Investigation on low-loss hollow-core anti-resonant terahertz fiber.

In this work, a hollow-core anti-resonant terahertz (THz) fiber with elliptical cladding and nested tubes is proposed and fabricated. It is an effective way to reduce the loss of THz waves by transmitting them in an air core and breaking the material absorption. After parameter optimization of the initial structure, multiple transmission windows exist in the 0.2-0.8 THz band, where confinement loss is as low as 3.47×10-3cm-1 at 0.8 THz. At 0.2-0.7 THz, confinement losses lie between 10-3 and 10-2cm-1. The 3D printed samples are characterized by a THz time-domain spectroscopy system. Experimental results showed that the designed fiber structure transmits loss coefficients up to 10-2cm-1 in the 0.2-0.8 THz band (the minimum value is located at 0.46 THz, corresponding to a loss coefficient of 0.0284cm-1). The experiments show that the designed THz fiber achieves a good transmission effect.

Enhancement of lateral Casimir force on a rotating particle near hyperbolic metamaterial.

Enhancement of weak Casimir forces is extremely important for their practical detection and subsequent applications in variety of scientific and technological fields. We study the lateral Casimir forces acting on the rotating particles with small radius of 50 nm as well as that with large radius of 500 nm near the hyperbolic metamaterial made of silicon carbide (SiC) nanowires. It is found that the lateral Casimir force acting on the small particle of 50 nm near hyperbolic metamaterial with appropriate filling fraction can be enhanced nearly four times comparing with that acting on the same particle near SiC bulk in the previous study. Such enhancement is caused by the coupling between the resonance mode excited by nanoparticle and the hyperbolic mode supported by hyperbolic metamaterial. The results obtained in this study provide an efficient method to enhance the interaction of nanoscale objects.

Supermode noise suppression with polarization-multiplexed dual-loop for active mode-locking optoelectronic oscillator.

The active mode-locking (AML) technique has been widely used in erbium-doped fiber lasers to generate picosecond pulse trains. Here we propose a novel active mode-locking dual-loop optoelectronic oscillator (AML-DL-OEO), which can generate microwave frequency comb (MFC) signals with adjustable comb spacings. Based on this scheme, the order of harmonic mode-locking is dramatically decreased for a certain AML driving frequency compared with a single-loop AML-OEO. Thus, the supermode noise caused by harmonic mode-locking can be efficiently suppressed. In addition, the sidemodes are well suppressed by the dual-loop architecture. An experiment is performed. MFC signals with different comb spacings are generated under fundamental or harmonic mode-locking states. AML-DL-OEO systems with different length differences between two loops are implemented to evaluate supermode noise suppression capability. The performance of the generated MFC signals is recorded and analyzed.

Linearity improvement of analog photonic link based on a phase-coherent orthogonal light wave generator.

This Letter presents a novel, to the best of our knowledge, linearized analog photonic link (APL) based on a phase-coherent orthogonal light wave generator that consists of a polarization-dependent Mach-Zehnder modulator (MZM) and a polarization controller (PC). By adjusting the PC and bias voltage of MZM, the third-order intermodulation (IMD3) terms can be suppressed while retaining a high gain for the fundamental terms, which indicates that the spurious free dynamic range (SFDR) of the proposed APL can be much improved. To further verify the feasibility of the proposed APL, a proof-of-concept experiment is performed, and the performances are compared with conventional APL. The experimental results demonstrate that a 14 dB improvement in the fundamental to IMD3 power ratio and an SFDR of 100.2dB⋅Hz2/3 or 119.1dB⋅Hz2/3 for a noise floor of -139dBm/Hz or -163.9dBm/Hz are achieved. In addition, an orthogonal frequency division multiplexing signal with 30 MHz bandwidth centered at 2.5 GHz is delivered by our proposed APL, whose signal-to-noise ratio is increased by 10 dB, compared to conventional APL.

Mechanical modulation of spontaneous emission of nearby nanostructured black phosphorus.

In this study, we investigate the spontaneous emission of a quantum emitter nearby black phosphorus (BP) sheet. The spontaneous emission can be modulated mechanically by rotating the BP sheet when the quantum emitter is placed parallel to the sheet. The spontaneous emission is dependent on the electron doping and rotation angle of BP with respect to the x-axis. The Purcell factor decreases with the increase in rotation angle under smaller electron doping. The Purcell factor increases with the increase in rotation angle under larger electron doping. The spontaneous emission of quantum emitter nearby two types of BP ribbon arrays tailored along armchair (type I) and zigzag (type II) directions is studied in detail. The spontaneous emission of quantum emitter parallel to type I is enhanced compared with that parallel to BP sheet. The spontaneous emission decreases remarkably for the quantum emitter parallel to type II compared with that parallel to BP sheet. The spontaneous emission can be flexibly modulated by rotating BP ribbon arrays mechanically in two types. The results obtained in this study provide a new method to actively modulate the spontaneous emission.

High sensitivity demodulation of a reflective interferometer-based optical current sensor using an optoelectronic oscillator.

A novel, to the best of our knowledge, interrogation scheme based on an optoelectronic oscillator (OEO) with high sensitivity and high speed response for a fiber optical current sensor utilizing a reflective interferometer is proposed and experimentally demonstrated. Due to the Faraday effect, a magneto-optic phase shift induced by current variation is generated between two orthogonal light waves. The polarization-dependent properties of the Mach-Zehnder modulator are used to convert the magneto-optic phase shift into the phase difference between the optical carrier and sideband, which is then mapped to the oscillating frequency shift by closing an OEO loop. A high current sensitivity of 152.5 kHz/A with a range of 0-2.5 A is obtained in the experiment.

Magnetic field sensor based on a dual-frequency optoelectronic oscillator using cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot and fiber Bragg grating-Fabry Perot filters.

A magnetic field sensor using a dual-frequency optoelectronic oscillator (OEO) incorporating cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot (MA-FBG-FP) and FBG-FP filters is proposed and demonstrated. In the OEO resonant cavity, two microwave signals are generated, whose oscillation frequencies are determined by the FBG-FP filter and MA-FBG-FP filter filters with two ultra-narrow notches and two laser sources. Due to the characteristics of MA and FBG, the two generated microwave signals show different magnetic field and temperature sensitivities. By monitoring the variations of two oscillating frequencies and the beat signal using a digital signal processor, the simultaneous measurement for the magnetic field and temperature can be realized. The proposed sensor has the advantages of high-speed and high-resolution measurement, which make it very attractive for practical magnetic field sensing applications. The sensitivities of the proposed OEO sensor for magnetic field and temperature are experimentally measured to be as high as -38.4MHz/Oe and -1.23 or -2.45 GHz/°C corresponding to the MA-FBG-FP filter and FBG-FP filter, respectively.

High-sensitivity measurement of angular velocity based on an optoelectronic oscillator with an intra-loop Sagnac interferometer.

A novel scheme for angular velocity measurement is proposed and demonstrated by using an optoelectronic oscillator (OEO) incorporating a Sagnac interferometer. In the OEO resonant cavity, the optical carrier (OC) and the first-order sidebands propagate in opposite directions in the Sagnac loop. Thus, the rotation-induced Sagnac phase difference between the OC and first-order sidebands will produce an oscillating frequency shift of the OEO which is proportional to the rotation angular velocity. Then a high-sensitivity angular velocity measurement is realized by monitoring the oscillating microwave frequency. The system is free from the lock-in problem, and the sensitivity scale is measured to be 51.8 kHz/(rad/s) which is equivalent to a minimally detectable angular velocity of 3.98°/h with a frequency shift of 1 Hz.

Edge technique for direct detection of strain and temperature based on optical time domain reflectometry.

A hybrid technique for real-time direct detection of strain and temperature along a single-mode fiber is proposed. The temperature is directly detected from the Raman backscattering in the time domain. To retrieve the strain profile from the Brillouin backscattering, an edge technique is introduced and a response function of the Fabry-Perot interferometer for the Brillouin backscattering is defined for the first time to our knowledge. The outgoing laser and the Brillouin backscattering are measured on different interference orders through different channels of the Fabry-Perot interferometer. A low-resolution reference channel and a high-resolution Brillouin channel are designed to keep both a high measurement sensitivity and a wide dynamic range. The measurement is based on detecting the bandwidth changes and the frequency shifts of the Brillouin backscattering; thus the resulting measurement is insensitive to the power fluctuation of the backscattering and the laser frequency jitter or drift. Neither time-consuming frequency scanning nor heavy data processing is needed, which makes real-time detection possible. The dynamic range of the edge technique can be increased substantially by using a piezoelectric tunable and capacitive-servo-stabilized Fabry-Perot interferometer. We highlight the potential of this technique by numerical simulations. Given that the uncertainty of the temperature measurement is 0.5 degrees C and that the spatial and temporal resolutions are 10 cm and 1 s, the strain uncertainty is less than 20 microepsilon within a 2 km distance when the strain is below 0.4%, and it is not more than 110 microepsilon within a 4 km distance when the strain is below 0.6%.