Phase-modulated Optical Signals Research Articles

Photonic Generation of Background-Free Phase-Coded Pulses With Low Power Fluctuation Over the Multi-Octave Frequency Range

In this article, a photonic microwave signals generation scheme for background-free phase-coded (PC) radar pulse with multioctave tuning is experimentally demonstrated. Firstly, an optical frequency comb (OFC) demultiplexing technology based on optical injection locking (OIL) provides a power-equalized coherent optical local oscillator (LO) with ultra-broadband tuning capability. Then, a phase-modulated optical signal is produced based on the conversion intensity modulation to phase modulation. Finally, a background-free phase-coded signal can be obtained by beating the optical signal with LO at a balanced photodetector (BPD). The proposed scheme features a flat frequency response over an ultra-wideband frequency range without needing a broadband modulator and radio frequency (RF) synthesizer. A proof-of-concept experiment demonstrates that the power fluctuation of 0.5 Gbps background-free PC pulses is lower than 3.12 dB within 3∼24 GHz carrier frequency. Furthermore, the pulse compression performance and system stability are investigated, respectively.

Microwave frequency measurement using a silicon integrated microring resonator.

Photonics-assisted instantaneous frequency measurement of a microwave signal using a silicon integrated microring resonator (MRR) is proposed and experimentally demonstrated. The frequency of a microwave signal has a unique relationship with the power ratio between the two microwave signals at the outputs of two microwave photonic filters (MPF) with complementary frequency responses. The key device to implement the MPFs is a silicon integrated MMR, which is employed to convert a phase-modulated optical signal to an intensity-modulated optical signal by placing two optical carriers at the complementary slopes of the MRR. For a given frequency measurement range and resolution, an MRR is designed and fabricated, and its use for instantaneous microwave frequency (IMF) measurement is implemented. For the fabricated MRR, an IMF measurement range of 14-25 GHz with a measurement accuracy of ±0.2GHz is achieved.

Photonic Generation and Antidispersion Transmission of Background-Free Multiband Arbitrarily Phase-Coded Microwave Signals

A novel type of photonic background-free multiband arbitrarily phase-coded microwave signal generator with power fading elimination capability is reported. An <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -polarized phase modulated optical signal and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -polarized radio frequency (RF) modulated harmonic single-sideband (HSSB) optical signal are produced based on a dual-polarization quadrature phase shift-keying modulator (DP-QPSKM). After polarization modulation to intensity modulation conversion, a microwave signal with arbitrarily phase encoding in multiple frequency bands is generated. Benefiting from the phase modulation state of the driven electrical coded signal, the generated arbitrarily phase-coded signal has no baseband components. Moreover, the equivalent HSSB modulation ensures the system can realize antidispersion transmission of one-to-multi base stations (BSs). The main distinct highlights of our generator are multifunction. The proposed single-modulator-based photonic scheme can achieve phase encoding in arbitrary format over multiple frequency bands and transmit over long-distance single-mode fiber with the elimination of power fading simultaneously. This work is pretty promising in multiband radar systems for one-to-multi BSs transmission and can increase the detection range, range resolution, Doppler tolerance, and sensitivity of objective recognition of radars.

A Novel Spectral-Efficient Coherent Radio-Over-Fiber Link With Linear Digital-Phase Demodulation

In this paper, a novel spectral-efficient coherent radio-over-fiber (RoF) link with linear digital phase demodulation is proposed and experimentally demonstrated. At the transmitter, to make an efficient use of the optical power and spectra, an intensity-modulated optical signal serving as the optical reference signal and a phase-modulated optical signal are polarization-multiplexed on a single optical carrier. At the receiver, the two optical signals are coherently detected with an optical local oscillator (OLO) and demodulated free of laser phase fluctuation through digital signal processing. Owing to simple and linear digital phase demodulation, an RF input signal is linearly demodulated from the optical phase without approximations and preconditions, which preserves the linearity of the phase-modulated RoF optical link from the transmitter end to the receiver end. The proposed scheme is experimentally verified by 25-km single-mode-fiber (SMF) transmission of two 16-QAM microwave vector signals at 2 GHz and 2.4 GHz, both with a symbol rate of 50 Msymbol/s. The transmission performance in term of error vector magnitude (EVM) is evaluated. Additionally, 25-km SMF transmission of the phase-modulated input signal with a spurious-free dynamic range (SFDR) of 112.8 dB·Hz2/3 is obtained.

Effects of frequency chirping and finite extinction ratio of optical modulators in microwave photonic IFM receivers

Frequency-to-power mapping approach based on chromatic dispersion is frequently used in microwave photonic (MWP) instantaneous frequency measurement (IFM) receivers. In this technique, carrier frequency of an unknown microwave signal is estimated according to a specific relationship between the unknown microwave frequency and the ratio of two optical or microwave power functions which is known as amplitude comparison function (ACF). The ACF can be constructed using chromatic dispersion-induced power fading functions of intensity- or phase-modulated optical signals transmitting through dispersive mediums. Practical non-ideal intensity modulators have finite extinction ratio and suffer from frequency chirping. Despite previously published numerous works, the contribution of these parameter in MWP IFMs have not been already studied. In this paper, effects of chirp parameter and finite extinction ratio of intensity modulators on the performance of chromatic dispersion-based MWP IFM receivers are theoretically investigated. It is shown that in general the ACFs of these MWP IFM receivers depend on the chirp parameter of their intensity modulators which is a function of unknown amplitude of the microwave signal whose frequency is intended to be measured and varies with time. This problem cannot be resolved by calibrations and degrades the measurement accuracy. Besides, it is shown that ACFs are different for different finite extinction ratios which degrades the measurement accuracy. Finally, we propose using Mach–Zehnder interferometer (MZI)- and Sagnac interferometer (SI)-based intensity modulators with zero chirp in these MWF IFMs. Analytical results are verified by computer simulations using OptiSystem software.

Generation of linear frequency-modulated signals with improved time-bandwidth product based on an optical frequency comb.

A generation scheme for the linear frequency-modulated (LFM) signals with tunable carrier frequency and improved time-bandwidth product (TBWP) using an optical frequency comb (OFC) and a fiber Bragg grating (FBG) time delay line is proposed and demonstrated by simulation. In the scheme, intensity modulation is used to convert the continuous time-domain waveform to an optical pulse with a duty cycle of one-sixteenth, and two cascaded polarization modulators driven by two different radio frequency signals are used to generate 16-line OFC without an optical filter. Then the 16-line OFC is injected into the FBG time delay line, which consists of 16 discrete FBGs, and each spectral line is reflected by the specific FBG after different time delays. By properly designing the position of the FBGs, the 16 spectral lines of the 16-line OFC are separated in the time domain. The time-separated OFC is then modulated by a phase modulator driven by a periodic parabolic signal for introducing parabolic phase modulation. Subsequently, after the heterodyne beating between the phase-modulated optical signal and the local oscillator light wave emitted from a tunable laser source (TLS) in a photodiode, an electrical LFM signal with improved TBWP is generated. Simulation results show that the generated LFM signal has the bandwidth of 31.78GHz and TBWP of 1365.33, and its center frequency can be tuned by varying the wavelength of the TLS.

A Spectral-Efficient Self-Homodyne-Detected Microwave Photonic Link With an Extended Fiber-Reach

In this letter, a simple and spectral-efficient self-homodyne-detected microwave photonic link is proposed and experimentally demonstrated. At the transmitter, a phase-modulated optical signal and an intensity-modulated optical local oscillator signal are polarization-multiplexed and delivered in a single fiber over long distance. At the receiver, the two signals are coherently detected by a self-homodyne interferometer and then demodulated through simple digital signal processing. The proposed scheme is experimentally validated by 25-km fiber transmission of the 50-Msymbol/s 16-QAM microwave vector signals at 2 and 2.5 GHz. The transmission performance compared with the conventional homodyne detection scheme in terms of error vector magnitude and bit error rate is evaluated. The proposed scheme can be a potential solution to wireless access networks for both high spectral efficiency and low-cost implementation.

A reconfigurable microwave photonic filter with flexible tunability using a multi-wavelength laser and a multi-channel phase-shifted fiber Bragg grating

We propose a photonic scheme to realize a reconfigurable microwave photonic filter (MPF) with flexible tunability using a multi-wavelength laser (MWL) and a multi-channel phase-shifted fiber Bragg grating (PS-FBG). The proposed MPF is capable of performing reconfigurability including single bandpass filter, two independently bandpass filter and a flat-top bandpass filter. The performance such as the central frequency and the bandwidth of passband is tuned by controlling the wavelengths of the MWL. In the MPF, The light waves from a MWL are sent to a phase modulator (PM) to generate the phase-modulated optical signals. By applying a multi-channel PS-FBG, which has a series of narrow notches in the reflection spectrum with the free spectral range (FSR) of 0.8 nm, the +1st sidebands are removed in the notches and the phased-modulated signals are converted to the intensity-modulated signals without beating signals generation between each two optical carriers. The proposed MPF is also experimentally verified. The 3-dB bandwidth of the MPF is broadened from 35 MHz to 135 MHz and the magnitude deviation of the top from the MPF is less than 0.2 dB within the frequency tunable range from 1 GHz to 5 GHz.

Optical mapping of oscillatory stresses in transparent media

The phase-modulated optical signal produced by the light propagation through the stressed transparent media is detected using the non-steady-state photoelectromotive force technique. The mechanical system including the glass plate and piezoelectric transducer demonstrates the resonant behavior in the vicinity of 100 kHz. The measured distribution of the optical phase is a bell-shaped surface for these frequencies. The simulation of the piezooptic response shows the qualitative coincidence with the experimental data and provides the maps of the stress field. The characterization of the β-Ga2O3 adaptive detector is performed for the light wavelength λ = 532 nm.

Visualization of oscillatory stresses in transparent media using a β-Ga2O3 adaptive detector.

The phase-modulated optical signal produced by light propagation through stressed transparent media is detected using the non-steady-state photoelectromotive force technique. The mechanical system, including the glass plate and piezoelectric transducer, demonstrates resonant behavior in the vicinity of 100 kHz. The measured distribution of the optical phase is a bell-shaped surface for these frequencies. The simulation of the piezo-optic response shows qualitative coincidence with the experimental data and provides maps of the stress field. The characterization of the β-Ga2O3 adaptive detector is performed for the light wavelength λ=532 nm.

Photonic-assisted chirped microwave pulses generation with a flexible and fine parameter manipulation.

A photonic approach for generating chirped microwave pulses with a flexible and fine parameter manipulation is proposed and experimentally demonstrated. In the proposed system, an intensity modulator (IM) biased at the minimum transmission point is used to generate two ± 1st-order optical sidebands which are then sent to a phase modulator (PM) for implementing large-signal phase modulations. A de-interleaver combined with an optical variable delay line (OVDL) is utilized to introduce a time delay between two phase-modulated optical signals. A second IM that acts as a time domain intensity switch (TDIS) is used to select different phase modulation ranges of the two phase-modulated optical signals. After the optical-electrical conversion in a photodetector (PD), chirped microwave pulses are generated. The key feature of this approach is that the parameters of the generated chirped microwave pulses including central frequency, pulse repetition frequency, and chirp rate can be flexibly and precisely manipulated by the radio frequency (RF) signals applied to modulators. A proof-of-principle experiment is carried out to verify the proposed approach. Consequently, positive or negative chirped microwave pulses with different central frequencies at 20, 22, 24 or 26 GHz and different pulse repetition frequencies at 1.5 or 2 GHz are generated, respectively.

Tunable Dual-Passband Microwave Photonic Filter Using Orthogonal Polarization Modulation

A microwave photonic filter (MPF) with two independently tunable passbands using a polarization modulator (PolM) and a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. By applying two orthogonally polarized optical waves from two tunable laser sources (TLSs) to the PolM, two phase-modulated optical signals that are orthogonally polarized are generated, which are sent to the PS-FBG that acts as an optical notch filter to suppress one sideband of the orthogonally polarized phase-modulated signals. Thus, the two phase-modulated signals are converted to two intensity-modulated single-sideband signals. The overall operation corresponds to a dual-passband MPF with the central frequencies of the two passbands independently tunable by changing the wavelengths of the optical carriers from the two TLSs. The proposed MPF is experimentally evaluated. A dual-passband MPF with a bandwidth for each passband of 140 MHz and a frequency tunable range of $\sim 6$ GHz is demonstrated. The insertion loss and the dynamic range of the filter are also studied and measured experimentally.

Frequency- and Notch-Depth-Tunable Single-Notch Microwave Photonic Filter

A single-notch microwave photonic filter with tunable frequency and tunable notch depth is proposed and experimentally demonstrated. In the proposed filter, two phase-modulated optical signals at two different wavelengths are generated at a phase modulator. One phase-modulated signal is filtered by a phase-shifted fiber Bragg grating with an ultra-narrow notch to remove the upper sideband. The other phase-modulated signal is filtered by a wideband optical bandpass filter to remove the lower sideband. Since the remaining lower and upper sidebands in the two optical signals are out of phase, the combination of the beat signals at a photodetector will lead to the cancelation of the detected microwave signals, to generate an ultranarrow notch. An experiment is performed. A single-notch microwave photonic filter with a 3-dB bandwidth of ~180 MHz, a tunable frequency from 1.5 to 6.6 GHz, and a tunable notch depth from 0 to 42 dB is experimentally demonstrated.

Nonlinear dynamics of optical Costas loop with inherent time delay

Nonlinear dynamics of optical Costas loop (OCL) has been studied in the presence of loop delay. Analytical predictions show that, when loop delay exceeds some critical value, the loop fials to lock to the input optical signal. For large value of loop delay, loop phase error can oscillate chaotically by generating chaotically phase modulated optical signal. Analytically, it is possible to predict the different values of delay and loop parameters, for which different nonlinear effects are observed. All the analytical results have been verified properly by solving the system equation with the help of numerical integration.

Experimental Demonstration of Nonlinearity and Phase Noise Tolerant 16-QAM OFDM W-Band (75–110 GHz) Signal Over Fiber System

We propose a nonlinearity and phase noise tolerant orthogonal frequency division multiplexing (OFDM) W-band signal over fiber system based on phase modulation and photonic heterodyne up-conversion techniques. By heterodyne mixing the phase-modulated optical OFDM signal with a free-running laser in the photodiode, the constant envelope OFDM W-band wireless signal is obtained to suppress the nonlinear impairments. Moreover, the phase noises of the beating lasers appear as additive terms to the desired signal, and could be easily filtered out without complex phase noise estimation and compensation algorithms. In our experiment, 4 Gb/s QPSK and 8 Gb/s 16-QAM constant envelope OFDM W-band signals are transmitted over 22.8 km single mode fiber and 2.3 m air distance with achieved bit-error-rate performance below the forward error correction limit.

Tunable Microwave Frequency Multiplication by Injection Locking of DFB Laser With a Weakly Phase Modulated Signal

We have demonstrated in this paper a novel tunable microwave frequency multiplication by injecting a weakly phase-modulated optical signal into a DFB laser diode. Signals with multiple weak sidebands are generated by cross-phase modulation of a continuous wave (CW) with short pulses from mode-locked fiber laser. Then, frequency multiplication is achieved by injection and phase locking a commercially available DFB laser to one of the harmonics of the phase modulated signal. The multiplication factor can be tuned by changing the frequency difference between the CW and the free oscillating wavelength of the DFB laser. The experimental results show that, with an original signal at a repetition rate of 1 GHz, a microwave signal with high spectral purity and stability is generated with a multiplication factor up to 60. The side-mode suppression ratio over 40 dB and phase noise lower than -90 dBc/Hz at 10 kHz are demonstrated over a continuous tuning range from 20 to 40.

Ring resonator-based on-chip modulation transformer for high-performance phase-modulated microwave photonic links

In this paper, we propose and experimentally demonstrate a novel wideband on-chip photonic modulation transformer for phase-modulated microwave photonic links. The proposed device is able to transform phase-modulated optical signals into intensity-modulated versions (or vice versa) with nearly zero conversion of laser phase noise to intensity noise. It is constructed using waveguide-based ring resonators, which features simple architecture, stable operation, and easy reconfigurability. Beyond the stand-alone functionality, the proposed device can also be integrated with other functional building blocks of photonic integrated circuits (PICs) to create on-chip complex microwave photonic signal processors. As an application example, a PIC consisting of two such modulation transformers and a notch filter has been designed and realized in TriPleX(TM) waveguide technology. The realized device uses a 2 × 2 splitting circuit and 3 ring resonators with a free spectral range of 25 GHz, which are all equipped with continuous tuning elements. The device can perform phase-to-intensity modulation transform and carrier suppression simultaneously, which enables high-performance phase-modulated microwave photonics links (PM-MPLs). Associated with the bias-free and low-complexity advantages of the phase modulators, a single-fiber-span PM-MPL with a RF bandwidth of 12 GHz (3 dB-suppression band 6 to 18 GHz) has been demonstrated comprising the proposed PIC, where the achieved spurious-free dynamic range performance is comparable to that of Class-AB MPLs using low-biased Mach-Zehnder modulators.

All-Optical Photonic Microwave Phase Shifter Based on an Optical Filter With a Nonlinear Phase Response

A new photonic microwave phase shifting structure that can realize a continuous 0° to 360° phase shift with only little frequency dependent amplitude and phase variation over a wide frequency range is presented. It is based on controlling the amplitude and phase of the optical carrier and two antiphase RF modulation sidebands of two phase modulated optical signals by adjusting the wavelength of the optical carriers that fall on the rising and falling edges of an optical bandpass filter with a nonlinear phase response. The new photonic microwave phase shifter has a simple structure. It only requires one normal electro-optic phase modulator and one low-cost commercially available optical filter. It does not involve any electrical component, and has the advantages of small size, lightweight, and robust performance. Experimental results are presented which demonstrate the realization of a continuous 0° to 360° phase shift, with close to flat phase and amplitude response performance over a wideband microwave range.

Subcarrier-Multiplexed Phase-Modulated Optical Signal Amplification by a Synchronized Quantum Cascade Laser

This paper investigates theoretically the amplification performance of a synchronized quantum cascade laser for an input subcarrier-multiplexed phase modulated lightwave. The output optical phase-modulation amplitude corresponding to a given microwave subcarrier has been calculated. The third order intermodulation distortion of the desired optical phase modulation by a microwave subcarrier has been calculated and its dependence on the input phase modulation (PM) index has been studied analytically.

Colorless Regenerative Amplification of Constant Envelope Phase-Modulated Optical Signals Based on Injection-Locked Fabry–Pérot Lasers

An efficient phase preserving amplitude noise limiter suitable for constant envelope phase-modulated signals is experimentally demonstrated for 10-Gb/s differential phase-shift-keying (DPSK) signals exploiting injection locking in Fabry-Pérot lasers. The limiter operates successfully over a 16.4-nm tuning range leading to 12 dB of power penalty reduction for 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> of bit-error-rate (BER) performance.