Many optical applications of fused quartz exploit its wide transparency range, which can extend well into the ultraviolet and into the near-mid infrared. Fused quartz is the key starting material for optical fiber, used for telecommunications.
Because of its strength and high melting point (compared to ordinary glass), fused quartz is used as an envelope for halogen lamps and high-intensity discharge lamps, which must operate at a high envelope temperature to achieve their combination of high brightness and long life. Some high-power vacuum tubes used silica envelopes whose good transmission at infrared wavelengths facilitated radiation cooling of their incandescent anodes.
Because of its physical strength, fused quartz was used in deep diving vessels such as the bathysphere and benthoscope and in the windows of crewed spacecraft, including the Space Shuttle and International Space Station. Fused quartz was used also in composite armour development.
In the semiconductor industry, its combination of strength, thermal stability, and UV transparency makes it an excellent substrate for projection masks for photolithography.  Its UV transparency also finds use as windows on EPROMs (erasable programmable read only memory), a type of non-volatile memory chip which is erased by exposure to strong ultraviolet light. EPROMs are recognizable by the transparent fused quartz (although some later models use UV-transparent resin) window which sits on top of the package, through which the silicon chip is visible, and which transmits UV light for erasing.
Due to the thermal stability and composition, it is used in 5D optical data storage and in semiconductor fabrication furnaces.
Fused quartz has nearly ideal properties for fabricating first surface mirrors such as those used in telescopes.  The material behaves in a predictable way and allows the optical fabricator to put a very smooth polish onto the surface and produce the desired figure with fewer testing iterations. In some instances, a high-purity UV grade of fused quartz has been used to make several of the individual uncoated lens elements of special-purpose lenses including the Zeiss 105 mm f/4.3 UV Sonnar, a lens formerly made for the Hasselblad camera, and the Nikon UV-Nikkor 105 mm f/4.5 (presently sold as the Nikon PF10545MF-UV) lens. These lenses are used for UV photography, as the quartz glass can be transparent at much shorter wavelengths than lenses made with more common flint or crown glass formulas.
Fused quartz can be metallised and etched for use as a substrate for high-precision microwave circuits, the thermal stability making it a good choice for narrowband filters and similar demanding applications. The lower dielectric constant than alumina allows higher impedance tracks or thinner substrates.