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Optical Parametric Devices
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
In the QPM process, the orientation of the electric dipoles in the non-linear material is periodically reversed by 180° along the pump propagation direction (Figure 23.9). A practical method to achieve such domain reversal is through periodic poling of ferroelectric materials during the fabrication process by applying a high electrical field (several kV) across the crystal using patterned electrodes. The result of periodic poling is that the radiated optical waves from the oscillating dipoles in consecutive domains become out of phase by π in both time and space. The domain reversal is equivalent to a periodic change in the sign of the effective non-linear coefficient between +deff and –deff. If the poling period, Λ, is made to correspond to two coherence lengths for the non-linear interaction (Λ = 2ℓc), then the phase of the generated waves is periodically reversed by π for every two coherence lengths. This periodic re-adjustment of phase preserves a constructive relative phase between the pump and parametric waves (albeit in a quasi-continuous manner) as they propagate through the material. This prevents the waves from slipping out of phase after only one coherence length, as would be the case under normal dispersion.
Background of Physical and Geometrical Optics for Holography
Published in Raymond K. Kostuk, Holography, 2019
Quantitative measurements of the temporal coherence length can be made using a Michelson interferometer as shown in Figure 2.6. In this device, incident beam from the source is collimated and then divided by a beam splitter sending half the power to two mirrors M1 and M2. The light is reflected by the mirrors, passes again through the beam splitter, and is combined and displayed on a screen. The mirrors start out at equal distances from the beam splitter ensuring equal optical path lengths (i.e., z1−z2=0). One of the mirrors is then moved to increase the difference in optical path length. If one of the mirrors is tilted, a set of fringes will appear across the screen allowing the maximum and minimum intensity of the interference pattern to be measured. The variation in intensity can be expressed in terms of the visibility defined as: () V=Imax−IminImax+Imin.
C
Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
coherence length distance over which the amplitude and phase of a wave can be predicted. coherence time the time over which the effect of communication channel can be assumed constant. Signals of duration less than this can be transmitted without significant distortion. coherent integration where magnitude and phase of received signals are preserved in summation. coherent acousto-optical processor acoustooptical (AO) signal processor where the light is amplitude-modulated by the acoustic wave in the AO device as opposed to intensity or power modulated. coherent detection detection technique in which the signal beam is mixed with a locally generated laser beam at the receiver. This results in improved receiver sensitivity and in improved receiver discrimination between closely spaced carriers. coherent illumination a type of illumination resulting from a point source of light that illuminates the mask with light from only one direction. This is more correctly called "spatially coherent illumination."
Statistical properties of a spatiotemporally partially coherent vector cosine-Gaussian-correlated pulsed beam with radial polarization in atmospheric turbulence
Published in Waves in Random and Complex Media, 2021
Yan Li, Ming Gao, Hong Lv, Liguo Wang, Bin Li, Shenhe Ren, Pengli Wu
Figure 9 illustrates the modulus of the two-point, single-frequency spectral DOC of a radially polarized CGCSMP beam for different values of at L = 10 km. It can be inferred that the spectral DOC of a conventional radially polarized GSMP beam is a typical circular symmetry distribution and the spatial coherence length decreases as increases (Figure 9(a1–a3)). For the case in which n ≠ 0, both the shape and the symmetry of the spectral DOC change on propagation in turbulence, and eventually it exhibits a Gaussian shape, which quite differs distinctly from its free space behavior. In addition, owing to the diffraction, the spatial coherence length increases considerably.
Blind structured illumination as excitation for super-resolution photothermal radiometry
Published in Quantitative InfraRed Thermography Journal, 2020
Peter Burgholzer, Thomas Berer, Mathias Ziegler, Erik Thiel, Samim Ahmadi, Jürgen Gruber, Günther Mayr, Günther Hendorfer
Usually, a reflection mode set-up is needed for NDE applications or for biomedical imaging, as there is access only to the front side of the sample. The structured heating could be performed inside the sample, e.g. by speckled illumination inside a light scattering sample/tissue. In [10] we have used ‘far-field’ speckles from a diffusor outside of the sample for excitation, which could be changed by rotating the diffusor. In turbid scattering media, a coherent illumination from an excitation laser produces unknown speckle-patterns inside the sample [16]. In a reflection mode set-up, such ‘near-field’ speckles, which emerge when coherent light propagates through a scattering sample, could be the illumination pattern used for structured heating. Absorbing structures inside the sample do not ‘see’ a homogeneous illumination but such a speckle-pattern, which is unknown (blind structured illumination). The size of near-field speckles inside the scattering sample material is quite small, approximately half the wavelength of the light [16]. Therefore, speckles from mid-infrared lasers or microwaves could be used to get larger speckles. The coherence length of the lasers should be significantly higher than the penetration depth to assure interference of the scattered light.