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We propose and experimentally demonstrate a high-power eight-wavelength distributed feedback (DFB) laser array with 100 GHz spacing using the grating reflector (GR). The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of a random grating phase at the HR facet. Therefore, more output power, better single-longitudinal-mode operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with HR/AR facet coatings. The maximum output power and side mode suppression ratio of the fabricated eight-channel GR-DFB laser array as high as 70 mW and 55 dB can be achieved, which may benefit wavelength division multiplexing transmission systems, gas sensors, optical computing systems, and microwave photonic signal processing. [ABSTRACT FROM AUTHOR]

A unified data communication and radar sensing system towards millimeter-wave (mm-wave) applications with extended coverage is proposed and experimentally demonstrated. In the unified system, the transmission and reception of mm-wave signals are implemented by microwave photonics technologies, and the orthogonal frequency division multiplexing (OFDM) signal is employed for communications and radar sensing simultaneously. The superheterodyne optical carrier suppression (HeteroOCS) is used to up-convert the intermediate frequency (IF) OFDM signal to the mm-wave band in the central office (CO). The optical mm-wave signal is transmitted to the remote antenna units (RAUs) via optical fibers. The echo signal is received by the RAU which is remotely connected to the CO by an analog photonic link based on a polarization modulator (PolM). In the transmitter and receiver links, the chromatic dispersion of optical fibers is overcome by the HeteroOCS and the PolM, respectively. The baseband units (BBU) centralized in the CO is used to process the radar sensing signal. A pre-delayed trigger signal is proposed to overcome the fiber length limitation. A proof-of-concept experiment is performed. Two targets with distances of 1.64 m and 3.40 m are successfully detected separately. The communication function of the system is also verified. 1 m wireless transmission of 1.56 Gb/s data rate is achieved below the forward error correction limit. [ABSTRACT FROM AUTHOR]

A novel approach for generating RF vector signals with equivalent single sideband (SSB) modulation based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator is proposed and experimentally demonstrated. One QPSK modulator in the DP-QPSK modulator is driven by a single-frequency RF signal. By properly setting its biases, a SSB carrier suppressed (SSB-CS) signal is generated. The other QPSK modulator in the DP-QPSK modulator is modulated by $I/Q$ data streams, resulting in a baseband vector signal carried by the optical carrier. A polarizer is placed after the DP-QPSK modulator to combine the two signals with orthogonal polarization, leading to the generation of an equivalent SSB signal, which is free from the dispersion-induced power fading effect. By adjusting the angle of the polarizer, the carrier-to-sideband ratio can also be flexible tuned to maximize the transmission performance. The proposed system is verified by an experiment. The generation of a 1.25-GBd vector signal with QPSK modulation format at 10.5 GHz and the transmission of the signal over 25-km single-mode fiber are evaluated. An error-free transmission is achieved at a received optical power of −12 dBm and the power penalty is <1 dB. The proposed approach features all-optical and compact configuration. [ABSTRACT FROM PUBLISHER]

A novel approach to generating microwave frequency shift keying (FSK) signals using a polarization modulator (PolM) and a dual-polarization Mach–Zehnder modulator (DPol-MZM) is proposed and experimentally demonstrated. In the proposed system, the PolM is employed to modulate the polarization state of an input linearly polarized lightwave to switch between two orthogonal directions, leading to the generation of a polarization shift keying (PolSK) signal. Then the PolSK signal is sent to a DPol-MZM, which is made up of two polarization multiplexed sub-MZMs by a polarization controller (PC). Two microwave signals with different frequencies are applied to the sub-MZMs. After aligning the orthogonal directions of the PolSK signal with the axes of the DPol-MZM by the PC, the PolSK signal can be converted into a microwave FSK signal. A proof-of-concept experiment is carried out to verify the proposed system. When the sub-MZMs in the DPol-MZM are biased at a quadrature point, a double sideband microwave FSK signal at 3/6.5 GHz with a bit rate of 1.25 Gb/s is generated and transmitted over 10 km of single-mode fiber. When the sub-MZMs are biased at minimum point, an optical carrier suppression microwave FSK is achieved at frequency-doubled 6/14 GHz with a bit rate of 2.5 Gb/s. [ABSTRACT FROM PUBLISHER]

A novel technique to transport a microwave signal over an optical fiber based on phase-modulation and coherent in-phase (I) and quadrature-phase (Q) demodulation with an extended transmission distance is proposed and experimentally demonstrated. In the transmitter, a Sagnac loop incorporating a phase modulator (PM) is used to generate two orthogonally polarized optical signals with one being phase modulated and the other with no modulation that acts as a remote optical reference signal. The orthogonally polarized optical signals are transmitted over a single-mode fiber (SMF) to a polarization and phase diversity coherent receiver, and are coherently detected with a free-running optical local oscillator at the receiver. Since the phase-modulated and the reference signals are transmitted over the same SMF, the optical phases are correlated, and the original signal can be recovered based on a digital signal processing algorithm. The proposed technique is experimentally evaluated. Compared with a phase-modulated coherent I/Q demodulated link without using an optical phase correlated reference signal, the transmission distance is extended from 50 m to 10 km, while providing a link gain and a spurious-free dynamic range (SFDR) of −9.5 dB and 115.8 dB $\cdot $ Hz ^{2/3} , respectively. [ABSTRACT FROM PUBLISHER]

A broadband chromatic dispersion (CD) compensation scheme using optical phase conjugation (OPC) based on a DFB semiconductor laser is proposed and experimentally demonstrated in analog photonic links (APLs). The DFB laser acts as a nonlinear medium to induce four-wave mixing and a pump laser simultaneously. Compared with the conventional OPC based on a semiconductor optical amplifier or a dispersion-shifted fiber, an extra pump laser is no longer required. Experimental results show that the power fading caused by the CD in a 50.4-km APL can be compensated with a 3-dB bandwidth up to 33 GHz. Meanwhile, the spur-free dynamic range is enhanced with 12.6 \textrm dB\cdot \textrm Hz^2/3 ; the transmission performance is also improved. [ABSTRACT FROM PUBLISHER]

A novel technique to generate a microwave vector signal based on a bidirectional use of a polarization modulator (PolM) and a phase modulator (PM) in a Sagnac loop is proposed and experimentally demonstrated. The I/Q data streams of a vector signal are mixed with a microwave carrier and then applied to the PolM and the PM, respectively. A microwave vector signal is generated by combining two microwave signals with one obtained by beating the optical carrier and the + 1^{\mathrm {st}} order sideband of a phase-modulated signal and the other obtained by beating the optical carrier and the + 1^{\mathrm {st}} order sideband of an intensity-modulated signal. A quadrature phase is introduced due to the inherent \pi/2 phase shift between the optical carrier and the + 1^{\mathrm {st}} order sideband of the phase-modulated signal. The generation of a 4.5 GHz microwave signal at 625 Msym/s with quadrature phase-shift-keying (QPSK) modulation and the transmission of the signal over a 25-km single-mode fiber are evaluated. An error-free transmission is achieved at a received optical power of 6 dBm. By incorporating I/Q$ imbalance compensation, an error-free transmission is achieved at a lower received power of 2 dBm. [ABSTRACT FROM PUBLISHER]

In this letter, a tunable single bandpass microwave photonic filter (MPF) with an improved spurious free dynamic range (SFDR) based on a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. A phase-modulated double-sideband (DSB) signal is sent to a PS-FBG; when one of the sidebands falls in the notch of the reflection band of the PS-FBG, the reflected signal is converted to an intensity-modulated single-sideband (SSB) signal which leads to a single passband of the MPF. By tuning the wavelength of the optical carrier, the center frequency of the passband can be tuned correspondingly. The SFDR is improved by employing polarization mixing technique in a LiNbO3 phase modulator and a polarization diversity receiver. Experimental results show that the passband width and the frequency-tunable range of the single bandpass MPF are 80 MHz and 5.5 GHz, respectively. An SFDR improvement of 13.1 dB is successfully achieved. [ABSTRACT FROM PUBLISHER]

A dual-passband microwave photonic filter (MPF) implemented based on phase-modulation to intensity-modulation (PM-IM) conversion using a phase modulator and an equivalent phase-shifted fiber Bragg grating (EPS-FBG) is proposed and experimentally demonstrated. The key component in the system is the EPS-FBG, which is designed and fabricated based on the equivalent phase-shift technique. The unique feature of the EPS-FBG is that equivalent phase shifts are introduced to both of the \pm \1^st channels, leading to a notch in each of the two channels. Thus, by implementing PM-IM conversion in the two channels, two passbands are produced. The central frequency of each passband is determined by the wavelength different between the notch and the optical carrier. In the design and fabrication, two phase shifts are introduced to the EPS-FBG to decrease the shape factor, which is defined as the ratio between the 20- and 3-dB bandwidths. In addition, a stimulated Brillouin scattering (SBS) assisted filter is incorporated in the system for carrier suppression to increase the spurious-free dynamic range (SFDR) and decrease the noise figure (NF) of the MPF. An experiment is performed. A dual passband filter with a 3-dB bandwidth and a shape factor of 167.3 MHz and 3.8, and 143.3 MHz and 3.3 for the \1^st and \2^nd passband is achieved. The frequency tunable ranges of the \1^st and \2^nd passbands are 5.4 and 7.4 GHz, respectively, with the magnitude variations of about \pm \ 0.5 dB during the turning. Due to the SBS-assisted filter, the SFDRs are increased by 7 dB and the NFs are decreased by 10 dB. [ABSTRACT FROM AUTHOR]

A wideband tunable single bandpass microwave photonic filter (MPF) based on the four‐wave mixing (FWM) nonlinear dynamics of a distributed feedback (DFB) semiconductor laser under optical injection is proposed and experimentally demonstrated. Utilising the wavelength‐selective optical gain of the optical‐injected DFB laser, phase‐modulation to intensity‐modulation conversion is obtained, leading to the realisation of a single bandpass MPF. By changing the injection parameters, an MPF with a frequency tuning range of 27.72 GHz has been achieved. The minimum 3 dB bandwidth and the out‐of‐band suppression ratio are measured to be 61.2 MHz and 25 dB, respectively. [ABSTRACT FROM AUTHOR]

A directly-modulated Fourier domain mode-locked optoelectronic oscillator (FDML_OEO) based on three-section mutual-injection integrated semiconductor laser (TS_MIISL) with dual-loop feedback structure is proposed and experimentally demonstrated in this paper. By driving the TS_MIISL to work under the period-one (P1) state and modulating the TS_MIISL with low-repetition-rate sawtooth drive signal, a linear frequency-modulated (LFM) signal can be generated. Through adjusting the delay time of the feedback loops to match with the time duration of drive signal, the LFM signal can be significantly optimized in terms of spectral purity. In such a proposed FDML_OEO, no external modulator, polarization controller (PC) and filter are utilized, leading to an outstanding advantage in terms of stable performance and compact structure. The high-purity LFM signal with center frequency and bandwidth of 14 GHz and 8 GHz is obtained. The maximum time-bandwidth product (TBWP) of generated LFM signal could reach up to 48800. Due to the introduction of dual feedback loop, the frequency comb contrast and linewidth could reach up to 45 dB and 5 kHz, respectively. The center frequency and bandwidth of the generated signals are both tunable. A demonstration is made to prove that the pre-compensation processing could improve the linearity of LFM signals. • We have also proposed an equivalent microwave photonics filter (MPF) and it can be observed when the TS_MIISL works at P1 state. • Because of the short distance of PS between the FL and RL, the TS_MIISL would go into the short-coupling regime. • When the delay matching is completed, the spectral purity of the obtained LFM signal is significantly optimized by the optoelectronic feedback loop. • When the delay matching is completed, the spectral purity of the obtained LFM signal is significantly optimized by the optoelectronic feedback loop. [ABSTRACT FROM AUTHOR]

Abstract: A coupled frequency-doubling optoelectronic oscillator (OEO) is proposed and experimentally demonstrated, which is constructed based on the perfect combination of polarization modulation and polarization multiplexing. A fundamental microwave signal at 9.95GHz or a frequency-doubled microwave signal at 19.9GHz is generated with a wavelength-independent sidemode-suppression ratio (SMSR) as high as 78dB obtained. The phase noise of the generated 19.9-GHz signal is −103.45dBc/Hz at 10-kHz frequency offset, indicating a good short-term stability. The proposed scheme is simple and flexible, which can find applications in radars and wireless communications. [Copyright &y& Elsevier]