Down-converted Signal Research Articles

Super-resolution electromagnetic imaging with an optical frequency comb sampling via multi-dimensional signal separation

Electromagnetic sources show wide distribution, broad frequency coverage, and numerous quantities, posing challenges for traditional sensing techniques to achieve ultra-wideband, large-scale detection and localization. The “electromagnetic eye” imaging technique, inspired by the human eye, utilizes a Luneberg lens and a wideband optoelectronic sensing array as the electromagnetic “lens” and “retina,” respectively. This technique utilizes femtosecond optical pulse sampling reception to down-convert wideband signals, facilitating rapid, large range, and wideband sensing of multiple targets in complex electromagnetic environments. However, the limited aperture of the Luneberg lens results in diffraction-limited blurring, and optical down-conversion may lead to spectral aliasing, causing time-frequency-space overlap and reduced system resolution. In this paper, the frequency variation of the point spread function (PSF) in the wideband degraded images is analyzed, and a multi-dimensional joint super-resolution algorithm is proposed, which involves joint time-frequency-space diagonalization of eigenmatrices based on convolutional mixing array model. The concept is demonstrated through a four-sources imaging simulation achieving 2° resolution, breaking the Rayleigh limit 7.25 times. Furthermore, experimental results show 4-10 GHz imaging breaks the Rayleigh limit 4.5 times.

Optimized domain-engineered lithium niobate for electro-optic spectral tuning in a multi-wavelength optical parametric oscillator

We report on an electro-optically (EO) spectrum tunable, multiline optical parametric oscillator (OPO) based on a nonperiodically poled lithium niobate (NPPLN). Besides being an efficient optical parametric down converter (OPDC) for multi-wavelength signal generation, the NPPLN can function as an EO modulator for fast spectral tuning based on a unique asymmetric-duty-cycle (ADC) equivalent domain structure. We have developed a genetic algorithm to construct and optimize such an ADC equivalent domain configuration in NPPLN to maximize its EO spectral tuning rate with the least compromise of its nonlinear gain for conducting the multiple down-conversions. When the novel EO NPPLN device with an equivalent ADC of 29%/71% works in an OPO intracavity pumped by a diode-pumped, 1064-nm Nd:YVO4 laser, we measured EO tuning rates of ∼0.5 and ∼0.53 nm/(kV/mm) for the down-converted dual signals at 1540 and 1550 nm, respectively. The tuning rates have been three orders of magnitude higher than a conventional one without an engineered domain asymmetry and 12% greater than another domain asymmetric LN device designed by a simulated annealing optimization method. The EO tuning technology developed in this study can be potentially implemented in micro/nano-photonic LN waveguide circuits.

Photonic architecture for remote multi-parameter measurement and transmission of microwave signals

A photonic architecture for remote multi-parameter measurement and transmission of microwave signals is proposed and demonstrated, which utilizes a dual-parallel dual-drive Mach-Zehnder modulator (DP-DDMZM) in the antenna unit and a dual-drive Mach-Zehnder modulator (DDMZM) in the processing unit. Doppler frequency shift (DFS) and angle of arrival (AOA) can be determined by analyzing the down-converted intermediate frequency signals. Introducing a reference signal in the processing unit ensures DFS measurement without directional ambiguity. The proposed architecture can also be applied for instantaneous frequency measurement based on down-conversion. Due to the use of optical single sideband modulation, long-distance transmission of radio frequency (RF) signals without dispersion-induced power fading can be achieved. Experiments for accurate and stable DFS and AOA measurement as well as long-distance RF signal transmission with dispersion-induced power fading are presented. The approach avoids the use of optical filters and polarization-related devices, facilitating wideband and stable operation, which is highly desirable. The proposed architecture is a potential solution for microwave photonic antenna remoting, offering support for both remote transmission and multi-parameter measurement.

Confirming X-ray parametric down conversion by time–energy correlation

We present measurements of X-ray Parametric Down Conversion at the Advanced Photon Source synchrotron facility. Using an incoming pump beam at 22 keV, we observe the simultaneous, elastic emission of down-converted photon pairs generated in a diamond crystal. The pairs are detected using high count rate silicon drift detectors with low noise. Production by down-conversion is confirmed by measuring time–energy correlations in the detector signal, where photon pairs within an energy window ranging from 10 to 12 keV are only observed at short time differences. By systematically varying the crystal misalignment and detector positions, we obtain results that are consistent with the constant total of the down-converted signal. Our maximum rate of observed pairs was 130/h, corresponding to a conversion efficiency for the down-conversion process of 5.3±0.5×10−13.

Experimental Evaluation of All-Optical Up- and Down-Conversion of 3GPP 5G NR Signals using an Optomechanical Crystal Cavity Frequency Comb

Experimental Evaluation of All-Optical Up- and Down-Conversion of 3GPP 5G NR Signals using an Optomechanical Crystal Cavity Frequency Comb

Microwave photonic I/Q mixer for wideband frequency downconversion with serial electro-optical modulations.

In this paper, we propose a serial electro-optical (EO)-modulation-based microwave photonic in-phase and quadrature (I/Q) mixer and investigate its performance for wideband frequency downconversion. The proposed I/Q mixer uses two EO modulators and a programmable optical processor in a serially cascaded structure, which ensures good phase stability and flexibility to achieve high-performance broadband frequency downconversion. A proof-of-concept experiment is carried out in which the frequency downconversion of the RF signals in the range from 10 to 40 GHz is demonstrated with an average image rejection ratio of 38.66 dB. The spurious-free dynamic range (SFDR) measured at around 15 GHz is 86 dBc·Hz2/3. Based on this microwave photonic I/Q mixer, a broadband radar receiver is established and de-chirping of linearly frequency modulated (LFM) radar echoes with an instantaneous bandwidth of 8 GHz (10-18 GHz) is achieved. The results verify its feasibility for high-performance I/Q mixing in practical applications.

High cost-effective photonic assisted in-band interference cancellation scheme for centralized radio access network towards future 5G communication

A photonic-assisted scheme capable of self-interference cancellation (SIC) and image rejection mixing (IRM) is demonstrated to solve the intractable problem of in-band interference for in-band full duplex (IBFD) systems with high frequency and large bandwidth. In the proposed scheme, a single optical path configuration is constructed based on an integrated modulator applied as the main device. The SIC operation is implemented in the optical domain making it immune to the influence caused by fiber dispersion. Owing to the intermediate frequency (IF) signal amplitude regulation mechanism under the combined action of fiber dispersion induced phase and double sideband (DSB) modulation, the scheme can realize IRM without increasing the system complexity. Experimental results show that the 50 MHz 16-quadrature amplitude modulation (16-QAM) signal of interest (SOI) centered at different frequencies are down-converted to 2.5 GHz. The depth of SIC and image-rejection (IR) is over 35 dB and 25 dB after transmitted through a span of 10.1 km optical fiber. The received 16-QAM down-converted signal can be recovered with an error vector magnitude (EVM) which satisfies the requirement of 3GPP-specified EVM limit. The signal recovery performance, SIC depth and image rejection ratio (IRR) under different signal to interference ratio (SIR) is also investigated. In addition, a comparison of relevant literature with this scheme from the perspective of technical discussion is also presented. The proposed scheme can realize effective functional integration with compact structure, simple parameter tuning mechanism, improved stability and high cost-effectiveness to solve the in-band interference problem which is of great value to be applied in IBFD centralized radio access networks (CRAN) towards future 5G communication.

Broadband frequency modulation signal downconversion using a monolithic integrated mutual injection laser.

We propose a new, to the best of our knowledge, broadband signal downconversion scheme implemented by a monolithic integrated mutual injection laser. A mathematical derivation, simulation, and experimental verification are carried out. Because the period-one oscillation frequency can be selectively operated on a large scale by controlling the current on the integrated laser, the tuning downconversion range is realized without changing the experimental equipment. The experiment verifies that the downconversion of the linear frequency modulation signal with a bandwidth of 0.5GHz from the center frequency of 18.75 to 0.85GHz, and the spurious-free dynamic range (SFDR) has reached 71.7d B/H z 2/3. Compared with the scheme based on discrete components, the system has no electric local oscillator or external modulator, which provides a method for radar signal downconversion.

Remote mmW photonic local oscillator delivery for uplink down-conversion in DML-based optical hybrid C-RAN fronthaul

In this work, we propose and demonstrate the remote delivery of a photonically generated local oscillator (LO) signal for uplink down-conversion in photonically generated millimeter-wave (mmW) signals over duplex centralized radio access network fronthaul. The technique, based on carrier-suppressed external modulation in a single Mach–Zehnder modulator, is employed for generating the mmW signals, which are distributed over hybrid optical links composed of fiber and free-space optics (FSO) prior to the radio wireless link at 42 GHz, and also for the uplink (UL) LO generation enabling the 38 GHz UL signal down-conversion. A 5G new radio (NR) quadrature phase-shift keying (QPSK) and 64-quadrature amplitude modulation (QAM) signal transmission is successfully demonstrated using low-frequency directly modulated lasers for both the downlink and the UL mmW transmission, and thus a low-cost and simple intermediate frequency over fiber and FSO UL deployment are presented. The lowest achieved error vector magnitudes with the 5G NR signals at 100 MHz bandwidth in the proposed mmW UL schemes, including 10 km of optical fiber and a 1.1 m long FSO link, are 5.8% and 6% for QPSK and 64-QAM, respectively.

Frequency stabilization of a terahertz quantum-cascade laser to the Lamb dip of a molecular absorption line.

We demonstrate the frequency stabilization of a terahertz quantum-cascade laser (QCL) to the Lamb dip of the absorption line of a D2O rotational transition at 3.3809309 THz. To assess the quality of the frequency stabilization, a Schottky diode harmonic mixer is used to generate a downconverted QCL signal by mixing the laser emission with a multiplied microwave reference signal. This downconverted signal is directly measured by a spectrum analyzer showing a full width at half maximum of 350 kHz, which is eventually limited by high-frequency noise beyond the bandwidth of the stabilization loop.

Photonic system for Doppler-frequency-shift and Angle-of-arrival simultaneous measurement using dual-parallel Mach–Zehnder modulator

A simple photonic system is proposed to measure the Doppler frequency shift (DFS) and Angle of arrival (AOA) of microwave signals simultaneously. The echo signals received by the two antennas are fed into two sub-MZM of a dual-parallel Mach–Zehnder modulator (DPMZM) respectively, and the reference signals are sent into one of the sub-MZM. The introduction of reference signals makes it easier to determine the direction of DFS. After the reference signal and echo signal pass through the low-speed PD, the beating signal is obtained. The value and direction of the DFS can be derived from the frequency of the down-converted signal. And the measurement of AOA can be calculated from the power of the output signal. The experimental results show that the echo signal measurement errors of DFS are less than ±0.5 Hz in the operating frequency range of 20 GHz, and the power sensitivity is less than -40 dBm. AOA has measurement errors of less than ±1.5° in the range of 0 to 90°. In addition, by adding only one test branch, the system can achieve the unambiguous measurement of AOA in the range of ±90°.

Photonic-assisted multi-frequency measurement based on frequency-to-power mapping and serial channelization

A novel photonic approach to multi-frequency measurement based on frequency-to-power mapping and serial channelization is proposed and demonstrated. In this proposal, a light wave from a laser diode (LD) is split into two branches. In the upper branch, a dual-port Mach–Zehnder interferometer (MZI) is used to perform frequency-to-power mapping. While in the lower branch, an optical wavelength scanner based recirculating frequency shifting (RFS) loop is used to generate multiple serial optical local oscillators (LOs) to down-convert the outputs of the dual-port MZI at the corresponding time window. Therefore, the unknown radio-frequency (RF) frequencies can be unambiguously estimated by the multiple monotonous amplitude comparison functions (ACFs) obtained from the power ratios between two down-converted electrical signals. Only two low-frequency photodetectors (PDs) with narrow bandwidth are employed, which guarantees a simple structure and low cost. In addition, the proposal has robustness to the bias drift of the MZM and input RF power. Simulation results indicate that the multi-frequency identification within 5 μs is achieved over the measurement range up to 19 GHz with an accuracy better than ± 20 MHz. Furthermore, the system is easily extended to higher frequency and larger bandwidth (e.g., 100 GHz).

Photonic-Assisted Scheme for Simultaneous Self-Interference Cancellation, Fiber Dispersion Immunity, and High-Efficiency Harmonic Down-Conversion.

A photonic approach to the cancellation of self-interference in the optical domain with fiber dispersion immunity and harmonic frequency down-conversion function is proposed based on an integrated, dual-parallel, dual-drive Mach-Zehnder modulator (DP-DMZM). A dual-drive Mach-Zehnder modulator (DMZM) is used as an optical interference canceller, which cancels the self-interference from the impaired signal before fiber transmission to avoid the effect of fiber transmission on the cancellation performance. Another DMZM is used to provide carrier-suppressed, local-oscillation (LO)-modulated, high-order double optical sidebands for harmonic frequency down-conversion to release the strict demand for high-frequency LO sources. By regulating the DC bias of the main modulator, the signal of interest (SOI) can be down-converted to the intermediated frequency (IF) band after photoelectric conversion with improved frequency-conversion efficiency, immunity to the fiber-dispersion-induced power-fading (DIPF) effect, and effective signal recovery. Theoretical analyses and simulation results show that the desired SOI in the X and K bands with a bandwidth of 500 MHz and different modulation formats can be down-converted to the IF frequency. The self-interference noise with the 2 GHz bandwidth is canceled, and successful signal recovery is achieved after a 10 km fiber transmission. The recovery performance of down-converted signals and the self-interference cancellation depth under different interference-to-signal ratios (ISRs) is also investigated. In addition, the compensation performance of DIPF is verified, and a 6 dB improvement in frequency conversion gain is obtained compared with previous work. The proposed scheme is compact, cost-effective, and thus superior in wideband self-interference cancellation, long-range signal transmission, and effective recovery of weak desired signals.

Architecture for integrated RF photonic downconversion of electronic signals.

Electronic analog to digital converters (ADCs) are running up against the well-known bit depth versus bandwidth trade off. Towards this end, radio frequency (RF) photonic-enhanced ADCs have been the subject of interest for some time. Optical frequency comb technology has been used as a workhorse underlying many of these architectures. Unfortunately, such designs must generally grapple with size, weight, and power (SWaP) concerns, as well as frequency ambiguity issues which threaten to obscure critical spectral information of detected RF signals. In this work, we address these concerns via an RF photonic downconverter with potential for easy integration and field deployment by leveraging a novel, to the best of our knowledge, hybrid microcomb/electro-optic comb design.

Stimulated Brillouin Scattering Based Image-Reject Microwave Signal Harmonic Down-Converter

We propose and demonstrate a photonic microwave signal harmonic down-converter with image rejection capability via stimulated Brillouin scattering (SBS). A dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) is driven by a radio frequency/image (RF/IM) signal and a local oscillation (LO) signal to realize equivalent phase modulation and 4th-order double sideband modulation respectively. The optical carriers from the two arms of the DP-DPMZM can cancel out each other after polarization combining, thereby increasing the useful optical signal power in photodetection. Since the beating notes between the IM-modulated signals and LO-modulated 4th-order sidebands interfere destructively, the down-converted IM signals disappear naturally, which makes the down-converted image parts be suppressed in optical domain. By using an SBS-based dual-band microwave photonic filter to suppress the RF modulated -1st-order sideband and amplify the +1st-order sideband, destructive interference can be broken. Therefore, the RF signal over frequency range of 21-28 GHz can be down-converted to intermediate frequency (IF) band, and the image rejection ratio (IRR) can be also improved to more than 55 dB. Under -160 dBm/Hz noise floor, the SFDR is 105.1 dB·Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> .

Ultra-Wide and Tunable Microwave Frequency Down-Conversion Based on Re-Circulating Opposite Frequency Shifting

A photonic-assisted scheme is proposed and demonstrated for microwave frequency down-conversion based on re-circulating opposite frequency shifting. The proposed method consists of a microwave signal receiver and a re-circulating opposite frequency shifter. The microwave signal under down-conversion is received by electro-optical modulation (EOM). The microwave frequency is down-converted with a ring-assisted re-circulating EOM, in which the 0th and 1st optical sidebands of the microwave signal are frequency shifted in the opposite direction with the frequency step of the local oscillator (LO) signal in optical frequency domain, respectively. Two optical frequency combs (OFCs) with the same frequency spacing and different center frequency are generated. After photodetection, the frequency of the down-converted IF signal is the frequency difference between the generated two OFCs. The re-circulating opposite frequency shifter enables accurately tunable and ultra-wide frequency operation of down-conversion by simply adjusting the low-frequency electrical LO. As compared with the conventional EOM-based methods, our method eliminates the bandwidth limitation of devices for the wideband LO signals EOM. Prior to the conventional OFC-based methods, ours enables flexible frequency down-conversion by tuning the frequency of LO. In the demonstrations, 6∼40 GHz radio-frequency (RF) signals are experimentally down-converted to IF band below 3 GHz with a low-frequency electrical LO in the range of 6∼6.5 GHz.

Sampling Parallel SOA-MZIs Configuration for All-Optical Simultaneous Frequency Down-Conversion

In this paper, we expound a modulation concept to contrive simultaneous frequency down-conversion based on a three parallel Semiconductor Optical Amplifier Mach-Zehnder Interferometers (SOA-MZIs) link by using a band pass sampling method in a Virtual Photonics Inc. simulator. Each SOA-MZI is deployed to achieve a down-converted signal, which has ten replicas related to the first ten harmonic ranks of the sampling signal, at the SOA-MZI outer port. Then, the admixture of the three down-converted signals yields a sampled signal, which is called a simultaneous down-converted signal that contains thirty different replicas. The positive down-conversion gains with top values are reached with the sampling parallel SOA-MZIs link. Moreover, we evaluated the quality of the parallel SOA-MZIs transmission system over orthogonal frequency division multiplexing (OFDM) complex modulated signals using the error vector magnitude values as a performance index. The utmost bit rate attained is 2 Gbit/s for OFDM modulations.

Quadrature amplitude modulation frequency up- and down-conversions using a positive-intrinsic-negative photodiode sampling mixer

Simulation and experimental performance analysis of a positive-intrinsic-negative photodiode (PIN-PD) sampling mixer used as frequency up and down-converters are presented. Besides, a frequency mixing with a PIN-PD can operate in a frequency range up to 47.3 GHz for up-conversion and 38.5 GHz for down-conversion. The design and operating regime peculiarities of a PIN-PD as an up and down-converter in a radio over fiber (RoF) system are also discussed. The conversion gain of a PIN-PD sampling mixer is simulated and measured. We showed that the gain may decrease from 27 dB at a mixing frequency of 8.3 GHz to 3 dB at the one with 47.3 GHz in an up-conversion mode and from 26.5 dB at 31.7 GHz to 7 dB at 38.5 GHz in a down-conversion mode, when the PIN-PD bias voltage was 0 V. The conversion gain degrades with the decrease of the bias voltage from 0 to −5 V. Error vector magnitudes (EVMs) of the quadratic amplitude modulation (M-QAM) up and down-converted signals are investigated. Quadrature phase shift keying (QPSK) data at bit rates between 312.5 Mbit / s and 5 Gbit / s are frequency-converted. The EVM is used a performance index to assess the mixing system. Exploitable EVMs in the range of 15% up to 34.9% are reached for up-conversion of the mixing frequency. For down-conversion, the EVM varies from 20% to 34.9% at 5 Gbit / s over the whole frequency range. The EVM of mixed signals is improved with 16-QAM and 64-QAM modulations.

A Pulsed-Coherent Lidar With Sub-10 μm Precision

This article presents a pulsed-coherent lidar that uses a 19-GHz pulse-modulated optical carrier and segmented time-of-flight (ToF) detection. The receiver analog frontend (AFE) employs a down-conversion chain that consists of a phase-invariant programmable-gain low-noise amplifier (PI-PGLNA), a sub-harmonic mixer (SHM), and an inverter-based phase-invariant programmable-gain amplifier (PI-PGA). The local oscillator (LO) is generated from a phase-locked loop (PLL) with a ring-type voltage-controlled oscillator (VCO). An analog-to-digital converter (ADC) subsamples the down-converted signal, and a digital signal processor (DSP) calculates the coarse, intermediate, and fine ToF. At the receiver input, a narrowband matching is implemented between the p-i-n photodiode and the PI-PGLNA with direct wire bonding. The system achieves 6- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> precision with a 5-MSa/s sampling rate at a 2.5-m distance. The 2-D scanning is achieved with this lidar with the use of an MEMS mirror.

Dual-Output Microwave Photonic Frequency Up- and Down-Converter Using a 90° Optical Hybrid Without Filtering

A dual-output microwave photonic frequency up- and down-converter using a 90° optical hybrid without filtering is proposed. By using a dual-parallel Mach-Zehnder modulator and a Mach-Zehnder modulator in parallel to realize carrier suppressed single sideband and carrier suppressed double sideband modulation, respectively, with a 90° optical hybrid and 90° electrical hybrid coupler, this scheme can implement two functions: simultaneous up- and down-conversion signal generation, or high image rejection in down-conversion. Simulation results show that the proposed structure can achieve an up-conversion with a tunable range of 17–23 GHz or a down-conversion with a tunable range of 1–7 GHz, respectively, based on a fixed 8 GHz LO signal. The unwanted signal suppression ratio of desired signal is about 27.2 dB for up-conversion and 30.0 dB for down-conversion. In addition, we have tested and analyzed the conversion gain, noise figure and spurs free dynamic range of the system. For image rejection in down-conversion, we can achieve an image rejection ratio (IRR) about 65.2 dB. Moreover, the impact of the phase drift, RF frequency, amplitude and phase variation on IRR is discussed. Finally, the influence of half-voltage imbalance of the DPMZM sub-modulators on IRR is analyzed, and a compensation method is given.