The Seattle Optical Rotation Experiments

To illustrate the procedures and problems encountered in optical rotation PNC experiments, we will discuss the University of Washington experiments in some detail. These experiments have yielded measurements of PNC in atomic bismuth and, more recently, in atomic lead. 

Of great importance in the optical rotation experiments is the laser source. The laser must be tunable, and should be highly monochromatic, stable in intensity, and emit a beam of good optical quality. Tunable dye lasers satisfying all these criteria operated well at the 648-nm bismuth line in 1974 when work began in several places. The groups at Oxford and Novosibirsk and later at Moscow have all used such lasers to measure the effect on this line. 

The linewidth of an individual mode varies dramatically between lasers, but the best appear to have about 20 MHz width. There is also fine structure on either side of the main mode about 1 to 2 GHz away. These fine structure modes can have as little as 0.1%, and up to 10% of the total light on them depending on the type of laser. These side modes can be quite troublesome. Luckily, at the 0.1% level, they do not add significantly to errors. 

Currently there are GaAlAs lasers operating in the wavelength region from 790 to 920 nm, and InGaASP lasers in the 1.1-1.6-jLtm region. While diode lasers have some drawbacks as spectral sources, their simplicity and low cost make them quite useful. Much work has been done also in putting these lasers in external cavities to decrease their linewidth and eliminate all extraneous mode structures. 

When the two polarizers are uncrossed slightly, the transmission T grows as the square of the angle of uncrossing f  

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