Open Access
Add to BibSonomy
Abstract
We demonstrate the first single-mode optical fiber couplers made with ZBLAN optical fiber. Couplers are fabricated using a controlled tapering procedure enabling high reproducibility while limiting glass crystallization. A coupling ratio of up to 41%/59% in cross/through ports with an excess loss of 2.5 dB is obtained at a wavelength of 2.73 μm. In addition, the stability of a coupler with traces of surface crystallization is tested at ambient atmosphere over a period of more than 90 days.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical fiber couplers (OFCs) are components broadly used in optical fiber devices and systems for power dividing/combining, wavelength multiplexing/de-multiplexing, and polarization beam splitting/combining. The vast majority of OFCs are made of silica glass, just like the optical fibers from which they are made of. However, the use of silica-based OFCs in the mid-infrared (2-20 $\mu$m) is limited due to their large intrinsic losses [1]. The mid-infrared is of particular interest to reveal fundamental vibrational resonances of molecules, resulting in a wide range of applications in medicine [2,3], material science [4], biotechnology [5], and environmental analysis [6,7].
In contrast to silica, soft-glass fibers transmit mid-infrared wavelengths, up to $5\,\mu$m for ZBLAN and up to $12\,\mu$m for $As_2Se_3$ fibers. This has provided the opportunity to develop multi-mode OFCs compatible with the mid-infrared using chalcogenide [8–12] and fluoride fibers [13]. In 2018, Rezaei et al. proposed the first single-mode chalcogenide OFCs including broadband OFCs, wavelength division multiplexers, and polarization beamsplitters [14,15]. Since then, chalcogenide OFCs have been demonstrated also in the form of nonlinear OFCs [16], and single-mode OFCs have served in the fabrication of all soft-glass fiber laser cavities [17,18].
Fluoride glass OFCs, on the other hand, have not been developed primarily because the glass tends to crystallize when heated near softening temperature [19–21]. Although there are significant challenges and the concurrent development of chalcogenide-based OFCs, creating fluoride glass OFCs is still desirable because they could provide a complementary technology due to their greater damage intensity threshold and lower optical nonlinearity when compared to chalcogenide glass [22,23]. The viscosity of fluoride glass has already been characterized as a function of temperature showing that viscosity doubles every $1.3\,^\circ$C at the glass processing temperature [24,25]. In 2016, G. Stevens et al. were first to report a fluoride glass OFC [13]. The multi-mode ZBLAN OFC had a coupling ratio of up to $50{\% }/50{\% }$ at a wavelength of $2\,\mu$m. Recently a fused fiber combiner based on multi-mode step-index fluoroindate optical fibers (InF3) has been proposed [26]. The presence of multiple modes in OFCs restricts their functionality in single-mode based optical fiber systems and devices. To fabricate a single-mode OFC, it is necessary to use single-mode fibers and ensure the presence of suitable adiabatic transition profiles on both sides of the OFC coupling section. The single-mode nature of an OFC can be monitored during the fabrication process, from the observation of a smooth and monotonous power transmission as a function of OFC extension. After fabrication, single-modedness is also validated from the observation of a smooth and monotonous transmission spectrum that is free from cyclic modal interference fluctuations [27].
Here, we demonstrate the successful fabrication of single-mode ZBLAN OFCs. The OFCs are fabricated using a multiple-sweep tapering technique that enables precise and repeatable control of the geometry. This results in single-mode OFCs with repeatable optical properties, while allowing to limit glass crystallization. The transmission spectra of the ZBLAN OFCs are single-mode over a wavelength range of $1.5-3.0\,\mu$m. The OFC coupling ratios at cross/through ports can be designed within the range of $0{\% }/100{\% }$ to $41{\% }/59{\% }$ with a maximum excess loss of 2.5 dB at a wavelength of $2.73\,\mu$m. Transmission at cross/through ports are also provided as a function of time for up to 90 days. Despite traces of crystallization triggered at their surface during fabrication, ZBLAN OFCs preserved at ambient atmosphere are chemically stable over time. This research is an important step toward the extension of optical fiber technologies in the mid-infrared.
2. Adiabaticity criteria
OFCs are formed by placing two fibers side by side and fusing them together through heating and stretching. To obtain single-mode OFCs and avoid propagation losses, the taper geometry that links the optical fiber mode to a coupler mode must prevent the propagating fundamental mode from coupling to higher-order modes, via an adiabatic taper profile. This adiabaticity is satisfied from a criteria [28]
where $dr/dz$ is the slope of transition regions on both sides of the OFC, $r$ is the cladding diameter, $\beta _1$ is the propagation constant of the fundamental mode, $LP_{01}$, and $\beta _2$ is the propagation constant of the mode to which the power is most likely to be coupled, $LP_{02}$. This criteria provides a limit on the sharpest allowed slope of the taper profile. Figure 1 shows the schematic of a typical OFC with identified waist region over which coupling happens, and transition regions where the mode is adiabatically converted in between optical fibers and waist region. Figure 2 plots the adiabaticity upper limit for a ZBLAN fiber with core/cladding diameters of $6.5/125\,\mu$m and a numerical aperture of 0.23 used in this experiment, and the slope of a designed transition profile for the OFC fabrication. Taking the right side of Eq. (1) as a theoretical upper limit, this experimental transition slope is designed to maintain a comfortable security margin while not making the OFC arbitrarily too long.
Fig. 2. Adiabaticity criteria and the slope of designed transition profile as a function of fiber cladding diameter.
3. Experiment and results
The OFC fabrication begins with cleaving and polishing two identical pieces of ZBLAN fiber (Le Verre Fluoré) with core/cladding diameters of $6.5/125\,\mu$m and a numerical aperture of 0.23. They are then coupled to SMF-28 using UV-cured epoxy. The two fibers are subsequently placed together on a tapering setup with a half-turn twist, which ensures that the fibers maintain close contact during the tapering process. A diagram of the tapering setup is shown in Fig. 3. This home-made setup has been developed throughout years of development, initially aimed for chalcogenide tapers and OFCs, and then adapted to fluoride glass processing [11,15,29,30]. More information can be found on these references. The fibers are then heated near their softening temperature ($290\,^\circ$C) using an electric resistance heater and stretched following an adiabatic tapering profile with a stretching precision of $0.1\,\mu$m. The temperature is monitored using a temperature sensor connected to the heating element. The choice of temperature for the tapering process is found experimentally, being the lowest possible at the limit of fiber breaking due to high viscosity, since processing at higher temperature sharply increases the presence of crystallization and associated propagation losses. The range of temperature over which tapering can be performed is of the order of $3-5\,^\circ$C. While tapering, the waist of the OFC is brushed ${\sim}100$ times by the heater, which moves back and forth between the two pulling stages at a velocity of $254\,\mu$m/s, resulting in a total tapering time of ${\sim}200$ min. Using this process, a smooth transition region adiabatically connects the fiber mode to a tapered fiber mode, as well as a waist region of constant diameter. To minimize crystallization, the tapering setup is confined inside a box and constantly purged with argon gas, as the presence of moisture in ambient air is known to act as a crystallization catalyst.
Fig. 3. Schematic of the tapering setup. PS: pulling stage; HE: heating element; TS-HE: translation stage of the heating element; PM: power meter.
Compared to single-sweep technique where the fiber is tapered in only one heating sweep and requires high temperature for low viscosity, the multiple-sweep technique tapers the fiber in more than a hundred sweeps with a small amount of stretching at each step [31]. This enables the use of a lower heating temperature and higher glass viscosity. Monitoring the fiber surface crystallization reveals that although the optimized multiple-sweep technique heats up the fiber over a longer time period, it causes less crystallization because of a lower tapering temperature. It appears that crystallization is triggered more easily with relatively brief exposition at high temperature than long exposition at lower temperatures. In addition, the slow and multiple-step process of the optimized mulitple-sweep technique enables better control and repeatability of the transition profile. Figure 4 shows the surface of ZBLAN fibers before and after tapering. Figure 4(a) is the untapered fiber with an initial diameter of $125\,\mu$m. In contrast, Figs. 4(b)-c show the surface of a fiber tapered down to a diameter of $40\,\mu$m with the single-sweep technique (Fig. 4(b)), and the multiple-sweep tapering technique (Fig. 4(c)). The surface crystallization created during tapering has been significantly suppressed thanks to the multi-sweep tapering recipe.
Fig. 4. Microscope images of (a) a ZBLAN fiber before tapering with a diameter of $125\,\mu$m, (b) a fiber tapered down to a diameter of $40\,\mu$m with the single-sweep technique, and (c) a fiber with a diameter of $40\,\mu$m tapered using the multiple-sweep technique.
To monitor the OFC transmission during tapering, continuous wave laser light at a wavelength of $1.55\,\mu$m, is sent into one arm of the OFC while the power output from though-port and cross-port is monitored. Figure 5 shows the OFC transmission as a function of taper extension of three OFCs (OFC #1, OFC #2, OFC #3) with the same tapering profile.
Fig. 5. Transmission as a function of extension of OFC #1, OFC #2, OFC #3 at the wavelength of $1.55\,\mu$m. T: through-port; C: cross-port.
The taper extension refers to the distance that the two pulling stages have moved away from each other since the start of the tapering process. Initially, at low extension, all the power remains in the through-port. However, as the extension increases, the fundamental modes guided by the two fiber cores increasingly overlap, resulting in increased coupling. The tapering process is stopped manually once the desired coupling ratio is attained. The three OFCs tapered with the same tapering recipe demonstrate similar coupling ratio and optical properties. Similar optical properties verify similarity in geometry and repeatability of the OFCs [32]. Although the tapering temperature has been optimized and a multi-sweep tapering approach has been used, traces of crystallization remain at the surface of the OFC. Pursuing the tapering to bring the fiber diameter below $30\,\mu$m would propagates the crystallization deeper into the bulk material and lead to more associated excess losses. There is therefore a trade-off to perform in between tapering more to increase coupling, and tapering less to limit excess losses.
Figure 6 shows a schematic of the characterization setup used to acquire the transmission spectra of OFCs post tapering. Supercontinuum light (NKT Compact) is launched into the input port of the OFC and the output power spectrum from through and cross ports is measured using an optical spectrum analyzer (Yokogawa AQ6375). Figure 7 shows the transmission spectra of OFC #3 made with a waist fiber diameter of $27\,\mu m$, a transition length of $4.8\,$cm, and a waist length of $3\,$cm. The coupling ratio increases with increasing wavelength due to a decreasing V-number and decreasing light confinement in both tapered fibers. The excess losses also increase with increasing wavelength because of the decreasing V-number and light confinement. OFC #3 provides a coupling ratio of $5{\% }/95{\% }$ with excess losses of 1.6 dB at a wavelength of $2.2\,\mu$m. Using the same ZBLAN fibers for the fabrication of an OFC with enhanced coupling requires increasing the waist length above $3\,$cm and/or decreasing the OFC waist diameter below 27 $\mu$m, relative to OFC #3. Increasing the waist length increases the coupling as it provides a longer interaction length in between both fiber cores. Reducing the waist fiber diameter also increases coupling as it brings the two cores closer to each other and increases the mode overlap. Despite the fact that both options are equivalent to increase the coupling, the onset of crystallization, however, plays a role, and experimental attempts have shown that reducing the fiber waist from a waist of 27 $\mu$m has a stronger triggering effect on crystallization than increasing the waist length. Figure 8 shows three tapered fibers with final waist diameters of 40, 30, and 20 $\mu$m. Surface crystallization is apparent on the fiber surface as tapering decreases the fiber diameter. The origin of ZBLAN glass crystallization remains unclear but variables like the exposure time to heat and the temperature of the hot zone increase the probability of crystallization [25].
Fig. 6. Schematic of the characterization setup. SC: supercontinuum source; OFC: optical fiber coupler; OSA: optical spectrum analyzer.
Fig. 7. Transmission spectra of OFC #3 with a waist length of $3\,$cm, and OFC #4 with a waist length of $6\,$cm.
Fig. 8. Microscope images of three tapered fibers with a final waist diameter of (a) 40 $\mu$m, (b) 30 $\mu$m, and (c) 20 $\mu$m.
Figure 7 shows the transmission spectra of OFC #4 with a waist fiber diameter of $27\,\mu m$, a transition length of $4.8\,$cm, and a waist length of $6\,$cm, that is, twice the waist length of OFC #3. In comparison with OFC #3, the coupling ratio of OFC #4 is increased to $14{\% }/86{\% }$ with excess losses of 1.8 dB at a wavelength of $2.2\,\mu$m. The absence of modal interference fluctuations and the smooth transmission spectra of the OFCs confirm single-modedness.
All OFCs fabricated up to now show transmission spectra that indicate an increase in coupling as a function of wavelength. It is thus extrapolated that the fabricated OFCs should provide a strong coupling at mid-infrared wavelengths i.e. $>2\,\mu$m for which ZBLAN glass is compatible with. A home-made supercontinuum source is built up to monitor the OFC spectra at wavelengths up to $3\,\mu$m. Seed pulses with a duration of $900\,$fs are launched from a commercial thulium doped fiber laser (Advalue) at a repetition rate of $30\,$MHz. The average power is $7.5\,$mW at a central wavelength of $1.94\,\mu$m. Pulses are amplified in a home-made thulium doped amplifier (TDFA) and reach an average power of $\sim \!340\,$mW. The TDFA is made with a $20\,$cm long thulium-doped fiber (Coractive DCF-TM-6/125) with a core/cladding diameters of $6/125\,\mu$m and a numerical aperture of 0.23. The fiber is pumped by 2.7 W of C-band light from two erbium-doped fiber amplifiers in co-propagating and counter-propagating directions via two wavelength division multiplexers. A cascade of silica (CorActive) and ZBLAN (Le Verre Fluoré, ZFG SM[1.95] 6.5/125) fibers of lengths 0.8 m and 4.5 m, respectively, with core/cladding diameters of $6.5/125\,\mu$m and numerical aperture of 0.23 in both cases is placed after the amplified signal. The output of the ZBLAN fiber is a spectrally stable supercontinuum spanning up to a wavelength of $3.0\,\mu$m (inset of Fig. 9). Figure 9 shows the transmission spectra of OFC #5 with a waist fiber diameter of $27\,\mu m$, a transition length of $4.8\,$cm, and a waist length of $6\,$cm. At a wavelength of $2.73\,\mu$m, at the low wavelength limit of the $OH^-$ absorption resonance, the coupling ratio raises to $41{\% }/59{\% }$ at cross/through ports, with an excess loss of 2.5 dB. It is at a wavelength of $2.85\,\mu$m, within that resonance, that the coupler provides equal output power for both the cross-port and through-port. The transmission spectrum shows a notch at wavelengths around $2.9\,\mu$m. This absorption band is attributed to $OH^-$ impurity in ZBLAN caused by $H_2O$ molecules that migrate in the glass during the tapering process [33]. This absorption band is absent from the untapered fibers and appears during tapering despite the controlled Argon atmosphere.
Fig. 9. Transmission spectra of OFC #5. Inset: Transmission spectrum of the home-made supercontinuum source.
One OFC has also been kept at room temperature to observe the impact of aging over transmission. Figure 10 shows the transmission of OFC #6 with a final fiber cladding diameter of 28 $\mu$m and a waist length of $3\,$cm as a function of time, monitored at a wavelength of $2\,\mu$m. If crystallization was to develop further after the OFC fabrication, it is expected that losses would be increasing on both through and cross ports. After 94 days of constant probing, no increasing loss is observed on the transmission of both output ports, indicating that the ZBLAN OFCs maintain a good chemical stability despite traces of crystallization at their surface.
The multiple-sweep tapering technique has extensively suppressed the crystallization but not completely eliminated it. Hence, the final OFC waist fiber diameter is limited to a minimum diameter of around 27 $\mu$m and as a result, the maximum coupled power is limited. It is expected that a fine tuning of the tapering parameters such as the heater temperature and pulling velocity will lower down the amount of crystallization even more, providing the possibility of tapering to smaller final diameters and fabrication of wavelength division multiplexing OFCs.
4. Conclusion
This work shows the first single-mode OFCs made of fluoride glass. A multiple-sweep tapering technique resulted into a successful fabrication of single-mode OFCs with high reproducibility and low level of crystallization. OFCs with a coupling ratio as high as $41{\% }/59{\% }$ at cross/through ports and excess losses of 2.5 dB at a wavelength of $2.73\,\mu$m have been fabricated. The stability of OFC transmission with respect to time was also investigated and no significant aging effect has been observed over 90 days. This work is a significant advancement in expanding the capabilities of optical fiber technologies into the mid-infrared range.
Funding
Natural Sciences and Engineering Research Council of Canada.
Acknowledgment
The authors express their gratitude to Le Verre Fluoré and CorActive for the optical fibers that have served this set of experiments.
Disclosures
The authors declare no conflict 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.
References
1. A. Hoffman and C. Gmachl, “Extending opportunities,” Nat. Photonics 6(7), 407 (2012). [CrossRef]
2. S. De Bruyne, M. M. Speeckaert, and J. R. Delanghe, “Applications of mid-infrared spectroscopy in the clinical laboratory setting,” Crit. Rev. Clin. Lab. Sci. 55(1), 1–20 (2018). [CrossRef]
3. A. B. Seddon, “Mid-infrared (IR)–A hot topic: The potential for using mid-IR light for non-invasive early detection of skin cancer in vivo,” Phys. Status Solidi B 250(5), 1020–1027 (2013). [CrossRef]
4. D. da Silva Andrade, J. H. da Silva Rego, P. C. Morais, and M. F. Rojas, “Chemical and mechanical characterization of ternary cement pastes containing metakaolin and nanosilica,” Constr. Build. Mater. 159, 18–26 (2018). [CrossRef]
5. M. A. Pleitez, A. A. Khan, A. Soldà, A. Chmyrov, J. Reber, F. Gasparin, M. R. Seeger, B. Schätz, S. Herzig, M. Scheideler, and V. Ntziachristos, “Label-free metabolic imaging by mid-infrared optoacoustic microscopy in living cells,” Nat. Biotechnol. 38(3), 293–296 (2020). [CrossRef]
6. S. Türker-Kaya and C. W. Huck, “A review of mid-infrared and near-infrared imaging: principles, concepts and applications in plant tissue analysis,” Molecules 22(1), 168 (2017). [CrossRef]
7. Y. Gong, L. Bu, B. Yang, and F. Mustafa, “High repetition rate mid-infrared differential absorption lidar for atmospheric pollution detection,” Sensors 20(8), 2211 (2020). [CrossRef]
8. I. Tugendhaft, A. Bornstein, Y. Weissman, and A. A. Hardy, “Directional multimode fiber couplers in the mid-infrared,” Opt. Eng. 34(10), 2846–2849 (1995). [CrossRef]
9. D. Schaafsma, J. Moon, J. Sanghera, and I. Aggarwal, “Fused taper infrared optical fiber couplers in chalcogenide glass,” J. Lightwave Technol. 15(12), 2242–2245 (1997). [CrossRef]
10. G. S. Athanasiou, E. Bereś-Pawlik, G. Semczuk, D. Furniss, A. B. Seddon, and T. M. Benson, “Large core, multimode, chalcogenide glass fibre coupler by side-polishing,” Opt. Quantum Electron. 45(9), 961–967 (2013). [CrossRef]
11. F. Tavakoli, A. Rekik, and M. Rochette, “Broadband and wavelength-dependent chalcogenide optical fiber couplers,” IEEE Photonics Technol. Lett. 29(9), 735–738 (2017). [CrossRef]
12. O. Benderov, I. Nechepurenko, B. Stepanov, T. Tebeneva, T. Kotereva, G. Snopatin, I. Skripachev, M. Spiridonov, and A. Rodin, “Broadband mid-IR chalcogenide fiber couplers,” Appl. Opt. 58(26), 7222–7226 (2019). [CrossRef]
13. G. Stevens and T. Woodbridge, “Mid-ir fused fiber couplers,” in Components and Packaging for Laser Systems II, vol. 9730 (International Society for Optics and Photonics, 2016), p. 973007.
14. M. Rezaei and M. Rochette, “All-chalcogenide single-mode couplers,” in IEEE Photonics Conference (IPC), (IEEE, 2018), pp. 1–2.
15. M. Rezaei and M. Rochette, “All-chalcogenide single-mode optical fiber couplers,” Opt. Lett. 44(21), 5266–5269 (2019). [CrossRef]
16. M. Rezaei, M. H. M. Shamim, M. El Amraoui, Y. Messaddeq, and M. Rochette, “Nonlinear chalcogenide optical fiber couplers,” Opt. Express 30(12), 20288–20297 (2022). [CrossRef]
17. M. Rezaei and M. Rochette, “All-chalcogenide ring fiber laser,” Opt. Fiber Technol. 71, 102900 (2022). [CrossRef]
18. I. Alamgir, M. Rezaei, and M. Rochette, “Fiber optical parametric oscillator made of soft glass,” Optics Letters (2022).
19. L. Moore, D. MacFarlane, and P. Newman, “Surface Crystallization of ZBLAN glasses,” J. Non-Cryst. Solids 140, 159–165 (1992). [CrossRef]
20. A. Torres and R. Barr, “A quantitative characterization of micro-gravity and unit-gravity processed ZBLAN glass,” Mater. Charact. 107, 386–397 (2015). [CrossRef]
21. A. Torres, J. Ganley, and A. Maji, “Experimental and analytical techniques for studying ZBLAN crystallization in microgravity,” Exp. Tech. 40(2), 501–512 (2016). [CrossRef]
22. C. Xia, Z. Xu, M. N. Islam, F. L. Terry, M. J. Freeman, A. Zakel, and J. Mauricio, “10.5 W time-averaged power Mid-IR supercontinuum generation extending beyond 4μm with direct pulse pattern modulation,” IEEE J. Sel. Top. Quantum Electron. 15(2), 422–434 (2009). [CrossRef]
23. J. Sanghera and I. Aggarwal, “Active and passive chalcogenide glass optical fibers for ir applications: a review,” J. Non-Cryst. Solids 256-257, 6–16 (1999). [CrossRef]
24. K. Matusita, M. Koide, and T. Komatsu, “Viscous flow mechanism of fluoride glasses over a wide temperature range,” J. Non-Cryst. Solids 140, 141–145 (1992). [CrossRef]
25. É. Ducharme, S. Virally, R. I. Becerra-Deana, C. Boudoux, and N. Godbout, “Viscosity of fluoride glass fibers for fused component fabrication,” Appl. Opt. 61(17), 5031–5039 (2022). [CrossRef]
26. A. Annunziato, F. Anelli, P. L. P. Du Teilleul, S. Cozic, S. Poulain, and F. Prudenzano, “Fused optical fiber combiner based on indium fluoride glass: perspectives for mid-IR applications,” Opt. Express 30(24), 44160–44174 (2022). [CrossRef]
27. V. Fortin, Y. O. Aydin, M. Bernier, R. Vallée, M. Rochette, F. Chenard, O. Alvarez, L. E. Busse, L. B. Shaw, R. R. Gattass, and J. S. Sanghera, “Post-processing soft glass optical fibers,” in Mid-Infrared Fiber Photonics, (Elsevier, 2022), pp. 233–302.
28. J. Love and W. Henry, “Quantifying loss minimisation in single-mode fibre tapers,” Electron. Lett. 22(17), 912–914 (1986). [CrossRef]
29. C. Baker and M. Rochette, “Highly nonlinear hybrid AsSe-PMMA microtapers,” Opt. Express 18(12), 12391–12398 (2010). [CrossRef]
30. L. Li, A. Al-Kadry, N. Abdukerim, and M. Rochette, “Design, fabrication and characterization of PC, COP and PMMA-cladded As2Se3 microwires,” Opt. Mater. Express 6(3), 912–921 (2016). [CrossRef]
31. C. Baker and M. Rochette, “A generalized heat-brush approach for precise control of the waist profile in fiber tapers,” Opt. Mater. Express 1(6), 1065–1076 (2011). [CrossRef]
32. A. W. Snyder, “Coupled-mode theory for optical fibers,” J. Opt. Soc. Am. 62(11), 1267–1277 (1972). [CrossRef]
33. S. R. Loehr, K.-H. Chung, and C. T. Moynihan, “Infrared absorption due to H2O and D2O in a fluorozirconate glass,” J. Am. Ceram. Soc. 71(1), C-46–C-48 (1988). [CrossRef]
