An apparatus for laser frequency conversion having a high damage threshold, comprising: a fundamental laser light source configured to generate fundamental wavelength laser light; a first nonlinear optical crystal configured to receive fundamental laser light from the fundamental laser light source, the first nonlinear optical crystal configured to generate first alternate wavelength light by frequency converting at least a portion of the received fundamental laser light to first alternate wavelength light; a second nonlinear optical crystal configured to receive first alternate wavelength light from the first nonlinear optical crystal, the second nonlinear optical crystal configured to generate second alternate wavelength light by frequency converting at least a portion of the received first alternate wavelength light to second alternate wavelength light; a set of Brewster angle wavefront processing optics configured to receive first alternate wavelength light and second alternate wavelength light from the second nonlinear optical crystal, the set of Brewster angle wavefront processing optics further configured to condition the first alternate wavelength light and second alternate wavelength light emanating from the second nonlinear optical crystal; and a harmonic separator configured to receive the first alternate wavelength light and the second alternate wavelength light from the set of Brewster angle wavefront processing optics, the harmonic separator configured to at least partially separate the first alternate wavelength light from the second alternate wavelength light; characterised in that the harmonic separator comprises: a harmonic separation element (218, 700,801,901), the harmonic separation element having a first Brewster angle surface (704, 804, 904), a total internal reflection surface (706, 806, 906), and a second Brewster angle surface (708, 814, 914), the TIR surface between the first Brewster angle surface and the second Brewster angle surface; wherein when the first alternate wavelength light and the second alternate wavelength light are received at the first Brewster angle surface the light is transmitted through the harmonic separation element and undergoes TIR on the TIR surface, at least a portion of the light exits the harmonic separation element through the second Brewster angle surface. a fundamental laser light source configured to generate fundamental wavelength laser light; a first nonlinear optical crystal configured to receive fundamental laser light from the fundamental laser light source, the first nonlinear optical crystal configured to generate first alternate wavelength light by frequency converting at least a portion of the received fundamental laser light to first alternate wavelength light; a second nonlinear optical crystal configured to receive first alternate wavelength light from the first nonlinear optical crystal, the second nonlinear optical crystal configured to generate second alternate wavelength light by frequency converting at least a portion of the received first alternate wavelength light to second alternate wavelength light; a set of Brewster angle wavefront processing optics configured to receive first alternate wavelength light and second alternate wavelength light from the second nonlinear optical crystal, the set of Brewster angle wavefront processing optics further configured to condition the first alternate wavelength light and second alternate wavelength light emanating from the second nonlinear optical crystal; and a harmonic separator configured to receive the first alternate wavelength light and the second alternate wavelength light from the set of Brewster angle wavefront processing optics, the harmonic separator configured to at least partially separate the first alternate wavelength light from the second alternate wavelength light; characterised in that the harmonic separator comprises: a harmonic separation element (218, 700,801,901), the harmonic separation element having a first Brewster angle surface (704, 804, 904), a total internal reflection surface (706, 806, 906), and a second Brewster angle surface (708, 814, 914), the TIR surface between the first Brewster angle surface and the second Brewster angle surface; wherein when the first alternate wavelength light and the second alternate wavelength light are received at the first Brewster angle surface the light is transmitted through the harmonic separation element and undergoes TIR on the TIR surface, at least a portion of the light exits the harmonic separation element through the second Brewster angle surface. a harmonic separation element (218, 700,801,901), the harmonic separation element having a first Brewster angle surface (704, 804, 904), a total internal reflection surface (706, 806, 906), and a second Brewster angle surface (708, 814, 914), the TIR surface between the first Brewster angle surface and the second Brewster angle surface; wherein when the first alternate wavelength light and the second alternate wavelength light are received at the first Brewster angle surface the light is transmitted through the harmonic separation element and undergoes TIR on the TIR surface, at least a portion of the light exits the harmonic separation element through the second Brewster angle surface.