Zhengbin Li

Analytical perspective for bound states in the continuum in photonic crystal slabs.

We investigate the formation of photonic bound states in the continuum (BICs) in photonic crystal slabs from an analytical perspective. Unlike the stationary at-Γ BICs which originate from the geometric symmetry, the tunable off-Γ BICs are due to the weighted destructive via the continuum interference in the vicinity of accidental symmetry when the majority of the radiation is precanceled. The symmetric compatible nature of the off-Γ BICs leads to a trapping of light that can be tuned through continuously varying the wave vector. With the analytical approach, we explain a reported experiment and predict the existence of a new BIC at an unrevealed symmetry.

Optically compensated dual-polarization interferometric fiber-optic gyroscope

We first introduce optical compensation into the field of interferometric fiber-optic gyroscopes (IFOGs), using the opposite polarity of the disturbance between two orthogonal polarization states to suppress polarization-fluctuation-induced noise. A dual-polarized IFOG, in which the fast and slow axes of polarization-maintaining fiber work simultaneously, is implemented and tested. Interference signals of the two axes are added to achieve optical compensation. Experiments show that the IFOGs sensitivity is effectively enhanced in compensated output: all Allan variance indices are improved, among which bias instability is reduced from 0.335 and 0.227 deg/h(fast and slow axes output, respectively) to 0.061 deg/h.

Quadrature demodulation with synchronous difference for interferometric fiber-optic gyroscopes

We propose a novel method of quadrature demodulation with synchronous difference for suppressing noise in interferometric fiber-optic gyroscopes (IFOGs). For an IFOG with sine wave phase modulation, an in-phase result and a quadrature result are obtained simultaneously by coherent detection. Eigenfrequency modulation is used and a phase shift of 45° is set between the modulation signal and the reference signal, so that two results have the same expectation of amplitude but with opposite signs. A synchronous difference procedure is carried out for output, in which signals are added up and common noise between two results is eliminated. Theoretical analysis and experimental results show that both short term noise and long term instability of the IFOG are reduced by this method. In experimental comparison with the traditional demodulation method on the same IFOG with a 1982 m fiber coil, this method reduces the bias drift from 0.040°/h to 0.004°/h.

All-Depolarized Interferometric Fiber-Optic Gyroscope Based on Optical Compensation

We propose and demonstrate a novel configuration for depolarized interferometric fiber-optic gyroscopes (IFOGs). This configuration utilizes optical compensation between two orthogonal polarizations for suppressing errors induced by polarization nonreciprocity. Theoretical analysis shows that it is a new approach different from conventional IFOGs where a polarizer is mandatory. An experimental demonstration of the proposed IFOG (2097-m coil, open-loop configuration) achieves a low bias drift of 0.016 °/h in detecting the Earths rotation rate. Furthermore, this configuration requires no polarizer or any other polarization-maintaining device.

Polarization nonreciprocity suppression of dual-polarization fiber-optic gyroscope under temperature variation.

Polarization nonreciprocity (PN) is one of the most critical factors that degrades the performance of interferometric fiber-optic gyroscopes (IFOGs), particularly under varying temperature. We present an experimental investigation of PN error suppression in a dual-polarization IFOG. Both experimental results and theoretical analysis indicate that the PN errors of the two orthogonally polarized light waves always have opposite signs that can be effectively compensated despite the temperature variation. As a result, the long-term stability of the IFOG has been significantly improved. This study is promising for reducing the temperature fragility of IFOGs.

Dual-polarization interferometric fiber-optic gyroscope with an ultra-simple configuration

We demonstrate a novel dual-polarization interferometric fiber-optic gyroscope (IFOG), which needs only one coupler and no polarizer. Polarization nonreciprocity (PN) errors in common IFOGs will increase significantly if the polarizer is absent, or if only one coupler is used. In our setup, however, PN errors are effectively compensated by using two balanced polarizations. The 2xa0km coil, open-loop configuration obtains a bias instability of 0.02°/h in detecting the Earths rotation rate. Its performance difference from the conventional two-coupler IFOG is only a stable bias, caused by coupler nonreciprocity.

Optically compensated polarization reciprocity in interferometric fiber-optic gyroscopes

Polarization reciprocity is studied both theoretically and experimentally in an optically compensated configuration of interferometric fiber optic gyroscope (IFOG). In conventional IFOGs based on the minimal scheme, the output port of the coil coupler cannot be used mainly because of its polarization nonreciprocity (PN), and thus it is usually called the nonreciprocal port. We show that the PN errors at the nonreciprocal port are effectively eliminated by optical compensation. With this unique property, the optically compensated IFOG can possess two low-PN ports for rotation sensing at the same time. From another perspective, one port IFOGs are possible to be constructed with less structural complexity.

Semi-analytical approach for guided mode resonance in high-index-contrast photonic crystal slab: TE polarization

In high-contrast (HC) photonic crystals (PC) slabs, the high-order coupling is so intense that it is indispensable for analyzing the guided mode resonance (GMR) effect. In this paper, a semi-analytical approach is proposed for analyzing GMR in HC PC slabs with TE-like polarization. The intense high-order coupling is included by using a convergent recursive procedure. The reflection of radiative waves at high-index-contrast interfaces is also considered by adopting a strict Greens function for multi-layer structures. Modal properties of interest like band structure, radiation constant, field profile are calculated, agreeing well with numerical finite-difference time-domain simulations. This analysis is promising for the design and optimization of various HC PC devices.

Three-dimensional coupled-wave theory for the guided mode resonance in photonic crystal slabs: TM-like polarization

A general coupled-wave theory is presented for the guided resonance in photonic crystal (PhC) slabs with TM-like polarization. Numerical results based on our model are presented with finite-difference time-domain validations. The proposed analysis facilitates comprehensive understanding of the physics of guided resonance in PhC slabs and provides guidance for its applications.

A multi-frequency signal processing method for fiber-optic gyroscopes with square wave modulation.

The bias stability and random walk coefficients (RWC) of interferometric fiber-optic gyroscopes (IFOGs) are directly affected by characteristic noises produced by optoelectronics interactions in optic sensors. This paper documents a novel demodulation method for square wave modulated IFOGs, a method capable of suppressing the white noise that results from optical intensity noises and circuit noises as well as shot noises. In addition, this paper provides a statistical analysis of IFOG signals. Through use of orthogonal harmonic demodulation followed by deployment of matched filters to detract the Sagnac phase from the IFOGs, these channels we then processed, using principle component analysis (PCA), to establish optimal independent synchronous quadrature signal channels. Finally a difference procedure was carried out for the outputs. Our results showed that an experimental sample of the proposed IFOG (1982 m coil under uncontrolled room temperature) achieved a real-time output variance improvement in detecting the Earths rotation rate, which is well matched with theoretical calculations of its Cramèr-Rao bound (CRB).