Research progress in phase modulation of partially coherent beam and its fundamental applications

Abstract

It is well known that the propagation properties of a laser beam are closely determined by its amplitude, polarization, phase and coherence. Usually the coherence length of a laser beam is extremely large and the laser beam is assumed to be a coherent beam, which is characterized by the electric field. A laser beam with small coherence length (i.e., partially coherent beam) is characterized by the correlation function of the electric field and is preferred in many applications, such as inertial confinement fusion, optical imaging, atom cooling, atomic recoil lasing, free-space optical communications, particle trapping, optical scattering, and second-harmonic generation. Partially coherent beam has many exciting optical properties, e.g., the degree of polarization and the state of polarization of a vector partially coherent beam may vary on propagation, spectral shift may appear in a polychromatic partially coherent beam on propagation, and the correlation function of a partially coherent beam can induce self-shaping, self-splitting and self-focusing properties. Partially coherent beam carrying vortex phase possesses orbital angular momentum, and such beam carries unique correlation singularities. Partially coherent beam carrying twist phase also possess orbital angular momentum. The twist phase opens up “a new dimension” in the area of partially coherent field, and it differs in many aspects from the customary quadratic phase factor. The twist phase is bounded in strength and is absent in a coherent beam, and such phase has an intrinsic chiral or handedness property and is responsible for the rotation of the beam spot on propagation. An essential aspect of the twist phase is that it is intrinsically two-dimensional, and it cannot be separated into a sum of one-dimensional contributions. The requirements on the phase properties of partially coherent beam are different in various applications, thus it is necessary to modulate the optical properties of such beam. In this paper, we present a review of the developments on phase modulation and fundamental application of partially coherent beam, and we mainly introduce the theoretical model, experimental generation, optical properties and related applications of partially coherent beam carrying vortex phase or twist phase. It is shown that the propagation properties of partially coherent beam are closely determined by its initial phase, and phase modulation has important and promising applications in beam shaping, particle trapping, free-space optical communication, optical imaging and nonlinear optics.