RESUMO
We report an all-fiber triple-wavelength single-frequency distributed Bragg reflector (DBR) laser for 1-10â GHz microwave (MW) generation. The DBR cavity contains a non-polarization-maintaining Er fiber and a pair of fiber Bragg gratings (FBGs) made using a femtosecond laser line-by-line (LbL) direct-writing method. Such a configuration combining a short cavity and radial asymmetry leads to frequency locking and phase controlling of multi-wavelength fiber lasers. A 1.59-µm triple-wavelength laser with high coherence, spectrum purity, and polarization purity has been demonstrated; 3-6â GHz triple-frequency MW was generated.
RESUMO
In the resonator fiber optic gyroscope (RFOG), the conventional laser feedback loop is realized by adjusting the laser central frequency to tracking the resonance point of the resonator. This method generally relies on the laser tuning coefficient and digital-to-analog converter, which inevitably produces quantization error and limits frequency-locking accuracy and gyro resolution. In addition, the output drift caused by low-frequency noise also is a problem with long-term lock-in tests. In this paper, a novel and simple combined frequency-locking technology based on a phase-modulated feedback loop and a laser feedback loop is proposed by using the high-precision tuning of a phase modulator to improve the resolution and suppress the low-frequency drift. Furthermore, it has been proved by experiments that the resolution is increased by 10 times, while the frequency-locking accuracy is improved from 10°/s to 0.5°/s by using the combined frequency-locking mode. In addition, the low-frequency drift is eliminated with the long-term lock-in of RFOG, and the system resolution from 1°/s to 0.1°/s is accurately displayed.
RESUMO
Nitrogen management through monitoring of crop nitrate status can improve agricultural productivity, profitability, and environmental performance. Current plant nitrate test methods require expensive instruments, time-intensive labor, and trained personnel. Frequent monitoring of in planta nitrate levels of the stalks in living plants can help to better understand the nitrogen cycle and the physiological responses to environmental variations. Although existing enzymatic electrochemical sensors provide high selectivity, they suffer from short shelf life, high cost, low-temperature storage requirement, and potential degradation over time. To overcome these issues, an artificial enzyme (vitamin B12 or VB12) and a two-dimensional material (graphene oxide or GO) are introduced into a conventional photoresist (SU8) to form a bioresin SU8-GO-VB12 that can be patterned with photolithography and laser-pyrolyzed into a carbon-based nanocomposite C-GO-VB12. The electrocatalytic activity of the cobalt factor in VB12, the surface enhancement properties of GO, and the porous feature of pyrolytic carbon are synergized through design to provide C-GO-VB12 with a superior ability to detect nitrate ions through redox reactions. In addition, laser writing-based selective pyrolysis allows applying thermal energy to target only SU8-GO-VB12 for selective pyrolysis of the bioresin into C-GO-VB12, thus reducing the total energy input and avoiding the thermal influence on the materials and structures in other areas of the substrate. The C-GO-VB12 nitrate sensor demonstrates a year-long shelf lifetime, high selectivity, and a wide dynamic range that enables a direct nitrate test for the extracted sap of maize stalk. For in situ monitoring of the nitrate level and dynamic changes in living maize plants, a microelectromechanical system-based needle sensor is formed with C-GO-VB12. The needle sensor allows direct insertion into the plant for in situ measurement of nitrate ions under different growth environments over time. The needle sensor represents a new method for monitoring in planta nitrate dynamics with no need for sample preparation, thus making a significant impact in plant sciences.