Analog Angle Blog

Quantum physics, lasers enable advanced instrumentation

A large part of the analog world is focused on sensors and their related interface circuitry. In many cases, the physical variable to be sensed is obvious, such as temperature, pressure, vibration, flow and so on, but the actual implementation is challenged by the realities of the installation and setting.

In the past few years, new components and techniques made available have started merging older and much-newer disciplines. The convergence of analog circuitry with quantum physics, lasers and gigahertz RF has enabled researchers to develop field instrumentation offering amazingly sophisticated sensing.

Much of this effort is driven by enormous opportunities provided by Earth-orbiting units such as CubeSat, while other parts are deployed on much more advanced mission such as NASA’s James Webb Space Telescope now “parked” at the L2 LaGrange point about a million miles from Earth, and its Parker Solar Probe, which is grazing the Sun while taking data. Still, other instrumentation is literally Earth bound, trying to make extremely accurate, super-precise measurements of gravity, a most basic and truly ubiquitous physical phenomenon.

Two very different examples show the scope of the advances and technology being used.

Compact Ocean Wind Vector Radiometer (COWVR)

It’s a passive microwave radiometer which measures variations in natural microwave emissions from Earth; it was delivered to the International Space Station in December 2021 as part of SpaceX’s 24th commercial resupply mission for NASA. It captures microwave emissions from Earth at up to 34 GHz through all latitudes visible to the space station (52⁰N to 52⁰S). These microwave frequencies provide weather forecasters with information on the strength of winds at the ocean surface, the amount of water in clouds, and the amount of water vapor in the atmosphere—an impressive example of remote sensing (Figure 1).

Figure 1 The passive COWVR system with its polarimetric receivers can provide a wide range of critical weather and climate data. Source: JPL/NASA

Radiometers need an antenna that rotates so that they can observe a wide scan of Earth’s surface rather than just a narrow line. In all other spaceborne microwave radiometers, not only the antenna but also the radiometer itself and the companion electronics rotate at about 30 revolutions per minute. While there are strong reasons for using a design with so many spinning parts, it’s a challenge to keep a spacecraft stable when there’s that much rotating mass.

COWVR is a complete rethinking of a classic instrument’s design. Its primary objective was not to improve accuracy, but to reduce overall mass and size while also minimizing rotating mass. The COWVR design weighs about 58 kilograms (130 pounds), which is less than one-fifth the mass of the standard microwave radiometer used by the U.S. military to measure ocean winds; in addition, less than one-third of its mass rotates. Power requirements are a modest 40 W versus 350 W.

Figure 2 The COWVR instrument has a couple of three-frequency polarimetric receivers operating at 18.7, 23.8, and 33.9 GHz, all using a single feedhorn antenna and reflector. Source: JPL/NASA

The new instrument uses a radically different design approach, with three main differences compared to the established unit:

  • It has a single multi-frequency feedhorn which facilitates the use of a simple antenna rotating around the feed axis, as opposed to having to spin the entire radiometer system and pass signals through the spin assembly. The feedhorn supports operation at 18.7 GHz/780 MHz bandwidth, 23.8 GHz/780 MHz bandwidth, and 33.9 GHz/1,975 MHz bandwidth.
  • Internal calibration sources enable fully polarimetric calibration—polarimetry is the measurement and interpretation of the polarization of transverse waves—by replacing the “warm target” used to calibrate a radiometer’s polarization measurements, with a noise source that generates known polarized signals. This eliminates the need for an external warm load and cold-sky reflector, thus simplifying the mechanical design and enabling a complete 360⁰ scan.
  • It uses a compact, highly integrated MMIC polarimetry-receiver combination, thus lowering the system mass and power which, in turn, makes the system well suited for deployment on smaller, lower-cost satellites.

The data acquired thus far indicates that the unit and sensor design are meeting the performance of the legacy sensor at a fraction of the cost and, perhaps more important, with significantly reduced mass and power needs.

Quantum Gravity Gradiometer at University of Birmingham UK

In stark contrast to the COWVR project, it’s designed to measure a much more mundane physical parameter: gravity. It’s not that gravity itself is hard to measure but, as with most such parameters, measuring it with extreme precision—and quantifying micro changes that value—is difficult for many reasons.

The project team says they are the first to test a quantum gravity gradiometer outside of laboratory conditions, as they used to find a tunnel buried outdoors in real-world conditions one meter below the ground surface. So, the team claims to have won an international “race” to take the technology outside the lab.

Why even care about this? By measuring small variations in the Earth’s gravitational field, it’s possible to gain useful information about what lies underground. This can be used for a range of applications, including mineral and resource exploration, aquifers, geological mapping, civil engineering and archaeology.

The sensor works by detecting variations in microgravity using the principles of quantum physics, which is based on manipulating nature at the sub-molecular level. The sensor creates two “clouds” of ultracold rubidium atoms held in separate magneto-optical traps in an hourglass configuration (Figure 3). The clouds are positioned such that one is 1 meter above the other and each cloud is then cooled to millikelvin temperatures before being released simultaneously, at which point, the clouds undergo freefall inside the vacuum chamber.

Figure 3 The system has a main sensor head and an enclosure for the laser and control systems, with the laser system showing the two modes of Raman beam delivery used with arrows representing the beams input to the chamber. The interferometer outputs are read out by measuring the atomic state populations of the two hyperfine ground states, using a fluorescence pulse delivered along the central axis with the light that is scattered by the atoms being captured on a photodiode. Source: University of Birmingham UK

That’s only the start of the complicated sequence. During freefall, a sequence of counterpropagating laser pulses is fired at the clouds to create two atom interferometers that can each measure the local acceleration due to gravity. The difference in local gravity between the two clouds is extracted by subtraction from the experiment.

To greatly reduce some error sources, the design uses an approach similar to the circuit and sensor technique of causing unavoidable errors to self-cancel—think Wheatstone bridge or matched, tracking resistors in an amplifier. Here, noise, such as vibration, which is common to both atom interferometers, is removed and the subtraction means misalignments from the vertical plane are correlated for the pair of clouds, giving a reduced susceptibility to tilt misalignment.

To demonstrate the sensor’s applicability to gravity cartography, the team found a 2 × 2 meters utility tunnel under a road on the university campus. They determined the horizontal position of the tunnel center to an accuracy of 0.19 meters and the depth to the accuracy of about 2 meters.

Figure 4 In the unit in the field, the main sensor head is in the blue cylinder, which weighs 75 kg. It’s connected to a flight case, which contains the lasers and control system, with a secondary case placed on top; the combined weight of these cases is approximately 250 kg. The system operates on a single AC wall outlet, requires approximately 800 W, and has an internal battery for short-term holdover. Source: University of Birmingham UK

Have you ever worked on a sensing-system project where the challenge was not accuracy improvement but instead significant decrease in weight, size, and cost? Alternatively, have you ever had to sense a physical variable which is “obvious,” but where the desired accuracy made the design a major challenge?

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