We propose and demonstrate a scheme for second-harmonic generations with ultra-broad acceptance bandwidth and flat-top spectrum shape by using quasi-phase-matching modified fan-out periodically poled crystals with transversely varying grating length, in combination with spatial spectral dispersion of fundamental pulses. It is shown that, theoretically, the scheme has the capability of broadening the bandwidth and tailoring the spectrum shape of the second-harmonic arbitrarily, by appropriately patterning the transverse variation of ...
