Femtosecond pulse interferometry of a laser plasma

Summary form only given. Interferometric techniques used to diagnose plasmas, being frequency selective, do not generally provide temporal-or even spatial-information. Electron density changes can be inferred from refractive index change measured through spectral interferometry. For spectral interferometry, a Mach-Zehnder interferometer is used to generate a two pulse sequence with a reference and a probe pulse. After traversing a sample the beam is sent into a high resolution spectrometer which temporally stretches the two ultrashort pulses such that interference occurs. An index change results in a change in spectral fringe spacing. In our method, the Mach Zehnder delay generator itself is used as the spectrum analyzer. After traversing a sample with a time-dependent refractive index n(t), the two pulse sequence is sent back through the same Mach Zehnder. That second transmission through the instrument consists ofa sequence of four pulses. Only the middle two of these four pulses interfere because they share a common path.