This paper presents results of the characterisation of type I GaSb/AlSb superlattices (SLs) with a thin GaSb layer and varying thicknesses of an AlSb layer. Nextnano software was utilized to obtain spectral dependence of absorption and energy band structure. A superlattice (SL) with an energy bandgap of ~ 1.0 eV and reduced mismatch value was selected for experimental investigation. SLs with single (sample A) and double (sample B) AlSb barriers and a single AlSb layer (sample C) were fabricated using molecular beam epitaxy (MBE). Optical microscopy, high-resolution X-ray diffractometry, and photoluminescence were utilized for structural and optical characterisation. The presence of satellite and interference peaks in diffraction curves confirms the high crystal quality of superlattices. Photoluminescence signal associated with the superlattice was observed only for sample B and contained three low-intensity peaks: 1.03, 1.18, and 1.25 eV. The first peak was identified as the value of the energy bandgap of the SL. Other two peaks are related to optical transitions between defect states located at the interface between the SL and the top AlSb barrier. The time-dependent changes observed in the spectral characteristics are due to a modification of the SL/AlSb interface caused by the oxidation and hydroxylation of the AlSb layer.