Suppressing the dephasing of optically trapped atoms inside a hollow-core fiber.

We demonstrate the suppression of inhomogeneous dephasing of cold 87Rb atoms optically trapped inside a hollow-core fiber. The differential light shift (DLS) for the clock transition caused by the trapping beam is reduced by one order of magnitude through the use of a weak compensation laser beam that is spatially mode-matched to the trapping beam. The coherence of the DLS-compensated system is characterized by microwave Ramsey interferometry, which shows Ramsey fringes with a contrast of over 0.6 at a separation time of 10 ms. The dephasing time, measured by Ramsey spectroscopy at different separation times, reaches tens of milliseconds after DLS cancellation, limited by the residual DLS caused by mode mismatching between the two laser beams. This work paves the way for compact and portable fiber-guided atom interferometers.

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers.

Stimulated Brillouin scattering (SBS) is effective for realizing a laser with an ultra-narrow linewidth. Although photonic crystal fiber (PCF) is considered an excellent medium to achieve SBS, it does not meet the requirements of low loss, large birefringence, and ease of fabrication. We propose a polarization-maintaining PCF (PM-PCF) structure and theoretically analyze the effects of the geometric structural parameters of the PM-PCF on various optical properties. Our theoretical analysis and experimental results contribute to the advancement of the field of ultra-narrow linewidth fiber lasers, distributed fiber sensing, and fiber-optic gyroscopes related to SBS.

Monolithic Saturable Absorber with Gallium Arsenide Nanowires Integrated on the Flexible Substrate for Optical Pulse Generation.

In this work, we demonstrated a kind of flexibly monolithic saturable absorber (SA) with GaAs nanowires (NWs) on polyimide (PI) plastic substrate for broadband optical modulation at 1.0 and 1.5 µm, separately. The monolithic SA sample was prepared by the metalorganic vapor phase epitaxy (MOVPE) method. The crystal structure and element analysis were examined carefully by high-resolution scanning transmission electron microscopy (HRSTEM) and energy-dispersive X-ray spectroscopy (EDX). We observed a high-density distribution of NWs on the flexible substrate by scanning electron microscopy (SEM). In addition, linear and nonlinear optical properties of the sample were examined by testing the photoluminescence and absorption properties, which showed its potential application as an optical switch due to the pure semiconducting properties. After the characterizations, we experimentally demonstrated this monolithic SA for laser modulation at 1.0 and 1.5 µm, which yielded the minimum optical pulse widths of 1.531 and 6.232 µs, respectively. Our work demonstrated such a kind of monolithic flexible NW substrate-integrated device used for broadband optical modulation, which not only eased the integration process of NWs onto the fiber endface, but also proved the potential of easily integrating with more semiconducting nanomaterials (e.g., graphene, MoS2, …) to realize monolithic active flexible photonic systems, such as a microscale phase modulator, delay-line, and so on, paving an easy avenue for the development of both active and flexible photonic devices.

Hollow-conical atomic beam from a low-velocity intense source.

We demonstrate, for the first time, a hollow-conical atomic beam from a standard low-velocity intense source. Experimental results and numerical simulations indicate that the hollow-conical feature is caused by the converging-diverging extraction process. The degree of hollowness can be reduced by using a weaker push beam and extending the length of transverse cooling. Analytical models are proposed to quantitatively describe the hollowness of the atomic beam. This study can find applications where a compact and solid atomic beam is needed, such as coupling cold atoms into matter waveguides or continuous cold atomic beam interferometers.

Small-core hollow-core nested antiresonant nodeless fiber with semi-circular tubes.

Hollow-core nested anti-resonant nodeless fibers (HC-NANFs) exhibit great performance in low loss and large bandwidth. Large core sizes are usually used to reduce confinement losses, but meanwhile, bring side effects such as high bending and coupling losses. This study proposes a small-core HC-NANF with a relatively low confinement loss. Semi-circular tubes (SCTs) are added to constitute the core boundary and reduce the fiber-core radius (R). Double NANFs tubes and single-ring tubes are added inside the SCTs to reduce loss. Simulation results show that the optimized structure with R of 5 µm has confinement loss and total loss of 0.687 dB/km and 4.27 dB/km at 1.55 µm, respectively. The bending loss is less than 10 dB/km at 1.4 ∼ 1.6 µm with a bending radius of 10 mm. The direct coupling loss with standard single mode fiber is greatly reduced to ∼ 0.125 dB compared to other HC-NANFs. The modified structure of HC-NANFs also shows a large bandwidth, effective single-mode operation, potentially high birefringence performance, and remarkable robustness of the optimized structure parameters, making it suitable for short-haul applications in laser-based gas sensing, miniaturized fiber sensing, etc.

Cryogenic temperature sensing based on the temperature dependence of color centers in optical fibers.

A cryogenic temperature sensor based on the temperature dependence of stable color centers in a commercial single-mode optical fiber is proposed. The radiation induced attenuation spectra at different temperatures are measured and decomposed by Ge-NBOHC and Ge(X) color centers. The configurational coordinate model is used to explain the temperature properties of the color centers. A series of experiments are conducted to evaluate its performance in the temperature range from 10°C to -196°C, and the results suggest that the temperature sensitivity is ∼0.17 dB/km/°C with a resolution of 0.034°C, and the nonlinearity and repeatability error are ±3.8% and 1.4%, respectively.

Hybrid photonic bandgap effect in twisted hollow-core photonic bandgap fibers.

A hybrid photonic bandgap effect in twisted hollow-core photonic bandgap fibers (HC-PBFs) is theoretically investigated for the first time, to the best of our knowledge. Due to the topological effect, twisting of the fibers changes the effective refractive index and lifts the degeneracy of the photonic bandgap ranges of the cladding layers. This twist-induced hybrid photonic bandgap effect shifts up the center wavelength and narrows the bandwidth of the transmission spectrum. A quasi-single-mode low-loss transmission is achieved in the twisted 7-cell HC-PBFs with a twisting rate α = 7-8 rad/mm, which has a loss < 30 dB/km and higher-order mode extinction ratio > 15 dB. The twisted HC-PBFs could be suitable for applications such as spectral and mode filters.

Investigation of longitudinal uniformity of the core structure in a hollow-core photonic bandgap fiber.

In this study, a low-noise Fabry-Perot interference-based method is promoted to measure the longitudinal uniformity of the distance between six pairs of opposite silica-air interfaces within the core of a seven-cell hollow-core photonic bandgap fiber. The experimental results demonstrate that the precision of the method is improved to the subnanometer scale. Based on the test results, a model is established to study the effect of the longitudinal uniformity of the core structure on the fiber loss, and the simulation results indicate that the fiber loss could reach ∼22.73 dB/km, which is consistent with the practical loss value.

Single-polarization single-mode hollow-core photonic-bandgap fiber with thin slab waveguide.

A novel single-polarization single-mode hollow-core photonic bandgap fiber with thin slab waveguide (TSW) was designed and simulated. Single-polarization guidance is achieved by the high loss of a polarized fundamental mode (FM) induced by mode coupling with a higher-order TE/TM mode of TSW and low loss of another polarized FM. We achieve a polarization loss ratio ∼ 46.9 dB, birefringence Δn ∼ 2.4 × 10-4, loss ∼ 35.9 dB/km and minimum higher-order mode extinction ratio > 15 dB. Moreover, the performance could be maintained when the guidance wavelength λ = 1.44 ∼ 1.56 µm and bending radius rc > 9 mm. The proposed model will be suitable for application as resonator sensing paths of miniaturized resonator fiber optic gyroscopes, high-performance interferometers, fiber lasers, frequency metrology, quantum communications, and laser-based gas sensing, etc.

Hollow-core mode propagation in an isomeric nested anti-resonant fiber.

We present a modified fiber model based on the nested hollow core anti-resonant fiber that enables the stable transmission of the orbital-angular-momentum mode HE21. By replacing a pair of nested anti-resonant tubes in the horizontal axis with resonant tubes, the coupling between core mode and cladding mode has been increased. Therefore, the relative strength of fundamental mode HE11 and the first higher mode HE21 has been modified. The numerical simulation results indicate that the loss ratio of the lowest loss HE11 to HE21 can be optimized to more than 187, while the HE21 still maintains a low confinement loss as 0.0027 dB/m. Our research has brought about a solution of low loss hollow core mode propagation in optical fiber. Those properties will make this fiber an ideal medium for blue-detuned atomic guidance.

High-precision photonic crystal fiber-based pressure sensor with low-temperature sensitivity.

A novel high-precision photonic crystal fiber-based pressure sensor with low-temperature sensitivity is proposed. The sensor is fabricated by fusion splicing a photonic crystal fiber with a hollow core fiber immersed in polydimethylsiloxane. Owing to the special structure of the photonic crystal fiber, the temperature cross-coupling effect can be minimized and the membrane shape can be controlled. Experimental results indicate that the pressure sensitivity of the FP pressure sensor is 2.47 nm/kPa, 5.37 times the temperature sensitivity of 0.46 nm/°C. The proposed FP pressure sensor has broad application prospects in chemical and biological detection for monitoring pressure in real time.

Fiber-optic distributed acoustic sensor utilizing LiNbO3 straight through waveguide phase modulator.

A novel fiber-optic distributed acoustic sensor (DAS) utilizing a LiNbO3 straight through waveguide phase modulator as phase generation carrier (PGC) modulation module for the detection of acoustic signal is presented. The sensitive principle and the phase demodulation method of the system based on phase-sensitive optical time domain reflectometer (Φ-OTDR) are described. This scheme solves the problems of low modulation frequency and unstable performance of piezoelectric transducer (PZT) in the traditional homodyne detection system and depends only on the pulse repetition frequency. The efficacy of the new approach is demonstrated experimentally, showing that the weak acoustic signal can be demodulated accurately. The noise level of the system is < 4.2×10-3 rad/√Hz, the signal to noise ratio (SNR) is > 16 dB, and the spatial resolution is 10 m, as well as a detection frequency can theoretically achieve 25 kHz at 2 km sensing fiber. It provides a new research idea for DAS and is expected to replace PZT to achieve a high-frequency response, which has good potential in the applications of low cost, long distance and high frequency detection.

Low loss hollow-core antiresonant fiber with nested supporting rings.

A hollow-core antiresonant fiber (HC-ARF) with nested supporting rings (NSRs) is designed and simulated. The HC-ARF with NSRs has advantages and benefits of low loss, large bandwidth, simple structure and a well bending characteristic, in which confinement loss (CL) is ∼ 0.15 dB/km @ 1.55 µm and the bandwidth is ∼ 220 nm @ CL < 1 dB/km. The bending loss (BL) is lower than ∼ 1 dB/km @ bend radius rc > 24 mm at 1.55 µm. Therefore, the HC-ARF with NSRs has potential applications of data transmission, sensing, high power delivery and so on.

Adaptive Kalman filter based on random-weighting estimation for denoising the fiber-optic gyroscope drift signal.

Reducing and suppressing the random noise and drift error is a critical task in an interferometric fiber-optic gyroscope (IFOG). In this paper, an improved adaptive Kalman filter (KF) based on innovation and random-weighting estimation (RWE) is proposed to denoise IFOG signals in both static and dynamic conditions. The covariance matrix of the innovation sequence is estimated using the random-weighted-average window. The KF gain is then adaptively updated by the estimated covariance matrix. To decrease the inertia of KF response in the dynamic condition, the covariance matrix of process noise is adjusted when discontinuous IFOG signals are detected by the innovation-based chi-square test method. The proposed algorithm is applied for denoising IFOG static and dynamic signals. Allan variance is used to evaluate the denoise performance for static signals. In the dynamic condition, root-mean-square error is considered as the performance indicator. Quantitative results reveal that the proposed algorithm is competitive for denoising IFOG signals when compared with conventional KF, RWE-based gain-adjusted adaptive KF, and RWE-based moving average double-factor adaptive KF.

High-performance fully differential photodiode amplifier for miniature fiber-optic gyroscopes.

Electrical cross coupling is regarded as a major obstacle to achieving high-performance miniature fiber-optic gyroscopes (FOGs), because it can cause dead bands, which are critical errors in FOGs. Using a differential photodiode amplifier has proven to be effective in rejecting coupled interference. However, the conventional three-op-amp instrumentation amplifier cannot provide a miniature FOG's bandwidth requirements, because of the large photodiode capacitance and parasitic capacitance. We present a high-performance, fully differential photodiode amplifier, where the bandwidth limitations are removed by applying a reverse bias to the photodiode and replacing the feedback resistor with a modified tee-network and a DC cancellation loop. For an experimental FOG with a 300 m fiber coil, we demonstrate a fully differential photodiode amplifier with 880 kΩ gain and 3.5 MHz bandwidth. In the FOG performance test, it not only reduces the angular random walk and bias drift, but also eliminates the approximately 1°/h dead band observed in the same FOG using a PINFET receiver, demonstrating its effectiveness in suppressing coupled interference.

Dynamics simulation and experiment of smooth end face grinding on photonic crystal fiber.

A smooth end face is the critical factor for the application of photonic crystal fiber (PCF) to enhance coupling efficiency. A number of air holes distributed on the cladding make the surface a discontinuous structure. For the sake of acquiring a high-quality end surface of PCF, this paper proposes a detailed study first on dynamics simulation of the grinding process of PCF using a finite element model with a verification experiment later. To facilitate the calculation, the grinding is simplified as a single grain to cut the material on the location between two holes. The process of material removal is analyzed on the basis of force curve obtained by changing the cutting depth and position, respectively. Theoretical and experimental results illustrate that the earliest material collapse occurs on the rim of the air hole, and brittle cracks extend along circumferential and axial direction of the hole; the damage on the edge of the hole is more severe with the increment of grain tip radius and cutting depth. Keeping the grinding force stable can achieve the process in the ductile regime, and a smooth surface can be obtained by polishing with abrasive paper of fine grit.

Nondestructive determination of the core size of a hollow-core photonic bandgap fiber using Fabry-Perot interference.

In this Letter, we propose a simple and nondestructive method for the determination of the core size of a hollow-core photonic bandgap fiber (HC-PBF) and its axial uniformity based on a Fabry-Perot cavity induced by a pair of opposite silica-air interfaces within the hollow core. The experimental results indicate that the core size test of the HC-PBF has a nanometer-level precision, and its axial uniformity test has an ultimate spatial resolution of tens of microns. The method provides an effective and precise tool for the investigation of the hollow-core size and its longitudinal evolution.

Error Analysis of the K-Rb-21Ne Comagnetometer Space-Stable Inertial Navigation System.

According to the application characteristics of the K-Rb-21Ne comagnetometer, a space-stable navigation mechanization is designed and the requirements of the comagnetometer prototype are presented. By analysing the error propagation rule of the space-stable Inertial Navigation System (INS), the three biases, the scale factor of the z-axis, and the misalignment of the x- and y-axis non-orthogonal with the z-axis, are confirmed to be the main error source. A numerical simulation of the mathematical model for each single error verified the theoretical analysis result of the system's error propagation rule. Thus, numerical simulation based on the semi-physical data result proves the feasibility of the navigation scheme proposed in this paper.

Experimental Investigation of the Influence of the Laser Beam Waist on Cold Atom Guiding Efficiency.

The primary purpose of this study is to investigate the influence of the vertical guiding laser beam waist on cold atom guiding efficiency. In this study, a double magneto-optical trap (MOT) apparatus is used. With an unbalanced force in the horizontal direction, a cold atomic beam is generated by the first MOT. The cold atoms enter the second chamber and are then re-trapped and cooled by the second MOT. By releasing a second atom cloud, the process of transferring the cold atoms from MOT to the dipole trap, which is formed by a red-detuned converged 1064-nm laser, is experimentally demonstrated. And after releasing for 20 ms, the atom cloud is guided to a distance of approximately 3 mm. As indicated by the results, the guiding efficiency depends strongly on the laser beam waist; the efficiency reaches a maximum when the waist radius (w0) of the laser is in the range of 15 to 25 µm, while the initial atom cloud has a radius of 133 µm. Additionally, the properties of the atoms inside the dipole potential trap, such as the distribution profile and lifetime, are deduced from the fluorescence images.

Modeling of Thermal Phase Noise in a Solid Core Photonic Crystal Fiber-Optic Gyroscope.

A theoretical model of the thermal phase noise in a square-wave modulated solid core photonic crystal fiber-optic gyroscope has been established, and then verified by measurements. The results demonstrate a good agreement between theory and experiment. The contribution of the thermal phase noise to the random walk coefficient of the gyroscope is derived. A fiber coil with 2.8 km length is used in the experimental solid core photonic crystal fiber-optic gyroscope, showing a random walk coefficient of 9.25 × 10-5 deg/√h.