Interferometric gravimeters
Interferometric gravimeters are highly precise instruments used to measure the acceleration due to gravity. They operate by tracking the motion of a freely falling test mass in a vacuum using laser interferometry. This technique involves splitting a laser beam into two paths, reflecting them off mirrors, and then recombining them to create an interference pattern. Variations in this pattern can be used to calculate the distance the test mass falls, and consequently, the gravitational acceleration.
Challenges with gravimeters
While single-wavelength gravimeters have proven highly effective, they come with certain challenges. One of the key contributors to the uncertainty in gravity measurements is the reference wavelength. To address this, it is customary to use an iodine-stabilized He-Ne laser, resulting in an uncertainty of about 2–3 μGal. This type of laser is used in for instance the FG5X which makes it one of the most accurate commercially available absolute gravimeters today.
In metrology, the most stringent accuracy requirements for gravity measurements are linked to the realization of the kilogram using the Kibble balance. The uncertainty in absolute gravity measurements must not exceed 5 μGal. Therefore, it is essential to analyze potential systematic errors in gravimeters and explore methods to minimize these errors
Investigating uncertainty of gravimetry
The Czech Metrology Institute and the Geodetic Observatory Pecný are both involved in research aimed at improving the accuracy of gravimetric measurements. In collaboration with DFM, they sought to investigate whether the FG5X gravimeter could be operated using a highly stable fiber laser, such as the Stabiλaser 1542ε, operated at a wavelength of 771 nm. Additionally, they explored whether the system could analyze measurements at two wavelengths simultaneously
Measurement set-up
The measurements were carried out with the FG5X-251 gravimeter using the WEO model 100 iodine-stabilized 633 nm He–Ne laser along with the 771 nm second harmonic of the acetylene-stabilized fiber laser, the Stabiλaser 1542ε. The gravity measurements were conducted with both lasers operating simultaneously in the interferometer, as well as with each laser operating separately.
Comparison of the stability
First, it was found that the Stabiλaser 1542ε is more frequency stable than the WEO for time averages comparable to the free fall duration (0.25 s). The Allan standard deviation was around 5×10-13 for the 771 nm stabilaser and 3×10-11 for the 633 nm WEO.
Gravity measurements
The experimental results demonstrate that using two optical wavelengths in the absolute gravimeter allows for the identification of potential error sources in interferometric measurements at specific wavelengths. It was found that the AR coating contributed to the uncertainty budget by about 0.25 μGal, 2 μGal and 13 μGal at the laser wavelengths of 633 nm, 771 nm and 1542 nm, respectively. This illustrates that the optics in the gravimeter was optimized for 633 nm.
The results furthermore, show agreement in gravity acceleration measurements within a range of 2–4 μGal, confirming the applicability of the Stabiλaser 1542 in gravimeters. The 771 nm measurements exhibited improved noise characteristics at high frequencies, though low-frequency noise increased due to the gravimeter’s optical optimization for the 633 nm wavelength.
Read the full story in Measurement Science and Technology, Volume 35, Number 3 here