Shuyang Hu

Time-Division Multiplexing Fiber Grating Sensor With a Tunable Pulsed Laser

A novel time-division-multiplexing fiber Bragg grating (FBG) sensor with a tunable pulsed laser is proposed and demonstrated. A tunable pulsed fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting maximum voltages with a photodiode, which avoids a complex demodulation process. The advantages of the sensor include simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages

A hydrophone based on high-birefringence fiber loop mirror

A new type of hydrophone based on high-birefringence fiber loop mirror (HiBi-FLM) is proposed and demonstrated. The sensing part is a section of high-birefringence fiber wound onto a hollow metal cylinder fiber. A narrow band light source is used and the detection is done by measurement of light intensity. The hydrophone is theoretically researched and experimentally verified. The hydrophone is 32 times more sensitive than FBG hydrophone and has the advantages of polarization indent, cheap cost and easy interrogation.

A novel method for demodulation of FBG sensor

Because of the characteristic of wavelength encoding, fiber Bragg grating (FBG) has the advantages of immunity to light power fluctuation, variation in polarization and connecting loss, so it has high sensing precision. However, for the demodulation of FBG, wavelength signal is usually converted to electric amplitude signal. By measuring the amplitude signal, the sensing result is obtained. It is well know that the amplitude signal is easy to be disturbed in sense. For this reason, amplitude demodulation limits the effect of wavelength encoding of FBG. This paper presents a novel method of counting wavelength demodulation for FBG sensors using a high birefringent fiber (HBF) loop mirror. This demodulator has simple structure, high precision, low cost and convenient to use. The resolution of the loop mirror device with 30 meter long of HBF is 0.067 nm. This counting wavelength demodulating method has the significance for widespread practical application of FBG sensors.

A highly precise fiber sensor by use of counting demodulation

Wavelength encoding sensors, such as fiber Bragg grating, have the advantage of strong antijamming ability. However, the wavelength encoding signal is usually converted into electric intensity signal by demodulators in most measurements. The intensity signal is easy to be disturbed by environmental factors, so the advantage of wavelength encoding fails due to the instable intensity demodulation. In this paper, a novel wavelength demodulating method is presented. This method demodulates the wavelength encoding signal directly by means of counting using a Sagnac interferometer with birefringent fiber without the encoding conversion from wavelength to intensity. Through the interferometer, the changed wavelength signal from sensor becomes the kind of output light, the intensity of which is changed with wavelength periodically. The intensity can easy be disturbed, but its period is very stable. In other words, the change of wavelength in one intensity period is stable. With this character, we count the number when the rising edge and falling edge of intensity appear, and then calculate the accurate change of wavelength signal. In experiment, we get the 0.01 nm wavelength resolution by use of a Sagnac interferometer with 200 meter birefringent fiber. It can be proved that the intensity frequency (the reciprocal of intensity period) changed with wavelength is proportional to the length of birefringent fiber. The length is fixed, the frequency is determined. For farther increasing the wavelength resolution, it is only necessary to increase the length of birefringent fiber. The wavelength demodulator by counting method has high stability and high precision.

A novel Sagnac fiber sensor

It is well known that as a measurement device, Sagnac fiber interferometer has the advantages of high measuring sensibility and strong antijamming ability. However, it is precisely because of the high sensibility, the measured change range of sensed quantities is usually very small. And also because of the strong antijamming ability, the fiber of Sagnac interferometer can not be used as a sensor. The reason is that the sensed signal well be eliminated by the signal cancellation effect between the two directions in Sagnac interferometer like the disturbance signal. This paper presents a novel Sagnac fiber sensor. It is added two devices into the common Sagnac fiber loop, one is the phase compression device and another is the birefringent fiber sensor. The phase compression device consists of a time delay fiber and a piezoelectic ceramic twisted around by Sagnac fiber. This device can realize the phase compression by which the measured range of sensed quantities is widely extended. The birefringent fiber sensor consists of a short section of high birefringent fiber. The method of adding the birefringent fiber sensor is selecting suitable position on Sagnac fiber loop and breaking off the fiber, then melting and linking the two break surfaces with the short section of high birefringent fiber. Not like the common fiber, there is not cancellation effect on the birefringent fiber. So the Sagnac interferometer can become a sensor. This kind of Sagnac fiber sensor has the advantages of high measuring precision, wide measured range, strong antijamming ability and simple structure.

Research of surface acoustic wave acousto-optic effect

A strengthened acousto-optic (AO) coefficient of piezoelectric crystal is defined. AO coupled-wave equations of surface acoustic wave (SAW) to plane guide optic wave (PGOW) and to fiber guide optic wave (FGOW) are given respectively. A diffractive efficiency formula of SAW PGOW AO interaction and a back-wave efficiency formula of SAW FGOW AO interaction are given. The formulas indicate that under condition of weak AO interaction intensities of the diffractive wave and the back-wave are proportional to ultrasonic powers. Lithium Niobate (LN) is optimum crystal of SAW PGOW AO device, quartz is optimum crystal of SAW FGOW AO device. A SAW PGOW AO modulator using LN and a SAW FGOW AO modulator using quartz are designed and manufactured. Some curves of the diffractive wave power and the back-wave power vs. electric powers of signals driving the modulators are measured. SAW AO modulators have many advantages, for example small volume, good stability, low energy consumption and it is easy to integrate. The modulators can be used as intensity modulators. The modulators have applications in optic communication and real time signal processing.

A novel variable optical attenuator based on fiber Sagnac interferometer

A novel variable optical attenuator (VOA) based on fiber Sagnac interferometer is proposed and demonstrated. This VOA is constructed with a 3dB coupler and a polarization controller. By tuning the polarization controller, the output power can be adjusted easily. With the advantage of all fiber design, simple structure and essentially polarization independent, The VOA has an attenuation range more than 20dB, a wavelength ripple of 0.07 and its insertion loss is lees than 0.7dB.

Study on the demodulation technique of fiber grating sensor arrays based on a tunable pulsed laser

Time-division-multiplexing(TDM) demodulation technique of Fiber Bragg Grating(FBG) sensor arrays based on a tunable pulsed laser is studied. A tunable fiber laser based on a matched FBG is applied, and the wavelengths of the sensing FBGs are obtained by determining the maximum voltages by using a photodiode. The advantages of this scheme include simple structure, high signal-to-noise ratio, and the sensing signals are obtained by determining the maximum voltages.

Research of surface acoustic wave acousto-electro-optic effect

In this paper a strengthened acousto-optic (AO) coefficient of piezoelectric crystal is proposed. It shows the influences of AO effect, electro-optic (EO) effect and piezoelectric effect in piezoelectric crystal. Through correcting acoustic momentum mismatch into sum of the acoustic and electric momentum mismatch, the coupled wave equation group of the surface acoustic wave (SAW) AO effect can be changed as that of SAW acousto-electro-optic (AEO) effect. A diffraction efficiency formula of SAW AEO effect is given through solving the equation group. The formula indicates that the diffraction efficiency is related to power of SAW, acoustic frequency deviation and direct current (DC) voltage. We designed and manufactured a SAW AEO device using Lithium Niobate (LN). Some curves of relative diffracted efficiency vs power of electric signals driving the device, acoustic frequency deviation and DC voltage are measured. Experimental results are consistent with theory. SAW AEO device can be used as a deflector or modulator. When it is used as deflector, its bandwidth is wider than that of SAW AO deflector. When it is used as modulator, its center frequency can be changed. SAW AEO device has smaller volume, less energy consume and is easy to integrate. It can be used in optic communication and real time signal processing, for example correlation, convolution, spectrum analysis and optic vector calculation and so on.

Precise wavelength demodulator used for fiber sensitivity

Wavelength encoding sensors, such as fiber Bragg grating, have the advantage of strong antijamming ability. However, the wavelength encoding signal is usually converted to electric intensity signal by demodulators in most measurements. The intensity signal is easy to be disturbed by environmental factors, so the advantage of wavelength encoding fails due to the instable intensity demodulation. In this paper, a novel wavelength demodulating method is presented. This method demodulates wavelength encoding signal directly by means of counting using a Sagnac interferometer with birefringent fiber without encoding conversion from wavelength to intensity. Through the interferometer, the changed wavelength signal from sensor becomes the kind of output light, the intensity of which is changed with wavelength periodically. The intensity can easy be disturbed, but its period is very stable. In other words, the change of wavelength in one intensity period is stable. With this character, we count the number at the rising edge and falling edge of intensity appear, and then calculate the accurate change of wavelength signal. From experiment, we get 0.067 nm wavelength resolution by use of a Sagnac interferometer with 30 meter birefringent fiber. It is proved that the intensity frequency (the reciprocal of intensity period) changed with wavelength is proportional to the length of birefringent fiber. If the length is fixed, the frequency is determined. For farther increase of wavelength resolution, it is only necessary to increase the length of birefringent fiber. The wavelength demodulator is characteristic of high stability and high precision.