Phone signals spend at least some time traveling over fiber-optic cables. To ensure that the information gets where it needs to go, and to help researchers find better ways to ferry this information around, it’s necessary to reliably measure radiation power through these fibers. In order to calibrate a radiation power meter, researchers currently have to use a bulky cryogenic system and transfer the measurements to at least one other intermediate system. Each of these transfers increases uncertainties in the measurements, and the cryogenic systems are relatively rare and expensive to use and maintain.
A need exists for a sensor that could measure laser power accurately and be self-calibrating. It also would be small and inexpensive enough for one-time use. Because the sensors would be inexpensive to produce, light, accurate, and with a wide range, they could be deployed in bulk both on land and in space for networks of small detectors working together to measure the light entering and exiting the planet — particularly infrared light.
Scientists completed a prototype of a small, chip-based sensor designed to perform these tasks. Though it specializes in detecting light power in the infrared, the sensor should be capable of measuring power across a broad range of wavelengths, from visible light at about 300 nanometers, to light in the far infrared at 500 micrometers. The light at the long-wavelength end of this spectrum corresponds to terahertz (THz) radiation. The power range for these measurements includes microwatts (millionths of a watt) for fiber power measurements, and thousandths of a watt (milliwatts) for remote sensing for climate studies. The prototype contains a chip whose carbon nanotube (CNT)-coated surface is smaller than the diameter of a pencil eraser, and a compact cryogenic housing.
Optical radiation power is a measure of energy per unit time emitted by a source of light. Typically, sensors gauge this kind of power by measuring heat — the light comes in, the detector heats up, and the power is determined by the temperature rise. The primary standard for the U.S. is the cryogenic radiometer. This device is usually a cavity coated very black for maximum absorption of light. The cavity and electronics are housed inside a cryostat to keep them cooled to just a few degrees above absolute zero.
These traditional devices, though highly accurate, are large and expensive to maintain, and their accuracy only extends to a limited range of powers. In order to calibrate a detector that is sensitive to powers beyond that range, researchers need to transfer the standard through a chain of calibrations, with one type of device being used to assess the accuracy of another. Each link in the chain of calibrations adds uncertainty to the measurements.
The chip-based sensor would potentially replace many of these devices, shortening or entirely doing away with the chain. Like traditional cryogenic devices, the new sensors also measure power by gauging changes in heat, but they are designed to do so with high efficiency on a very small surface. The efficiency is due in part to the coating of CNTs, which are very black, allowing the chips to absorb up to 99.98% of the light within the target wavelength range. For comparison, the best optical cavity-based approach to power measurements would give just a hundredth of a percent better absorption.
An optical fiber is built into the detector package with a fairly rigid and precise alignment. There is a tiny heater built into the chip that produces a predictable amount of heat — which can be compared to the optical heat that the device is designed to sense — giving the ensemble the ability to calibrate itself. The whole package would be housed in a cryostat and kept at temperatures just a kelvin or so above absolute zero.
The team is currently testing the chip to characterize its performance, particularly at lower frequencies of light. While the researchers continue to develop the sensor for its various applications at cryogenic temperatures, the team also hopes to do these kinds of chip-based optical power measurements at room temperature. These chips would be optimized for the remote sensing measurements that could allow them to be used for Earth- and space-based climate applications.