An approach of phase-shift transform is proposed and demonstrated to suppress the interference fading in phase sensitive optical time domain reflectometry (-OTDR). The phase-shift transform is executed in the digital domain and the complicated frequency modulation is not required. The operation principle is carefully analyzed and a signal with complementary amplitude can be obtained with this method. Through synthesizing the complementary signals, the false phases can be corrected. Benefiting from the flexibility of this approach, multiple transforms are implemented to make sure that all phases are retrieved accurately. Experimental results show that the intensity fluctuation over 60 dB is reduced to 15 dB, which decreases the standard deviation of differential phase to 0.0224. Besides, 30 dynamic measurements are tested and all distorted phases have been corrected, indicating the high reliability of this method.

In this paper, a tilted fiber Bragg grating (TFBG) plasmonic sensor based on thymine (T) - Hg - thymine (T) (T-Hg-T) base asymmetric pairing and Au nanoparticles (AuNPs), which can be used for highly sensitive and selective ultra-low-concentration Hg2+ detection, has been demonstrated. As is known, TFBG can provide an effective mean to easily excite the surface plasmon resonance (SPR) phenomenon while the fiber is coated with metal layer. By measuring the evolution of spectra, the SPR properties hence the surface perturbation could be detected accurately. To achieve highly sensitive and selective Hg2+ detection, the two specially designed single-strand probe DNAs (ssDNAs) with rich T base that can specifically forming stable T-Hg-T asymmetric pairing are served as a binding element for AuNPs labelled-target DNA conjugates. In the present of Hg2+, these two ssDNAs that modified respectively on the surface of the sensor and the AuNPs will create a double helix T-Hg-T structure between the two DNA oligonucleotides and result in the surface refractive index perturbation amplified by the AuNPs. To overcome the low reusability of the AuNPs based DNA biosensor, by using iodide ions that can form more stable compounds with mercury ions over the T-Hg2+-T structure, a regenerable DNA biosensor that can repeatedly detect mercury ions is developed. The experimental results demonstrate that this sensor shows an excellent ultra-low-concentration and selectivity detection capability for Hg2+ with the limit of detection (LOD) of ${3.073 \times 10^{-12} M \ (3.073 pM)}$ as well as a large dynamic range of 8 order of magnitude from ${1.0\times 10^{11}\rm \ M \ to \ {1.0\times 10^{3}} M}$ simultaneously. Thus, the experimental results in various actual sample from ultrapure/tap water to even clinical human serum, also reveal that it has potential application for Hg2+ detection in environmental samples and in clinic biological samples.

We present a dual-resonance fiber surface plasmon resonance (SPR) sensor for biological analysis. The sensing element was fabricated by sequentially sputtering layers of indium tin oxide (ITO) (100 nm thickness) and Au (35 nm thickness) on the surface of an optical fiber. The refractive index dispersion effect of ITO material led to resonances in the near infrared and visible wavelength regions. The refractive index of ITO is larger than the optical fiber in visible spectral area (400 to 733nm), such that the structure is a typical Kretschmann configuration surface plasmon resonance sensor. However, an Otto configuration is observed in the near infrared area (NIR) due to the ITO refractive index being smaller than the fiber core. We characterized the sensor performance by measuring bulk refractive index (RI) sensitivity in the two configurations, which were 1345 nm/RIU in the Kretschmann configuration and 1100 nm/RIU in the Otto configuration. In addition, this sensor was applied for real-time and label-free monitoring of the IgG/anti-IgG biomolecular interaction. As a robust and ultra-compact SPR sensor, which possesses wide detection range and is highly sensitive, this fiber SPR sensor can be applied for real-time biological analysis and monitoring.

A phase demodulation algorithm in phase-sensitive optical time domain reflectometry (Φ-OTDR) with self-interference balance detection is proposed. With a simple differentiate-and-cross-multiply (DCM) operation to acquire the perturbation waveform, and a further second-order operation to compensate the amplitude fluctuation of the phase, the phase can be extracted accurately. The self-interference balance detection structure can equivalently achieve the function of dual-pulse detection, but avoid the negative impact of the common-mode noise and the difference between the dual pulses on the demodulation. In experiment, several types of vibrations are retrieved correctly under the condition of single channel and low sampling rate, which demonstrates the validity of this demodulation approach.

Phase-sensitive optical time domain reflectometry (φ-OTDR) is often required in many large-scale monitoring applications. However, the frequency response of the system is restricted by the round-trip time transmitted along the sensing fiber. In this paper, we propose a new system which can realize ultrasound sensing with a very long sensing distance. The new system uses a weak reflector array as the sensing part to realize a quasi-distributed measurement. An orthogonal frequency division multiplexing (OFDM) method is used to multiplex multiple probe pulses into one interrogating period. This method has a high spectrum utilization efficiency of the transmission bandwidth. Vernier effect is also used to detect the frequency higher than the repetition rate of the probe pulses. With the combination of these two proposed methods, a quasi-distributed measurement of ultrasound signal with a response frequency up to 25 kHz can be realized with a very long sensing distance up to 51 km.