MODIFICATION OF KGW CRYSTAL TRANSMISSION SPECTRUM BY CONTINUOUS-WAVE DIODE LASER RADIATION
I.A.Khodasevich1, A.A.Kornienko2, E.B.Dunina2, A.S.Grabtchikov*1
1B.I.Stepanov Institute of Physics NAS, Minsk, Belarus
2 Vitebsk State University Vitebsk, Belarus
Crystal of potashium tungstate (KGW) is widely used in laser systems. It can be doped by rare earth elements as a neodymium and serves as a laser medium for generation of radiation at the 1067 and 1351 nm at the lamp and diode pumping. Often KGW crystal is used as a Raman medium in particular, allowing Raman conversion of laser radiation at the 1351 nm wavelength to the eye-safe spectral region.
As a rule at the longitudinal pumping the excitation of active medium is implemented by continuous-wave (CW) radiation in the absorption band of neodymium ions near 808 nm. It is traditionally supposed, that radiation absorbed by ions provides the laser generation and partially spent on heating the Nd:KGW crystal leading to degradation of output characteristics. To our knowledge the effect of CW pump intensity on KGW matrix was not considered.
In this report we describe results on excitation effect of the 808 nm radiation on the transmission of the undoped KGW crystal.
The KGW crystal of 4×4×2 mm size, oriented along the b-axis, was used in experiment. The preliminary measurements of the crystal transmission showed no obvious bands of absorption in the 350-600 nm range. The short-wavelength absorption edge began near 350 nm. For observation of transmission changes the KGW crystal was placed in the USB 650UV Ocean Optics spectrometer. The crystal was illuminated in a horizontal plane, along the b-axis by the lamp source with the continuous radiation spectrum. The full spectral bandwidth of the lamp source amounted to 200-850 nm. Part of this spectrum falls within the absorption band on the KGW crystal.
The color glass filter SZS-21 was inserted between the crystal and lamp to eliminate the effects due to absorbed ultraviolet radiation. The 350-600 nm spectral part was transmitted by this filter. Excitation of the KGW crystal by CW diode laser radiation was carried out in a vertical direction at the 808 nm wavelength. The second glass filter SZS-21 was introduced between KGW crystal and the spectrometer detector to suppress the signal diode laser radiation in measurements. The beam of diode laser radiation was focused in the center of KGW crystal by the objective. The beam waist diameter was 150 microns and provided the intensity of up to 100 kW/cm2.
Modification for the spectral contour of transmition with grows of CW diode laser intensity is registered. Experimental data demonstrate the shift of the absorption edge to the longer wavelengths with creation of broad absorption bands in the registered part of spectrum for crystal transmission. Modification of transmition is accompanied by the appearance of two emission bands at 525 and 550 nm. The appearance of emission has a threshold character. Observed effect is interpreted as a result of absorbing some laser photons by the KGW crystal matrix followed by emission in the visible region and the development of thermal and other processes leading to change of the transmition characteristics. These data are important for development of diode CW pumped solid-state laser systems partially on KGW and understanding the processes of energy transfer at non-resonant interaction of CW laser radiation on crystal mediums.
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