Heterodyne Efficiency Research Articles

Spatial phase distortion compensation technology based on combinatorial optimization method in heterodyne detection

The spatial phase distortion caused by turbulent atmosphere, target speckle, or optical system aberration causes decoherence of heterodyne signals. The decoherence effect is the key problem that imposes restrictions on the application of laser heterodyne techniques. We propose an approach for spatial phase distortion correction based on a genetic algorithm by establishing a one-to-one correspondence between combination of spatial phase compensation amounts and heterodyne efficiency of the system. The spatial phase distortion compensation problem is converted into a combinatorial optimization problem. The experiment results show that the method achieves excellent compensation performance for the spatial phase distortion, significantly contributing to the applications of heterodyne techniques.

Demonstration on the performance enhancement of the 1.645 µm coherent Doppler lidar for long-range wind measurements with modification of laser transmitter and optical antenna

We have developed and experimentally investigated a long-range 1.645 µm coherent Doppler wind lidar (CDWL) system. A compact 1.645 µm single-frequency Er:YAG laser is utilized as the laser transmitter. The impact of laser transmitter parameters on wind detection was assessed using the figure of merit (FOM) concept. To enhance the measurement efficiency, the influence of wave aberrations on the heterodyne efficiency was analyzed. A Galilean telescope with an optical aperture of 100 mm is designed as the optical antenna based on the analysis. The line of sight (LOS) detection range exceeds 30.42 km with a data rate of 1 Hz at an elevation angle of 3.5°. To evaluate the effectiveness of the CDWL, comparison experiments were conducted between the 1.645 µm CDWL and a calibrated 1.55 µm CDWL, revealing a correlation coefficient of 0.9816 for the whole detection path in the wind velocity measurement.

An Effective Method for Enhancing Heterodyne Efficiency by Comparing the Effect of Degree of Polarization on an Uplink Path and a Downlink Path

By analyzing the effect of the degree of polarization (DoP) of the partially coherent Gaussian Schell-model (GSM) beam on a heterodyne system of an uplink path and a downlink path, we developed an innovative and noteworthy theory according to which σsy (signal beam waist radius in the y direction component) and δSyy (coherence length of the signal beam in the yy direction component) had a more significant impact on heterodyne efficiency and DoP than the turbulence term on uplink and downlink paths. Namely, the DoP and heterodyne efficiency of an uplink path are higher than that of a downlink path when σsy > 0.02 m or δSyy ≥ 0.03 m. This innovative rule provides an efficient way for increasing the heterodyne efficiency of a signal beam propagating along an uplink or a downlink path channel in satellite-ground communication links in free-space optical heterodyne detection communication.

Coherent Detection With Double-Side Vertically Illuminated Photodiodes for Spatially Resolved Ranging Applications

We theoretically investigate and demonstrate coherent detection with double-side vertically illuminated photodiodes by injecting signal and local oscillator (LO) collinearly from opposite sides of the photodetector chip. This avoids optical combiners in front of the detector surface resulting in a 4.5 dB advantage over conventional coherent detection with 50-50 beam splitters to combine signal and LO. The heterodyne efficiency in the case of counter-propagating signal and LO exhibits a penalty of 1.5 dB at optimal absorption layer thickness compared to co-propagating signal and LO and has the same pronounced dependency on the relative propagation angle.A first proof of concept for using this dependency for spatially resolved coherent lidar systems is demonstrated.

An innovative law for improving the heterodyne efficiency of heterodyne detection in atmospheric turbulence

An innovative law for improving the heterodyne efficiency of heterodyne detection in atmospheric turbulence

Factors influencing the applications of active heterodyne detection

Heterodyne efficiency reflects the matching states of phase and amplitude between a local oscillator (LO) beam and a received signal beam and is, therefore, an indicator of system performance. Many factors can cause sensitivity degradation or decoherence in active heterodyne detection applications, such as target speckle, atmospheric turbulence, and optical system aberrations. This paper presents a method for calculating the heterodyne efficiency by mixing LO and customized signal beams in the plane of the detector surface. Results show that the heterodyne efficiency is fixed if the amplitude distribution of the signal light obeys a certain distribution, but the phase mismatches can significantly degrade the system sensitivity. Furthermore, a high-speed camera heterodyne system was developed to evaluate the effectiveness of genetic algorithms for correcting the spatial phase mismatch in applications involving active heterodyne detection. The array detector method improves the heterodyne system sensitivity and benefits active detection applications such as Synthetic Aperture Ladar, Laser Vibrometry, and Long-range Coherent Ladar.

Multi-beam OCT imaging based on an integrated, free-space interferometer

While it is a common practice to increase the speed of swept-source optical coherence tomography (OCT) systems by using a high-speed source, this approach may not always be optimal. Parallelization in the form of multiple imaging beams is an alternative approach, but scalable and low-loss multi-beam OCT architectures are needed to capitalize on its advantages. In this study, we demonstrate an eight-beam OCT system using an interferometer architecture comprising planar lightwave circuits (PLC) splitters, V-groove assemblies (VGA), and optical ribbon fibers. We achieved an excess loss and heterodyne efficiency on each channel that was close to that of single-beam systems. In vivo structural imaging of a human finger and OCT angiography imaging of a mouse ear was performed to demonstrate the imaging performance of the system. This work provides further evidence supporting multi-beam architectures as a viable strategy for increasing OCT imaging speed.

Phase-coded modulation 3D ghost imaging

A three-dimensional ghost imaging system based on the phase-coded modulation is proposed. The system utilizes two high speed electro-optic modulators to encode and decode the pulsed laser with the M-sequence so as to accomplish correlation calculation in optical domain to solve the range information of targets. Compared with the conventional optical heterodyne ghost imaging system using complex detection devices to guarantee heterodyne efficiency, the phase-coded modulation three-dimensional ghost imaging is capable of easy implement and similar detection performance. The results of numerical simulation and experiment have verified the feasibility of the proposed phase-coded modulation three-dimensional ghost imaging.

Effect of beam mode of partially Gaussian Schell-model beam on a heterodyne detection system in turbulence.

A mathematical model considering the transmission of a partially coherent Gaussian Schell-model (GSM) beam in slant turbulence atmosphere of heterodyne detection was established. A closed-form expression of the weighting factor for the partially GSM beam at the receiving end was derived. The effect of the beam mode on the performance of the proposed detection system was theoretically investigated. The results show that the proportion of the fundamental mode and heterodyne efficiency can be optimized by controlling the waist radius of the signal and local oscillator beams. The inner scale of turbulence significantly affects the heterodyne efficiency and normalized M2. With a larger mode order, the proportion of the fundamental mode and heterodyne efficiency are lowered.

Theoretical and experimental investigation on the performance of heterodyne detection system affected by the beam mode of partially coherent beams in atmospheric turbulence

The mathematical model of the effect of beam mode (It contains fundamental mode and other higher orders mode) on the heterodyne efficiency of heterodyne detection system was established, and the effect of beam mode on the performance of heterodyne detection system was theoretically investigated for partially coherent GSM beam in slant atmospheric turbulence conditions. The results show that the inner scale of turbulence has a greater effect on the heterodyne efficiency and normalized M2. (It is used to evaluate beam quality.) The larger the mode orders are, the lower the proportion of the fundamental mode and heterodyne efficiency are. Finally, the changes of beam mode caused by turbulence was obtained by a heterodyne detection system in an experiment investigating optical communication at a distance of 1.3 km. The results of this experiment verified that the mode orders are increased by turbulence effect and the beam mode increases with an increase in the turbulence intensity, resulting in a decrease in the corresponding the peak-to-peak value of the IF signal.

Modeling the Heterodyne Efficiency of Array Detector Systems in the Presence of Target Speckle

To solve the speckle effect problem in heterodyne detection, an array detector is proposed; heterodyne efficiency is modeled by accounting for speckle and using an additional parameter to simulate coherent summation processing. Heterodyne efficiency is derived by mixing a Monte Carlo simulated speckle field with a planar local oscillator field on a photodetector surface to evaluate the array detector performance at improving signal-to-noise ratio (SNR) of the coherent optical system. From the simulation results, the array detector is shown to effectively improve the SNR of the heterodyne system. For low SNR, the heterodyne efficiency of an N × N array detector increases linearly with N .

Practical Security Analysis of Self-Referenced CV-QKD System in the Presence of Polarization Aberration

How to remove local oscillator (LO) side channel attacks has been a notoriously hard problem in continuous-variable quantum key distribution (CV-QKD). In the self-referenced CV-QKD schemes, the LO signal is locally generated at the receiver by an independent laser so that it is not co-transmitted with the quantum signal. This simple solution removes all LO side channels. However it also introduces some other practical vulnerabilities. Especially the polarization states of the quantum signal and LO signal may not be identical across the detector because of the presence of the polarization aberrations. Thus, the detection efficiency which is arguably the most critical experiment parameter of the practical implementation will be impaired. In this paper, we analyze the impact of polarization aberrations on the detection efficiency for CV-QKD and propose a self-referenced CV-QKD scheme in the presence of polarization aberrations by using an off-axis optical system. In the proposed scheme, the polarization states of the quantum signal would change with the off-axis optical system, thus impairing the heterodyne efficiency. Our security analysis shows a gap between the theory and practice of CV-QKD.

The quantitative relationship between the target surface speckle phase echo parameters and heterodyne efficiency

In applications of active detection, the speckle effect caused by the rough surface of the target is still the major obstacle restricting the practical application of the optical heterodyne detection. The heterodyne efficiency reflects the matching between the received beam and the local oscillator beam on the detector surface, and this property can be used to evaluate the alignment of the heterodyne system. In this paper, we derive an expression for the heterodyne efficiency of mixing planar local oscillator (LO) beam and simulated Monte Carlo (MC) speckle field over the plane of the detector surface in terms of analyzing the quantitative influence of the relationship between the detector size and speckle statistical parameters on heterodyne detection. Numerical results are given to show the variation in the heterodyne efficiency with relationships of the size of detector and speckle echo statistical parameters. Based on the simulation results and our theoretical works, the influence of Rayleigh speckle field on heterodyne detection are discussed, and a criterion on Rayleigh speckle statistics in heterodyne detection is given.

Large-aperture synthetic aperture imaging ladar

A strip-mode side-looking large-aperture synthetic aperture imaging ladar (SAIL) based on free-space optics is designed to experimentally verify the key performances for further airborne application. The optical difficulties in the spatial domain, including optical antenna and imaging resolution for large-aperture SAIL, are considered systematically. The optical antenna consists of a circulated duplex including the tunable transmission and reception channels for the flexible wavefront control. An irradiating beam with a spatial quadratic phase-bias for the suitable and controlled phase history can be transmitted from the transmission channel, and the wavefront aberration in the reception channel resulted from the diffraction can be eliminated for the optimized heterodyne detection. The construction and 2-D imaging algorithm of the SAIL are also proposed. The performances, including wavefront, heterodyne efficiency, field of view, imaging resolution, and 2-D imaging algorithm, have been systemically verified in the short-distance of laboratory space. The well-focused 2-D SAIL imaging of a diffusely scattering target with submillimeter imaging resolution is given. The experimental results are in good agreement with the theoretical design. The results have the great significance for the future development of airborne large-aperture SAIL.

Photoconductive antenna as local oscillator in terahertz frequency measurement: heterodyne efficiency and bias effect

In a photoconductive antenna (PCA), femtosecond-laser-excited carriers will form a broadband terahertz photon-carrier (PC) comb, and the terahertz PC comb can be used as a multi-frequency local oscillator to carry out heterodyne detection of continuous terahertz sources with high frequency accuracy. In this paper, the heterodyne efficiency and the bias effects of a PCA terahertz PC comb are investigated. The results show that the pair beat signals (with the beat frequencies lower than the repetition frequency of femtosecond laser) of a continuous terahertz source and the two adjacent comb teeth do not decrease with the increase of beat frequency. Applying a bias voltage to the PCA can effectively enhance the terahertz emission efficiency. However, such a bias voltage has no positive effects on the heterodyne detection responsivity because the heterodyne detection is intrinsically based on the terahertz rectification effect that is proportional to the photo-excited electrons. In addition, by using a reference terahertz source with high frequency stability, it is possible to measure the fluctuation and the drift of the repetition frequency of femtosecond lasers with higher accuracy. The results are helpful for improving the performance of terahertz frequency measurement system based on PCA PC combs.

Modified heterodyne efficiency for coherent laser communication in the presence of polarization aberrations.

Heterodyne efficiency is referred as a measure of the quality for the coherent laser communication. The heterodyne efficiency not only reflects the matching of phase and amplitude between the received signal and the local oscillator, but also reveals the polarization matching between the two beams. Different from the common heterodyne efficiency, a revised heterodyne efficiency is proposed by considering the polarization aberrations of optical system. Based on the Polar and Pauli-Zernike decomposition algorithms, the effects of polarization aberrations on the output polarization states are analyzed and shown graphically. The variations of the heterodyne efficiency are investigated by including the separate component of polarization aberrations in mixing of two perfectly matched Gaussian beams. Depending on the modified heterodyne efficiency, an off-axis optical system with a periscopic scanner is modeled and used to discuss the variations of the heterodyne efficiency. A further investigation for three different coatings is accomplished to verify the effects the varied polarization aberrations have on the heterodyne efficiency. The analysis indicates that the modified heterodyne efficiency not only can provide a comprehensive description of the coherent detection system, but also can be used to evaluate and minimize the polarization aberrations of optical system.

光斑尺寸偏差和光轴偏转对空间光混频器混频效率的影响分析

光斑尺寸偏差和光轴偏转对空间光混频器混频效率的影响分析

The effects of polarization changes of stochastic electromagnetic beams on heterodyne detection in turbulence

The performance of heterodyne systems is discussed for stochastic electromagnetic beams in turbulent atmospheres by introducing a turbulence spectrum of refractive-index fluctuations. Several analytic formulae for the heterodyne detection system are presented by employing the beam coherence polarization matrix of an electromagnetic Gaussian Schell-model beam. Based on the Tatarskii spectrum model, some numerical results are given for the variation of heterodyne efficiency with the misalignment angle, detector diameter, turbulence conditions and parameters of the overlapping beams. According to the numerical results, we find that the change in heterodyne efficiency is dependent on the initial polarization of the beams, and that a turbulent atmosphere degrades the heterodyne efficiency significantly for a lager detector diameter. The change in heterodyne efficiency in a turbulent atmosphere is basically the same as that in free space for a relatively short distance zs. However, the change in heterodyne efficiency is different for a sufficiently long distance. For the deterministic received signal and the detector, the performance of the heterodyne detection can be adjusted by controlling the local oscillator signal parameters.

Impact of turbulent phase noise on frequency transfer with asymmetric two-way ground-satellite coherent optical links

Bidirectional ground-satellite laser links suffer from turbulence-induced scintillation and phase distortion. We study how turbulence impacts on coherent detection capacity and on the associated phase noise that restricts clock transfer precision. We evaluate the capacity to obtain a two-way cancellation of atmospheric effects despite the asymmetry between up and down link that limits the link reciprocity. For ground-satellite links, the asymmetry is induced by point-ahead angle and possibly the use, for the ground terminal, of different transceiver diameters, in reception and emission. The quantitative analysis is obtained thanks to refined end- to-end simulations under realistic turbulence and wind conditions as well as satellite cinematic. These temporally resolved simulations allow characterizing the coherent detection in terms of time series of heterodyne efficiency for different system parameters. We show that Tip/Tilt correction on ground is mandatory at reception for the down link and as a pre-compensation of the up link. Good correlation between up and down phase noise is obtained even with asymmetric apertures of the ground transceiver and in spite of pointing ahead angle. The reduction to less than 1 rad2 of the two-way differential phase noise is very promising for clock comparisons.

Impact of turbulence on high-precision ground-satellite frequency transfer with two-way coherent optical links

Bidirectional ground-satellite laser links suffer from turbulence-induced scintillation and phase distortion. We study the impact of turbulence on coherent detection and the related phase noise that restricts time and frequency transfer precision. We evaluate the capacity to obtain a two-way cancellation of atmospheric effects despite the asymmetry between up- and downlink that limits the link reciprocity. For ground-satellite links, the asymmetry is induced by point-ahead angle and possibly the use, for the ground terminal, of different transceiver diameters, in reception and emission. The quantitative analysis is obtained thanks to refined end-to-end simulations under realistic turbulence and wind conditions as well as satellite kinematics. These temporally resolved simulations allow characterizing the coherent detection in terms of time series of heterodyne efficiency and phase noise for different system parameters. We show that tip-tilt correction on ground is mandatory at reception for the downlink and as a pre-compensation of the uplink. Besides, thanks to the large tilt angular correlation, the correction is shown to be efficient on uplink despite the point-ahead angle. Very good two-way compensation of turbulent effects is obtained even with the asymmetries. The two-way differential phase noise is reduced to $1{\mathrm{rad}}^{2}$, with the best fractional frequency stability below $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ after 1-s averaging time.