Bragg Reflection of Laser Light From a Phase Grating in a Q-Switching Liquid

Abstract

It is postulated that in a saturable dye absorber used for giant pulse laser switching a phase grating is formed. To prove this experimentally a frequency doubled part of the laser beam was reflected from this ``Lippman plate'' at the Bragg angle.

Laser Q-switching by organic solvents

“…The resultant phase grating will then decay by thermal diffusion, with a decay time ~-~ 10 -7 sec (for ,~ = 10 -4 cm, n o = 1.5 and diffusivity D ~ 10 -3 em 2 sec-1). Experimental evidence lending support for these conclusions is provided by the Bragg reflection studies with variable time delay [14].…”

“…The stationary periodic refractive index structure, with a spatial period of half the optical wavelength in the medium, will decay thermally with a characteristic time ~-~ 10 -7 sec and should be observable both during and after the pulse by Bragg reflection techniques [14].…”

“…Such a structure has been observed by Bragg reflection obtained in the dye cell of a giant pulse ruby laser [14]. This periodicity of refractive index will act as a spectrally selective reflector in exact analogy with the Lippmann plate, thus providing a dynamic increase in the cavity Q.…”

Thermal effects in phase-conjugation in saturable absorbers with picosecond pulses

“…Phase-conjugation reflection with lower efficiency, has also been measured [3] in an intra-cavity saturable absorber dye cell in the experimental arrangement conunonly used for passive mode-locking of pulsed and CW dye lasers [4,5], Photoisomer effeets [6] in the saturable absorber dye were found [3] to be important in the phase-conjugation process, particularly when the láser was tuned to longer wavelengths. Martin and Hellwarth [7] have shown that thermally induced refractive Índex changes [8][9][10][11] were the dominant mechanism in a variety of liquids employed in a four-wave mixing process for image conversión from infrared to visible and more recently the influence of the thermal grating in phase conjugation was discussed by Heilweil et al [12]. We have investigated the role played by thermal effeets in phase conjuga tion reflection of picosecond pulses from saturable absorbers.…”

“…In a dye with quantum yield of fluorescence 0, a fraction (f~ 1 -0) of the energy absorbed is thermalized very quickly by the solvent molecules and the resulting temperature rise produces a phase gratin [10,11]. In the geometry of the present experiment, the relevant diffraction grating is created by the Ínter action of 7j and / 2 with a thermal decay time of 5 ns (calculated for an angle of 8o between / j and / 2 in ethanol).…”