TADPOLE measures ultrashort pulses with zeptojoule energies
Investigators at the Combustion Research Facility of Sandia National Laboratories (Livermore, CA) have developed a method to measure femtosecond laser pulses with average energies of hundreds of zeptojoules (10-21 J). Termed temporal analysis by dispersing a pair of light E-fields (TADPOLE), the technique is 1 billion times more sensitive than the laboratory`s frequency-resolved optical-gating (FROG) method, from which TADPOLE is derived.
In a FROG device, the laser pulse is first split to create two "replicas" of itself. Mirrors direct these pulses to arrive at a nonlinear crystal so they overla¥in time (see Laser Focus World, Nov. 1994, p. 9). The spectra produced by the overlapping pulses change with differences in the time delay and provide the intensity and phase versus time of the initial pulse. Because a nonlinear optical crystal is required, FROG sensitivity is limited to measuring pulses having picojoule energies or more. Many researchers, however, need to measure pulses that are much weaker. The key to TADPOLE, explains Sandia`s Rick Trebino, is that "you never have a weak pulse by itself. The nonlinear processes that occur in short-pulse lasers require that they emit strong pulses, which eventually make the weak ones." Developed in conjunction with Ian Walmsley at the University of Rochester (NY), TADPOLE combines a FROG measurement of the strong pulse--used as a reference--with spectral interferometry, which yields the phase difference between the reference and the unknown pulse being measured. The technique has measured pulses with average energies as low as 42 zeptojoules.