An Open-Loop Test of a Resonator Fiber Optic Gyro

The resonator fiber optic gyro （R-FOG） ,which utilizes a resonance frequency change due to the Sagnac effect,is a promising candidate for the next generation inertial rotation sensor. In this study, an open-loop R-FOG is set up using phase modulation spectroscopy. First,the demodulation curve is obtained using a lock-in amplifier. From the demodulation signal,a gyro dynamic range of ± 4.2rad/s is obtained. Then,using different phase modulation frequencies,the open-loop gyro output signal is measured when the gyro is rotated clockwise or counterclockwise. The bias drift as a function of time is also measured. The fluctuation of the output over 5s is about 0.02rad/s. The drift can be reduced by taking countermeasures against system noise.

Ultrasonic Pulse Signal Resonance Features in Layered CFRP Within Voids

The ultrasonic pulse signal resonance features in layered carbon fiber reinforced plastic（CFRP） within voids were researched. The frequency domain model of acoustic wave propagation in multilayered medium was established. Then the reflection coefficient of multilayered CFRP within voids was numerically calculated. The results are as follows. When the CFRP laminate is tested by ultrasonic whose center frequency is close to the CFRP inherent resonant frequency, the ultrasonic may generate resonance phenomenon in CFRP. If CFRP contains evenly distributed voids, the frequency of resonant signal and its amplitude all decrease with the increase of porosity. For the thick section CFRP within local concentrated voids, the local concentrated voids near testing surface will cause signal frequency reduction and the decrease of its amplitude. But the voids which exist in layers far away from testing surface almost have no influence on signal resonance. The ultrasonic pulse echo testing was conducted for thick section CFRP specimen. The analysis results of testing signals were in accordance with the results of the numerical calculation, showing that the reflection coefficient frequency response model can effectively explain the ultrasonic resonance phenomenon in layered CFRP within voids.

Fiber resonator using negative-curvature anti-resonant fiber with temperature stability

The coupling efficiency of hollow-core fiber changes with temperature,which leads to the decrease of the finesse(F)of fiber resonator and limits the performance of the resonant fiber optic gyroscope(R-FOG)system.Negative-curvature antiresonant fiber(ARF)can maintain single-mode characteristics under the condition of large mode field diameter,achieve efficient and stable fiber coupling,and significantly improve the consistency of the F of the spatial coupling resonator in variable temperature environment.A new type of ARF with a mode field diameter(MFD)of 25μm is used to fabricate a fiber resonator with a length of 5.14 m.In the range of 25℃-75℃,the average F is 31.45.The ARF resonator is used to construct an R-FOG system that shows long-term bias stability(3600 s)of3.1°/h at room temperature,4.6°/h at 75℃.To our knowledge,this is the best reported index of hollow-core fiber resonator and R-FOG system within the temperature variation range of 50℃ test.

Photonic crystal fiber polarization filter with two large apertures coated with gold layers

A novel photonic crystal fiber（PCF） polarization filter is designed and fabricated; it consists of two large apertures coated with gold. The asymmetric structure separates the resonance position in the vertical direction well. Due to the metal layer covering, loss is greatly improved. Finite element method is applied for numerical simulation. The influences of varying gold thickness and varying the diameters and the center positions of the larger apertures on filtering performance are evaluated. Theory of coupling between surface plasma and core mode is introduced. By modulating the parameters, we realize a single polarization filter at 1.31 μm and 1.55 μm. The basal mode loss in the y direction can reach 1408.80 dB/cm at 1.31 μm and 1911.22 dB/cm at 1.55 μm respectively, but basal mode loss in the x direction is relatively small, 0.82 dB/cm and 1.87 dB/cm. In addition, two kinds of broadband polarization filters are proposed. If the fiber length is set to 200 μm,the extinction ratio is greater than 20 dB with width of 570 nm and 490 nm. The filter has simple structure and excellent performance.

Dynamic Free-Spectral-Range Measurement for Fiber Resonator Based on Digital-Heterodyne Optical Phase-Locked Loop

We propose a novel scheme, based on digital-heterodyne optical phase-locked loop with whole-fiber circuit, to dynamically measure the free-spectral-range of a fiber resonator. The optical phase-locked loop is established with a differential frequency-modulation module consists of a pair of acousto-optic modulators. The resonance-tracking loop is derived with the Pound-Drever-Hall technique for locking the heterodyne frequency of the OPLL on the frequency difference between adjacent resonance modes. A stable locking accuracy of about 7 × 10?9 and a dynamic locking accuracy of about 5 × 10?8 are achieved with the FSR of 8.155 MHz, indicating a bias stability of the resonator fiber optic gyro of about 0.1?/h with 10 Hz bandwidth. In addition, the thermal drift coefficient of the FSR is measured as 0.1 Hz/?C. This shows remarkable potential for realizing advanced optical measurement systems, such as the resonant fiber optic gyro, and so on.

Planning for selective amygdalohippocampectomy involving less neuronal fiber damage based on brain connectivity using tractography

Temporal lobe resection is an important treatment option for epilepsy that involves removal of potentially essential brain regions. Selective amygdalohippocampectomy is a widely performed temporal lobe surgery. We suggest starting the incision for selective amygdalohippocampectomy at the inferior temporal gyrus based on diffusion magnetic resonance imaging（MRI） tractography. Diffusion MRI data from 20 normal participants were obtained from Parkinson＇s Progression Markers Initiative（PPMI） database（www.ppmi-info.org）. A tractography algorithm was applied to extract neuronal fiber information for the temporal lobe, hippocampus, and amygdala. Fiber information was analyzed in terms of the number of fibers and betweenness centrality. Distances between starting incisions and surgical target regions were also considered to explore the length of the surgical path. Middle temporal and superior temporal gyrus regions have higher connectivity values than the inferior temporal gyrus and thus are not good candidates for starting the incision. The distances between inferior temporal gyrus and surgical target regions were shorter than those between middle temporal gyrus and target regions. Thus, the inferior temporal gyrus is a good candidate for starting the incision. Starting the incision from the inferior temporal gyrus would spare the important（in terms of betweenness centrality values） middle region and shorten the distance to the target regions of the hippocampus and amygdala.

Harmonic dissipative soliton resonance pulses in a fiber ring laser at different values of anomalous dispersion

The pulse dynamics of harmonic mode-locking in a dissipative soliton resonance(DSR) region in an erbiumdoped fiber ring laser is investigated at different values of anomalous dispersion. The fiber laser is mode-locked by a nonlinear polarization rotation technique. By inserting 0–200 m anomalous dispersion single-mode fiber in the laser cavity, the cavity length is changed from 17.3 to 217.3 m, and the corresponding dispersion of the cavity ranges from -0.27 to-4.67 ps^2. The observed results show that the tuning range of repetition rate under a harmonic DSR condition is highly influenced by the cavity dispersion. Furthermore, it is found that, by automatically adjusting their harmonic orders, the lasers can work at certain values of repetition rate, which are independent of the cavity length and dispersion. The pulses at the same repetition rate in different laser configurations have similar properties, demonstrating that each achievable repetition rate represents an operation regime of harmonic DSR lasers.

High sensitivity D-shaped hole fiber temperature sensor based on surface plasmon resonance with liquid filling

A high sensitivity D-shaped hole double-cladding fiber temperature sensor based on surface plasmon resonance(SPR)is designed and investigated by a full-vector finite element method.Within the D-shaped hole doublecladding fiber,the hollow D-section is coated with gold film and then injected in a high thermo-optic coefficient liquid to realize the high temperature sensitivity for the fiber SPR temperature sensor.The numerical simulation results show that the peaking loss of the D-shaped hole double-cladding fiber SPR is hugely influenced by the distance between the D-shaped hole and fiber core and by the thickness of the gold film,but the temperature sensitivity is almost insensitive to the above parameters.When the thermo-optic coefficient is -2.8×10^(-4)∕℃,the thickness of the gold film is 47 nm,and the distance between the D-shaped hole and fiber core is 5μm,the temperature sensitivity of the D-shaped hole fiber SPR sensor can reach to -3.635 nm∕℃.

Residual intensity modulation in resonator fiber optic gyros with sinusoidal wave phase modulation

We present how residual intensity modulation(RIM) affects the performance of a resonator fiber optic gyro(R-FOG) through a sinusoidal wave phase modulation technique. The expression for the R-FOG system's demodulation curve under RIM is obtained. Through numerical simulation with different RIM coefficients and modulation frequencies, we find that a zero deviation is induced by the RIM effect on the demodulation curve, and this zero deviation varies with the RIM coefficient and modulation frequency. The expression for the system error due to this zero deviation is derived. Simulation results show that the RIM-induced error varies with the RIM coefficient and modulation frequency. There also exists optimum values for the RIM coefficient and modulation frequency to totally eliminate the RIM-induced error, and the error increases as the RIM coefficient or modulation frequency deviates from its optimum value; however, in practical situations, these two parameters would not be exactly fixed but fluctuate from their respective optimum values, and a large system error is induced even if there exists a very small deviation of these two critical parameters from their optimum values. Simulation results indicate that the RIM-induced error should be considered when designing and evaluating an R-FOG system.

Suppression of backscattering induced noise by the sideband locking technique in a resonant fiber optic gyroscope

Polarization fluctuation induced noise and backscattering-induced noise are the dominant noises in resonant fiber optic gyroscopes. This Letter proposes a new method to suppress the carrier and backscattering induced noise by the sideband locking technique. Besides choosing an optimized modulation depth and different clockwise and counterclockwise modulation frequencies, the sideband is locked to the cavity resonance. With the proper modulation frequency, the carrier frequency component locates at a position far away from the resonant frequency, and then it is suppressed by the cavity itself, which can be taken as a bandpass filter. The amplitude of the carrier frequency can be suppressed by 20–25 d B additionally by the cavity and the total intensity suppression ratio can reach 115.74 d B. The backscattering induced noise can be eliminated for the adoption of different frequencies. The method can realize a stable and high suppression ratio without high requirements for parameter accuracy or device performance.

Curvature Sensor Based on a Long-Period Grating in a Fiber Ring Resonator Interrogated by an OTDR

The proposed technique demonstrates a fiber ring resonator interrogated by an optical time domain reflectometer(OTDR),for intensity sensing.By using this methodology,a cavity round trip time of 2.85ms was obtained.For a proof of concept,a long-period grating was inserted in the resonant cavity operating as a curvature sensing device.A novel signal processing approach was outlined,regarding to the logarithmic behavior of the OTDR.Through analyzing the experimental results,an increase in the measured sensitivities was obtained by increasing applied bending.With curvatures performed from 1.8 m^(-1) to 4.5 m^(-1),the sensitivity values ranged from 2.94 dB·km^(-1) to 5.15 dB·km^(-1).In its turn,the sensitivities obtained presented a linear behavior when studied as a function of the applied curvature,following a slope of 0.86×10-3 dB.The advantages of applying this technique were also discussed,demonstrating two similar fiber rings multiplexed in a series of configurations.

Controllable time delay using slow/fast light in active fiber ring resonators with gain manipulation

We propose to generate controllable time delay using slow/fast light in fiber ring resonators with gain manipulation.The dispersion and group delay of active fiber ring resonator can be controlled by manipulating its gain level below the lasing threshold.Controllability of the negative group delay in the undercoupled regime and the positive group delay in the overcoupled regime is theoretically demonstrated.Besides,large group delay can be obtained accompanied by signal gain in active ring resonators.In addition,we describe wide bandwidth and large group delay in 4-stage cascaded ring resonators.

Nonlinear Refractive Index Measurement Utilizing Bistable Behavior of Double Coupling Optical Fiber Ring Resonator

A novel approach for measuring the nonlinear refractive index of an optical fiber utlizing the bistable behavior of the double coupling optical fiber ring resonator was proposed and investigated. The switch-off or switch-on power decreases with an increase in the nonlinear refractive index n2 （m2/W）, and the dependence of swith-off or switch-on power on the nonlinear refractive index was analyzed numerically. Simulation results showed that the switch-off power and switch-on power （in dBW） decreased linearly with loglo （n2） in a 100-m-length fiber ring resonator, when n2 changed from 3.2 ×10^-20 m2/W to 2.5 × 10^-17 m2/W or nearly n2 = 3.2 × 10^-20 m2/W. These mean that high accuracy as well as large-scale nonlinear refractive index measurement can be achieved by the proposed approach.

Stopping and storing light pulses within a fiber optic ring resonator

A simple all optical system for stopping and storing light pulses is demonstrated. The system consists of an erbium-doped fiber amplifier （EDFA）, a semiconductor optical amplifier （SOA）, and a fiber ring resonator. The results show that the multisoliton generation with a free spectrum range of 2.4 nm and a pulse spectral width of 0.96 nm is achieved. The memory time of 15 min and the maximum soliton output power of 5.94 dBm are noted, respectively. This means that light pulses can be trapped, i.e., stopped optically within the fiber ring resonator.

Reduced thermal sensitivity of hybrid air-core photonic band-gap fiber ring resonator

A novel hybrid air-core photonic band-gap fiber(PBF) ring resonator with twin 90° polarization-axis rotated splices is proposed and demonstrated. Frist, we measure the temperature dependent birefringence coefficient of air-core PBF and Panda fiber. Experimental results show that the relative temperature dependent birefringence coefficient of air-core PBF is 1.42×10^(-8)/℃, which is typically ~16 times less than that of Panda fiber. Then, we extract the geometry profile of air-core PBF from scanning electron microscope(SEM) images. Numerical modal is built to distinguish the fast axis and slow axis in the fiber. By precisely setting the length difference in air-core PBF and Panda fiber between two 90° polarization-axis rotated splicing points, the hybrid air-core PBF ring resonator is constructed, and the finesse of the resonator is 8.4. Environmental birefringence variation induced by temperature change can be well compensated, and experimental results show an 18-fold reduction in thermal sensitivity, compared with resonator with twin 0° polarization-axis rotated splices.