Spectral broadening of picosecond laser pulses in optical fibres

Abstract

Picosecond light pulses of a passively mode-locked ruby laser (pulse duration At L « 35 ps) are spectrally broadened in optical fibres of core diameters from 4//m to 600j/m. Combining the effects of self-phase modulation, stimulated Raman scattering, and parametric four-photon interaction in an 8-/im core fibre of 4 m length with the effect of selective spectral attenuation in a ruby rod resulted in rather smooth spectra extending from 685 nm to 830 nm (spectral width « 2300cm" 1 ).

Theoretical investigation of noncollinear phase-matched parametric four-photon amplification of ultrashort light pulses in isotropic media

“…Only the small-signal approximation limit is considered where the pump pulse intensities remain constant. Neglecting optical absorption, the collinear parametric four-photon amplification in the slowly varying amplitude approximation is described by Equations 1 and 2 [1,10,12,14,15] where E s , 2s x , E x and E 2 are the amplitudes of the electrical field strengths at the angular frequencies u; s (signal), UJX (idler), UJX and UJ2 (pump pulses); c 0 is the light velocity in vacuum; z is the propagation direction; and Ak z = k Sz + k Xz -k x z -k 2z -k$ + k\ -k x -k 2 is the wavevector mismatch. The wavevectors k t (i = 1,2, S, I) are given by k t = HiUii/cQ = lirriii/i/cQ = lirriii/i == Inni/Xi, where the n t are the refractive indices at the angular frequencies uj i9 frequencies v i9 wavenumbers v t or wavelengths A;.…”

“…(/ = L or SH) causes selfphase modulation of temporally bell-shaped pump pulses of frequency uj t [1,14,53]. The carrier frequency D c of the pulses becomes chirped according to [62,63] ^t ] = -7^l^r ( 3 1 ) The spectral broadening A£ S P M of a Gaussian pulse of peak intensity I 0ji is approximately given by [53,62] The wavevector mismatch coefficient |^SPM,ZI = I AA: Z |/Az/ SPM versus signal frequency D s is displayed in Fig.…”

“…requirement Ak x d Q^ < ir is fulfilled and the influence of transversal phase mismatch may be neglected. The pump pulse propagation through the medium modifies the refractive index at the signal frequency z/ s according to An s = 2^2Ii (i = L or SH) (the factor of 2 arises because of the lifting of frequency degeneracy) [14,54]. This refractive index change causes a signal frequency chirping and a signal frequency broadening of twice the magnitude of the pump pulse chirping and broadening.…”

“…It should be noted that media with a large figure of merit for parametric four-photon amplification (small Xn\ responsible for self-phase modulation, small dispersion characterized by large Abbe number, high damage threshold intensity) and low Raman gain factor are widely used for picosecond and femtosecond light continuum generation (H 2 0 [11,12,56,72], D 2 0 [11,12,15,56], mixtures of H 2 0 and D 2 0 [73], ethylene glycol [74,75], mixtures of ethylene glycol and glycerin [76], fused silica [11,12], optical silica fibres [10,11,13,14,77], NaCl crystal [12], compressed xenon gas [78,79], compressed nitrogen gas [78,79]). …”

“…Parametric four-photon interaction is possible in all media [1][2][3][4][5][6][7][8][9][10]. Parametric four-photon generation (also called stimulated four-photon mixing [10,11], stimulated parametric fourphoton interaction [12], or stimulated four-wave parametric emission [3]) plays an important role in the generation of picosecond light continua [3,[10][11][12][13][14][15] and references therein]. Four-photon parametric oscillators have been realized by applying optical fibres [16] and atomic vapours [6,[17][18][19][20][21].…”