Femtosecond pulse-shape modulation at kilohertz rates

We demonstrate a new scanning femtosecond pulse-shaping technique that allows pulse shapes to be modulated at kilohertz rates. This technique is particularly useful for lock-in measurements in which the signal is synchronized with the alternating pulse shapes. The pulse-shape lock-in technique is demonstrated in resonant coherent anti-Stokes Raman scattering, where it is shown to significantly improve the ratio of the resonant signal to both the nonresonant background and to noise.