Qizhen Sun

Refractive index sensor using microfiber-based Mach–Zehnder interferometer

A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159  μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.

Experimental demonstration of wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) in LiNbO3 waveguides.

All-optical wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) is proposed and experimentally demonstrated in periodically poled LiNbO3 (PPLN) waveguides. The signal pulse with 40-GHz repetition rate and 1.57- ps pulse width is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. No obvious changes of the pulse shape and width, also no chirp are observed in the converted idler pulse. The results imply that single-to-multiple channel wavelength conversions can be achieved by appropriately tuning the two pump wavelengths.

All-optical 40 Gbit/s CSRZ-DPSK logic XOR gate and format conversion using four-wave mixing

We report simultaneous logic XOR gate for carrier-suppressed return-to-zero differential phase-shift keying (CSRZ-DPSK) and format conversion from CSRZ-DPSK to return-to-zero differential phase-shift keying (RZ-DPSK) by exploiting non-degenerate four-wave mixing (FWM) in a highly nonlinear fiber (HNLF). We derive analytical solutions to complex amplitudes under non-depletion approximation to show the principle of operation and verify the CSRZ-DPSK logic XOR gate and format conversion by theoretical analyses. Three converted idlers are obtained by three non-degenerate FWM processes. All-optical 40 Gbit/s simultaneous multicasting CSRZ-DPSK logic XOR gate and CSRZ-DPSK to RZ-DPSK format conversion are successfully demonstrated in the experiment.

Proposal for All-Optical Switchable or / xor Logic Gates Using Sum-Frequency Generation

All-optical logic gate based on parametric processes in periodically poled lithium niobate (PPLN) waveguides is a promising technique in future high-speed all-optical signal processing. A simple realization of switchable or/xor logic gates at 40 Gb/s is proposed and numerically demonstrated using sum-frequency generation in a PPLN waveguide. By appropriately adjusting the input signal power and choosing the waveguide length, or and xor logic gates can be obtained. The operation performance is simulated, including eye diagrams and Q-factor. The input signal powers and waveguide length are optimized, providing a theoretical basis for achieving the optimal performance for the switchable or/xor logic gates

Experimental observation of a 1.5 μm band wavelength conversion and logic NOT gate at 40 Gbit/s based on sum-frequency generation

We have experimentally verified a new idea for 40 Gbit/s wavelength conversion within the 1.5 microm band based on sum-frequency generation (SFG) in a periodically poled LiNbO3 waveguide. The spectrum and the temporal waveform of the output pump reveal that the input cw pump is converted to an optical pulse during SFG. Not only wavelength conversion but also a logic NOT gate at 40 Gbit/s are experimentally observed.

Simultaneous measurement of refractive index and temperature using multimode microfiber-based dual Mach–Zehnder interferometer

A multimode microfiber (MMMF)-based dual Mach-Zehnder interferometer (MZI) is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. By inserting a section of MMMF supporting a few modes in the sensing arm of the MZI setup, an inline interference between the fundamental mode and the high-order mode of MMMF, as well as the interference between the high-order mode of MMMF and the reference arm, i.e., the dual MZI, is realized. Due to different interference mechanisms, the former interferometer achieves RI sensitivity of 2576.584  nm/RIU and temperature sensitivity of -0.193  nm/°C, while the latter one achieves RI sensitivity of 1001.864  nm/RIU and temperature sensitivity of 0.239  nm/°C, demonstrating the ability to attain highly accurate multiparameter measurements.

Twist sensor based on axial strain insensitive distributed Bragg reflector fiber laser

A novel fiber-optic twist sensor based on a dual-polarization distributed Bragg reflector (DBR) fiber grating laser is proposed and experimentally demonstrated. By beating the signal between the two polarizations of the laser which operates at 1543.154 nm, a signal of 30.78 MHz in frequency domain is observed. The twist will change the fiber birefringence, and resulting in the beat frequency variation between the two polarization modes from the fiber laser. The result shows the beat frequency shifts as a Sinc function curve with the twist angle and both the measuring curve period and twist sensitivity depend on the twist length of the laser cavity. A high twist sensitivity of 6.68 MHz/rad has been obtained at the twist length of 17.5 cm. Moreover, the sensor is insensitive to the environmental temperature, as well as strain along the fiber axis with ultralow beat frequency coefficients, making temperature and axial strain compensation unnecessary.

Novel Miniaturized Fabry–Perot Refractometer Based on a Simplified Hollow-Core Fiber With a Hollow Silica Sphere Tip

In this paper, we report and demonstrate a novel miniaturized intrinsic Fabry-Perot interferometer (IFPI) based on a simplified hollow core fiber ended with a hollow silica sphere tip. The hollow core fiber is spliced to single mode fiber and subsequently applied with fusion arc at the end, forming a hollow sphere with a thin silica wall. The reflection spectrum is modulated by the environment of the sensor head such as refractive index and temperature. By monitoring and measuring the fringe visibility, the measurement of refractive index of the calibrated solution is carried out experimentally. The achievable refractive index (RI) resolution is about 6.2 × 10-5 by measuring the fringe visibility. This sensor offers key features and advantages of IFPI including easy fabrication, low loss, low cost, and good fringe visibility. Furthermore, the sensor head is made of single material and therefore has the potential for measurements in harsh environments. Measurements at high temperatures up to ~ 1000°C are carried out. Results reveal that the sensor has low temperature.

Microfiber Fabry–Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor

We propose a novel fiber Fabry-Perot interferometer (FPI) that incorporates a length of microfiber as its cavity and two fiber Bragg gratings (FBGs) as reflectors. The microfiber FPI is simply fabricated by flame-heated taper-drawing the central spot of an FBG into a section of microfiber. Ambient refractive index (RI) influences the effective index of microfiber, and thus the free spectrum range of the microfiber FPI, resulting in RI sensing. A dual-wavelength fiber laser based on the microfiber FPI is constructed, enabling radio frequency interrogation with high resolution. RI sensitivity of 911 MHz/RIU is experimentally demonstrated for microfiber FPI with equivalent diameter of 1.455 μm. Simulation results indicate that the sensitivity can be further enhanced by reducing the diameter of the microfiber.

Hybrid TDM/WDM-Based Fiber-Optic Sensor Network for Perimeter Intrusion Detection

A distributed fiber-optic sensor system is proposed and demonstrated for long-distance intrusion-detection, which employs the hybrid time/wavelength division multiplexing architecture. The sensing elements are single-mode telecommunication fiber which can be hanged on the fence and hedge or buried along the monitored perimeter. The pulsed light generated by the superluminescent diode was filtered by the filter which has multichannel (m=6), and then amplified by erbium-doped fiber amplifiers. A 1×(n+1) (n=20) splitter of which every port has a fiber delay coil except the first port splits the amplified light. The fiber delay coils have different lengths, which generate different delay time and produce n time zones. By utilizing the m channel dense wave-length division multiplexing modules, every fiber sensing unit (OSU)-based unbalance Mach-Zehnder Sagnac interferometer technology occupy a time zone and a wavelength. By utilizing 20 time zones and 6 wavelengths, the system contains up to 120 OSUs, of which the distributed sensing distance is from 0 to 500 m. So, the whole sensing distance of this system could reach 60 km. The system has been demonstrated to stably run over six months with the false alarm rate of less than 4%.