Temporal coherence control of Nd:YAG pumped Raman shifter Wilson Garcia, Jonathan Palero, Caesar Saloma * National Institute of Physics, University of the Philippines, Diliman 1101, Quezon City, Philippines Received 14 March 2001; received in revised form 27 June 2001; accepted 5 July 2001 Abstract We study the temporal coherence properties of a Nd:YAG pumped Raman shifter pump wavelength  355 nm) and report the control of its temporal coherence length z c with hydrogen gas pressure P for dierent input pump energies E in . Within the range: 3:1 6 E in 6 6:5 mJ, and for: 0:41 6 P 6 4:83 MPa, z c exhibits an inverse hump-like dependence with P. The shortest z c occurs near a critical) pressure P  P c  0:69 MPa, that is independent of E in . At P  P c , the eight Stokes and anti-Stokes lines exist at their strongest in the Raman output spectrum. For P < P c , a sharp increase in z c is obtained with decreasing P due to the relative dominance of the 355 nm Rayleigh line in output spectrum. For P > P c , a slow increase in z c is observed with P due to the dominance of the second-order Stokes line and the weakening of the anti-Stokes lines that is caused by the deterioration of the phase-matching condition for four-wave mixing. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.55.Ye; 42.65.Dr; 33.20.Fb; 42.25.Kb Keywords: Raman laser; Stimulated Raman scattering; Coherence 1. Introduction Stimulated Raman scattering SRS) is a pow- erful technique for shifting the wavelength of laser radiation into another spectral region [1]. SRS is capable of simultaneously generating multiple spec- tral lines from the vacuum ultraviolet to the far in- frared [2±4] and it has already found a number of interesting applications in the area of image ampli- ®cation [5±7]. Laser sources with controllable temporal co- herence properties are valuable in spectroscopy and optical microscopy [8±11]. They enable the ecient and simultaneous excitation of multiple ¯uorescence probes ± a capability that is poten- tially useful in spectral karyotyping [12] and exci- tation of biological samples labeled with multiple site-speci®c ¯uorescence probes [13,14]. With its highly directional beam outputs, such sources can be ecient illuminators in conventional optical mi- croscopes for generating images with acceptable levels of coherent optical noise caused by speckles, edge ringing and shifting, and point diractions. Optical noise reduction by temporal coherence control has been demonstrated for semiconductor 15 September 2001 Optics Communications 197 2001) 109±114 www.elsevier.com/locate/optcom * Corresponding author. Fax: +632-92-05-474. E-mail address: csaloma@nip.upd.edu.ph C. Saloma). 0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0030-401801)01424-9