Intensity Profile Reshaping of a Spectrally Broadened Gaussian Beam

Research into the spatial reshaping of monochromatic laser beams grew significantly in the late 1990s due to improvements in the fabrication of diffractive optical elements. Nowadays, some applications, such as optical coherence tomography, necessitate the use of broadband light beams with a spectral width of hundreds of nanometers. The difficulty in reshaping such spectrally broadened beams lies in the wavelength dependence of the beam shaping process. This paper presents a numerical study of the wavelength dependence of two beam shaping techniques that allow a Gaussian beam to be transformed into a flat-top or doughnut intensity profile in the focal plane of a focusing lens. The first technique is based on the diffraction of an incident Gaussian beam passing through a simple binary diffractive optical element. The second technique can be described as an interferometric method, as it involves the coaxial superposition of two Gaussian beams emerging from a Michelson interferometer. We compared the stability of these two techniques’ ability to reshape the beam versus the spectral bandwidth of the incident Gaussian beam. We showed that the interferometric method is more resilient than the diffractive method to changes in the spectral bandwidth of the Gaussian beam. We also considered the case of a quasi-monochromatic beam delivered by a widely tunable laser and reshaped using the interferometric method, where the dispersion of beam reshaping could be mitigated by two programmable liquid lenses that enable control of the curvature of the Michelson interferometer mirrors.