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We report the first observation of tunable stimulated optical emission from a nonlinear crystal without the use of an external resonator, and with greater than 50% conversion of the pump beam to the tunable frequency. Opposite faces of a LiNbO3 crystal were polished flat and parallel to provide for multiple reflections of the signal radiation (Stokes) inside the crystal. The A1 symmetry 248-cm−1 polariton mode was excited with a Q-switched ruby laser, and the signal frequency was tuned by varying the angle of incidence of the laser beam relative to the normal to the crystal surface. Operation was at room temperature; no crystal damage was observed over most of the tuning range; and laser depletion occurred within the first few nanoseconds of the Q-switched pulse. The difference between the Stokes frequency and laser frequency was varied from 42 to 200 cm−1, and the corresponding idler wavelength was varied from 238 to 50 μ.

Objectives-To summarise and to facilitate comparison of three major studies of electric utility workers that examined the relation between quantitative measurements of occupational exposure to magnetic fields and risk of brain cancer and leukaemia. These studies have been interpreted as providing conflicting evidence. Methods-A common analytical approach was applied to data from the five cohorts included in the three studies based on original data from four of the cohorts, and published data from one additional cohort. A nested case-control design with conditional logistic regression was used to estimate the relative risk/10 microteslayears (µT-years) for each of the contributing cohorts and for the combined data. The homogeneity of these estimates among the studies was also evaluated. Results-Apparent inconsistencies in the findings of these studies can be explained by statistical variation. Overall, the studies suggest a small increase in risk of both brain cancer and leukaemia. DiVerent methodological choices had little impact on the results. Based on a combined analysis of data from all five studies, the relative risk/10 µT-years was 1.12 (95% confidence interval (95% CI) 0.98 to 1.28) for brain cancer, and 1.09 (95% CI 0.98 to 1.21) for leukaemia. Conclusions-The combined estimates seem to provide the best summary measures of the data from all studies. However, fluctuations in risks among studies may reflect real diVerences, and the exposure measurements in diVerent studies may not be entirely comparable. (Occup Environ Med 1999;56:567-574)

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The power-versus-wavelength characteristic for a high-power, continuously tunable far-infrared source has been experimentally determined and compared with theory. Utilizing a Q-switched ruby laser as the pump and stimulated polariton scattering in the crystal LiNbO3 as the scattering mechanism, peak powers outside the crystal of ∼ 3 W at 200 μ are observed without crystal damage. Linewidth measurements indicate a bandwidth of < 0.5 cm−1 for the radiation over the observed tuning range 66–200 μ.