channel waveguides (original) (raw)

Author: the photonics expert (RP)

Definition: waveguides with the form of a channel at the surface of a host medium

Alternative terms: strip waveguides, ridge waveguides

Category: fiber optics and waveguides

• waveguides
  • planar waveguides
  • channel waveguides
  • optical fibers

Related: waveguidesphotonic integrated circuitssilicon photonicswaveguide lasers

Opposite term: planar waveguides

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DOI: 10.61835/awj Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Channel Waveguides?

Channel waveguides (also called strip waveguides or ridge waveguide) are a class of waveguides having the form of a channel running along the surface of some solid transparent host medium — a dielectric or a semiconductor. In contrast to planar waveguides, they provide guidance of light not only in one dimension, but in two dimensions — similar to an optical fiber, only that channel waveguides are usually stiff, i.e., they cannot be bent. However, they are not necessarily straight, but can also define a curved path.

Figure 1: A simple channel waveguide, made on a surface.

Some channel waveguides are directly at the surface of the device, so that vertical light guidance is influenced by the high refractive index contrast between the waveguide material and air. Possible disadvantages arise from the asymmetry of the waveguide modes and from the high sensitivity to irregularities of the surface, which can cause high propagation loss due to light scattering. Therefore, one sometimes realizes buried waveguides (or embedded waveguides) by covering the waveguides with an additional material, often having a similar refractive index as the substrate.

Channel waveguides are often designed to support only a single guided mode at the intended operating wavelength, just as single-mode fibers.

Fabrication Methods

Channel waveguides can be made on different kinds of materials, and the used fabrication methods can be very different for different materials.

Waveguides in Semiconductors

In the case of semiconductors, epitaxial techniques in combination with masks are generally used for generating waveguide structures — for example, those in various kinds of laser diodes and semiconductor optical amplifiers (SOAs). Usually, the waveguides have a relatively high numerical aperture. Often, they are covered by an electrode for injecting an electric current into the active region of a laser diode or an amplifier.

Waveguides in Dielectric Materials

Channel waveguides can be fabricated in various dielectric (insulator) materials, both in crystalline and amorphous materials — for example, in silica and in nonlinear crystal materials such as lithium niobate and lithium tantalate. Profoundly different fabrication techniques are available for such purposes — some examples:

• One may apply ion exchange techniques. Here, a strip on the surface of the substrate is exposed to some liquid over some time, and certain ions in the substrate are exchanged by others in a region just below the exposed strip surface. As a result, the refractive index is increased, allowing that region to guide light.
• Another possibility is to deposit a metallic strip (e.g. of titanium) on a substrate and then apply a high temperature to the whole device over a couple of hours or even days. One obtains some degree of diffusion of the metal into the dielectric, which also leads to an increase of refractive index.

Waveguide Properties

The following waveguide properties are particularly relevant for applications:

• The transverse shape and size of the guided waveguide modes are relevant in various respects. For example, this determines the required beam profile for efficiently launching light into the waveguide. Also, the effective mode area is relevant for the strength of any nonlinear interactions in the waveguide.
• Largely depending on the numerical aperture (and thus on the refractive index contrast), a waveguide is to some extent sensitive to bend losses. For example, waveguides with very high tolerance to bending, down to bend radii far below 1 mm, can be made on silicon and other semiconductors, which is important for photonic integrated circuits.
• The propagation losses tend to increase with increasing refractive index contrast. However, relatively high propagation losses (e.g. >1 dB/mm) may often be tolerated in devices where the waveguides are rather short.
• For some applications, it is essential to have means for influencing the light propagation in the waveguide — for example, to apply electric fields for modulating the radiation or electric currents for obtaining gain in a semiconductor.

Applications of Channel Waveguides

Some typical applications of channel waveguides are explained in the following:

• Photonic integrated circuits contain channel waveguides just as electronic integrated circuits contain tiny wires. The waveguides connect optical inputs, various components and optical outputs with each other. Such circuits are used for signal processing in optical fiber communications, for example.
• In active semiconductor devices such as laser diodes, semiconductor optical amplifiers and electroabsorption modulators, channel waveguides confined the light to a small cross-section where it is exposed to laser gain or a modulating influence.
• Other kinds of waveguide lasers and amplifiers can be made with waveguides which are doped with rare earth ions or transition metal ions. Due to the smaller mode area, lasers with very low laser threshold power can be made.
• Waveguides in nonlinear crystal materials allow for nonlinear frequency conversion (e.g. frequency doubling or optical parametric oscillation) or for electrooptic modulation.
• There are optical sensors, where the active sensor element is a channel waveguide, the top surface of which is exposed to external influences. For example, an evanescent optical field may penetrate a liquid above the waveguide, so that the guided light can interact with chemicals in the liquid. One may then apply laser absorption spectroscopy for identifying chemical species and measuring their concentrations.

Frequently Asked Questions

What is a channel waveguide?

A channel waveguide, also called a strip waveguide, is a structure that guides light in two dimensions along a channel on the surface of a solid transparent material.

What is the purpose of a buried channel waveguide?

A buried waveguide is covered with an additional material to protect it from surface irregularities. This reduces propagation loss from light scattering and makes the guided light mode more symmetric.

How are channel waveguides fabricated?

Fabrication methods depend on the material. For semiconductors, epitaxial techniques are common. For dielectric materials, methods include ion exchange or the diffusion of a metallic strip into the substrate at high temperatures.

Why is a high refractive index contrast useful in channel waveguides?

A high refractive index contrast results in strong light confinement, allowing for waveguides with very small bend radii (far below 1 mm). This is particularly important for compact photonic integrated circuits. However, it also tends to increase scattering losses.

What are some key applications of channel waveguides?

They are fundamental for routing light in photonic integrated circuits. They are also used in active devices like laser diodes and modulators, in waveguide lasers, and for optical sensors.

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