1. Introduction

Broadband optical fiber sources operating in the near-infrared range (NIR) are a very attractive area of research due to their potential applications in many fields. Applications for gas sensors, LIDAR, amplified spontaneous emission (ASE) sources or diagnostics in medicine are some of the areas most worth mentioning [1–3]. Moreover, a lot of work is put into lasers operating in the “eye-safe” region at wavelengths longer than 1.4 µm [4]. These solutions are realized using rare earth (RE) ions such as thulium (Tm³⁺) or holmium (Ho³⁺) operating in the 1.8 µm and 2.0 µm regions [5,6]. An attractive research pathway is silica optical fibers fabricated by the modified chemical vapor deposition (MCVD) method doped with rare-earth ions. Moreover, low losses, silica glass fibers are characterized also by good mechanical properties. However, it should be also mentioned about a limitation in the form of high phonon energy (∼1100 cm⁻¹), which negatively affects the efficiency of energy transfer between the dopants (donor-acceptor) in the fiber core [7]. This problem can be solved with gallo-germanate optical fibres that represent a compromise between the low phonon energy of heavy metal oxide (HMO) and good mechanical properties of silica fibers [8–15]. Their excellent optical quality provides an attractive material for low loss (even c.a 0.25 dB/m) optical fibers (for ASE sources and lasers) fabricated by rod-in-tube method [12,16–18] [x]. Moreover, the low phonon energy (∼800 cm⁻¹) compared to fused silica optical fibers provides the opportunity to use multiple dopants in the fiber core while maintaining high energy transfer efficiency between them. In the case of upconversion fibers, this problem can be easily solved by the application of antimony-based glasses [19]. As mentioned before broadband NIR luminescence can be achieved by using co-doped systems containing 2 or 3 rare-earth dopants [3,20–22]. The most study is focused on Yb³⁺/Tm³⁺/Ho³⁺ and Er³⁺/Tm³⁺/Ho³⁺ systems, that operate in 1.5 µm (Er³⁺), 1.8 µm (Tm³⁺) and 2.0 µm (Ho³⁺) bands [23–26]. Due to the lack of cheap high-power laser sources that can efficiently direct pump of holmium, ytterbium, and thulium ions are used as a donor in the energy transfer process to obtain emission corresponding to the ⁵I₇→⁵I₈ transition [12]. The emission bandwidth of active optical fibers can be varied within a relatively wide range by changing the dopants and their concentrations in the core, but multi-core and multi-section designs gives wider possibility that allow superposition of the emission bands from individual cores of the fiber [27–29]. It is worth to note that obtaining flat emission needs an additional optimisation in terms of the structure of fiber and dopant concentrations in individual cores [30]. However, in multicore optical fibers, the amount of doped rare-earth ions can be significantly higher than in typical single-core fibers, which is proportional to the number of cores obviously. Thus such constructions enable the reduction of the length of the fiber which is necessary to absorb the pumping radiation and enables to the achievement of higher power output in comparison to common double-clad fibers [31].

In this work, we present novel double-core optical fiber with broadband emission in the near-infrared spectral range. Based on our previous work with gallo-germanate glasses we have chosen optimal dopant concentrations to obtain flat emission under 940 nm and 976 nm pump excitation. Obtained broadband ASE emission is an effect of the superposition of emission bands from both cores doped with Er³⁺ and Yb³⁺/Tm³⁺/Ho³⁺ ions, respectively. Presented work shows new approach in profiling ultra-broadband emission by proposing the construction of multicore optical fiber.

2. Experimental

Set of gallo-germanate core glasses (GGB) with a composition (mol%): 48.85GeO₂-25Ga₂O₃-10BaO-15Na₂O-0.5Yb₂O₃-0.4Tm₂O₃-0.05Ho₂O₃ and 49.8GeO₂-25Ga₂O₃-10BaO-15Na₂O-0.2Er₂O₃ were prepared by standard melting and quenching method. Glass composition and dopants concentrations have been optimized based on our previous work [23]. Reagents (purity 99.99%) were melted in a platinum crucible in an electric furnace at a temperature of T = 1500°C for 30 minutes. The molten glass was poured into a stainless steel form and annealed at 610°C for 12 h. Absorbance spectra of both core glasses were measured using the Stellarnet Green-Wave spectrometer in a range of 300 nm to 900 nm. Luminescence properties of core glass samples were measured under 940 nm and 976 nm pump excitation (LIMO laser diode, P_opt(max)= 1-30W) with Acton 2300i monochromator. 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 oscilloscope. Two-core, double-clad optical fiber was fabricated using the modified rod-in-tube technique. Fiber preform consists of two germanate glass rods located in the mechanically drilled internal cladding. This structure was placed in a glass tube (outer cladding). Inner cladding and outer cladding refractive indexes were as follows: 1.62 and 1.51, respectively. The optical fiber was drawn from a 7 cm long preform in the temperature range of 890-930°C and coated with low refractive index coating. Spectroscopic properties of the optical fiber have been measured in the range of 1400–2300 nm by using Yokogawa AQ6375B optical spectrum analyzer and high-power LIMO laser diodes with optical fiber output, P_{opt. max}= 30W).

3. Results and discussion

3.1 Spectroscopic properties of the germanate core glasses

Absorbance spectra of 2.8 mm thickness glasses have been measured in the range from 300 nm to 1000 nm (see Fig. 1). In the case of Er³⁺ - doped GGB glass the following absorption bands are located at wavelengths of around 380 nm, 500 nm, 530 nm, 550 nm, 660 nm, and 970 nm. In case of other dopants we observed transitions from the ground state ³H₆ to the ¹G₄,³F₂ (³F₃), ³H₄, energy levels attributed to Tm³⁺ ions and ¹D₂, ⁵S₂ (⁵F₄), ⁵S₂ (⁵F₄), ⁵I₅ absorption bands corresponding to the Ho³⁺ ions. Judd–Ofelt spectroscopic parameters of the Er³⁺, Tm³⁺, and Ho³⁺ doped gallo-germanate glasses are known as beneficial for near-infrared emission and presented in the literature [14,32,33].

 

Fig. 1. Absorbance spectra of the gallo-germanate glasses (further fiber cores) co-doped with Er³⁺ ions and Yb³⁺/Tm³⁺/Ho³⁺ ions.

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Figure 2 presents luminescence properties of both glass samples under 976 nm pump excitiation. In singly Er³⁺ doped glass we can observe standard emission at 1550 nm corresponding to the ⁴I_13/2→⁴I_15/2 transition with full width at half-maximum equal to 35 nm. In case of glass co-doped with Yb³⁺/Tm³⁺/Ho³⁺ ions, we measured broadband luminescence of Tm³⁺ and Ho³⁺ ions with FWHM value reaching 343 nm. Obtained intensity ratio of I₂₀₀₀/I₁₈₀₀ is equal to 1.25. Figure 3 presents energy level diagrams for both fabricated glass samples. The obtained band for triply-doped glass is the result of energy transfer from the Yb³⁺:²F_5/2 level to the Tm³⁺:³H₅ and Ho³⁺:⁵I₆ levels. In addition, further transitions between energy levels result in energy transfer from the Tm³⁺:³F₄ level to Ho³⁺:⁵I₇ (Fig. 3) giving important contribution to near-infrared emission of Ho³⁺ ions at 2 µm. Previous studies for barium gallo-germanate glass indicated that near-infrared emission due to transition ⁵I₇→⁵I₈ (Ho³⁺) near 2 µm sensitized by Tm³⁺ ions through the energy transfer process is much stronger than that of the direct excitation of holmium. Ultimately, as a result of radiative transitions ³F₄→³H₆ of Tm³⁺ ions and ⁵I₇→⁵I₈ of Ho³⁺ ions, ultra-broadband emission covering from the 1.6 µm to nearly 2.2 µm is possible to detect in core gallo-germanate glasses co-doped with Er³⁺ ions and Yb³⁺/Tm³⁺/Ho³⁺ ions under 976 nm excitation (see Fig. 2).

 

Fig. 2. Luminescence spectra of the core gallo-germanate glasses co-doped with Er³⁺ ions and Yb³⁺/Tm³⁺/Ho³⁺ ions under 976 nm excitation.

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Fig. 3. Simplified energy level diagram for Yb³⁺/Tm³⁺/Ho³⁺ and Er³⁺ co-doped glass with the energy transfer mechanisms.

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Figure 4 presents luminescence decay times for glass samples doped with Er³⁺, Yb^3+, and Yb³⁺/Tm³⁺/Ho³⁺ under 976 nm pump excitation. In the case of the GGB glass sample doped with erbium ions, obtained decay time monitored at λ_em= 1530 nm equals to 7.991 ms.

 

Fig. 4. Luminescence decay time curves for GGB glasses doped with Er³⁺, Yb³⁺, and Yb³⁺/Tm³⁺/Ho³⁺ ions.

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The value of lifetime is higher than the presented results for fluorotellurite or lead silicate glasses [34,35]. Single doped glass with Yb³⁺ ion was characterized by 954 µs decay time. Introduction of Tm³⁺ and Ho³⁺ ions into glass matrix reduced ²F_5/2 level lifetime to 274 µs at the wavelength of 1020 nm. It is caused by energy transfer from ytterbium ions to thulium and holmium ions. Obtained lifetimes corresponding to the thulium and holmium emission levels were 1.992 ms and 0.332 ms, respectively. We obtained higher lifetimes of ³F₄:Tm³⁺ and ⁵I₇:Ho³⁺ excited state compared to silicate-germanium and tellurite glasses reported before [20,36]. Based on the experimental results we calculated energy transfer efficiency from Yb³⁺ ions to Tm³⁺ and Ho³⁺ based on the equation presented below:

3.2 Double-core optical fiber

Developed double-core optical fiber was fabricated using a modified rod-in-tube method. Figure 5 presents the designed refractive index profile, scheme, and cross-section photo of the drawn fiber. The refractive indexes of internal cladding glass and core glass were 1.62 and 1.74, respectively. This gives a numerical aperture (NA_core) value of 0.63. The refractive index of external cladding was 1.51 (Na_cladding= 0.59). Dimensions of the fabricated optical fiber were as follows: 275 µm of cladding diameter and 15 µm for both cores. Background losses measured by the cutback method (@1310 nm) were calculated to be 7 dB/m. ASE spectra were measured under “one end” pumping with the use of 940 nm or 976 nm pumps. For both used pumps we observed broad near-infrared ASE spectra being a superposition of Er³⁺ and Tm³⁺/Ho³⁺ emission bands.

 

Fig. 5. Cross section of fabricated double-core glass fiber.

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Luminescence spectra of double-core fiber with core glasses doped with Er³⁺ and Yb³⁺/Tm³⁺/Ho³⁺ are presented in Fig. 6. Luminescence spectrum is superposition of emission bands from both cores, first doped with Er³⁺ and second doped with Yb³⁺/Tm³⁺/Ho³⁺. In the case of using a 940 nm pump for a 1 m optical fiber, a broad ASE with a bandwidth of 326 nm for −3 dB and 634 nm for −10 dB were observed. Increasing the fiber length to 5 m resulted in widening of the −3 dB bandwidth to 355 nm, while the −10 dB bandwidth narrows to a value of 539 nm. Observed emission bands are corresponding to the transitions ³F₄→³H₆ of Tm³⁺ ions and ⁵I₇→⁵I₈ of Ho³⁺. It is caused due to reabsorption of the emitted light by Tm³⁺ ions and ground state reabsorption of Ho³⁺ ions [37,38]. It worth to mention that cross relaxation of Tm³⁺ ions was not observed. It is in a good agreement with the previous results suggesting that cross-relaxation process is evidently weaker than the energy migration rate from the ³H₄ state of Tm³⁺ ions in barium gallo-germanate glass [18]. Additionally, in both fiber lengths, we can observe emission at 1550 nm which is contributed to the ⁴I_13/2 →⁴I_15/2 transition of Er³⁺ ions. In the case of using a 976 nm pump for both measured lengths (1 m and 5 m), we obtained significantly narrowed emission bands. In the case of a 1 m length of the fiber, it was 28 nm bandwidth for −3 dB and 155 nm for −10 dB. Additionally, in comparison to the 940 nm pump the registered emission of Er^3 +(1550 nm) ions was significantly higher than Tm³⁺ (1800nm) and Ho³⁺ (2000nm) ions. This is the result of direct Er³⁺ pumping which negatively affects the flattening of the emission for −3 dB and −10 dB bands. Compared to our previous work, which concerned Er³⁺/Tm³⁺/Ho³⁺ triply doped single-core optical fiber, obtained slightly narrower emission bands for all investigated fiber lengths and pump wavelengths [25]. However, developed fibers enables their efficient pumping of Tm³⁺ and Ho³⁺ ions because of Yb³⁺ sensibilization. Introduction of the second core doped with Er³⁺ ions allows to expand luminescence spectrum due to additional emission peak at 1550 nm, compared to previously reported single core Tm³⁺/Ho³⁺ optical fibers [39–41]. It is worth to be mentioned that emission spectra can be profiled by the number of active cores in the fabricated fiber. Moreover, the clear advantage of the presented multicore construction is that there is lack of unwanted energy transfer between Er³⁺ and Ho³⁺ ions. For a detailed analysis of the flattening, Fig. 7 presents luminescence intensities in linear scale for all presented pumps and optical fiber lengths.

 

Fig. 6. ASE spectra of fabricated double-core fiber under 940 nm and 976 nm pump excitation for different fiber lengths (1 m and 5 m).

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Fig. 7. Luminescence intensities I_1550nm, I_1800nm, I_2000nm for different fiber lengths and pumps configurations.

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In the case of 940 nm pump, a dominating emission band corresponded to the Tm³⁺ ions (1800nm), whereas for the 976 nm excitation to the Er³⁺ ions (1550 nm). Increasing the optical fiber length to 5 meters for both used pumps has resulted in similar intensities.

4. Conclusions

This paper investigates the luminescence properties of gallo-germanate, double-core optical fiber. As a result of doping cores with Er³⁺ and Yb³⁺/Tm³⁺/Ho³⁺ ions, we obtained a broadband spectrum under 940 nm pump excitation due to the superposition of emission bands from both cores. We obtained the widest emission bandwidth (355 nm, −3 dB band) for the 5 m length optical fiber excited with 940 nm radiation. In the case of 10 dB bandwidth, the widest value (634 nm) was obtained for a 1 m length with the same excitation wavelength. It was observed that high-intensity peak at 1550 nm which corresponds to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions under 976 nm pumping, has the negative impact on the emission bandwidths at 3 dB and 10 dB. The results presented in this study indicate that the developed gallo-germanate dual-core optical fiber presents new opportunities in construction broadband ASE sources operating within the 1.5-2.1 µm spectral range.