Pulsed, coherent light is generated by optical mixing which takes place inside the resonator of a pulsed laser oscillator. One of the beams to be mixed is generated by the pulsed laser, and the other by a distinct, external laser oscillator. If the light from the external oscillator is modulated, that modulation will be transferred onto the light generated by the optical mixing. Using sum frequency generation, light for sodium excitation, such as for a guide star, can be generated with the optimal modulation of spectral and temporal properties. If the pulse repetition frequency is chosen to be at or near the magnetic (Larmor) resonance of the sodium, then the amount of scattered light due to sodium excitation is enhanced, and the magnetic field at the location of the sodium atoms can be measured.