An adjustable beam directing optical system for a focused laser differential interferometer (FLDI) instrument according to various aspects of the present technology may include an optical half waveplate to achieve an incident linear polarization orientation with equal components of laser intensity aligned to the vertical and horizontal axis of the optical system, and an optical prism for splitting these components of an incident laser beam into two orthogonally-polarized beams equally about an optical axis of the FLDI instrument. A series of beam realignment devices positioned downstream of the optical prism are configured to selectively direct each beam to a predetermined location.