We present a flexible and cost-effective Z-scan-based method, referred to as Beam Size Relative Variation (BSRV), for the determination of either the nonlinear refractive index \({n}_{2}\) or the femtosecond pulse duration τ in the visible spectral range (650–900 nm). The technique, implemented using a tunable optical parametric amplifier (OPA), accommodates arbitrary beam profiles, thus removing the constraints imposed by traditional Gaussian or circular symmetry assumptions. Spatial characterization is performed via CCD imaging, while temporal information can be inferred directly from Z-scan measurements when a reference material is used. As a proof of concept, the method is applied to fused silica, a widely used reference material in nonlinear optics. A total of 108 Z-scan measurements were conducted across the spectral range, allowing for a robust statistical analysis of the extracted parameters. A new reference value is established for fused silica: \(n_{2} = \left( {1.8 \pm 0.4} \right) \times 10^{ - 20} m^{2} /W\) . Notably, the pulse durations extracted from BSRV Z-scan data are statistically validated through independent measurements based on two-photon absorption (2PA) autocorrelation, with both methods revealing consistent spectral trends. All simulation tools and analysis codes are provided in the Supplementary Information to ensure reproducibility. This approach offers a reliable and accessible framework for nonlinear optical characterization, particularly in experimental contexts where conventional autocorrelation systems may be unavailable or cost prohibitive.
