Quartz Glass Engineered for Light Itself

Plane-parallel windows in fused silica for viewports, beam entry and exit windows, pressure barriers, and environmental protection for optical systems. The defining characteristic of a good optical window is minimum wavefront distortion — a function of glass homogeneity, surface flatness, and parallelism between the two faces. FGQuartz windows are ground and polished to customer-specified surface quality and figure requirements, with both standard circular and rectangular geometries plus custom shapes available. AR coatings for UV, visible, and NIR bands can be applied on request through partner coating houses.

Fused silica lenses are specified wherever UV transmission, laser damage resistance, or environmental durability rules out conventional optical glass. Plano-convex lenses focus or collimate beams; plano-concave lenses diverge beams. Cylindrical lenses focus light to a line — essential for laser scanning, barcode reading, and laser sheet illumination. FGQuartz produces all three types in JGS1 and JGS2 grades, with diameters from a few millimetres to several hundred millimetres. Custom radii and aspherical surfaces are available on request.

Optical prisms redirect, invert, or disperse light through total internal reflection or refraction. Common types in fused silica include right-angle prisms for beam deflection and retro-reflection, equilateral prisms for dispersion and wavelength separation, Dove prisms for image rotation, and Porro prisms for optical path folding. Because fused silica has low chromatic dispersion across the visible and near-UV, it produces less chromatic aberration than flint glass alternatives when used as a dispersing element. FGQuartz machines and polishes prisms with the number of optically active faces required by the application.

Telescope mirrors, laser cavity mirrors, and interferometer reference flats all begin as optically polished glass blanks before the reflective coating is applied. Fused silica is favoured as a mirror substrate for precision applications because its near-zero thermal expansion means the mirror surface figure changes negligibly as ambient temperature fluctuates — critical for astronomical telescopes, ring-laser gyroscopes, and high-stability optical systems. FGQuartz supplies polished mirror blanks and substrate discs in circular and rectangular formats, with both solid and lightweight core-drilled options for large astronomical applications.

An optical flat is a window polished to exceptionally tight flatness, used as a reference surface in interferometric metrology, as the reference mirror in Fabry-Pérot cavities, and as the output window in etalon filters. Etalons are parallel-plate Fabry-Pérot cavities used for laser linewidth narrowing, wavelength selection, and spectral filtering. The cavity spacing must be stable against temperature changes — which makes the low thermal expansion of fused silica a fundamental advantage over other glass types. FGQuartz produces optical flats and etalon substrates with the parallelism and surface figure required by the application.

Analytical spectroscopy instruments measure the absorption, emission, or scattering of light by samples in precisely dimensioned transparent cells. The optical path length directly determines analytical sensitivity, so dimensional precision of the light-path walls is analytically critical. Fused silica cuvettes transmit far further into the UV than quartz-blend or borosilicate glass cells, enabling spectroscopy at analytical wavelengths used for proteins, nucleic acids, and aromatic compounds. FGQuartz produces standard 10 mm path-length cuvettes plus custom flow cells, high-pressure cells, and micro-volume cells for specialised applications.

UV germicidal lamps, excimer lamps, and deuterium arc lamps rely on fused silica envelopes that transmit the short-wavelength UV generated inside the discharge without absorbing it. Standard silica glass and borosilicate glass are opaque below approximately 280 nm, making them unsuitable for germicidal 254 nm UV-C lamps. Only fused silica or very high-purity natural quartz glass transmits these critical wavelengths. FGQuartz manufactures lamp envelope tubes, dome-end discharge vessels, and custom envelope geometries for UV lamp and plasma discharge applications.

Solid quartz rods act as efficient light guides through total internal reflection, coupling light from a source to a remote location or homogenising the spatial profile of a beam. Ground and polished rod diffusers convert the concentrated output of a laser diode or fibre optic into a uniform line or area illumination. For UV applications — stage lighting, curing systems, and photolithographic alignment — fused silica rods transmit wavelengths that plastic or standard glass waveguides absorb. FGQuartz produces polished rods from 1 mm diameter upward and custom light guide geometries to drawing.

Not every optical application fits a standard catalogue shape. Instrument designers frequently need optical components with mounting holes, flanges, slots, or non-standard profiles that cannot be produced by polishing alone. FGQuartz combines precision CNC grinding and milling with optical polishing to produce complex optical assemblies from a single piece of fused silica, eliminating adhesive joints that would introduce scatter or stress birefringence. Customers supply drawings in DXF, STEP, or IGES format; prototype quantities down to a single piece are accepted with no minimum order constraint.

JGS1 is a synthetic fused silica produced by flame hydrolysis of a silicon-containing precursor, resulting in a material with high hydroxyl content. The high-OH network structure extends UV transmission deeper into the ultraviolet than any natural quartz glass, making JGS1 the standard choice for applications at or below 250 nm. It is the default grade for excimer laser optics at 193 nm and 248 nm, for UV spectroscopy instruments measuring nucleic acid and protein absorption at 260–280 nm, and for germicidal UV lamps at 254 nm. The trade-off is that the high-OH content introduces an absorption feature in the near-infrared around 2.7 µm, making JGS1 unsuitable for applications requiring transmission in that wavelength region.

JGS2 is manufactured from high-purity natural quartz crystals melted in an electrically heated furnace, producing a material with much lower hydroxyl content than synthetic grades. The lower OH concentration reduces absorption in the near-infrared, making JGS2 the right choice for near-infrared laser applications — Nd:YAG at 1064 nm and 1550 nm telecom wavelength windows — and for any application where transmission beyond 2.5 µm matters. JGS2 transmits usably from approximately 260 nm through to beyond 2.5 µm. Its UV cut-off is slightly less deep than JGS1, which generally does not matter for applications above 260 nm. Most general-purpose optical quartz components — windows, lenses, prisms for visible and NIR systems — are manufactured from JGS2.

Standard JGS1 fused silica undergoes a slow densification or rarefaction process under extended excimer laser irradiation — particularly at ArF 193 nm — because the high-energy photons break and reform Si-O bonds, gradually changing the glass network density. This compaction manifests as a slow change in refractive index that introduces wavefront error accumulating over millions of pulses. Radiation-hardened grades of synthetic fused silica are produced with controlled impurity balance — specifically the ratio of OH content to dissolved oxygen and reactive sites — to minimise the magnitude of the compaction effect under excimer laser irradiation. FGQuartz can supply these grades and advise on expected compaction rate for a given fluence exposure profile.

Birefringence means a material has a different refractive index for two orthogonal polarisation states — caused by a non-isotropic crystal structure. Crystalline quartz (α-SiO₂) is birefringent. Fused silica is amorphous — its silicon dioxide network has no long-range order — and therefore has essentially zero intrinsic birefringence. This makes fused silica the correct choice for any application where polarisation state must be preserved through an optical element, including polarimetry instruments, interferometers, ellipsometers, and polarisation-sensitive laser systems. It is also why fused silica is used for retardation plates in the near-UV, where crystal quartz would introduce large retardance from its birefringence.

Optical surface quality is specified in two complementary ways. Scratch-dig notation (such as 40-20 or 10-5) is a legacy US military standard describing the visual appearance of surface defects under controlled illumination — a lower number means fewer and smaller visible defects. Surface roughness, measured by interferometry or atomic force microscopy and quoted as Ra or RMS roughness in nanometres, measures the statistical height variation of the polished surface at a microscopic scale. Both matter because they affect scattered light at different spatial frequencies — scratches scatter at large angles; nanometre-scale roughness generates forward-directed scatter that reduces the Strehl ratio of an optical system. FGQuartz polishes optical surfaces to customer-specified scratch-dig and roughness requirements appropriate to the application.

Even in an amorphous glass with no intrinsic birefringence, stress-induced birefringence can be introduced by uneven cooling during glass fabrication, by mechanical grinding stress not subsequently relieved, or by temperature gradients during use. Stress birefringence in an optical window or lens is equivalent to a spatially varying waveplate — it rotates the polarisation of transmitted light by different amounts at different points on the aperture, degrading the polarisation purity of the transmitted beam. For polarisation-critical applications, optical blanks should be annealed to release residual stress, and residual stress birefringence measured by polarimetry before components enter the finishing process. FGQuartz uses annealed blanks for optical components and can arrange polarimetric inspection for applications requiring low-birefringence assurance.

All optical materials produce some fluorescence when illuminated by UV or visible light, as impurity centres absorb excitation light and re-emit at longer wavelengths. This fluorescence background sits underneath the signal of interest in Raman spectroscopy and can obscure weak fluorescence signals in biological assays. Fused silica has an exceptionally low fluorescence background compared to other glass types because the SiO₂ network itself has few intrinsic fluorescence centres, and because the high-purity grades used for optical components contain minimal levels of transition metal impurities — iron, chromium, cobalt — which are the most common fluorescence sources in optical glass. This is why fluorescence cuvettes, Raman spectroscopy probe windows, and fluorescence microscope components are routinely made from fused silica rather than standard optical glass.

Optical quartz surfaces are sensitive to contamination that would be trivial on a structural component. Hydrocarbon contamination from fingerprints or pump oils deposits a film that absorbs UV laser radiation, heating the surface and eventually causing catastrophic laser-induced surface damage. The standard cleaning sequence is solvent wipe with acetone followed by isopropanol, then dry with clean filtered-air or nitrogen blower — never use lens tissue in a dragging motion, which deposits fibres and scratches the surface. For deep-UV components exposed to excimer lasers, even molecular-level organic contamination can cause damage; these optics should be handled only in class-100 environments with gloves and stored in sealed containers purged with dry nitrogen or argon. Any UV-exposed surface showing clouding, pitting, or deposits should be immediately removed from service.

Borosilicate glass is frequently used for viewports, cuvettes, and protective windows in non-demanding applications because it is significantly cheaper than fused silica. The relevant differences are transmission, thermal stability, and homogeneity. Borosilicate transmits adequately from approximately 300 nm, covering the visible range and near-UV. Below 300 nm it absorbs strongly. Borosilicate also has a much higher coefficient of thermal expansion than fused silica, making it more susceptible to thermal shock and to figure change with temperature. For applications requiring UV performance below 300 nm, laser exposure, high-temperature environments, or consistent optical figure across a temperature range, the performance advantage of fused silica justifies its higher unit cost.

Optical components begin as sawn blanks shaped by grinding — removing material with progressively finer diamond abrasive until the component reaches its target geometry. FGQuartz operates CNC grinding centres with diamond wheels for both surface and cylindrical grinding of fused silica. CNC control enables consistent reproduction of complex shapes across a production run. Compound curves, non-circular apertures, mounting flats, and integrated structural features are achievable in the grinding step before the part moves to polishing.

Polishing converts a ground surface — with a rough, light-scattering structure at the nanometre scale — into an optically smooth surface with the required surface figure. Classical polishing using pitch laps and cerium oxide or colloidal silica slurry is still used for the highest-quality surfaces because a well-controlled pitch lap self-corrects surface errors through selective material removal. CNC subaperture polishing tools allow deterministic material removal from specific surface regions, converging on the target figure much faster than classical polishing when surface errors are localised.

Every polished optical surface is measured before shipment. Surface figure is characterised by interferometry using a Fizeau interferometer against a reference flat or sphere. Surface roughness is measured by phase-shifting or white-light interferometry. Defect inspection is performed under controlled dark-field illumination. FGQuartz provides measurement data sheets with finished optical components on request, documenting as-measured wavefront error, surface roughness, and defect map for incoming inspection at the customer facility.