Spectral extent and pulse shape of the supercontinuum for ultrashort laser pulse (original) (raw)

1986, IEEE Journal of Quantum Electronics

The pulse shape and the generated supercontinuum spectral distribution associated with the propagation of an ultrashort intense pulse in a cubic (@') nonlinear medium are obtained by solving the electromagnetic field wave equation using the method of multiple scales. New forms for the set of quasi-linear partial differential equations describing the system, pulse distortion, and the spectral extent of the supercontinuum are the focus of this paper. S I. INTRODUCTION UPERCONTINUUM generation is the production of nearly white continuous spectrum by propagating picosecond and subpicosecond laser pulses through nonlinear media. Alfano and Shapiro [ l ] were the first to experimentally observe this phenomenon more than 15 years ago. They and others [2] used this superbroadening as a means to produce ultrashort pulses in the spectral range from ultraviolet to infrared. The shape, fine structure, and extent of the spectrum produced are functions of the nonlinear index of refraction of the medium, the shape, wavelength, duration, intensity, and phase modulation of the pump laser pulse, and the interaction length of the pulse in the medium. Typically, the observed broadened spectrum consists of larger frequency extent towards the blue than the red by factors of approximately two, a feature commonly referred to as spectral asymmetry. This coherent and ultrafast superbroad frequency band has been mainly used as a spectral tool for time-resolved absorption spectroscopy [3], [4] and nonlinear optical effects [3], [5]. Recently, new uses in engineering applications [6], such as ranging, 3-D imaging, atmospheric remote sensing, and optical fiber characterization, have been proposed. The supercontinuum was explained either as a result of self-phase modulation (SPM) following self-focusing and optical breakdown [7] or as a result of a four-wave parametric process [8]. The asymmetry in the Stokes and anti-Stokes regions was attributed to contributions from plasmas and/or the time response of the nonlinear index of refraction. Recent experiments [9] suggest, however, that the supercontinuum can be observed in experimental conditions where self-focusing and optical breakdown are ab-Manuscript received April 10, 1985.