Hybrid Bragg Grating Enables Optical Current Sensor

Breck Hitz

Engineers at City University of Hong Kong have demonstrated a compact, inexpensive optical sensor for alternating and direct currents. The sensor, based on a fiber Bragg grating, has a low susceptibility to temperature variations and offers an approach to relay protection in electric power systems and in other applications that do not require high-accuracy measurements.

There are several approaches to designing an optical sensor for electrical currents. In the Faraday effect, for example, the magnetic field generated by an electric current rotates the polarization of light passing through an optical fiber or through a piece of bulk glass. Measurement of the effect, however, requires complex -- and expensive --polarimetric detection.

Fiber Bragg grating current sensors are based on the principle that the wavelength reflected from a grating changes when the structure is stretched or compressed. Thus, a Bragg grating may be mounted on a piezoelectric transducer or on a resistive heating element to measure voltage or current, respectively. Alternatively, it may be mounted on a magnetostrictive material and placed inside a coil so that a current passing through the coil creates a magnetic field that stretches or compresses the grating. All of these approaches, however, are subject to temperature errors because fiber Bragg gratings are very sensitive to thermal changes.

Photonics researchers offered a solution to the temperature problems several years ago, using a pair of fiber Bragg gratings mounted on materials with different magnetostrictive but similar thermal characteristics. At a given initial temperature, and in the absence of a magnetic field, the two gratings reflect the same wavelength. As the temperature changes, the reflectivity of both gratings changes equally, but as the magnetic field changes, their reflectivities separate. Thus, a temperature-independent measurement of the magnetic field -- or of the current in the coil generating the magnetic field -- can be inferred from the shape of the spectrum of reflected light from the two Bragg gratings.

The Hong Kong engineers have refined the two-grating technique to make it inexpensive, compact and perhaps suitable for real-world applications. They have eliminated the need for complex spectral measurements of the reflected light by measuring its intensity. When the two fiber Bragg gratings reflect the same wavelength, a given intensity is observed, but as the reflections separate, more total light is reflected. From a knowledge of the line shape of the gratings' reflectivity, the scientists calculate the amount of separation and, hence, the amount of current in the coil.

Figure 1. A new optical current sensor employs a fiber Bragg grating in two materials with different coefficients of magnetostriction. In the presence of a magnetic field, the reflectivity of the half of the grating mounted on Terfenol-D shifts to a longer wavelength.

Another refinement is their use of a single Bragg grating mounted on a substrate of two joined materials, Terfenol-D and Monel 400, which have a high and a low coefficient of magnetostriction, respectively (Figure 1). In the absence of a magnetic field, both halves of the grating re-flect the same wavelength, but as the Terfenol-D stretches in the presence of a magnetic field, the light reflected from that half shifts to a longer wavelength.

Figure 2. In an experimental arrangement, the researchers launched broadband light from an erbium-doped-fiber amplified-spontaneous-emission source into the setup and analyzed the spectra of the light reflected from the hybrid fiber Bragg grating without an applied magnetic field and under a magnetic field.

The intensity of the light reflected from the fiber Bragg grating is linearly dependent on the current passing through the coil (Figure 3). As the two reflected lines become increasingly separated, the slope of the curve diminishes, ultimately flattening out when there is no overlap. The observed dynamic range in this case was approximately 2 to 22 A, but the range could be increased -- at the expense of sensitivity -- by increasing the linewidth of the Bragg grating's reflectivity.

Figure 3. The researchers monitored the output voltages of the sensor for different magnetic flux values (circles) and for different temperatures (triangles). The solid curve is a polynomial fit to their experimental measurements.

The straight line across the bottom of the data plot indicates the intensity of the reflected signal as a function of temperature. The sensor shows good independence of environmental temperature. Observable on both lines is a slight oscillation of the measured data points. The researchers attribute this to interference between the fiber Bragg gratings. Essentially, the two form a low-finesse Fabry-Perot etalon, and the resonances of this etalon are visible in the data.