The current‐density‐profile diagnostic system developed for the Texas experimental tokamak (TEXT) employs a cw dye laser system to induce fluorescence in a Li beam injected into a tokamak. This laser has spectral and polarization modulation requirements not available in commercial systems. The required spectrum is a bandwidth of approximately 5 GHz at a wavelength of 667 nm. The intensity within the bandwidth should be as uniform as possible, i.e., a quasicontinuum within the 5‐GHz bandwidth. The polarization of the laser must remain linear, but the plane of polarization must be rotated at a high frequency, 50 kHz. The details of the Li atomic spectroscopy in a tokamak and the requirements of the diagnostic system that lead to these desired laser parameters will be addressed. The laser system and polarization modulator which have been developed will be discussed in detail, including both design and performance parameters.