Ultrafast Laser Laboratory
Ultrashort-laser-pulse measurement: 2-photon photoconductivity
2-photon photoconductivity has recently been used to measure the duration of ultrashort laser pulses. With the advance of 100-femtosecond and sub-10-femtosecond solid state lasers, particularly the Ti:sapphire laser, measurements of sub-10-fs laser pulses are conventionally and routinely done using second-harmonic generation (SHG) technique where a nonlinear frequency conversion crystal is used and the SH signal detected by a photomultiplier tube (PMT). However, the thickness of a nonlinear crystal has to be made unconventionally thin, <20 micron, to accommodate for the large bandwidth of an ultrashort laser pulse. Subsequently, the available harmonic signal photons decrease with thinner crystal. Furthermore, due to the large bandwidth of the laser, the intrinsic dispersive property of a nonlinear crystal will distort a pulse measurement somewhat.
These shortcomings can be overcome using 2-photon photoconductivity in a semiconductor. Specifically, using GaAsP (or GaP) photodiode which has a bandgap of 2.5 eV and negligible 1-photon contribution, the pulse duration of a mode-locked 17-fs Ti:sapphire laser (1.55 eV) is measured. Elsewhere, pulse as short as 4.5 fs has also been measured using this 2-photon induced photoconductivity. The simplicity of the design, a UV photodiode and a preamplifier, is far more superior than the conventional approach of using a nonlinear crystal, a PMT, and a high voltage power supply. Furthermore, no phase-matching condition is needed, no input polarization restriction, and no group-velocity dispersion distort the pulse measurement. However, like any other high-order autocorrelation techniques, the phase of an optical pulse is not precisely measured. The quality of the 2-photon photoconductivity signal outperforms the SHG, resulting in a pulse measuring device ideal for online monitoring and diagnostic. A compact, palm-size, repetitive scanning autocorrelator is constructed using miniature rather than discrete optical components. Other than a 15 volt dc power, the picture of the compact autocorrelator shown below contains all the electronics and the optics including a HeNe passed photodiode for generating a calibrated time delay. The autocorrelator has an order of magnitude reduction in size compared to a conventional device, yet the sensitivity and integrity of a measurement is maintained. The autocorrelator shown here has a usable scan range of ~1 ps, limited by the micro-electromechanical scanning delay line. A resolution of sub-fs and an integral linearity of 2% over the full width of a measured 17-fs pulse.
The simplicity of the optical arrangement and the comparison with the conventional SHG autocorrelation technique are shown here.
For more information or preprint request contact Thomas Y. F. Tsang
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Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST