Kun Qu

Dynamic Range Improvement for an Analog Photonic Link Using an Integrated Electro-Optic Dual-Polarization Modulator

A novel analog photonic link with enhanced spurious-free dynamic range (SFDR) based on an integrated electro-optic dual-polarization modulator is presented and demonstrated. The dual-polarization modulator, which consists of a polarization beam splitter, a polarization beam combiner, and two z-cut LiNbO3 MZMs, can achieve complementary intensity modulation along the orthogonal polarization directions with different modulation indexes. By properly adjusting the power relationship of the orthogonal polarization directions, two third-order intermodulation distortions (IMD3) have equal intensity and opposite phase and cancel each other out. Combined with a low-biasing technique to reduce the noise power, the SFDR of the link can be further improved. A theoretical analysis is presented and validated by a simulation. The IMD3 can be suppressed by 30.6 dB, giving an improvement in SFDR of about 22 dB compared with a conventional optical double-sideband modulation link.

An Optical Frequency Shifter Based on High-Order Optical Single-Sideband Modulation and Polarization Multiplexing

We propose and experimentally demonstrate a novel electro-optic frequency shifter with frequency multiplication operation based on high-order optical single-sideband (SSB) modulation and polarization multiplexing using an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator. By changing the dc bias phases of the modulator and the phase difference between the input RF signals, two-high-order SSB modulation signals are generated from the two QPSK modulators. After that, the two optical signals are polarization multiplexed and then projected to one direction to remove the low-order sidebands while only the high-order sideband is reserved. The scheme is numerically analyzed with the modulation indices of the modulator for optimal operation. A distinct advantage of this approach is the capability to generate a frequency shifting with a continuous and ultrawide tunable range (more than ±100 GHz), and the tuning operation is accurate, simple, and flexible. An experiment is carried out to achieve a frequency-doubled optical frequency shifter. A continuous frequency shifting ranged from -36 to 36 GHz is experimentally verified.

Photonically Assisted Microwave Signal Generation Based on Two Cascaded Polarization Modulators With a Tunable Multiplication Factor

Filterless photonic generation of microwave signals with various frequency multiplication factors via series connected polarization modulators is proposed and demonstrated experimentally. Frequency octupling, sextupling, or quadrupling microwave signal can be obtained when the polarization state of the light wave, the modulation indices of the modulators, and the phase of the radio frequency (RF) driving signal are suitably set. The optical sideband suppression ratios and RF spurious suppression ratios of the generated frequency multiplication signals are measured. Besides, the frequency-tunable ability and phase noise are analyzed.

Generation of Flat Optical Frequency Comb Based on a DP-QPSK Modulator

An approach to generate flat optical frequency comb (OFC) is proposed and experimentally demonstrated using a single integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator. By adjusting six dc bias points and two modulation indices of the modulator, the OFC with adjustable comb number and exactly the same intensity can be theoretically generated according to the equations derived from the DP-QPSK modulation model. Experimental results show that the proposed OFC generator can produce 7, 9, 11, and 13 lines with a spectral flatness of 0.96, 1.13, 2.01, and 1.17 dB, respectively. The unwanted-mode suppression ratio of the generated 7, 9, 11, and 13 lines OFCs are 10.33, 9.54, 6.79, and 10.45 dB, respectively. In addition, the generated OFC exhibits good frequency tunability. The nine-lines OFCs with frequency spacing of 6.02, 7.02, 8.02, and 9.02 GHz are experimentally generated.

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.

Microwave photonic image rejection mixer based on a DP-QPSK modulator

Abstract A simple image rejection mixer based on a dual-polarization quadrature phase shift keying modulator is presented without a 90° hybrid coupler in electrical field. By properly adjusting DC bias points in the integrated modulator and performing balanced photo-detection, the influence of image components can be well rejected. The simulation results show that a 52.24 dB image suppression ratio (ISR) and a conversion efficiency (CE) of −9.17 dB can be achieved by the proposed mixer. Besides, another research on the ISR and CE against non-ideal parameters is carried out. According to the simulation results, it can be figured that the main factor to influence the ISR and CE is the DC drifting which results both of them changed rapidly as the DC bias points deviate from the settled values. On the other hand, with different extinction ratios and LO signal powers and RF signal powers, the mixer still performs good ISR and CE.

Ultra-Flat and Broadband Optical Frequency Comb Generator via a Single Mach–Zehnder Modulator

An ultra-flat and broadband optical frequency comb (OFC) generator is proposed and experimentally demonstrated by directly modulating a dual-driven Mach–Zehnder Modulator (MZM) with only one RF signal. The proposed scheme can generate two forms of OFCs by simply adjusting the bias voltages of the MZM and the power of the RF signal. One form is a 20-line OFC with a frequency spacing equals the frequency of the RF signal, the other form is a 16-line OFC with a frequency spacing of two times the frequency of the RF signal. The generated OFCs have low power variation and broad bandwidth. The proposed scheme is verified by an experiment. A 20-line OFC with a frequency spacing of 10 GHz and flatness of 0.7 dB, and a 16-line OFC with a frequency spacing of 20 GHz and flatness of 0.3 dB are experimentally demonstrated.

Photonic Generation of Frequency and Bandwidth Multiplying Dual-Chirp Microwave Waveform

A photonic approach to generate a frequency and bandwidth multiplying dual-chirp microwave waveform using a single dual-polarization quadrature phase shift keying (DP-QPSK) modulator is proposed and demonstrated. In the proposed scheme, an RF carrier is applied to one QPSK modulator and a baseband single-chirp waveform is applied to another. By setting the driving signals applied to the modulator and adjusting the dc bias phases in the modulator, high-order optical sidebands can be obtained with the optical carrier suppressed. After optical to electrical conversion, a frequency-doubling and bandwidth-quadrupling, or frequency-quadrupling and bandwidth-octupling dual-chirp microwave can be generated. The approach is verified by simulation. Dual-chirp microwave waveforms with 16-GHz central frequency, 2048-MHz bandwidth and 32-GHz central frequency, 4096-MHz bandwidth are generated through an 8-GHz RF carrier and 512-MHz bandwidth baseband signal. The generated dual-chirp microwave waveform can be used in a radar system to improve its range-Doppler resolution.

Photonic microwave frequency conversion scheme with flexible phase shift

ABSTRACT In this paper, a single, flexible and stable optical frequency conversion scheme with tunable phase shift is proposed. It mainly consists of a dual-parallel binary phase shift keying (DP-BPSK) modulator and a polarization modulator (PolM). Properly setting the values of two DC bias points in the DP-BPSK modulator, the frequency conversion ability can be achieved. Meanwhile, the phase shift can also be realized by controlling the DC voltage that drives the PolM or tuning the polarization controller (PC3). The simulation results show that a 20 GHz RF signal can be converted into a 10 GHz IF signal with the conversion efficiency of −4.45 dB and a full 0° to 360° phase shift. It should be noted that the system performs well against different non-ideal factors such as extinction ratio, DC drifting or different LO and RF amplitudes. In addition, spurious free dynamic range of 82.14 dB·Hz2/3 is achieved and it can be greatly improved if a better extinction ratio is available.

A Linearized Analog Photonic Link Based on a Single z-Cut LiNbO3 Dual-Output Mach–Zehnder Modulator

We propose a linearized analog photonic link (APL) using a single z-cut LiNbO3 dual-output Mach–Zehnder modulator (MZM) to remove the third-order intermodulation distortion (IMD3). A polarizer, a z-cut LiNbO3 dual-output MZM, and a polarization beam combiner are combined to realize orthogonal polarized complementary intensity modulation with diverse modulation depth. Two IMDs3 with identical amplitude and adverse phase are eliminated destructively via appropriately controlling the optical power between two orthogonal polarized paths. A theoretical analysis is carried out and demonstrated by a simulation. Simulation results show that the spurious-free dynamic range of the proposed linearized APL is 24.5 dB and 18.5 dB higher than that of the conventional optical double sideband modulation link in back-to-back transmission and after 20 km transmission in single-mode fiber, respectively. The proposed scheme features simple structure with good performance.