A spontaneous Raman scattering optical fiber sensor was developed for a specific need of the National Aeronautics and Space Administration (NASA) for long-term detection and monitoring of the purity of liquid oxygen \left({\mathrm{LO}}_2\right) in the oxidizer feed line during ground testing of rocket engines. The Raman peak intensity ratios for liquid nitrogen \left({\mathrm{LN}}_2\right) and {\mathrm{LO}}_2 with varied weight ratios \left({\mathrm{LN}}_2/{\mathrm{LO}}_2\right) were analyzed for their applicability to impurity sensing. The study of the sensor performance with different excitation light sources has helped to design a miniaturized, cost-effective system for this application. The optimal system response time of this miniaturized sensor for {\mathrm{LN}}_2/{\mathrm{LO}}_2 measurement was found to be in the range of a few seconds. It will need to be further reduced to the millisecond range for real-time, quantitative monitoring of the quality of cryogenic fluids in a harsh envioronment.