At present, limit the use of passive optical clock program to further improve the stability factor is used to excite the atom probe spectral linewidth of the laser, and the stability of the laser frequency, and these limiting factors directly related to the laser cavity.Currently our group proposed scheme is based on the active optical clock bell novel optical cavity mode bad, stimulated radiation in the atomic excitation is amplified with bad cavity mode selected, and directly as the quantum frequency standards.Since the active optical clock that, due to the active optical clocks have a greater advantage in the stability and accuracy, widespread international attention.
Good mold cavity laser cavity width is much smaller than the bandwidth of the gain medium, the gain medium due to the mode competition is closest to the peak output of the cavity mold, but due to the external environment, the laser cavity length jitter will occur, which while causing jitter cavity mold, so that the output of the laser mode linewidth and frequency stability is not good.And in bad mode cavity, the cavity mold width is much larger than the bandwidth of the gain medium, then even if the cavity length jitter can guarantee narrow linewidth output signal, this program weakens the cavity pulling effect, line width laser output can theoretically reach mHz magnitude.
Fig.1. The working mechanism and bad cavity coefficient of the passive optical clock and the active one. (a) Traditional optical clock. (b) Active optical clock .
In bad cavity modes ,adjusting the resonant cavity length ,so that the cavity-mode mobile1.336GHz,the active optical output clock signal frequency shifts 30MHz,the cavity resonance frequency change of1/45.5.
Light clock concept, each team presented a variety of programs in the world, however, based on a three-level active optical clock output is limited by the width of the pump light frequency shift caused by the light, in order to solve this problem, we study group proposed four-level initiative Cs atom optical clocks based solution .By 455 nm laser pump the atoms to 7P3 / 2 state, the 7S1 / 2 state to the 1469.9 nm laser 6P3 / 2 state as the clock laser output .By pumping energy level associated with the separation of stimulated emission levels, reduce the light frequency shift caused by the pumping light problem .Our study group in 2013 first in the world to achieve the active optical clock output by stimulated emission of radiation, in the case of cavity length is not locked in the experimental study of Cs atoms on a four - level active optical clock by four sets of Cs atoms active optical clocks beat level, measured by stimulated emission linewidth output 1kHz.
This program is expected to be the stability of optical lattice clock by 2 orders of magnitude, so that the light clock applications tend to be more extensive.
2. Active optical clocks four-level cesium
Based Cs atom optical clocks four-level program of active energy level structure as shown below .By 455 nm laser pump the atoms to 7P3 / 2 state, the 7S1 / 2 state to the 1469.9 nm laser 6P3 / 2 state as the clock laser output .By pumping energy level associated with the separation of stimulated emission levels, reduce the light frequency shift caused by the pumping light problem .Our study group in 2013 first in the world to achieve the active optical clock output by stimulated emission of radiation, in the case of cavity length is not locked in the experimental study of Cs atoms on a four - level active optical clock by four sets of Cs atoms active optical clocks beat level, measured by stimulated emission linewidth output 1kHz.
Fig.2 Relevant atomic energy levels of Cesium.
Fig.2 The power and temperature charicteristics of 1469.9 nm laser.
To further enhance the stability and pressure narrow linewidth output clock signal,we will make further improvements in the experimental group:
(1)the integration of all switch to the cage, reducing external environmental interference;
(2)shielding the external magnetic field;
(3)changing there flectivity mirror,mirror symmetry study the impacton the output signal.
Constantly optimize the experimental results, the pressure of narrow linewidth output, improve the stability of the active light Zhong Zhong xin number.
3.Faraday Active optical clocks
As a unique optical clock,the active optical clock having a clock stability is better than the conventional optical potential advantages.The study group is nowan improved experimental research initiative Faraday optical frequency standard cesiumand459 nmlaserpumping.The program improves the original with 852 nm laser pumped a result , reducing there sidual Doppler broadening.This significantly reduces the bandwidth and reduce bad cavity length Faraday optical filter for active cesium Faraday optical frequency standard.We can help suppress good traction effect chamber cavity that to a great extent.
Energy level can be seen on the map 459 nm and 852 nm light frequency the same ground state ,459 nm laser action on atoms 62S1 / 2, and probed with 852 nm laser in order to obtain a narrow linewidth FADOF signal.
In this study,the optical path is built pumped 459 nm , 852 nm detection optical path.
Data currently obtained as shown below,
The upper half of the picture is saturated absorption of 459 nm lasers pectroscopy experiment data ,the lower half of the picture is at 10.7 G magnetic field for 852 nm laser FADOF signal .According to the experimental data we need further experiments is to improve the magnetic shielding effect of the magnetic field uniformity, the intensity of the probe light and pumping light and relevance.
4.The quality of the integration of dual-wavelength cavity Active optical clocks
Of cesium atom susing 455 nm laser pump,on an experimental basis has taken the initiative to implement 1470 nm light output clock, and with the 455 nm and 459 nm laser pump different than added to the active optical clock in the dual-wavelength resonator specially designed 633 nm He-Ne laser device to achieve 633 nm and 1470 nm dual-wavelength cavity while total output and the common active optical clock systems in the same cavity.Where the population numbers 633 nm He-Ne laser wavelength corresponding to the reversalofa quantum system gain linewidth of about 1.5 GHz,significantly larger than the cavity mold width(ie,in the range offine cavity),and the use of frequency stabilization program will PDH 633 nm linewidth output pressure to narrow the order of 10 Hz ,in order to achieve long active optical clock lock resonant cavity.At the same time, the number of population cesium atomic clock four-level master optical output wavelength 1470 nm corresponding inversion gain linewidth of quantum system is only about 10 MHz, significantly smaller than the width of the mold cavity (ie, in the range of bad cavity), enabling the chamber weakening pulling effects (specific objectives to reduce noise resonator length about 20 times).Thus, as the main action of light Zhongzhong Yue moved to 1470 nm output signal in PDH frequency stabilization based on the use of active optical clock cavity pulling effect of weakening mechanism to achieve the order of 1 Hz output line width.Based on 633 cesium atoms four-level quantum systems nm and 1470 nm or bad co-chamber cavity dual-wavelength optical clocks initiative in addition to its ultra-narrow linewidth and traction advantage of the weakening effect of the chamber, but also has a light frequency shift is small, the number of anti-populationturn easily, high atomic number density by large stimulated emission intensity, can be extended to atomic laser cooling and imprisoned superior advantages, so as to lay a solid foundation for the eventual realization of the order of 10-18 frequency instability.
How to test: The cesium four-level quantum system with He-Ne laser combined to build 633 nm and 1470 nm or bad co-chamber cavity dual-wavelength optical clock active experimental system, using 455 nm and 459 nm laser pump (ratio performance), 633 nm and 1470 nm to achieve the quality of co-chamber cavity dual wavelength output.
Cesium has a population inversion easy,high atomic number density by large stimulated emission intensity,it can be extended to atomic laser cooling and imprisoned superior advantages.
The use of 455 nm blue laser glass chamber of cesium atoms from the 6S1 / 2 ground state to the continuous pumping 7P3 / 2 state, to achieve population inversion between the number of 7S and 6P.Our previous theoretical calculations show design special Fabry-Perotcavity(cavity mold cavityl aser linewidth laser gain mediumis much larger than the gain linewidth that bad cavity),the cesium atoms placed in bubble chamber being bad.Achieve 7S1 / 2 state and 6P3 / 2 states of atomic population inversion of atoms produced by stimulated emission of radiation in the cavity under the bad effect,after the stimulated emission threshold is reached in the cavity to form a stable oscillation output 7S1 / 2 state transition to 6P3 / 2 state corresponding to 1470 nm active optical clock output.While carrying out research with 6S1 / 2 state to the 7P1 / 2 state corresponding to the 459 nm laser pump,thus to build two different independent system scan be compared.
On this basis,the clock in the master optical resonant cavity joined He-Ne 633 nm laser tube assembly,to make He-Ne mixed gas gain medium 633 nm wavelength stimulated emission and cesium atomic four-level systemfor the gain medium the 1470 nm wavelength of stimulated emission share the same resonator optical clocks to achieve active dual wavelength output.Where in the gain of the output light 633 nm line width of about 1.5 GHz, 1470 nm light output of the gain medium is to achieve population inversion cesium 455 nm pump speed after the election, the gain linewidth of about10 MHz.Through the two resonator mirrors for specialized requirements of different wavelength scoating design, we can achieve 633 nm light output cavity mold width is much smaller than its 1.5 GHz gain linewidth, and 1470 nm light output cavity mold linewidth much greater than its width to 10 MHz gain, thus ensuring 633 nm and 1470 nm output signals are in good and bad cavity region cavity area.
633 nm and 1470 nm or bad co-chamber cavity dual-wavelength active optical clock experiment system diagram shown in Figure2.