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Journal of North University of China

Improvement of Photon-Counting Signal-to-Noise Ratio Based on Wavelength Modulating Spectroscopy

Jia Suo-Tang

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

Photon counting has been widely used in weak light even to single photon detection.However,the Shot noise of photon-counting N for Poisson statistics seriously affects the sensitivity.A method using wavelength modulation spectroscopy technique to improve signal-to-noise ratio of photon-counting is presented.The light from an external cavity diode laser is wavelength modulated and then chopped to a series of laser pulses by using a 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 is the 6S_1/2(F=4)→6P_3/2 transition of Cs D_2 line and the detected probability of photon is 35%.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 cold molecules or single molecule.

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Photon counting has been widely used in weak light even to single photon detection.However,the Shot noise of photon-counting N for Poisson statistics seriously affects the sensitivity.A method using wavelength modulation spectroscopy technique to improve signal-to-noise ratio of photon-counting is presented.The light from an external cavity diode laser is wavelength modulated and then chopped to a series of laser pulses by using a 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 is the 6S_1/2(F=4)→6P_3/2 transition of Cs D_2 line and the detected probability of photon is 35%.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 cold molecules or single molecule.
The wavelength modulation indices are measured based on harmonic amplitude ratio of 4f_(amp)/6f_(amp) (4f_(amp) and 6f_(amp) are the 4- and 6-th harmonic central peak amplitudes correspondingly) with the Doppler-free selective reflection modulation spectroscopy. The experiments for the 6S_(1/2)(F = 4) ? 6P_(3/2)(F' = 5) transition of cesium D_(2) line with 30-MHz linewidth were carried out. The 4f- and 6f-harmonic signals were detected with two digital lock-in amplifiers separately. The maximum error for modulation indices measurement was ±0.1 within the range of m from 3 to 6. The non-linear modulation behaviour of an external cavity diode laser induced by voltage tuning was studied with this method. The method for modulation indices measurement does not require a solid etalon as usual for measuring the wavelength modulation depth and the absorption linewidth correspondingly.
Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The technique is especially adaptable to diode lasers, which can be modulated easily, and has been used extensively in the near-infrared and infrared spectral regions. The availability of blue diode lasers now means that the accessible wavelength region can be increased. We successfully demonstrate wavelength-modulation spectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line of potassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated 6(p)(2 3)P(2) metastable level. Information on the modulation parameters is obtained with a fitting procedure. Experimental signal-to-noise ratios at different absorption levels are compared with theoretical signal-to-noise ratios and show good agreement. Detection sensitivities of 2 x 10(-6) and 5 x 10(-6) for wavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth are demonstrated.
A technique for reducing background and improving the signal-to-noise ratio in optical spectroscopy of weak atomic transitions is described. In this technique an incident laser is square-wave amplitude modulated and the resonance fluorescence from the atom is then detected out of phase with the excitation when the laser is off. Due to the finite lifetime of the atom’s excited state, appreciable signal is observed when the laser is off and instrumental scattering background is virtually eliminated. Applying this technique to an experiment on single laser-cooled In+ ions, we have observed a factor of 500 reduction in background and a factor of 5 increase in the signal-to-noise ratio.
Employing grating extended-cavity diode lasers as the cooling/trapping and repumping lasers for preparing and manipulating single atoms, we have implemented a large-magnetic-gradient caesium magneto-optical trap (MOT). To detect and evaluate single caesium atoms trapped in MOT, laser-induced-fluorescence (LIF) photons of trapped atoms driven by MOT lasers are collected and counted by an avalanched photodiode worked in photon-counting mode. The dependences of LIF photon-counting signals of single atoms on a cooling laser's intensity, frequency detuning and frequency fluctuation are analysed and investigated. Remarkable improvement of the signal-to-noise ratio of LIF photon-counting signals is achieved by optimizing the cooling laser's intensity and frequency detuning and using the modulation-free polarization spectroscopic technique with feedback to both the slow channel (piezoelectric transducer channel with typical bandwidth of ∼2 kHz in the grating extended cavity) and the fast channel (current modulation channel with typical bandwidth of ∼200 kHz in the current driver).
Fabry-Peort (FP) cavity-based quantum key distribution has some advantages in the phase-coded quantum key distribution (QKD). Locking the frequency of the FP cavity transmission to the frequency of single photon carrying the key information determines the security of QKD. In this paper, we propose an FP cavity locked to a single photon (the mean photon number is about 0.1) frequency using single-photon modulation/demodulation method. The single photon detector working in the Geiger mode is used to detect modulated single photons then direactly demodulated by the lock-in amplifier. The discrete single photon response pulses accumulated fulfills the phase sensitive detection, and the signal-to-noise ratio of signal frequency discrimination is up to 112. The transmission frequency fluctuation of the locked FP is limited to 2 MHz.
Optical wavelength standards, based on the optical transitions of atoms or molecules, provide ultra-high precision frequency references and are the core technology of next-generation optical clocks. Their stability far exceeds that of the microwave frequency range, laying the foundation for precision metrology and cutting-edge scientific research. The wavelength standard device based on the 5S1/2-5D3/2 hyperfine level two-photon transition of 87Rb atoms exhibits significant advantages such as no Doppler broadening, high signal-to-noise ratio (SNR), and potential for miniaturization and chip integration, showing important application potential in precision measurement fields like rubidium atomic clocks and optical communication. In this paper, an experimental setup for two-photon transition was constructed using an external cavity diode laser (ECDL), achieving stable locking of the 778 nm wavelength by modulating the laser current. The effects of the temperature and length of the atomic vapor cell on the blue fluorescence linewidth, SNR, and frequency stability were systematically studied, further optimizing the system. By using an atomic vapor cell designed at Brewster's angle, optical reflection loss was effectively reduced, compensating for the impact of shortened optical path on frequency stability in miniaturized atomic vapor cells. This provides new research directions and performance enhancement solutions for the miniaturization and chip integration of two-photon transition systems. In the improved system, a Brewster-angle glass vapor cell with a length of 30 mm was used and heated to 110°C, achieving a linewidth of 1.69 MHz full width at half maximum (FWHM) and a SNR of 20 dB. The system demonstrated a instability of 9.86×10-11 over a 1-second averaging time and a minimum instability of 5.79×10-11 over a 5.7-second averaging time. This achievement provides key technological support for the practical application of the next generation of miniaturized optical atomic clocks, with the potential for widespread use in portable metrology equipment and space science through integrated design and noise suppression optimization in the future.