S.K. Dixit

Performance of a copper vapor laser with self filtered unstable resonator

Abstract A self-filtered unstable resonator (SFUR) has been used for the first time with a copper vapor laser. An output beam with divergence of 0.19 mrad was obtained. This is about two times the diffraction limit.

A generalized diffraction filtered resonator with a copper vapor laser

Abstract The diffraction filtered resonator (DFR) offers large volume utilization while maintaining the diffraction limited nature of the output. Large active volume utilization demands a DFR with large resonator length and/or very small pinhole diameter. Increase in the resonator length results in lesser number of transits through the gain medium. Reducing the diameter of the pinhole, on the other hand, would reduce the feedback and hence increase the amplified spontaneous emission. In this paper a generalized DFR scheme is proposed, in which the mode volume is increased without either increasing the resonator length or reducing the pinhole diameter. The performance of these resonators with a high-gain, self-terminating laser like copper vapor laser is studied and compared with an equivalent semiconfocal DFR.

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})

The paper discusses the influence of the evolution of the spatial coherence of the fundamental beam on second harmonic generation (SHG) of copper vapor laser beams in beta-barium borate (BBO) crystals. It is established that a coherence parameter, defined, as the ratio of the coherence width to the full beam cross-section, can uniquely be used to compare the performance of beams from different resonators for SHG. Fundamental beams with average coherence widths varying from a fraction of the beam cross-section to the full cross-section were used. It is established, from time-resolved and averaged studies, that in line-focusing geometry, SHG is primarily governed by this coherence parameter.

Stability of Type-IIa FBG Written by 255 nm Beam

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.

On the role of the coherence width and its evolution in a short-pulse fundamental beam in second harmonic generation from beta-barium borate

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.

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

The results of a study on the spatial coherence of a generalized diffraction-filtered resonator (GDFR) copper vapor laser (CVL) for various magnifications are presented. The coherence width and output power are compared with that of unstable resonators (URs) of equivalent magnifications. It is established, by use of reversal shear interferometry, that the GDFR CVL beam has better spatial coherence and average power characteristics than the UR CVL beam for equivalent resonator magnifications.

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 an experiment and analysis on the factors affecting nonlinear evolution of Bragg wavelength with change in temperature in typical bare and embedded fiber Bragg grating-based (FBG) temperature sensors. The purpose of the study was to find the constants in the function required to evaluate temperature from Bragg wavelength shift. The temperature sensitivity of bare FBGs was found to increase with temperature elevation, and is different for FBGs written in different fiber types. The average temperature sensitivity increased by about 20% when the bare FBG temperature was elevated from 25°C to 525°C. The average temperature sensitivity of the embedded FBG sensor, investigated in the temperature range of 30°C-90°C, was a factor of 2-3 times larger than for bare FBG, depending on its fastened length with the substrate. Analytically, it is shown that the nonuniform behavior of temperature sensitivity in bare FBGs is the result of both the thermal expansion effect of the fiber and the temperature derivatives of the effective refractive index. The strain transfer and temperature coefficients of thermal expansion of the substrate affect the nonuniform behavior of temperature sensitivity in embedded FBG sensors.

Spatial coherence of the generalized diffraction-filtered resonator copper vapor laser.

We present results of comparative time-resolved coherence studies on a single pulse from amaster oscillator power amplifier (MOPA) copper vapor laser with generalized diffraction-filtered and unstable resonators as master oscillators. It is shown that, unlike the conventionally used unstable-resonator MOPA reported in literature, the coherence of a generalized diffraction-filtered resonator MOPA pulse is fairly independent of the delay between the oscillator and the amplifier. It also remains constant throughout the pulse, with the result that the flux is constant over a large range of the delay.

Study of the nonuniform behavior of temperature sensitivity in bare and embedded fiber Bragg gratings: experimental results and analysis.

The conversion efficiency in second-harmonic generation of an amplified beam in a master-oscillator power amplifier copper-vapor laser (CVL) is lower than that of the oscillator beam alone. This lower efficiency is often vaguely attributed to wave-front degradation in the amplifier. We investigate the role of wave-front degradation and thermal dephasing in the second-harmonic generation of a CVL from a beta-barium borate crystal. Choosing two beams with constant intrapulse divergence, one from a generalized diffraction filtered resonator master oscillator alone and other obtained by amplifying oscillator by use of a power amplifier, we show that at low flux levels the decrease in efficiency is due to wave-front degradation. At a fundamental power above the critical power for thermal dephasing, the decrease is due to increased UV absorption and consequent thermal dephasing. Thermal dephasing is higher for the beam with the lower coherence width.