What is the use of distributed Bragg reflector?

Distributed Bragg Reflector

A Distributed Bragg Reflector (DBR) is an optical structure that is used to reflect specific wavelengths of light while transmitting others. It is essentially a mirror with a high degree of wavelength selectivity, formed from a structure known as a one-dimensional photonic crystal, which consists of multiple layers of alternating high and low refractive index materials.

Working Principle

The working principle of a DBR is based on the Bragg's law of diffraction, which takes place at the interfaces of different refractive indices. When light encounters the boundary between different layers, part of the light is reflected and part is transmitted. The reflected light from all interfaces can constructively interfere if the optical thickness (refractive index times physical thickness) of each layer is one-quarter of the desired wavelength of light. This constructive interference greatly enhances the reflection at the particular design wavelength while other wavelengths are less affected.

Applications

• Laser diodes: DBRs are often used in the construction of laser diodes, especially vertical-cavity surface-emitting lasers (VCSELs), to form the resonant cavity that allows for lasing at a specific wavelength.
• Optical filters: By designing the layer structure appropriately, DBRs can act as optical filters to selectively filter out certain wavelengths while allowing others to pass.
• Sensors: Due to their sensitivity to changes in refractive index, DBRs can be employed in sensor applications, where they can detect biological or chemical substances by monitoring shifts in the reflected wavelength.

Advantages

• High reflectivity: Designed correctly, DBRs can achieve close to 100% reflectivity for certain wavelengths, making them very efficient reflectors.
• Wavelength specificity: The precise control over reflected wavelengths is advantageous for creating narrow-bandwidth mirrors.
• Compactness: Their thin-film nature makes DBRs ideal for integration into compact optical devices.

Fabrication

Fabrication of DBRs usually involves methods such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), or sputtering. These techniques allow for the control of layer thickness and composition, critical to achieving the desired optical properties.

In conclusion, Distributed Bragg Reflectors are versatile optical components that are widely used in photonics. Their design and functionality are integral to a variety of applications where specific control over light reflection is necessary.