text: As the pump and the probe beam must have the same exact frequency, the most convenient solution is for them to come from the same laser.  The probe beam can be made of a reflection of the pump beam passed through neutral density filter to reduce its intensity.  To fine-tune the frequency of the laser, a diode laser with a piezoelectric transducer that controls the cavity wavelength can be used.  Due to photodiode noise, the laser frequency can be swept across the transition and the photodiode reading averaged over many sweeps.
  In real atoms, there are sometimes more than two relevant transitions within the sample's Doppler profile (e.g. in alkali atoms with hyperfine interactions). This will generate the apparition of other dips in the absorption feature due to these new resonances in addition to crossover resonances.