1. Introduction

Tellurite glass fiber is of interest for use in the near-and mid-infrared [1–3]. Its high nonlinearity and excellent transmission properties in the infrared make it an ideal candidate for both linear and nonlinear optical devices. These can be devices for sensing [1], nonlinear optical signal processing [2], Raman amplification [3,4] or continuum generation [6]. Tellurite glasses, which also are excellent hosts for rare earth (RE) ions due to their large solubility of such ions and low phonon energy, have led to many demonstrations of amplification and lasing from different RE systems [7–11]. Recently, Fabry-Perot cavities were demonstrated in tellurite fiber by splicing the soft glass fiber onto standard silica fiber with low loss, thus demonstrating the possibility of integration into existing systems [12]. Another topic currently of great interest in fiber optics is that of microstructured fibers and in particular multicore fibers (MCFs). MCFs can offer new device designs such as arrayed fiber lasers and amplifiers [13] which can scale up laser output powers and bend sensors that can provide multi-axis measurements [14]. One of the main obstacles to the practicality of MCF has been the coupling of light into and out of such fiber. Previously, laborious etching processes using hydrofluoric acid were needed to fabricate MCF couplers that enable such fibers to be integrated to standard fiber geometries [14]. This problem has been addressed by recent work that used ultrashort-pulse laser waveguide inscription to fabricate a three dimensional fan-out device; this proof-of-concept work demonstrated that arbitrary core geometries can be addressed with standard fiber arrays [15]. With this obstacle to the use of MCF eliminated, more advanced MCF designs should be explored. Active fiber using multiple RE systems has the potential of providing emission or gain across many spectral regions with a common optical pump, by the correct engineering of the RE doping levels in each core it should be possible to fabricate efficient amplifiers in all three cores using pump coupling from a common large area pump core.

In this paper we demonstrate major progress towards such a device and report on the fabrication and characterization of a RE doped tellurite glass multiple core fiber (MCF) using a soft glass extrusion technique. Soft glass extrusion has previously been demonstrated and has even been used to fabricate a tellurite photonic crystal fiber [4], we recently reported on the fabrication of an un-doped tellurite multiple core fiber that was made via shaped extrusion [5]. We have now for the first time (to the best of our knowledge) fabricated an actively doped multiple core fibre having different RE dopants in each core. Emission from Ho³⁺- Tm³⁺-Yb³⁺, Er³⁺-Ce³⁺, and Tm³⁺-Yb³⁺ codoped cores has been characterized from the visible to the mid-infrared.

2. Fabrication

The fabrication of a three core fibre via shaped extrusion has been described by the authors in a previous work and a similar method was used for the active MCF fabrication [5]. In order to produce the three core fiber preform, a cylindrical bulk tellurite glass (TZN) was prepared and placed within the chamber of a homemade extruder system. A die was designed so that an arrangement of 3 stainless steel pins of 1.5 mm diameter could sit equilaterally spaced around the central axis of the die. A 250 mm rod was extruded. The resultant shaped preform had three equally spaced holes of diameter 1.2 mm. The core glass rods of diameter 1 ± 0.1 mm were fabricated by stretching 10 mm diameter rods of RE-doped TZN in the drawing tower. An additional tellurite glass jacketing tube of 6.7mm inner diameter and 10 mm outer diameter was made by the rotational casting technique. The core glass rods were placed into the holes of the inner cladding extruded rod which was put into the jacketing tube. The preform was then drawn into fiber at a speed of 5.4 m·min-1 in a laboratory atmosphere. The glass compositions and dimensions of the drawn fiber are described in Table 1. The inner cladding index is designed to be higher than that of the jacket to enable cladding pumped configurations to be tested in future work.

Table 1. Fiber composition.

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A 3.2 cm section of this fiber was prepared to study the core transmission properties. An optical micrograph of the cleaved end-face of this section is shown in Fig. 1(a). The cleave quality is evident in the relatively few features seen, with only a slight scarring present between cores 2 and 3. For the rest of this paper we identify the three cores as follows: c1 (Tm Ho Yb doped TZN), c2 (Er Ce doped TZN) and c3 (Tm Yb doped TZN). An environmental-SEM image of the cleaved fiber is shown in Fig. 1(b). The fiber diameter was measured to be 140 µm. The core separations were found to be 39.0 µm, 40.0 µm and 40.5 µm for c₁-c₂, c₂-c₃ and c₃-c₁ respectively. The quality of the fiber interface was very high with only very small defects observed as shown in Fig. 1(c). These are likely due to contamination present on either the jacketing tube surface or the shaped preform surface prior to drawing. These defects were measured to be less than 300 nm in diameter and will contribute to the overall transmission loss in fiber via scattering.

 

Fig. 1. Optical microscope image of three-core fiber (a) with different core doping giving different colour cores in transmission, also shown are SEM images of the fiber (b) and of the interface of core 1 (c).

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3. Transmission Properties

Light from a 1550 nm band ASE source was coupled into each core by butt-coupling a standard singlemode silica fiber up to the 3.2 cm test fiber. It was relatively straightforward to couple light into any of the individual cores by translating the silica fiber mounted on a micro-positioner. The output from the three-core fiber was imaged onto an infrared camera. Near single-mode output was observed for each of the three cores in turn at 1550 nm.

Due to the high index of this glass, air-guided outer cladding light is easily observed over substantial distances. Figure 2(a) shows such cladding light, which also gives us an image of the geometry of the fiber and allows us to calibrate mode profiles. The raised inner cladding index is demonstrated by the strong coupling to this area as shown in Fig. 2(b). The inner cladding glass composition has been described in previous work [5] in which the refractive index of bulk glasses of this composition was measured using a commercial prism coupler to be 2.006 at 1300 nm. At 1550 nm none of the cores were singlemode although the low order of other modes observed suggests that they are close to singlemode. All three fundamental modes were of near-circular gaussian profile with negligible interaction expected for the given core separation, and their images are shown in Fig. 2(c–e), with fundamental 1/e² mode field diameters (MFD) measured as 17 µm, 13 µm and 26 µm for core 1, 2 and 3 respectively. Using the core diameters given in Table 1 and the refractive index of the inner cladding, we estimate the core indices to be 2.0072, 2.0096 and 2.0066 for c1, c2 and c3 respectively. This has been estimated by modeling the fundamental MFD using a commercial finite element package (Comsol Multiphysics) and comparing to that measured for each core.

 

Fig. 2. Imaged output at 1550 nm from 3.2 cm of the active three core fiber for light coupled into the outer cladding (a), inner cladding (b) and cores 1 - (c) - (d).

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The transmission losses of this proof of principle fiber were not directly measured but are estimated are estimated to be over 6 dBm^-1 at wavelengths outside any absorption band in the near infrared. This estimation is based upon previous work on an undoped multicore fibre that used the same extruder design and fiber geometry [5].

4. Infrared emission via down conversion

The three RE systems as detailed in Table 1 were chosen as they all exhibit emission in different spectral regions when optically pumped at 980 nm. The energy level diagrams for the three RE systems are shown in Fig. 3(a)–(c) for c1, c2 and c3 respectively. The Er³⁺-Ce³⁺ co-doped system in tellurite glass fiber was reported in one of our previous works [7]. By controlling the pump excited state absorption (ESA) at 980 nm, the presence of Ce³⁺-ions enhances gain at 1550 nm by nearly 50% in comparison with Er³⁺-doped tellurite fiber. This enhancement in pump inversion at 980 nm was possible via a fast cross-relaxation process between the Er³⁺:⁴I_11/2→⁴I_13/2 and Ce³⁺:²F_5/2→²F_7/2 levels, and yielded an internal gain of 2.5 dB cm-1. In a paper by Richards et al. [9] separate tellurite fibers were fabricated with cores doped with Tm³⁺, Tm³⁺:Yb³⁺ and Tm³⁺:Ho³⁺. It was shown that using 800 nm pumping the Tm³⁺ and Tm³⁺:Ho³⁺ co-doped cores had strong 1.8 µm and 2.0 µm emissions, respectively. By comparison, under 980 nm and 1080 nm pumping schemes strong 1.9 µm emission and lasing were observed, respectively in a Tm-Yb co-doped fiber. We have chosen the Yb³⁺-co-doping of Tm³⁺ for aiding efficient energy transfer between Tm³⁺:³F₄→Ho³+:⁵I₇→⁵I₈ for 2.05 µm emission. Such a pumping scheme will allow us to use a multimode high-power 980 nm laser for pumping all three cores simultaneously.

 

Fig. 3. Energy level diagrams and pumping schemes for the three cores 1 - 3 (a)-(c) [Refs 7, 9].

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4.1 Near infrared emission properties

The near infrared emission properties were studied using a fiber coupled Ocean Optics NIR512 spectrometer. A fiber-coupled diode laser emitting 980 nm radiation was used as a pump source. The single mode pump fiber, (Corning SMF28) was butt-coupled to the MCF using micro-positioners that enabled light to be coupled into each core in turn. In this way 200 mW of pump power was incident on the MCF fiber input. The spectra obtained are shown in Fig. 4. It can be seen that c2 and c3 exhibit the expected emission peaks at 1550 nm and 1460 nm associated with the Er³⁺: ⁴I_13/2 - ⁴I_15/2 and Tm³⁺: ³H₄ - ³F₄ transitions respectively. Core c1 shows peaks at the same wavelength as c2. As there is not expected to be an efficient 1550 nm emission from the Ho³⁺ system this peak is attributed to cross-talk from c2. The similarity of the shape of the emission peaks in c1 and c2 indicates that this is the most likely explanation. There may be two reasons for the cross-talk between the cores. The pump launch condition is the first reason, allowing certain modes to leak out which then excite the rare-earth ions in other cores and as signal collection was performed using a 600 µm fiber output from all the fiber is recorded. The second reason may be the geometrical proximity of cores themselves, which are predetermined by the die-geometry, however, with a 40 µm core separation, evanescent coupling between the cores is expected to be negligible over 3.2 cm. A customized fan-out device is currently in preparation to minimise pump scatter into more than one core and to better address individual cores for detection. The 1550 nm emission from c2 is the strongest in this spectral region and investigations are underway to ascertain if there is any internal gain at this wavelength for c2.

 

Fig. 4. Plots (a)–(c) show near infrared emission spectra for cores 1–3 respectively.

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4.2 Mid-infrared emission properties

To examine the mid-infrared emission from this fiber a Bentham M300 monochromator was used for wavelength scanning. The output face of the short, cleaved fiber was placed at the entrance slit of the monochromator and imaged onto a cooled 3 mm×3 mm Pb-Se detector array. The input face of the MCF was butt-coupled to the pump fiber which delivered 200 mW at a wavelength of 980 nm. The pump was modulated and used to trigger a lock-in amplifier. A silicon filter was used to block any pump light from the detector and the resulting spectra can be seen in Fig. 5.

 

Fig. 5. Plots (a)–(c) show mid-infrared emission spectra for cores 1–3 respectively.

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Figure 5 shows clear emission peaks at the expected wavelengths for the three RE co-doped fibers. Core 1 has a peak at 2050 nm corresponding to the Ho³⁺: ⁵I₇–⁵I₈ transition, core 2 has its main peak at 1545 nm corresponding to the Er³⁺:⁴I_13/2 - ⁴I_15/2 transition and the core 3 emission peak is centred on 1805 nm corresponding to the Tm³⁺: ³F₄ - ³H₆ transition. There is a strong evidence for cross-talk between cores c1 and c2, since neither Er³⁺ nor Ce³⁺ has emissions above 1600 nm, which suggests that the ASE at 1805–2050 nm from c1 is detected whilst c2 is pumped. As the whole of the fiber output face is imaged by the spectrometer, any emission generated by scattered pump light will be observed as well. Mid-infrared-transmitting fiber collection will be investigated to reduce this signal cross-talk.

5. Visible emission properties

Tellurite glasses have much lower phonon energy (~780 cm^-1) than silicate (1100 cm^-1) and phosphate glasses (1200 cm^-1), as such, the probability of non-radiative transition is much smaller. The slow non-radiative transition prolongs the lifetime of metastable state and promotes pump ESA at 980 nm. For the Er³⁺ Ce³⁺ codoped c2, two-photon ESA allows population inversion at ³S₂ resulting in upconversion emission at 552 nm. On the other hand, the sensitizer Yb³⁺-ions in c1 and c3 favors population inversion at ¹G₄ level in Tm³⁺ via the three-photon ESA, resulting in ¹G₄→³H₆ (494 nm), ¹G₄→³F₄ (651 nm) and the strongest near infrared emission at 804 nm. In c1 the upconversion in Ho³⁺ ions is mediated via the cross-relaxation between Tm³⁺:³F₄→ ³H₆ and Ho³⁺:⁵I₈→ ⁵I₇ and ⁵I₇→ ⁵I₅ transitions and a 1020 nm photon from Yb³⁺:²F_5/2→²F_7/2, which populate the Ho³⁺:⁵F₃ level. The upconversion emissions from ⁵F₃→⁵I₈ (490 nm) and ⁵S_2,3-⁵I₈ (550 nm) then repopulate the Ho³⁺ ground state. The observed green upconversion transition is consistent with the observation made by Singh and co-workers [1].

 

Fig. 6. Visible upconversion luminescence when pumping Cores 1, 2 and 3 (a, b, and c) with a fiber coupled 980 nm laser diode. The high degree of visible upconversion fluorescence is evident in images of the fiber when pumping each of the cores (d).

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When the MCF was pumped with the 980 nm source each core exhibited characteristic upconversion emissions, the spectra are shown in Fig. 6(a)–(c) and were taken using an Ocean Optics HR2000 spectrometer. Figure 6(d) shows images of the fiber aligned to each of the different cores in turn with 200 mW of incident pump power at 980 nm. These images were taken using color glass filters to eliminate the 980 nm pump light. We have also identified the optical transitions for the dopants in each visible emission spectrum. The large amount of upconversion luminescence indicates the possibility of laser action at these wavelengths – a multicore RGB upconversion fiber laser would be an interesting prospect for display applications, however the efficiency of these transitions is currently under investigation and the core-cladding interface quality would have to be improved to decrease the amount of scattering at these shorter wavelengths.

6. Conclusions

To the best of our knowledge, we have demonstrated for the first time, emission from three different RE systems in a multicore fiber fabricated using an extrusion technique. The characteristic emission peaks of a Tm³⁺-Ho³⁺-Yb³⁺, Er³⁺-Ce³⁺ and Tm³⁺-Yb³⁺ doped tellurite glass fiber were observed in the visible, near infrared and mid-infrared spectral regions. The near- and mid-infrared spectra show evidence of a small degree of cross-talk between the three cores, which is believed to be due to pump light scattering into the other cores at high intensities. Future experiments will use an ultra-short pulse laser inscribed fan-out as detailed in [15] to ensure high isolation at both the input and output ends of the fiber; this will enable cutback emission spectra to be measured to ascertain the optimum doping concentrations in each of the cores. Improvements to the fiber quality will be made by reducing the diameter mismatch between the core and inner cladding to reduce transmission loss and scattering. Also, the fiber core sizes will be reduced to achieve singlemode guiding at near-infrared wavelengths. The results show promise for potential amplification and lasing at 1550 nm, 1800 nm and 2050 nm in the infrared and the high upconversion luminescence can be investigated for visible laser applications. The visible spectra indicate that with correct doping concentrations, emissions at red, green and blue wavelengths could be optimized.