Choosing Materials for Dielectric Waveguides in Integrated Optic Circuits

Key Takeaways

• Optical fibers are a well-known example of dielectric waveguides.

• Generally, techniques such as diffusion, chemical vapor deposition, sputtering, etching, ion implantation, and epitaxial growth are used to fabricate dielectric waveguides.

• The electro-optical and acoustic-optical properties of lithium niobate make them the best candidates for dielectric waveguide materials.

Optical fibers are an example of a dielectric waveguide

Dielectric waveguides are fundamental components in integrated optic circuits. The optical field in integrated optics, called an optical waveguide, is formed by dielectric waveguides to guide waves in the optical spectrum. With the emergence of monolithic integrated circuits, dielectric waveguides replaced classical optical systems consisting of lenses, light sources, and mirrors.

The optical functionalities of dielectric waveguides are used in integrated optic applications such as sensors, biomedical devices, printers, optical data storage systems, and optical fiber communications. There are a variety of materials that can be used to make dielectric waveguides, each with its own unique properties. When choosing a material to fabricate a dielectric waveguide, it is important to note these properties to ensure you choose a material that fits your intended application.

Let’s further explore how dielectric waveguides work as well as the various materials they are made of. 

Dielectric Waveguides  

The simplest form of dielectric waveguide consists of a planar film of material with refractive index nr sandwiched between a substrate and cover. The light is guided by the phenomenon of total internal reflection and is internally reflected between the film-substrate and film-cover interfaces, propagating forward to a target destination.

The refractive indices of the materials used in a dielectric waveguide are critical in guiding the light from source to destination. The refractive index of substrate ns and cover nc should be lower than the refractive index nr. Usually, the cover material is air, which is of a refractive index equal to unity. Typically, the refractive index value differences are in the range of 10-3 to 10-1 and the film thickness is 1µm.

A dielectric waveguide can be made of various materials and the refractive index is an important parameter that determines the characteristics of the dielectric waveguide structure.

Dielectric Waveguide Materials 

There are an array of materials available that can be used to fabricate dielectric waveguides. Generally, techniques such as diffusion, chemical vapor deposition, sputtering, etching, ion implantation, and epitaxial growth are used for fabricating dielectric waveguides. Depending on the materials used to construct the dielectric waveguide, the method of fabrication changes. 

Some common materials that are used in the fabrication of dielectric waveguides are discussed below. 

Glass

Diffusion and deposition technology are used for the fabrication of dielectric waveguides in glass. Glass-made waveguides have a large cross-section, and this allows easy coupling with optical fibers. The low refractive index contrast makes the size of glass dielectric waveguides larger than other types of waveguides. 

Lithium Niobate (LiNbO3)

The electro-optical and acoustic-optical properties of lithium niobate make it the best candidate for dielectric waveguide material. Lithium niobate dielectric waveguides are usually used for optical switches due to their strong electro-optical properties. The method of fabrication for this type of waveguide is usually diffusion technology. It is worth noting that waveguides made of lithium niobate are polarization-dependent. 

Indium Gallium Arsenide Phosphide / Indium Phosphide (InGaAsP/InP)

Dielectric waveguides made of InGaAsP/InP are relatively small in size due to their high refractive index and high index contrasts. They are usually used in devices where laser diodes and photodiodes are integrated with passive components. 

Aluminium Gallium Arsenide/ Gallium Arsenide (AlGaAs/GaAs)

Dielectric waveguides made of AlGaAs are typically used for short-distance optical communication.

Silicon-on-Insulator (SOI)

Silicon-on-insulator waveguides are fabricated in the silicon layer, which is connected to a silicon wafer through an intermediate silica layer. There are two variants of silicon-on-insulator dielectric waveguides: fiber-matched dielectric waveguides and high-contrast dielectric waveguides. With silicon-on-insulator materials, the fabrication process can be done using standard CMOS technology.

According to the wavelength of electromagnetic waves, the material choices for dielectric waveguides vary.  Cadence software offers tools to select and manage materials for fabricating dielectric waveguides. 

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