- Velocity of light 4/21/2024 Definition: The speed of a particle is a very simple and clear concept, but the speed of light (and other wave phenomena) is a very complex issue. There are several different concepts of speed (especially when light travels through a medium), and they have different numerical values: Phase velocity is the speed at which the phase wavefront propagates. The group velocity is the speed at which ...
- Gouy phase shift 4/14/2024 Definition: Additional phase shift produced during propagation of a focused Gaussian beam. (Often incorrectly written as "Guoy Phase Shift") Gaussian beams produce an additional phase shift in the direction of propagation, which is different from the phase shift produced by plane waves of the same frequency. This different phase shift is called the Guyi phase shift: where zR is the Rayleigh...
- Beam parameter product 4/6/2024 Definition: Abbreviation: BPP Product of beam radius at focus and far-field divergence angle. The beam parameter product (BPP) of a laser beam is the product of the beam radius (at the beam waist) and the half divergence angle (far field). Commonly used units are mm mrad (millimeters times milliradians). BPP is usually used to characterize the beam quality of laser beams: the greater the beam ...
- Beam parameter product 3/31/2024 Definition: Abbreviation: BPP Product of beam radius at focus and far-field divergence angle. The beam parameter product (BPP) of a laser beam is the product of the beam radius (at the beam waist) and the half divergence angle (far field). Commonly used units are mm mrad (millimeters times milliradians). BPP is usually used to characterize the beam quality of laser beams: the greater the beam ...
- Spectrometers 3/24/2024 Definition: A device for recording spectra. A spectrometer is an instrument used to record the spectrum of a light source. That is, it measures the power spectral density (PSD) of light as a function of wavelength or frequency. Not all spectrometers give calibrated PSDs; often the intensity is not calibrated and the calibration factor (responsivity) is wavelength dependent. Using the spectral...
- Superluminal transmission 3/17/2024 Definition: Certain quantities travel faster than the speed of light in a vacuum. Under certain conditions, the phase velocity or group velocity of light may be greater than the vacuum speed of light c. It seems that information can also be transmitted faster than the speed of light, but this is contrary to Einstein's theory of relativity, because this theory holds that super-light transmissio...
- Sellmeier formula 3/10/2024 Definition: Equation for calculating the refractive index as a function of wavelength. When characterizing the refractive index as a function of wavelength in transparent optical materials, the Sellmeier formula [1] (also known as the Sellmeier equation or the Sellmeier dispersion formula) is usually used. A typical form is: This form is derived from a simple physics model that is only suit...
- Brewster windows 3/3/2024 Definition: A transparent disk pointed at Brewster's angle. In some cases when a beam of light passes through some transparent window, the loss in the window must be very small. A typical case is a helium-neon laser sealed by a glass tube and containing an external cavity mirror, as shown in Figure 1, where the glass window isolates the gas mixture in the laser from the outside air. Assuming t...
- Bragg gratings 3/3/2024 Definition: Reflective structures containing periodic refractive index modulation. An optical Bragg grating is a transparent device with a periodically changing refractive index. The reflectivity is very large in a wavelength region (bandwidth) near a specific wavelength, which satisfies the Bragg condition: where λ is the vacuum wavelength, n is the refractive index, θ is the propagation a...
- Wavenumber 2/16/2024 Definition: Phase delay per unit length, or this value divided by 2π. There are different definitions of wavenumber. The physical definition is: where λ is the wavelength in the medium (not the vacuum wavelength). The wave number is the size of the wave vector and the phase delay per unit length during the propagation of a plane wave. Another definition: The unit is cm-1, which is commo...
- Flat-top beams 2/4/2024 Definition: The beam has a flat intensity cross-section. A flat-top beam refers to a laser beam (usually a converted laser beam) whose intensity cross-section is flat over a large area. This is different from a Gaussian beam, where the intensity of a Gaussian beam decreases gently from a maximum value outward along the beam axis to 0. This type of beam is required in some laser applications. ...
- Gaussian beams 1/28/2024 Definition: The electric field of a beam on a plane perpendicular to the optical axis can be expressed by the Gaussian equation, sometimes with an additional parabolic phase curve. Gaussian beams and resonator modes If the resonator is stable, the optical medium in the resonator is isotropic, and the medium surface is either flat or parabolic, then the lowest-order mode (TEM00 or transverse f...
- Ultraviolet light 1/21/2024 Definition: Invisible light with a wavelength less than about 400nm. Ultraviolet light is light with a wavelength less than about 400 nm (the lower limit of visible light wavelengths). There are several different definitions for distinguishing different spectral regions: Near UV spectrum region from 400nm-300nm. Mid-UV light ranges from 300nm-200nm, while 200nm-10nm belongs to the far UV reg...
- Instantaneous Frequency 1/15/2024 Definition: The differential of the oscillation phase with time divided by 2π. When describing non-monochromatic signals, instantaneous frequency needs to be used, and its definition is: That is, the differential of the oscillation phase φ with respect to time. (Removing the factor 1/2π gives you the instantaneous angular frequency.) Unlike the Fourier frequency, the instantaneous frequenc...
- Collimated beams 1/7/2024 Definition: A laser beam with a small divergence angle. A collimated beam is a beam (usually a laser beam) that has a small beam divergence angle so that the beam radius does not change significantly after a certain propagation distance. The simplest and most common case is a Gaussian beam, which means that the Rayleigh length is longer than the assumed propagation distance. A Shack-Hartman...
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