ABSTRACT

Interference is the ability of light waves when combined or interfered to produce interference fringes. Monochromatic light easily produces interference fringes and speckle patterns over a wide variety of situations. Incoherent light, such as sunlight and light from LEDs, only produces interference fringes in very restricted conditions where the optical path lengths OPL of the combined beams differ by less than a few wavelengths. The visibility of interference fringes is highest when the polarization states of the interfering beams are aligned and decreases as the angle between polarization vectors increases. The fringe visibility becomes zero when orthogonal polarizations interfered. Polarization fringes are a modulation of the polarization state. Orthogonal polarization states do not produce interference (intensity) fringes, only polarization fringes. Even though the fringe visibility may be zero, an interference pattern may recovered by introducing a polarizer oriented at an intermediate angle. The interference of right and left circularly polarized plane waves produces polarization fringes with a rotating linearly polarized state. Young’s double slit generates interference by division of wavefront, taking two separate pieces from a wavefront and combining them. Most interferometers operate by division of amplitude, splitting the amplitude of a wave with a beamsplitter or diffractive optical element such as a grating, to create two or more beams. Beamsplitters can be designed to be non-polarizing, where the polarization changes are minimal. Polarizing beamsplitters transmit the p-polarized component is transmitted and reflect the s-polarized component. To produce holograms with good fringes visibility, the polarization states of the reference beam and the test beam should be nearly the same, which is polarized orthogonal to the test and reference propagation directions. Generally, polarization in the plane of the test and reference beams produces holograms of poor visibility. The addition of monochromatic beams of two different frequencies does not produce a polarization ellipse but a more general shape, a time-varying polarization state which appears as an evolving ellipse. When combining polychromatic beams, the degree of polarization is generally reduced since most of the pairs of frequency components in two white light beams are different. Mode locked laser pulses with time varying polarization states are used in quantum optics and spectroscopy to get fine control over quantum states and transitions. Complex pulses with rapidly changing frequencies and polarizations can put atoms or molecules into particular quantum states with unique density matrices.

A monochromatic beam resembles a single note from a piano, while an incoherent beam is similar to the sound played by leaning on many piano keys with your arm. Adding Jones vectors is similar to playing the exact same note on several different pianos. These sound waves will constructively and destructively interfere, and interference fringes of sound can be set up in the room. Adding Stokes parameters is like having several small children banging on pianos in the same room.