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The Strip-Loaded Slot Waveguide: A Combination of Slot and Strip-Loaded Waveguides

Key Takeaways

  • A channel waveguide is a non-planar waveguide with two-dimensional transverse optical confinement.

  • A strip-loaded waveguide offers advantages like easy fabrication and high tolerance ability against fabrication imperfections. 

  • Advantages of strip-loaded slot waveguides include the excellent confinement of fields, the absence of scattering, simple fabrication, low waveguide losses, and single-mode and multimode waveguide operation.

Photonic circuits

The propagation of high-frequency electromagnetic waves is usually executed using waveguide structures. Among waveguides, dielectric waveguides have become popular due to their useful propagation behavior and dispersion-free transmission over long distances. Dielectric waveguides are classified into planar and non-planar waveguides. There are two subcategories of non-planar dielectric waveguides: channel waveguides and optical fibers.

Channel waveguides have two-dimensional transverse optical confinement and are used in photonic circuits, optical sensors, and lasers. The combination of two types of channel waveguides—strip-loaded waveguides and slot waveguides—is called a strip-loaded slot waveguide.

Strip-loaded slot waveguides are an emerging trend in photonic integrated circuits. To better understand this type of hybrid waveguide, let’s first discuss the advantages and characteristics of the strip-loaded waveguides and slot waveguides that strip-loaded slot waveguides come from. 

Understanding Channel Waveguides

In most channel waveguides, there are two materials of different refractive index, namely core and cladding. The cladding surrounds the core in all transverse directions. The index of refraction, n(x,y), in the channel waveguide is dependent on both x and y coordinates. 

The Classifications of Channel Waveguides

Channel waveguides can be classified into the following types:

  1. Buried channel waveguides
  2. Ridge waveguides
  3. Rib waveguides
  4. Diffused waveguides
  5. Strip-loaded waveguides
  6. Slot waveguides

Strip-Loaded Waveguides

In a strip-loaded waveguide, the optical confinement in the x-direction is provided by a planar waveguide. The planar waveguide is made of material with a refractive index n1. The planar waveguide is loaded with a dielectric strip of refractive index n3. The refractive index of loading dielectric strip n3 is less than the refractive index of planar waveguide n1 to avoid the leakage of electromagnetic waves.

A loading dielectric strip provides the optical confinement in the y-direction. The waveguiding region in a strip-loaded waveguide is the region with index n1. The strip-loaded waveguide is defined by thickness, d, given by the thickness of the planar waveguide, and width, w, given by the width of the loading dielectric strip.

A strip-loaded waveguide can be called a low refractive index contrast waveguide, as it offers features such as easy fabrication and high tolerance ability against fabrication imperfections. The strip-loaded waveguide offers a lot of flexibility in terms of materials, features, and fabrication.

The guiding layer in a strip-loaded waveguide is not patterned and does not require any processing after deposition. Therefore, any material can be utilized for fabricating the guiding layer. Typically used materials include lithium niobate, titanium dioxide, and glasses. Polymer strips are suitable to use as loading dielectric strips in a strip-loaded waveguide, as their refractive index is low. The ease of patterning increases the use of polymer strips in strip-loaded waveguides.

Next, let’s discuss another common type of channel waveguide—slot waveguides.

Slot Waveguides

Slot waveguides allow an electromagnetic wave to be guided through a thin gap. They consist of two rails separated by a thin slot. The refractive index of the rails is greater than the refractive index of the slot. A slot waveguide offers an extremely confined wave propagation mode in the low refractive index medium.

Slot waveguides are fabricated by an etching process. Therefore, the height of the rails is finite in slot waveguides. The structure of slot waveguides makes it possible to use them in integrated optical devices, non-linear optics, telecommunication systems, and sensors.

Horizontal Slot Waveguides

A horizontal slot waveguide is a type of slot waveguide where the layers are etched for the lateral confinement of the guided mode. Horizontal slot waveguides are lossy due to the overlapping of electromagnetic waves with rough boundaries. The solution to avoid these losses is to keep the edges of the waveguide apart from the guided modes. Introducing the strip-loading technique to a horizontal slot waveguide forms a strip-loaded slot waveguide, and this waveguide structure is effective at reducing waveguide losses. 

Strip-Loaded Slot Waveguides

In a strip-loaded slot waveguide, a horizontal slot waveguide forms the base of the entire structure. A dielectric strip is loaded on top of the slot waveguide, which increases the effective index of the wave propagating mode. This leads to the formation of a strip-loaded slot waveguide with the two-dimensional confinement property. Vertical confinement is provided by a slot waveguide, whereas a loaded dielectric strip gives lateral confinement.

Advantages of strip-loaded slot waveguides include the excellent confinement of fields, the absence of scattering, simple fabrication, low waveguide losses, and single-mode and multimode waveguide operation. In integrated photonic circuits or optical sensors, strip-loaded slot waveguides are recommended over slot or strip-loaded waveguides. 

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