Shankar V. Nakhe

Enhanced Temperature

This letter reports a first time study on enhanced temperature (~800°C) stability of type-IIa as compared with type-I fiber Bragg gratings (FBGs) written by high repetition rate (5.5 kHz) 255 nm UV beam. Both the FBGs were written in high photosensitive Ge doped (~10 mole%) single mode fiber without hydrogen loading. Both the FBGs survived 800°C over 9 h, however, time rate of fall of type-IIa FBG reflectivity was ~3 times lower as compared with type-I. The high temperature stability of 255 nm written FBGs is attributed to enhanced core-cladding stress release in the strong seed grating written by highly stabilized interference fringes and high cumulative fluence of 255 nm, 5.5 kHz UV pulses.

({\sim}{800}^{\circ}{\rm C})

Single pulse time resolved studies on the development of dye laser characteristics are conspicuous by their absence. In this paper we report, for the first time, studies on the temporal variation of bandwidth and divergence of a transverse pumped, prism beam expander grazing incidence grating dye laser within a single pulse. It is shown that the characteristics of the dye laser pulse depend on the divergence of the pump beam and its temporal variation.

Stability of Type-IIa FBG Written by 255 nm Beam

We present an analysis on the saturation of refractive index modulation of fiber Bragg gratings written in nonhydrogenated Ge-B co-doped single-mode photosensitive optical fiber by partially coherent pulsed UV beams. The UV beams of different spatial coherence properties were generated by second harmonic conversion of high repetition rate, high average power copper vapor laser (CVL) oscillators with different optical resonators. It is observed that for UV beams of higher spatial coherence, the fiber Bragg grating reflectivity growth was faster and saturation of refractive index modulation was higher. The experimental results are explained with the help of a physical model based on exponential decay of defect centers per unit volume on UV absorption in the fiber core. The subsequent increase in the refractive index was attributed to the structural modification and densification of the fiber core.

Single pulse time resolved studies on the characteristics of a transverse pumped HMP-GIG dye laser and the role of pump beam divergence

CuBr master oscillator-power amplifier (MOPA) laser system fitted with a generalized diffraction filtered resonator (GDFR) is reported to produce high-quality beam of throughout-pulse diffraction-limited laser emission at the repetition rate of /spl sim/19 kHz. The comparison with typical (stable and unstable) resonator configurations for CuBr lasers has featured out its high brightness accompanied by a very low beam divergence permanent within laser pulse. This intrinsic characteristic of GDFR-MOPA laser radiation makes it a perfect light source for fine laser applications.

Analysis on the saturation of refractive index modulation in fiber Bragg gratings (FBGs) written by partially coherent UV beams.

Abstract Generation of short duration (≈4 ns full width at half maximum) copper vapor laser pulses of high peak power, using an oscillator-amplifier setup is described. It is observed that the amplifier output is nearly constant for 40 ns delay variation between oscillator and amplifier.

High-brightness CuBr MOPA laser with diffraction-limited throughout-pulse emission

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.

Amplification of short (4 ns) copper vapor laser pulses

The optogalvanic (OG) effect has been observed in a Eu/Ne hollow cathode discharge lamp using pulsed laser irradiation. An OG spectrum is recorded in dye laser wavelength region 574–602 nm using a boxcar-averager. In total 41 atomic lines are observed. Of these, 38 lines are assigned to neon transitions. Two lines observed corresponding to wavelengths 576.519 and 601.815 nm are assigned to europium transitions; (4f 7 6s 2 , S 8 7/2 →4f 7 6s6p , zP 6 7/2 ) and (4f 7 6s 2 , S 8 7/2 →4f 7 6s6p , zP 8 9/2 ), respectively, and the remaining line at 582.475 nm could not be assigned. The effect of the discharge current on europium as well as neon OG signals is also studied. At moderate discharge current values, an extra positive peak is observed in neon OG signal for the transition (1s 5 →2p 2 ) at 588.189 nm, which is explained by Penning-ionization process using the quasi-resonant energy transfer interactions between excited neon and europium atoms lying in 2p 2 and D 10 9/2 states, respectively.

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

This paper presents studies on the pulsed optogalvanic effect and isotope-selective excitation of Yb 555.648 nm (0 cm−1 → 17 992.007 cm−1) and 581.067 nm (17 992.007 cm−1 → 35 196.98 cm−1) transitions, in a Yb/Ne hollow cathode lamp. The Yb atoms were excited by narrow linewidth (500–1000 MHz) Rh110 and Rh6G dye based pulsed lasers. Optogalvanic signal inversion for ground state transition at 555.648 nm was observed beyond a hollow cathode discharge current of 8.5 mA, in contrast to normal optogalvanic signal at 581.067 nm up to maximum current of 14 mA. The isotope-selective excitation studies of Yb were carried out by recording Doppler limited optogalvanic signals as a function of dye laser wavelength. For the 581.067 nm transition, three even isotopes, 172Yb, 174Yb, and 176Yb, and one odd isotope, 171Yb, were clearly resolved. These data were compared with selective isotope excitation by 10 MHz linewidth continuous-wave dye laser. For 555.648 nm transition, isotopes were not clearly resolved, although isotope peaks of low modulation were observed.

Studies on pulsed optogalvanic effect in Eu/Ne hollow cathode discharge.

Abstract We report the use of a positive branch self-filtering unstable resonator (PB/SFUR) with a copper vapor laser (CVL). The performance of this resonator is compared with that of a positive branch unstable resonator (PBUR) CVL. In the case of the PB/ SFUR CVL about 70% of the output power is contained in the diffraction limited divergence angle as compared to only about 10% for PBUR CVL. The analysis of the temporal behaviour of the output pulses indicates that the PB/SFUR starts influencing the laser pulse characteristics very early during the pulse.

Studies on the Optogalvanic Effect and Isotope-Selective Excitation of Ytterbium in a Hollow Cathode Discharge Lamp Using a Pulsed Dye Laser

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.