Nageshwar Singh

Multiphoton photoconductivity and optical nonlinearities in ZnSe and CdSe direct band gap crystals

The multiphoton photoconductivity properties of ZnSe and CdSe direct band gap crystals have been studied using frequency-doubled Nd:YAG laser excitation. Two-photon absorption in both crystals has been observed. The photoconductivity measurements and nonlinearity in Z-scan traces confirms the existence of a lower valence band, from where one-photon transitions are forbidden but two-photon transitions are allowed. The value of the two-photon absorption cross section γ2 in both crystals has been calculated by using second-order time-dependent perturbation theory.

Influence of optical inhomogeneity in the gain medium on the bandwidth of a high-repetition-rate dye laser pumped by copper vapor laser

Fluctuations in output of a pulsed dye laser are inevitable and seriously affect the realization of their full potential as a narrow-bandwidth tunable source. High-repetition-rate dye lasers require the flow of the dye solution to be at sufficiently high speed across the pump region so as to replace the heated medium and move it away from the dye laser axis. This introduces instabilities and fluctuations in the lasing medium, thereby affecting the bandwidth and wavelength of the dye laser. We report the effect of optical inhomogeneity in the gain medium on the bandwidth of a rhodamine 6G dye laser transversely pumped by a copper vapor laser. It is observed that the bandwidth of the dye laser decreases with increasing Reynolds number within the laminar region of flow.

Determination of nonlinear absorption and refraction in direct and indirect band gap crystals by Z-scan method

A systematic investigation on nonlinear optical properties such as three photon absorption (3PA) wavelength dependent of Kerr type nonlinear refraction in direct and indirect band gap crystals has been reported in the present work. The Z-scan measurements are recorded for both ZnO and CdI(2) with femtosecond laser pulses while the wavelength dependent of the Kerr nonlinearity are in agreement with a two band model. The wavelength dependence of the 3PA is determined by [(3E(photon)/E(g))-1](5/2)[(3E(photon)/E(g))](-9) in the case of direct band gap crystal and [(3E(photon)±ℏΩ/E(g))-1](5/2)[(3E(photon)±ℏΩ/E(g))](-9) in the case of indirect band gap crystal. In the present investigation the value of 3PA in the case of indirect band gap crystal is lower than the direct band gap crystal which is due to the phonon assisted transition. The materials of large band gap with optical nonlinearity and fast response speed should be dominating factor for further photonic devices such as optical limiters, optical switches and optical modulators. The higher order nonlinear optical effects have also been determined in the present study.

Fluctuations in near 360? curved and straight channel dye cells for high repetition rate copper vapour laser pumped dye laser

A study of wavelength and bandwidth fluctuations of a rhodamine 6G dye laser, transversely pumped by high repetition rate copper vapour laser, using a near 360° curved and a straight duct dye cell, is presented. The axial modes, which are indistinguishable in the straight duct cell, are clearly distinguishable in the curved duct cell dye laser. The curved duct dye cell has smaller fluctuations in wavelength and bandwidth as compared with the straight dye cell.

Design, modeling, and performance evaluation of a novel dye cell for a high repetition rate dye laser

In this paper, a new dye cell for transverse pumping was designed, modeled, and its performance in a narrow spectral width dispersive resonator, pumped by a high repetition rate copper vapor laser, was investigated. The scheme essentially involves the profiling of the cubical glass and stainless steel cylindrical surface such that convex-plano contour be present near the optical pumping region. The design is an amalgamation of straight and curved periphery to enhance the dye solution flow stabilities near the dye laser axis. A computational fluid dynamics analysis of the liquid flow through this dye cell has been carried out. The dye laser outputs such as optical average power, spectral width and wavelength stability, tuning range, pulse shape, through this new dye cell was evaluated. The dye laser average power about 30 mW was fairly steady over the observation period of more than an hour. The dye laser short-term (1 min) spectral width was within 0.824 ± 0.075 GHz, while, in a long-term, more than an hour, drifted by about 180 MHz. The dye laser wavelength in short-term fluctuates within ±0.0065 nm whereas in a long-term, more than an hour, drifted by about 0.0105 nm. The dye laser tuning range was 10 nm with a sub GHz spectral width operation. The pulse shape of the dye laser follows the pump laser pulse profile. Thus, the dye laser has demonstrated fairly long-term stability, without the use of either low expansion material or close loop control on the output.

Influence of buffer gas pressure on the spectral width of the 510.6-nm line of an atomic copper vapor laser

Abstract. The effect of buffer gas pressure on the intensity and thespectral width of the 510.6-nm line of an atomic copper vapor laser CVL is reported. A self-discharge heated CVL tube of internal diameter47 mm and length 1500 mm was used. The buffer gas pressure wasvaried from 5.0 to 470.0 mbar. A Fabry–Perot etalon-based setup wasused to measure and analyze the spectral width of 510.6-nm lines ofCVL. The relative intensity and spectral width of the hyperfine compo-nents changes with operating buffer gas pressure. The buffer gas pres-sure reduces the number of hyperfine components and hence overalllinewidth .

A study of flow characteristics of a high repetition rate dye laser gain medium

The gain medium of a high repetition rate liquid dye laser is made to flow at a sufficiently high velocity through the dye cell in order to minimize the thermal inhomogeneity. In this paper, temperature contours as a function of distance from the pump beam entrance dye cell window wall at various flow velocities in the laminar and turbulent region of gain medium flow were evaluated using computational fluid dynamics (CFD). Appreciable temperature gradients subsist at low Reynolds numbers due to inefficient heat transfer from the gain medium. However, at high Reynolds numbers >6000, the substantial effects of local density turbulent fluctuations degrade the optical quality of the gain medium. These computational studies were used to elucidate the characteristics of observed spectral fluctuations of a dye laser as a function of the Reynolds number.

Spectral fluctuations of a high repetition rate dye laser through a flowing gain medium

In this paper, wavelength and bandwidth fluctuations of a Rhodamine 6G dye laser, transversely pumped by CVL, through a flowing gain medium are investigated. The gain medium mass flow rates were varied in the range 1?6?l?min?1 (LPM). Analysis of the wavelength and bandwidth fluctuations of the dye laser was carried out using a high resolution Fabry?Perot etalon based setup and composite images generated from spectral profiles. The wavelength varied within 0.0290, 0.0240, 0.0200, 0.0120, 0.0110 and 0.0200?nm while the bandwidth was within 1.340, 0.912, 0.679, 0.301, 0.296 and 0.509?GHz at flow rates of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0?LPM, respectively. The minimum fluctuations, which depend on the experimental conditions, were observed at ?5?LPM. Spectral fluctuations at particular flow rates depend mainly on the microscopic fluctuations in the temperature and flow velocity inside the boundary layers.