Lithium Niobate

Overview

Thin film lithium niobate (TFLN) is the successor to ion-diffused lithium niobate, which has been used for decades for high-speed modulators. Although commercially available only for a few years, TFLN is now offered in several open-access foundries worldwide. The core value proposition of TFLN is fast, highly linear electro-optic modulation with down to zero static power consumption.

Characteristics

Lithium niobate is a birefringent crystal with a refractive index between 2.2 and 2.3. TFLN consists of a single monocrystalline slice on top of an oxidized silicon substrate, typically 200-600 nm thick, depending on the foundry and process. Due to birefringence, the refractive index differs for TE and TM polarization, and depending on the crystal cut, it might even vary between two different directions of propagation.

TFLN has a higher optical confinement than ion-diffused lithium niobate, and can therefore be used for more complex devices. Because of the high confinement, the modulation is more efficient, which allows for shorter modulators with higher bandwidths. With a driving voltage, its Vπ can be below 2V.

Lithium niobate is chemically inert, which leads to angled sidewalls. This leads to fabrication challenges limiting the minimum feature and gap size, which need to be carefully taken into account. This could also affect directional couplers, edge couplers and grating couplers. Lensed or small-core fibers are recommended for high-efficiency coupling.

TFLN is transparent for wavelengths between 350 and 5200 nm; however, it suffers from photorefractive effects at lower wavelengths.

Applications

Because of its wide transparency window, TFLN is the only PIC platform that offers low loss and high speed modulation below 1.1 µm. This unique capability makes it especially interesting for quantum applications.

Additionally, by periodically poling TFLN, it can be used for nonlinear frequency conversion, which can be harnessed for quantum applications such a the generation of entangled photons and quantum key distribution.

Like ion-diffused lithium niobate, TFLN can be used for high-speed modulators, for example for telecom applications. Its higher confinement allows direct integration with splitters and couplers, such that coherent or datacenter optical communication can be implemented on a single chip.

Bring your PIC project to life

At Epiphany Design, we work with all PIC material platforms and always advise our customers on the best choice for their particular applications. When it comes to lithium niobate photonics, we know how to make the most of its strengths to create solutions across a wide range of use-cases. We specialize in PIC Design, but also provide support at all stages of PIC development when need.