The First Experimental Setup of 822 nm System

Builder: Chieng Ming Wu and Tzu Wei Liu

The 822 nm ECDL serves as a master laser source. The purpose is to investigate the cesium 6S-8S two-photon transition (TPT). The output beam from the ECDL is shaped to TEM00 mode by a spatial filter so that this beam can be injected into the tapered amplifier. After the tapered amplifier, the laser power is amplified to 200 mW. For laser linewidth narrowing, Dr. Wu installed an EOM and a ULE cavity based on the PDH technology. With the same EOM, the laser frequency could be stabilized on the cesium transition line. So far, the frequency-stabilized laser system is created. With this system, the cesium spectrum is scanned with precise spacing. Eventually, Wu compared the laser frequency to the comb-laser frequency, thus the absolute frequency of the cesium spectrum is obtained.

Block diagram of 822 nm spectroscopy system and beat note experiment
Left: beat frequency between 822nm laser and comb laser; Right: the stability of beat frequency

Based on the experimental results of the beat note measurement, the laser linewidth is about 70 kHz. The stability of the laser is close to 10-13 in 100-second integrating time.

The error budget for determining the absolute frequency of Cs 6S–8S hyperfine transitions (a) Two sample Allan deviations (comb laser beat against master laser); inset, resettability (Δfr, relative to mean value). Log scale on both axes. (b) Typical light shift at room temperature, relative to statistical zero-light-intensity frequency (ΔfL); “Light power” means total laser power per area in the waist (center of the cell). (c) Longitudinal magnetic field (by solenoid current) versus frequency shift: β, residual magnetic field; ΔfB, frequency deviation from zero current; black solid line, ΔfB=α(β+B)2 where β is the fitted residual field. (d) Collision shift and broadening

Influence of atmospheric helium on secondary clocks

  1. Helium Diffusion through Cs Pyrex Glass Cell
Advanced Setup of 822 nm Laser System

Builder: Chieng-Ming Wu , Shu-Rong Wu and Ko-Han Chen

For improving the stability of 822 nm system, Dr. Wu, et al. replaced the homemade EOM with the fiber EOM in the “Crossover signal” region in the above figure. Because the resonant frequency range of the homemade EOM is narrow, the magnitude of the EOM sidebands changes a lot during frequency scanning. That is to say, the error signal for frequency stabilization also changes which leads to inaccuracy of the scanning frequency.
In addition, Dr. Wu, et al. built the slave laser system. With the injection locking technology, the slave laser possesses the same frequency stability as the master laser. Therefore, we could receive the fluorescence of two Cs spectra at the same time.

Absolute frequency of 6S-8S TPT among 10 cells

With the master and slave system, Dr. Wu compared the absolute frequency of 10 cells two by two. Dr. Wu found that the results show discrepancies among 10 cells even with high precision. In 2014, Nathan D Zameroski et al. published one paper about pressure shift and broadening of rubidium transition lines by the noble gas and mentioned that atmospheric helium might be the prime culprit in Wu’s experiment.

For proving this suspicion, we investigate the pressure effect on 6S-8S TPT with a test cell placed in a 2-atm helium vacuum chamber. After a period of time, we exactly found the pressure effect and proved that glass cells can’t avoid the diffusion of helium.

builder: Ko-Han Chen

For further discussion, we directly measure the pressure effect. Without a glass cell, cesium atoms are filled in a vacuum chamber in 10-7 torr. With a wide-range gauge and a needle valve, we could put helium gas into the vacuum chamber to complete the quantitative analysis of the pressure shift and broadening. Eventually, we obtained a trend that frequency shift and linewidth are in a linear relation. Surprisingly, the measuring parameter of the 10 cells could smoothly fit the trendline. The pure cesium in the vacuum chamber possesses the narrowest linewidth, thus we treat it as a frequency zero point. Based on the experimental results, we could deduce that the mystery of the frequency discrepancies is formally solved. In future experiments, we could easily check the condition of the Cs cells by this helium-collision slope, whenever we don’t know the strange shift of the transition frequency.

page editor: Ko-Han Chen (2023)