across the passes of the signal; broadband amplification by dumping the idler and optionally individually detuning the crystals; complete pump depletion by offsetting the pump and signal in time and space at every pass and feeding one pump pulse through all passes; high gain with BBO, since BBO is only available in small dimensions. Since the direction of the beams is fixed, multiple passes cannot be overlapped into a single small crystal like in a Ti:Sa amplifier. Unless one uses noncolinear geometry and adjusts amplified beams onto the parametric fluorescence cone produced by the pump pulse. Relationship to parametric amplifiers in electronics The idea of parametric amplification first arose at much lower frequencies: AC circuits, including radio frequency and microwave frequency (in the earliest investigations, sound waves were also studied). In these applications, typically a strong pump signal (or "local oscillator") at frequency f passes through a circuit element whose parameters are modulated by the weak "signal" wave at frequency fs (for example, the signal might modulate the capacitance of a varactor diode). The result is that some of the energy of the local oscillator gets transferred to the signal frequency fs, as well as the difference ("idler") frequency f-fs. The term parametric amplifier is used because the parameters of the circuit are varied. The optical case uses the same basic principle—transferring energy from a wave at the pump frequency to waves at the signal and idler frequencies—so it took the same name. See also Optical parametric oscillator Footnotes and references 1. Boichenko, V.L.; Zasavitskii, I.I.; Kosichkin, Yu.V.; Tarasevich, A.P.; Tunkin, V.G.; Shotov, A.P. (1984). "A picosecond optical parametric oscillator with amplification of the tunable semiconductor laser radiation". Sov. J. Quant. Electronics 11 (1): 141–143. 2. Magnitskii, S.A.; Malakhova, V.I.; Tarasevich, A.P.; Tunkin, V.G.; Yakubovich, S.D. (1986). "Generation of bandwidth-limited tunable picosecond pulses by injection-locked optical parametric oscillator". Optics Letters 11 (1): 18–20. External links NOPA and Group Velocity Rainbow in photo Nonlinear optics Electronic amplifiers Laser science