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VARIATION OF REFRACTIVE INDEX DURING LASER OPERATION.

Jerald R. Izatt, Harold A. Daw, Mitchell,Robert C.

Published Apr 1, 1965• 0 citations
OpenAlex

Abstract

High-resolution measurements have been made of the anomalous dispersion associated with the two Stark components of the R sub l absorption line in ruby. These preliminary measurements suggest that the R sub l dispersion obeys the Kronig-Kramers relations; however, it is superposed on a background which is itself anomalous because the refractive index increases with increasing wavelength. These measurements cover a wavelength interval of about 0.5 A, and the change in background index is of the order of 0.001 while the R sub l dispersion curve deviates from the background value by about 0.0001 at the maximum. A second surprising feature of the measurements is the observation that coherent radiation from a ruby laser, which was used to illuminate the ruby sample, is absorbed less in traversing the sample than would be expected on the basis of absorption measurements made with incoherent light. Highly accurate measurements have been made of the temperature dependence of the ruby laser emission between 63K and 116K and in a second interval between 196K and 207K. Light shielding is described which has been designed and tested to minimize pump light and ruby fluorescence at the detector in experiments with the sample optically pumped. (Author)

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Applied Physics Letters•Mar 15, 1977•124 citations
High-resolution measurements have been made of the anomalous dispersion associated with the two Stark components of the R sub l absorption line in ruby. These preliminary measurements suggest that the R sub l dispersion obeys the Kronig-Kramers relations; however, it is superposed on a background which is itself anomalous because the refractive index increases with increasing wavelength. These measurements cover a wavelength interval of about 0.5 A, and the change in background index is of the order of 0.001 while the R sub l dispersion curve deviates from the background value by about 0.0001 at the maximum. A second surprising feature of the measurements is the observation that coherent radiation from a ruby laser, which was used to illuminate the ruby sample, is absorbed less in traversing the sample than would be expected on the basis of absorption measurements made with incoherent light. Highly accurate measurements have been made of the temperature dependence of the ruby laser emission between 63K and 116K and in a second interval between 196K and 207K. Light shielding is described which has been designed and tested to minimize pump light and ruby fluorescence at the detector in experiments with the sample optically pumped. (Author)
The real and imaginary parts of the complex index of refraction in the R-line region of ruby have been measured near liquid-nitrogen temperature with sufficient resolution that the R-line doublet structure was well resolved. A modification of the usual Babinet compensator technique was used for the refractive-index measurements, which were found to correlate with the absorption measurements as predicted by the Kramers-Kronig relations. An attempt at direct quantitative verification of the dispersion relations for an optically pumped sample is also described.
The real and imaginary parts of the complex index of refraction have been measured in the R1 line of optically pumped ruby. The experiments, which employed a temperature-controlled, single-mode ruby laser as a variable-frequency monochromatic source, were performed near liquid-nitrogen temperature, where the R1 doublet structure is well resolved. R1 absorption profiles were measured for several optical-pumping conditions, ranging from zero up to a pump power that excited a population in the 2E state equal to about 60% of the population of the 4A2 ground state. For each pumping condition, the anomalous-dispersion curve was measured interferometrically. The spectral profiles of the absorption coefficient and the anomalous dispersion display a dependence on the optical-pumping conditions that is more complicated than simple saturation of the R1 transitions, but for given pumping conditions the two quantities are correlated as predicted by the Kramers–Kronig relations.
Studies of the resonant interaction of radiation from a liquid-nitrogen-cooled ruby laser with a ruby crystal in the temperature range 4.2°–100°K are described. The interaction was measured by observation of the change which occurs in the Cr3+ ground state EPR absorption on laser irradiation. At low radiation intensities, the temperature dependence of this change is shown to be determined by the well-known thermal tuning behavior of the R lines. Precise measurement of the relative laser and ruby-sample absorption frequencies by this technique of double-resonance spectroscopy is described. Under optical saturation conditions, additional temperature-dependent pumping effects occur at low temperatures. In particular, as the ruby temperature is increased from 4.2° to 40°K, the laser-induced change in EPR absorption is observed to undergo a large increase. This increase cannot be explained by a relative thermal shift of the laser and absorption frequencies which is known to be very small in this temperature range. The phenomena are attributed in part to the temperature-dependent homogeneous linewidth which determines the widths of the holes burnt into the inhomogeneous strain-broadened R lines. Calculations indicate that the related temperature-dependent thermalization process which redistributes excited Cr3+ among the 2E levels quantitatively accounts for most of the increase.
A high resolution study has been made of the R(1) doublet in the absorption spectrum of pink ruby, using a temperature-controlled ruby laser as a variable frequency monochromatic source. The source and sample crystals were oriented for measurement of the absorption of alpha-polarized radiation, and the measurements were made near liquid nitrogen temperature. The observed absorption coefficient was found to decrease as the intensity of the incident beam was increased, indicating that significant depletion of the ground state was effected by the resonant pumping action of the laser beam. The highest laser beam intensity employed was approximately 100 J/cm(2). At this intensity level a significant reduction in the spectral spacing of the R(1)(+/-3/2) and R(1)(+/-(1/2)) components of the absorption doublet was also observed. Time-resolved spectral measurements showed, moreover, that at similar intensity levels, temporally smooth incident beams at frequencies near the absorption line peaks emerged from the sample temporally structued. The observed reduction in the absorption coefficient agrees well with calculations based on a simple model for ground state depletion, but no detailed theoretical explanation of the other observed phenomena has yet been developed.
We describe an experiment to be carried out by advanced students as part of their laboratory practice. Its aims are (i) to measure simultaneously the frequency dependence of both the refractive index n and the extinction coefficient of an atomic gas interacting with a resonant laser beam and (ii) to examine that n and satisfy the Kramers-Kronig relations. The medium to be probed consists of a dilute 85 Rb vapour exposed to collisions with a buffer gas. The D 2 resonance line is thereby broadened to such an extent that its hyperfine components merge into single Lorentzian dispersion and absorption profiles, respectively. The main features of the apparatus are a tunable laser diode, a Mach-Zehnder interferometer at nearly zero path difference, an array of photodiodes to measure the interference pattern as a function of laser frequency and Rb density in one of the interferometer arms, an absorption monitor, computer-controlled electronics to read out and analyse the data, and a video camera for visual control and adjustment of the interference pattern.
Bistability, differential gain, discriminator, clipper, and limier actions were observed using a plano-concave Fabry-Perot cavity containing ruby. Input powers were approximately 20 mW from a cw ruby laser. Operation wa anticipated near 77 °K where the R1 transition was nearly resonant with the laser. However, the device functioned at room temperature. The refractive index of ruby depends on the fractional ground-state population. Driving the R lines leads to a nonlinear refractive index due to dispersive contributions from nonresonant pump and charge-transfer bands, which explains the room-temperature operation.