Many lasers produce pulses or groups of photons in bursts at some frequency rather than a continuous stream of light. Included in this group are the “ultrafast” lasers (usually based on titanium–sapphire) used in two-photon microscopy; these typically produce pulses of approximately 140 fs duration and at 80 MHz repetition rate. Ultrafast pulsed lasers effectively “pack” the energy of continuous radiation into very short pulses, thus producing extremely high instantaneous intensity of light necessary for efficient non-linear excitation.