Material reference

Lithium niobateLiNbO₃

The electro-optic material. A strong Pockels effect, wide transparency, and ferroelectric poling made bulk LiNbO₃ the telecom modulator standard for decades; thin-film LiNbO₃ has rebuilt the platform at chip scale.

Lithium niobate earned the nickname "the silicon of photonics" long before silicon photonics existed. Its linear electro-optic (Pockels) response is fast, pure, and low-loss: applying a field changes the refractive index directly, with none of the absorption penalty that carrier-based silicon modulators pay. Every long-haul fiber system built between the 1990s and the 2010s put its data on light through a diffused-waveguide LiNbO₃ Mach-Zehnder.

Key properties

Property	Value	Conditions
Ordinary index $n_o$	2.211	1550 nm, congruent, 300 K
Extraordinary index $n_e$	2.138	1550 nm, congruent, 300 K
Electro-optic coefficient $r_{33}$	~31 pm/V	low frequency
Transparency window	~0.35–5 μm
Thermo-optic coefficient $dn_e/dT$	$\approx +3.3 \times 10^{-5}$ K⁻¹	1550 nm
TFLN waveguide loss	~0.1 dB/cm typical	0.03 dB/cm demonstrated
TFLN modulator $V_\pi L$	~1.5–3 V·cm	with >100 GHz bandwidth demonstrated

What the numbers mean in practice

The action is along the crystal's $z$ -axis: $r_{33}$ couples an applied $E_z$ to $n_e$ , so devices are oriented to keep both the optical polarization and the RF field aligned with $z$ . The material is also strongly birefringent ( $n_o - n_e \approx 0.073$ ), which matters for phase matching and for keeping polarization straight at the bench.

Being ferroelectric is the second superpower. Domain orientation can be flipped lithographically, and periodically poled lithium niobate (PPLN) turned quasi-phase-matched frequency conversion (second-harmonic generation, parametric oscillators, quantum photon-pair sources) into a catalog product.

Thin-film lithium niobate (TFLN, or LNOI) is the modern chapter: submicron crystalline films transferred onto oxide-on-silicon carriers, etched into ridge waveguides. Confinement improves by orders of magnitude over the old titanium-diffused waveguides, which is what pushed $V_\pi L$ from ~10–15 V·cm down to a few V·cm and bandwidths past 100 GHz at CMOS-compatible drive voltages.

Two practical cautions: the photorefractive effect can slowly distort transmission at visible wavelengths and high intensities (MgO doping suppresses it), and DC bias drift is a real operational behavior of LiNbO₃ modulators. Commercial devices ship with bias controllers for a reason.

References

D. E. Zelmon, D. L. Small, D. Jundt, "Infrared corrected Sellmeier coefficients for congruently grown lithium niobate and 5 mol.% magnesium oxide-doped lithium niobate," J. Opt. Soc. Am. B 14, 3319 (1997).
R. S. Weis, T. K. Gaylord, "Lithium niobate: summary of physical properties and crystal structure," Appl. Phys. A 37, 191 (1985).
M. Zhang, C. Wang, R. Cheng, A. Shams-Ansari, M. Lončar, "Monolithic ultra-high-Q lithium niobate microring resonator," Optica 4, 1536 (2017).
C. Wang et al., "Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages," Nature 562, 101 (2018).