Single-mode Fiber Coupling Efficiency Research Articles

Selection and control strategies of nonuniform coherence structures to further improve single-mode fiber coupling efficiency in turbulence

We introduce a general and effective strategy for selecting and controlling nonuniform coherence structures to further improve fiber-coupling efficiency (FCE) of nonuniformly randomly structured beams in turbulence. Our results show that adjusting the weight distribution of such beams is the key to mastering the strategy. Selecting the beams with weight distribution concentrated at short/long focal length modes possess higher/lower maximum FCE, which is suitable for short/long-distance transmission situations. Furthermore, adjusting the initial coherence length and beam order of the selected beam will help users further improve the FCE.

Free-space to single-mode fiber coupling efficiency with optical system aberration and fiber positioning error under atmospheric turbulence

Benefiting from the rapid development of fiber-optic devices, high-speed free-space optical (FSO) communication systems have recently used fiber-optic components. The received laser beam in such a system couples into single-mode fiber (SMF) at the input of the receiver module. This work is oriented to common problems in actual FSO coupling systems, such as atmospheric turbulence, optical system aberration (OSA), and fiber positioning error. We derive the statistical expectation models of SMF coupling efficiency with OSA in the presence of atmospheric turbulence and the statistical expectation models of SMF coupling efficiency with fiber positioning error in the presence of atmospheric turbulence. The influences of OSA and fiber positioning error on the coupling efficiency under different turbulence strengths are also analyzed in this paper.

Experimental optimization of the fiber coupling efficiency of GaAs quantum dot-based photon sources

We present an efficient experimental method to optimize the combined extraction efficiencies and the far-field emission patterns of solid state-based single and entangled photon pair sources for efficient coupling to single mode fibers. This method is demonstrated for emitters based on droplet etched GaAs quantum dot nanomembranes attached to gallium phosphide solid immersion lenses using an adhesive layer of poly(methyl methacrylate). By varying the thickness of the latter, the optimization of both the extraction efficiency and the far-field emission pattern for single mode fiber coupling is facilitated. The applied method of far-field characterization is validated by benchmarking it against direct measurements of the single mode fiber coupling efficiency. Using this scheme, devices with a more than 150-fold enhanced free-space intensity compared to an unprocessed sample as well as a fiber coupling efficiency of 64% are achieved. In addition, the optimized device has been employed for on-demand generation of maximally entanglement photon pairs using two-photon excitation of the quantum dot bi-exciton exciton cascade. This universal approach for experimental optimization can be applied to other photonic nanostructures, including circular Bragg grating and micropillar cavities as well as monolithic microlenses.

Adaptive optics technology and urban horizontal link laser communication system

The wavefront distortion caused by atmospheric turbulence seriously affects the performance of free-space optical communication (FSOC) system. Adaptive optics (AO) technology has been proven to be an effective method to suppress atmospheric turbulence. Two laser communication terminals equipped with AO system are designed and manufactured, which are used in the horizontal link near the ground. In the laboratory simulation, 100-Gbps data transmission can be realized under moderate turbulence (D / r0 < 10), the bit error rate (BER) is better than 1 × 10 − 6, and the measured single-mode fiber coupling efficiency is 42% to 48%. 10-Gbps data transmission can be realized under strong turbulence (D / r0 > 10), and the BER is better than 1 × 10 − 6. The BER is high in the practical communication test under the urban horizontal link of 1 km (D / r0 = 10), the BER is 1 × 10 − 3, and the coupling efficiency is 12% to 20%. Finally, to achieve the goal of 100-Gbps FSOC for near ground and long distance, based on the experimental performance of the existing two sets of laser communication system, the solution of laser communication system is proposed, which provides a reference for relevant researchers.

Experimental Research on Automatic Alignment and Control Algorithm of Spatial Light-Fiber Coupling

This study aims to solve the difficulties in the coupling between space light and single-mode fiber (SMF) in free-space optical communication. A fiber coupler based on two-dimensional (2D) piezoelectric ceramics was developed, which uses the stochastic parallel gradient descent (SPGD) algorithm to realize the automatic coupling of space light-SMF. In addition, a spatial light-SMF alignment experimentation platform was built indoors to verify the effectiveness and practicality of the 2D piezoelectric ceramic fiber coupler. The results show that the use of the SPGD algorithm can realize the automatic alignment of fiber position coupling, and the SMF coupling efficiency reaches 52.58% when the system is closed loop. 2D piezoelectric ceramic fiber couplers have unique advantages of low cost, simplified structure, and easy array expansion and can effectively solve the difficulty in the alignment of spatial light-SMF coupling. This study will serve as a significant reference for the research on spatial fiber-coupled array technology.

Study on Probability Distribution of Single-Mode Fiber Coupling Efficiency Distorted by Turbulence

Study on Probability Distribution of Single-Mode Fiber Coupling Efficiency Distorted by Turbulence

Single-mode fiber coupling with a M-SPGD algorithm for long-range quantum communications.

Satellite-based quantum communication is a promising approach for realizing global-scale quantum networks. For free-space quantum channel, single-mode fiber coupling is particularly important for improving the signal-to-noise ratio of daylight quantum key distribution (QKD) and compatibility with standard fiber-based QKD. However, achieving a highly efficient and stable single-mode coupling efficiency under strong atmospheric turbulence remains experimentally challenging. Here, we develop a single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent (M-SPGD) algorithm and test its performance over an 8 km urban terrestrial free-space channel. Under strong atmospheric turbulence, the M-SPGD AO system obtains an improvement of about 3.7 dB in the single-mode fiber coupling efficiency and a significant suppression of fluctuation, which can find its applications in free-space long-range quantum communications.

Probability enhancement of fiber coupling efficiency under turbulence with adaptive optics compensation

Abstract Statistical distribution of single mode fiber (SMF) coupling efficiency η c with atmospheric compensation is important to evaluate the bit error rate performance of the satellite to ground laser communication (SGLC). The statistical distribution of SMF coupling efficiency with adaptive optics (AO) compensation under different turbulence strength is investigated. When the AO is turned on, the signal beam wavefront root mean square (RMS) value decreases from 0.42 λ to 0.04 λ under the turbulence strength D / r 0 = 7. Simultaneously, the average of coupling efficiency increases from 2.2% to 35.4%. The statistical distribution of SMF coupling efficiency with and without AO correction is consistent with the gamma distribution. In addition, the AO correction can help greatly improve SMF coupling efficiency with high probability. For example, the cumulative probabilities of coupling efficiency P ( η c > 15 % ) is 56.4%, 35.8%, 14.8%, 3.0% and 0.1% without AO correction when the D / r 0 is 3, 5, 7, 9 and 11, respectively. The value is improved to 100%, 100%, 99.9997%, 99.72% and 97.95% with AO correction in the corresponding D / r 0 . The results provide a quantifiable parameter to evaluate the SGLC link performance with AO correction.

Method to enhance the single mode fiber coupling efficiency for obscured receiver by beam shaping

Since the aperture obstruction contributes to an energy leak from the core to the sidelobe of the far field spot, the Gaussian mode of the single-mode fiber (SMF) will receive less energy leading to the coupling efficiency (CE) degradation. A method based on diffraction optical element (DOE) and wavefront corrector is introduced to reshape the plane wave with hollow uniform intensity into the plane wave with uniform intensity to improve the core energy of spot. The SMF CE with and without DOE for different central obscuration ratio is studied by numerical simulation. The DOE is effective to improve the SMF CE. Without the DOE, the CE decreases from 81% to 54% when the central obscuration ratio increases from 0 to 0.4. Compared with 54% CE at the of 0.4, the CE can increase to 72%, 78% and 80% with 4 level, 8 level and 16 level quantized DOE, respectively. It is not necessary to use the DOE beam shaper when the central obscuration ratio is small. The 4 level, 8 level and 16 level quantized DOE can obtain benefits of the CE when the central obscuration ratio is above 21.6%, 11.5% and 7%, respectively.

Surface-Emitting Surface Plasmon Polariton Laser in a Second-Order Distributed Feedback Defect Cavity

The coupling between photons and lattice plasmons in metallic nanostructure arrays makes them good candidates to build radiative surface plasmon polariton (SPP) lasers. We report lasing action from the defect state in a second-order distributed feedback SPP cavity. In this device, a silver nanoslit array confines the SPPs in its second-order bandgap, while a phase shift section in the array introduces a high-quality-factor defect state for SPP lasing. At the same time, the fundamental order grating diffraction of the nanoslit array provides a high extraction efficiency from SPPs to photons. Using optical pumping, we demonstrate pulsed lasing at room temperature, with a collimated emission in the surface-normal direction and a high single-mode fiber coupling efficiency. Together with its compact two-dimensional geometry and lasing wavelength tunability, the proposed device is a promising candidate for small footprint and low-cost surface-emitting lasers.

Experimental demonstration of single-mode fiber coupling over relatively strong turbulence with adaptive optics.

High-speed free-space optical communication systems using fiber-optic components can greatly improve the stability of the system and simplify the structure. However, propagation through atmospheric turbulence degrades the spatial coherence of the signal beam and limits the single-mode fiber (SMF) coupling efficiency. In this paper, we analyze the influence of the atmospheric turbulence on the SMF coupling efficiency over various turbulences. The results show that the SMF coupling efficiency drops from 81% without phase distortion to 10% when phase root mean square value equals 0.3λ. The simulations of SMF coupling with adaptive optics (AO) indicate that it is inevitable to compensate the high-order aberrations for SMF coupling over relatively strong turbulence. The SMF coupling efficiency experiments, using an AO system with a 137-element deformable mirror and a Hartmann-Shack wavefront sensor, obtain average coupling efficiency increasing from 1.3% in open loop to 46.1% in closed loop under a relatively strong turbulence, D/r0=15.1.

大气湍流像差对单模光纤耦合效率的影响分析及实验研究

大气湍流像差对单模光纤耦合效率的影响分析及实验研究

Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber (SMF) with high and steady coupling efficiency is crucial for fiber-based free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler (AFC) which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent (SPGD) algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80 μrad. The compensation bandwidth is 35 Hz at sine-jitter of 15 μrad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error (MSE) of coupling efficiency drops from 7.43% to 1.75%.

Efficient single-spatial-mode periodically-poled KTiOPO_4 waveguide source for high-dimensional entanglement-based quantum key distribution

We demonstrate generation of high-purity photon pairs at 1560 nm in a single spatial mode from a periodically-poled KTiOPO_4 (PPKTP) waveguide. With nearly lossless spectral filtering, the PPKTP waveguide source shows approximately 80 % single-mode fiber coupling efficiency and is well suited for high-dimensional time-energy entanglement-based quantum key distribution. Using high-count-rate self-differencing InGaAs single-photon avalanche photodiodes configured with either square or sinusoidal gating, we achieve > 1 Mbit/s raw key generation with 3 bits-per-photon encoding, and, to the best of our knowledge, the highest reported Franson quantum-interference visibility of 98.2 % without subtraction of accidental coincidences.

Degradation of single-mode fiber coupling efficiency due to localized wavefront aberrations in free-space laser communications

Coupling efficiency of coherent plane waves into single-mode fibers is degraded by the localized deformation of the optics antenna used for reception of optical beams. For transmission-type and reflection-type antennas, the evolution of coupling efficiency with the change of characteristic parameters of the localized distortion are investigated. It is found that the coupling efficiency varies regularly as the increase of the normalized center deepness h/R, the normalized radius a/R, and the normalized center distance d/R. The key principles for selecting optics are determined. The results obtained will be useful in estimation and design of fiber-coupled optical systems in free-space communications.

Optimization of single-mode fiber coupling efficiency with an adaptive membrane mirror

We propose and demonstrate the use of an adaptive membrane mirror to increase the efficiency of coupling light into optical single-mode fibers. The membrane mirror has an active area of 12 mm in diameter and is electrostatically activated by 37 electrodes. Two optimization algorithms are compared. The first one is a maximization algorithm, while the second one is an evolutionary algorithm. These algorithms are used either to optimize the electrode voltages or to optimize a set of low-order Zernike polynomials describing the membrane deformation.

Single-mode fiber coupling efficiency with graded-index rod lenses

Losses are calculated for the single-mode graded-index- (GRIN-) lens coupler. The main advantage of this coupling system is large separation between fibers with small power loss. The excess loss of the GRIN-lens coupler is due primarily to the misalignments of the GRIN lenses and is most sensitive to angular tilt rather than lateral offset or end separation. The excess loss is calculated from the overlap of two differently sized or misaligned Gaussian beams. Experimental results show that this method is adequate for predicting losses that are due to misalignments of the GRIN lenses.

Loss analysis of laser diode to single-mode fiber couplers with glass spheres or silicon plano-convex lenses

The single-mode laser diode to single-mode fiber coupling efficiency is analyzed for couplers with glass ball lenses and silicon plano-convex lenses, respectively. The contributions of the different origins of loss are given. It is found that in nearly all cases, most of the total loss is determined by the spherical aberration of the lends that collects the divergent laser diode (lens 1). This loss is calculated for a variety of glass ball lenses and silicon plano-convex lenses are shown to be dependent on the laser diode spot size (or divergence angle). The minimum loss of a coupling configuration is determined with good accuracy by the parameters of lens 1; a single additional parameter, the effective laser diode spot size (or effective divergence angle) for the laser diode; and the coupling arrangement. The calculated losses are compared with experimental values. In most cases, theory and measurement agree very well; differences are discussed. >

High-power 1.3 μm superluminescent diode

Superluminescent diodes with high output power (10 mW at 175 mA), wide spectral width (28 nm), low spectral modulation depth (&amp;lt;15%), wide frequency modulation bandwidth (570 MHz), and high single-mode fiber coupling efficiency (40%) are reported. The structure is based on a buried crescent laser structure with an antireflection coating and a ‘‘short-circuit’’ absorber to suppress lasing.

1.3 ¿mLD Module with Optical Isolator Using YIG Crystals by the Floating Zone Method

This paper reports on fabrication of an LD (laser diode) module with an optical isolator using YIG crystals in which the optical parts are fixed by soldering and YAG laser welding. The LD to single-mode fiber coupling efficiency was less than -4.6 dB, and the isolator insertion loss was 0.1 dB. Output variation in the 0 to 50°C temperature range was as low as 0.4 dB and after heat treatment, coupling loss variation was less than 0.25 dB, a very stable value. This seems to be due to the fact that the optical fiber and the lens were fixed by the metal fixing methods. Laser output noise induced by the reflected laser beam from the far end of the single-mode fiber is reduced by the optical isolator. Using YIG made by the FZ method and fabricating the module using the metal fixing methods realizes high isolation ratio and high reliability.