Xiaodong Wen

Photonic generation of triangular waveform signals by using a dual-parallel Mach-Zehnder modulator.

A photonic approach to generate triangular waveform signals is proposed and analyzed. With active bias control, two sub-MZMs (MZ-a and MZ-b) of a dual-parallel Mach-Zehnder modulator (DP-MZM) operate at minimum transmission point, leaving the main MZM (MZ-c) at quadrature transmission point. Triangular waveform can be observed by a parameter setting of modulation index. The proposal is first analyzed and then validated by simulation. The key significance of the scheme is that it is capable of generating triangular waveform signals via a sinusoid local oscillator.

Liquid level measurement by applying the Mach–Zehnder interferometer based on up-tapers

A novel scheme for liquid level detection using an all-fiber Mach-Zehnder interferometer, based on two up-tapers, is proposed and experimentally investigated. The effective refractive indices of the axisymmetric modes LP(0n) are analyzed while it is partly immersed in liquid. The sensitivity of the sensor is greatly dependent on the fiber length between the two up-tapers, as well as the order of the cladding modes through the calculation. The experimental results show that the sensitivity of the Mach-Zehnder interferometer reaches a 22.5 nm wavelength shift per meter of level change with the middle fiber length of 14.3 cm. The proposal might be attractive since the up-tapered liquid level sensor could easily be fabricated and used for a large measuring range.

Dumbbell-Shaped Mach–Zehnder Interferometer With High Sensitivity of Refractive Index

We proposed and fabricated a dumbbell-shaped Mach-Zehnder interferometer (MZI) on a single mode fiber using a revised program of the fusion splicer. To the best of our knowledge, it is the first time one has produced and applied the MZI of this configuration to detect liquids refractive indexes. The dumbbell shape has a length of 1.9 mm and a minimum waist diameter of 63 μm. The wavelengths shift with the changing of surrounding refractive indexes, and the refractive index sensitivity is as high as 251 nm per refractive index unit with the linear range of 1.333-1.391 that is tens of times higher than the MZI based on two fiber tapers.

Ultrasensitive temperature fiber sensor based on Fabry-Pérot interferometer assisted with iron V-groove.

A fiber extrinsic Fabry-Pérot interferometer (EFPI) assisted with iron V-groove for temperature measurement is proposed and investigated by means of both numerical simulation and experiment for the first time to our best knowledge. The main temperature sensing component is acted by the iron V-groove whose coefficient of linear thermal expansion (CLTE) is much higher than that of the silica glass. Two fibers are stuck to the V-groove with two glued points, respectively. Maximum sensitivity of 260.7 nm/°C, which is the highest value for a fiber interferometric sensor up to now, has been achieved experimentally. It is worth noting that the temperature sensitivity of this sensor can be improved limitlessly via implementing a smaller gap size of the EFPI, longer distance between the two glued points or material with higher CLTE of the V-groove, theoretically.

Optical temporal differentiator using a twin-core fiber

Abstract. Twin-core fiber (TCF) can provide the required spectral response for implementing an optical temporal differentiator. It is shown that the output temporal waveform from one core of the TCF providing full energy coupling is proportional to the first derivative of the optical temporal signal launched at the input. Moreover, TCF can also be used as the first order optical temporal differentiator for multiwavelength optical signals, in which the position and the number of central wavelength can be tunable simply by changing the fiber length. Our numerical results confirmed that the TCF has a good accuracy by calculating the first time derivatives of the input optical signals with temporal features in the picosecond and subpicosecond ranges.

A stable wavelength-tunable single frequency and single polarization linear cavity erbium-doped fiber laser

We report the configuration and operation of a wavelength-tunable single frequency and single polarization erbium-doped fiber laser (EDFL) with a stable and high optical signal to noise ratio (OSNR) laser output. A narrow-band fiber Bragg grating (NBFBG), a FBG-based Fabry–Perot (FP) filter, a polarization controller (PC) and an unpumped erbium-doped fiber (EDF) as a saturable absorber (SA) are employed to realize stable single frequency lasing operation. An all-fiber polarizer (AFP) is introduced to suppress mode hopping and ensure the single polarization mode operation. By adjusting the length of the NBFBG using a stress adjustment module (SAM), four stable single frequency and single polarization laser outputs at wavelengths of 1544.946, 1545.038, 1545.118 and 1545.182 nm are obtained. At room temperature, performance with an OSNR of larger than 60 dB, power fluctuation of less than 0.04 dB, wavelength variation of less than 0.01 nm for about 5 h measurement, and degree of polarization (DOP) of close to 100% has been experimentally demonstrated for the fiber laser operating at these four wavelengths.

Multi-Wavelength Optical Single-Sideband Modulated WDM Radio-over-Fiber Systems by Employing a Twin-Core Fiber

Multi-wavelength optical single-sideband (OSSB) modulation for wavelength-division-multiplexed radio-over-fiber (WDM-RoF) systems using a twin-core fiber (TCF) is proposed and demonstrated in this study. A TCF shows a periodic transmission spectrum due to the inherent coupling between the cores. Optical double-sideband (ODSB) signals generated by a Mach-Zehnder modulator pass through the TCF to realize multi-wavelenth OSSB modulation for WDM-RoF systems. Due to the negative slope of the TCF transmission spectrum, the upper-sideband experiences higher attenuation than the lower-sideband for each wavelength channel. In addition, the optical carrier-to-sideband ratio (OCSR) also can be optimized, simultaneously, which can enhance the transmission performance of RoF links. The performance analysis of the proposal is carried out via simulations, results show that the receiver sensitivity is improved 10.35 dB with OCSR reducing from 12.48 to 6.89 dB. The RF power fluctuation also can be optimized by employing the TCF to suppress the upper-sidebnd (or lower-sideband).

High-Sensitive Microdisplacement Sensor Based on Fiber Mach–Zehnder Interferometer

A fiber-optic sensor based on Mach-Zehnder interferometer, which is composed of two cascaded fiber directional couplers, is investigated and applied for high-sensitive microdisplacement detecting. The free spectrum range of the transmission spectrum varies with the gap sizes in the sensing arm, which is used to sense displacement for the first time to the best of our knowledge. The experimental result is well agreed with the numerical simulation, and shows the maximum displacement sensitivity of 66 nm/μm which is at least forty times higher than that of the previous displacement fiber-optic sensors. Furthermore, the proposed sensor has the potential for higher sensitivity by enlarging the wavelength range of both optical source and signal detector.

Design and performance analysis of high-order optical temporal differentiator with twin-core fiber

A simple and general approach for implementing all-fiber high-order optical temporal differentiator based on twin-core fiber (TCF) is presented and demonstrated. Specifically, the core 2 (or core 1) of the TCF should be cut in N sections with the same length for achieving N ’th-order optical temporal differentiator, which can be considered to consist of N cascaded first-order optical temporal differentiators based on TCF. Our simulations show that the proposed approach can provide optical operation bandwidths in the several THz regime, which is capable of accurately processing time features as short as subpicoseconds. Performance analysis results show a good accuracy calculating the high-order time differentiation of the optical signal launched at core 2 (or core 1).

CCD fiber Bragg grating sensor demodulation system based on FPGA

A CCD fiber Bragg grating sensor demodulation system based on FPGA is proposed. The system is divided into three units: spectral imaging unit, signal detection unit and signal acquisition and processing unit. The spectral imaging unit uses reflective imaging system, which has few aberration, small size, simple structure and low cost. In the signal detection unit, information of spectrum are accessed by CCD detector, the measurement of spectral line is converted into the measurement of the pixel position of spot, multi point can be simultaneously measured, so the systems reusability, stability and reliability are improved. In the signal acquisition and processing unit, drive circuit and signal acquisition and processing circuit are designed by programmable logic device FPGA, fully use of programmable and high real-time features, simplified system design, improved the systems real-time monitoring capabilities and demodulation speed.