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Abstract
In this paper 1400 nm–2200 nm broadband emission under 808 nm and 976 nm laser diodes excitation in germanate glasses co-doped with (0.35–0.7) Er2O3/(0.35–0.7) Tm2O3/(0–0.15) Ho2O3 (mol%) are presented. Flattened luminescence profiles were achieved as a result of RE content optimization in glasses co-doped with 0.35Er2O3/0.7Tm2O3/0.15Ho2O3, (λexc = 808 nm) and 0.7Er2O3/0.35Tm2O3/0.07Ho2O3, (λexc= 976 nm). The second, glass characterized by the widest emission range, has been used for the fabrication of double-clad optical fiber with 15 µm/250 µm (core/outer cladding) diameters. The optimization of broadband spectrum in glass and optical fiber was performed as the result of the superposition of 1550 nm (Er3+: 4I13/2 → 4I15/2), 1800 nm (Tm3+: 3F4 → 3H6), and 2000 nm (Ho3+:5I7 → 5I8) emission bands. The influence of optical fiber length and pump wavelength on near-infrared amplified spontaneous emission (ASE) features have been determined. As a result, the broadest bandwidth of ASE (415 nm - 3 dB and 731 nm – 10 dB) was obtained in 5 m length optical fiber under 808 nm pumping. It is significantly broader than in silica and tellurite fibers, which showed the advantage of RE triply-doped germanate optical fiber.
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1. Introduction
Rare-earth (RE) doped optical fiber lasers and amplified spontaneous emission sources (ASE) operating in the near-infrared spectral range are widely investigated. The interest is derived from their applications at 2 µm including atmospheric pollution monitoring, eye-safe laser surgery, and sensors [1–5]. In recent years, silica glass fibers co-doped with rare-earth ions are used for near-infrared (NIR) emission. However, their high phonon energy (∼1100 cm−1), reduces efficient energy transfer (ET) between donor and acceptor. Besides, the ET optimization is hardly possible as the concentration of rare-earth ions is limited due to the known clustering and phase separation effects. On the other side, low phonon energy oxide glasses (450 – 750 cm−1), i.e. heavy metal oxide or tellurite-based glasses are known from a higher energy transfer efficiency than silica glass. Unfortunately, it has been clearly stated that the quality of their optical fibers is relatively low [6–10]. Despite the fact, that ASE and lasing at 2 µm have been analyzed in silica, silicate, fluorophosphate, tellurite, and antimony fibers recently [11–15]. Analyzing the results it is shown that the advantages of relatively low phonon energy (800 cm−1) and thermal stability of gallo-germanate oxide glasses make them an attractive material for RE-doped optical fibers [12,13,16–21]. Additionally, due to the wide transmission window, germanate glasses offer good luminescent properties in the broad spectrum from the visible up to 3 µm range under doping with Tb3+, Pr3+, and Ho3+ ions [19,22–24]. Lasing and ASE have been presented in Tm3+-doped and Tm3+/Ho3+-co-doped germanate optical fibers [24,25]. Concerning ASE the interesting direction is triply doping of the glass (further fiber) as a broadband emission in the 1.4–2.1 µm range can be obtained as a result of Tm3+: 3H4→3F4 (1.45 µm), Er3+: 4I13/2→4I15/2 (1.55 µm), Tm3+: 3F4→3H6 (1.8 µm) and Ho3+: 5I8→5I7 (2 µm) simultaneous radiative transitions. Herein under one wavelength laser pumping donor-acceptor energy transfer leads to simultaneous radiative transitions in particular RE. Thus, triply-RE doped glasses and optical fibers open new possibilities in the development of novel broadband optical fiber ASE sources. Recently, Er3+/Tm3+/Ho3+ triply doped silicate, tellurite glasses and α-SiAlON ceramics have been analyzed [26–28]. The broadband eye-safe emission in Er3+/Tm3+/Ho3+ co-doped gallo-germanate glasses and double-clad optical fibers have not been examined yet.
In this paper, the effect of the Er3+/Tm3+/Ho3+ co-doping on the NIR luminescent properties, energy transfer processes in germanate glasses, and double-clad optical fiber under 808 and 976 nm laser excitation are presented. The novelty of this work is the optimization of Er3+/Tm3+/Ho3+ content in germanate glass to obtain broadband 1400–2100 nm emission and its application as core glass in a double-clad optical fiber. Moreover, elaborated Er3+/Tm3+/Ho3+ co-doped double-clad optical fiber presents in the eye-safe region the broadest bandwidth of ASE i.e 415 nm for - 3 dB and 731 nm for – 10 dB level.
2. Experimental
The gallo-germanate glass samples (including optical fiber core) were prepared with molar compositions: (60-RE)GeO2−15Ga2O3-10BaO-15Na2O-RE, where RE = (0.35–0.7)Er2O3 – (0.2–0.7)Tm2O3/(0-0.15Ho2O3), (in mol%) by standard melting and quenching method. A homogenized power was placed in the platinum crucible and melted at 1450° C for 30 minutes in a dry nitrogen atmosphere. The melted glass was then cast in stainless form, kept in the annealing furnace at 600 ° C for 12 hours, and then slowly cooled to room temperature to remove internal thermal stress. Luminescence measurements in the range of 1400–2300 nm were carried out by using Yokogawa AQ6375B optical spectrum analyzer and high-power LIMO laser diodes with optical fiber output, Popt. max = 30W). Luminescence decay measurements were performed using a system PTI QuantaMaster QM40 coupled with a tunable pulsed optical parametric oscillator (OPO), pumped by the third harmonic of an Nd:YAG laser (OpotekOpolette 355 LD). The laser system was equipped with a double 200 monochromator, a multimode UV-VIS photomultiplier tube (PMT) (R928), and Hamamatsu H10330B-75 detectors controlled by a computer. Luminescence decay curves were recorded and stored by a PTI ASOC-10 [USB-2500] oscilloscope. Double-clad optical fibers were manufactured using the modified rod-in-tube technique. Fabricated germanate glass rods were located in drilled internal cladding structure and were placed in a glass tube. Inner cladding and outer cladding refractive indexes were 1.62 and 1.51, respectively. Finally, the double-clad optical fiber was drawn from an 8 cm long preform in the temperature range of 890–930°C and coated with low refractive index coating.
3. Results and discussion
3.1 Spectroscopic properties of the germanate core glass
Figure 1 presents absorbance spectra of the fabricated Er3+ - doped and Er3+/Tm3+, Er3+/Tm3+/Ho3+ co-doped germanate glass. Each assignment corresponds to the ground state absorption (GSA) to the exciting level of Er3+, Tm3+, and Ho3+. The absorption bands centered at 420, 446, 486, 536, 646, 1210, 1940nm correspond to the transition from Ho3+: 5I8 to the 5G5, 5G6+5F1, 5F3, 5F4+5S2, 5F5, 5I5, 5I6, 5I7 levels (blue colour). Analogically, bands and assignments for Er3+ (red color) and Tm3+ (black color) GSA transitions can be seen in Fig. 1. Absorption bands around 800 nm correspond to the Er3+: 4I15/2→4I9/2 as well as Tm3+: 3H6→3H4. The band at 980 nm is due to Er3+: 4I15/2→4I11/2 transition. These, two absorption bands overlap with common laser diodes. Thus, in the case of 976 nm laser pump, a direct excitation of the Er3+ occurs and due to the Er3+→ Tm3+/Ho3+ energy transfer, thulium and holmium exited levels are populated. For 808 nm, direct pumping of Er3+, Tm3+ and Tm3+→ Ho3+ takes place. Judd–Ofelt spectroscopic parameters of the Er3+, Tm3+, and Ho3+ doped gallo-germanate glasses are favorable for near-infrared emission and can be found in the literature [29–31].
Fig. 1. Absorbance spectra of the fabricated germanate glass: Er3+ - singly doped and co-doped with Er3+/Tm3+, Er3+/Tm3+/Ho3+ (glass sample thickness = 2.5 mm).
In order to investigate the profile of luminescence spectra and ultra-broadband emission ability in the Er3+/Tm3+ and Er3+/Tm3+/Ho3+ co-doped glasses with different RE-dopant content, the excitation by 808 nm and 976 nm laser diodes have been analyzed. Summarised results are presented in Fig. 2 and fig. 3. The broadband infrared emission (λexc = 808 nm, Fig. 2) aroused as a result of the superposition of Er3+:4I13/2 → 4I15/2, Tm3+:3F4 → 3H6, and Ho3+:5I7 → 5I8 radiative transitions. The population mechanism of the Er3+/Tm3+/Ho3+ excited levels under 808 nm laser pumping can be described as presented in Fig. 4(a). It should be clearly noted that the upconversion process wasn’t observed, therefore, the energy scheme can be simplified. Moreover, an excited state absorption and upconversion energy transfer have been also neglected.
Fig. 2. Luminescence spectra of the germanate glass co-doped with Er3+/Tm3+/Ho3+ under 808 nm excitation.
Fig. 3. Luminescence spectra of the germanate glass co-doped with Er3+/Tm3+/Ho3+ under 976 nm excitation.
Fig. 4. Simplified energy diagram with energy transfer mechanism in germanate glass co-doped with Er3+/Tm3+/Ho3+, a) λexc = 808 nm, b) λexc = 976 nm.
Analyzing energy scheme under the 808 nm pump, firstly both Er3+ and Tm3+ ions are excited to the Er3+:4I9/2 and Tm3+:3H4 by GSA. Next, after nonradiative relaxation to the Er3+:4I11/2, the part of Er3+ ions generates luminescence at 1550 nm due to the Er3+:4I13/2 →4I15/2 transition. Simultaneously, another part of the energy is transferred to the neighboring Tm3+ and Ho3+ ions. Further, due to a small energy difference of 700 cm-1 between Er3+:4I13/2 and Tm3+:3F4 multiples the overlapping of emission cross-section of Er3+ at 1550 nm and absorption cross-section at 1600 nm of Tm led to the quasi-resonant energy transfer and enhancement of the emission at 1800 nm. It is well-known that the Ho3+ ions cannot be excited by 808 nm due to the absence of an absorption band. Excited electrons localized at 4I11/2 erbium levels relax nonradiatively to the 4I13/2 level and simultaneously transfer their energy to the Ho3+:5I7 level via phonon-assisted energy transfer (ET2). Finally, the luminescence band at 2000 nm occurs due to the 5I7→5I8 transition. At the same time, the resonant Tm3+ → Ho3+ energy transfer (ET4) populating the Ho3+:5I7 level occurs.
In the case of 976 nm pumping the broadband 1400–2200 nm luminescence was measured and shown in Fig. 3. However, herein only Er3+ ions are excited directly. The population of the Tm3+:3F4 is realized by ET1, ET3 (Fig. 4(b)) energy transfers exclusively. Holmium 5I7 level is populated analogically as for 808 nm pumping by ET2 and ET4 energy transfers (Fig. 4(a)).
Detailed analyses of the luminescence profiles are presented in Fig. 5. Luminescence intensities at particular 1550, 1800, and 2000nm transitions clearly showed that obtaining broadband, flattened emission spectra depends on the pump wavelength. Optimization of the RE content-enabled obtaining flattened luminescence profiles in glasses co-doped with: 1) 0.35Er2O3/0.7Tm2O3/0.15Ho2O3 (λexc = 808 nm), 2) 0.7Er2O3/0.35Tm2O3/0.07Ho2O3 (λexc= 976 nm). The achieved results indicated a possibility to obtain a wide emission with full width at half-maximum (FWHM) over 700 nm. The broadband NIR emission in triply Er3+/Tm3+/Ho3+ co-doped system was also reported in tellurite glasses where flattened emission reached 374 nm in glass co-doped with 0.1Er2O3/0.8Tm2O3/0.05Ho2O3 pumped by 808 nm laser diode. However, in this case, the emission range was limited to 1600-2200 nm [28].
Fig. 5. Luminescence intensities I1550nm, I1800nm, and I2000nm under pumping at a) 808 nm, b) 976 nm.
In order to determine the efficiency of energy transfer between lanthanide ions, the lifetimes were measured in two analyzed excitation schemes. Figure 6(a) and 6(b) show the luminescence decay curves monitored at 1550 nm emission from Er3+:4I13/2 energy level as a function of Tm3+ and Ho3+ ions concentration recorded under laser excitation at 808 nm (4I15/2→ 4I9/2) and 976 nm (4I15/2→ 4I11/2), respectively. The lifetime of 4I13/2 energy level in all fabricated germanate glass is characterized by single-exponential behavior (R2 = 0.99) and for reference glass sample doped with 0.7Er2O3, equals 5.6 ms. In both cases, the introduction of other ions (Tm3+, Ho3+) led to the fast reduction of the measured lifetimes of the 4I13/2 level as a result of energy transfer from Er3+→ Tm3+ ions and Er3 +→Ho3+ ions assigned to ET1 and ET2 (Fig. 4). This fast reduction of lifetimes in glasses co-doped with 0.7Er/0.35Tm to 500 µs (Fig. 7(a)) confirms the efficient energy transfer in the Er3+:4I13/2 → Tm3+:3F4 channel.
Fig. 6. Luminescence decay curves from Er3+: 4I13/2, as a function of Tm3+ and Ho3 + content, a), (λexc = 808 nm), b), (λexc = 976 nm).
Fig. 7. a) Lifetime of Er3+: 4I13/2, level, b) Er3+→Tm3+, Ho3+ energy transfer efficiency as a function of Tm3+ and Ho3 + content.
It is worth to note, that in triply-doped samples the slight decrease of 4I13/2 lifetime occurred when the concentration of Ho2O3 increased (Fig. 7(a)). It is connected with ET2 and ET4 possible mechanisms. Also, we observed that a double increase in Tm2O3 concentration leads to a further decrease of 4I13/2 lifetime, and for glass co-doped with 0.7Er2O3/0.7Tm2O3/0.15Ho2O3 we obtained the shortest value. Moreover, when the concentration of both thulium and holmium ions is lowest (0.2Tm2O3/0.02Ho2O3) then the lifetime of the 4I13/2 (Er3+) level is longer than in other glass samples. This fact can also confirm that energy transfer is observed between Er3+ and Tm3+ ions mainly. To estimate the efficiency of energy transfer in fabricated glasses we used the following equation:
where: $\tau _{\textrm{Er}}^{\textrm{Er}/\textrm{Tm}/\textrm{Ho}}$ is the lifetime of Er3+:4I13/2 energy level in the presence of Tm3+ and Ho3+ ions, ${\tau _{\textrm{Er}}}$ is the lifetime of singly Er3+ -doped germanate glass. As a result of calculations, the maximum efficiency of energy transfer (Er3+ → Tm3+/Ho3+) was determined to be >90% in both excitation schemes (fig. 7(b)). Slightly higher efficiency was observed under 808 nm laser pumping. Besides in each fabricated sample, the ET efficiency was more than 80%, without quenching phenomena, which confirms that the concentration of RE ions was optimized.3.2 Double–clad optical fiber
As a core material glass co-doped with 0.7Er2O3/0.35Tm2O3/0.07Ho2O3 with the broadest emission was used. The double-clad optical fiber was fabricated using a modified rod-in-tube technique. Figure 8 presents the cross-section of the developed optical fiber. The refractive indexes of outer cladding and inner cladding were 1.5, and 1.62, respectively. That gives numerical aperture (NA) values for core approx. 0.51 and for the cladding NA = 0.49. Optical fiber with outer cladding diameter = 250 µm and core diameter = 15 µm has a background loss c.a 7 dB/m measured at 1360 nm. ASE spectra of 1 and 5-meter lengths of optical fibers under “one end” cladding pumping at 808 nm and 976 nm laser are presented in Fig. 9 (for detailed analysis, a linear Y scale was used). In case of both pumping wavelengths, a broadband emission is observed as a result of the superposition of bands related to the Er3+:4I13/2 → 4I15/2, Tm3+:3F4 → 3H6, and Ho3+:5I7 → 5I8. transitions. It can be seen that the ASE profile strongly depends on the pump wavelength and length of the optical fiber.
Fig. 9. ASE spectra of double-clad germanate optical fiber co-doped with 0.7Er2O3/0.35Tm2O3/0.07Ho2O3, Effect of fiber length and pumping wavelength on ASE profile.
In the case of 808 nm pumping of 1 m fiber length, the −3 dB bandwidth was measured to be 381 nm and 696 nm for - 10 dB (Fig. 9(a)). For the 5 m, fiber length the ASE flattening (Fig. 9) resulted in enhancement of the ASE bandwidth (415 nm - 3 dB and 731 nm – 10 dB). This effect can be related to the partial reabsorption of the ASE signal along the fiber due to the Tm3+:3H6→3F4 and Ho3+:5I8→5I7. A different situation was observed for 976 nm pumping where for both optical fiber lengths the emission band at 1550 nm was dominant (Fig. 9(c),d). For 1 m and 5 m fiber lengths ASE parameters are as follows: 245 nm (1 m), 91 nm (5 m) – 3 dB bandwidth, 690 nm (1 m), 678 nm (5 m) – 10 dB bandwidth. Direct Er3+ pumping results in favorable conditions for 1550 nm ASE and finally obtaining less flattened emission, which strongly influences the −3 dB bandwidth. Comparing the above result with silica optical fibers co-doped with Tm3+/Ho3+ where the bandwidth achieved at the level of −10 dB was 645 nm (combining the forward and backward ASE) it is clearly showed the advantage of RE triply-doped optical fiber [15]. Due to the comparable low-phonon energy of the tellurite glass optical fiber, it is worth to mention about ASE from tellurium-based construction co-doped with Tm3+/Ho3+ (FWHM = 210 nm) [14]. It should be also stated that to the best of the Authors knowledge Er3+/Tm3+/Ho3+ triply doped, double-clad optical fibers haven’t been presented before.
4. Summary
In conclusion, as a result of RE content optimization in germanate glasses co-doped with 0.35Er2O3/0.7Tm2O3/0.15Ho2O3, (λexc = 808 nm) and 0.7Er2O3/0.35Tm2O3/0.07Ho2O3, (λexc= 976 nm) flattened luminescence profiles were presented. Double-clad optical fiber triply-doped 0.7Tm2O3/0.35Tm2O3 /0.07Ho2O3 (mol. %) germanate core, has been developed. The broadband ASE with −10dB bandwidth of 731 nm (λexc = 808 nm) was obtained under one end cladding pumping. The obtained ASE spectrum is the result of direct excitation of erbium and thulium ions, resonant Tm3+ → Ho3+, phonon-assisted Er3+ → Ho3+ energy transfer, and superposition of Er3+ (1550 nm), Tm3+ (1800 nm), and Ho3+ (2000nm) emission bands which are broader compared to silica and tellurite fibers. In consequence, the cladding-pumped Er3+/Tm3+/Ho3+-co-doped double-clad germanate core optical fiber is the attractive active element for construction broadband ASE optical fiber sources operating in the eye-safe spectral region. Besides, optimization of RE co-doped germanate glasses showed it very promising in terms of their use for the fabrication of novel dual-wavelength and tunable NIR lasers.
Funding
Narodowe Centrum Nauki (2019/35/B/ST7/02616).
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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