Abstract <p>Contactless laser transillumination method is promising for studying the flow structure in turbulent supersonic jets. However, interpretation of the experimental results is a nontrivial task, especially in terms of turbulence, due to the spatial inhomogeneity of the fields of gas-dynamic characteristics. In this work, the spectral composition of turbulence in axially symmetric jets in the root region (before closing mixing layers) is analyzed based on laser sensing results. Intensity fluctuations of a laser beam propagating through a submerged supersonic jet are experimentally studied. The spectral density of the power of the fluctuations fully corresponds to the known experimental and calculated spectra for a moving turbulent medium, with the exception of the high-frequency range, where the spectrum is a superposition of two exponential components: the first is close to the known value −14/3 and the second is lower. This is shown to be due to the difference in motion speeds and the structure of vortex flows in the external and internal regions of the mixing layers. The results can be used in calculations of gas-dynamic characteristics and the acoustic emission of jet engines.</p>