Case story: Interferometric length calibration with Stabiλaser 1542ε
The importance of accurate length measurements in metrology
Accurate length measurements are fundamental to the field of metrology. These measurements are crucial in various areas, including manufacturing, engineering, and scientific research. Precise length calibration ensures that components fit together correctly, machines operate smoothly, and research results are reliable. For national metrology institutes like the Danish National Metrology Institute (DFM), maintaining high standards of accuracy in length measurements is essential. It helps set benchmarks that industries rely on for quality assurance and regulatory compliance.
Calibration of gauge blocks
Industrial companies typically use calibrated gauge blocks with precisely defined dimensions to calibrate their length measurement devices. However, these gauge blocks change over time and need to be regularly inspected and recalibrated. As the Danish National Metrology Institute, DFM provides accredited calibration of gauge blocks for length measurement equipment using various methods. The most accurate method is laser interferometry, where the length is determined from the interference patterns produced by two lasers at different wavelengths.
Interferometric length calibration
Interferometric setups for accurate gauge block calibration typically use two laser beams of different wavelengths to achieve a wide absolute measurement range. As the beams reflect and interfere, they generate interference patterns from which the exact length of the gauge block is determined with high accuracy.
Challenges faced with traditional interferometric set-ups
Traditionally, DFM and other metrology institutes have used commercial Zeeman-stabilized 633 nm Helium-Neon (HeNe) lasers and custom-made stabilized green HeNe lasers at 543 nm for length measurements. However, especially the latter laser presents certain challenges. A major issue is the availability of the 543 nm HeNe laser, which is no longer commercially produced. Additionally, HeNe lasers rely on gas tubes that require regular replacement and recalibration, a time-consuming process that can introduce inconsistencies in measurements.
Introducing DFM’s Stabiλaser 1542ε
DFM decided to replace the 543 nm HeNe laser with the 2nd harmonic 771 nm output from the Stabiλaser 1542ε. The Stabiλaser 1542ε is an acetylene-stabilized fiber laser that offers a narrow linewidth, excellent long-term stability, and high accuracy. By integrating the Stabiλaser 1542 into DFM’s interferometric length calibration system, we achieved a reliable, robust, and future-proof solution for length calibration.
Advantages of Using 771 nm Laser Line for Calibration
Besides replacement of the obsolete 543 nm laser, the 771 nm laser line from the Stabiλaser 1542ε offers numerous additional benefits for length calibration. First and foremost, the fiber laser technology ensures excellent long-term reliability. Unlike HeNe lasers, which rely on gas tubes needing frequent replacement, fiber lasers are more durable and require less maintenance. The Stabiλaser 1542ε operates according to the CIPM List of Recommended Frequency Standard Values, enabling its use as a primary length standard with direct traceability for the interferometric meter realization. When traceability relies on a Zeeman-stabilized HeNe laser, that HeNe laser must be regularly calibrated against another primary length standard, typically an iodine-stabilized HeNe laser. Hence, using a Stabiλaser 1542ε with the 771 nm option for interferometric gauge block calibration removes this extra calibration step and a single Stabiλaser 1542ε can effectively replace both a Zeeman-stabilized and an iodine-stabilized HeNe laser.
Additionally, the Stabiλaser 1542ε produces a cleaner interference pattern, leading to more robust and precise measurements. This improvement enhances the overall accuracy of the calibration process, ensuring that DFM’s measurements remain at the forefront of metrological standards.
Learn more about Stabiλaser 1542ε
The Stabiλaser 1542ε is available as a product from DFM. Read more about the features and specifications on the product page or contact DFM to discuss how Stabiλaser 1542ε can be integrated into your interferometric setup.
Case story: Using Stabilaser for dual wavelength gravimeter
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 5x10-13 for the 771 nm stabilaser and 3x10-11 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
Case story: Atomic clock using off-the-shelf Stabiλaser
Optical atomic clocks and frequency combs
Atomic clocks are essential for a wide range of applications, including GPS systems, telecommunications, and scientific research requiring ultra-precise timekeeping. These clocks are typically based on the high-frequency energy transitions of atoms, such as cesium or rubidium. A key advancement in atomic clock technology is the use of optical frequency combs, which enable precise measurements of optical frequencies in the THz range. Compared to traditional microwave-based atomic clocks, optical clocks can provide significantly more precise timekeeping. Frequency combs are used to link optical frequencies to radio frequencies, which, unlike optical frequencies, can be measured by electronic systems.
Challenges of optical clocks
Optical clocks offer unprecedented precision and stability, far surpassing traditional atomic clocks. However, for the ultimate performance these optical clocks are largely confined to laboratory use due to their complexity and sensitivity. They require ultra-stable lasers, intricate cooling and trapping techniques for atoms or ions, and precise environmental control to prevent disturbances. The delicate nature of these systems makes them challenging to deploy outside controlled settings, limiting their practical use in everyday applications.
Vescent and DFM: A case study in optical source innovation
To demonstrate the feasibility of building a highly stable optical clock using commercial off-the-shelf components, Vescent and DFM combined Vescent's FCC-100 frequency comb with DFM's acetylene-stabilized fiber laser, Stabiλaser 1542ε, and evaluated the performance of the resulting 100 MHz clock signal.
Performance results
The performance of clocks is often evaluated from their relative frequency instability over time in terms of the so-called Allan deviation.
The black data points in the figure show the measured Allan deviation of the frequency difference between the 100 MHz output of DFM’s passive hydrogen maser used for realization of the national timescale UTC(DFM) and the output from a DFM-Vescent optical clock. Thus, the black data points are the quadratic sum of the frequency instability of both the maser and the optical clock. The blue line is the manufacturer's specifications for the maser. The data shows that the optical clock performs significantly better than the hydrogen maser specifications up to 10 000 s, but clearly the maser performance is also better than its specifications. These data underscore the potential of compact reliable optical clocks in pushing the boundaries of atomic clock performance.
Additional data can be found in the white paper here.