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Near Field and Far Field in Photonics

Radiating Dipole

An oscillating electric dipole emits electromagnetic radiation. The near field is close to the dipole (less than one wavelength), while the far field is much farther away. Mathematical expressions for the far field are simpler than for the complete radiation field. Objects in the near field can directly affect the antenna.

Diffraction at an Aperture

When light diffracts at an optical aperture, the near field occurs when the Fresnel number is much larger than unity, while the far field is characterized by Fresnel numbers well below unity.

Imaging

Imaging devices like microscopes are limited in resolution to about half the optical wavelength in the far field. However, optical near field microscopes can achieve resolutions far below the wavelength by using a tapered fiber tip very close to the sample.

Evanescent Fields

Evanescent fields are confined to the close vicinity of an object, as seen in total internal reflection or light scattering at small dielectric particles.

Laser Beams

For a laser beam, the near field is around the focus within one effective Rayleigh length, while the far field is far from the focus where the beam radius increases linearly with distance.

Optical Fibers

The near field for light exiting an optical fiber is where the field profile has not expanded significantly. The far field is obtained at distances much larger than the Rayleigh length, where the beam profile shape is determined by the 2D Fourier transform of the near-field profile.

Source: SIEMENS Community
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