Measurement of χ⁽²⁾ symmetry in a poled fiber

We measure the values of individual χ⁽²⁾ tensor components in a birefringent periodically poled silica fiber through spectrally separated type I and type II second-harmonic generation. We demonstrate that the χ⁽²⁾ tensor symmetry is consistent with that of χ⁽²⁾ in silica and thereby provide experimental evidence that this χ⁽²⁾ originates from a χ⁽³⁾ process.