Secure optical communication system based on polarization regulation of the data fragmentation multipath transmission technology.

We propose and demonstrate a data fragment multipath transmission scheme to achieve a secure optical communication based on polarization regulation. A dual-polarization Mach-Zehnder modulator (DPMZM) is driven by digital signals which are scattered by field-programmable gate array (FPGA) and transmitted in multiple paths. By utilizing two orthogonal polarization states, we have achieved a signal transmission under different optical parameters, and the transmission rate of the two paths can reach over 10 Gbps through a 20 km fiber with 2.5 Gbps hopping rate. In addition, we establish a theoretical model to analyze the security of the system and simulate brute force cracking; the probability of cracking the minimum information unit is 1.53 × 10-53. This proves that it is difficult to obtain a user data even using the fastest computers. Our scheme has provided, to our knowledge, a new approach for physical layer security.

100 Gb/s NRZ OOK signal regeneration using four-wave mixing in a silicon waveguide with reverse-biased p-i-n junction.

A silicon waveguide with reverse-biased p-i-n junction is used to experimentally demonstrate all-optical regeneration of non-return-to-zero (NRZ) on-off keying (OOK) signal based on four-wave mixing. The silicon waveguide allows a high conversion efficiency of -12 dB. The 0.22 dB (1.1 dB) quality (Q) factor and 0.74 dB (6.3 dB) extinction ratio (ER) improvements on average are achieved for 100 Gb/s (50 Gb/s) NRZ OOK signal regeneration at different receiving powers via the optimal match between the input signal optical power and input-output transfer curve. To the best of our knowledge, this silicon-based all-optical regenerator exhibits superior regeneration performance, including large ER and Q factor improvements, and the highest regeneration speed of NRZ OOK signal, and it has wide applications in 5 G/6 G networks.

Ultrahigh spectral resolution single passband microwave photonic filter.

Microwave photonic filters (MPFs) with only one ultra-narrow passband are able to provide high frequency selectivity and wide spectral range, and they are of great importance in radio-frequency (RF) signal processing. However, currently all MPFs are limited by trade-offs between key parameters such as spectral resolution and range, tunability, and stability. Here, we report the first demonstration of a single passband MPF with unprecedented performance including ultrahigh spectral resolution of 650 kHz, 0-40 GHz spectral range, and high stability of center frequency drifting within ±50 kHz. This record performance is accomplished by breaking the amplitude equality of a phase-modulated signal via a Brillouin dynamic grating (BDG) which has an ultra-narrow reflection spectrum of sub-MHz. The results point to new ways of creating high performance microwave photonic systems, such as satellite and mobile communications, radars, and remote-sensing systems.

Photonic generation of quadruple bandwidth dual-band dual-chirp microwave waveforms with immunity to power fading.

A photonic method to generate and transmit quadruple bandwidth dual-band dual-chirp microwave waveforms with immunity to fiber chromatic dispersion induced power fading is proposed and experimentally demonstrated, which is suitable for Doppler blind-speed elimination, small target detection, and multiband detection in multiband radar systems. A dual-polarization dual-parallel Mach-Zehnder modulator is utilized to realize carrier-suppressed harmonic single-sideband modulation of a radio frequency carrier and carrier-suppressed DSB modulation of a baseband single-chirped waveform at two orthogonal polarization states. After photoelectronic conversion, dual-band bandwidth-quadrupling dual-chirp waveforms are generated. Moreover, different from traditional DSB-based dual-chirp signal generation, the generated dual-chirp microwave waveforms can be transmitted over fiber without power fading, which is significant in dual-band radars for one to multiple base station transmissions.

Dual-chirp microwave waveform transmitter with elimination of power fading for one-to-multibase station fiber transmission.

We present a photonic approach to generate and transmit a dual-chirp microwave waveform with antidispersion performance. Traditionally, the microwave signal generated based on double-sideband (DSB) modulation suffers from power fading significantly. We propose a DSB-based dual-chirp microwave waveform transmitter that can eliminate the chromatic dispersion-induced power fading (CDIP) over fiber transmission. The CDIP elimination, rather than compensation, ensures that the working bandwidth of the dual-chirp waveform is not limited by the periodical power fading. The proposed signal modulation scheme makes the signal transmitter free from the direct current bias drifting of the modulator. Moreover, thanks to the phase modulation, the generated waveform is background-free. The proposed dual-chirp waveform transmitter features a compact structure, polarization independence, and CDIP elimination, which has great potential in radars for one-to-multibase station fiber transmission.

Chromatic-dispersion-induced power-fading suppression technique for bandwidth-quadrupling dual-chirp microwave signals over fiber transmission.

We report a technique to overcome chromatic-dispersion-induced power fading (CDIP) for bandwidth-quadrupling dual-chirp microwave signals over fiber transmission. Normally, dual-chirp microwave signals are generated using double-sideband (DSB) modulation. However, DSB modulation suffers from CDIP significantly. We propose a carrier-frequency shift method to suppress the CDIP based on a dual-polarization dual-parallel Mach-Zehnder modulator. In addition, the bandwidth of the dual-chirp signal can be quadrupled by properly setting the biases of the modulator. The proposed technique is theoretically analyzed and experimentally verified. Our technique is promising for improving the range-Doppler resolution of radars for one-to-multi base stations transmission.

Photonic generation of background-free binary phase-coded microwave pulses.

We report a photonic scheme to generate background-free frequency-doubled binary phase-coded microwave pulses. The key component is a dual-polarization dual-parallel Mach-Zehnder modulator that is used to realize phase modulation and carrier-suppressed double second-order sideband modulation at two orthogonal polarization states, respectively. The π phase jump of the frequency-doubled phase-coded microwave pulse is dependent on the polarity of the coding signal rather than its amplitude. Besides, the generated microwave pulses are free from the baseband-modulated signals, because the optical power launched to a photodetector (PD) keeps constant all the time. Since no electrical or optical filters are involved, the photonic generator can ensure a broad operation bandwidth and wide tunability. Our scheme is theoretically analyzed and experimentally verified. The 4 Gb/s at 16 GHz and 7 Gb/s at 28 GHz background-free frequency-doubled phase-coded microwave pulses have been successfully generated.

Ultrahigh-Q and tunable single-passband microwave photonic filter based on stimulated Brillouin scattering and a fiber ring resonator.

A scheme to perform ultrahigh-Q and single passband microwave photonic filter (MPF) with tunability is proposed and experimentally demonstrated. The single passband of the MPF is implemented by the stimulated Brillouin scattering (SBS) in an optical fiber. The ultrahigh-Q of the MPF is realized due to the ultra-narrow resonant linewidth of the fiber ring resonator, which compresses the bandwidth of the SBS dramatically. A MPF with a single passband and an ultra-narrow 3 dB bandwidth of 825±125 kHz over the wide center frequency tuning range of 2-16 GHz are achieved. The MPF also shows high out-of-band suppression exceeding 25 dB and small amplitude fluctuation of ±1.8 dB within the tuning range. The maximum Q-factor of the MPF is 17778. To the best of our knowledge, this is the highest Q single-passband MPF reported to date, with wide tunability, high out-of-band suppression, and a simple structure.

Transmission of dual-chirp microwave waveform over fiber with compensation of dispersion-induced power fading.

We propose a microwave photonic link to transmit a dual-chirp microwave waveform over fiber with compensation of dispersion-induced power fading. In a center office (CO), we use a polarization-dependent dual-parallel Mach-Zehnder modulator (DPMZM) which is driven by a radio frequency (RF) carrier and a baseband single-chirped waveform to realize carrier-suppressed dual-sideband (CS-DSB) modulation at two orthogonal polarization states. When the CS-DSB optical signal is detected by a photodetector (PD) at a base station (BS), the dual-chirp waveform is generated. However, along with the transmission to different BSs before emission, the chromatic dispersion of fiber will cause power fading. In our link, we can simply adjust the polarization controller (PC) in each BS to compensate for the dispersion-induced power fading for different transmission distances and RF carrier frequencies. Besides, the link can realize one to multitransmission with shared dual-chirp microwave source in CO for radars. The proposed scheme is theoretically analyzed and experimentally verified.

Simultaneous frequency upconversion and phase coding of a radio-frequency signal for photonic radars.

We report a photonic approach to simultaneously realize frequency upconversion and phase coding of a radio-frequency (RF) signal based on polarization manipulation of optical signals. An intermediate frequency (IF) signal is upconverted to the local frequency (LO) band using a dual-polarization dual-parallel Mach-Zehnder modulator, while a high-speed polarization modulator is used to realize high-speed phase coding of the upconverted signal. The key advantage of the proposed method is that no optical or electrical filters are required to remove the residual IF, LO, and undesired downconverted signals, which ensures a broad operation bandwidth, excellent isolation, and wide tunability. The proposed scheme is theoretically analyzed and experimentally verified.

Optical vector network analyzer based on double-sideband modulation.

We report an optical vector network analyzer (OVNA) based on double-sideband (DSB) modulation using a dual-parallel Mach-Zehnder modulator. The device under test (DUT) is measured twice with different modulation schemes. By post-processing the measurement results, the response of the DUT can be obtained accurately. Since DSB modulation is used in our approach, the measurement range is doubled compared with conventional single-sideband (SSB) modulation-based OVNA. Moreover, the measurement accuracy is improved by eliminating the even-order sidebands. The key advantage of the proposed scheme is that the measurement of a DUT with bandpass response can also be simply realized, which is a big challenge for the SSB-based OVNA. The proposed method is theoretically and experimentally demonstrated.

Phase-coherent orthogonally polarized optical single sideband modulation with arbitrarily tunable optical carrier-to-sideband ratio.

We propose and experimentally verify a novel approach to achieve phase-coherence orthogonally polarized optical single sideband (OSSB) modulation with a tunable optically carrier-to-sideband ratio (OCSR). In our scheme, the orthogonally polarized OSSB signal is achieved using a dual-polarization quadrature phase shift keying (DP-QPSK) modulator without an optical band-pass filter (OBPF). Therefore, the proposed method is wavelength independent. The DP-QPSK modulator includes two parallel QPSK modulators locating on its two arms. The upper QPSK modulator of the DP-QPSK modulator is driven by two quadrature sinusoidal microwave signals and works at the frequency shifting condition whose bias voltages are optimized to suppress the optical. The lower QPSK modulator of that works at the maximum transmission point and the optical carrier is not modulated. The OCSR is continuously tunable by simply adjusting the bias voltages of the lower modulator. The frequency shifting optical signal from the upper QPSK modulator and the optical carrier from the lower QPSK modulator are combined together at the output of the DP-QPSK modulator. The optical carrier and sideband are polarized orthogonally. The generated OSSB signals could be used to shift and code the phase of the microwave signal and generate ultra-wideband (UWB) microwave pulse. The proposed method is analyzed and experimental demonstrated.

Optical vector network analyzer with improved accuracy based on polarization modulation and polarization pulling.

We report a novel optical vector network analyzer (OVNA) with improved accuracy based on polarization modulation and stimulated Brillouin scattering (SBS) assisted polarization pulling. The beating between adjacent higher-order optical sidebands which are generated because of the nonlinearity of an electro-optic modulator (EOM) introduces considerable error to the OVNA. In our scheme, the measurement error is significantly reduced by removing the even-order optical sidebands using polarization discrimination. The proposed approach is theoretically analyzed and experimentally verified. The experimental results show that the accuracy of the OVNA is greatly improved compared to a conventional OVNA.

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.

Stable radio-frequency phase distribution over optical fiber by phase-drift auto-cancellation.

We report a new radio-frequency (RF) phase stabilization approach for long-haul optical fiber distribution. The phase drift of an RF signal induced by fiber-length variations can be canceled out automatically via RF mixing without using active phase discrimination and dynamic phase tracking. A key significance of our approach is that no assistant local oscillator (LO) signal is needed. Consequently, frequency estimation of the received RF signal, as well as frequency locking between the LO and the received RF signal, is no longer required, which simplifies the system. A proof-of-concept experiment shows that the phase drift of the received RF signal at 9.6 GHz is significantly reduced using the proposed method. The root mean square (RMS) timing jitter is 0.76 ps when a tunable optical delay line (TODL) inserted between the remote antenna unit (RAU) and local station is changed from 0 to 300 ps.

Generation of triangular waveforms based on a microwave photonic filter with negative coefficient.

We report a novel approach to generating full-duty-cycle triangular waveforms based on a microwave photonic filter (MPF) with negative coefficient. It is known that the Fourier series expansion of a triangular waveform has only odd-order harmonics. In this work, the undesired even-order harmonics are suppressed by the MPF that has a periodic transmission response. A triangular waveform at fundamental frequency can be generated by setting the bias of a Mach-Zehnder modulator (MZM) at quadrature point. However, it is found that a broadband 90° microwave phase shifter has to be used after photodetection to adjust the phases of odd-order harmonics. Alternatively, a frequency doubling triangular waveform can be generated by setting the bias of the MZM at maximum or minimum transmission point. This approach is more promising because the broadband microwave phase shifter is no longer required in this case but it is more power consuming. The proposed approach is theoretically analyzed and experimentally verified.

Generation of FCC-compliant and background-free millimeter-wave ultrawideband signal based on nonlinear polarization rotation in a highly nonlinear fiber.

We propose a novel approach to generating millimeter-wave (MMW) ultrawideband (UWB) signal based on nonlinear polarization rotation (NPR) in a highly nonlinear fiber (HNLF). The MMW UWB signal is background-free by eliminating the baseband frequency components using an optical filter. The proposed scheme is theoretically analyzed and experimentally verified. The generated MMW UWB signal centered at 25.5 GHz has a 10-dB bandwidth of 7 GHz from 22 to 29 GHz, which fully satisfies the spectral mask regulated by the Federal Communications Commission (FCC).

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.

All-optical frequency upconversion for radio-over-fiber applications based on cross-gain modulation and cross-polarization modulation in a semiconductor optical amplifier.

We propose a novel all-optical frequency upconversion technique for radio-over-fiber (RoF) systems based on cross-gain modulation (XGM) and cross-polarization modulation (XPolM) in a semiconductor optical amplifier (SOA). A local oscillator signal is carried onto a continuous wave probe beam with orthogonally polarized single-sideband (SSB) modulation using a polarization modulator and a tunable optical filter. The intermediate frequency signal carried by a pump beam is only intensity modulated onto the sideband of the probe beam while the carrier of the probe beam is unmodulated thanks to the joint use of the XGM and XPolM effects in the SOA. This SSB upconversion scheme is inherently free from the chromatic-dispersion-induced power fading after transmission over single-mode fiber. The proposed scheme is theoretically analyzed and experimentally verified.

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.