Achromatic complex holograms for laser mode conversion.

While conventional complex phase masks are chromatic, we present an achromatic holographic phase mask capable of performing optical beam transformations in a spectral range exceeding 1000 nm. The system consists of a holographic phase mask fabricated by encoding the desired phase profiles into volume Bragg gratings, inserted in between two surface gratings. This device automatically adjusts each spectral component diffracted by the surface grating to the Bragg angle of the volume Bragg grating and equalizes phase incursion for all diffracted wavelengths. Transverse mode conversion is demonstrated and compared with theory for multiple narrow line laser sources operating from 488 to 1550 nm and for a broadband femtosecond source.