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Novel semiconductor laser based optical sensors are presented which allow to implement highly sensitive systems for monitoring of different biomarkers in medical applications, of trace gases in environmental diagnostics, of explosives in security systems and of dangerous volatile organic compounds in buildings in the future. A novel detection principle is presented and investigated. Mode degeneracy of lasers oscillating on two modes simultaneously is used to enhance the sensitivity of the sensor and Relative Intensity Noise (RIN) is chosen for monitoring the intensity difference between the modes to considerably reduce sensor cost. The effect of wavelength tuning and mode intensities on RIN spectra has been investigated. RIN has been discussed in the frame of mode competition and mode partition noise theories. The results show that wavelength tuning has no effect on the RIN spectra which is very beneficial for our sensor principle. On the other hand side, RIN is found to be an excellent indicator for mode intensity ratio variations which strongly supports our sensor methodology. Combining spectral tuning of the mode degeneracy across the spectral finger prints of the molecules to be monitored with RIN read-out, it is shown that our sensors have a high potential to be highly sensitive and selective.