A measurement system for measuring at least one mode spectrum of a waveguide formed in a top surface of a substrate and having a depth-of-layer DOL of greater than 50 microns, the system comprising: a coupling prism having an input surface, an output surface and a coupling surface, a refractive index n p, and a prism angle α between the output surface and the coupling surface, and wherein the coupling surface interfaces with the waveguide at the substrate top surface, thereby defining a substrate-prism interface; a light source system configured to illuminate the substrate-prism interface through the input surface of the prism, thereby forming reflected light that includes mode lines of the at least one mode spectrum, wherein the reflected light exits the output surface of the coupling prism; a photodetector system having a detector and arranged to receive the reflected light from the coupling prism and detect the mode lines of the at least one mode spectrum on the detector, the mode lines having a distorted mode-line spacing due to refraction of the reflected light at the output surface of the coupling prism; and the coupling prism angle α being in the range 0.81 α max ≦ α ≤ 0.99 α max, wherein α max defines a maximum coupling prism angle associated with total internal reflection occurring at the output surface of the coupling prism, wherein α max = arcsin n s n p + arcsin 1 n p with n s being the refractive index of the substrate; and a controller being configured to process the detected mode lines of the at least one mode spectrum to correct the distorted mode-line spacing to obtain a corrected mode spectrum. a coupling prism having an input surface, an output surface and a coupling surface, a refractive index n p, and a prism angle α between the output surface and the coupling surface, and wherein the coupling surface interfaces with the waveguide at the substrate top surface, thereby defining a substrate-prism interface; a light source system configured to illuminate the substrate-prism interface through the input surface of the prism, thereby forming reflected light that includes mode lines of the at least one mode spectrum, wherein the reflected light exits the output surface of the coupling prism; a photodetector system having a detector and arranged to receive the reflected light from the coupling prism and detect the mode lines of the at least one mode spectrum on the detector, the mode lines having a distorted mode-line spacing due to refraction of the reflected light at the output surface of the coupling prism; and the coupling prism angle α being in the range 0.81 α max ≦ α ≤ 0.99 α max, wherein α max defines a maximum coupling prism angle associated with total internal reflection occurring at the output surface of the coupling prism, wherein α max = arcsin n s n p + arcsin 1 n p with n s being the refractive index of the substrate; and a controller being configured to process the detected mode lines of the at least one mode spectrum to correct the distorted mode-line spacing to obtain a corrected mode spectrum. with n s being the refractive index of the substrate; and a controller being configured to process the detected mode lines of the at least one mode spectrum to correct the distorted mode-line spacing to obtain a corrected mode spectrum.