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IEEE Transactions on Instrumentation and Measurement

A novel method to compensate for the effect of light shift in a rubidium atomic clock pumped by a semiconductor laser

Minoru Hashimoto, Motoichi Ohtsu

Published Jun 1, 1990• 24 citations
Publisher

Abstract

Precise measurement of the shift (i.e. microwave frequency shift induced by the electric field of the pumping light) in a rubidium atomic clock pumped by a semiconductor laser is discussed. The spectral lineshape of the microwave resonance, which is used as a frequency discriminator for the atomic clock in the optical microwave double resonance experiment, depends strongly on the spatial distribution of the laser beam intensity, laser frequency detuning, and modulation parameters of the microwave frequency. Based on measurements of the deformation of the resonance lineshape, a self-tuning system was built to compensate for the effect of light shift. As a result of controlling the laser frequency with this system, long-term drift of the microwave frequency as low as 6.3*10/sup -13//h was obtained.>

DOI
10.1109/19.106272

Related papers

Modulation transfer and optical Stark effect in a rubidium atomic clock pumped by a semiconductor laser

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Novel spectral line shapes of optical-microwave double resonance with an extremely narrow line width were observed in a rubidium atomic clock pumped by a semiconductor laser. It was confirmed that these novel spectral line shapes of double resonance were due to the modulation-transfer effect. These spectral line shapes were calculated by solving the equation of motion of the density matrix that describes the modulation-transfer process. The calculated results agree with the experimental results. By using the calculated results of the influence of the optical Stark effect on these spectral line shapes, characteristics of the inhomogeneous light shift were evaluated quantitatively for the frequency-modulated microwave. Furthermore, optimum values of modulation parameters were found by computer simulation in order to obtain the highest microwave frequency stability of the atomic clocks. An experiment was carried out employing these optimum parameters, and a short-term frequency stability as high as σy(τ) = 7.9 × 10−13τ−1/2 was obtained, where τ is an integration time of measurement.

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ln recent years, laser pumped Rb atomic clocks have been numerously studied and developed to reduce "light shift" and increase the accuracy and stability of the frequency. Previously, we reoorted on the measurement of the linewidths and S/N of the optical-microwave double resonance signal and the light shift in a Rb atomic clock pumped by a stabilized diode laser. In this paper, the light shift was numerically calculated by using the theory proposed by Cohen-Tannoudji & Barrat. Furthermore, contributions of the laser power and frequency detuning to the light shift were also estimated to attain the microwave frequency stability as high as 10^<-12>.

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Light shift and vapor-cell temperature shift are the two most significant factors dominating the long-term instability of compact atomic clocks. Due to the different physical mechanisms, there is not yet a solution that can effectively suppress the frequency shifts induced by these two effects. Here, we propose a 'resonance-offset' locking approach that compensates for the two physical frequency shifts. In this approach, the additional offset locking shift can effectively counteract the atomic resonance shifts arising from changes in vapor-cell temperature and light power, reducing the net impact on the clock's frequency to nearly zero. We have demonstrated this strategy on the 778 nm Rubidium two-photon optical frequency standard, successfully compensating for light shift and cell-temperature shift, respectively. This general method is particularly appealing for compact vapor-cell microwave and optical atomic clocks designed for the excellent stability rather than accuracy.

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Simple method of light-shift suppression in optical pumping systems

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We report a simple method to suppress the light shift in optical pumping systems. This method uses only frequency modulation of a radio frequency or microwave source, which is used to excite an atomic resonance, to simultaneously lock the source frequency to the atomic resonance and lock the pumping light frequency to suppress the light shift. We experimentally validate the method in a vapor-cell atomic clock and verify the results through numerical simulation. This technique can be applied to many optical pumping systems that experience light shifts. It is especially useful for atomic frequency standards because it improves long-term performance, reduces the influence of the laser, and requires less equipment than previous methods.

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Abstract To measure microwatt-level laser power, this paper presents a method based on the atomic light shift effect. A 795 nm semiconductor laser is directed into a rubidium atomic clock with an output frequency of 10 MHz. To minimize the influence of laser frequency on power measurements, an experimental setup is implemented to precisely lock the laser frequency to the wavelength meter. After stabilizing the laser frequency, we analyze its correlation with the output frequency of the rubidium atomic clock. The experimental results indicate that, during free-running, the laser undergoes a frequency drift of -90 MHz over 2 hours, which is subsequently reduced to ±0.9 MHz over 40 hours through frequency locking. Moreover, by fixing the laser wavelength at 794.99 nm, we observe an increase in the atomic clock frequency shift with rising laser power. Specifically, a laser power of 2 μW corresponds to a frequency shift of 0.13 mHz at 10 MHz. This study achieves quantum measurement of weak laser power, significantly enhancing the accuracy of low-power laser measurements.

Journal of Physics Conference Series•Nov 1, 2024•0 citations
Precise measurement of the shift (i.e. microwave frequency shift induced by the electric field of the pumping light) in a rubidium atomic clock pumped by a semiconductor laser is discussed. The spectral lineshape of the microwave resonance, which is used as a frequency discriminator for the atomic clock in the optical microwave double resonance experiment, depends strongly on the spatial distribution of the laser beam intensity, laser frequency detuning, and modulation parameters of the microwave frequency. Based on measurements of the deformation of the resonance lineshape, a self-tuning system was built to compensate for the effect of light shift. As a result of controlling the laser frequency with this system, long-term drift of the microwave frequency as low as 6.3*10/sup -13//h was obtained.>
Novel spectral line shapes of optical-microwave double resonance with an extremely narrow line width were observed in a rubidium atomic clock pumped by a semiconductor laser. It was confirmed that these novel spectral line shapes of double resonance were due to the modulation-transfer effect. These spectral line shapes were calculated by solving the equation of motion of the density matrix that describes the modulation-transfer process. The calculated results agree with the experimental results. By using the calculated results of the influence of the optical Stark effect on these spectral line shapes, characteristics of the inhomogeneous light shift were evaluated quantitatively for the frequency-modulated microwave. Furthermore, optimum values of modulation parameters were found by computer simulation in order to obtain the highest microwave frequency stability of the atomic clocks. An experiment was carried out employing these optimum parameters, and a short-term frequency stability as high as σy(τ) = 7.9 × 10−13τ−1/2 was obtained, where τ is an integration time of measurement.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
ln recent years, laser pumped Rb atomic clocks have been numerously studied and developed to reduce "light shift" and increase the accuracy and stability of the frequency. Previously, we reoorted on the measurement of the linewidths and S/N of the optical-microwave double resonance signal and the light shift in a Rb atomic clock pumped by a stabilized diode laser. In this paper, the light shift was numerically calculated by using the theory proposed by Cohen-Tannoudji & Barrat. Furthermore, contributions of the laser power and frequency detuning to the light shift were also estimated to attain the microwave frequency stability as high as 10^<-12>.
Light shift and vapor-cell temperature shift are the two most significant factors dominating the long-term instability of compact atomic clocks. Due to the different physical mechanisms, there is not yet a solution that can effectively suppress the frequency shifts induced by these two effects. Here, we propose a 'resonance-offset' locking approach that compensates for the two physical frequency shifts. In this approach, the additional offset locking shift can effectively counteract the atomic resonance shifts arising from changes in vapor-cell temperature and light power, reducing the net impact on the clock's frequency to nearly zero. We have demonstrated this strategy on the 778 nm Rubidium two-photon optical frequency standard, successfully compensating for light shift and cell-temperature shift, respectively. This general method is particularly appealing for compact vapor-cell microwave and optical atomic clocks designed for the excellent stability rather than accuracy.
We report a simple method to suppress the light shift in optical pumping systems. This method uses only frequency modulation of a radio frequency or microwave source, which is used to excite an atomic resonance, to simultaneously lock the source frequency to the atomic resonance and lock the pumping light frequency to suppress the light shift. We experimentally validate the method in a vapor-cell atomic clock and verify the results through numerical simulation. This technique can be applied to many optical pumping systems that experience light shifts. It is especially useful for atomic frequency standards because it improves long-term performance, reduces the influence of the laser, and requires less equipment than previous methods.
Abstract To measure microwatt-level laser power, this paper presents a method based on the atomic light shift effect. A 795 nm semiconductor laser is directed into a rubidium atomic clock with an output frequency of 10 MHz. To minimize the influence of laser frequency on power measurements, an experimental setup is implemented to precisely lock the laser frequency to the wavelength meter. After stabilizing the laser frequency, we analyze its correlation with the output frequency of the rubidium atomic clock. The experimental results indicate that, during free-running, the laser undergoes a frequency drift of -90 MHz over 2 hours, which is subsequently reduced to ±0.9 MHz over 40 hours through frequency locking. Moreover, by fixing the laser wavelength at 794.99 nm, we observe an increase in the atomic clock frequency shift with rising laser power. Specifically, a laser power of 2 μW corresponds to a frequency shift of 0.13 mHz at 10 MHz. This study achieves quantum measurement of weak laser power, significantly enhancing the accuracy of low-power laser measurements.