Development Of Lasers With 10^-16 Frequency Instability And Theoretical Exploration On 10^-18 Laser Frequency Instability Via Four-wave Mixing

Precision control of laser frequency is the foundation of precision spectroscopy and precision measurement since it determines the resolution and accuracy of precision spectroscopy and precision measurement.Lasers with ultra-narrow linewidth and high frequency stability are the key component for developing optical atomic clocks-the new generation of time/frequency standards,high-resolution spectroscopy,laser interferometers for gravitational wave detection,low-noise microwave sources,and optical frequency synthesizers.These applications urgently require lasers with higher frequency stability and narrower linewidth.Taking optical atomic clocks as an example,when the frequency instability of narrow-linewidth lasers as local oscillators is reduced from 10^-15 to 10^-16,the frequency instability of optical clocks has been improved from 10^-15/?? to 10 16/????is the averaging time?,enabling 10^-18 frequency precision in an averaging time of less than one day.In order to develop optical atomic clocks with even better frequency stability and detect gravitational wave in space based on laser interferometer with more than 10⁸-meters-long arms,it is necessary to develop narrow-linewidth lasers with a frequency instability of 10^-17 or even 10^-18.Towards this goal,in this thesis we construct narrow-linewidth lasers with 10^-16 frequency instability experimentally and theorectically investigate the possibilities of 10^-18 laser frequency instability.To obtain a thermal-noise-limited laser frequency instability better than 10^-15,we construct two frequency-stabilized laser systems at 578 nm by stabilizing their frequencies to the resonance of stable optical reference cavities via the Pound-Drever-Hall technique.The reference cavities are designed to have a thermal-noise-limited length instability of 1.6 x 10^-16.Environmental vibration noise has a great effect on the length instability of the reference cavities,and thus the frequency instability of the cavity-stabilized lasers.We obtain the optimal structure and support positions of the reference cavities by combining numerical simulation calculations with experimental measurements.The vibration sensitivities of the optical reference cavities in all directions are reduced to be less than 5 × 10^-10 g^-1.Based on the vibration insensitive supporting,the frequency noise of the cavity-stabilized lasers induced by vibration noise is close to the cavity-thermal-noise-limited frequency noise.In addition,we reduced the laser frequency noise arisen from residual amplitude modulation of EOMs,random phase noise introduced by optical fibers and light power fluctuations below the thermal-noise-limited length instability of the reference cavities.Based on the frequency comparison between two similar laser systems,the most probable linewidth of each laser is measured to be 0.2 Hz,and the fractional frequency instability of each laser system is 1.8 × 10^-16 at 1 s averaging time,approaching to the thermal-noise-induced length instability of the reference cavitiesSo far,the frequency instability of the cavity-stabilized lasers can reach to 4 x 10^-17 level,which are stabilized to independent 21-cm-long silicon Fabry-Perot cavities operated at 124 K.Achievement of the ultrastable lasers with a frequency instability of 10^-18 would be a technical challenge.Thus,we propose a novel scheme for generation of ultra-narrow linewidth laser based on four-wave mixing in cold strontium?Sr?atoms.Benefiting from the natural linewidth of the clock transition of Sr atoms,the mixing laser light can generate with narrow linewidth and high frequency instability.We show that the linewidth of the mixing laser light is ultimately limited by the natural linewidth of the atomic clock transition rather than by the linewidth of the input lasers.Based on the theoretical analysis,it is possible to generate a 4 mHz-linewidth laser light with a frequency instability of 10^-18 and a power of 10-12 W when the input lasers with a relative intensity instability of 10^-4 and a spectral width of 1 Hz interact with Sr atoms with a density of 1 × 10¹¹ cm^-3.The theoretical exploration described here is a basis for the future experimental research.