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In this paper, the performance of Optical Wireless Communication (OWC) under atmospheric turbulence and pointing errors is enhanced by employing MIMO transceivers and heterodyne BPSK and DPSK modulations. We have derived new closed-form expressions for the bit error rate (BER) of MIMO optical wireless communications based on BPSK and DPSK with heterodyne detection for atmospheric turbulence only, pointing error only, and both effects combined. Numerical simulations reveal that the effect of turbulence and pointing errors increases with Rayleigh variance and normalized jitter variance, while it diminishes with normalized pulse width. The BER values for SISO-BPSK and SISO-DPSK, considering a normalized pulse width of 4, normalized jitter of 2, Rayleigh variance of 2, and SNR of 16 dB, are 7.50 × 10 - 3 and 1.48 × 10 - 2 , respectively. It is evident that the BER of SISO OWC experiences significant degradation for both modulation techniques. However, through the employment of a MIMO transceiver with an increased number of receivers and transmitters, the BER experiences a substantial reduction. The BER values for MIMO-BPSK and MIMO-DPSK under the same conditions are remarkably low, standing at 1.24 × 10 - 12 and 2.38 × 10 - 10 , respectively. These results underscore that, for both SISO and MIMO systems, BPSK outperforms DPSK. The power penalty of SISO-BPSK, for Rayleigh variance and normalized pulse width, is twice that of MIMO-BPSK under the same conditions. The results show that we can use MIMO with heterodyne BPSK to reduce the BER caused by atmospheric turbulence and pointing errors. [ABSTRACT FROM AUTHOR]

Satellite‐to‐ground quantum communication constitutes the cornerstone of the global quantum network, heralding the advent of the future of quantum information. Continuous‐variable quantum key distribution is a strong candidate for the space‐ground quantum communication due to its simplicity, stability, and ease of implementation, especially for the robustness of space background light noise. Recently, the discrete‐modulated continuous‐variable protocol has garnered increased attention, owing to its lower implementation requirements, acceptable security key rate, and pronounced compatibility with extant infrastructures. Here, key rates are derived for discrete‐modulated continuous‐variable quantum key distribution protocols in free‐space channel environments across various conditions through numerical simulation, revealing the viability of its application in satellite‐to‐ground communication. [ABSTRACT FROM AUTHOR]

In recent times, photonic synthesis and processing of arbitrary microwave waveforms have gained interest due to their high frequency and huge time–bandwidth product. This study reviews methods for producing dual‐chirp microwave waveforms, focusing mostly on system topologies based on external modulation techniques, such as cascaded Mach–Zehnder modulator (MZM), dual parallel MZM, and dual polarized MZM. Other photonic approaches, like using semiconductor laser and optical heterodyne detection methods, are also discussed. These techniques are preferable because of high frequency, enhanced time–bandwidth product, greater chirp rate, and many more advantages. Additionally, the difficulties associated with integrating the systems into real‐world applications are also addressed. [ABSTRACT FROM AUTHOR]

This paper proposes an approach to analyze the performance of radio frequency/underwater wireless optical communication (RF/UWOC) systems using Mellin inverse transform. This analysis combines heterodyne detection (HD) and direct detection /intensity modulation (IM/DD). The RF link from land ground stations to relay stations follows the Nakagami-m distributed channel. The UWOC link from the relay station to the underwater vehicle is modeled using the double generalized gamma (GG) distribution with pointing errors. By employing the Fixed Gain Amplification and Forwarding (AF) protocol, the probability density function (PDF) of the RF/UWOC link is derived using the Mellin inverse transform, expressed in the form of the Meijer-G function. In addition, expressions for the outage probability and bit error rate of the mixed system were derived. Monte Carlo simulations validated the analysis results, which demonstrated the superior performance of HD technology. [ABSTRACT FROM AUTHOR]

We have studied THz heterodyne detection in sub-micrometer MgB2 hot electron bolometer (HEB) mixers based on superconducting MgB2 films of ∼ 5 nm (HEB-A), corresponding to a critical temperature (Tc) of 33.9 K, and ∼ 7 nm (HEB-B), corresponding to a T c of 38.4 K. We have measured a double sideband (DSB) receiver noise temperature of 2590 K for HEB-A and 2160 K for HEB-B at 1.6 THz and 5 K. By correcting for optical losses, both HEBs show receiver noise temperatures of ∼1600 K referenced to the front of anti-reflection (AR)-coated Si lenses. An intermediate frequency (IF) noise bandwidth of 11 GHz has been measured for both devices. The required local oscillator (LO) power is about 13 μW for both HEBs. We have also measured a DSB receiver noise temperature of 3290 K at 2.5 THz and 5 K but with an AR-coated lens optimized for 1.6 THz. Besides, we have observed a step-like structure in current voltage (IV) curves, which becomes weaker when the LO power increases and observable only in their differential resistance. Such a correlated structure appears also in the receiver output power as a function of voltage, which is likely due to electronic inhomogeneities intrinsic to the variations in the thickness of the MgB2 films. Different behavior in the IV curves around the low bias voltages, pumped with the same LO power at 1.6 and 5.3 THz, was observed for HEB-B, suggesting the presence of a high-energy σ-gap in the MgB2 film. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Heterodyne detection based on interband cascade lasers (ICL) has been demonstrated in a wide range of different applications. However, it is still often limited to bulky tabletop systems using individual components such as dual laser setups, beam shaping elements, and discrete detectors. In this work, a versatile integrated ICL platform is investigated for tackling this issue. A RF-optimized, two-section ICL approach is employed, consisting of a short section typically used for efficient modulation of the cavity field and a long gain section. Such a laser is operated in reversed mode, with the entire Fabry–Pérot waveguide utilized as a semiconductor optical amplifier (SOA) and the electrically separated short section as detector. Furthermore, a racetrack cavity is introduced as on-chip single-mode reference generator. The field of the racetrack cavity is coupled into the SOA waveguide via an 800 nm gap. By external injection of a single mode ICL operating at the appropriate wavelength, a heterodyne beating between the on-chip reference and the injected signal can be observed on the integrated detector section of the SOA-detector. [ABSTRACT FROM AUTHOR]

We report room temperature heterodyne detection of a quantum cascade laser beaten with a local oscillator on a unipolar quantum photodetector in two different atmospheric windows, at 4.8 µm and 9 µm. A noise equivalent power of few pW is measured by employing an active stabilization technique in which the local oscillator and the signal are locked in phase. The measured heterodyne noise equivalent power is six orders of magnitude lower than that obtained with direct detection. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communication is a wireless alternative to fiber-based communication as it may provide a backbone emergency communication link in disaster-hit areas. This paper proposes a full duplex millimeter wave (mm-wave) enabled FSO system to provide a high data rate communication link for search and rescue operations. The proposed system consists of a central unit which is connected to multiple unmanned aerial vehicles (UAVs) through an optical-amplify and forward relay over an FSO channel represented by Log-Normal channel model. An optical comb is generated and Laguerre-Gaussian modes of each wavelength of the optical comb are exploited to carry 10 Gbps differential phase shift keying modulated signals. 60 GHz mm-wave signals are generated at the remote UAVs by employing optical heterodyne detection for downlink transmission towards the user equipment. For the uplink transmission, a dedicated wavelength is used to carry 10 Gbps on-off keying baseband data for live streaming of the disaster-hit area. Furthermore, a single hop scheme is employed to counter the non-line of sight issue of the FSO link. The performance of the proposed model is evaluated under different weather conditions and refractive index structure parameter values. A maximum distance of 3.3 km is achieved for clear sky under low turbulence conditions. Moreover, the effect of misalignment due to hovering of the UAVs on the system performance is also investigated and tolerance in the x (horizontal) and y (vertical) directions is observed at the forward error correction BER limit of 10 - 9 . The simulation results reveal that the proposed system has the potential to provide reliable and quick emergency communication services in disaster-struck areas. [ABSTRACT FROM AUTHOR]

A new approach to the creation of millimeter-wave radio imaging systems is proposed. This approach is based on the use of an array receiver consisting of a densely packed (pixel size of 4 mm) array of planar mixers located in the focal plane of a quasi-optical objective, with application of the frequency-modulated continuous-wave radar technique. It has been demonstrated that the implementation of the heterodyne type of reception makes it possible to increase the distance range of the array radio imaging system up to ~100 m while maintaining the angular resolution at the previous level. [ABSTRACT FROM AUTHOR]

The ground‐based laser heterodyne radiometer (LHR), which exhibits the advantages of small size, high spectral resolution, and easy integration, has been used for the remote sensing detection of several gases to meet a wide range of needs. This study aims to optimize the laser heterodyne system for detecting CO2 gas by focusing on existing research. Firstly, using the all‐fiber laser heterodyne detection system built by our research group, the power spectrum associated with the radio frequency signals of the detection system is discussed under different conditions: under no irradiation, under sunlight only, under sunlight and laser irradiation at the absorption peak, and under a filter in the spectrum range of 185–270 MHz. Signal‐to‐noise ratios (SNRs) of the high‐resolution spectrum have been obtained using different filter bands of 185–270, 225–270, and 225–400 MHz. Finally, the filter in the 225–270 MHz band, which has the highest SNR, is selected. Consequently, the resolution is improved and the system is further optimized. Furthermore, an optical fiber attenuator is used to change the power of the local oscillator light entering the system, and hyperspectral spectra with varying percentages of input energy and total energy are obtained. When the laser attenuation reaches 40%, the optimal SNR of the system is 486 and can be further improved to meet the expected requirements. This study will provide insights for improving the applicability of laser heterodyne technology in atmospheric sounding. [ABSTRACT FROM AUTHOR]

This paper investigates the performance analysis of radio frequency/free space optical-visible light communication (RF/FSO-VLC) system, where a hybrid RF/FSO link is connected to an indoor VLC subsystem using decode-and-forward (DF) relay. The selection combining (SC) technique is used at the DF relay. We have considered Nakagami-m fading, Double Generalized Gamma (DGG) distributed atmospheric turbulence with Rayleigh distributed pointing errors and Lambertian pattern of the light emitting diode (LED) for the RF, FSO, and VLC links, respectively. To analyze the performance of the considered co-operative system, different performance metrics such as outage probability, bit-error-rate (BER), and ergodic capacity of the system are taken into consideration. For deriving the performance metrics, novel expressions for cumulative distribution function and probability density function of the instantaneous signal-to-noise-ratio (SNR) of hybrid RF/FSO-VLC system in terms of Meijer-G function are obtained and used. The numerical results show that the outage probability of the system is more in the strong turbulence conditions and intensity modulation/direct detection (IM/DD) technique in comparison to weak turbulence and heterodyne detection scenario. It is also seen from the results, that the BER of the system is better in case of coherent binary modulation techniques in comparison with non-coherent binary and M-ary modulation techniques. Moreover, the capacity of the system increases as the number of LEDs increase and as the height of the LEDs reduce. The asymptotic and simulation results match with the derived analytical results. [ABSTRACT FROM AUTHOR]

Laser heterodyne detection (LHD) is a key velocimetry technique that provides better accuracy and sensitivity than direct laser detection. However, random phase noise can be introduced by the surface topography of the moving target undulation or atmospheric turbulence during transmission. The random phase noise causes the target echo to undergo decoherence, resulting in degradation of the signal-to-noise ratio (SNR). Here, we propose a novel LHD method based on photon time–domain differential detection. It can infer the heterodyne spectrum of the target echo and the local oscillator light from the time intervals of the photon arrival. The time interval statistic is a relative quantity, which can effectively avoid the effect of random phase noise in LHD. With our method, the SNR of LHD can be improved in application scenarios where the target echo is decoherent. We developed a complete solution model for acquiring the heterodyne spectrum based on photon time–domain differential detection and performed proof-of-principle experiments. The experimental results show that in the presence of random phase noise, the SNR and velocity measurement error of our method are significantly better than that of the conventional method, and the larger the phase noise is, the more the SNR and velocity measurement error of our method are improved. Moreover, along with the increase in phase noise, the SNR of our method is basically unchanged, which also indicates that our method is not affected by random phase noise. This advantage is significant for photon-level weak echoes that require long detection times to be detected. [ABSTRACT FROM AUTHOR]

In this paper, the performance analysis of a cooperative radio frequency-free space optical communication-visible-light-communication (RF–FSO–VLC) system using decode-and-forward (DF) relays is presented. In the proposed model, the RF link acts as the core network, the FSO link is used for providing last mile connectivity for the indoor cell users communicating through VLC environment. The RF link is characterized by Nakagami-m-distributed fading, while the FSO link undergoes double generalized gamma distributed atmospheric turbulence and pointing errors and the VLC link is characterized by the Lambertian pattern of the LED. The signal at each DF relay is decoded , re-encoded and again decoded before sending to the next relay. The signal to the first relay is sent by an RF link. The received signal is then decoded, re-encoded and again decoded to send to the second relay via a FSO link. From the second relay the signal is transmitted to the end users present in the indoor environment via an VLC link. The closed-form expressions of the statistical characteristics of signal-to-noise-ratio of the DF based triple-hop system are evaluated using which various performance metrics, i.e, outage probability, bit error rate (BER), and capacity of the considered system are obtained. The numerical results show that the system's performance gets severely affected by the RF fading, atmospheric turbulence, and pointing errors. The outage performance of the system decreases as field-of-view and the height of the light-emitting-diode (LED) increase. It is also seen from the results that the BER performance of the system is better in case of heterodyne detection as compared to direct-detection. It is also observed that the capacity of the system decreases when the effect of atmospheric turbulence and pointing error is high and it increases when the no. of LEDs increases. [ABSTRACT FROM AUTHOR]

This paper presents systematic modeling and performance analyses of a 340-GHz dual-polarized antenna-coupled single-sideband (SSB) high-Tc superconducting (HTS) mixer for terahertz (THz) communication systems. The mixer features a fully monolithic-integrated circuit layout, which enables both sensitive heterodyne detection and high image rejection by appropriate polarization-manipulating operation. To predict the comprehensive performance of such a HTS mixer with relatively complicated network architecture and polarization relationships, an innovative modified three-port modeling approach is presented and developed by theoretical derivations. Using the measured Josephson junction parameters of our previous experimentally demonstrated HTS mixer module at the same band, the best conversion gain and SSB noise temperature of the presented mixer are predicted to be around −5 dB and 1200 K at the operating temperature of 40 K, respectively. In addition, the mixer exhibits a flat frequency response over a wide intermediate-frequency range from 15 to 25 GHz and high image rejection of more than 17 dB over the radio frequency operation range. The SSB HTS Josephson mixer will be useful for THz wireless communication applications. [ABSTRACT FROM AUTHOR]

The LIGO analogy lab [Am. J. Phys. 87, 44 (2019)] includes a complicated pair of circuits to sense and control the length of an optical cavity. This Note describes an alternate, Arduino-based controller that is significantly less complex and easier to troubleshoot. The LIGO analogy lab [Am. J. Phys. 87, 44 (2019)] includes advanced experiments in which op-amp-based mixing and feedback circuits are used to lock the length of an optical cavity using heterodyne detection. This Note describes an alternate, Arduino-based controller that implements the heterodyning technique in a significantly less complex and easier to troubleshoot manner. This simplified approach makes the advanced LIGO analog experiments accessible to a wider audience in advanced instructional and optics laboratory courses. [ABSTRACT FROM AUTHOR]

The outage probability performance of a hybrid free space optical (FSO)/radio frequency (RF) system with a reconfigurable intelligent surface (RIS) assisted full-duplex relay is presented in this paper. The FSO link follows the Gamma-Gamma distribution over pointing error and atmospheric turbulence with random foggy impairments. The RF link between the relay and the destination is subject to Nakagami-m distributions, while the RIS links and relay self-interference (SI) link follow Rayleigh fading. As a result, the RIS-to-relay link's cumulative distribution function (CDF) of the signal to interference plus noise ratio (SINR) is obtained. On the basis of this, the system's outage probability is determined according to the decode and forward relay protocol. Thus, Monte-Carlo simulations are utilized to verify the obtained expression's accuracy. Our findings show how atmospheric turbulence, pointing errors, fog conditions, and the number of RIS reflecting elements affect the system performance. Furthermore, it is concluded that, under the identical channel conditions, heterodyne detection performs better than intensity modulation/direct detection (IM/DD). [ABSTRACT FROM AUTHOR]

Nitrogen vacancy (NV) centers are a major platform for the detection of nuclear magnetic resonance (NMR) signals at the nanoscale. To overcome the intrinsic electron spin lifetime limit in spectral resolution, a heterodyne detection approach is widely used. However, application of this technique at high magnetic fields is yet an unsolved problem. Here, we introduce a heterodyne detection method utilizing a series of phase coherent electron nuclear double resonance sensing blocks, thus eliminating the numerous Rabi microwave pulses required in the detection. Our detection protocol can be extended to high magnetic fields, allowing chemical shift resolution in NMR experiments. We demonstrate this principle on a weakly coupled 13C nuclear spin in the bath surrounding single NV centers, and compare the results to existing heterodyne protocols. Additionally, we identify the combination of NV-spin-initialization infidelity and strong sensor-target-coupling as linewidth-limiting decoherence source, paving the way towards high-field heterodyne NMR protocols with chemical resolution. Heterodyne detection is vastly used to overcome the intrinsic electron spin lifetime limiting the spectral resolution in NMR experiments based on nitrogen vacancy platforms, but the application of this technique at high magnetic fields is yet a challenge. The authors introduce heterodyne detection method applicable at high magnetic fields. [ABSTRACT FROM AUTHOR]

A detector of electromagnetic radiation may benefit - if its sensor area is small - from the application of a substrate lens, which focusses the radiation onto the active sensing area of the device and thus enhances its responsivity. The use of such a lens, attached directly onto the detector backside in order to avoid reflection losses, requires that the detector substrate and backside be transparent to the radiation. However, if this is not the case, one may like to place instead a superstrate lens onto the front side of the detector. It may even be of interest to use both a substrate and a superstrate lens if the detector needs to be illuminated with two beams, e.g. for heterodyne detection, where one beam provides the local-oscillator signal. The use of a superstrate lens is, however, often hindered or impeded by an uneven surface topography or by the presence of bonding wires on the front side of the detector. Here, we address this issue and explore the use of paraffin wax to form or attach superstrate lenses. In the first case, which is the main topic of this contribution, we exploit the surface tension of liquid paraffin, brought onto the detector, to sculpt the wax itself into a lens. In the second case, only addressed conceptually here, we use paraffin to form a thin intermediate layer which also acts as an adhesive for the attachment of a plastic or silicon lens. In both cases, the application of liquid paraffin allows one to fill out an uneven detector surface and to embed wires without breaking them. We investigate the use of wax for the case of CMOS TeraFETs - detectors of terahertz radiation based on field-effect transistors - embedded into antenna structures. We describe the processing steps and analyze the performance of a TeraFET equipped with such a wax superstrate lens for front-side beam coupling. [ABSTRACT FROM AUTHOR]

Dual-comb interferometry harnesses the interference of two laser frequency combs to provide unprecedented capability in spectroscopy applications. In the past decade, the state-of-the-art systems have reached a point where the signal-to-noise ratio per unit acquisition time is fundamentally limited by shot noise from vacuum fluctuations. To address the issue, we propose an entanglement-enhanced dual-comb spectroscopy protocol that leverages quantum resources to significantly improve the signal-to-noise ratio performance. To analyze the performance of real systems, we develop a quantum model of dual-comb spectroscopy that takes practical noises into consideration. Based on this model, we propose quantum combs with side-band entanglement around each comb lines to suppress the shot noise in heterodyne detection. Our results show significant quantum advantages in the uW to mW power range, making this technique particularly attractive for biological and chemical sensing applications. Furthermore, the quantum comb can be engineered using nonlinear optics and promises near-term experimentation. [ABSTRACT FROM AUTHOR]

An analytical model is presented for light scattering associated with heat transport near a cell membrane that divides a complex system into two topologically distinct half-spaces. Our analysis is motivated by experiments on vibrational photothermal microscopy which have not only demonstrated remarkably high contrast and resolution, but also are capable of providing label-free local information of heat transport in complex morphologies. In the first Born approximation, the derived Green's function leads to the reconstruction of a full 3D image with photothermal contrast obtained using both amplitude and phase detection of periodic excitations. We show that important fundamental parameters including the Kapitza length and Kapitza resistance can be derived from experiments. Our goal is to spur additional experimental studies with high-frequency modulation and heterodyne detection in order to make contact with recent theoretical molecular dynamics calculations of thermal transport properties in membrane systems. [ABSTRACT FROM AUTHOR]

In this work, we have carried out a performance analysis of the Decode and Forward (DF), dual-hop hybrid Radio Frequency-Free Space Optical Communication (RF-(RF/FSO)) link. The Source to Relay (S → R) link is considered as an RF link, and Relay to Destination (R → D) link is a parallel RF and FSO link. The selection combining technique is employed at the destination. The RF links are modeled by Nakagami-m distribution, and the FSO link is modeled by and Malaga channel model with pointing errors. For the FSO link, both Intensity Modulation/Direct Detection (IM/DD) and Heterodyne detection schemes are considered. The closed form expressions of Probability Distribution Function (PDF), Cumulative Distribution Function (CDF), and Moment Generating Function (MGF) of end-to-end SNR are derived in form of Meijer's G function. Utilizing the above results, The expressions for outage probability, bit error rate (BER), and ergodic capacity have been obtained. The numerical results are compared for different strengths of fading of RF links, the strength of atmospheric turbulence, the severity of pointing errors, and detection techniques. The numerical results closely match with Monte-Carlo simulations which validate the analytical work. The RF-(RF/FSO) link's performance is found to be better than the equivalent RF-FSO link. The link (S → R or R → D) experiencing the stronger fading dominates the overall link performance. [ABSTRACT FROM AUTHOR]

Wideband radio frequency (RF) channelization is essential for the reception and detection of cross-band RF signals in various applications, including communications, radar, and spectrum sensing. However, digital channelizers are inefficient at performing RF channelization over a working bandwidth above 10 GHz. Meanwhile, current photonic RF channelizers face challenges in simultaneously considering a wideband, multi-channel, and a high crosstalk suppression ratio. In this work, we proposed and demonstrated a wideband image-reject RF channelization scheme based on integrated dual-soliton microcombs. The dual-soliton microcombs are used for RF spectral copies and heterodyne detection, respectively. Supported by image-reject mixers, the RF channelization is verified with an 8–37 GHz working bandwidth, a 1.2 GHz channel bandwidth, and 25 channels. The image suppression ratio is higher than 34 dB for single-tone signals and 20 dB for wideband signals. Our approach provides an innovative architecture of integrated photonic RF channelizers with high performance, which can benefit a wide range of RF applications by miniaturizing the systems. [ABSTRACT FROM AUTHOR]

Femtosecond laser-based terahertz (THz) sources have gained attention for their potential in various applications. As for any electromagnetic radiation, the wave polarization is a critical parameter, which needs to be under control in a wide class of those applications. However, characterizing the polarization of broadband THz pulses remains challenging due to their limited efficient optics. THz air-biased coherent detection has emerged as a promising candidate. The technique employed is heterodyne detection, utilizing second-harmonic generation induced by THz radiation in laser-induced air plasmas. This approach provides exceptional spectral bandwidth and an unbounded power detection limit, rendering it highly suitable for ultra-broadband and high-power THz sources. It enables a very efficient polarization measurement too. However, recent findings have revealed that the laser-induced air plasma generated in this technique can exhibit birefringence, which in turn adds systematic errors to the polarization-state determination. In this Letter, we propose a simplified approach that utilizes a weak probe beam and avoids high-voltage DC bias-fields. Unlike the terahertz (THz) air-biased coherent detection scheme, our approach yields a unipolar, intensity-proportional signal for second-harmonic generation. The experimental results reported in this Letter show the absence of induced birefringence in air and, hence, demonstrate accurate measurements of the polarization state of ultra-broadband THz pulses. Therefore, our technique may provide valuable results in applications where the polarization state, and not the full electric field waveform, is required for analysis or characterization. Finally, we discuss a possible application of our method to the emergent field of THz singular optics. [ABSTRACT FROM AUTHOR]

In this study, we experimentally showcase the microfiber evanescent-field photothermal gas detection by exploiting all-fiber MHz-level frequency shift scheme. Based on the acousto-optic interaction effect, the low-frequency shifts of 0.9 MHz and 1.83 MHz can be obtained through the cyclic conversion between the transverse core modes LP01 and LP11 in the few-mode fiber. Our proposed all-fiber frequency shifters show flexible MHz-level up(down) frequency shifts with superior sideband rejection ratio (over 40 dB) and low insertion loss (less than 1 dB). Furthermore, an all-fiber heterodyne interferometric detection system is implemented by leveraging the above low-frequency shifters, in which around 1-μm-diameter microfiber is investigated for photothermal gas detection. A pump-probe configuration is employed to obtain the photothermal effect induced by the gas absorption of the modulated evanescent field. By demodulating the phase of the beat signal output by the interferometer, an equivalent detection limit (1σ) of 32 ppm and a response time of 22 s are achieved for ammonia, as well as 0.24 % instability within 48 pump cycles. Given its compact all-fiber configuration and high sensitivity with fast response, the experimental results can pave the way for widespread applications like heterodyne detection, fiber optical sensors, and interplanetary coherent communications. [ABSTRACT FROM AUTHOR]

With the completion of the standardization of fifth-generation (5G) networks, the researchers have begun visioning sixth-generation (6G) networks that are predicted to be human-centric. Hence, similar to 5G networks, besides high data rates, providing secrecy and privacy will be the center of attention by the wireless research community. To support the visions beyond 5G and 6G, in this paper we propose a secure radio frequency (RF)-free space optical (FSO) mixed framework under the attempt of wiretapping by an eavesdropper at the RF hop. We assume the RF links undergo α - η - μ fading whereas the FSO link exhibits a unified Málaga turbulence model with pointing error. The secrecy performance is evaluated by deducing expressions for three secrecy metrics i.e. average secrecy capacity, secrecy outage probability, and probability of non-zero secrecy capacity in terms of univariate and bivariate Meijer's G and Fox's H functions. We further capitalize on these expressions to demonstrate the impacts of fading, atmospheric turbulence, and pointing errors and show a comparison between two detection techniques (i.e. heterodyne detection (HD) and intensity modulation with direct detection (IM/DD)) that clearly reveals better secrecy can be achieved with HD technique relative to the IM/DD method. The inclusion of generalized fading models at the RF and FSO hops offers the unification of several classical scenarios as special cases thereby exhibiting a more generic nature relative to the existing literature. Finally, all the analytical results are corroborated via Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

We introduce a spectrometer design that uses Brillouin lasing to perform broadband spectral compression. This approach enables the entire C-band (optical frequencies covering 4 THz, or 25 nm) to be compressed into a 230 MHz wide RF spectrum—allowing a 4 THz wide optical spectrum to be monitored continuously using a standard analog-to-digital converter. This technique is based on the linear dependence of the Brillouin frequency shift on optical wavelength. To use this dependence for spectral analysis, we couple the input optical spectrum into a fiber ring cavity where it acts as an optical pump, exciting a frequency-shifted Brillouin lasing spectrum. The Brillouin lasing spectrum is then referenced to a copy of the original optical spectrum using heterodyne detection, converting the Brillouin frequency shift associated with each lasing mode to the RF domain. The narrow linewidth of the lasing modes enables a precise measurement of individual lines with an uncertainty of 28 MHz (0.2 pm) across a 4 THz band at an update rate of 200 Hz. This simple approach, constructed using standard fiber-coupled telecom components, provides an efficient method for high-resolution, broadband spectral analysis. [ABSTRACT FROM AUTHOR]

Summary: Due to the rapid development of satellite laser communication technology, free‐space optical (FSO) links present a promising alternative to traditional radio frequency (RF) links. In this paper, taking the influence of weather factors into consideration, we investigate the performance of the hybrid FSO/RF links where the feeder link operates in the FSO band and the user link operates in the hybrid FSO/RF band. Specifically, the FSO feeder link is modeled by the gamma–gamma distribution in the presence of beam wander and pointing error, and the detection method adopts either the intensity modulation with direct intensity (IM/DD) or heterodyne detection. The RF user link is assumed to follow the shadowed Rician model. In addition, in order to improve the transmission rate of the link under the time‐varying satellite–terrestrial channel, a rate adaptation scheme is proposed. The performance of the system under study is evaluated in terms of the outage probability, average bit error rate (BER), and average transmission rate. Our results provide some important insights, for example, (1) due to the constraints of the feeder link and weather factors, there is an upper limit on the outage performance and bit error rate of the hybrid link; (2) the adaptive transmission strategy can significantly improve the transmission rate of the link compared with traditional design. [ABSTRACT FROM AUTHOR]

We investigate second-harmonic generation (SHG) light from a Pt surface in atmosphere under terahertz (THz) pulses. THz pulse-modulated SHG intensity, Δ I 2 ω , shows a clear time profile of the THz field, which is similar to that of the conventional electro-optic sampling. The result can be explained by interference between THz field-induced second-harmonic light from air molecules in an optical path and a local oscillator from a Pt surface, whereby heterodyne detection of the THz waveform can be achieved. Using numerical calculations of a wave equation, we discuss the contribution of the Gouy phases of all the pulses, including near-infrared, SHG, and THz pulses, and identification of effective nonlinear susceptibility of the Pt surface. Our method, simply using a polished metal surface and air molecules, does not suffer from phonons or phase matching in solid-state optics and does not require any power supply, bias voltage, or fabrication process, but it offers a simple and gapless sampling method for broadband THz pulses. Here, we demonstrate the gapless detection of a broadband THz pulse in the region of 0.2–20 THz using this method. [ABSTRACT FROM AUTHOR]

Coherent detection (homodyne detection/heterodyne detection) and single photon detection are commonly applied in classical communication and quantum communication/receiver respectively. In this paper, we firstly propose a quantum-enhanced receiver based on conjugated homodyne detection, which is consisted of two classical balanced homodyne detectors, for discrimination among quadrature-phase-modulated weak coherent states. Our detection part works as a photon counter in every single-shot measurement during the process of detection and feedback, which could help our quantum-enhanced receiver surpass the standard quantum limit when an optimized detection threshold τ from 0.05 to 0.3 is selected. Moreover, our proposed quantum-enhanced receiver with conjugated homodyne detectors can obtain the same low error probability as conventional quantum-enhanced receivers (with its superconducting nanowire single photon detectors' detection efficiency 60% for pulse separation M = 8 and 70% for M = 12 under the same adaptive feedback countermeasures.) Meanwhile, our scheme of the quantum-enhanced receiver gains its incomparable advantages on room-temperature working condition and low cost due to the application of conjugated homodyne detectors. As far as we concerned, this is the first time to explore the performance of the quantum-enhanced receiver with commercial homodyne detectors. And the analysis in this paper may pave the way to reduce the cost of the quantum-enhanced receiver and make it more adaptable for long-distance optical communication systems. [ABSTRACT FROM AUTHOR]

The wavelength modulation‐active laser heterodyne spectroscopy (WM‐ALHS), which introduces wavelength modulation technique into active laser heterodyne spectroscopy, is reported to enhance the gas standoff detection capability of weak return optical power. The ALHS gas standoff detection system is developed using a 1.65 μm butterfly packaged distributed feedback laser with tail fiber and an all‐fiber structure, which successfully detects methane concentrations at a distance of 35 cm using an aluminum plate as the reflective surface. Allan variance analysis shows that the system has good stability, and the detection limit at 29.2 s integration time is 0.84 ppm m. Standoff detection experiments with direct absorption‐active laser heterodyne spectroscopy (DA‐ALHS) and wavelength modulation spectroscopy (WMS) are carried out on the basis of the WM‐ALHS system and the detection limits of 4.49 and 1.64 ppm m were achieved within integration times of 18.6 and 29 s for DA‐ALHS and WMS, respectively. The superiority of the WM‐ALHS method compared to these two spectral detection methods is demonstrated. Furthermore, the feasibility of applying WM‐ALHS to gas standoff detection has been verified, laying the foundation for the development of this spectroscopic technology. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence causes signal beam wavefront distortion at the receiving end of a coherent detection system, which decreases the system mixing efficiency. Based on the coherent detection theory, this study establishes a mathematical model of wavefront distortion with mixing efficiency and mixing gain. It also analyzes the improvement limits of wavefront correction on mixing efficiency and mixing gain under different atmospheric turbulence intensities and experimentally measures them. Simulation results show that the mixing efficiency can be improved to 51%, 55%, and 60% after correcting for tilt, defocus, and astigmatism terms, respectively, when turbulence intensity D/r0 is 2. The mixing gain with homodyne detection is 3 dB higher than heterodyne detection. Meanwhile, the wavefront correction orders required for optimal mixing efficiency are higher than the heterodyne correction order. In the experiment, Haso4 NIR + DM 40 was used, and the turbulence intensity D/r0 was 2. After the closed-loop control algorithm corrects the tilt, defocus, and astigmatism terms, the indoor experimental results showed that the mixing efficiency is improved to 36%, 47%, and 62%, respectively. The outdoor experimental results showed that the mixing efficiency improved to 36%, 51%, and 68%, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we examine the performance of a free-space optical (FSO) communication system under imperfect Fisher-Snedecor F atmospheric turbulence channels. Taking into consideration the atmospheric turbulence, channel estimation errors, and pointing errors, simple and accurate analytical expressions for the probability density function and cumulative density function under intensity modulation direct detection (IM/DD) and heterodyne detection techniques are derived. Consequently, new analytical expressions for useful performance metrics such as outage probability, average bit error rate, and ergodic capacity are derived. To get more insight into the system diversity gain, asymptotic analytical expressions, in the high signal-to-noise ratio regime, for the outage probability and average BER are obtained. Our analysis is validated by means of numerically and Monte-Carlo simulations under various turbulence conditions and system parameters. The numerical results demonstrate that the derived analytical expressions are very accurate and provide precise evaluations for the proposed performance analyses. In addition, the results show that imperfect CSI seriously affects the FSO communication systems. However, the performance of heterodyne detection outperforms its counterpart the intensity modulation/direct-detection under all scenarios. [ABSTRACT FROM AUTHOR]

A novel centimeter-wave (CMW) and millimeter-wave (MMW) ultra-wideband (UWB) signal generator based on optical up-conversion technique is presented. A dual-parallel Mach–Zehnder modulator is employed to modulate the electrical baseband UWB signal to an optical carrier and eliminate the local oscillator component. Meanwhile, a Mach–Zehnder modulator and an optical bandpass filter produce a carrier-suppressed single sideband (CS-SSB) signal. Then, the UWB modulated optical signal is combined with the CS-SSB signal for photonic frequency up-conversion. Subsequently, the combined optical signal is injected into a balanced photodetector that is used for both heterodyne detection and low-frequency components suppression. A theoretical simulation and a proof-of-concept experiment are carried out to verify the feasibility of the proposed scheme. Eventually, a CMW-UWB signal is generated having a spectral power efficiency (SPE) of 54.25% and a 10-dB bandwidth of 5.89 GHz. An MMW-UWB signal is generated having an SPE of 54.12% and a 10-dB bandwidth of 5.67 GHz. Compared with other methods, the proposed generator can generate high-order CMW-UWB signal in a simpler structure since the up-conversion technique is employed. And the SSB up-conversion technique used in this generator overcomes the instability problem existing in the uncorrelated heterodyne method. The generated UWB signals both fulfil the requirements specified by the Federal Communications Commission. [ABSTRACT FROM AUTHOR]

In order to ensure the security of the local oscillator (LO) in continuous variable quantum key distribution (CV-QKD), the LO in CV-QKD based on the local LO (LLO) scheme no longer passes through the quantum channel. For the LLO scheme using homodyne detection, two orthogonal quadratures of reference pulse can only be measured sequentially, which, as a result, each signal pulse should be accompanied by a pair of identical reference pulses. However, due to the phase shift between two adjacent pulses, it is difficult to prepare dual reference pulses with the same phase relative to the LO. In this paper, we propose an improved method to get around this difficulty, which is demonstrated experimentally for a transmitter and a receiver connected by a 50 km single mode fiber. The phase noise measured in the experiment is 2.0 × 10 − 3 rad 2 , which is comparable to the recent experimental results using heterodyne detection. This method simplifies the preparation of phase reference pulses and removes the obstacles for the application of homodyne detection in the LLO scheme. In addition, this method can also be used for the compensation of quadrature imbalance in heterodyne detection, then the performance of heterodyne detection can be improved. [ABSTRACT FROM AUTHOR]

We demonstrate heterodyne detected transient vibrational sum frequency generation (VSFG) spectroscopy and use it to probe transient electric fields caused by interfacial charge transfer at organic semiconductor and metal interfaces. The static and transient VSFG spectra are composed of both non-resonant and molecular resonant responses. To further disentangle both contributions, we apply phase rotation to make the imaginary part of the spectra be purely molecular responses and the real part of the spectra be dominated by non-resonant signals. By separating non-resonant and molecular signals, we can track their responses to the transient electric-fields at interfaces independently. This technique combined with the phase sensitivity gained by heterodyne detection allows us to successfully identify three types of photoinduced dynamics at organic semiconductor/metal interfaces: coherent artifacts, optical excitations that do not lead to charge transfer, and direct charge transfers. The ability to separately follow the influence of built-in electric fields to interfacial molecules, regardless of strong non-resonant signals, will enable tracking of ultrafast charge dynamics with molecular specificities on molecular optoelectronics, photovoltaics, and solar materials. [ABSTRACT FROM AUTHOR]

Simulations were carried out to study the uniqueness range of Heterodyne Lock-in Carrierography (HeLIC) imaging measurements of carrier transport parameters (bulk lifetime, diffusion coefficient, and front and back surface recombination velocities) of silicon wafers. Theoretically, HeLIC can resolve all four transport parameters; however, in practice, the presence of experimental noise does not allow all those parameters to be measured reliably and uniquely from HeLIC images alone. An extended range of simultaneously determined unique transport parameters was attained through the combination of HeLIC camera pixel and photocarrier radiometry (PCR) experimental frequency response data via a multi-parameter fitting procedure. Two n- and p-type wafers with different surface recombination velocities were used to illustrate the simulation results. Quantitative HeLIC images of diffusion coefficients and surface recombination velocities were obtained, with bulk lifetime values determined from PCR frequency scan measurements. The combined approach was shown to be able to resolve all four transport parameters uniquely and thus reliably. [ABSTRACT FROM AUTHOR]

Polarization dependence of heterodyne-detected chiral vibrational sum frequency generation (VSFG) was examined for thin films of polylactic acids and neat limonene liquid far from electronic resonance. The enantiomers of polylactic acid films on silica substrates were successfully distinguished, and their chiral VSFG signals were ascribed not to bulk but to the interfaces by comparing chiral signals observed in reflection in the S-polarized VSFG, P-polarized visible, and P-polarized infrared and P-polarized VSFG, S-polarized visible, and P-polarized infrared polarization combinations with theoretical model calculations. In the same way, the chiral VSFG signal of neat limonene was assigned to bulk, which is consistent with the previous assignment. The method employed for assigning the source of chiral signals to the bulk or the interface may be useful for organic films on substrates with low refractive indices and thick samples. [ABSTRACT FROM AUTHOR]

A phase-sensitive optical time domain reflectometer based on coherent heterodyne detection of geometric phase in the beat signal of light, is reported for the first time to our knowledge. The use of the geometric phase to extract strain makes it immune to polarisation diversity fading. This is because a polarisation mismatch between the interfering beams is not a hindrance to its measurement. The geometric phase is calculated using the amplitude of the beat signal and individual beam intensities without any need for phase unwrapping. It is measured per beat period and can be equated with the traditionally measured dynamic phase with appropriate scaling. The results show that the system based on the geometric phase successfully measures strain, free from polarisation mismatch fading and phase unwrapping errors, providing a completely novel solution to these problems. [ABSTRACT FROM AUTHOR]

A three-color laser interferometer consisting of three Mach–Zehnder-type, one-color laser interferometers with heterodyne detection and coaxial laser beams is demonstrated. The three-color laser interferometer is considered as three sets of a two-color laser interferometer. From the two sets of the two-color laser interferometer, the value consisting only of the noise floor can be assessed. The noise floor can be reduced by subtracting the value consisting only of the noise floor from the measurement value obtained with the other two-color laser interferometer. In the case of the three lasers with wavelengths 9.25 μm, 10.59 μm, and 532 nm, a 15% noise reduction was obtained compared to the two sets of the two-color laser interferometers contained in the three-color laser interferometer. The 100-Hz noise reduction by 53% was achieved, and the other frequency noises were equal to or less than the smallest noise achieved by the two-color laser interferometers. The 100-Hz noise floor is caused by the vibration noise, which remains because of the non-coaxiality of the three beams. [ABSTRACT FROM AUTHOR]

The evolution of a quantum system subject to continuous weak measurement is described by a stochastic differential equation (SDE). It has been shown that the conventional density matrix ρt$\rho _t$ characterized by such SDE, under experimentally relevant settings, remains within a deterministically evolving manifold of low dimension, whose associated equations can be computed explicitly. In this paper, as a complement, it is demonstrated that the effect matrix Et$E_t$, associated to a quantum system at the time t to express its measurement during the interval [t,T]$[t,T]$, also remains confined to a deterministically evolving manifold. In particular, by using the Bloch sphere representation together with the properties of Pauli matrices (as the quantum representation of quaternions), deterministic submanifolds for the effect matrix concerning a monitored qubit are given explicitly. The qubit stays on a deterministically evolving curve in the homodyne amplitude detection scenario, and on a deterministically moving surface under heterodyne amplitude detection. This deterministic characterization should be of interest for parameter estimation and towards obtaining computationally efficient quantum filters for real‐time quantum control. [ABSTRACT FROM AUTHOR]

Since the start of the European Space Agency's Aeolus mission in 2018, various studies were dedicated to the evaluation of its wind data quality and particularly to the determination of the systematic and random errors in the Rayleigh-clear and Mie-cloudy wind results provided in the Aeolus Level-2B (L2B) product. The quality control (QC) schemes applied in the analyses mostly rely on the estimated error (EE), reported in the L2B data, using different and often subjectively chosen thresholds for rejecting data outliers, thus hampering the comparability of different validation studies. This work gives insight into the calculation of the EE for the two receiver channels and reveals its limitations as a measure of the actual wind error due to its spatial and temporal variability. It is demonstrated that a precise error assessment of the Aeolus winds necessitates a careful statistical analysis, including a rigorous screening for gross errors to be compliant with the error definitions formulated in the Aeolus mission requirements. To this end, the modified Z score and normal quantile plots are shown to be useful statistical tools for effectively eliminating gross errors and for evaluating the normality of the wind error distribution in dependence on the applied QC scheme, respectively. The influence of different QC approaches and thresholds on key statistical parameters is discussed in the context of the Joint Aeolus Tropical Atlantic Campaign (JATAC), which was conducted in Cabo Verde in September 2021. Aeolus winds are compared against model background data from the European Centre for Medium-Range Weather Forecasts (ECMWF) before the assimilation of Aeolus winds and against wind data measured with the 2 µm heterodyne detection Doppler wind lidar (DWL) aboard the Falcon aircraft. The two studies make evident that the error distribution of the Mie-cloudy winds is strongly skewed with a preponderance of positively biased wind results distorting the statistics if not filtered out properly. Effective outlier removal is accomplished by applying a two-step QC based on the EE and the modified Z score, thereby ensuring an error distribution with a high degree of normality while retaining a large portion of wind results from the original dataset. After the utilization of the described QC approach, the systematic errors in the L2B Rayleigh-clear and Mie-cloudy winds are determined to be below 0.3 m s -1 with respect to both the ECMWF model background and the 2 µm DWL. Differences in the random errors relative to the two reference datasets (Mie vs. model is 5.3 m s -1 , Mie vs. DWL is 4.1 m s -1 , Rayleigh vs. model is 7.8 m s -1 , and Rayleigh vs. DWL is 8.2 m s -1) are elaborated in the text. [ABSTRACT FROM AUTHOR]

The use of a heterodyne detection scheme in Kelvin probe force microscopy (KPFM) is an effective way for enhancing the performance of KPFM. However, this detection scheme generally has difficulty in detecting the first- and second-harmonic electrostatic forces simultaneously. To overcome this problem, we propose dual-bias modulation heterodyne frequency modulation KPFM (DM-hetero-FM KPFM), in which dual AC biases at 2 f 1 ± f m are applied between the tip and the sample. DM-hetero-FM KPFM enables us to measure the contact potential difference and capacitance gradient simultaneously at high frequencies (in the MHz range) beyond the bandwidth of phase-lock loop. Moreover, the present method allows us to perform it in the open-loop mode, which is highly desired for performing KPFM on semiconductors or in liquids at high frequencies. [ABSTRACT FROM AUTHOR]

Photoacoustic endoscopy shows promise in the detection of gastrointestinal cancer, inflammation, and other lesions. High-resolution endoscopic imaging of the hemodynamic response necessitates a small-sized, high-sensitivity ultrasound sensor. Here, we utilize a laser ultrasound sensor to develop a miniaturized, optical-resolution photoacoustic endoscope. The sensor can boost the acoustic response by a gain factor of ωo/Ω (the frequency ratio of the signal light and measured ultrasound) by measuring the acoustically induced optical phase change. As a result, we achieve a noise-equivalent pressure density (NEPD) below 1.5 mPa·Hz−1/2 over the measured range of 5 to 25 MHz. The heterodyne phase detection using dual-frequency laser beams of the sensor can offer resistance to thermal drift and vibrational perturbations. The endoscope is used to in vivo image a rat rectum and visualize the oxygen saturation changes during acute inflammation, which can hardly be observed with other imaging modalities. The authors developed an optical-fiber-based photoacoustic endoscope for gastroenterology. It can noninvasively view the internal vascular structures and visualize the hemodynamic response in a lesion. [ABSTRACT FROM AUTHOR]

Existing laser active imaging detection methods are limited by the detection mechanism, which can only capture the intensity image and distance image of the target, and lack practical detection solutions for acquiring images of the target motion state characteristics (non-macroscopic centroid motion, such as rotation, rolling, and other motion characteristics around the centroid). In this paper, a detection method using Geiger mode avalanche photodiode (GM-APD) array detector for heterodyne imaging to obtain images of the target motion state is proposed. An innovative method based on photon counting time interval calculation (PCTIC) is proposed to obtain the Doppler frequency shift of the target echo for each GM-APD pixel, which effectively eliminates useless data unrelated to the signal. With limited data storage and processing speed, the heterodyne imaging performance has been effectively improved to achieve high-precision heterodyne imaging detection of very weak target echoes and to support heterodyne imaging detection of large arrays with more image pixels in parallel. Experiments show that the method can achieve video-level detection at an imaging resolution of 32×32, and with a paucity of echo photons (detection probability of echo is 0.0032), a millimetre-level velocimetry measurement error can be achieved by accumulating only 6 echo photons per image element. The system also shows good resistance to turbulence interference, with target motion state characteristics detected based on velocity images remaining virtually unaffected under turbulence conditions caused by strong flames. This advancement is crucial for target detection in complex working environments such as smart manufacturing and autonomous driving. • Supports a larger array of heterodyne imaging detection with more image pixels in parallel. • Velocity images provide features of the target motion dimension in addition to distance and intensity. • PCTIC method can break the limitation of phase noise on conventional heterodyne imaging detection. [ABSTRACT FROM AUTHOR]

This paper investigates the ergodic capacity performance of hybrid free space optics (FSO)/ radio frequency (RF) system by employing an adaptive-combining-based switching scheme. In the adaptive combining scheme, the FSO and RF links are combined using the maximal-ratio combining (MRC) at the output, when the instantaneous signal-to-noise ratio (SNR) of the FSO link drops below a predefined threshold SNR value. The FSO link experiences the atmospheric turbulence, which is modeled using the generalized Malaga distribution, non-zero boresight misalignment or pointing errors, and atmospheric attenuation, under two types of detection techniques, i.e. heterodyne detection (HD) and intensity modulation/direct detection (IM/DD). Similarly, the fading of the RF link is characterized using the generalized $\kappa$–$\mu$ distribution. In particular, we derive the probability density function (PDF) of the instantaneous SNR of the adaptive combining scheme. Capitalizing on the SNR statistics, we obtain the unified closed-form ergodic capacity expressions for the adaptive-combining-based hybrid FSO/RF system. In addition, the asymptotic expressions for the ergodic capacity of the system at the high-SNR region are presented. All the derived analytical expressions are validated by using Monte-Carlo simulations. Numerical results and discussions are provided to compare the ergodic capacity performance of the adaptive-combining-based hybrid FSO/RF system with the existing system models such as single-link FSO, MRC-based hybrid FSO/RF, and hard-switching-based hybrid FSO/RF systems.

We consider the problem of recovering the physical parameters of ejecta from data of photon Doppler velocimetry (PDV). The results of PDV measurements of ejecta production from shock-loaded Pb samples are presented. Shockwaves in the samples were generated by an accelerated flyer-plate. Depending on the sample thickness, the shock-breakout pressure varied from 27.5 to 43 GPa. The ejecta transport occurred in the air. The spectral profile J ( ω ) of heterodyne beats underlies our analysis of the experiments. The noise-free component of | J ( ω ) | 2 is shown to obey the transport equation which accounts for multiple scattering and absorption of the probing beam in the cloud of moving particles. This provides a means for theoretical modeling of PDV data. Varying the values of the free-surface velocity and the parameters of the velocity distribution of ejecta, we fit the calculated spectrum to the PDV-measured one. We apply this method to time-resolved PDV spectra and, based on the ejecta optical model, recover the values of the free-surface velocity, the areal density, and the density-velocity distribution of ejecta. [ABSTRACT FROM AUTHOR]