In-fiber modal interferometers have been widely used in the applications of biochemical sensing, mine safety and health monitoring of buildings. The temperature feature of sensors is one of the most important characteristics, but the studies are rarely reported under the condition of subzero temperature. In this paper, through core-mismatch fiber splicing method, three in-fiber Mach-Zehnder interferometers (MZIs) are fabricated based on single-mode fiber (SMF), erbium-doped fiber (EDF, with core diameter of 3.6 μm) and multimode fiber (MMF, with core diameter of 50 μm), respectively. Their interference patterns are investigated through beam propagation method and Fast Fourier Transform analysis. The comprehensive tests of temperature are then performed in the range from -40 to 0°C. The experimental results show that, in subzero temperature, the transmission spectrums of MZI sensors based on single mode fiber (SMF) and MMF are worsened in terms of fringe visibility and intensity. And the sensitivity of MMF-based structure is 68.8 pm/°C with a 12.3-dB deduction of fringe visibility. Comparatively, the EDF-based MZI presents ideal sensitivity due to negative gain-temperature feature. By calculation, the 124.7 pm/°C sensitivity is gained with the linearity of 0.9892. Moreover, 10-dB enhancement in intensity and over-20-dB fringe visibility are demonstrated, which indicates that the EDF-based sensor is potential and promising for the applications of cryogenic sensing.
In this paper, a novel fiber ring laser (FRL) is proposed and investigated based on modal interference. Through core-offset splicing technique, an in-fiber Mach-Zehnder interferometer (MZI) is fabricated based on thin-core fiber and single mode fibers. Its distribution of light filed is comprehensively analyzed by beam propagation method. The FRL is then setup, in which the fabricated MZI is used as a band-pass filter. The output of laser is controlled and optimized by accurately adjusting the state of polarization controller. The experimental results show that, the extinction ratio of lasing wavelength reaches 39.8 dB, and the line width is less than 0.1 nm. Moreover, the proposed FRL is applied in temperature sensing, and the tested sensitivity reaches 122.7 pm/°C with the linearity of 0.9982. In addition, by calculation, the amplitude noise and the spectrum resolution are 8.84×10<sup>-3</sup> nm and 2.89×10<sup>-3</sup> nm, respectively. Therefore the detection limit in this laser sensor is about 0.07°C, which is obviously higher than that in passive fiber optic sensor.
This type of sensor is manufactured by using the KF-FBT type fusion taper machine to uniformly pull the same single mode optical fiber. Double tapered section cascade type singles mode fiber. When the signal light passes through the concatenation fiber cone, the cladding mode is excited after passing through the first stage cone region. After a certain distance transmission, the second stage cone region interferes with the core mode. The change of the waveform is seen from the spectrometer. When the two cones are bent by 90, the article explores a double-cone-section cascading sensor that is easy to operate and easy to perform multi-point measurements from both theory and experiment. The spectral peak-to-valley contrast of the interference fringe is more obvious, and it can serve as a sensor head to test the ambient temperature. The sensor has a temperature sensitivity of 60 pm/°C.
The stability of gain-clamped erbium-doped fiber amplifiers (EDFAs) may be affected by the parameter drift of active and passive devices. We propose a hybrid gain-control scheme to enhance the working stability, which consists of an optical dual-fiber-Bragg-grating (FBG)-based linear cavity and an electrical fuzzy-based controller. The principle of optical gain clamping is then depicted and a detailed design about dual-channel fuzzy controller is given. The experimental results show that, in the dual-FBG configuration, with 20.2-dB mean gain, the designed L-band EDFA simultaneously guarantees the ±0.1-dB stability (with 30-dB dynamic range) and the ∼0.4-dB flatness in the range of 1570 to 1610 nm. In addition, under the electrical feedback control, the flatness stability of gain spectrum reaches ±0.18 dB within 60 min and the mean gain stability is about ±0.34 dB within 10-h continuous operation.