1. Introduction

In contrast to bulk optical devices, all-fiber active and passive components are featured with singlemode, low optical losses, easy alignment/coupling, good environment stability, compact size, flexibility, long interaction length, strong power confinement, low thermal load, and high laser beam quality. In order to achieve various optical functions like nonlinearity, photosensitivity, optical gain, special dispersion and so forth, different kinds of ions are usually doped into fiber core or different matrix glasses are employed as the host medium to attain the above targets. For core dopants, the lead [1,2] and germanium [3,4] can be used to significantly improve the nonlinearity, germanium [5] and boron [6] can respectively be used to introduce and enhance photosensitivity, cerium together with silver can be used to initiate the photo-thermal refractive properties [7], silver anisotropic nanoparticles can be used to generate strong birefringence, erbium or ytterbium can be used to give optical gain [8], sodium can be used to enlarge poling efficiency [9], neodymium can be used to increase the Brillouin spectral width [10], samarium can be used to provide saturable absorption [11], while the boron and fluorine can be respectively used to make fiber more dispersive and less dispersive [12]. On the other hand, the fluoride and chalcogenide matrix glasses can respectively expand the gain bandwidth due to its low phonon energy [13] and extend the transmission widow to mid-infrared region [14] while the phosphate is also renowned as a matrix glass with a very high doping solubility for compact waveguide amplifiers [15]. In contrast, the most popular fused silica is a well-known high phonon- and high bandgap-energy glass due to the stringent molecule networks it has. The standard silica fiber is therefore robust and with low loss over visible/near infrared region, but is dispersive and with low heavy ions solubility, low quantum efficiency of doped ions, short energy level lifetime for upconversion emission, and narrow gain bandwidth [16]. Consequently, the silica fiber is good for transmission but is disadvantageous for serving as compact functional devices like nonlinear or amplifying components, which usually comprise giant atoms or molecules inside the host glass. Though the nonlinearity in silica fiber can be enlarged and had been proved to generate the supercontinuum lights [17,18] through a tight power confinement using tapered silica fibers with diameters of down to the wavelength scale, the slope of group velocity dispersion is still large due to the intrinsically high dispersive characteristics of silica. Hence, a pump laser with a very high peak power is always necessary for supercontinuum generation in silica fiber but in general the supercontinuum spectrum is still spanning over less than two octaves. In addition, the core dopants are usually diffused during tapering and which could be a drawback to make waveguiding ineffective for functional devices. Of course, the uses of optical fibers made of multiple component glasses could be a solution [19]. However, it is usually very expensive to design and draw a special fiber for the demands of only centimeter-long devices [20–22]. Moreover, a special splicing between optical fibers with different host glasses, core diameters, cleaved angles, and numerical apertures (NA) is always a big perplexity and is usually difficult to be accomplished by the popular fusion splicers. More recently, the nonlinear or amplifying photonic micro-/nanowires using multicomponent glasses like fluoride, phosphate, bismuthate, chalcogenide, and tellurite glasses are extensively studied. The optical characteristics in the sub-wavelength scale [23–26], enhancement of nonlinearity and gain efficiency, dispersion engineering, decrease of free carrier lifetime of semiconductor, and minimization the footprint of integrated devices [27] had been investigated. However, in the above methods, the power delivering based on evanescent coupling between the silica fiber and the nanowire could be inefficient, unstable, and mechanically weak [24]. Besides, a high evanescent coupling efficiency only stringently occurs when the two fibers are identical. It will come along with a very limited transmission bandwidth due to the wavelength dependent coupling effect [28]. A photonic bridging wire made of multicomponent glass for providing special optical functions and simultaneously connecting the standard silica fibers could be an important evolution for achieving in-line novel and compact fiber active, passive and functional devices.

In this work, we demonstrate the power delivering between two silica fiber tapers and the multicolor upconversion emissions of blue-violet, green, and red lights simultaneously based on a 21-mm-long non-silica photonic wire made of Tm³⁺/Er³⁺ codoped tellurite glass [29–32] under a cw 1064 nm Nd:YAG pump laser light. The blue-violet lights are known to be the important medical fluorescence excitation source and the essential light sources for blue-ray disc and display. In contrast to the bulk tellurite glass, the photons by multicolor upconversion emission processes can be efficiently generated based on this Tm³⁺/Er³⁺ codoped tellurite photonic wire due to the stronger optical field confinement and a longer interaction length. However, the index difference between the silica (n = 1.45) and tellurite glass (n > 2) is so high that the catastrophe optical losses deriving from the intra-cavity resonances can be encountered. This can be substantially suppressed by an angled-cleaved fiber tapers at the splicing points. The angled-cleaved fiber tapers are made by a twist-and-cut method and the achieved angle is 8.3°. Accordingly, the preliminary result of the insertion loss can be less than 8.85 dB. The longest tapered length (L_W) and the thinnest tapered diameter (D_W) of the tellurite bridging wire is respectively 21 mm and 5.3 μm. Besides the suppression of cavity resonances, the insertion loss can be further reduced by alleviating the excitations of higher order modes using silica fiber tapers with a proper tapered diameter (D_T) of around the wavelength scales and a cleaved angle at taper tip. This is because the electromagnetic fields passing through the wire can be more smoothly transferred between two silica fiber tapers. Moreover, the angled silica fiber tapers gives a stronger juncture while the photonic microwire bridging them. The 21-mm-long tellurite microwire can afford the maximal tensile strength when a counterpoise with the weight of 3.72 grams is used. Consequently, a stronger optical field confinement for achieving high nonlinearity or high optical gain is highly promising. Different kinds of optical host glasses like germanate or tellurite/floride/chalcogenide can be further used for inscribing fiber gratings [33], generating supercontinuum [4,20,21,34,35], and nanowire laser [36], respectively, based on this novel, simple, cost-effective, non-silica micro-wire-bridging technique.

2. Fabrication and experiments

A high nonlinear or high gain multicomponent glass usually has a quite large index when compared with the silica. A high index difference between the interfaces is known to introduce strong cavity resonances like in a Fabry-Perot resonator [37]. In order to deliver a broadband white light from one silica fiber taper to another, the catastrophe optical losses coming from the cavity resonances must be carefully handled. It is known that an 8° angled-cleaved fiber end is usually employed as the APC (Angled Physical Contact) connector to avoid the reflection from the fiber end. Thus, the tapered singlemode fiber (Corning: SMF-28) is angled-cleaved at the central tapered region by the twist-and-cut method to extensively suppress the resonances, as shown in Fig. 1 . There could be also a minor benefit that the angled-cleaved fiber tip provides a larger contact area to improve the binding strength between silica fiber and photonic wire. In addition to the cavity resonances, the excitations of higher order modes due to the abrupt change of core diameter and NA can also lead to huge optical losses. The use of tapered fiber is to expand the mode field outside the original core until its evanescent field is thoroughly exposed to occupy the shrunken cladding. The optical mode field can therefore be transferred to the photonic bridging wire smoothly and the higher order modes excitations are substantially suppressed, as conceptually depicted in Fig. 1(b). Moreover, the D_W of bridging wire can be controlled by choosing a proper D_T of silica fiber taper as a seed during tapering and a high index Tm³⁺/Er³⁺ codoped tellurite wire with a diameter of around the wavelength scale is helpful to enhance the optical field overlaps to convert the cw 1064 nm Nd:YAG pump laser light into blue-violet, green, and red fluorescence lights. Also, this micro-wire-bridging technique, compared with the fiber drawing technique using standard perform, is advantageous to draw the glass into fiber at a temperature of much lower than the melting point. The melting point of the tellurite glass is typically less than 570°C and is much lower than the melting temperature above 1500°C of silica fiber. This wire drawing for glasses can be working well at the temperature of softening point (~350°C for tellurite) which is somewhat higher than glass transition temperature (~300°C for tellurite [31]) but is much lower than the melting point. Hence, this micro-wire-bridging technique is an efficient low temperature fabrication method. In contrast to bulk glass, the tapered fibers also have a merit to provide stronger optical field confinement in micro/nanowire to enhance the nonlinearity of nonlinear devices and improve the quantum efficiency of lasers and amplifiers. For lasers and amplifiers, the gain bandwidth as well as the upconversion energy level lifetime can be both improved, if a low phonon energy glass like tellurite or fluorozirconate is employed as the host for active ions [16,38].

 

Fig. 1 The mode fields of a singlemode fiber in (a) are converted into higher order modes to cause huge optical losses but in (b) are smoothly transferred into the blue-violet, green, and red lights through Tm³⁺/Er³⁺ codoped tellurite wire using fiber tapers. The angled-cleaved bridging wire can avoid resonances in high index tellurite wire.

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To fabricate the Tm³⁺/Er³⁺ codoped tellurite photonic bridging microwire, a tapered SMF-28 silica fiber is prepared and is then twist-and-cut at around the middle of taper to divide the tapered fiber into two angled-cleaved tips. A small piece of Tm³⁺/Er³⁺ codoped tellurite glass (Tm³⁺ ion concentration: 0.5 mol.% and Er³⁺ ion concentration: 0.2 mol.%) from a light yellow bulk glass, as the 5-mm-thick sample shown in the inset picture between Figs. 2(a) -2(b), is heated using a butane gas flame at about 600°C. The high index tellurite glass is easy to cluster into a sphere at the tip of one silica fiber taper, as shown in Fig. 2(a). The fiber tip of the second tapered fiber is subsequently moved inward and blended into the melted tellurite sphere by a stepping motor, as shown in Fig. 2(b). The tellurite sphere is then heated using a scanning hydrogen flame at about 500°C by carefully controlling the distance between flame and tellurite glass sphere. The two merged silica fiber tips are moving outward slowly and the melted tellurite sphere is accordingly pulled bilaterally to become a bridging wire between the silica fiber tapers, as shown in Fig. 2(c). When pulling the two tapered fibers outward, the glass sphere is gradually deformed and then reshaped into a stick. The stick will eventually turn out into a wire when the silica tapers are continuously moving outward. The hydrogen flame is subsequently moving back and forth for many times to smooth and uniform the tellurite microwire. The nodes formed with the clustered tellurite glass at splicing points, as shown in Fig. 2(d) can thus be alleviated. Though the nodes are more or less remained at the junction in Fig. 2(d) to cause extra optical losses, they can be further eliminated by using a moving flame scanning over a wider range covering all the nodes. In this work, the longest L_W of the tellurite bridging microwire is about 21.3 mm which is believed to be long enough for nanowire laser [36] using this Tm³⁺/Er³⁺ codoped tellurite microwire or for supercontinuum generation using a highly nonlinear tellurite wire [20–22]. The side view of the angled-cleaved silica fiber tip and the tapered tellurite microwire is respectively shown in Figs. 3(a) -3(b). Under a 1000x CCD microscope, the measured D_T and D_W is respectively 25 μm and 5.3 μm and the wire boundary is shown very smooth to benefit a low insertion loss. The silica taper has a cleaved angle of 8.3°. The junction between silica and tellurite microwire is firm and this microwire is finally fixed in a U-groove on a glass substrate using a UV-curing epoxy.

 

Fig. 2 Fabrication procedures of a tellurite glass wire between two silica fiber tapers under a 50x CCD microscope. (a) The melting tellurite glass is initially clustered into a sphere at one end of fiber taper. (b) Another fiber taper is attached to the tellurite glass sphere. (c) The tellurite glass is heated and stretched. (d) The tellurite wire is keeping stretching to a smaller desired diameter (D_W) and the nodes formed with tellurite glasses at the splicing junctions can thus be blurred. The inset picture with a pink frame between (a) and (b) is the 5-mm-thick tellurite glass.

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Fig. 3 Side view of (a) angled-cleaved fiber taper with a cleaved angle of 8.3° and (b) tellurite bridging wire with the thinnest tapered diameter (D_W) of 5.3 μm under a 1000x CCD microscope.

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3. Results and discussions

3.1 Upconversion emission of bulk glass

In measurement, it is important to investigate the upconversion emission spectra of the Tm³⁺/Er³⁺ codoped tellurite bulk glass to serve as a reference before the glass is transformed into the microwire. Figures 4(a) -4(c) show the upconversion emission lights of the Tm³⁺/Er³⁺ codoped tellurite bulk glass. In Fig. 4(a), a 5.6 Watt cw diode laser light at 808 nm with the focused laser beam diameter of about 1 mm is incident onto the glass and the bright red light can be observed. The energy levels scheme for Tm³⁺ and Er³⁺ ions is depicted in Fig. 5 [29,30,39]. The red light photons centered at 650 nm wavelength are generated by two-photon upconversion due to the transition between ¹G₄→³F₄ manifolds [29,30,40] of Tm³⁺ ions. An 8 Watt cw diode laser light at 915 nm out from a pigtailed fiber with core diameter of 105 μm is also subsequently launched into this glass but only a weak red light can be observed by naked eye. In Figs. 4(b) and 4(c), an 11 Watt cw Nd:YAG laser light at 1064 nm is focused onto the glass through a 10x objective lens and the blue-violet lights can be clearly captured by the CCD camera. The violet lights are from the wing of the transition over ¹D₂→³F₄ of Tm³⁺ ions. There was also very weak red and green light accompanying the blue-violet lights, observed by naked eye, but is not capable of being captured by the CCD camera. The multistep upconversion for visible lights generation are as follows. First, the 1064 nm light pumped the electrons to go from the ground state ³H₆ to occupy the excited state ³H₅, which is called ground state absorption (GSA). Second, the electrons will then populate at ³F₄ by releasing nonradiative emission and can be further pumped to ³F₂ by the same 1064 nm laser light. This is the process of excited state absorption (ESA). Accordingly, an intense 1064 nm laser light can pump the electrons to respectively occupy the ¹D₂ and ¹G₄ manifolds through 4-photon upconversion and 3-photon upconversion. From Fig. 5, the upconversion violet (centered at 450 nm) and blue (centered at 480 nm) lights are respectively generated from the transition over ¹D₂→³F₄ (4-photon upconversion) and ¹G₄→³H₆ (3-photon upconversion) manifolds [29,30,40,41] under the intense 1064 nm pump light. The upconversion green light is derived from the transition over ⁴S_3/2→⁴I_15/2 of Er³⁺ ions. Therefore, a powerful 1064 nm Nd:YAG laser or 1060 nm Yb fiber laser is highly promising to generate multicolor upconversion emissions including blue-violet, green, and red lights by successive ESA processes in the tellurite photonic wire.

 

Fig. 4 (a) The bright upconversion red light from the bulk tellurite glass under a cw pump power of 5.6 Watt at 808 nm wavelength. The upconversion blue-violet lights from (b) the cross-sectional view and (c) the side view of a tellurite bulk glass when a cw pump power of 11 Watt Nd:YAG laser light at 1064 nm wavelength is focused by a 10x object lens into the tellurite bulk glass.

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Fig. 5 Energy levels diagram of Tm³⁺ ions.

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3.2 Transmission characteristics of perpendicular-cleaved and angled-cleaved Tm³⁺/Er³⁺ codoped microwire

In order to obtain stronger intensity of upconversion emission in the Tm³⁺/Er³⁺ codoped tellurite microwire, a long interaction length and a low loss power transmission between silica taper and tellurite microwire are crucial. To check the insertion losses of this tellurite microwire, a white-light source comprising multiple superluminescent diodes spanning 1250-1650 nm is launched into the silica fiber taper. The transmitted output power from the other end of fiber taper is recorded by an optical spectrum analyzer under the optical resolution (RES) of 1nm. The normalized transmission losses are shown in Figs. 6(a) and 6(b). In Fig. 6(a), the cavity resonances can be clearly observed when the two silica fiber tapers for bridging the tellurite wire are perpendicularly cleaved. The D_W, D_T, and L_W of the tellurite wire for sample 1 and 2 are (15.7 μm, 30 μm, 20.2 mm) and (13 μm, 30 μm, 21.3 mm), respectively. Since the L_W can be as long as 21.3 mm, there should be a lot of cavity resonances existed in the microwire. The measured spectra with much less resonances are because the two end-reflection planes in microwires are not strictly parallel due to the microwire is slightly bent during tapering processes. From Fig. 6(a), the extinction ratio of resonant loss peak can even go above 22.5 dB to cause huge optical losses. As mentioned above, the angled-cleaved fiber tapers are advantageous to avoid the cavity resonances in such a high index photonic bridging microwire and the corresponding spectra are shown in Fig. 6(b). D_W, D_T, and L_W of the tellurite microwire for sample 3, 4, and 5 are (34.2 μm, 70 μm, 17.2 mm), (25.4 μm, 50 μm, 19.5 mm) and (17.1 μm, 30 μm, 21 mm), respectively. The cleaved angles are among 8° to 10°. The lowest transmission loss is about 8.85 dB at 1557.8 nm. The losses go beyond 20 dB at the wavelengths shorter than 1330 nm and only some minor resonances take place at around 1.27 μm. The insertion loss and the minor resonances both get even worse for the shorter wavelengths since the scattering losses or the Fresnel reflection losses from the junction as well as from the tellurite nodes are both higher for the shorter wavelengths. The problems of the losses and the minor resonances at around 1.27 μm are believed to be alleviated when the clustered tellurite nodes at splicing points are successfully removed in future works. Though the lowest insertion loss is still as high as above 8.85 dB at this preliminary stage, the high index tellurite photonic bridging microwire with L_W of 21 mm can still successfully deliver the broadband optical power between silica fiber tapers and efficiently generate the multicolor upconversion emission lights as discussed later. The D_W is proportional to D_T since a smaller D_T can help generate a smaller D_W. The average losses decrease when the D_W gradually decreases. This is because a smaller D_T cannot only enlarge the guiding core to smoothly transfer the fundamental mode power into the tellurite wire but also avoid the big clustered tellurite nodes to scatter the guiding lights. Accordingly, the insertion loss could be significantly reduced by using silica tapers and bridging wire with diameters of around a few tens of micrometers. The remained clustered tellurite nodes at the junction should be further blurred or removed to significantly reduce the insertion loss as well as the minor resonances at short wavelengths by a scanning flame.

 

Fig. 6 Spectral responses of the tellurite wire with (a) D_W, D_T, and L_W of (15.7 μm, 30 μm, 20.2 mm) and (13 μm, 30 μm, 21.3 mm), respectively, when bridging two perpendicular cleaved and (b) D_W, D_T, and L_W of (34.2 μm, 70 μm, 17.2 mm), (25.4 μm, 50 μm, 19.5 mm) and (17.1 μm, 30 μm, 21 mm), respectively, when bridging the two angled-cleaved silica fiber tapers. (RES: 1nm).

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3.3 Upconversion emission spectra of Tm³⁺/Er³⁺ codoped microwire and the bulk glass

Since the photonic microwire can provide a much longer interaction length as well as a better optical field confinement than that in the bulk glass, the intensity of upconversion fluorescence is expected to be enhanced in the microwire. To compare the upconversion emission spectra of photonic microwire and bulk glass, a 975 nm laser light is respectively launched into the sample 5 in Fig. 6(b) with pump power of 103 mW and a 5-mm-thick bulk glass with pump power of 308 mW to excite the Tm³⁺ and Er³⁺ ions. The upconversion lights were measured using a spectrometer (Stellarnet: EPP2000-VIS-10) whose reflection probe with bundled fiber inside is respectively placed right after the output end of silica fiber and the bulk glass. For the Tm³⁺/Er³⁺ codoped tellurite microwire, the generated upconversion lights are collected and delivered to the spectrometer through a 1.5-cm-long lead-out silica fiber. For the bulk glasses, the emitted upconversion lights are measured by the spectrometer directly. In Fig. 7 , the 975 nm pump light makes the gain medium to give upconversion emissions over the red wavelength band. The fluorescences at the red and near-infrared wavelengths are the transitions of Tm³⁺ ions over ¹G₄ →³F₄ and ³H₄→³H₆ manifolds respectively under a 975 nm pump wavelength [29]. The intensity of upconversion from photonic microwire is stronger than that from bulk glass. From Fig. 8(a) , the blue light can be clearly observed from the side view of the entire 21-mm-long Tm³⁺/Er³⁺ codoped tellurite microwire by launching a cw 270 mW 1064 nm Nd:YAG pump laser light into the sample 5 in Fig. 6(b) and under a CCD camera. In contrast to Fig. 4(c), the desired pump power for blue light generation in microwire than in bulk glass is substantially reduced. However, the violet lights are easier to be seen at the tellurite nodes since the discontinuity of index and core diameter at the nodes can make the shorter wavelengths more likely be diffracted. Similar to the condition of bulk glass, there was also very weak red and green light accompanying the blue-violet lights, observed by naked eye, but is not capable of being captured by the CCD camera. It is worth mentioning that only 270 mW cw 1064 nm pump laser light is enough to simultaneously produce the blue-violet, green, and red upconversion lights, shown in Fig. 8(b), through a thin Tm³⁺/Er³⁺ codoped tellurite microwire. The upconversion emission from a 5-mm-thick bulk glass, which was pumped by a 16 Watt cw Nd:YAG laser light at 1064 nm through a 10x objective lens, is also shown in Fig. 8(b). From Fig. 5 and the references [29,30,40–42], the fluorescence wavelengths of the Tm³⁺/Er³⁺ codoped tellurite microwire are corresponding to the following transitions. (1) 378 nm (Tm³⁺: ¹D₂→³H₆), (2) 435 nm (Tm³⁺: ¹D₂→³F₄), (3) 477 nm (Tm³⁺: ¹G₄→³H₆), (4) 543 nm (Er³⁺: ⁴S_3/2 → ⁴I_15/2), (5) 650 nm (Tm³⁺: ¹G₄→ ³F₄), (6) 800 nm (Tm³⁺: ³H₄→ ³H₆). The fluorescence wavelengths do not match to the central wavelength of each transition in Fig. 5 since the stark splitting of energy sublevels, the doping concentration, and the host glass with different phonon energy can make the fluorescence wavelengths slightly different from each other. Clearly, the blue-violet, green, and red lights are all enhanced in the photonic microwire than in the bulk glass. This is because the photonic wire cannot only provide a longer interaction length but also a better optical field confinement. Thus, it would be an exciting result that the thin Tm³⁺-doped tellurite photonic microwire could efficiently decrease the pump power threshold of upconversion emissions for the practical applications. This tellurite photonic bridging microwire can be further replaced by other special multicomponent glasses in future works for various novel in-line nonlinear or amplifying components.

 

Fig. 7 The red upconversion emissions of the Tm³⁺/Er³⁺ codoped photonic microwire and bulk glass under a 975 nm pump laser light.

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Fig. 8 (a) The blue-violet upconversion fluorescence along the Tm³⁺/Er³⁺ codoped tellurite bridging microwire indicating the transition over ¹D₂→³F₄ (4-photon upconversion) and ¹G₄→³H₆ (3-photon upconversion) manifolds when a cw 270 mW 1064 nm Nd:YAG laser light is launched into the 21-mm-long tellurite wire. (b) The measured spectra of upconversion emissions for the photonic microwire and bulk glass.

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3.4 Test of tensile strength for Tm³⁺/Er³⁺ codoped microwire

The tensile strength of the photonic bridging microwire is also an important issue in practical applications. The test is progressed using a counterpoise hanging over one end of the lead-out silica taper. The weight of counterpoise is gradually added and the corresponding spectral responses are measured and shown in Fig. 9 . The microwire under test is sample 6 with the D_W and L_W of 17 µm and 16 mm, respectively. The original transmission loss of microwire before loading the counterpoise is shown as the orange solid line in Fig. 9. The transmission loss is initially uneven over 1250-1650 nm wavelength range and the losses at each peak and valley gradually grow when the loaded counterpoise becomes heavier. The photonic wire can be endurable until a counterpoise with the weight of 3.72 g is applied.

 

Fig. 9 Tensile strength test of the photonic wire with the D_W and L_W of 17 µm and 16 mm, respectively. The photonic wire can be endurable until a counterpoise with the weight of 3.72 g is applied.

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4. Conclusion

In conclusion, we have demonstrated a Tm³⁺/Er³⁺ codoped tellurite photonic bridging microwire that can connect two angled-cleaved silica fiber tapers for broadband (1250-1650 nm) power delivery and simultaneously generate multicolor upconversion emissions including the blue-violet, green, and red lights using a 270 mW 1064 nm Nd:YAG pump laser light. The intensity of upconversion lights is significantly improved in microwire than in bulk glass due to a longer interaction length and a better optical field confinement. The maximal length of photonic bridging microwire can be as long as 21.3 mm (sample 2). The uses of angled-cleaved silica fiber tapers are shown to be helpful in avoiding cavity resonances and delivering the broadband white light toward the other fiber. The maximal tensile strength for the bridging microwire can be endurable under the weight of up to 3.72 grams. These photonic bridging microwires made of non-silica glass materials have silica lead-in and lead-out fibers to be compatible to standard fibers. They are highly promising to be employed for novel fiber components like blue-violet lasers for biophotonic applications.