This paper studies the use of laser diodes and single element photo-detectors in mechanical alignment. Considering Gaussian beam theoretical model, the effect of laser beam diameter is studied using computer simulation. Optimum conditions that give the highest sensitivity are proved to be where:
(i) the laser beam diameter matches the diameter of the sensitive area of the detector.
(ii) the laser beam waist is located at the sensitive area of the detector.
The photo-detector with two-complement sensitive area has been studied as a better replacement of a single area photodetector. It was proved that the use of this type of detectors , with all of the other parameters unchanged, doubles the alignment sensitivity. Finally, an experimental verification has been performed which gives a good agreement with the results obtained by computer simulation, where an alignment sensitivity of ± 0.3558 ttm was obtained.