Observation of high-contrast coherence fringes in high-order harmonic generation

We present a direct measurement of the coherence length in the high-order-harmonic generation process up to orders as high as 25. The harmonic signal generated in an argon gas cell by a 805-nm, 6-mJ, 30-fs laser is studied as a function of medium length and shows very high-contrast coherence fringes. From a one-dimensional time-dependent model, we unravel the conditions for which such Maker fringes can appear in high-order harmonic generation.