The Role of Planar Waveguides in Sensing Applications

Key Takeaways

• Optical sensors can be classified into two main types: fiber optic sensors and planar waveguide sensors. 

• Planar waveguides are optical structures that confine the optical radiation in the direction of propagation.

• There are two independent electromagnetic modes in planar waveguides: transverse electric (TE) mode and transverse magnetic (TM) mode.

Integrated optical sensors that depend on planar waveguides are often used in sensing applications

Integrated optics find numerous applications in sensor technologies due to advantages such as precise sensitivity, compactness, and wide dynamic range accuracy. In integrated optical sensors, the sensing region consists of an optical waveguiding system that helps in sensing the measurements accurately.

Integrated optical sensors that depend on planar waveguides are popular for merits such as robustness, electromagnetic immunity, compactness, low cost, short response time, and high sensitivity. They also exhibit the inherent capability of integrating with fiber-optic networks. In this article, we will discuss the types and modes of planar waveguides, which are the basic building blocks of planar waveguide optical sensors.  

Optical Sensors

Optical sensors utilize the principles of optics for sensing mechanisms. Sensing is achieved with the change of light energy through chemical, biochemical, or mechanical processes. During these processes, light energy is converted into a detectable signal, thus fulfilling sensing requirements.

Compared to existing sensing technologies, optical sensors are much more effective due to their accuracy, versatility, and strength. Optical sensors can be used independently or in combination with other types of sensors. In either case, optical sensors provide a vast amount of information about measurement quantities. Optical sensors are often used in pharmaceuticals, the biotechnology industry, petroleum adulteration detection, clinical diagnostics, and medical systems.

Optical sensors can be classified into two main types:

1. Fiber optic sensors - Rely on optical fibers.
2. Planar waveguide sensors - Use planar waveguides to guide light in the sensor.

The usability of planar waveguide optical sensors is high. The principle of optical waveguides forms the basis for the optical sensing mechanisms in planar waveguide sensors. Let’s take a look at planar waveguides and planar waveguide modes in the next sections.

Planar Waveguides

In integrated optical sensors, planar waveguides are used as input devices. These waveguides are optical structures that confine the optical radiation in the direction of propagation. The structure of planar waveguides consists of a core layer sandwiched between two layers of cladding. The core layer of a planar waveguide is also known as the film, which has a refractive index value denoted by n_f. Upper cladding and lower cladding are often referred to as the cover and substrate, with refractive indices n_c and n_s, respectively.

Based on the refractive index of the core, planar waveguides are subdivided into various types:

1. Symmetric planar waveguide - In a symmetric planar waveguide, the optical constants of the upper and lower cladding are equal.
2. Asymmetric planar waveguide - When the optical constants of the upper and lower cladding mediums are different, then the structure forms an asymmetric planar waveguide.
3. Step-index planar waveguide - The type of planar waveguide with the uniform refractive index for the core is called a step-index planar waveguide. The step-index planar waveguide consists of a homogeneous film surrounded by two dielectric mediums of lower refractive indices forming a cover and substrate, respectively. Usually, in a step-index planar waveguide, the refractive index of cover (nc) is less than or equal to the substrate’s refractive index (ns). 
4. Graded-index planar waveguide - If the core is made of non-homogenous material and its refractive index is dependent on depth, then such a structure forms a graded-index waveguide. In a graded-index waveguide, the interface between the core and cladding is smooth and continuous, with a gradual change in refractive indices. The refractive index is maximum at the top surface and lowest at the substrate. Graded-index planar waveguides are used to reduce modal dispersion and ease the coupling and alignment problems commonly seen in single-mode waveguides. 

Planar Waveguide Modes

The modes of planar waveguides represent the discrete waves characterized by discrete amplitudes and discrete velocities propagating through a waveguide structure. The number of supported modes depends on the thickness of a planar waveguide. There are two independent electromagnetic modes: the transverse electric (TE) mode and the transverse magnetic (TM) mode.

In guided TE mode, the electric field component along the z-axis is equal to zero (Ez=0). The only non-vanishing components in the guided TE mode are H_x, E_y, and H_z. Similarly, in guided TM mode, the magnetic field component along the z-axis is equal to zero (H_z=0). The only non-vanishing components in the guided TM mode are E_x, H_y, and E_z.

Due to their high sensitivity, imperviousness to electromagnetic interference, short detection time, compactness, low cost, and ease of interface, planar waveguide optical sensors are often used for sensing applications. Cadence’s suite of design and analysis tools can help in developing integrated electronic/photonic design automation (EPDA) environments suitable for designing integrated optical sensors based on planar waveguides as well as optical fibers.

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