  We propose non-phase-matched sum frequency generation (SFG) as a method for
characterizing broadband multimode light. We demonstrate its advantages using
high-gain parametric down conversion (PDC) as an example. By generating the sum
frequency in the near-surface region of a nonlinear crystal, we increase the
SFG efficiency and get rid of the modulation caused by chromatic dispersion,
known as Maker fringes. The obtained results interpreted in terms of the
Schmidt mode formalism reveal the sensitivity of the SFG spectrum to the number
of frequency modes. Furthermore, we demonstrate efficient non-phase-matched
three- and four-frequency summation of broadband multimode light, barely
possible under phase matching.
