摘要
相干布居囚禁(coherent population trapping, CPT)原子钟因其体积小,功耗低的特点在通信、时间同步、导航定位、实验研究等领域具有广泛的应用前景。当前国外已生产出小型化和微型化CPT原子钟产品,并已得到实际应用,同时还发展出了一些改善CPT共振信号质量进而提高原子钟的频率稳定度的新方案。为了研制具有自主知识产权的实用型CPT原子钟,我们开展了小型化CPT原子钟的工程转化和新方案探索工作。本论文工作期间取得的主要进展和研究成果如下：
为CPT原子钟工程样机设计、研制了一款物理系统。在该款物理系统中消除了之前研制的物理系统中存在的频率跳变隐患,定型的物理系统体积18.4mL,功耗1.4W。采用所研制的物理系统的原子钟已实现小批量生产,产品的典型频率稳定度优于4×10-11,τ1/2(1s≤τ≤100s),整体性能达到了国际同类产品先进水平。
针对CPT原子钟在不同温度条件下的应用设计、研制了一款物理系统。该款物理系统体积16mL,功耗1.6W,应用该物理系统实现的原子钟的环境温度适应范围更大,这款产品现已获得应用。
研究了平行线偏振光激发CPT共振方案(lin//lin CPT方案)。lin//lin CPT方案消除了传统圆偏振光方案(σ+-σ+CPT方案)中由于光抽运作用形成的极化暗态,增加了有效参与工作的原子数量。我们开展与σ+-σ+CPT方案的对比实验,在较低的原子泡工作温度下获得了质量更优的用于原子钟鉴频信号的CPT共振微分信号,因此通过该方案可以实现体积与传统方案CPT原子钟相当,功耗更低,频率稳定度更好的原子钟。
研究了偏振选择CPT共振方案。该方案采用椭圆偏振光激发CPT共振,椭圆偏振光可以看作幅度不同的左右旋圆偏振光的线性叠加,利用原子对左右旋圆偏振光吸收强度不同压制本底光噪声幅度,由此获得的CPT共振信号对比度(CPT信号幅度与本底光噪声幅度的比值)大幅度改善,实验中采用本方案获得的信号对比度与传统CPT原子钟方案的信号对比度相比约提高了6倍,因此采用该方案可望大幅度改善原子钟的频率稳定度。
提出并研究了频率切换Ramsey-CPT方案。该方案通过周期性地用微波信号调制垂直腔面发射激光器(vertical-cavity surface-emitting laser, VCSEL)的驱动电流,使VCSEL的出射光周期性地在多色光与单色光之间切换,这样的激光与原子作用实现周期性地与原子CPT共振,从而实验记录到了Ramsey-CPT条纹。由于不需要之前的常规Ramsey-CPT原子钟方案中用于产生脉冲光的声光调制器(acousto-optic modulator, AOM),该方案实现的物理系统体积大大减小,而对比两种Ramsey-CPT原子钟方案获得的Ramsey-CPT条纹,应用新方案实现的原子钟频率稳定度预期改善46%。另外,与传统CPT原子钟相比,新Ramsey-CPT原子钟方案的物理系统体积、功耗基本相同,而原子钟电路只需增加一个微波开关,所以体积、功耗也非常接近,因此采用所研究的方案可以实现体积功耗与传统CPT原子钟相当,频率稳定度优于之前的常规Ramsey-CPT原子钟的新型Ramsey-CPT原子钟。
提出并研究了正交偏振探测-频率切换Ramsey-CPT方案。该方案采用lin//lin双ACPT共振构型,频率切换方案实现Ramsey,并在光信号探测上引入正交偏振探测方法,利用磁光旋转效应降低本底光噪声。通过该方案我们已经实验获得了对比度为22%和线宽为280Hz的Ramsey-CPT条纹,与仅应用频率切换方案所获得的Rasmey-CPT条纹相比,对比度提高为8倍。因为只在物理系统的器件设置上有所调整,并不需要增加体积,因此采用该方案可以实现性能更高的小型Ramsey-CPT原子钟。
Because of its small size and low power consumption, coherent population trapping (CPT) atomic clock has been applied in many fields, such as communication, time synchronization, navigation, and experimental research. At present, compact and chip-scale CPT atomic clock products have been applied to practical projects abroad, and some new schemes that can improve the quality of CPT resonance signal have been developed to further improve the frequency stability of CPT atomic clock. In order to develop CPT atomic clocks with independent intellectual property rights, we have carried out product transformation and new scheme exploration on compact CPT atomic clock. The main progress and research results of this paper are as follows:
For CPT atomic clock engineering prototype, we designed and developed a physics package. The hidden trouble of frequency-hopping in former physics packages was erased in this physics package. The volume and power consumption of this physics package are respectively18.4mL and1.4W. The CPT atomic clocks that use these physics packages have been realized to small batch production, and the products can achieve a typical frequency stability better than4×10-11τ1/2for integration time1s≤τ≤100s, the overall performance of atomic clock reaches the international advanced level of like product.
For the CPT atomic clock working under different temperature, we designed and developed a physics package with the volume of16mL and the power consumption of1.6W. The CPT atomic clock based on the physics package can work steadily in a large ambient temperature range, now it has been applied to an actual project.
We have experimentally studied the scheme of realizing CPT resonance through the interaction between atoms and parallel linearly polarized bichromatic coherent light field (lin//lin CPT scheme). By eliminating the leaky-trap state due to optical pumping effect in conventional scheme which uses circularly polarized bichromatic coherent light field (στ-στ CPT scheme), more atoms will contribute to CPT resonance. We experimentally studied the two schemes, and better CPT resonance differential signal which used as frequency discriminating signal in atomic clock was obtained by lin//lin CPT scheme at lower temperature. Therefore, comparing to σ+-σ+CPT scheme, the lin//lin CPT scheme can be used to implement an atomic clock with comparable volume, lower power consumption and better frequency stability.
We have experimentally studied the polarization-selectve CPT scheme of exciting CPT resonance with elliptical polarized bichromatic coherent light field. The elliptical polarized light can be seen as a linear superposition of σ+andσ-circularly polarized light with different amplitude. Due to the different light absorption intensity of atoms for σ+and σ-, it can suppress the background of CPT resonance signal, and the scheme provides a CPT resonance signal with greatly improved contrast (contrast is defined as the amplitude ratio of CPT signal to background). The experimental results show that the CPT signal contrast is about6times higher than that of conventional CPT atomic clock, so the frequency stability of atomic clock based on our studied scheme will be greatly improved.
We have proposed and experimentally studied a frequency-switching Ramsey-CPT scheme (FS scheme). Driven by the periodically microwave modulated current, a vertical-cavity surface-emitting laser (VCSEL) emits continuous laser which switches between monochromatic and multichromatic laser, and Ramsey-CPT fringes has been studied with the frequency switching continuous laser. With elimination of the acousto-optic modulator (AOM) which is used to generate pulsed laser in conventional Ramsey-CPT atomic clock, the volume of physics package of the proposed scheme greatly decreases. By comparing Ramsey-CPT fringes obtained from the two Ramsey-CPT schemes, the frequency stability of the proposed scheme is expect to be46%better than that of the scheme with AOM. Additionally, comparing to conventional CPT atomic clock, the physics package of the proposed scheme is basically the same, while the resource budget of the electronics will be slightly increased as a microwave switch shall be added to it. Our experimental results suggest that it is feasible to implement a compact Ramsey-CPT atomic clock with the volume and power consumption of the conventional CPT atomic clock, and the frequency stability comparable to or even better than that of the conventional Ramsey-CPT atomic clock.
We have proposed and experimentally studied a crossed-polarizers frequency-switching Ramsey-CPT scheme. It excites CPT resonance with lin//lin CPT configuration and implements Ramsey interference with FS scheme. A crossed-polarizers method is introduced in light signal detection, and it significantly reduces the background signal level due to the magneto-optical rotation effect. Typical Ramsey-CPT fringes with a linewidth of280Hz and a contrast of22%were observed. By comparing the detected signal with the Ramsey-CPT atomic clock realized only through the FS scheme, the contrast of the proposed scheme increased by8times. Because only the components setting of physics package need to be slightly adjusted, the volume will not increase, our proposed scheme could be implemented compact Ramsey-CPT atomic clock with higher performance.
引文
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