Production of coherent radiation of frequency $k\nu$($k=2,3,...$) from coherent radiation of frequency. In general, this effect is obtained through the interaction of laser light with a suitable optical medium with non-linear polarizability. The case $k=2$ is referred to as frequency doubling, $k=3$ is frequency tripling and $k=4$ is frequency quadrupling. Even higher integer values of $k$ are possible.