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Physical Review A

Loss of spectral memory in the relaxation oscillations of a multimode solid-state laser

D. Romanini, Alexander A. Kachanov, Éric Lacot, F. Stoeckel

Published Jul 1, 1996• 8 citations
Publisher

Abstract

To test the application of solid-state lasers to intracavity laser absorption spectroscopy (ICLAS), we studied a neodymium-doped glass laser, in the presence of extreme relaxation oscillations (up to ${10}^{8}$ intensity contrast). The contrast of these oscillations could be varied by changing the Q-switch transition time. The recorded absorption spectra accurately follow the regular ICLAS behavior except when the intensity in the first relaxation oscillation minimum becomes too low. In such a case, we find that the ICLAS absorption spectrum accumulated during the first spike is ``forgotten'' by the system. Numerical simulations were done which accurately describe the observed transition to a regime of memory loss, when the number of photons circulating in each mode drops close to 1. These simulations show that the mechanism of memory loss is the prevailing of spontaneous over stimulated emission. This effect sets the fundamental limit of the sensitivity of an ICLAS spectrometer where the nonlinear mode interactions are negligible. This effect was observed before only in a diode laser system with high internal losses. In this paper we report an observation of ICLAS sensitivity saturation by spontaneous emission during relaxation oscillations in a laser with low cavity losses. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.920

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To test the application of solid-state lasers to intracavity laser absorption spectroscopy (ICLAS), we studied a neodymium-doped glass laser, in the presence of extreme relaxation oscillations (up to 108{10}^{8}108 intensity contrast). The contrast of these oscillations could be varied by changing the Q-switch transition time. The recorded absorption spectra accurately follow the regular ICLAS behavior except when the intensity in the first relaxation oscillation minimum becomes too low. In such a case, we find that the ICLAS absorption spectrum accumulated during the first spike is ``forgotten'' by the system. Numerical simulations were done which accurately describe the observed transition to a regime of memory loss, when the number of photons circulating in each mode drops close to 1. These simulations show that the mechanism of memory loss is the prevailing of spontaneous over stimulated emission. This effect sets the fundamental limit of the sensitivity of an ICLAS spectrometer where the nonlinear mode interactions are negligible. This effect was observed before only in a diode laser system with high internal losses. In this paper we report an observation of ICLAS sensitivity saturation by spontaneous emission during relaxation oscillations in a laser with low cavity losses. \textcopyright{} 1996 The American Physical Society.
In their review about intracavity laser spectroscopy, P. E. Toschek and V. Baev [in Laser Spectroscopy and New Ideas, edited by W. M. Yen and M. D. Levenson, Springer Series in Optical Science (Springer, Berlin, 1987)] have shown that multimode lasers are needed to obtain a high sensitivity. In this paper we show that two modes is enough. We discuss first the ideal case where only the spontaneous emission limits the sensitivity. In a second part we take into account the spatial hole burning, which couples the two modes and has for effect a lower limitation of the sensitivity. Analytical expressions are given for the modified absorption coefficient and the time evolution of the intensity of the two modes is calculated by numerical integration.
We study the dynamics of a laser oscillating on L optical transitions connecting a common upper level and L lower sublevels. Each transition is single mode. The minimal generalization of the rate equations to explain the experimental observations includes the low-frequency coherence dynamics [Europhys. Lett. 45, 307 (1999)]. These equations are studied analytically. We prove that such a laser has a low intensity self-pulsing threshold corresponding to the destabilization of the lowest of the relaxation oscillation frequencies. It also displays antiphased dynamics in the time-periodic regime, resulting in a vanishingly small oscillation amplitude of the total intensity while modal intensities have finite amplitude oscillations. The physical origin of this behavior is found to be the enhancement of stimulated absorption by quantum interference.
A model using rate equations which allow for quantum noise is used to make a theoretical analysis of the steady-state lasing regime of a cw dye laser. Equations describing the laser performance are derived in the adiabatic approximation neglecting spatial inhomogeneities in the burnup of the active medium. An investigation is made of the stability of steady-state multimode laser action and it is found that this is weak in certain modes. Calculations are made of correlation functions describing fluctuations in the numbers of photons in the modes and fluctuations in the total intensity. Although the fluctuations in individual modes are large, the integrated intensity fluctuates negligibly. An analysis is made of the influence of quantum intensity fluctuations on the attainment of maximum sensitivity in intracavity laser spectroscopy.
We investigate the limits of intracavity absorption spectroscopy with nonlinear media. Using a common theoretical framework, we compare the detection of a trace gas within (i) an undriven cavity with gain near and above threshold; (ii) a driven cavity with gain kept just below threshold; and (iii) a cavity driven close to the saturation point of a saturable absorber. These phase-transition-based metrology methods are typically quantum limited by spontaneous emission, and we compare them to the empty-cavity shot-noise-limited case. Although the fundamental limits achievable with nonlinear media do not surpass the empty-cavity limits, we show that nonlinear methods are more robust against certain technical noise models. This recognition may have applications in spectrometer design for devices operating in nonideal field environments.
An experimental investigation was made of fluctuations of the spectral distribution of the intensity in the wide-band spectrum of a ring cw traveling-wave dye laser. Allowance for the averaging within the limits of the instrumental function of a spectrometer was made by analyzing the noise power spectra obtained for the recorded intensity distributions. A good agreement with the theory was observed. The threshold of detection of an intracavity spectrometer, denned in terms of the absorption coefficient, was found to be governed by the quantum nature of the radiation used in the spectrometer and amounted to ~ 3 X 10 − 17 cm − 1 when the accumulation time was 20 s.
We study a Nd-doped microchip glass laser that emits in two polarizations and in many longitudinal modes. Saturation of the inversion by standing waves causes spatial inhomogeneity of both the longitudinal and azimuthal distributions of the laser gain. These nonlinear inhomogeneities couple the modes and result in low-frequency oscillations (10--500 kHz) of the light flux in the individual laser modes. These oscillations are steadily driven by quantum noise and appear as 15% fluctuations of the power in each mode. In-phase fluctuations in the laser modes appear at the frequency of the main relaxation oscillation of the total laser power. Antiphase fluctuations appear at other frequencies in individual longitudinal and polarization laser modes only. The dominant frequency of these fluctuations is determined by the light power in the mode. Numerical simulations of rate equations, including Langevin forces, satisfactorily reproduce the experimental results. These phenomena must be taken into account when lasers are applied as stable coherent optical light sources, and also with sensitive absorption measurements in the laser cavity.