As a primary switching device in modern power systems

vacuum circuit breakers have become dominant in medium-voltage applications. However

the lack of reliable online vacuum degree monitoring methods limits routine operation and maintenance

as well as the wider adoption of vacuum circuit breakers in higher voltage levels. In this study

a fiber-optic laser-induced breakdown spectroscopy(FO-LIBS) approach is proposed for online vacuum degree detection in vacuum circuit breakers

effectively addressing the issues of limited flexibility and complex field operation inherent in conventional LIBS systems

while enhancing protection for both detection and analysis equipment. By employing fiber optics for laser delivery and coaxial plasma signal collection

a long-focus double-lens refocusing probe with a mirror-to-sample distance of 110 mm was designed to suit the vacuum circuit breaker application scenario. The focused beam has a diameter of 930 μm

featuring lower peak intensity and more uniform energy distribution. The plasma generated under varying vacuum degrees exhibits distinct expansion dynamics that differ from those in conventional open-path LIBS. Through plasma spectral and imaging diagnostics

the influence of ambient pressure on plasma morphology and temporal spectral evolution was investigated. The pressure-response characteristics of four spectral features are analyzed

and these features are further used for inverse estimation of vacuum degree. Furthermore

a random forest-based spectral classification algorithm was developed

achieving an accuracy of 91.3%. The proposed FO-LIBS system also demonstrates excellent flexibility and adaptability under complex operational conditions.