Researchers at the Max Planck Institute for the Science of Light (MPL; Erlangen, Germany) have created a hollow-core photonic-crystal fiber (PCF) through which they send a glass microbead that senses different physical quantities such as electric field, temperature or vibration with a high spatial resolution as it moves along the fiber. The fiber can be quite long and used in harsh conditions such as those created by an aggressive chemical substance or inside an oil pipeline.
"In the beginning, the idea was to develop a radioactivity sensor for inside a nuclear power station," says Tijmen Euser, one of the researchers. When used for sensing radioactivity, conventional glass fibers with embedded fiber-optic sensors are gradually darkened by the radiation so that light can no longer propagate within them. But the PCF's air-filled cavity cannot be darkened by radioactive radiation, making them suitable for this task.
The researchers examined whether hollow-core PCFs are suitable as sensors by initially using the fibers to measure electric field, vibration, and temperature. To do this, they guided a glass microbead measuring probe through the hollow core of the PCF, which is only a few micrometers across, by sending a laser beam through the channel from each end of the fiber. The bead reflects the light and thereby experiences photonic pressure from both sides. By setting the power of the two laser beams to different strengths, the bead can be pushed in one direction more than in the other and moved through the fiber at a specific speed.
To measure the strength of an electric field, the researchers exploited the fact that the bead becomes electrically charged by rubbing against other beads before they send it through the hollow fiber. In an electric field, it is therefore deflected from the center of the channel to its edge, and thus reflects more laser light to the side than in the central position. This light attenuation is measured by a photodiode at one end of the fiber.
To find out to what resolution the spatial distribution of the field strength can be measured with the flying bead, the researchers passed the glass fiber close to fine electrodes, the thinnest measuring 200 µm. The researchers accurately reproduced the fine structure of the electrodes with their measurements to a 100 µm resolution.
"We were also able to measure magnetic fields with a magnetic bead with extremely high precision," says Dmitry Bykov, doctoral student at the Max Planck Institute in Erlangen and lead author of the study. Vibrations could also be similarly determined, as they also deflect the particle from the center of the fiber. Electric fields and vibrations can be distinguished by the behavior of beads carrying different levels of charge.
Bykov and his colleagues have also demonstrated that their PCF can measure temperature. For this, they use the fact that the viscosity of air decreases with increasing temperature, and the particle thus migrates faster through the channel of the fiber. To measure its speed, the physicists illuminate the bead with an additional, weak laser, using the Doppler effect to determine its velocity.
In their experiment, the researchers used an oven to heat part of the fiber to temperatures of several hundred degrees Celsius. They were able to measure this temperature with an accuracy of around five degrees. Fluctuations in the speed meant the spatial accuracy of this method was only a few centimeters. "With the aid of a rotating particle, whose rotational frequency depends on the viscosity of the air, it would be possible to measure with micrometer accuracy, however," says Euser.
Fluorescent bead senses radioactivity
"Next, we want to realize the radioactivity sensor," says Bykov. To do this, the researchers want to use fluorescent beads; information on the strength of the radioactivity at the location of the bead would then be provided by the changes in the intensity of the fluorescence.
The maximum length of the sensor fiber is currently around 400 m, as the laser light experiences losses as it is transmitted in the PCF, and thus the glass bead can no longer be trapped above a certain length. However, PCFs with significantly lower losses do exist. These could be used to increase the range of the fiber sensors to several tens of kilometers.
1. D. S. Bykov et al., Nature Photonics 9, 461–465 (2015); doi: 10.1038/nphoton.2015.94