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Gas-dynamic and chemical lasers: gas dynamics
Published in E R Pike, High-power Gas Lasers, 1975, 2020
Consider a beam of radius a, wavelength λ, which propagates a distance L between mirrors. One can then define the Fresnel number NF = a2/Lλ for the cavity. The Fresnel number is a measure of diffraction effects; for example a small Fresnel number indicates a large beam spread due to diffraction. Large amounts of cross-coupling occur across the optical beam for a rapidly spreading beam. Such a cross-coupling locks the phases of the emitting molecules and in that fashion one can obtain a phase-coherent mode.
Lasers
Published in Daniel Malacara-Hernández, Brian J. Thompson, Advanced Optical Instruments and Techniques, 2017
Vincente Aboites, Mario Wilson
The diffraction losses in a laser are characterized by the resonator Fresnel number N. This is a dimensionless parameter given as N=a2Lλ, where α is the radius of the mirror resonator. A large Fresnel number implies small diffraction losses.
Three-dimensional Waves
Published in Jakob J Stamnes, Waves in Focal Regions, 2017
which shows that the larger the Fresnel number N, the smaller the focal volume. For a sufficiently large N, SN will be so small that within the focal volume (u, v) and (u′, υ′) will be practically the same.
A precise approach to determining the optimum location of observation plane or optimum size of array aperture to achieve maximum power-in-the bucket in the coherent beam combining
Published in Journal of Modern Optics, 2020
Naser Siahvashi, Moslem Hamdami, Atoosa Sadat Arabanian, Reza Massudi
Accordingly, Figures 2, 4 and 5 demonstrate that there is a relation between the size of the array aperture and the distance of the observation plane when PIB is maximum. On the other hand, the concept of Fresnel zones and Fresnel number provides a measure of the resulting field in the observation plane due to the interference of emitted fields from different Fresnel zones. The Fresnel number is defined as follows: where R and Z are the zone radius and location of the observation plane, respectively. One can formulate the relation between the observation plane distance and the array aperture size in the coherent beam combining so that R and Z correspond to the array aperture size and the observation plane distance, respectively.