May 6 2014
Optical coherence is associated with interference effects of light. A field of light is said to be coherent when the electric field values at different locations are in a fixed phase.
The light waves that come in to contact with each other are said to be ‘coherent’ if they produce an interference pattern, ‘incoherent’ if they do not produce the interference pattern, and ‘partially coherent’ if they produce a ‘less-than-perfect’ pattern.
Under the conditions of high optical coherence, images develop interference fringes resulting from the interference of coherent wavefronts that are reflected by optical components, such as mirrors, lenses, and so on. This pattern appears as highly defined concentric circles superimposed on the image, thereby obscuring the details of the image.
Types of Optical Coherence
There are two different types of optical coherence, which are the following:
- Spatial coherence is a strong correlation between the values of electric field at different points in the beam profile. For instance, electric fields at different locations of a laser beam cross-section, having diffraction-limited beam quality, will oscillate in a correlated manner - even in the presence of superposition of various frequency components. Spatial coherence is necessary to achieve strong directionality of laser beams
- Temporal coherence, by contrast, is a correlation between the electric field values at a single point, but at different time intervals. For instance, the high temporal coherence of a single-frequency laser output is due to the temporal sinusoidal oscillation of electric field over an extended time period
Determination of Optical Coherence
The degree of optical coherence can be quantified as a function of a spatial or temporal distance by means of correlation functions. These functions are of different orders, such as the first order correlation function related to the optical spectrum, and the second-order function associated with intensity of photons.
The coherence time quantifies the degree of first-order temporal coherence, and the coherence length indicates the vacuum velocity of light.
Fringe visibility parameters, on the other hand, define the visibility of the interference pattern formed as a result of the superposition of two electric fields. The line width of a laser is related to temporal coherence - a narrow line width specifies high temporal coherence.
Applications of Optical Coherence
Optical coherence principle is employed in the following applications:
- Holography
- Optical sensors, such as fiber optic sensors
- Interferometry
- Laser projection displays
- Imaging applications
References