Direct measurement of the critical power of femtosecond Ti:sapphire laser pulse in air

We report a simple experiment to directly determine the critical power for self-focusing in air by measuring the focal shift of the focused femtosecond Ti:sapphire laser pulses. The measured critical power is 10 GW for the 42 fs laser pulse; it gradually decreases to 5 GW for (chirped) pulse duration longer than 200 fs.