Modulation transfer spectroscopy in atomic rubidium

We report modulation transfer spectroscopy on the D2 transitions in Rb-85 and Rb-87 using a simple home-built electro-optic modulator (EOM). We show that both the gradient and amplitude of modulation transfer spectroscopy signals, for the Rb-87 F = 2 -> F ' = 3 and the (8)5Rb F = 3 -> F ' = 4 transitions, can be significantly enhanced by expanding the beams, improving the signals for laser frequency stabilization. The signal gradient for these transitions is increased by a factor of 3 and the peak to peak amplitude was increased by a factor of 2. The modulation transfer signal for the Rb-85 F = 2 -> F ' transitions is also presented to highlight how this technique can generate a single, clear line for laser frequency stabilization even in cases where there are a number of closely spaced hyperfine transitions.