波长调制光谱用于提高光子计数测量的信噪比

Photon counting has been widely used in weak light even to single photon detection. However, the Shot noise of photon-counting ( N is the mean counts) for Poisson statistics seriously affects the sensitivity. In this letter, we present a new method that using wavelength modulation spectroscopy technique to improve signal-to-noise ratio of photon-counting. After the light from an external cavity diode laser is wavelength modulated and then chopped to a series of laser pulses by use an acoustic-optical modulator, the counts of fluorescence photon are recorded and then converted to an analog signal. The analog signal is added to a lock-in amplifier for harmonic demodulation using lock-in method. The fluorescence spectrum of Cs D2 line for the 6S1/2(F=4)→6P3/2 transition is studied here. The experimental demonstration has achieved a 51 dB signal-to-noise improvement factor in 2f harmonic detection. The minimum detectable count here is 3×10-4 with the residual noise 7.54 μV. The results show a potential application in fluorescence detection of single molecules.