Photonic-assisted multiple microwave frequency measurement with improved robustness.

A multiple microwave frequency measurement approach based on frequency-to-time mapping (FTTM) is reported. The FTTM is constructed by optical sideband sweeping and electric-domain intermediate frequency envelope monitoring. Two optimized operations are implemented. First, the use of balanced photodetection cancels out the beat components generated by the signals under test (SUT) themselves, so as to exclude frequency misjudgment. Second, a reference signal is introduced to map the SUT frequency to a relative time difference instead of an absolute time value, avoiding the measuring bias caused by time synchronization. As a result, the proposed scheme with improved robustness could be attractive for future practical applications. An experiment is performed. Microwave frequency measurement from 16 to 26 GHz is demonstrated, with an average error of 7.53 MHz.

Influence of Source Parameters on the Polarization Properties of Beams for Practical Free-Space Quantum Key Distribution.

Polarization encoding has been extensively used in quantum key distribution (QKD) implementations along free-space links. However, the calculation model to characterize channel transmittance and quantum bit error rate (QBER) for free-space QKD has not been systematically studied. As a result, it is often assumed that misalignment error is equal to a fixed value, which is not theoretically rigorous. In this paper, we investigate the depolarization and rotation of the signal beams resulting from spatially-dependent polarization effects of the use of curved optics in an off-axis configuration, where decoherence can be characterized by the Huygens-Fresnel principle and the cross-spectral density matrix (CSDM). The transmittance and misalignment error in a practical free-space QKD can thus be estimated using the method. Furthermore, the numerical simulations clearly show that the polarization effect caused by turbulence can be effectively mitigated when maintaining good beam coherence properties.

Free Space Measurement Device Independent Quantum Key Distribution with Modulating Retro-Reflectors under Correlated Turbulent Channel.

Modulating retro-reflector (MRR), originally introduced to support laser communication, relieves most of the weight, power, and pointing requirements to the ground station. In this paper, a plug-and-play measurement device independent quantum key distribution (MDI-QKD) scheme with MRR is proposed not only to eliminate detector side channels and allow an untrusted satellite relay between two users, but also to simplify the requirements set-ups in practical flexible moving scenarios. The plug-and-play architecture compensates for the polarization drift during the transmission to provide superior performance in implementing the MDI-QKD on a free-space channel, and the MRR device is adopted to relax the requirements on both communication terminals. A double-pass correlated turbulent channel model is presented to investigate the complex and unstable channel characteristics caused by the atmospheric turbulence. Furthermore, the security of the modified MDI-QKD scheme is analyzed under some classical attacks and the simulation results indicate the feasibility under the situation that the system performance deteriorates with the increase of fading correlation coefficient and the turbulence intensity, which provides a meaningful step towards an MDI-QKD based on the moving platforms to join a dynamic quantum network with untrusted relays.

Photonic 2-D angle-of-arrival estimation based on an L-shaped antenna array for an early radar warning receiver.

Early radar warning is a significant step to lessen the fine scanning range of a receiver. The small size two-dimension (2-D) angle-of-arrival (AOA) estimation part with moderate accuracy and sensitivity is important for an early radar warning receiver. In our method, we specially design an L-shaped antenna array (L-sAA) and connect it with dual-polarization binary phase shift keying modulator (DP-BPSKM). The dual-sideband (DSB) modulation is performed to transfer most of the optical power to electrical, so as to increase the sensitivity. It is also possible to map the AOA information of the incoming beam to photo-detected electrical power without a high extinction ratio modulator or optical filter. During the estimation, the 2-D AOA is firstly measured, whose measurement range is 18.22°âˆ¼90° and the measurement error is lower than 1°. Then, based on the 2-D AOA estimation results, the third one is mathematically calculated to construct 3-D location of the target. Noteworthy, the amplitude comparison function (ACF) is adopted in this method to make the system response irrelative to the received signal power, which endows the system with signal power fluctuation immunity. Experimental results show that this method is capable of measuring a single-tone signal and a bandwidth signal. This structure is very concise and meets the potential of on-chip integration.

Photonics-based multi-band linearly frequency modulated signal generation and anti-chromatic dispersion transmission.

A photonics-based anti-chromatic dispersion transmission scheme for multi-band linearly frequency modulated (LFM) signals is proposed and experimentally demonstrated. In the central station (CS), the key component is an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator, of which the up-arm and down-arm are driven by a microwave reference signal and an intermediate-frequency (IF) LFM signal respectively. By properly adjusting the DP-QPSK modulator, optical frequency comb (OFC) and frequency shift lightwave are generated. After polarization coupling and remote transmission, the orthogonal-polarization optical signals are introduced into balanced photodetector for heterodyne detection. Thence, multi-band LFM signals are generated and transmitted to remote base stations (BS) with the largest power for the anti-chromatic dispersion ability. Experiments are conducted to verify the analysis. Multi-band LFM signals at L (1.5 GHz), C (7 GHz), X (10 GHz), Ku (15.5 GHz) and K (18.5 GHz) bands with flatness of 1.9 dB are simultaneously obtained in the CS after 50 km fiber transmission, while the normally double-sideband modulation approach experiences a significant power fading for the fiber dispersion. Tunability of the system is evaluated, and detection performances of the generated signals are also analyzed.

Frequency downconversion with independent multichannel phase shifting and zero-intermediate-frequency receiving based on optical frequency shift and polarization multiplexing.

Photonic microwave frequency downconversion with independent multichannel phase shifting and zero-intermediate-frequency (IF) receiving via an integrated polarization multiplexing dual-parallel Mach-Zehnder modulator (MZM) is proposed. Based on the ideas of optical frequency shift and polarization multiplexing, the radio frequency (RF) signal is frequency downconverted to multichannel IF signals with the phases independently and arbitrarily tuned by adjusting the polarization controllers or even frequency downconverted to baseband directly by choosing two quadrature channels. In the simulation, the gain of our proposed frequency downconversion system is higher than that of the conventional two cascaded MZMs' system, and the phase shift with the range of 360° can be obtained concurrently. Furthermore, 2.5 Gbit/s RF vector signals centered at 10 GHz with different modulation formats are successfully demodulated.

Generation of a frequency-quadrupled phase-coded signal using optical carrier phase shifting and balanced detection.

A novel approach for photonic generation of a frequency-quadrupled phase-coded signal using optical carrier shifting and balanced detection is proposed and demonstrated. The key component of the scheme is an integrated dual-polarization quadrature phase shift-keying (DP-QPSK) modulator. In the modulator, an RF signal is applied to the upper QPSK modulator to generate high-order optical sidebands, while an electrical coding signal is applied to the bottom QPSK modulator to perform optical carrier phase shifting. After that, a frequency-quadrupled phase-coded signal with an exact π-phase shift is generated through balanced detection. The proposed scheme has a simple, compact structure and good tunability. Besides, a phase-coded pulse can be directly obtained when a three-level rectangular coding signal is applied. A proof-of-concept experiment is carried out. The generation of a 2-Gbit/s phase-coded signal with a frequency tuning from 12.12 to 28 GHz is experimentally demonstrated, and the generation of a phase-coded microwave pulse is also verified.

Displacement damage in bit error ratio performance of on-off keying, pulse position modulation, differential phase shift keying, and homodyne binary phase-shift keying-based optical intersatellite communication system.

Displacement damage (DD) effect induced bit error ratio (BER) performance degradations in on-off keying (OOK), pulse position modulation (PPM), differential phase-shift keying (DPSK), and homodyne binary phase shift keying (BPSK) based systems were simulated and discussed under 1 MeV neutron irradiation to a total fluence of 1×1012 n/cm2 in this paper. Degradation of main optoelectronic devices included in communication systems were analyzed on the basis of existing experimental data. The system BER degradation was subsequently simulated and the variations of BER with different neutron irradiation location were also achieved. The result shows that DD on an Er-doped fiber amplifier (EDFA) is the dominant cause of system degradation, and a BPSK-based system performs better than the other three systems against DD. In order to improve radiation hardness of communication systems against DD, protection and enhancement of EDFA are required, and the use of a homodyne BPSK modulation scheme is a considered choice.

Broadband linearized analog intersatellite microwave photonic link using a polarization modulator in a Sagnac loop.

A novel orthogonal polarization optical carrier suppression with carrier (OCS+C) modulation and a coherent balanced detection intersatellite microwave photonic link with improved signal-to-noise and distortion ratio (SNDR) is proposed. By bidirectional use of a polarization modulator in a Sagnac loop in conjunction with a polarization beam splitter and two polarization controllers, only the light wave along the clockwise direction is effectively modulated while the counterclockwise light wave is not modulated due to the velocity mismatch, which generates the orthogonal polarization OCS+C modulation signal to mitigate the third-order intermodulation distortion (IMD3) and the signal-amplifier spontaneous emission beating noise. By demultiplexing and adjusting the polarization of the orthogonal polarization OCS+C modulation signal, coherent balanced detection can be realized without a local oscillator signal in the receiver, which suppresses the second-order distortions. Thus, a broadband linearized intersatellite microwave photonic link with high SNDR is achieved. Simulation results show that the maximum SNDR of 36.2 dB can be obtained when the optimum modulation index is 0.26, which is 8 dB higher than our previously proposed intersatellite microwave photonic link with an optical preamplifier.

Filterless frequency 12-tupling optical millimeter-wave generation using two cascaded dual-parallel Mach-Zehnder modulators.

A novel frequency 12-tupling optical millimeter-wave (mm-wave) generation using two cascaded dual-parallel Mach-Zehnder modulators (DP-MZMs) without an optical filter is proposed and demonstrated by computer simulation. By properly adjusting the amplitude and phase of radio frequency (RF) driving signal and the direct current (DC) bias points of two DP-MZMs, a 120 GHz mm-wave with an optical sideband suppression ratio (OSSR) of 25.1 dB and a radio frequency spurious suppression ratio (RFSSR) of 19.1 dB is shown to be generated from a 10 GHz RF driving signal, which largely reduces the response frequency of electronic devices. Furthermore, it is also proved to be valid that even if the phase difference of RF driving signals, the RF driving voltage, and the DC bias voltage deviate from the ideal values to a certain degree, the performance is still acceptable. Since no optical filter is employed to suppress the undesired optical sidebands, a high-spectral-purity mm-wave signal tunable from 48 to 216 GHz can be obtained theoretically when a RF driving signal from 4 to 18 GHz is applied to the DP-MZMs, and the system can be readily implemented in wavelength-division-multiplexing upconversion systems to provide high-quality optical local oscillator signal.

Impact of pointing errors on performance of an intersatellite microwave photonics link with an optical preamplifier under dual-tone modulation.

The intersatellite microwave photonics link with an optical preamplifier is affected by third-order intermodulation distortion under dual-tone modulation and pointing errors due to beam wander, which would greatly degrade the link performance. An exact analytical expression for signal-to-noise and distortion ratio (SNDR) is derived considering the signal fade caused by the pointing errors of transceiver. It is shown that, given the desired SNDR and the rms random pointing jitter, an optimum modulation index of Mach-Zehnder modulator exists that minimizes laser output power. Moreover, an optimized model for laser output power and modulation index is established. The effects of the optical preamplifier gain and noise figure on the optimum link performance are also examined. Numerical results show that the minimum laser output power required to achieve the desired SNDR is more sensitive to the preamplifier noise figure. For an SNDR of 20 dB, doubling the preamplifier noise figure results in an 8.95 dB increase in minimum laser output power at the rms pointing jitter of 0.5 µrad.

Photonics-Based Simultaneous DFS and AOA Measurement System without Direction Ambiguity.

A novel scheme that can simultaneously measure the Doppler frequency shift (DFS) and angle of arrival (AOA) of microwave signals based on a single photonic system is proposed. At the signal receiving unit (SRU), two echo signals and the reference signal are modulated by a Sagnac loop structure and sent to the central station (CS) for processing. At the CS, two low-frequency electrical signals are generated after polarization control and photoelectric conversion. The DFS without direction ambiguity and wide AOA measurement can be real-time acquired by monitoring the frequency and power of the two low-frequency electrical signals. In the simulation, an unambiguous DFS measurement with errors of ±3 × 10-3 Hz and a -90° to 90° AOA measurement range with errors of less than ±0.5° are successfully realized simultaneously. It is compact and cost-effective, as well as has enhanced system stability and improved robustness for modern electronic warfare systems.

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.

Optimization of an analog intersatellite microwave photonics link with an optical preamplifier.

An exact analytical expression of the signal-to-noise ratio (SNR) for an intersatellite microwave photonics link with an optical preamplifier is derived considering the signal fade caused by the pointing errors of the transceiver, and an optimized model for laser output power and direct current (DC) bias phase shift of the Mach-Zehnder modulator is established. It is shown that, given the desired SNR and the root mean square (rms) random pointing jitter, an optimal DC bias phase shift exists that minimizes laser output power. The effects of the optical preamplifier parameters on the minimum laser output power and optimal DC bias phase shift are also examined. Numerical results show that the preamplifier noise figure determines the minimum laser output power needed to achieve the desired SNR but affects the optimal DC bias phase shift little. For a SNR of 20 dB, doubling the preamplifier noise figure results in a 6.36 dB increase in minimum laser output power for rms pointing jitter of 0.4 µrad.

Optimization of intersatellite microwave photonic links by utilizing an optical preamplifier under dual-tone modulation.

An optical preamplifier is utilized to improve the signal-to-noise and distortion ratio (SNDR) of intersatellite microwave photonic links employing a Mach-Zehnder modulator under dual-tone modulation. The resulting SNDR at an appropriate direct current (DC) bias phase shift is additionally investigated without small-signal approximation in order to optimize the performance of all the links. It is observed that the most limiting factor degrading the SNDR performance is changed, and the fundamental power is seen to increase more compared with the power of third-order intermodulation (IM3) plus noise due to the optical preamplifier. Thus, SNDR can be improved with respect to the case of a nonoptical preamplifier. For the preamplifier gain of 20 dB and noise figure of 3 dB, an increase of about 24 dB in optimum SNDR is accessible. In addition, the optimum DC bias phase shift is found to be insensitive to the preamplifier gain and noise figure, while the optimum SNDR is sensitive to the preamplifier gain and noise figure.