Wide-band acousto-optic light modulator for frequency domain fluorometry and phosphorimetry

Multifrequency-phase and modulation fluorometry allows for accurate analysis of fluorescence decay in the frequency domain. Essential to these frequency domain methods is a high-frequency modulation of the light source. Techniques for generating wide-band modulation of light are currently limited to the use of Pockel's cells and intrinsically modulated sources such as mode-locked lasers and synchrotron radiation. We present a method that employs two acousto-optic modulators in series for use with cw light sources. This modulator system gives two orders of magnitude more intensity output than the Pockel's cell modulator and requires less than one-tenth of the rf driving power. In addition, the Pockel's cell system is limited to modulation frequencies less than 250 MHz, whereas the particular implementation discussed here gives a quasicontinuous distribution of modulation frequencies from dc to 320 MHz. To obtain this range of frequencies, acoustic standing waves are set up simultaneously in each modulator, and the desired modulation frequency is achieved by choosing the proper combination of the two standing-wave frequencies. Light modulation is obtained at twice each of the individual standing-wave frequencies and at the sum and difference of twice the two acoustic frequencies. Data are presented to illustrate the use of this system for the measurement of phosphorescence as well as fluorescence decay.