Wen Hui Sun

Triangular Microwave Waveforms Generation Based on an Optoelectronic Oscillator

We demonstrate a novel scheme to generate full-duty-cycle triangular microwave waveforms using an optoelectronic oscillator (OEO). The OEO produces high-quality local oscillator signal. Thus, an external microwave source, which is usually involved in orthodox approaches obviates the need. A polarization modulator is involved in both the OEO loop and a microwave photonic filter (MPF) structure. To generate a triangular waveform, the undesired electrical harmonics are suppressed by the MPF. The proposed method is theoretically analyzed and experimentally verified. Triangular waveforms at fundamental tone of 4.6 GHz and frequency doubling tone of 9.2 GHz are generated, which fit well with the stimulated ones. The root mean square errors between the generated and the simulated triangular waveforms are 6.6e-5 and 1.61e-3 for triangular pulses at 4.6 and 9.2 GHz, respectively.

Widely tunable single bandpass microwave photonic filter based on Brillouin-assisted optical carrier recovery

A widely tunable single bandpass microwave photonic filter (MPF) based on Brillouin-assisted optical carrier recovery in a highly nonlinear fiber (HNLF) with only one optical filter is proposed and experimentally demonstrated. The fundamental principle lies in the fact that the suppressed optical carrier of the phase modulated optical signal could be recovered by the stimulated Brillouin scattering (SBS) amplification effect. When phase modulated optical signals go through an optical filter with a bandpass response, the optical carrier and the upper sidebands suffer from the suppression of the optical filter because they fall in the stopband of that. In our system, the optical carrier could be recovered by the SBS operation around 38 dB. The MPF is achieved by one-to-one mapping from the optical domain to the electrical domain only when one of phase modulated sidebands lies in the bandpass of the optical filter. It shows an excellent selectivity with a 3-dB bandwidth of 170 MHz over a tuning frequency range of 9.5-32.5 GHz. The out-of-band suppression of the MPF is more than 20 dB. Moreover, the MPF shows an excellent shape factor with 10-dB bandwidth of only 520 MHz. The frequency response of the MPF could be widely tuned by changing the frequency difference between the frequency of the optical carrier and the center frequency of the bandpass of the optical filter. A proof-of-concept experiment is carried out to verify the proposed approach.

Reduction of Measurement Error of Optical Vector Network Analyzer Based on DPMZM

We propose a new method to reduce the measurement error of an optical vector network analyzer (OVNA) based on a dual-parallel Mach-Zehnder modulator (DPMZM). Even-order sidebands suppressed single-sideband modulation is realized by the joint use of the DPMZM and an optical filter. The transmission response of a device-under-test is characterized based on one-to-one mapping between the optical and electrical domains. The measurement error is significantly reduced because the even-order sidebands, which are the dominant error source, are suppressed. The proposed scheme is theoretically analyzed and experimentally verified. The experimental results show that the proposed OVNA has excellent measurement accuracy and improved dynamic range.

Generation of Flat Optical Frequency Comb Using a Single Polarization Modulator and a Brillouin-Assisted Power Equalizer

We propose a novel approach for the generation of flat optical frequency comb (OFC) using a single polarization modulator (PolM) and a Brillouin-assisted power equalizer (BAPE). The reference radio-frequency (RF) signal applied to the PolM is provided by an optoelectronic oscillator (OEO). The spectra of the optical signals launched to the BAPE are optimized in advance for the generation of OFCs with different number of lines. The BAPE is introduced to flatten the uneven OFC lines by attenuating the optical lines having power beyond the threshold of the stimulated Brillouin scattering (SBS) and amplifying the ones having power below the SBS threshold with the amplifier inside the BAPE. In addition, the OFC generator is optical wavelength independent since there is no optical filter involved in our approach. We experimentally demonstrated that the proposed OFC generator can produce 5, 7, 9, and 11 lines within a spectral flatness of 0.5, 1.5, 2, and 4.1 dB, respectively.

Photonic generation of background-free millimeter-wave ultra-wideband pulses based on a single dual-drive Mach–Zehnder modulator

We propose a novel photonic approach for generating a background-free millimeter-wave (MMW) ultra-wideband (UWB) signal based on a conventional dual-drive Mach-Zehnder modulator (DMZM). One arm of the DMZM is driven by a local oscillator (LO) signal. The LO power is optimized to realize optical carrier suppressed modulation. The other arm is fed by a rectangular signal. The MMW UWB pulses are generated by truncating the continuous wave LO signal into a pulsed one in a photodetector (PD). The generated MMW UWB signal is background-free by eliminating the baseband frequency components because the optical power launched to the PD keeps constant all the time. The proposed method is theoretically analyzed and experimentally verified. The generated MMW UWB signal centered at a frequency of 26 GHz meets the Federal Communications Commission spectral mask very well.

Optically controlled microwave phase shifter based on nonlinear polarization rotation in a highly nonlinear fiber

This Letter reports an optically controlled microwave phase shifter with an ultra-wideband working bandwidth and a full 360° phase shifting range based on nonlinear polarization rotation (NPR) in a highly nonlinear fiber (HNLF). A continuous wave probe light is modulated by a polarization modulator (PolM) that is driven by a microwave signal to be phase shifted. The optical carrier and the first-order sidebands of the probe light experience different phase shifts due to the NPR induced by the control light in the HNLF. An optical bandpass filter is used to realize single-sideband modulation of the probe light by removing one of the first-order sidebands, as well as to reject the control light. After detecting by a photodetector, the phase of the recovered microwave signal is continuously tunable by adjusting the power of the control light. The proposed approach is theoretically analyzed and experimentally verified. A full 360° tunable phase shift is realized over an ultra-wideband frequency range from 8 to 38 GHz when the power of the control light is tuned from 0 to 570 mW.

Photonic generation of arbitrarily phase-modulated microwave signals based on a single DDMZM

We propose and demonstrate a compact and cost-effective photonic approach to generate arbitrarily phase-modulated microwave signals using a conventional dual-drive Mach-Zehnder modulator (DDMZM). One arm (arm1) of the DDMZM is driven by a sinusoidal microwave signal whose power is optimized to suppress the optical carrier, while the other arm (arm2) of the DDMZM is driven by a coding signal. In this way, the phase-modulated optical carrier from the arm2 and the sidebands from the arm1 are combined together at the output of the DDMZM. Binary phase-coded microwave pulses which are free from the baseband frequency components can be generated when the coding signal is a three-level signal. In this case, the precise π phase shift of the microwave signal is independent of the amplitude of the coding signal. Moreover, arbitrarily phase-modulated microwave signals can be generated when an optical bandpass filter is attached after the DDMZM to achieve optical single-sideband modulation. The proposed approach is theoretically analyzed and experimentally verified. The binary phase-coded microwave pulses, quaternary phase-coded microwave signal, and linearly frequency-chirped microwave signal are experimentally generated. The simulated and the experimental results agree very well with each other.

Photonic-assisted microwave phase shifter using a DMZM and an optical bandpass filter

We propose and demonstrate a photonic-assisted wideband 360° microwave phase shifter based on a conventional dual-drive Mach-Zehnder modulator (DMZM) and an optical bandpass filter (OBPF). The two arms of the DMZM are driven by the fundamental microwave signal to be phase shifted and its frequency doubled component, respectively. The OBPF followed after the DMZM is used to remove the optical carrier and the sidebands at either side of the optical carrier. As a result, only two sidebands corresponding to the fundamental microwave signal and its frequency doubled component, respectively, are left. Moreover, the phase shift between the two sidebands can be continuously tunable by adjusting the bias voltage of the DMZM. This phase shift is mapped to the fundamental microwave signal which is recovered by beating the two sidebands in a photodetector (PD). The proposed approach is theoretically analyzed and experimentally verified.

Photonic generation of triangular pulses based on nonlinear polarization rotation in a highly nonlinear fiber

We propose a novel method to generate triangular pulses based on the nonlinear polarization rotation (NPR) effect in a highly nonlinear fiber. A continuous wave probe beam is polarization-rotated by an intensity-modulated control beam via the NPR effect. A polarization-division-multiplexing emulator is exploited to split the probe beam into two orthogonally polarized states with imbalanced time delay. After detection by a photodetector, a 90° microwave phase shifter is used to compensate the phases of the fundamental and the third-order harmonic components in order to generate triangular pulses. Triangular pulses at 5 and 6 GHz with full duty cycles are experimentally generated. The root mean square errors between the generated and the simulated waveforms are 3.6e-4 and 1e-4 for triangular pulses at 5 and 6 GHz, respectively.

Optical Vector Network Analyzer With Improved Accuracy Based on Brillouin-Assisted Optical Carrier Processing

We propose a new method to improve the accuracy of an optical vector network analyzer (OVNA) based on stimulated Brillouin scattering (SBS) in a dispersion-shifted fiber (DSF). Generally, the measurement error of the OVNA mainly derives from the beat signals between adjacent optical sidebands. In this paper, the measurement error is significantly suppressed using a two-step measurement process. For the first measurement, both the transmission response of an optical device-under-test (ODUT) and the measurement error are recorded. For the second measurement, only the measurement error is obtained by suppressing the optical carrier using SBS in the DSF. The accurate transmission response of the ODUT is subsequently extracted by subtracting the error from the first measurement. The proposed method is theoretically investigated and experimentally demonstrated. The experimental results show that the accuracy of the OVNA is significantly improved.