Using Buried Channel Waveguides in Integrated Optical Circuits

Key Takeaways

• A buried channel waveguide consists of a core and cladding. 

• Buried channel waveguides offer low propagation loss.

• Buried channel waveguides come with two-dimensional optical confinement, similar to any other channel waveguide. 

Optical devices need waveguiding structures to transmit electromagnetic waves

Optical communication systems and optical devices require waveguiding structures to transmit electromagnetic waves. Optical waveguides are the spatially inhomogeneous structures used to guide light. A channel waveguide is a basic optical waveguide used in most integrated optical devices. These waveguides offer two-dimensional optical confinement. 

Let’s take a look at a specific type of channel waveguide: the buried channel waveguide. 

Buried Channel Waveguides

A buried channel waveguide consists of a core and cladding. The core of the high refractive index (n₁)  is buried in a low refractive index (n₂) medium called cladding. A buried channel waveguide can also be called an embedded waveguide or embossed waveguide. 

The Advantages of Buried Channel Waveguides

Buried channel waveguides offer low propagation loss, typically below 1dB/m with smooth surfaces. The smooth wall surfaces of buried channel waveguides provide low transverse scattering loss, thereby increasing performance. These waveguides come with two-dimensional optical confinement, similar to any other channel waveguide. 

Where Are Buried Channel Waveguides Used?

Buried channel waveguides are used in: 

• Integrated optical devices
• Interferometers
• Splitters
• Switches
• Polarizers

They are also used as optical interconnects in bends and joints. Another common application of buried channel waveguides is in couplers. 

Buried Channel Waveguide Couplers

Buried channel waveguide-based couplers are widely used in integrated optic systems. These couplers can be based on single-core buried channel waveguides or multi-core buried channel waveguides. 

In a buried channel waveguide coupler with two parallel cores, there are two main modes of propagation—fundamental supermode and odd supermode. 

Fundamental Supermode

If there are no depressed regions in the refractive index profile of the coupler, the fundamental supermode can support the propagation of all electromagnetic waves. There is no cutoff frequency for the fundamental supermode in a buried channel waveguide coupler with two parallel cores.  

Odd Supermode

In odd supermode, there is a cutoff frequency, and above this frequency, the propagation of electromagnetic waves is not possible. 

Cutoff Wavelength

The cutoff wavelength of each supermode of a buried channel optical waveguide coupler is dependent on the distance between the parallel cores, the wavelength of electromagnetic waves, the refractive index of the core, and the refractive index of the cladding.

Next, let’s see how the characteristics of buried channel waveguides function in real-life applications, particularly in silicon-on-insulator structures and in the porous silicon substrate. 

Buried Oxide Silicon-On-Insulator Optical Waveguides

Buried oxide silicon-on-insulator optical waveguides are often used in microelectronics due to their robustness and low cost. These structures consist of a silicon dioxide layer of low refractive index buried in a cladding of high refractive index silicon substrate. Due to the high indexed refraction of the cladding, buried oxide silicon-on-insulator optical waveguides have no lossless guided mode. They support low-loss leaky optical waveguide modes.

Porous Silicon Buried Waveguides

In integrated optics and telecommunications, the optical properties of porous silicon and buried channel technology are combined to form porous silicon buried waveguides. Buried waveguides made of  porous silicon show the optical transparency of the guide. The use of porous silicon has reduced absorption loss in the visible light frequency range. The core-cladding interface quality influences the vertical confinement of porous silicon buried waveguides. 

As we have discussed, the buried channel waveguide technologies incorporated in various structures, such as silicon-on-insulators or couplers, help achieve high quality performances in optical and telecommunication applications. The possibility of limiting absorption loss, scattering loss, and propagation loss in buried channel waveguides with precise fabrication has increased their demand in integrated optics. 

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