Active noise reduction for a differential Helmholtz photoacoustic sensor excited by an intensity-modulated light...

A highly sensitive differential Helmholtz photoacoustic sensor with active noise reduction was reported. Coupled to one cavity of the photoacoustic cell, an intensity-modulated excitation light would reflect multiple times to produce photoacoustic signal, and meanwhile cause the solid-state photoacoustic effect forming differential mode noise with the frequency same as the photoacoustic signal, which could not be suppressed by conventional differential technology. Wavelength modulation technology is a splendid method to restrain this effect, which is not suitable for light sources with not adjustable wavelength. To suppress this kind of noise, an intensity-modulated compensation light was coupled to another cavity, whose central wavelength was at the non-absorption line of the measured gas. The compensation light was of the same frequency, phase, and power as the excitation light, by which the solid-state photoacoustic effects were produced to form destructive interference called active noise reduction. The experiment results showed that the active noise reduction significantly improved the signal-to-noise ratio and signal-to-background ratio. Compared with the differential, the differential with active noise reduction improved signal-to- noise ratio by about 1.2 times and signal-to-background ratio by about 9.4 times. When low-power near-infrared lasers were employed as the two light sources, the minimum detection limits for acetylene and methane reached 21 and 200 ppb, respectively.