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Annual Meeting Optical Society of America

Oscillatory emission from the Cu-vapor laser

Jagdish Rai, Kiegon Im, J. J. Kim

Published Jan 1, 1985• 0 citations
Publisher

Abstract

Our experimental studies of the optical emission from the Cu-vapor laser indicate a breakup of laser pulse into oscillatory structures in certain conditions. We believe this phenomenon is similar to the optical ringing reported earlier.1 We attribute this breakup of the pulse to cooperative effects involved in emission from the Cu-vapor laser.2 This effect is further enhanced by propagation through the system. We earlier explained the high gain of Cu-vapor due to radiation trapping and Rabi oscillation between the 2P and 2S states.3 This analysis further shows that a large percentage of Cu atoms in the lasing medium are in the excited state (2P) most of the time. Therefore, there is a very low probability for total depletion of this level. This implies that the phenomena like gain switching and relaxation oscillation may not play a significant role in the breakup of the laser pulse. The oscillatory structures for both the 510.6-nm and 578.2-nm lines were strongly dependent on the changes in operating temperature. We observed the breakup of the pulse mainly around the temperature of 1600°C.

DOI
10.1364/oam.1985.tht2

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Soviet Journal of Quantum Electronics•Mar 31, 1980•25 citations
Our experimental studies of the optical emission from the Cu-vapor laser indicate a breakup of laser pulse into oscillatory structures in certain conditions. We believe this phenomenon is similar to the optical ringing reported earlier.1 We attribute this breakup of the pulse to cooperative effects involved in emission from the Cu-vapor laser.2 This effect is further enhanced by propagation through the system. We earlier explained the high gain of Cu-vapor due to radiation trapping and Rabi oscillation between the 2P and 2S states.3 This analysis further shows that a large percentage of Cu atoms in the lasing medium are in the excited state (2P) most of the time. Therefore, there is a very low probability for total depletion of this level. This implies that the phenomena like gain switching and relaxation oscillation may not play a significant role in the breakup of the laser pulse. The oscillatory structures for both the 510.6-nm and 578.2-nm lines were strongly dependent on the changes in operating temperature. We observed the breakup of the pulse mainly around the temperature of 1600°C.
Since Karras et al. published a paper on the observation of optical cooperative emissions from copper vapor,1 no other convincing experimental evidence has been reported on this puzzling phenomenon. We reported an observation of a similar behavior of the stimulated emission from the copper-vapor laser in the context of Dicke superradiance.2 Subsequently we have carried out a detailed experimental investigation on this pulsing phenomenon in a transverse-discharge copper-vapor laser system. By means of a precision spectral analysis we can identify two principal mechanisms which are responsible for the multiple-pulsing phenomenon in a high-gain condition: (1) the superimposition of a few pulses corresponding to different regions of the spectrum of the laser line, and (2) superradiant ringing due to the coherent collective effect. The conditions in which one or the other of the two mechanisms is manifested are subtle and are not completely understood as yet. But the characteristics of the optical pulses resulting from each mechanism are quite distinct. We describe the difference between the two mechanisms and present the experimental results that exhibit an interesting spectral behavior of the laser pulses in each case.
Recently we reported the observation of stimulated emission in atomic copper vapor that is excited by a resonant tunable laser beam.1 One important result obtained in the experiment is that the two upper levels involved in the laser transitions of the atomic copper vapor are collisionally mixed in stimulated emission. This observation is particularly important for studies of high-pressure transverse-discharge copper vapor lasers. We conducted an optical pumping experiment to measure the collisional mixing cross sections (CMCSs) below the threshold of stimulated emission. We observed all four transitions: 2P1/2−2S1/2 (327.4 nm); 2P3/2−2S1/2 (324.8 nm); 2P1/2−2D3/2 (578.2 nm); and 2P3/2−2D5/2 (510.6 nm), when either one of the two levels 2P3/2,1/2 was excited. From the measurements of the intensities of the resonantly excited and sensitized fluorescences, we estimate the CMCSs Q34(2P1/2→2P3/2) and Q43(2P1/2←2P3/2) at 2 × 10−16 and 1.3 × 10-16 cm2 for Cu + Ne collisions, respectively. We also measured the linewidths of the optically pumped green and yellow lasers as a function of the buffer gas pressure and of detuning of the pump frequency. We observe that the linewidths change in an interesting way, which can be explained in terms of the gain narrowing.
Computational results show that the conical emission and Rabi sidebands observed when a nearly resonant laser pulse propagates through an atomic vapor are the result of the breakup of the pulse into solitary waves. For an incident pulse with a transverse as well as a temporal profile, the crests and troughs of the 2-D solitary waves are curved in a time-radius plane. The temporal modulation associated with pulse breakup appears as Rabi sidebands in the spectrum, while the curvature of the solitary waves in the time-radius plane results in a transverse spatial modulation, resulting in conical emission in the far field. These results were obtained by performing detailed numerical calculations of the time-dependent paraxial propagation of a cylindrically symmetric laser pulse through a vapor of two-level atoms. Our results can be interpreted readily in terms of optical nutation on the Bloch sphere and in terms of noncollinear phase matching through the curvature of the solitary waves rather than through additional parametrically generated waves. The cone opening angle depends on the initial transverse profile of the pulse and the process of beam breakup as well as self-focusing.
We report computational results that show that the conical emission observed when a nearly resonant laser pulse propagates through an atomic vapor is the result of the breakup of the pulse into solitary waves. For an incident pulse with a radial as well as a temporal profile, the crests and troughs of the two-dimensional solitary waves are curved in a time-radius plane. The temporal modulation associated with pulse breakup appears as Rabi sidebands in the spectrum, while the curvature of the solitary waves in the time-radius plane results in a transverse spatial modulation that leads to conical emission in the far field. These results were obtained by performing detailed numerical calculations of the time-dependent paraxial propagation of a cylindrically symmetric laser pulse through a vapor of two-level atoms under the rotating-wave and slowly-varying-envelope approximations in the limit of no collisional damping or Doppler broadening. Our results can be interpreted readily in terms of optical nutation on the Bloch sphere and in terms of noncollinear phase matching through the curvature of the solitary waves rather than through additional parametrically generated waves. We find that self-focusing is not required for the generation of conical emission, although it would be difficult to separate these effects experimentally.
Computational results show that the conical emission and Rabi sidebands observed when a nearly resonant laser pulse propagates through an atomic vapor are the result of pulse breakup into temporal-spatial (2-D) solitary waves. The temporal modulation associated with pulse breakup appears as Rabi sidebands in the spectrum. Because the process of pulse breakup depends on the pulse area, the 2-D solitary waves are curved in the time-radius plane due to the transverse profile of the pulse. The curvature of the solitary waves in the time-radius plane results in a transverse spatial oscillation and a corresponding transverse wave vector that leads to conical emission in the far field.
An investigation was made of the spectral composition of the spontaneous radiation, superradiance, and laser radiation emitted by a pulsed copper vapor laser. The time resolution was about 3 nsec. It was found that the observed spectral structure of the laser radiation and superradiance was governed by the hyperfine and isotopic splitting of the active levels of the copper atom. In the absence of saturation of the active medium a narrowing of the spectral line profiles was observed during a superradiance pulse. When saturation was achieved for superradiance and lasing, the line profiles broadened to the width of the spontaneous emission line during a radiation pulse. It was found that the temperature of the gas in the copper vapor laser did not exceed 2500°K.