Reduction of Fringe Noise in a Multi-pass Absorption Cell by Using the Wavelength Modulation Technique

The wavelength modulation technique was adopted to reduce the spurious fringe noise that arises from the interference between reflected lights in the multi-pass cell. The experimental results show that the fringe noise was reduced by half and the signal-to-noise ratio was doubly improved without any other additional equipment such as a lock-in amplifier, which is generally used for the phase-sensitive detection. Even though phase-sensitive detection was not employed, sufficient sensitivity for the detection of the coolant leakage in the pressurized heavy water reactor could be obtained. This simple method has advantages in applications to the real coolant detection in nuclear power plants. Also, this method can be utilized in other applications using absorption spectroscopy with a multi-pass cell.