We propose a novel scheme of achromatic phase matching for second-harmonic generation (SHG) in tilted quasi-phase matching gratings. The spectral angular dispersion is introduced in fundamental waves such that each frequency component satisfies the two-dimensional quasi-phase matched condition. This is equivalent to simultaneous quasi-phase- and group-velocity-matched SHG for ultrashort pulses. Equations to describe achromatic conditions are derived and applied specifically to periodically poled lithium niobate (PPLN). The phase-matching bandwidth for 10 mm-long PPLN increases by factors of 110 and 32 at wavelengths of 0.8 and 1.55 μm, respectively, compared with those for the conventional quasi-phase matching.
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