Ultrasensitive direct-field retrieval of femtosecond pulses by modified interferometric field autocorrelation

We report on spectral phase retrieval of 50 MHz, 374 fs optical pulses at 1560 nm with 28 aJ coupled pulse energy by measuring two modified interferometric field autocorrelation traces using a 5-cm-long periodically poled lithium niobate waveguide. The corresponding sensitivity is 1.1 x 10−7 mW^2, improving on the previous record by about 20 times. The same data traces can also be used to retrieve the power spectrum, given that the ratio of powers at two specific wavelengths is known.