Power and Bandwidth of Spontaneous Parametric Emission

Theoretical and experimental results concerned with the spectral and angular distribution of spontaneous parametric emission are reported. Such emission results when light from a pumping laser is incident on a nonlinear crystal, and does not require that the parametric gain exceed the single-pass crystal loss. The emission is somewhat like spontaneous Raman or Brillouin scattering, but differs in two major respects. First, in a lossless crystal, the bandwidth of the emission is controlled by the crystal length and accepted angular aperture as opposed to a loss mechanism in the Raman or Brillouin case. Second, by changing the crystal temperature or orientation, the spontaneous emission may be tuned over several thousand angstroms. The reported experiments employed a 4880 Å laser as a pump and a 1-cm crystal of LiNb${\mathrm{O}}_3$ as the non-linear material. The application of parametric spontaneous emission to the measurement of crystal non-linearities is considered, and the advantages of this method over second-harmonic generation are discussed.