Volume 57, number 5 OPTICS COMMUNICATIONS 1 April 1986 WIDEBAND FREQUENCY TUNING BY PHASE-MATCHED STIMULATED FOUR-PHOTON MIXING IN LOW MODE-NUMBER GeO2-SiO 2 FIBERS V.P. KALOSHA and A.P. KHAPALYUK A.N. Sevchenko Scientific Research Institute for Applied Physics Problems, V.L Lenin Belorussian State University, 220106 Minsk, USSR Received 2 September1985; revised manuscript received3 December 1985 We investigatethe frequencyshift for phase-matchedfour-photonmixing processesin low mode-numberGeO2-dopedsilica glass fibers as a function of dopant concentration,core radius, index profile and Nd:YAG laser pump wavelengthfor different mode combinations.The ways for the Stokes-anti-Stokesfrequency tuning over wide visible and near-IR spectral region are presented. 1. Introduction Among the various nonlinear optical processes in lowdoss glass fibers, phase.matched stimulated four. photon mixing (FPM) as well as stimulated Raman scattering was found to be the most promising for the frequency tuning of laser radiation [1-9]. They pro- vide for the efficient conversion of the input light into Stokes and anti-Stokes light for the small pump inten- sities in comparison with the bulk material case. In single-mode fibers phase matching of the FPM compo- nents is achieved due to intramodal dispersion in the narrow frequency band near zero material dispersion wavelength [5] or due to birefringence [6]. In these fibers the frequency tuning of output light is carried out by the choice of the fiber parameters [7] or by the birefringence change [8] which, however, are pos- sible only within limited regions. In low-mode-number fibers the material dispersion is compensated in the visible and near.IR region by intermodal dispersion of the FPM components. It gives considerably greater possibilities for frequency tuning due to parameter change in wider regions, changes of mode combina- tions participating in FPM as well as pump wavelength, although light conversion in single-mode fibers has ad- vantages in energy efficiency of nonlinear coupling and spatial intensity distribution (TEM00-1ike) of out- put radiation. Experimentally the Stokes and anti- 314 Stokes generation frequencies were measured only for a limited number of low-mode-number fibers [ 1--4]. Meanwhile one must know generation frequencies in wide intervals of the fiber parameters for the expan- sion of the practical application of this phenomena. The purpose of the present communication is to show the computed Stokes and anti-Stokes frequencies for various practical parameters and pump wavelengths. The series of the fibers with GeO2-doped SiO 2 cores and pure SiO 2 claddings was investigated which are low-mode-number ones for Nd:YAG laser pump with wavelengths kp = 1/Vp = 532 and 1064 nm. We considered FPM processes at mode combinations ob- served in refs. [ 1-4]. Generation frequencies were calculated from phase matching condition/3p1 + ~2 = /3 s +/3 A [1,3] for the propagation constants of LP modes. The computer procedure consists in numerical solution of the dispersion equation of LP modes for fiber material parameters on pump, Stokes and anti- Stokes frequencies ~, VS, vA and phase matching con- dition as an equation in Au = Up - us -- uA - Up for given fiber geometrical parameters, doping and pump wavelength. For the core and cladding indices as func- tions of frequency the three-term Sellmeier equation was used with coefficient sets from ref. [lO] for GeO2-SiO 2 glass with different GeO2 dopant concen- trations (3.1, 3.5, 5.8 and 7.9 mol%) and from ref. [11] for pure SiO2 glass. Thus unlike [1-4,12] we did not 0 030-4018/86/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)