Laser Frequency Stabilization With Iodine Molecules And Investigation On Residual Amplitude Modulation

In an iodine-stabilized laser system,the laser frequency is stabilized by locking the laser to a hyperfine transition of iodine molecules.This system is a cost-effective approach to a stable laser system or secondary optical frequency standard in that it is relatively simple,has small footprint and good portability,and yet maintains frequency stabilities at the order of 10-14～10-15 at averaging times of 1～10000 s.These features make the iodine-stabilized lasers suitable for many applications such as laser interferometry,gravimetry,gravitational wave detection,and precision laser spectroscopy.In these applications several competing factors,namely compactness,robustness and stability,are all emphasized for a laser system.Although much work has been done to develop iodine-stabilized lasers,in order to further release its potential and broaden its application,several key issues are needed to be addressed,such as a systematic investigation on one of the major noise sources,namely the residual amplitude modulation,and how to reduce its influence on frequency stability.Moreover,iodine stabilized lasers show a good prospect for many space applications.In addition to improving the reliability,its stability potential needs to be explored in the future.The current work is part of a research program that develops two similar,highly stable 532-nm systems adopting modulation transfer spectroscopy of iodine molecules in their gaseous phase,aiming at identifying the ultimate stability-limiting factor(s)in the systems.The first part of this thesis devotes to the intensity stabilization of multiple laser beams used in two experimental systems of iodine-stabilized lasers at 532 nm and high-precision temperature control for their vital components.Firstly,we review the existing works on similar systems,including the progress on the system adopting hyperfine lines of iodine molecules near 532 nm.Secondly,the basic principle and setup of iodine frequency stabilization are introduced.Then our experimental works on laser intensity stabilization and high-precision temperature control are introduced.The fluctuation of laser power is tested and compared between before and after stabilization,and the effect of optical intensity stabilization to the system is also evaluated.With intensity stabilization,the residual intensity fluctuation of the pump and probe beams are controlled to below 2.7 × 10-4 at 6.5 hours.The temperature fluctuations of electro-optic modulators in the two iodine systems have been reduced to 0.5 mK(NORTH)and 0.4 mK(SOUTH),while the temperature fluctuations at the cold finger of iodine cell are controlled to be within 1.6 mK(NORTH)and 0.2 mK(SOUTH).With this level of temperature stability,the resultant frequency instability is estimated to be 3 × 10-17 at 1000-s averaging time.The second part of the thesis describes our in-depth investigation on the mechanism of the spatially inhomogeneous residual amplitude modulation.Residual amplitude modulation is one of the key factors limiting the long-term stability of molecular iodine optical clock.A non-uniform distribution of residual amplitude modulation across a transverse plane of the laser beam has long been observed,but the exact cause of this spatial inhomogeneity is unknown.We perform measurement and analysis of this spatial inhomogeneity using several electro-optic crystals of different types.Unlike our previous investigations,here the demodulation phase is continuously varied,and a large number of intermediate states are detected.Two distinct components are identified in the spatial distributions,and their detailed properties are mapped out and analyzed,showing that the spatial inhomogeneity can be explained by acousto-optic interaction inside the crystal.Unique properties of this spatial inhomogeneity,which are previously not reported,are measured and analyzed in detail,such as the evolution of the spatial pattern with probe direction as well as the increasing modulation depth.Moreover,this spatial inhomogeneity can be further suppressed,improving the 1000-s stability of residual amplitude modulation to 3 ×10-7(8 × 10-8)at modulation frequency of 11 MHz(120 kHz),corresponding to a frequency instability of 1 × 10-17(3 × 10-18),estimated for a cavity-stabilized laser with a Pound-Drever-Hall discrimination slope of 1 × 10-4 V/Hz.Overall,the two iodine-stabilized lasers operating at 532 nm are upgraded by implementing(1)the intensity stabilization of the pump and probe beams,and(2)multi-channel,precision temperature control.The upgrade allows for the evaluation of individual contributions from the intensity fluctuation,the thermal instability,and the residual amplitude modulation.Moreover,the mechanism underlying the spatial inhomogeneity of residual amplitude modulation is identified by experimental measurement and theoretical calculation.These works not only provide information for the noise analysis and evaluation of the frequency stability,but also lay a solid foundation for performance improvement on the iodine-stabilized lasers.