Complex sentence: Without a sample there is no phase difference for the two beams in detector 1, thus yielding constructive interference: both have incurred wavelength + "k" phase shift due to two front-side reflections and one transmission through a glass plate.
Split into simpler sentences: Without a sample, there is no phase difference in the two beams in detector 1, yielding constructive interference. Both beams will have undergone a phase shift of wavelength + "k" due to two front-side reflections and one transmission through a glass plate.