Spectral Stability Assessment of an nJ-Class Ultrafast Femtosecond Fiber Laser Using Ensemble Statistics and Principal Component Analysis

This article presents a structured spectral-stability assessment of an nJ-class ultrafast fiber laser generating femtosecond pulses with an approximate pulse duration of 115 fs, based on an ensemble of 61 consecutively acquired optical spectra. The study is motivated by the practical need to extract reliable short-sequence stability information from routine compact-spectrometer exports without requiring a separate pulse-diagnostic instrument at the initial assessment stage. For each spectrum, peak wavelength, centroid wavelength, FWHM bandwidth, integrated spectral area, correlation with the ensemble mean spectrum, and RMS deviation were calculated. Principal component analysis (PCA) was then applied to reduce the full spectral ensemble into a compact diagnostic space and to identify the dominant modes of residual spectral variation. The analyzed spectra yielded a peak wavelength of 775.31 ± 0.19 nm, a FWHM bandwidth of 7.95 ± 0.20 nm, an integrated spectral area of 10.43 ± 0.42 a.u.·nm, and a correlation with the mean spectrum of 0.99957 ± 0.00019, confirming a highly repeatable spectral envelope. PCA showed that PC1 and PC2 explained 66.50% and 12.60% of the variance, respectively, while PC3 contributed only 1.90%, indicating that the measured variability was weak and largely low-dimensional. These results demonstrate that consecutively exported optical spectra can provide a defensible and physically interpretable short-sequence stability assessment of ultrafast femtosecond fiber lasers, offering a practical route for routine monitoring, early-stage diagnostics, and future integration with simultaneous temporal and spectral characterization.