Quartz Glass at the Core of Every Optical Fiber

Est. 2005
Lianyungang, Jiangsu, China

Global Fiber OEMs
Americas · Europe · Asia

High-Purity SiO₂
MCVD / VAD / OVD grade

Custom Dimensions
Any lathe, any platform

Specialty Fiber
PM, LMA, rare-earth, PCF

The MCVD substrate tube is the most geometrically critical component in inside-deposition preform manufacturing. It rotates on the glass lathe while the torch traverses along its length, oxidising halide precursors inside to deposit successive glass layers that will become the cladding and core of the finished fiber. The substrate tube’s outer diameter, inner diameter, wall thickness uniformity, and bow determine the lathe setup, the deposition layer geometry, and ultimately the refractive index profile of the finished preform. Any eccentricity in the tube wall — the variation of wall thickness around the circumference — translates directly into core ellipticity and polarisation mode dispersion in the drawn fiber. FGQuartz produces MCVD substrate tubes with controlled wall thickness uniformity and consistent OD/ID matched to the lathe chuck specifications of the customer’s glass lathe platform.

In the VAD process, deposition burners directing flame hydrolysis reactions build a porous soot preform axially at the tip of a rotating starting rod. The reaction tube surrounding the deposition zone provides the controlled atmospheric environment and thermal insulation that enables the growth to proceed with consistent radial composition profile. The starting rod — which will eventually become the seed for the preform’s core glass — must be made from high-purity fused silica free of the trace metals and hydroxyl content that would degrade the core refractive index profile. FGQuartz supplies both the reaction tube assemblies used in VAD deposition furnaces and the starting rods from which preform growth initiates, in low-OH grade fused silica appropriate for telecommunications fiber manufacture.

In outside vapour deposition, a cylindrical mandrel is rotated while deposition burners coat it with successive layers of soot. The mandrel is subsequently removed after sintering, leaving the hollow soot preform that will be collapsed to form a solid glass rod. The mandrel must survive the deposition temperatures while remaining removable without damaging the preform — which requires careful material and surface engineering. FGQuartz produces OVD mandrels in configurations suited to the specific deposition system geometry, along with the soot-deposition lathe components, burner housing parts, and handling fixtures that make up the peripheral quartz hardware of an OVD production station.

After the core-cladding assembly is deposited by MCVD, VAD, or OVD, the resulting preform rod is typically too small in diameter to produce a useful length of drawn fiber without additional jacketing. Overclad tubes — large-diameter high-purity fused silica tubes with precisely controlled inner bore — are sleeved over the core-cladding rod and fused onto it in a jacketing furnace to build up the outer diameter to the target drawing size. The optical quality of the jacketing tube directly affects the final cladding glass composition, since the jacket fuses into the fiber cladding. FGQuartz supplies overclad jacketing tubes in a range of bore sizes designed to accept standard preform rod diameters, with inner surface quality and purity appropriate for direct fusion into the cladding glass.

Single-mode fiber and certain specialty fiber designs require a cladding with a refractive index below that of pure silica to achieve the required waveguide geometry. This is accomplished by doping the cladding glass with fluorine, which lowers the refractive index below the fused silica baseline. Fluorine-doped quartz tubes are used as inner cladding elements in MCVD and as surrounding cladding tubes in rod-in-tube preform assembly. The fluorine dopant concentration must be tightly controlled because it directly determines the refractive index depression and therefore the fiber’s numerical aperture and cut-off wavelength. FGQuartz supplies fluorine-doped fused silica tubes in grades appropriate for standard single-mode, dispersion-shifted, and polarisation-maintaining fiber designs.

Glass lathes used for MCVD preform manufacture require quartz chuck inserts, tail-stock couplings, and collet fittings that interface between the metal lathe machinery and the quartz substrate tube. These fittings must be dimensionally precise for tube concentricity, thermally compatible with the quartz tube to minimise fracture risk, and chemically pure to avoid contaminating the deposition chemistry at the tube ends. FGQuartz supplies complete sets of lathe interface hardware for the major glass lathe platforms used in fiber preform production, along with the rod-support fixtures, preform holders, and draw-tower feed chucks used in downstream processing steps.

After soot deposition in VAD and OVD processes, the porous soot preform is consolidated in a high-temperature sintering furnace by zone-heating in a controlled atmosphere. The furnace tube and liner inside which consolidation takes place must be made from fused silica to avoid contamination of the preform surface during the sintering process, and to survive the temperatures required for full densification of the soot into glass. FGQuartz supplies sintering furnace muffle tubes and liners for both batch and continuous-draw sintering configurations. These items are manufactured in clear fused silica for single-zone furnaces and in opaque-grade quartz where thermal baffles between hot and cold zones are needed.

Hydroxyl content in the fiber cladding glass creates a strong absorption peak at 1383 nm — the water peak — that historically limited the transmission bandwidth of early single-mode fiber. Modern low-water-peak fiber achieves this by treating the soot preform with chlorine gas during consolidation to remove residual OH from the glass network. The dehydration and chlorine treatment step requires a quartz tube that is both compatible with the chlorine chemistry and sufficiently pure to avoid reintroducing metallic or OH contamination into the preform being treated. FGQuartz supplies high-purity reaction tubes for dehydration and chlorine-treatment stages with inner surface quality and material purity appropriate for this demanding application.

Specialty fiber types — polarisation-maintaining fiber, photonic crystal fiber, large-mode-area fiber, rare-earth-doped fiber for amplifiers, hollow-core fiber, and multi-core fiber — all require preform assemblies that deviate significantly from standard telecommunications preform geometry. PM fiber requires stress-applying parts (SAPs) made from borosilicate glass positioned within a fused silica preform. Photonic crystal fiber requires a stack of capillary tubes and solid rods assembled with precision. FGQuartz supplies the fused silica tube and rod components used in all of these specialty preform assemblies, producing capillary tubes, solid rods, large preform cylinders, and custom-machined quartz elements to the exact dimensions required by the specific fiber design. Prototype quantities for new fiber designs and production volumes for established specialty fiber types are both accommodated.

The backbone of global telecommunications infrastructure. ITU-T G.652 (standard SMF), G.654 (low-loss large-effective-area), and G.657 (bend-insensitive) fibers are drawn in enormous quantities from MCVD, VAD, and OVD preforms. The quartz substrate tubes, jacketing tubes, and process components used in their manufacture must maintain consistent quality across production lots of millions of metres. FGQuartz supplies substrate tubes and jacketing tubes to the dimensional specifications required by the major SMF production lines, with the material consistency needed for tight lot-to-lot performance control.

Multi-mode fiber with graded-index core profiles is used in data centre interconnects, local area networks, and short-reach optical links. OM3, OM4, and OM5 grades support 10G, 25G, and 100G Ethernet over standard link lengths and are the dominant fiber type for intra-data-centre connectivity. The preforms for high-bandwidth graded-index MMF are manufactured primarily by MCVD and VAD with precise radial index control. MCVD substrate tubes for MMF preforms must be dimensioned to produce the correct 50 µm or 62.5 µm core diameter upon collapse and draw. FGQuartz supplies substrate tubes for both 50 µm and 62.5 µm core MMF production lines.

PM fiber is essential for coherent optical communications, fiber-optic gyroscopes, interferometric sensors, and polarisation-sensitive fiber components. PANDA, bow-tie, and elliptical cladding PM fiber types all require specialty preform assemblies with stress-applying parts embedded in fused silica jackets. FGQuartz supplies the jacket tubes and overclad components for PM fiber preform assembly, with the dimensional precision needed for correct SAP positioning relative to the core. Fiber-optic gyroscopes for navigation and inertial measurement are one of the largest volume applications for PM fiber, and a significant fraction of FGQuartz’s PM fiber component supply goes into the gyroscope supply chain.

Erbium-doped fiber amplifiers (EDFAs) are the technology that made the modern internet possible by amplifying optical signals directly without optoelectronic conversion. Ytterbium-doped fiber lasers are now the dominant technology for industrial laser cutting and welding at kilowatt power levels. Both technologies require specialty preforms where the core is doped with rare-earth ions (Er³⁺, Yb³⁺, Nd³⁺) during the MCVD deposition process, and the surrounding cladding glass must have precisely controlled geometry for efficient pump light coupling. FGQuartz supplies large-mode-area jacketing tubes and overclad components for high-power fiber laser preforms, where the cladding diameter is much larger than standard SMF to allow multi-mode pump injection from diode laser arrays.

Photonic crystal fiber enables optical properties that conventional step-index fiber cannot achieve — endlessly single-mode guidance, anomalous dispersion at visible wavelengths, hollow-core guidance through photonic bandgap effects, and extremely high nonlinearity for supercontinuum generation. These fibers are assembled from stacks of capillary tubes and solid rods at the preform stage. FGQuartz produces the high-dimensional-uniformity capillary tubes and matching solid rods required for PCF stack-and-draw preforms. Applications span nonlinear wavelength conversion, medical delivery of high-power laser radiation, chemical gas sensing in hollow-core fiber, and quantum optics experiments requiring single-photon fiber delivery.

Distributed temperature sensing (DTS), distributed acoustic sensing (DAS), and distributed strain sensing systems use optical fiber as a continuous sensing element, measuring the Brillouin or Rayleigh backscatter signature along the entire fiber length to reconstruct temperature, strain, or vibration profiles over tens of kilometres. These applications require fibers with tight spectral and modal characteristics, and in harsh environments — oil well downhole monitoring, subsea pipeline, high-voltage electrical infrastructure — the fiber must additionally survive elevated temperatures and chemically aggressive surroundings. FGQuartz supplies the specialty preform substrate tubes and jacketing tubes used in the manufacture of sensing fiber for these applications, including carbon-coated and hermetically sealed fiber variants where the preform geometry determines the coating uniformity.

Hydroxyl (OH) groups incorporated into the fused silica network during glass synthesis create absorption bands in the infrared that translate directly into fiber transmission loss. The fundamental OH absorption peak at 2.73 µm has overtones at 1.383 nm (the water peak), 1.24 µm, and 0.95 µm — all within the telecommunications transmission bands. Low-water-peak fiber (G.652.D) achieves an attenuation below 0.4 dB/km at 1383 nm by using ultra-low-OH glass throughout the cladding, including the substrate tube. MCVD substrate tubes and jacketing tubes for low-water-peak fiber must therefore be manufactured from low-OH or dry-grade fused silica. High-OH grade quartz glass — which has better deep-UV transmission and is used in optical quartz for UV applications — is not suitable as a substrate tube for low-water-peak telecommunications fiber.

The MCVD substrate tube is not a passive container — it is a precision optical component. The tube’s wall thickness uniformity around its circumference (eccentricity) determines the roundness of the deposited layer cross-section and therefore the core ellipticity of the finished fiber. Core ellipticity causes polarisation mode dispersion (PMD) — a fundamental limit on high-speed data transmission in long-haul fiber links. For coherent 100G and 400G transmission systems where PMD budgets are tight, the eccentricity of the substrate tube is a direct contributor to system margin. Similarly, the bow of the tube — its axial straightness — affects the deposition uniformity along the tube length and can cause longitudinal variation in the core profile that manifests as chromatic dispersion variation in the drawn fiber.

The MCVD process deposits glass layers at moderate temperatures where OH from ambient humidity and precursor impurities can be incorporated into the glass network. Even with dry precursor gases and a controlled deposition atmosphere, residual OH is incorporated at low but measurable levels during each deposition pass. To achieve the sub-0.4 dB/km water peak attenuation of G.652.D fiber, the completed soot preform is treated with chlorine gas before full consolidation — a dehydration step in which Cl₂ reacts with OH groups in the glass to form HCl, which diffuses out of the glass. The tube used for this dehydration treatment must be high-purity fused silica compatible with hot chlorine chemistry, and must not itself be a source of OH contamination into the preform being treated. This is a demanding application for the quartz tube — the combination of high temperature, halide chemistry, and strict purity requirements rules out lower grades of quartz glass.

During MCVD collapse, the inner surface of the substrate tube becomes the glass-glass interface between the deposited cladding layers and the outer cladding provided by the tube wall. If this interface has surface contamination, crystalline inclusions, or significant roughness, it will scatter light propagating through the cladding region, increasing the cladding mode loss and potentially coupling scattered light back into the core. For single-mode fiber, cladding mode loss at the inner tube surface is less critical than for multi-mode fiber where a significant fraction of the guided modes travel in the outer cladding region. For specialty fiber applications where the cladding mode is the actively used guided mode — hollow-core fiber and anti-resonant fiber being extreme examples — the inner surface quality of the substrate or jacketing tube is directly performance-determining. FGQuartz controls the inner surface finish of substrate tubes through the grinding and polishing stages of tube manufacture and can provide polished inner bore surface on request for specialty applications.

Optical fiber is one of the purest glass objects ever manufactured at industrial scale. The fiber core glass used in single-mode telecommunications fiber has transition metal contamination at the parts-per-trillion level, because even sub-ppb levels of iron, chromium, copper, or nickel create absorption centres that increase fiber attenuation above the intrinsic Rayleigh scattering limit of pure silica. While the substrate tube and jacketing tube are not as purity-critical as the core glass — since they are drawn into the outer cladding where mode intensity is negligible in single-mode operation — any contamination that diffuses from the tube glass into the deposited layers during the high-temperature deposition and collapse cycles can degrade core glass quality. This diffusion risk means that substrate and jacketing tubes for telecommunications fiber should be manufactured from high-purity grade fused silica rather than lower-purity quartz glass, even though the tube glass is nominally outside the optical core.

The draw process converts the millimetre-scale dimensions of the preform into the 125 µm outer diameter of the finished fiber by a combination of viscous flow and diameter control. In principle, every dimensional and optical feature of the preform is preserved in the fiber — the core-to-cladding diameter ratio, the refractive index profile shape, the eccentricity, and any macro- or micro-defects in the glass. A bubble or inclusion in the preform glass produces a fiber break or a local optical discontinuity. A periodic variation in the preform diameter — caused by draw tension fluctuations during MCVD collapse — produces a longitudinal variation in the fiber’s group delay, known as periodic dispersion. This high-fidelity copying of preform quality into fiber quality is why the quartz glass components used to manufacture the preform — the substrate tube, the jacketing tube, the deposition vessels — must meet the same exacting purity and dimensional standards that would be expected of the fiber itself.

Optical fiber substrate tubes and jacketing tubes are produced by drawing fused silica from a controlled melt or by redrawing from a larger preform tube, with the outer and inner diameters set by the draw die geometry and controlled by in-line dimensional monitoring. The critical quality parameters — outer diameter, inner diameter, wall thickness, eccentricity, bow, and surface roughness — are measured along the full tube length at the end of the draw process. FGQuartz measures tube geometry on all substrate tube production with the same rigour applied to fiber draw in telecommunications production, because the causal link between substrate tube geometry and fiber optical performance is direct and well-established.

Chuck fittings, lathe inserts, tail-stock components, and custom preform handling fixtures require the kind of dimensional precision that flame forming alone cannot achieve. FGQuartz’s CNC turning and milling capability machines fused silica components to the tolerances required for interference-fit lathe chuck assemblies and concentricity-critical preform holders. PCF capillary tubes and solid rods require outer diameter uniformity over their full length that is produced by CNC grinding rather than flame drawing alone.