Self-calibrated atom-interferometer gyroscope by modulating atomic velocities.

Atom-interferometer gyroscopes have attracted much attention for their long-term stability and extremely low drift. For such high-precision instruments, self-calibration to achieve an absolute rotation measurement is critical. In this work, we propose and demonstrate the self-calibration of an atom-interferometer gyroscope. This calibration is realized by using the detuning of the laser frequency to control the atomic velocity, thus modulating the scale factor of the gyroscope. The modulation determines the order and the initial phase of the interference stripe, thus eliminating the ambiguity caused by the periodicity of the interferometric signal. This self-calibration method is validated through a measurement of the Earth's rotation rate, and a relative uncertainty of 162 ppm is achieved. Long-term stable and self-calibrated atom-interferometer gyroscopes have important applications in the fields of fundamental physics, geophysics, and long-time navigation.

Compact multi-channel radio frequency pulse-sequence generator with fast-switching capability for cold-atom interferometers.

Cold-atom interferometers have matured into a powerful tool for fundamental physics research, and they are currently moving from realizations in the laboratory to applications in the field. A radio frequency (RF) generator is an indispensable component of these devices for controlling lasers and manipulating atoms. In this work, we developed a compact RF generator for fast switching and sweeping the frequencies and amplitudes of atomic-interference pulse sequences. In this generator, multi-channel RF signals are generated using a field-programmable gate array (FPGA) to control eight direct digital synthesizers (DDSs). We further propose and demonstrate a method for pre-loading the parameters of all the RF pulse sequences to the DDS registers before their execution, which eliminates the need for data transfer between the FPGA and DDSs to change RF signals. This sharply decreases the frequency-switching time when the pulse sequences are running. Performance characterization showed that the generated RF signals achieve a 100 ns frequency-switching time and a 40 dB harmonic-rejection ratio. The generated RF pulse sequences were applied to a cold-atom-interferometer gyroscope, and the contrast of atomic interference fringes was found to reach 38%. This compact multi-channel generator with fast frequency/amplitude switching and/or sweeping capability will be beneficial for applications in field-portable atom interferometers.

1200x broadband modal converter using a subwavelength self-focusing structure.

Efficient modal interconversion between optical manipulation of cold atoms in free space and transmitted light within an integrated waveguide remains a challenge in the area of integrated atomic photonics. Here, a 1200x modal converter with a footprint on the order of millimeters is proposed based on a Si3N4 subwavelength self-focusing structure. The 2.8µm×1.7µm subwavelength structure enables efficient single modal conversion. The transmission efficiency is 84% at a wavelength of 830 nm with a working bandwidth of 240 nm. The device can work in dual polarization states.

An actively compensated 8 nT-level magnetic shielding system for 10-m atom interferometer.

We design and develop a high-performance magnetic shielding system for a long baseline fountain-type atom interferometer. The shielding system is achieved by a combination of passive shielding using permalloy and active compensation with coils. An 11.4 m-long three-layer cylindrical shield is completed by the process of welding, local annealing, and entire annealing. The active compensations compress the residual magnetic field to 8.0 nT max-to-min and the corresponding gradient below 30 nT/m over 10 m along the axial direction in which external compensation, internal compensation, and constant magnetic field (C-field) compensation reduce the inhomogeneities to 25.0, 12.6, and 1.7 nT (standard deviation) sequentially. We estimate that this system could reduce the systematic error of the quadratic Zeeman shift to the 10-13 level for the weak equivalence principle test with a simultaneous 85Rb-87Rb dual-species atom interferometer.

Laser frequency shift up to 5 GHz with a high-efficiency 12-pass 350-MHz acousto-optic modulator.

We demonstrate a novel laser frequency shift scheme using a 12-pass 350-MHz acousto-optic modulator (AOM). This AOM system shows better performance compared to ordinary acousto-optic modulation schemes. The frequency of the incident laser beam is shifted by 4.2 GHz with the total diffraction efficiency as high as 11%, and the maximum frequency shift is 5 GHz. Combining the ±1st order diffraction, laser signals with up to 10 GHz frequency difference can be obtained, which fulfill most frequency shift requirements of laser cooling and coherent manipulation experiments with alkali metal atoms.

Characterization and optimization of a tapered amplifier by its spectra through a long multi-pass rubidium absorption cell.

We present a method to characterize and optimize a tapered-amplifier laser system (TALS) by its spectral quality through a long multi-pass rubidium absorption cell. A thermal vapor cell is used to measure the non-resonant spectrum of TALS, including the broadband amplified spontaneous emission (ASE), which is its main spectral noise. This method gives a simple quantified measurement to optimize various working parameters of a TA, including current and temperature online. It can as well be used to compare various TA chips based on their usage time during our precision measurement experiments. The results of this method are compared and found in sync with traditional methods of Fabry-Perot cavity and beat measurements. Such characterization and optimization are important for noise control in atom interferometers, atomic clocks, and other atomic manipulations. It can very well be used for investigating nonlinearity and ASE inside amplifying chips and can be utilized in other applications of ASE using bioimaging.

High-resolution ex vacuo objective for cold atom experiments.

We present a versatile and cost-efficient objective with a five-lens configuration, which consists completely of commercial singlets. The home-built objective has a numerical aperture (NA) of 0.44 and a long working distance of 35.9 mm, making it suitable for ex vacuo utilization. A diffraction-limited resolution of 1.08 µm and a field of view of about 210 µm are achieved when a 780 nm light passes through a 5 mm thick vacuum window. Moreover, such a design can be well adapted to a broad range of laser wavelengths (560-1000 nm) and vacuum window thicknesses (0-6 mm) by simply modifying one lens spacing, while maintaining a NA of above 0.43. The characteristics of the objective are evaluated experimentally, which are in good agreement with the simulations. Also, the objective has been successfully used for single-atom trapping and detecting in experiments. We believe that it will find more applications in various cold atom experiments.

Note: A compact low-vibration high-performance optical shutter for precision measurement experiments.

The optical shutter is an important component for atom-interferometry-based precision measurements. To decrease its vibration noise and improve its performance, we present a compact design with a direct current motor and a photodetector inserted in a compact aluminum structure. The photodetector in the blade is driven by the motor to move either in or out of the way of the laser beam. This design can suppress laser intensity fluctuation down to 0.26% over 6 h in the method of sample and hold proportional-integral-derivative feedback. It is also quiet via an electrical braking process which not only reduces its vibration noise by an order of magnitude but also quickens and stabilizes its switching performance. The shutter has a switching time of 0.8 ms and an activation delay of 8 ms with low jitters. Besides, the shutter can work for over ten million cycles normally and reliably.

[Direct determination of hyperfine structures and isotope shifts in the 6s5d 3D --> 6p5d 3F transitions of Ba I].

In the present experiments, a new method of atomic spectral measurement using states optical pre-selection was proposed and demonstrated, and the measurement of hyperfine-structures and isotope shifts in the 6s5d 3D --> 6p5d 3F transitions of Ba I was taken as an example. A 791 nm laser was used to excite the different hyperfine transitions 6s6s 1S0 --> 6s6p 3P1 for different isotopes of barium and different hyperfine energy-levels of 6s5d 3D2 for these isotopes were populated by the 6s6p 3P1 --> 6s5d 3D2 spontaneous radiations, then the corresponding fluorescence spectra of 6s5d 3D2 --> 6p5d 3F2 transitions excited by another 778 nm laser were recorded. Comparing of these spectra, 22 lines were recognized and the hyperfine-structure constants of 6s5d 3D2 and 6p5d 3F2 of 135Ba and 137Ba were evaluated consequently while the isotope shifts of these transitions were also determined.

[Frequency stabilization of diode laser using zeeman spectra].

Zeeman split takes place for the hyperfine level structure of neutral atoms where magnetic field exists. In addition, being excited by right circularly polarized light and left circularly polarized light, atoms obey different transition rules. So, the absorption peak between the hyperfine Zeeman level shifts with respect to the absorption peak without magnetic field. Accordingly, the authors demonstrated a kind of simple and flexible method to stabilize the frequency of diode laser. The linewidth of diode laser was reduced to less than 1 MHz using this method. Through analyzing the experiment results, the authors found that when the sum of both shifts of the hyperfine level of atoms excited by right circularly polarized light and left circularly polarized light was equal to the FWHM (Full Width at Half Maximum) of the saturated absorption peak, the laser frequency was stabilized best.

[Constant scaled-energy spectroscopy of Rydberg atoms in a static electric field].

In the past years, scaled energy spectroscopy is under active investigation because this method can simplify the analysis of atomic spectra in the external field based on classic mechanics. A fully computer-controlled experimental system to study the constant scaled-energy spectroscopy was established and described in this paper. The excitation energy E and the strength of the external electric field F were controlled synchronously to keep the scaled-energy epsilon = E/square root of F constant. With this system, constant scaled-energy spectra of Strontium Rydberg atoms at epsilon = -3.0 in a static electric field were successfully recorded for the first time, and the recurrence spectra were obtained by a Fourier transform.