The most natural choice was TEOS (tetraethyl orthosilicate,. Si(OC2H5)4), a material widely used in electronic industry. TEOS easily converts to silica by ...
Fabrication of Specialty Optical Fibers Using Flash Vaporization Method Borut Lenardič*a, Miha Kvedera, Hervé Guillonb, Samuel Bonnafousb a Optacore, Trpinčeva ulica 39, 1000 Ljubljana, Slovenia b Kemstream, Rue de la vieille poste, PIT de la Pompignane, Montpellier Cedex 34055, France ABSTRACT Rare earth and metal ion doped optical fiber preforms have been produced using a novel flash vaporization method for precursor delivery [1] into substrate tube. TEOS and solutions of organometallic compounds, containing lanthanide, Al, Bi, Fe or Co were used. Use of TEOS and organometallic precursors makes this process similar to aerosol and sol-gel processes, but glass is laid down in thin layers in MCVD fashion, strongly relying on thermophoretic forces. Preforms with fully consolidated core layers have been made and were in most cases drawn to fibers. Results of these fibers and preform glass composition are discussed. Keywords: rare earth-doped fibers, fiber lasers, MCVD process, collapse, specialty fibers, flash vaporization
1. INTRODUCTION Rare-earth doped optical fibers based on fused silica glass are commonly produced using MCVD process by deposition of thin silica layers inside high-purity quartz substrate tube[1]. Most commonly used method is so –called "solution doping", invented in the mid 1980's [2], optimized over years and now providing for the large part of commercially sold fibers. Its inconvenience is in complex handling and processing of preforms, as they have to be removed from glass working lathe for soaking and then reset, dried and chlorinated before finally collapsed. Such complexity is limiting productivity and influences overall yield, while active core diameter is limited by practically achievable porous layer thickness. Soaking of porous layers can be repeated several times, increasing final core thickness but adding to the complexity of this fabrication process. Chelate doping method, based on high-temperature sublimation of solid rare-earth β-diketonate precursor, was reported by Tumminelli et al in 1990 [3]. A method using aerosol delivery of precursors to MCVD or OVD deposition zone was proposed in 1986 by T.F. Morse [4]. The advantage of chelate and aerosol methods over solution doping is the possibility of "in-situ" preform fabrication. While both techniques can produce highly rare earth-doped fibers, they never achieved the wide acceptance of solution doping process, one of the possible reasons being lack of viable fabrication equipment. A device design and process conditions were sought that could provide well-controlled precursor vapor generation and stable, repeatable "in-situ" vapor phase layer deposition for fabrication of LMA laser or amplifier fibers. A possibility to combine aerosol-like process with standard MCVD functionality was studied by introduction of so-called "flash vaporization" method. In this method, precursor vapor is generated from solution by a novel evaporation technique, using direct liquid injection into heated evaporation vessel. Vapors of complex organometallic molecules can thus be obtained and were introduced into hot zone of a standard substrate tube on a typical preform lathe. Reaction between precursors and oxygen in the hot zone caused by burner of furnace resulted in generation of silica soot particles, doped by other elements like lanthanides or transition metals, or with standard dopants like germanium or phosphorous. Relationship between doping device design, processing parameters and resulting glass composition has been investigated by fabrication of a series of RE3+ or metal ion co-doped preforms. Rare earth and metal ion doped optical fiber preforms have been produced using TEOS and solutions of organometallic compounds (containing lanthanide, Al, Bi, Fe or Co precursors) as core raw materials. Use of TEOS and organometallic precursors makes this process similar to aerosol and sol-gel processes, but glass is laid down in thin layers in MCVD fashion, strongly relying on thermophoretic forces.
*
borut.lenardic @ optacore.si, phone +386 1 546 1268, www . optacore. si
Preforms with fully consolidated core layers have been made and were in most cases drawn to fibers. Results of these fibers and preform glass composition are discussed.
2. GENERATING PRECURSOR VAPOR BY FLASH VAPORIZATION CVD (Chemical Vapor Deposition) technology in different forms has played and will continue to play a leading role in microelectronic and certain segments of fiber optics. In microelectronics it has enabled obtaining numerous materials containing elements from the right columns of the periodic table (B, C, N, O, Al, Si, P, Ge…) as thin amorphous, polycrystalline or epitaxial layers. In optical fiber fabrication, silica glass is often doped by germanium, phosphorous, boron and fluorine. For active fibers and special products, doping by lanthanides or metal ions is required. If precursors are available in gaseous (like BCl3 or SF6) or in liquid form (SiCl4, GeCl4, POCl3), sufficiently high precursor vapor pressures can be reached under normal conditions of temperature and pressure. For these elements, precursors are available only in solid form, with melting temperatures high above room temperature. To enable doping by such precursors via a gas phase process, high or very high temperature evaporation has to be used. Alternative method is to use solutions by soaking or aerosol, as described above (Sec. 1). To be able to use vapor phase MCVD deposition with alkaline earth, transition metal and lanthanide precursors, a new vaporization method had to be proposed. Suitable MCVD precursors are for example, organometallic compounds, containing the element that will be deposited and also carbon, hydrogen, oxygen and/or nitrogen, which are low vapor pressure liquids or solids. The most straightforward vaporization solution is to heat directly the liquid or the solid in a container (a bubbler for a liquid or a crucible or sublimator for a solid) under an inert or neutral carrier gas (N2 or Ar) flow. Unfortunately, this technique is not adaptable to some precursors (especially those of alkaline earths and lanthanides) and does not provide stable and reproducible vapor flows. Under long heat treatment, some precursors partially decompose and polymerize. Therefore, for evaporation of precursors that do not support direct contact heating, a method using atomization of liquid or precursor solution inside a hot stainless steel vaporizer is proposed. In basic versions of such evaporators clogging of the vaporizer is observed after a while due to non-volatile residues.
Fig. 1. Flash vaporization device operation principle, single vaporization channel shown for clarity, in practice each Vapbox vaporization cell is equipped with dual channels.
Direct liquid injection (DLI) sources have overcome these problems for some precursors critical for MCVD preform fabrication. In DLI sources, the precursors should be either pure liquids or dissolved solutions in an organic solvent (for solids or liquids). The liquid or the solution is delivered from a container at a controlled flow inside a heated stainless steel vaporizer that is connected to the deposition chamber by a heated gas line. Unlike bubbling technologies, only the precursors delivered inside the vaporizer are heated and evaporated, precursors in source containers remain at room temperature, preventing undesirable and premature thermal decomposition.
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Fig. 2. a) Photographic images taken inside vaporization chamber showing complete injection cycle, lasting several tens of milliseconds. Photos follow from top left to bottom right and show how injected aerosol evaporates; b) view of Vapbox flash vaporizer device with two injection valves sets for two precursors.
To overcome these problems, new type of high performance vaporizers were developed for ALD, CVD, MOCVD (Metal Organic CVD), PECVD(Plasma Enhanced CVD), PEALD (Plasma Enhanced ALD) and all other gas phase processes containing vapors of solid or liquid compounds. The vaporizers can vaporize pure liquid compounds and solid ones dissolved in a carrier liquid (organic solvent). Their operation is based on a very fine atomization of a pulsed liquid flow, mixed with carrier gas prior to injection into vaporizer chamber. Generated vapors can be used, for instance, for the synthesis by ALD and CVD of thin films, nanoparticles and nano-objects of numerous functional materials such as: dielectrics (low-k and high-k), barrier layers (Ta, Ti and Nb nitrides), ferroelectrics, piezoelectrics, metals (Cu, Ru, Rh, Pd, Ag, Ir, Pt…), III-V, II-V, chalcogenides (GST, CIS, CIGS,...), transparent conductive oxides, photovoltaic ones, low friction coatings, hard coatings and superconductors. In this described case we employed vaporizers as vapor delivery. Vapbox vaporizers employ a proprietary way to deliver and atomize the liquid inside the vaporizer. They feature what is termed an injection head (see Figure 1). The injection head consists of multiple components: one liquid injector, one mixing chamber and one mixture injector. These three components are surrounded by a heat sink with a cooling fan. This arrangement keeps liquid or solution at room temperature, as long as it is not injected in the vaporizer. Premature precursor decomposition is thus avoided. Both the liquid and mixture injectors are normally closed fast solenoid valves working in a pulsed regime. The liquid to be vaporized is stored at room temperature in a pressurized tank at a higher pressure than the mixing chamber that is maintained under constant inert carrier gas pressure. The liquid injector pulses the cool fluid into the mixing chamber. The mixing chamber blends this fluid with the inert carrier gas. Finally, the mixture injector injects this mixture into the vaporizer in a pulsed regime. Pulsed flow combined with the blasting effect of the carrier gas pressure allows atomization of the blended fluid into an aerosol with extremely small droplets, typically 10 µ m in diameter, with narrow distribution (see Figure 2a) The surface of evaporation is thus very large. Furthermore, the droplets are very uniformly distributed over the carrier gas volume. Since the carrier gas serves as a heat transfer medium between the hot vaporizer walls and the droplets, heat transfer is very eficient. This provides efficient and truly non-contact flash vaporization to happen. The pulsed two-phase flow regime at the outlet of the mixture injector is highly turbulent and therefore the mean residence time of the droplets inside the vaporizer is largely increased, as compared to a stationary flow situation, avoiding the presence of non-
vaporized droplets at the outlet of the vaporizer. A large amount of the droplets vaporize before touching the hot vaporizer walls of the vaporization chamber, limiting the clogging risks and allowing for generation of particle-free vapors. It is thus possible to efficiently vaporize thermally unstable compounds without clogging. These vaporizers can operate from vacuum to atmospheric pressure. Vapbox vaporizer was adopted as vapor source for novel MCVD-like process for deposition of silica layers highly doped by lanthanides or metal ions. Figure 2b shows Vapbox device with two injection heads.
3. RAW MATERIALS AND PRECURSORS Flash vaporization process relies on use of organometallic compounds as precursors for deposition process. It is not possible to combine standard halides like SiCl4 in combination with organometallic materials or organic solvent vapors. Therefore a different precursor for silicon had to be found. The most natural choice was TEOS (tetraethyl orthosilicate, Si(OC2H5)4), a material widely used in electronic industry. TEOS easily converts to silica by hydrolysis (sol-gel process) or at higher than 600 °C in presence of oxygen to SiO2 and diethyether. TEOS is readily available in ultra high purity and does not require complex handling. There are other options, among which octamethylcyclotetrasiloxane (OMCTS, also called D4, used in OVD preform fabrication) or tetramethylcyclotetrasiloxane (TMCTS, precursor for gate dielectrics in thin-film transistors) have reportedly been used in specialty optical fiber preform fabrication by Morse [5]. They both have much higher content of silicon than TEOS and their reaction with oxygen is strongly exothermic. β-diketonates have been chosen as organometallic precursors for lanthanide and transition metal ions. The reason for this was good availability from several commercial sources, relatively high purity, simple handling (with exception mentioned below) and good solubility in organic solvents. 2,2,6,6-tetramethyl-3,5-heptanedionates (tmhd) or acetyl acetonates (acac) were used, with typical purity of 99.9%. Organic solvents for preparation of solutions were, for example, diglyme, 1,3-dimethoxyethane, tetrahydrofuran (THF) in p.a. purity. Solution preparation and filling of precursor containers was made under dry and neutral (nitrogen) atmosphere, when necessary. All other raw materials used in preform and fiber fabrication were of the standard type and quality (substrate tubes, chlorides, gases, coatings, etc.)
4. EXPERIMENTAL Standard MCVD preform system has been used for fabrication of preforms. Lathe was equipped with induction furnace instead of H2/O2 burner to speed up collapse process (see Figure 4a). Flash vaporization doping system, connected to preform lathe as add-on device was a prototype of a commercial device (Vapbox 1500). Schematic view of the complete set-up is shown on Figure. 3.
Fig. 3. Schematical view of flash vaporization equipped MCVD deposition system
Precursor liquid or solution was injected into Vapbox vaporizer, volatized at temperatures between 100°C to 220°C, transported and injected into reaction hot zone by a system of heated conduits, valves, specially constructed hightemperature rotary seal and sliding precursor vapor injection tube An inert gas was used as a carrier to prevent chemical reaction between constituents. Precursors were allowed to mix with oxygen and other gases (i.e. fluorine containing gas) delivered from standard MCVD gas cabinet only in the hot zone. Vapor entering substrate tube is shown on Figure 4b.
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Fig. 4. a) MCVD preform lathe with flash vaporization deposition process running - a small burner was used as heat source to start chemical reaction; b) injection of precursor vapor from Vapbox into substrate tube is shown using sliding injection tube.
Substrate tubes with larger than usual outer diameter and thin wall (30/27 or 25/22 mm) were used to permit heated injection tube to slide into hot zone area (also seen inside substrate tube on Figure 4b). Standard cladding layers were first deposited on synthetic fused silica tube as barrier against hydroxyl impurity diffusion. Cladding was followed by active core deposition using flash vaporization doping system. Core was deposited in multiple thin layers. While deposition of large number of core layers was possible (up to 20 or more) usually 5 – 9 layers were used to reach core diameter of approx. 2 mm. Direct comparison with standard MCVD deposition is not possible, as TEOS was used in flash vaporization process as silicon precursor with evaporation rates between 0.25 and 2 g/min. Carriage traverse speed was 100 mm/min. Deposition was typically made over 600 mm and preforms with useful length between 250 and 350 mm and OD of 12 – 15 mm were produced.
Fig. 5. Top: flash vaporization deposition process, inner injection tube is seen inside substrate tube, as well as the flame from burning TEOS and solvent vapors. Bottom: side view of substrate tube shows thin porous deposited layer, furnace was kept at approx. 1450 °C.
Core layers were deposited as thin porous doped silica layers (carriage forward motion) with low hot zone temperature of approx. 1450°C. At higher deposition rates, layers were filled with carbon soot (see Figure 6a.) mainly from organic solvents and also partially from organometallic materials. At very high deposition rates, silica soot was filled with carbon to such an extent that the adherence to substrate tube wall was minimized and deposited layers detached themselves from the tube inner wall randomly.
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Fig. 6. a) highly carbon-filled layer is seen during deposition using a small burner, with high vapor flows resulting in carbon-filled layer; b) consolidation of carbon-filled layer by small burner, carriage direction was reversed and high partial pressure of oxygen was provided to promote carbon burn-off, resulting in fully transparent layer.
When heat source was in backward traverse, vapor delivery was stopped and porous layer was fully vitrified, while eventual carbon soot was burnt away, leaving transparent and solid layer of doped silica (see Fig. 6b). Chlorine was sometimes added to gas flow at this stage to remove impurities from deposited layers, and to reduce background loss [1]. After deposition of several core layers, vapor injection tube was retracted and substrate tube was collapsed into solid rod preform. In these experiments furnace was used as heat source, permitting significant acceleration of collapse process (more than twice faster than with H2/O2 burner) with generally improved optical and geometrical characteristics.
5. RESULTS AND DISCUSSION 5.1 Analytical procedures Precursor evaporation rate was checked by precise weighing of sublimator charges before and after process runs (Mettler PC2200 balance). Preform refractive index profile was measured by PK2600 analyzer and fiber attenuation by cut-back method using monochromator and optical spectrum analyzer Ando 6317C or Photon Kinetics 2200 tool. Ytterbium content was evaluated by peak absorption strength at 915 and 975 nm [6]. Aluminum content was evaluated from preform core refractive index difference using [7]. Some specific laser optical fiber measurements were provided by ORC Southampton and bismuth luminescence was checked by FORC Moscow. Glass composition by EDS analysis was provided by Jožef Stefan Institute, Ljubljana. 5.2 Experiment scope A number of experiments were carried out during evaluation of flash vaporization process and device. Preforms for laser fiber, co-doped by Yb3+ and Al3+, were of most interest as they are currently very intensely investigated. A few experiments were made with other rare earth dopants (Er3+, Tb3+) to compare results of flash vaporization process with that of direct chelate evaporation / sublimation. Bismuth-doped fibers are also of scientific and practical interest lately, therefore a significant effort was spent on finding proper precursor and process conditions.
Metal ion doping was studied using iron and cobalt co-doping with the goal to demonstrate flash vaporization capability for doping by a wide range of precursors. Table 1. summarizes main characteristics of produced preforms that were analyzed as preforms or drawn to fibers (permitting further geometrical and optical measurements to be done). Table 1: List of 7 analyzed preforms made by flash vaporization method (total 12 made)
5.3 Yb3+/Al3+ co-doped preform results From the series of rare earth-doped preforms 3 were drawn to fibers and tested. Refractive index profiles for P0125 and P0144 are shown on Figure 7. P0125 preform showed Yb3+ concentration between 1.1 and 3.2% wt over the preform length. Al3+ concentration was evaluated at approx. 2.5 %wt. A natural quartz tube was used as substrate tube and no attempt was made to reduce hydroxyl content or influence of other impurities from raw materials or processing. Preform P0144 (refractive index on Figure 7.) was produced in a similar way as P0125, but using F-300 synthetic quartz substrate tube. After each porous core layer has been deposited, it has been vitrified under atmosphere of high purity oxygen and chlorine gases without precursor in the tube.
Fig. 7. Refractive index profiles of preforms P0125 and P0144, measured at the exhaust end of preforms. Grey circle highlights the two fluorinated layers made by reacting TEOS vapors with O2 and SF6 gas (see text).
In this preform the possibility to deposit fluorine-doped layers with flash vaporization process was tested. Two layers were laid down after cladding deposition, using approx. 0.8 g/min TEOS vapor flow with SF6 gas added from MCVD gas cabinet, mixed with reaction oxygen (O2 flow 2000 sccm and SF6 flow 50 sccm). These two layers can be seen in P0144 refractive index profile between core and depressed cladding (Figure 7. circled detail)
Preform P0125 was drawn to single- and double-clad fiber and characterized for losses (courtesy of FORC, Moscow). Core losses (~ 1.6 dB/m at 1060 nm) and pump signal absorption from cladding (approx. 2 dB/m) were measured. Preform P0144 was drawn to single-clad fiber and attenuation was measured on several different lengths of fiber by cutback method. Figure 8. shows absorption peaks for both drawn fibers, measured on short fiber length by cut-back method. Peak profiles correspond well to data reported in literature for vapor phase made fibers.
Figure 8. Absorption peaks in 800 – 1000 nm window for fibers drawn in single-clad configuration from P0125 and P0144 preforms.
Figure 9. shows attenuation coefficient comparison for these fibers in the wavelength region between 1000 and 1600 nm. The difference in measurement results is evident and shows that vitrification of core layers under increased chlorine vapor pressure significantly reduces attenuation over the whole spectral region. This improvement is assumed to be the result of chlorination of deposited layers and change of the substrate tube to synthetic quartz. Strong influence of tube material on background attenuation of rare earth-doped fibers has been previously reported by Kirchof et al.[7].
Figure 9: Attenuation of chlorinated (P0144) vs. non-chlorinated fiber (P0125). P0144 was produced on F-300 synthetic substrate tube, while P0125 was produced using natural quartz substrate tube.
Preform P0144 was also drawn to dual-clad fiber, using low index polymer, suitable for cladding pumping (clad OD 125 µm, core OD 13.5 µm). Cladding absorption measurements showed approx. 0.6% wt Yb3+ concentration. Background
loss in the fiber measured by OTDR at 1280nm was 9dB/km. The lasing efficiency with respect to absorbed pump power was 76%, using 915nm pump diodes (courtesy of ORC, Southampton). 5.4 Bismuth-doped preform results Large interest in bismuth-doped fibers is related to research related to white light sources. Attempt to produce such fibers by flash vaporization process was made and several preforms have been produced. Results were compared to other vapor phase methods and to literature results on fibers produced by solution doping method. Finding a Bi precursor for flash vaporization process is more complex than for RE precursors. Bismuth chelate Bi(tmhd)3 is a very moisture sensitive compound which has to be handled under strict inert atmosphere and with anhydrous and deoxygenated solvents. Other possible precursor is triphenyl bismuth Bi(C6H5)3. After careful study of the problem, it was decided to use solution of [Bi(tmhd)3(1,2-ethylenediamine)] in n-octane. Al(acac)3 was dissolved in bismuth solution to which a small amount of chloroform was added [8]. Figure 10 shows refractive index profiles for P0182 and P0185 preforms. P0182 was the first trial and core was deposited on natural quartz substrate tube with no deposited cladding. Bi and Al precursors were carefully placed into one solution container under protective atmosphere. In total 9 layers of core were deposited, using different ratio of TEOS and precursor vapor flow. Profile shows irregularity in the central section of the core that was made intentionally. Two thin layers of pure silica were deposited after Bi/Al co-doped core to prevent Bi evaporation from core layers [9] with the idea to use gas phase etching before the last stage of collapse to remove them.
Fig. 10: Refractive index profiles for P0182 (see text about irregularities) and P0185 preform are shown.
P0185 preform was produced using similar conditions with increased precursor vapor flow. Cladding was deposited prior to core deposition (8 layers) and overdoping technique was used to prevent evaporation of Bi2O3 from inner core layers. Analysis of preform samples (slices) by X-ray microanalysis (EDS) has shown that bismuth content is below or at the detection level of the mentioned method (0.02 %at). P0185 has shown some bismuth luminescence (measured courtesy of FORC Moscow) but insufficient to proceed with preform or fiber analysis. Report of Bi luminiscence with similar level of doping has been published earlier by Dvoyrin [9] for Bi/Al co-doped core, while latest publications report Bi luminescence in phospho-germanate glasses (Umnikov [10], Bufetov [11]). Based on presented results and publications new Bi dopants and modifications to precursor preparation process are currently investigated.
5.5 Metal ion-doped preform results To demonstrate capability of flash vaporization system doping by metal ions was attempted. Among the elements that are of interest cobalt and iron were chosen. Fe(tmhd)3 and Co(acac)3 were used as metal ion precursors and they were dissolved in CHCl3 solvent (chloroform) with the added advantage of having chlorine present during chemical reaction and deposition of core layers. Deposited layers were very strongly filled with carbon soot (Figure 6.) and preform P0178 showed strong phase separation and multitude of bubbles in the core area.
Figure 11. Refractive index profile measured at exhaust end of P0183 preform co-doped by Fe and Co.
With proper processing during deposition and consolidation of core layers (proper burn-out of carbon during consolidation phase), fully vitrified preform P0183 was produced. Figure 12 shows finished preform P0183 with core of deep blue color typical for Co doping.
Figure 12. Preform P0183 after collapsing and outer surface fire polishing, blue tinted core indicates presence of Co ions, the bright yellow section is still heated to high temperature.
Fiber was drawn from P0183 preform and cut-back measurement was attempted, but low refractive index difference did not ensure proper waveguide characteristics for exact measurements to be made. Fabrication of Fe/Co co-doped preform shall be repeated with another dopant added (possibly Al) to provide proper single mode guide structure for single clad fiber.
6. CONCLUSIONS Preforms doped by rare earth and metal ions have been fabricated using novel flash vaporization precursor delivery system. It was demonstrated that fully vitrified transparent preforms without inclusions or bubbles can be fabricated using organometallic precursors for silicon, lanthanides, metal and transition element ions. RE-doped fibers have been produced with suitable refractive index profiles and composition for use in laser fibers. Background loss was controlled by choice of raw materials and process conditions. Drying and purification of deposited layers was demonstrated, using chlorine gas as drying and purification agent. Best background attenuation results were comparable to that of fiber produced by solution doping or chelate evaporation process. Doping by fluorine during deposition by organometallic reagents was also demonstrated, providing the possibility to adjust numerical aperture of the produced fibers. Fabrication of Bi-doped fiber did not succeed and additional work is necessary. Doping by Fe and Co was successful and preforms were produced that were drawn to fiber. Wide range of precursors permits fabrication of diverse glass compositions thus making flash vaporization a versatile and reliable research and development tool, suitable for development of novel glass structures and compositions.
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