| In recent years,lanthanides have been widely studied in the fields of timefrequency reference,dipole quantum gas and novel physical phase transition.Eribum atoms possess a special electron configuration,a so-called submerged shell,in which the 6s sub-shell is filled,while the lower-lying 4f or 5d sub-shells are open,being partially unoccupied.This leads to a large variety of optical transitions in these elements,whose linewidths range from tens of μHz to tens of MHz.Studies have shown that Erbium atoms have ultra-narrow linewidth energy levels and have potential applications in optical frequency standards.Theoretical calculations predict a linewidth of the 1299 nm transition of 2.1 Hz by J.J.Mc Clelland’s group in 2005,and F.Ferlaino obtained a linewidth of 0.9 Hz by measuring the lifetime of the excited state in 2021.Moreover,in contrast to most narrow transitions in other atomic species,the wavelength of 1299 nm lies within the telecom-wavelength window,which is advantageous for the transmission in fiber communication systems.The narrow linewidth laser is usually transmitted to the ultra-stable cavity,optical frequency comb and cold atom system by fiber in the cold atom spectroscopy precision measurement system.However,the equivalent length and refractive index of the fiber are easily affected by environmental factors(vibration,temperature,airflow,etc.),which introduces additional phase noise when transmitting the optical frequency signal,resulting in the broadening of the linewidth and the reduction of the frequency stability after transmission.At the same time,insufficient power or space limitation of narrow linewidth laser is common in single-point to multi-point transmission.Based on the above difficulties,this paper carries out experimental research and obtains the following research results:(1)We analyzed the principle and method of phase noise suppression,and developed a method of dual-wavelength laser fiber phase noise suppression.We have built a fiber phase noise suppression system based on short fiber(> 3 m)transmitting sub-Hertz linewidth laser(1299 nm).The phase noise introduced in the transmission process is obtained by the fiber round-trip two-way transmission method.The phaselocked loop and acoustic optic modulator are used to dynamically compensate the laser frequency signal before the input fiber.The experimental results show that the phase noise suppression system can narrow the linewidth broadening introduced by fiber transmission from 30.1(0.3)Hz to 0.58(0.09)m Hz,and stability of optical frequency transmission is increased from 7.4E-16 to 9.0E-17 in 1 s and from 5.0E-17 to 9.3E-20 in 1000 s.(2)In this paper,phase noise suppression in dual-wavelength laser lights propagating through fibers based on ultracold erbium atoms is implemented.The additional phase noise of a sub-Hz linewidth laser transmitted in the fiber for exciting the Hz-linewidth transition of erbium,is actively compensated by using the fundamental laser of the cooling laser in the ultracold erbium atom system.To carry on the heterodyne beat detection and the compensation feedback without affecting the power of the original sub-Hz linewidth laser(1299 nm),we inject the broad-linewidth fundamental laser of the cooling light at a similar wavelength(1166 nm)from the output end of the fiber.The phase noise of narrow-linewidth laser caused by temperature and vibration in fiber transmission is suppressed,when the noise of the two lasers is almost the same.The linewidth of the beat frequency signal of the transmitted laser is narrowed from 13.8(2.5)Hz to 11.6(1.3)m Hz and the stability of the opticalfrequency transmission link is improved from 1.6E-16 to 6.5E-19 in 1000 s,which meets the optical-frequency transfer needs of the start-of-art optical clock.This opticalfrequency transfer scheme can be used as an alternative where the power of the transmitted laser is insufficient or the physical space is limited.The scheme is also applicable for simplifying the source setup on branching optical-fiber networks. |
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