This research can be divided into two main parts. Firstly, the main challenge was to design and test a non-invasive in situ stress monitoring system, to be placed in a vacuum chamber, based on a novel approach using a laser-fibre system. Secondly, two materials, namely zinc sulphide (ZnS) and yttrium fluoride (YF3) , were to be deposited on CMX glass substrates, under different conditions, and analysed both in situ using the newly developed monitoring system and ex situ using various characterisation techniques. The design of the current monitoring system went through 3 different phases. It was characterised as an in situ stress monitoring of thin films based on the bending of the monitored substrate. Three criteria had to be considered during the development of the system, namely, the performance of the laser-fibre system, the accuracy of the scanning system, and the influence of the monitored substrate and the deposited layers. For a substrate length of 58 mm and a substrate thickness of 80 μm, the final prototype allowed a resolution of vertical displacement within ± 6 μm resulting in a maximum detection of± 67 m for the radius of curvature of the substrate. On average, in situ stress measurements using the monitoring device proved to have an accuracy within ± 15 MPa for layer thicknesses between 300 nm and 1.2 μm, which is generally sufficient for commonly reported stress values. Below a layer thickness of 100 nm, the stress in the layer cannot be detected accurately. The designed system also allowed monitoring the layer thickness, within ± 10 nm, in situ using optical interference effects. Single layers of YF3 and ZnS were deposited on CMX and float glass substrates. A tensile and a compressive residual stress was observed in the deposited YF3 and ZnS layers, respectively, generally in accordance with that reported in the literature and ex situ residual stress measurements, using the designed in situ stress monitor. A successful reduction in the compressive residual stress of ZnS single layers, deposited at ambient temperature, was achieved by depositing YF3/ZnS double layers at ambient temperature.