When coherent electromagnetic radiation generated by powerful laser system is tightly focused, pursuing the aim to achieve the highest value of intensity possible, it may be challenging estimating this intensity with a sufficient degree of accuracy. If the energy of a laser pulse, its duration, time profile and focal spot radius are known, evaulation of the maximal intensity is straightforward. However, for high power (sub-petawatt and above) femtosecond laser systems, the inherent uncertainties of these four parameters (except maybe the pulse duration) are rather high, so that different estimation models of the laser intensity in the focus may substantially disagree. Presently, the question of whether or not intensities above 10^21 W/cm2 have ever been achieved remains debatable, although values of this order and above are the main goal of the two Extreme Light Infrastructure (ELI) pillars.
In this context, a reliable method allowing to calibrate ultrahigh laser intensities becomes of even higher demand. Here we discuss the reliability of a method for the measurement of ultrahigh laser intensities, based on the effect of tunneling field ionization of heavy atoms and ions. To this end, we employ the highly nonlinear dependence of tunneling ionization rates on the laser intensity. This nonlinearity leads to the emergence of steep plateaus in the distribution of charge states in the laser focus and in such a way to allowing estimate, with a high degree of certainty, the laser intensity at focus.