Method and apparatus for frequency doubling a laser beam

DETAILED DESCRIPTION OF THE INVENTION The apparatus used for practicing the invention is shown schematically in the drawing.
The various beams are shown as separated from one another for purposes of clarity.
It will be appreciated, however, that the parallel beams are in fact coincident.
A laser active medium 10 is provided for emitting a beam 12 having a desired fundamental frequency.
In this particular embodiment of the invention, a YAG laser is utilized for emitting a beam having a wavelength of 1064 nanometers.
This wavelength is in the infrared range.
The system employed herein provides a beam 14 having a wavelength of 532 nm.
A second harmonic generator 16, which is a non-linear crystal, is positioned within the laser resonator cavity in the path of the beam 12 from the laser source.
The crystal may, for example, be made from potassium titanyl phosphate.
A dichroic Brewster plate 18 is positioned between the laser source 10 and the crystal 16 to insure the fundamental frequency polarization.
The dichroic Brewster plate 18 is designed to transmit substantially 100% of the p-polarized 1064 nm beam and to reflect substantially 100% of a 532 nm beam in arbitrary polarization, either p or s.
A quarter wave plate 20 at the second harmonic frequency is positioned above the dichroic dielectric mirror 18 in the path of the light reflected therefrom.
The plate 20 has its optic axis oriented at 45.
degree.
to the polarization of the harmonic beam.
A mirror 22 is provided to reflect the beam 14 passing through the quarter wave plate 20 back through the quarter wave plate to the Brewster plate 18.
The mirror 22 and quarter wave plate 20 may actually be superimposed.
An output mirror 24 is positioned at the front end of the apparatus to transmit all light having a 532 nm wavelength and reflect all beams of 1064 nm wavelength.
A rear mirror 26 is provided at the opposite end of the apparatus which is designed for 100% reflection of the 1064 nm wavelength.
In operation, the YAG laser emits a beam 12 having a wavelength of 1064 nm