Ramakanta Biswal

Study on the quality of interference fringes from a pulsed UV source for application in a biprism based fiber Bragg grating writing

This paper presents a study on the quality of interference fringes formed from a pulsed UV (255 nm, 5.6 kHz, and 40 ns) source for an application in writing fiber Bragg gratings (FBGs). The interference fringes of separation of about 8 microm, formed by a biprism of apex angle 2 degrees , were studied for their contrast, evolution of contrast, and positional and intensity stability over a period of 5 min (over 10(6) pulses). Second harmonic UV (255 nm) sources of different spatial coherence and pointing stability characteristics were employed as the inputs. It is established that the UV fringes contrast and interference pattern stability with time is largely controlled by the optical resonator geometry of the fundamental wavelength (510 nm) copper vapor laser (CVL) oscillator. In particular, the generalized diffraction filtered resonator (GDFR) CVL produced the highest quality second harmonic beam with the highest fringes contrast and stability. The implications of these results were studied by employing these UV sources for the fabrication of the C-band FBGs by a 24 degrees apex angle biprism.

A comparative study on thermal lensing characteristics of low (∼500 °C) and high (∼1500 °C) temperature variants of copper vapor laser

This paper presents a comparative study on thermal lensing characteristics of a low temperature copper vapor laser (CVL) (LT-CVL as Copper-HBr laser) and a high temperature CVL (HT-CVL as elemental CVL). Interferometric techniques were used to study the combined thermal lens power of active gaseous medium and discharge sealing optical windows as well as that of the optical windows separately at different electrical input powers. As the input power varied from 2.7 to 4.6 kW, the combined thermal lens power varied from −1.4 to +0.94 km −1 for LT-CVL and from +1.4 to +13 km−1 for HT-CVL. The thermal lens power of the windows varied from +3 to +15 km −1 for HT-CVL. On the other hand, for LT-CVL, the thermal lens due to windows was very weak and could not be measured. It was observed that the origin of net thermal lensing mostly resides in the discharge tube windows owing to its higher temperature variation of refractive index as compared to that of gaseous active medium. The weaker thermal lens characteristics of an LT-CVL was attributed to its much lower working temperature and relatively flatter radial gas temperature profile than that of an HT-CVL.

Flow and thermal characteristics of high Reynolds number (2800–17000) dye cell: simulation and experiment

This paper presents computational and experimental studies on wavelength/frequency fluctuation characteristics of a high pulse repetition rate (18 kHz) dye laser pumped by a frequency-doubled Nd:YAG laser (532 nm). The temperature gradient in the dye solution is found to be responsible for wavelength fluctuations of the dye laser at low flow rates (2800<Re(d)<5600). The turbulence Reynolds number (ReT) and the range of eddy sizes present in the turbulent flow are found to be responsible for the fluctuations at high flow rates (8400<Re(d)<17,000). A new dimensionless parameter, dimensionless eddy size (l(+)), has been defined to correlate the range of eddy sizes with the experimentally observed wavelength fluctuations. It was found that fluctuations can be controlled by keeping ReT≈10 and lmax(+)≈1. The simulated result explains the experimental observation and provides a basis for optimizing the dye solution flow rate for high PRR pumping.

Generation of 1.5 W average power, 18 kHz repetition rate coherent mid-ultraviolet radiation at 271.2 nm

This paper presents to our knowledge a first time study on the generation of 1.5 W average power, 18 kHz repetition rate coherent mid-ultraviolet (UV) radiation at 271.2 nm. The work is based on frequency summing of coherent green (G: 510.6 nm) and yellow (Y: 578.2 nm) radiations of a copper-HBr laser in a β-barium borate crystal. Average and peak sum frequency conversion efficiencies of about 13% and 16%, respectively, are obtained. The sum frequency results are experimentally analyzed in terms of the extent of matching of green and yellow pump radiations in space, time, and frequency domains. The result is of high significance for many applications in photonics components fabrication, semiconductor technology, and spectroscopy.

Studies on Spatial, Spectral, and Energy Characteristics of Copper-HBr Laser Radiations

This paper presents, for the first time, a study on the pulse-to-pulse variation in the spatial, spectral, and energy characteristics (pointing instability, divergence, intensity, as well as the emission line-width and their fluctuations) of both green and yellow radiations of a copper-HBr laser (Cu-HBrL). The experimental study was performed in a high power (specific power: 70 W/L, total power: 72 W), 18 kHz repetition rate Cu-HBrL for different wall plug electrical input powers (6-9.5 kW) and HBr concentrations (5%-9%). These observations are supplemented with studies on near field spatial intensity and temporal profiles under all working conditions. It is observed that the characteristics are widely different for the two radiation components. The experimental observations are supported by comprehensive analysis in terms of input power coupling, laser gain and its radial distribution, thermally induced wave-front aberrations, optical noise contribution, and laser kinetics.

Study on the purification of hydrogen bromide gas by fractional distillation technique and its effect on improvement of copper–hydrogen bromide laser performance

Abstract. This paper presents a purification process of hydrogen bromide (HBr) gas by fractional distillation technique and its use for performance enhancement of copper–HBr laser (Cu–HBrL). The residual impurities in HBr were suppressed by a two-step distillation process at temperatures of −196°C and −20°C. The lowering of the impurities was confirmed by comparing the mass spectrograph of the HBr gas before and after distillation, using a quadrupole mass spectrometer. The effect of the purified HBr gas on the performance of Cu–HBrL average output power as well its electrical discharge characteristics were studied. More than 37% improvement in the laser average output power (40 to 55 W) was observed with the use of this distilled HBr gas compared to undistilled gas. The underlying mechanism of the enhancement was analyzed by comparing electrical discharge characteristics in the two cases. The improvement in Cu–HBrL performance with distilled HBr gas was attributed to lowering of bromine and hydrogen concentration, mainly manifested as increased average electrical energy coupling to the discharge. This resulted in increased laser gain as well as gain volume, which were reflected in increases in laser output power and beam diameter.

Effect of buffer gas pressure on the spatiotemporal evolution of small-signal gain in copper vapor laser

We report the effect of buffer gas neon pressure on the spatiotemporal small signal gain evolution characteristics of a copper vapor laser CVL. The CVL discharge tube of diameter 47 mm and length 150 cm is studied for neon pressure from 20 to 70 mbar. The gain evolution is mapped by a specially generated 6-ns, 510.6-nm probe pulse. The evolution of gain at the center and at the edge are found to be widely different for different gas pressures. At lower pressure 20 mbar, the gain at the edge is higher compared to that at the center, however, the situation is reversed at higher pressure 70 mbar. The increased laser-circuit impedance matching and the reduced discharge plasma skin effect with increase in buffer gas pressure are the possible mecha- nisms responsible for such a changeover.

Linewidth of a high pulse repetition rate (~20 kHz) class dye laser

A theory is proposed for estimation of linewidth of an ~20 kHz class dye laser transversally pumped by nanosecond pulse width pump lasers such as frequency doubled Q-switched Nd:YAG laser (λ ~ 532 nm) and copper hydrogen bromide laser (λ ~ 510 nm). The experimental results are explained on the bases of dye solutions thermal and flow parameters obtained by computational fluid dynamics simulation and the proposed theory. The fusion of experimental investigations, computational fluid dynamics simulation and the proposed theory has led to establishment of an empirical relationship to predict the linewidth of the dye laser.