  We report modulation transfer spectroscopy on the D2 transitions in 85Rb and
87Rb using a simple home-built electro-optic modulator (EOM). We show that both
the gradient and amplitude of modulation transfer spectroscopy signals, for the
87Rb F=2 to F'=3 and the 85Rb F=3 to 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 85Rb F=2 to 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.
