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IEEE Journal of Quantum Electronics

Microwave modulation of a ruby laser output by absorption

Di Chen

Published Jun 1, 1965• 1 citation
Publisher

Abstract

Ruby exhibits the Zeeman effect in the energy levels responsible for the absorption of the R1line. When a ruby laser is directed to pass through a section of unexcited ruby rod, the intensity of the transmitted beam is strongly dependent upon the magnetic field applied to the absorber. This effect was used to modulate the amplitude of a pulsed ruby laser in the microwave frequency region. The experimental modulator consisted of a 0.2- inch long ruby absorber located at one end of anS-band TE111mode cylindrical cavity pumped by a 3.2-Gc/s microwave source. A dc bias magnetic field was also applied to the absorber. When both the laser and the absorber rods were cooled to 80°K, the modulated beam attained maximum modulation index with a bias field of 2.5 kOe. This and other experimental results agree well with calculations made from theoretical analysis of this modulation technique, and indicate that the various known relaxation mechanisms in the spin system of ruby do not impose a frequency limit to this method of modulation.

DOI
10.1109/jqe.1965.1072198

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Ruby exhibits the Zeeman effect in the energy levels responsible for the absorption of the R1line. When a ruby laser is directed to pass through a section of unexcited ruby rod, the intensity of the transmitted beam is strongly dependent upon the magnetic field applied to the absorber. This effect was used to modulate the amplitude of a pulsed ruby laser in the microwave frequency region. The experimental modulator consisted of a 0.2- inch long ruby absorber located at one end of anS-band TE111mode cylindrical cavity pumped by a 3.2-Gc/s microwave source. A dc bias magnetic field was also applied to the absorber. When both the laser and the absorber rods were cooled to 80°K, the modulated beam attained maximum modulation index with a bias field of 2.5 kOe. This and other experimental results agree well with calculations made from theoretical analysis of this modulation technique, and indicate that the various known relaxation mechanisms in the spin system of ruby do not impose a frequency limit to this method of modulation.
The Zeeman effect of the B and R absorption lines of ruby was observed photographically with strong pulsed magnetic fields up to 230 kOe at 77°K. This was achieved by flashing a Xe-lamp at the time when a magnetic field reached its peak. The duration of the pulsed field was made to be about 1 m sec which was much longer than that of the pulsed light, about 20 µ sec. The g -values of the excited states responsible for the B and R lines were measured and the comparison was made with those reported by other authors. For the Zeeman patterns of the R lines with magnetic fields perpendicular to the optic axis, the Paschen-Back effect was calculated and compared with the observed patterns.
The Zeeman effect of R-absorption lines in ruby was observed in a high pulsed magnetic field up to 520 kOe using an optical multichannel analyzer. The experiment was done under a thermal equilibrium condition ( T =130 K) and the Zeeman line intensity is satisfactorily explained. When a magnetic field is applied perpendicular to the C 3 -axis, the excited \barE and 2\barA states show a level mixing and the observed pattern indicates a clear Paschen-Back effect The observed g // -values of the excited states are -2.49±0.05 and 1.49±0.05 for \barE and 2\barA, respectively. g ⊥ -values for these levels were estimated to be smaller than 0.1 at 500 kOe.
Lasing without population inversion was studied in a three-level solid state medium of ruby. The two closely-spaced lower levels are the Zeeman split sublevels of the ground state 4A2 of ruby, corresponding to M=−3/2 and M=+l/2. The transition between the lower levels is the electric dipole-forbidden. When the angle between the constant magnetic field and C-axis of ruby is 90°, the transition matrix element of the electric dipole-forbidden transition is quite large.1 A strong microwave field which operates at K-band coherently couples the two lower levels.2 Changing the constant magnetic field intensity or adjusting the frequency of the pumping microwave field, the exact resonant excitation can be established. The upper level of the lasing process is the Zeeman split sub-level M=−l/2 of 2E state. The ruby crystal, with about 0.05% chromium concentration, is placed at the center of a cylindrical cavity and cooled by liquid helium. With 500 Gauss magnetic field, the coupling microwave frequency is 13.1 Ghz and the required power is around 100 W/cm2.
Ruby lasers, when operated near threshold at 77°K with a low Q cavity (one end silvered for maximum reflectivity and the other end uncoated) oscillated at two frequencies. The frequency differences near threshold, measured with Fabry-Perot etalons, were in good agreement with the splitting of the R1 doublet, previously measured by microwave methods. As the power of the laser increased, the frequency difference between the oscillating components [2Ē → 4A2(±32, ±½)] decreased. These results are accounted for on the basis of saturation broadening of overlapping Lorentzian lines. The observed variations in the splitting are in good accord with the theory. The rise in temperature of the ruby rods produced a negligible change in the splitting of the R1 doublet. The same mechanism which produces the variation in the splitting should broaden the output spectrum of the laser.
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.