ and Er3þ ions and acceptor are Ho3þ ions for HYE0.75 glass after the introduction of the Er3þ ions. The energy transfer microscopic properties of Yb3þ/ Ho3þ in HY glass and Yb3þ/ Ho3þ, Er3þ/ Ho3þ in HYE0.75 glass are respectively calculated using Eqs. (3e8) and listed in Table 1. Moreover, the number of phonons and phonon assisted are also summarized in Table 1. The result shows that the Yb3þ/ Ho3þ transference (ET1) in HY glass is a multiphonon mechanism and needs the assistance of 2(89.37%), 1(10.56%) and 0 (0.17%) phonon emissions and the energy transfer coefficient can be as high as

Table 1 Calculated energy transfer microscopic parameters CDA for ET1 in HY glass and ET1, ET3 and ET4 in HYE0.75 glass. The number of phonons necessary to assist the energy transfer and the percentage of each phonon participated in the process in HY and HYE0.75 samples. Samples

Energy transfer

HY HYE0.75 HYE0.75 HYE0.75

ET1 ET1 ET3 ET4

(Yb3þ: 2F5/2 / Ho3þ: (Yb3þ: 2F5/2 / Ho3þ: (Er3þ: 4I11/2 / Ho3þ: (Er3þ: 4I13/2 / Ho3þ:

N (No. of phonons) (%phonon-assist) 5

I6) I6) 5 I6) 5 I7)

5

0 0 0 0

(0.17) (0.21) (1.99) (10.46)

CDA (1040 cm6/s) 1 1 1 1

(10.56) (10.54) (10.66) (6.32)

2 2 2 2

(89.37) (89.35) (87.35) (83.22)

6.33 6.29 1.43 0.37

Y. Lu et al. / Optical Materials 60 (2016) 252e257

6.33  1040 cm6/s in energy transfer process. Meanwhile, energy transfer coefficient can be also as high as 6.29  1040 cm6/s in HYE0.75 glass with the assistance of 2(89.35%), 1(10.54%) and 0 (0.21%) phonon emissions. It is obvious that the value of the energy transfer coefficient from Yb3þ to Ho3þ (ET1) is similar in the two samples, which indicates the energy transfer efficiency from Yb3þ to Ho3þ, has been unchanged after the introduction of Er3þ ions. However, the Ho3þ: 5I6 and 5I7 level can be further populated by the added two energy transfer processes (ET3, ET4) and energy transfer coefficient can be up 1.43  1040 cm6/s and 0.37  1040 cm6/s in the HYE0.75 glass, respectively. Moreover, the ET3 energy transfer coefficient is four times larger than that of ET4 process. Therefore, further population inversion between 5I7 and 5 I8, 5I6 and 5I7 levels can be achieved and beneficial for 2 mm and 2.85 mm emission.

4. Conclusions In conclusion, enhanced Ho3þ 2.0 and 2.85 mm emission as well as 2.0 and 2.85 mm emission cross-sections have been obtained in Ho3þ/Er3þ/Yb3þ triply-doped germanate-tellurite glass with extralow hydroxyl content vOH (0.118 cm1). Details of ET processes are elucidated by the investigation of the enhanced visible upconversion emission spectra. Meanwhile, 53% longer lifetime (8.65 ms) of 5 I7 about HEY sample has also indicated Er3þ ions have a positive influence on the 5I7 population. Besides, the Ho3þ: 5I6 and 5I7 level can be further populated by the added two energy transfer processes (ET3, ET4), although the energy transfer coefficient from Yb3þ to Ho3þ in Ho3þ/Er3þ/Yb3þ triply-doped sample is similar in Ho3þ/Yb3þ co-doped sample (HY). Results suggest that this glass system is promising for the improvement of Ho3þ ~2.0 and 3 mm fiber laser performance.

Funding This research was financially supported by the Chinese National Natural Science Foundation (No. 51372235, 51272243, 51472225 and 61308090), Zhejiang Provincial Natural Science Foundation of China (No.LR14E020003), the International Science & Technology Cooperation Program of China (Grant no. 2013DFE63070), and Public Technical International Cooperation project of Science Technology Department of Zhejiang Province (2015c340009).

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