• Jogy George

      Articles written in Pramana – Journal of Physics

    • Thermal lens measurement technique in end-pumped solid state lasers: Application to diode-pumped microchip lasers

      Rakesh Kapoor P K Mukhopadhyay Jogy George S K Sharma

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      A simple derivation, which relates the thermal lens focal length in solid state lasers to pump power and a method for direct estimation of thermal lens focal length, is reported. This method is applicable to any type of stable resonator. The method is used for the measurement of the thermal lens focal length with an accuracy of 8% in an axially pumped microchip laser. The variation of focal length with pump power is also measured.

    • An alternative approach to determine the spot-size of a multi-mode laser beam and its application to diode laser beams

      Rakesh Kapoor PK Mukhopadhyay Jogy George SK Sharma

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      An alternative approach is suggested to determine the spot-size of a multi-mode laser beam. It has been shown by simulations that the suggested approach can give the beam quality factor and characteristic radius with less than 5% error. Unlike the power content method, the proposed method is applicable to the beams even with diameter one tenth of the CCD size. The new approach has been applied to a multi-mode diode laser output and it is shown that the ABCD matrix analysis can be used for beam propagation, with the measured parameters of the laser.

    • Measurement of στ product of solid state laser materials by an alternative method: Application to Nd3+ doped YVO4 crystal for 4F3/24I11/2 transition

      PK Mukhopadhyay Jogy George SK Sharma Rakesh Kapoor TPS Nathan

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      In this paper an alternative approach for measurement of στ product for 4F3/24I11/2 transition of Nd3+ doped YVO4 crystal is reported. In this method a microchip laser is formed by keeping a small piece of the sample in plane-plane resonator and a diode laser (808 nm) is used for pumping. The pump power induced thermal lensing effect is used to make the cavity stable. The cavity mode area is estimated by measuring the thermal lens focal length at the threshold and the average pump area is measured by Gaussian fit to the intensity profiles of the pump beam. The value of στ product of Nd:YVO4 crystal obtained by this method is within 10% of the reported values. The advantage of this method is that it is a simple method for direct measurement of στ product of laser crystals.

    • Operational characteristics of dual gain single cavity Nd:YVO4 laser

      Pranab K Mukhopadhyay Jogy George S K Sharma P K Gupta T P S Nathan

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      Operational characteristics of a dual gain single cavity Nd:YVO4 laser have been investigated. With semiconductor diode laser pump power of 2 W, 800 mW output was obtained with a slope efficiency of 49%. Further, by changing the relative orientation of the two crystals the polarization characteristics of the output could be varied. In particular by keeping the two Nd:YVO4 crystals with their c-axes orthogonal to each other and adjusting the gain of the crystals so that both operate at approximately the same power level, completely unpolarized beams could be obtained.

    • Effect of absorbed pump power on the quality of output beam from monolithic microchip lasers

      Pranab K Mukhopadhyay K Ranganathan Jogy George SK Sharma TPS Nathan

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      The dependence of the beam propagation factor (M2 parameter) with the absorbed pump power in the case of monolithic microchip laser under face-cooled configuration is extensively studied. Our investigations show that the M2 parameter is related to the absorbed pump power through two parameters (α and β) whose values depend on the laser material properties and laser configuration. We have shown that one parameter arises due to the oscillation of higher order modes in the microchip cavity and the other parameter accounts for the spherical aberration associated with the thermal lens induced by the pump beam. Such dependency of M2 parameter with the absorbed pump power is experimentally verified for a face-cooled monolithic microchip laser based on Nd3+ -doped GdVO4 crystal and the values of α and β parameters were estimated from the experimentally measured data points.

    • Effect of Nd3+ concentration on CW and pulsed performance of fiber-coupled diode laser pumped Nd:YVO4 laser at 1064 nm

      Pranab K Mukhopadhyay K Ranganathan Jogy George SK Sharma TPS Nathan

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      The effect of Nd3+ concentration on the CW and Q-switched laser performances at 1064 nm from Nd: YVO4 has been studied under diode laser pumping in identical laser configuration. The Nd3+ concentrations used were 1, 2 and 3 at.% in YVO4 crystals. Under the CW operations we have compared the thermal lensing effect, slope efficiencies and also the beam quality at the fourth-order degeneracy configuration. Q-switching was done with the help of an acousto-optic modulator and we have compared the pulses obtained from Nd: YVO4 laser with different doping concentration. It was found that the 1 at.%-doped crystal is the best, offering highest optical-to-optical conversion efficiency (55%), lowest fractional heat load (24%), highest pulse energy (80 µJ) and shortest pulse width (20 ns). It was also found that there was not much difference in performances for 2 and 3 at.%-doped crystals both in CW and Q-switched configurations.

    • An alternative method to specify the degree of resonator stability

      Jogy George K Ranganathan T P S Nathan

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      We present an alternative method to specify the stability of real stable resonators. We introduce the degree of optical stability or the 𝑆 parameter, which specify the stability of resonators in a numerical scale ranging from 0 to 100%. The value of zero corresponds to marginally stable resonator and $S < 0$ corresponds to unstable resonator. Also, three definitions of the S parameter are provided: in terms of $A&D$, $B&Z_{\text{R0}}$ and $g_{1}g_{2}$. It may be noticed from the present formalism that the maximum degree of stability with $S = 1$ automatically corresponds to $g_{1}g_{2} = 1/2$. We also describe the method to measure the 𝑆 parameter from the output beam characteristics and 𝐵 parameter. A possible correlation between the 𝑆 parameter and the misalignment tolerance is also discussed.

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