Optics & Laser Technology 34 (2002) 253 – 258 www.elsevier.com/locate/optlastec An alternative approach to determine the fractional heat load in solid state laser materials: application to diode-pumped Nd:YVO 4 laser Pranab K. Mukhopadhyay * , Jogy George, K. Ranganathan, S.K. Sharma, T.P.S. Nathan Diode Pumped Solid State Laser Group, Center for Advanced Technology, Indore, MP 452013, India Received 7 November 2001; received in revised form 8 January 2002; accepted 9 January 2002 Abstract A simple approach is described and used for on-line measurement of the fractional heat-load parameter  in an operating diode-pumped Nd:YVO4 laser at 1:06 m wavelength for dierent doping concentrations. The method is based on the fact that if the thermo-optical properties of the sample are known then the -parameter can be estimated from the measured eective focal length induced by the pump beam and any other lensing eect due to mechanical mounting of the sample for a given pumping conguration. The value of the fractional heat-load parameter estimated by our technique was in excellent agreement with the earlier reported values. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fractional heat load; Thermal lens; Diode pumping; Nd:YVO 4 crystal 1. Introduction In solid state laser material a portion of the absorbed pump power is lost in the form of heat due to non-radiative processes. The primary mechanism is the inherent quantum defect, which arises due to the dierence in energy between the pump and the laser photon. Moreover, cross-relaxation and upconversion processes can also give rise to heating of the gain medium [1,2]. Specially, in case of diode-pumped solid-state lasers where crystals with high doping concentra- tion are used, the cross-relaxation processes are of concern due to the uorescence quenching eect [3]. For high-power laser operation the temperature gradient caused by heat deposition in the laser crystals creates a variation in the refractive index of the material as well as mechanical defor- mation and stress. The changes result in lensing and aberra- tion in the laser cavity [4,5]. The pump-power-induced heat loading in the laser crystal has deleterious eects on the performance of solid state lasers, for example, it reduces the intrinsic slope eciency of the system and increases the diraction losses due to the thermal lensing and aberration [6,7]. Hence, there is a need to characterize the heat load- ing in solid-state laser crystals in order to choose proper material for diode pumping. ∗ Corresponding author. Fax: +91-0731-488760. E-mail address: pkm@cat.ernet.in (P.K. Mukhopadhyay). In general, the pump-power-induced heat load in solid-state gain media is described by a parameter, , known as the fractional heat load parameter. The  parameter is dened as the ratio of the heat generated in the crystal to the absorbed pump power [2]. Recently, there have been two methods reported in the literature to determine the frac- tional heat load in an operating Nd:YVO 4 laser under diode laser pumping [8,9]. In one method, thermally induced de- polarization of the second harmonic output was analyzed to determine the  parameter in a diode-pumped Nd:YVO 4 laser in an intra-cavity second harmonic conguration [8]. The second method applies the fact that the output power of a laser, operating at the boundary of stability when us- ing an intra-cavity lens, quenches at a critical pump power [9]. The fractional heat loads measured by these techniques were 0.24 for 1 at% doped and 0.43–0.48 for 3 at% doped Nd:YVO 4 crystal under operating conditions. Though both the methods are eective techniques to determine the frac- tional heat load in solid state laser materials they used intra-cavity components which make the cavity dicult to align. Moreover, in the second harmonic depolarization method, the wave plating action on the second harmonic beam by the non-linear crystal has to be known accurately. In this paper, we report a simple approach for on-line measurement of the fractional heat load in solid state laser materials. In our method, we used a plane–plane resonator conguration using a thin sample of the laser crystal as the 0030-3992/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII:S0030-3992(02)00008-7