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Abstract
Here, we present an innovative preform manufacturing technique for specialty optical fibers based on a carbon monoxide laser heating a rotating preform. The setup performance is evaluated with the aid of finite element modeling. The fabrication process is described in detail using silicon core preforms as a benchmark. The hybrid material nature of such a preform is addressed, together with the relevant characteristics, such as the difference in thermal conductivity and thermal expansion. Silicon core preforms with a wide range of core sizes were manufactured, proving the viability of this system for the development of specialty optical fibers based on novel materials.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
The most prominent use of optical fibers today is all-glass fibers in telecommunication. The enormous effort that has been put into researching and improving those fibers made them the gold standard in the near-infrared transmission region. Typically, the preforms from which those optical fibers are drawn are manufactured by techniques such as modified chemical vapor deposition (MCVD). In this way, the desired chemicals for the core material of the later drawn fiber are fused by a traveling burner onto the inside of a silica glass tube layer by layer. After deposition, the tube is collapsed into the final preform. MCVD, while leading to outstanding silica-based fibers, is limited in viable glass compositions [1] and is heavily focused on the deposition of oxides [2].
Extending the transmission window to the mid-infrared region by introducing new core materials, while maintaining the fiber qualities, would open up new possibilities for applications in biophotonics and medicine. Most biomolecules have their characteristic absorption band in this range [3], and the fiber platform could be used for minimally invasive in situ diagnostics and treatments [4,5]. Alternative core materials could be useful for THz waveguiding and nonlinear and optoelectronic devices [6]. For these purposes, silicon in particular has proven to be a promising and versatile core material [7,8].
These novel composite fibers can be obtained from preforms simply by stacking and inserting the desired core structure inside a silica tube that is usually sealed at the bottom. The preform is subsequently drawn into the fiber in a conventional draw tower. This is known as the stack and draw technique. It was shown that using this method, e.g., low-loss silicon core fibers could be drawn [9]. One disadvantage of this method is that the entire preform is exposed to temperatures above 1400°C for a long time, typically of the order of tens of minutes. During this time, oxides or other contaminants can diffuse from the cladding into the core. This diffusion can be reduced by introducing an interface layer at the core–cladding interface [10].
Another solution that does not require additional materials is to limit the exposure time of the preform to high temperature by utilizing very localized heating, such as that provided by laser sources. It has been demonstrated that ${{\rm{CO}}_2}$ lasers operating at a wavelength of 10.6 µm can be used to both draw and post-process optical fibers [11] to improve and modify their optical quality and composition [12–14]. When it comes to preform manufacturing, the high absorption of the ${{\rm{CO}}_2}$ laser radiation in silica presents the largest obstacle by limiting the processable preform dimensions. As most of the radiation is absorbed just at the surface, it is nearly impossible to heat preforms bigger than 3 mm in diameter without simultaneously evaporating the surface. However, commercially available carbon monoxide (CO) lasers operating at a wavelength near 5.5 µm can be a viable means to provide uniform heating even for larger silica preform diameters due to the larger penetration depth of this wavelength in silica. If their beam is protected from attenuation and thermal blooming due to water vapor absorption [15] by operating the laser in a dry air environment, they provide the benefits of localized laser heating with the advantages of uniform heating due to a large penetration depth.
Here, we present and evaluate a novel technique to fabricate preforms based on a CO laser. The preform setup was designed to rotate the preform tube around its longitudinal axis to distribute the heat from the unidirectional laser beam. The preform processing has been conducted using a single, unidirectional sweep along the preform length. The setup is used to fabricate multiple preforms with various constituent designs.
The scope of this paper covers the preform setup, the heating mechanisms, challenges of specialty fiber preform fabrication, and an evaluation of the manufactured preforms. The preforms produced by the setup were drawn into fiber in-house using a state-of-the-art experimental draw tower. The furnace design of the tower was also based on a CO laser to utilize the advantages provided by this technique. Drawn silicon-core fibers not only guided light, but also showed a record transmission loss of 0.2 dB/cm mark at telecom wavelengths. The tower and the resultant fiber are discussed in Ref. [16].
2. EXPERIMENTAL SETUP
The preform fabrication system was built using a modified dual spindle glass lathe (Gator, American Lathes). A schematic of the system is shown in Fig. 1. The headstock was fixed while the carrier table and lathe tailstock (further referred to as “pulling stage”) were equipped with stepper motors to enable computer controlled operation. The synchronous rotation of the lathe chucks could be operated up to a speed of 250 rpm. Instead of using traditional gas burners, heating was achieved by using a CO laser (Coherent Diamond J-3-5) operating with a central wavelength of 5.5 µm and maximum power output of 320 W. The laser beam was guided to the glass preform using three gold-coated mirrors M1–M3 (LBP Optics).
Mirror M3 was mounted onto the carrier table to enable traverse motion of the laser beam during processing. The carrier table motion used here was a single controlled sweep from left to right during preform processing. Additionally, a computer controlled lens revolver (LR) was mounted onto the carrier table. The LR enabled processing with laser spot sizes ranging from 0.6 mm up to 24 mm. To monitor the temperature at the focal spot during irradiation, a two-color infrared pyrometer was used (ENDURANCE, Fluke) capable of measuring temperature within the range of 1000°C–3000°C. Due to the small hot zone and the low emissivity of silica glass, temperature recordings were not possible for all-silica structures. However, it was possible when using core materials that possessed a higher emissivity. This implied that the recorded temperature was that of the core material at the center of the preform. For reference power measurements, a 1% beam tap (BT) was used. An aluminum housing placed on the carrier table was used to block any scattered radiation. Due to the high water absorption at 5.5 µm, which significantly deteriorates the beam shape and reduces power [17], a sealed enclosure was built around the preform fabrication system. The enclosure was then purged with dry air (KAESER DC 2.8 desiccant dryer) to achieve uniform humidity along the entire beam path with a dew point in the range of ${-}{{40}}^\circ {\rm{C}}$ to ${-}{{60}}{\rm{°C}}$. The purging time required to achieve a dew point of ${-}{{40}}{\rm{°C}}$ was approximately 1 h. Further, the open ended preform tube was connected to a flexible PVC tube to either purge gas or apply vacuum. Thereby the oxygen content was reduced, and splicing (welding) as well as collapsing tubes were facilitated. The PVC tube was accessed through the enclosure box using a rotating coupler. The whole system was computer controlled, and recipes for cutting, splicing, tapering, and preform fabrication were typically pre-programmed using machine code.
3. PREFORM MANUFACTURING
The fabrication of the preform was based on a number of processing steps that depended on the preform dimensions, available core materials, and targeted core–cladding ratio. Processing steps included cutting, splicing, collapsing, and tapering. Tapering could be used to assist the collapse of tubes, as well as to reduce the core size of the final preform by repetitive sleeving and tapering. The preform assembly was performed in several steps using individual components as illustrated in Fig. 2. These included the handle, initial preform tube, and exhaust tube. The core structure, placed inside the preform tube, was composed of a rod of the core material and two glass rod inserts. The first step was to splice the exhaust tube to the preform tube ensuring open throughput between the tubes. Once the preform tube was cut to the desired length (step 1, Fig. 2), the assembly was removed from the setup for surface cleaning. This was performed by immersing the preform tube into hydrofluoric acid (40%) for 5 min, followed by rinsing in deionized water and blow drying using pressurized nitrogen. This cleaning step was performed to remove any organic material or fumed silica that may have entered during the assembly process, and to remove any surface impurities on the inside of the preform tube. The second step was to insert the core material into the preform tube. Prior to insertion, the core rod and inserts were cleaned using an ultrasonic bath of acetone for 5 min, and subsequently methanol for 5 min followed by dry cleaning using compressed nitrogen gas. The third step was to splice the assembly to the glass handle. The position control of the inserts and core rod within the preform tube during step 3 was provided via a pusher inserted within the exhaust tube. The pusher was a custom made fiber-spring mechanism mounted within the exhaust tube extending to the chuck cavity that resides on the pulling stage (highlighted in Fig. 1). The linear actuator of the pusher was attached to the back side of the pulling stage. The pusher could be used either actively, by using the linear actuator, or passively, by applying pretension via a magnetic spring. This compensated for any thermal expansion or contraction of the core material during processing. The assembly was then collapsed, fusing the glass inserts with the preform tube. With the removal of the handle, the preform would then be ready to be drawn into a fiber. If a smaller core size was required, the preform assembly could be tapered down further (step 4). The taper could then be cut into suitable lengths and used as the core rod in a new assembly. The process steps (1–4) could then be repeated until the desired dimensions had been achieved.
A fused quartz exhaust tube with a length of 220 mm (4 mm outer diameter [OD] and 2 mm inner diameter [ID]) was used to enable the production of different preform lengths, fit the pusher, and to create a standard connection for the PVC exhaust tube system. The size of the preform tube defined the initial core–cladding ratio. Tubes with a diameter up to 10 mm were used in this system. The inner diameter of the preform tube was chosen to be as close to the insert and core rod diameter as possible. For the preforms discussed here, silica preform tubes with an inner diameter of 1.1 mm and outer diameters of 4 mm and 6 mm were used. Core rods and inserts had an outer diameter of 1 mm. The handle was a quartz rod with a diameter of 10 mm. An example of an assembled silicon core preform (step 3) is shown in Fig. 3.
4. SIMULATION OF HEATING DYNAMICS
The internal temperature distributions of a preform in this system are an important factor and were assessed here using finite element modeling in COMSOL Multiphysics. The simulations were conducted using the setup configuration described in Section 2. The limits of the preform size are a crucial factor in characterizing this system. For laser heating of silica on a preform scale, most of the energy is deposited at the surface, and conduction is the primary mechanism of heat transfer to the core. At higher surface temperatures, more energy is lost to convection and possible surface ablation before it can be conducted deeper. Thus, the achievable equilibrium temperature is heavily dependent on preform diameter. Also, the larger thermal mass requires longer heating times. To explore the capabilities of the preform system, a series of simulations was conducted with a range of preform diameters. The obtained thermal response times are shown in Fig. 4.
Fig. 4. Simulated heating time of different diameters of pure silica preforms irradiated from time $t = 0$ with 250 W from a CO laser. The temperature shown is the average temperature of the transverse, circular cross section of the preform at the point where the laser is incident.
The parameters used for these simulations were a 250 W laser beam with a diameter of 12 mm incident on a silica rod that was rotating at 100 rpm with no translational movement of the preform. The absorption coefficient of silica was estimated using the room temperature value of ${\alpha _{{\rm{RT}}}} = 12787\;{{\rm{m}}^{- 1}}$ [18], and it was assumed that all laser light was absorbed in the silica. Convection was approximated as natural convection of a horizontal cylinder, which was sufficient for a relative comparison of preform diameters. The actual convection properties of a spinning preform in the laboratory differ from this ideal.
Generally, the thermalization time increases and the equilibrium temperature decreases for larger preform diameters. For a preform of 12 mm in diameter, the heating time and temperatures were at the limits of what is feasible to process. This was consistent with experimental observations. For small preforms, the thermalization time was fast, as thermal mass was small. However, with smaller preform diameters, the overlap area of the preform and laser beam was small, so the absorption efficiency was low. The smaller preforms also had a larger surface area to mass ratio, which increased the relative amount of energy lost to convection. For preforms below 2 mm in diameter, these two mechanisms dominate, which sets the smallest workable preform diameter in this system. While some compensation could be made by reducing the diameter of the beam, the increased peak intensity resulted in greater surface ablation.
To provide a more detailed illustration of the thermal dynamics of a silicon preform in the system, a simulation was conducted with a 1 mm silicon core in a 6 mm outer diameter silica tube. This simulation used a 250 W beam with a diameter of 14.6 mm and a preform rotation speed of 100 rpm. Using the symmetry of the system modeled here, it was sufficient to model half of the preform in the longitudinal direction for efficiency. The longitudinal cross section of the preform is shown in Fig. 5.
Fig. 5. Snapshots of the axial cross section of a simulated silicon core preform during processing. The modeled preform has a core of 1 mm in diameter and a 6 mm outer diameter. Snapshots were taken in the material reference frame after 2 s, 7 s, and 12 s of CO laser exposure with the preform rotating at 100 rpm.
This shows the thermal gradients that were present during the processing of such a preform. The silicon core has a higher thermal conductivity that removes energy from the hotspot being processed. Additionally, the latent heat of silicon takes more energy as the core is heated to a molten state. These factors noticeably increased the laser power required to reach the same softening temperature at the hotspot. These simulations show that there are large transverse thermal gradients of many hundreds of degrees. This leads to large amounts of stress being applied to the silica as it was being processed in addition to the difference in thermal expansions of the materials upon cooling. The interplay of these effects contributes to the formation and propagation of cracks within the cladding, described in the following section.
5. MITIGATING CRACK FORMATION
This section describes the challenges encountered during the tapering process (step 4 in Fig. 2), in particular, relating to the formation of cracks in the preform and the developed solutions. The preform fabrication system was initially developed targeting semiconductor core materials, specifically silicon, for fabricating low-loss optical fibers. The material properties of silicon make it a useful benchmark material, as it provides several key challenges that need to be addressed for these hybrid structures containing both a crystalline semiconductor core material and a glass cladding. Silicon has a significantly higher thermal conductivity at room temperature ($149\;{\rm{W}}{{\rm{m}}^{- 1}}{{\rm{K}}^{- 1}}$) than silica ($1.3\;{\rm{W}}{{\rm{m}}^{- 1}}{{\rm{K}}^{- 1}}$). During laser processing, this causes a significant change in the observed heating dynamics when tapering preforms. Figure 6 shows a schematic of the preform during tapering.
Fig. 6. Schematic of a silicon core preform during tapering. Black is silicon, dark blue is solid silica, and light blue is the cavity of the silica tube.
The tapering process is initiated at point (a), with the laser stage and the pulling stage moving to the right. Here, there are three different states to be considered:
- 1. tapering in the region with only silica [points (a) and (e)],
- 2. tapering in the region with a silicon core [point (c)],
- 3. and tapering across the interface between these regions [points (b) and (d)].
Figure 7 shows the corresponding experimental temperature reading from the pyrometer during the tapering process.
Fig. 7. Measured temperature at the preform hotspot during the tapering procedure of a 210-mm-long preform processed at $0.3\;{\rm{mm}}\;{\rm{mi}}{{\rm{n}}^{- 1}}$. In this example, each point on the preform is above the melting temperature of silicon for 40 s. The blue line represents the moving average. Indicators b, c, and d correspond to those shown in Fig. 6.
The temperature at points (a) and (e) could not be recorded due to the low emissivity of silica. In the central silicon region [point (c) in Fig. 6], the temperature was relatively constant at around 1630°C. However, points (b) and (d) showed significantly higher temperatures. This implies that the temperature in the all-silica region was significantly higher even though it was not directly measurable. This temperature change was attributed to the higher thermal conductivity and latent heat of fusion for silicon. The effect of the higher thermal conductivity of silicon was explored with numerical simulations in the previous section and is illustrated in Fig. 5. This transition from processing a pure silica region to that with silicon resulted in a decrease in temperature with a subsequent change in viscosity. The sudden increase in viscosity of the preform tube alters the tapering conditions, thus forming a locally enlarged region at the start of the silicon core at point (b). The change in preform diameter is visible in Fig. 8.
Fig. 8. Cross-polarizer micrograph of the starting tapering point having (a) a detrimental propagating crack and (b) intentional crack formation. The white glow indicates the captured stress within the taper.
Another characteristic of silicon is the large, ${\approx} 10\%$, volume expansion during solidification to the crystalline phase [19]. The direct implication of this was a large stress buildup during processing. This was observed during tapering in the form of crack formation and crack propagation along the preform. Crack formation most frequently occurred at the material interfaces at point (b) or point (d). An example of such crack formations is shown in Fig. 8. At point (d), the silicon expanded, creating a sharp tapered tail, inducing additional stress on the cladding.
To properly handle the tapering at the final interface [point (d)], it was found that it was crucial to stop the tapering procedure immediately after this tail formation, but still within the laser melt zone (illustrated in Fig. 9).
Fig. 9. Silicon core tapering process at point (c). Red line indicates the Gaussian beam shape from the laser.
A controlled cooling of around 20°C/s was applied by stepping down laser power to prevent crack formation caused by the abrupt anomalous thermal expansion of silicon. This step proved to be vital for the prevention of catastrophic fracture propagation along the ingrained stress lines spanning the entire length of the preform. The combination of the tensile stress at the interface with a shear stress in the cladding (due to relatively cold tapering) often resulted in a spontaneous crack formation during tapering. This crack could transform into a longitudinal fracture that propagated with the melt zone during processing [see Fig. 8(a)]. In the case of spontaneous crack formation in a transverse configuration, the crack could self-seal, thereby creating stress points. To mitigate this process, a controlled fracture technique was implemented. This technique induced an intentional crack formation in the silicon core region by introducing a thermal perturbation approximately 10 mm after point (b) in Fig. 6. At this point, all stage movement was stopped, and the laser was blocked for a duration of 2 s. The preform was then exposed for a duration of 2 s prior to resuming the tapering process. The volume change from the rapid cooling and reheating of the silicon was sufficient to induce a thermal perturbation. This perturbation initiated an intentional crack, which released the embedded stress. Normal tapering could be resumed after this crack had been successfully created. This intentional crack formation is shown in Fig. 8(b). It is also of note that there was no inherent stress observed in the stock silica tubes prior to processing, so the crack formation was attributed to stresses induced during the processing procedure. Thus, no initial annealing of the silica tubes was used in this preform manufacturing process.
Crack formation was also an issue when cutting the taper into suitable lengths. No standard cutting techniques evaluated were adequate in reducing crack formation in these preforms. For this reason, a special laser cutting technique was developed. Here, the glass was slowly ablated using a focused laser beam with a spot size of 0.6 mm while rotating the preform. The laser cutting was terminated when the silicon core was reached. The final separation was performed by cleaving the silicon core after scoring it with a scriber. An example of such a cut is shown in Fig. 10(a).
Table 1. Tapering Parameters
6. EXPERIMENTAL RESULTS
A. Preform Fabrication
We fabricated a number of different preforms of various sizes using the described setup. In the following section, we discuss two specific preform examples.
Two different preform tubes were used here: a 6 mm OD (Goodfellow, fused quartz) and 4 mm OD (Heraeus, F300). Both of the preform tubes that were spliced to the exhaust tube had an inner diameter of 1.1 mm and were 55–60 mm in length to accommodate the inserts. The core insert was a cylindrical silicon rod, 20 mm long and 1 mm in diameter (Goodfellow SI007905). The inserts were 15-mm-long silica rods with a diameter of 1 mm. The preform tube was spliced to a 10 mm handle (Goodfellow, fused quartz). An example of the 6 mm preform assembly is shown in Fig. 3. The core rod and silica glass inserts were pretensioned using a spring-loaded pusher with a 2 mm allowance. This compensated for the silicon volume redistribution due to the difference in core and tube inner diameter. The vacuum applied to the exhaust tube resulted in a pressure in the range of 10–20 mbar. The preform rotation speed was set to 100 rpm. The laser spot size on the preform tube was 16.4 mm.
All preforms were tapered down to $1.8 \pm 0.1\;{\rm{mm}}$ for subsequent sleeving. The tapering diameter was defined by the precise speed ratio of the laser scanning stage (carrier table) and the pulling stage. The process required the laser processing speed to be faster than the pulling speed. The sets of parameters that resulted in successful tapering are summarized in line 1 and line 3 of Table 1. Figure 10 shows two examples of tapered preforms made of 4 and 6 mm OD tubes.
The usable lengths of the tapered preforms, defined by the 20 mm silicon insert, were 67 mm and 170 mm. The variations of core and outer diameter were within 5% along the length of the preform. The variations showed no indication of a periodic diameter variation, as would characteristic of Rayleigh–Plateau instability. It was believed that these diameter variations arose from fluctuations in the motor movement and laser power [20]. The core to cladding diameter ratio of the preform remained identical to the initial inner to outer diameter ratio of the preform tube. This varied along the tube length and was determined by the tube manufacturing quality.
To reduce dissolution of oxygen in the molten silicon it was beneficial to minimize heating time, and therefore faster tapering speeds were preferred. The most reliable results were achieved with a speed of $0.3\;{\rm{mm}}\;{{\rm{s}}^{- 1}}$ (for the 6 mm OD tube) and $0.5\;{\rm{mm}}\;{{\rm{s}}^{- 1}}$ (for the 4 mm OD tube). Under these conditions, the total manufacturing times were 700 s and 215 s, while the times at a single point on the preform above the melting temperature of silicon were 40 s and 24 s, respectively.
After tapering the preforms to a diameter of 1.8 mm, they were sleeved and tapered again to further reduce the core–cladding ratio of the final preforms. This process is often referred to as the rod-in-tube technique. The sleeving tubes used here (Goodfellow, fused quartz) all had an inner diameter of 2 mm, while the outer diameter was 4 mm, 8 mm, or 10 mm. As before, a final outer diameter of 1.8 mm was reached for each subsequent tapering step.
The inserts were held in place using the spring-loading insert shown in Fig. 3. After repeated sleeving, the final preform was tapered down to a diameter of 6 mm, which was a suitable diameter for our in-house laser based draw tower.
Compared to the first tapering step, which used a 1 mm diameter core, subsequent tapered preforms were found to be less susceptible to crack formation. This was attributed to the lower core to cladding volume ratio, reducing the overall effect of silicon expansion during solidification. Smaller cores also resulted in a lower temperature change during the tapering process at points (b) and (d) (see Fig. 7). However, when the core size was too small (below approximately 30 µm diameter) the core was more susceptible to beading arising from core fragmentation due to Rayleigh–Plateau instability [21].
Table 1 summarizes all manufacturing steps from the initial preform assembly (Fig. 3) to the final preform ready to be drawn to fiber. Two such preform examples, with silicon core sizes of 300 and 30 µm, are presented in Fig. 11. The preform with a 300 µm core was manufactured with two subsequent tapering steps, the parameters for which are given in Table 1, lines 1 and 2. The resulting core size at the end of the tapering process, given in line 1 (500 µm), became the input core size for the final process, given in line 2. In a similar way, the manufacturing sequence of the preform with a 30 µm core is listed. This comprised four steps, given in lines 3 to 6. Among other things, each line contains the speed ratio of the stages that was used to taper the sleeved tube down to a desired outer diameter (either 1.8 mm or 6 mm). Here, the laser processing speed is defined by the speed of the carrier table.
Fig. 11. Two 6 mm OD silicon core preform examples after two-stage tapering with (a) 300 µm core and (b) 30 µm core after four-stage tapering.
The column “Stock Tubes” in Table 1 catalogs all the tubes used here in the utilized sequence. The first number (4 or 6) represents the initial tube diameter prior to tapering, as discussed earlier, and the final number is the tube used for the final re-tapering step. The number of tubes named here corresponds to the total number of tapering steps in the process.
Tapering the 0.5 mm silicon core within a 10 mm quartz tube was possible but was at the limits of the system, due to the limitations of CO laser heating as discussed in Section 4. Although possible, the small tolerances required for tapering such a large mass resulted in greater preform diameter variation. The lengths of the produced preforms in this case were around 10 cm and 20 cm. A 10 cm long preform with a 6 mm outer diameter roughly corresponds to 230 m of drawn optical fiber with a diameter of 125 µm.
B. Analysis with Raman Spectroscopy
The stress in the preforms was characterized under cross-polarized light in an optical microscope (Nikon Eclipse LV100) and through Raman spectroscopy. Two examples of the obtained micrographs are shown in Fig. 10. Stress along the entire preform length is visible, characterized by the blue highlight around the core. Additionally, it can be seen that the stress is higher near the end of the preform. However, it appears to be uniform around the core and constant along the preform length.
The Raman spectra were acquired using a HORIBA iHR550 spectrometer. A diffraction grating with 1800 lines/mm was chosen, and the entrance slit size was set to 98 µm. A 20 mW He–Ne laser operating at a wavelength of 632.81 nm was focused to a spot size of ${\approx} 20\;{{\unicode{x00B5}{\rm m}}}$. The measured spectra are shown in Fig. 12.
Fig. 12. Raman scattering spectra of (gray) silicon rod and (blue and pink) silicon preforms after tapering. Negative Raman shift of the silicon peaks indicates that the silicon core of samples shown in Fig. 11 is under comparable tensile strain. The solid lines are Lorentzian fits.
First, a reference silicon spectrum of an unprocessed silicon rod was measured. The obtained Raman peak was centered at approximately $521\;{\rm{c}}{{\rm{m}}^{- 1}}$. The measured Lorentzian full width at half maximum (FWHM) of the reference sample was approximately $3.3\;{\rm{c}}{{\rm{m}}^{- 1}}$. Then, spectra of processed preforms were taken by probing the silicon in the preform from the side through the glass cladding. The Raman shift was measured to be ${-}2.5 \pm 0.2\;{\rm{c}}{{\rm{m}}^{- 1}}$ for the two preforms shown in Fig. 11, indicating that the silicon core was under a comparable amount of tensile stress [22]. Following Ref. [23], this corresponded to a tensile strain of approximately $0.32 \pm 0.02\%$. The symmetric shape of the silicon peak indicates a crystalline structure with a large grain size [24–26]. The FWHM of the measured spectra was approximately $3.3\;{\rm{c}}{{\rm{m}}^{- 1}}$ as well. Since there was no observable broadening of the silicon peak measured in the preform sample compared to the silicon reference, the crystalline structure of the silicon in the preform did not appear to degrade as a result of processing.
7. CONCLUSION
In this work, we have presented a novel computer controlled system for preform fabrication, specifically targeting research and development of new specialty optical fibers. The fabrication is based on the rod-in-tube technique, while using a CO laser for efficiently heating a rotating preform tube. With the laser operating at a wavelength of 5.5 µm, the system is enclosed and purged with dry air to reduce laser blooming. This system had the benefit of limiting local thermal exposure to tens of seconds, which minimized time for contaminant diffusion into the core. The low loss of the produced silicon fibers indicated successful reduction of contaminant diffusion. The viability of the preforms was confirmed by the quality of a silicon-core optical fiber subsequently drawn from these preforms in an in-house fiber draw tower [16]. In this initial work, we have focused on semiconductor-core preforms, specifically targeting silicon. A challenge with processing silicon core preforms is the large volume expansion of silicon during solidification and the large thermal conductivity. These factors contribute to a buildup of stress during processing and can often result in fracture and crack propagation throughout the preform, typically rendering the preform useless for further fiber drawing. In this work, we present a method to mitigate this effect by preventing crack propagation along the preform. With a high degree of control during processing, we have developed fabrication recipes and have fabricated preforms up to 30 cm in length with outer diameters in the range of 2–10 mm. Additionally, we have successfully produced crack-free preforms with silicon core diameters in a range of 1 mm down to 30 µm. Raman scattering measurements of preforms indicated the presence of tensile strain as well as a crystalline preform core.
Funding
Stiftelsen för Strategisk Forskning (RMA15-0135); Knut och Alice Wallenbergs Stiftelse (2016.0104).
Acknowledgment
We thank Prof. Mats Götelid for providing us access to the Raman spectrometer.
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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