Material reference

Fused silicaSiO₂

The quiet foundation of all of photonics: the fiber that spans oceans at 0.2 dB/km, the cladding on every integrated platform, and the substrate under most free-space optics.

Fused silica is easy to overlook because it is everywhere. It is the core and cladding of essentially all telecom fiber, the top and bottom cladding of silicon and nitride waveguides, the substrate of most mirrors and lenses, and the reason a photon can cross the Pacific and still be detected. No other material transmits light with anything close to its combination of loss, strength, and manufacturability.

Key properties

Property	Value	Conditions
Refractive index $n$	1.4440	1550 nm, 300 K
Refractive index $n$	1.4469	1310 nm, 300 K
Transparency window	~0.2–3.5 μm	bulk
SMF attenuation	0.18–0.20 dB/km	1550 nm, standard SMF
Zero-dispersion wavelength	~1310 nm	standard SMF
Thermo-optic coefficient $dn/dT$	$\approx +1.0 \times 10^{-5}$ K⁻¹	1550 nm
Kerr index $n_2$	$\approx 2.5 \times 10^{-20}$ m²/W	1550 nm
Thermal conductivity	1.4 W·m⁻¹·K⁻¹	300 K

What the numbers mean in practice

The famous 0.2 dB/km floor at 1550 nm sits at the crossover between Rayleigh scattering (falling as $\lambda^{-4}$ ) and the infrared multiphonon absorption edge, and it is the single number most responsible for the shape of global telecommunications. The historical "water peak" from OH absorption near 1383 nm has been engineered out of modern low-water-peak fiber, opening the E-band.

In integrated photonics, silica's role is defined by contrast with everything else: its low index makes it the universal cladding, its tiny $n_2$ makes it nonlinearly inert, and its small $dn/dT$ (an order of magnitude below silicon's, and famously exploited in athermal designs) makes silica-heavy waveguide geometries the temperature-stable ones. Its poor thermal conductivity is the flip side: the buried oxide under an SOI device layer is a thermal blanket, which helps microheaters and hurts high-power actives in equal measure.

Doping is how silica is tuned. Germanium raises the core index of standard fiber by a fraction of a percent; fluorine or boron lower it for depressed claddings; erbium turns a strand of it into the amplifier that made WDM practical.

References

I. H. Malitson, "Interspecimen comparison of the refractive index of fused silica," J. Opt. Soc. Am. 55, 1205 (1965).
K. Nagayama et al., "Ultra-low-loss (0.1484 dB/km) pure silica core fibre and extension of transmission distance," Electron. Lett. 38, 1168 (2002).
G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed., Wiley (2010).
R. Kitamura, L. Pilon, M. Jonasz, "Optical constants of silica glass from extreme ultraviolet to far infrared at near room temperature," Appl. Opt. 46, 8118 (2007).