A fiber-optic Fabry-Perot interferometer (FPI) is proposed and demonstrated as an electric current sensor based on ohmic heating with temperature self-compensation. The sensing head is fabricated by attaching a silicon pillar on the fiber tip as the FPI and inserting it into a molten Tin (Sn) sphere as the conducting medium. Two copper electrodes are also inserted into the Sn sphere and serve as the electrodes for current measurement. The electric current loaded on the Sn sphere will lead to a temperature rise that is measured with high resolution by the silicon FPI. A fiber Bragg grating (FBG) is incorporated in the vicinity of the sensing head for temperature compensation. Both theoretical analysis and experimental results indicate that the peak or valley wavelengths of the silicon FPI have a linear response to the square of the measured current. The current responsivity of 1.83 nm/A2 has been experimentally obtained, with a resolution of 1.28×10-3 A at around 0.3 A.

We report a sensitive and mechanically robust fiber-optic refractometer with temperature compensation based on a Fabry-Perot interferometer formed by two chirped fiber Bragg gratings on a seven-core multicore fiber. A small part of the fiber cladding at the sensing region is etched to expose the outer cores to the environment. While the optical modes supported by the outer cores are affected by both temperature and refractive index (RI) changes of the surrounding liquid, the optical mode in the central core of the fiber is affected by the temperature changes only. The wavelength shift due to RI change is obtained by subtracting the wavelength shift of the central core from that of one of the outer cores so that the temperature effect is removed. The demodulation can also remove the errors caused by the wavelength drift of laser. Due to the narrow spectral features of the sensor, the sensor shows a low detection limit of 2.4×10-6 at a background RI of 1.316 at wavelength of ~1550 nm. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.

We report a fiber-optic bolometer based on a high-finesse silicon Fabry-Perot interferometer (FPI). The silicon FPI absorbs and converts the incident radiation into temperature variations, which are interrogated by the shift of the reflection spectrum of the FPI. The FPI is a silicon pillar with one side coated with a high-reflectivity dielectric mirror and the other side coated with a gold mirror. A multimode fiber collimator is applied between the FPI and lead-in single-mode fiber to reduce the round-trip diffraction loss, giving rise to a high-finesse of 35 of the FPI. The sensor is demodulated using a low-cost distributed feedback diode laser. A dummy bolometer was used to effectively reduce the common noises from the laser wavelength drift and ambient temperature variations. Experimental results show that, compared with a previously reported fiber-optic bolometer, the one reported here has a 5-fold decrease in noise and a 7-fold increase in responsivity with a noise equivalent power density (NEPD) of 0.27 W/m², which is comparable with the NEPDs of the state-of-the-art resistive bolometers.

Wavelength interrogation has been widely used for fiber-optic sensors based on Fabry-Perot interferometers (FPIs) because of its high measurement resolution. However, phase ambiguity often limits the measurement range to be less than one free-spectral range (FSR) of the FPI. Here, we investigate a simple but effective method that can eliminate the phase ambiguity of a silicon FPI temperature sensor using a fiber Bragg grating (FBG) fabricated on the lead-in fiber close to the silicon FPI. The reflection spectrum of the sensor shows sinusoidal fringes from the FPI and a single peak from the FBG. Assuming the FBG and the silicon FPI have similar temperatures, the FBG peak can be used merely as a marker to identify an FPI fringe peak without ambiguity even when it shifts by more than an FSR. Although the temperature resolution of the FBG is lower than that of the silicon FPI, the measurement range of the silicon FPI can be greatly improved without sacrificing the measurement resolution. Using a temperature sensor based on a 75-μm-thick silicon FPI, we demonstrated temperature measurement over a range of -60 to 140 °C (corresponding to a spectral shift of about five FSRs) with a resolution of 0.003 °C. The measurement range with unambiguous peak recognition is expected to be larger than demonstrated in the experiment and only limited by the spectral bandwidth of the sensor system and the capability of materials used for the sensor construction.

Plasma radiation plays an important role in the operation and control of magnetic confinement fusion and is typically measured by bolometers. Here, we study the influence of mechanical vibration and magnetic field, which are typically present in magnetic confinement fusion devices, on the performance of a fiber-optic bolometer (FOB) based on silicon Fabry-Perot interferometer (FPI) demodulated by a scanning diode laser. It is found that both the vibration and magnetic field could cause large noise due to the birefringence of the silicon FPI. To mitigate the birefringence effects, we demonstrated two effective methods: polarization maintaining FOB and polarization scrambling. We show that two methods can be effective in mitigating the effect caused by the sensor birefringence: (1) replacing the regular single-mode fiber in the FOB system with polarization-maintaining fiber and (2) using a polarization scrambler after the laser source. For both methods, no significant increase in the noise was observed in the vibration and magnetic field tests. Our research highlights the importance of the polarization management for the FOB systems toward their practical applications in magnetic-confinement fusion systems.

A fiber optic bolometer (FOB) was demonstrated observing a fusion plasma for the first time at the DIII-D tokamak. A FOB uses a fiber optics-based interferometric technique that is designed to have a high sensitivity to temperature changes [75 mK/(W/m2) responsivity in high vacuum with 0.38 mK noise level] with a negligible susceptibility to electromagnetic interference (EMI) that can be problematic for resistive bolometers in a tokamak environment. A single-channel test apparatus was installed on DIII-D consisting of a measurement FOB and shielded reference FOB. The single-channel FOB showed a negligible increase in the noise level during typical plasma operations (0.39 mK) compared to the benchtop results (0.38 mK), confirming an insignificant EMI impact to the FOB. Comparisons to DIII-D resistive bolometers showed good agreement with the single-channel FOB, indicating that the FOB is comparable to a resistive bolometer when the impulse calibration is applied. The noise-equivalent power density of the calibrated FOB during a plasma operation was 0.55 W/m2 with an average sampling time of 20 ms. The major potential effect of ionizing radiation on the FOB would be the radiation-induced attenuation, which can be efficiently compensated for by adjusting the probing light power.