Phase Modulation To Intensity Modulation Research Articles

RF Frequency Selective Switch by Multiple PMIM Conversions

Nowadays, broadband and multi-channel radio frequency (RF) processing has been widely used in communication, radar, countermeasure, and other applications. At present, multiple-input and multiple-output (MIMO)-oriented microwave photonic signal processing technology is relatively scarce, so this paper proposes an RF frequency selective switch (FSS) based on multiple phase modulation to intensity modulation (PMIM) conversions. PMIM conversion has been used in narrowband microwave photonic filtering in the past. We extend it to a wideband and arbitrarily reconfigurable RF spectrum processing unit through an optical frequency comb and periodic optical filter. Although we use the incoherent combination of a multi-wavelength light source, we can obtain any frequency response including rectangles only by using all positive tap coefficients. Using an optical wavelength selective switch (WSS), we obtain RF FSS, and the spectral resolution of RF FSS is much better than that of optical WSS, which is improved by more than two orders of magnitude. The above principles, including single-channel reconfigurable filtering and multi-channel RF FSS, are verified by experiments. Our technology provides a stable solution for future RF MIMO signal processing.

A Multiband Microwave Photonic Filter Based on a Strongly Coupled Microring Resonator With Adjustable Bandwidth

A bandwidth-tunable multiband microwave photonic filter (MPF) using a strongly coupled microring resonator (MRR) with optical phase modulation is demonstrated theoretically and experimentally. Two notches of the MRR adjacent to the optical carrier break the intrinsic balance of the phase-modulated signal and then two sub-MPFs are formed thanks to the conversion from phase modulation to intensity modulation (PM-IM). The frequency responses of the two sub-MPFs are superimposed to form a passband with a fixed center frequency. Due to the periodicity of the MRR transmission spectrum, multiple passbands separated by a certain spacing appear in the frequency domain and combine to form a multiband MPF, whose bandwidth can be adjusted by tuning the wavelength of the carrier. The tunable bandwidth and shape factors ranging from 0.73 GHz to 2.73 GHz and 5.06 to 1.38 are experimentally performed. The number of passbands can be reconfigured. The proposed multiband MPF has the potential to be employed in modern multi-standard wireless communication systems.

A Photonics Approach for Microwave Waveform Generation Based on PM-IM Conversions Using Two Dual-Output Sagnac Interferometers

A photonics-assisted scheme for generating various microwave waveforms has been investigated in this paper. In the proposed scheme, two phase modulation to intensity modulation (PM-IM) conversions are realized using two cascaded Sagnac interferometers (SIs) to manipulate light in the optical domain. From the first SI, by adjusting the amplitude of the microwave signal and time delay of the tunable optical delay line (TODL), both half duty cycle square and full duty cycle triangular waveforms can be generated from the photodetector (PD) and balanced photodetection (BPD). Using the second SI and applying microwave signals with appropriate amplitudes and phases, as well as the BPD process, results in full duty cycle sawtooth and reversed sawtooth waveforms. Detailed theoretical analyses are presented and results are verified by simulations. In the simulations, square-, triangular- and sawtooth-shaped (or reversed sawtooth-shaped) waveforms are obtained successfully. To complete the simulation results, the effects of non-ideal microwave hybrid coupler and optical couplers on the performances of the proposed scheme have been studied and no destructive effects were observed. The proposed structure does not include any optical or electrical filters which result in a wide operational bandwidth.

Microwave photonic notch filter with integrated phase-to-intensity modulation transformation and optical carrier suppression.

We demonstrate for the first time, to the best of our knowledge, an on-chip microwave photonic (MWP) notch filter with high stopband rejection and integrated optical carrier suppression in a phase modulator-based system. The notch filter was achieved through phase modulation to intensity modulation (PM-to-IM) transformation and dual-sideband-processing using a network of three ring resonators (RRs) in a low-loss silicon nitride (Si3N4) platform. We show simultaneous PM-to-IM conversion and optical carrier processing for enhancing the filter performance using a single RR. We achieve filtering with a high stopband rejection of >55dB, an optical carrier suppression up to 3 dB, a radio frequency link gain of 3 dB, a noise figure of 31 dB, and a spurious-free dynamic range of 100dB⋅Hz2/3. These experiments point to the importance of vectorial spectral shaping of an MWP spectrum for advanced functionalities.

Reduction of Phase-Modulation to Intensity-Modulation Conversion Noise Effect Using Delayed Self-Homodyne Optical Orthogonal Frequency Division Multiplexing System

Self-homodyne optical orthogonal frequency division multiplexing (OFDM) transmission system is a variant of self-coherent systems with a relaxed laser linewidth requirement as opposed to a full coherent system. The system is designed by transmitting a carrier together with the subcarriers, ensuring a strong phase-correlation between them at a receiver compared to the coherent system where only the subcarriers are transmitted. Despite the strong phase-correlation, the laser phase noise (PN) can still degrade the self-homodyne system performance due to the interaction of the PN with a chromatic dispersion (CD) induces a phase-modulation to intensity-modulation (PM-to-IM) conversion noise which cannot be canceled using a balanced-receiver. This imposes a high noise pedestal underneath each subcarrier, with the highest pedestal's peak imposed beneath the subcarriers that are farthest away from the carrier. This in turn produces a phase-decorrelation between the carrier and subcarriers which degrades the Q-factor between the lowest and highest subcarrier frequencies. In this work, the effect of the PM-to-IM conversion noise is reduced for the transmission of up to 174 Gb/s using a simple delay-line assigned to realign the phase-decorrelation of the subcarriers. With this technique, the subcarriers Q-degradation can be improved and the total transmission length can be tripled with Q-improvements of 5.4, 5.3, and 5-dB for 16-quadrature amplitude modulation (QAM) over 1360 km, 32- QAM over 800 km, and 64-QAM over 480 km, respectively, using 10-MHz LW with a 15-GHz total signal bandwidth. For 29 GHz signal bandwidth, the phase-walk-off of the high frequency subcarrier becomes more severe where the introduced delay can increase the transmission length by 2.5 times than without the delay, which increased the signal tolerance to the conversion noise.

Wavelength division multiplexing secure communication scheme based on an optically coupled phase chaos system and PM-to-IM conversion mechanism

We propose a wavelength division multiplexing secure communication scheme based on an optically coupled phase chaos system and phase modulation to intensity modulation (PM-to-IM) conversion mechanism. In this scheme, three channels of intensity-modulated data signal are simultaneously distorted by chaotic phase noise. Dispersion-induced PM-to-IM conversion mechanism is used to mask the data signal in intensity dimension. An additional self-phase modulation feedback loop in each channel is used to further scramble the phase dimension, and a time-domain key is introduced by a fiber delay line. The numerical results indicate the good encryption and decryption performance as the fundamental communication characteristics. The wavelength spacing under different parameter settings is also investigated in the wavelength division multiplexing scenario. Moreover, time delay signature concealment is confirmed by using both autocorrelation function and delayed mutual information approaches. Finally, the sensitivity dependence on key parameters and robustness of the scheme are discussed in detail. Both the security and feasibility can be guaranteed. The scheme has the potential to be used in large capacity optical secure transmission systems.

Fibre axial strain sensor based on a microwave photonic filter with dual-passband

A fibre axial strain sensor based on a phase modulated microwave photonics filter (MPF) with dual-passband is proposed and experimentally demonstrated. According to the phase-modulation to intensity-modulation (PM-IM) conversion in the MPF, dual-passband can be generated by utilizing two cascaded fibre Bragg grating Fabry-Perot (FBG-FP) filters. The fibre axial strain will change the wavelength of the FBG-FP, and resulting in the frequency variation in the passband. Thus, with one FBG-FP serving as the sensing element and the other acting as the reference element, a highly sensitive strain sensor can be realized simply by monitoring the frequency difference variation between the two passbands. The result shows that the passband frequency changes linearly in response to the strain with a sensitivity of 1.40 × 108 Hz/με. The sensor reveals the advantages of highly sensitive, small hysteresis and good stability, providing great potential for practical applications.

Ultrahigh-Resolution Electro-Optic Vector Analysis for Characterization of High-Speed Electro-Optic Phase Modulators

We propose and experimentally demonstrate an electro-optic vector analyzer (EOVA), which measures not only the magnitude but also the phase responses of high-speed electro-optic phase modulators with potentially 100-GHz measurement range and sub-Hz frequency resolution, for the first time to the best of our knowledge. The key component in the EOVA is an accurately calibrated phase modulation to intensity modulation (PM-IM) convertor, which converts the phase-modulated signal generated by the phase modulator under test into an intensity-modulated signal. After simply direct photodetection in a photodetector, the magnitude and phase information is extracted in the electrical domain. Removing the responses of the PM-IM convertor and the photodetector from the measured overall responses, the magnitude and phase responses of the phase modulator are achieved. The proposed EOVA is immune to the residual amplitude modulation in the phase modulator when an optical Hilbert transformer serves as the PM-IM convertor. In an experiment, the magnitude and phase responses of a lithium niobate high-speed phase modulator from 100 MHz to 42 GHz are precisely measured with a frequency resolution of 10 MHz.

Silicon Photonic Integrated Optoelectronic Oscillator for Frequency-Tunable Microwave Generation

Photonic generation of a frequency-tunable microwave signal based on a silicon photonic integrated optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The silicon photonic chip includes a high-speed phase modulator (PM), a thermally tunable micro-disk resonator (MDR), and a high-speed photodetector (PD). When an external light wave is injected into the chip, by a joint use of the PM, the MDR, and the PD, a bandpass microwave photonic filter (MPF) based on phase modulation and phase-modulation to intensity-modulation (PM-IM) conversion is realized. If the output microwave signal from the MPF is fed to the microwave input port of the PM with a sufficiently large gain provided by an electrical amplifier, the MPF becomes an OEO. By controlling the electrical power applied to a micro-heater, the resonance frequency of the MDR is tuned, which leads to the tuning of the MPF, and thus, the OEO oscillation frequency. In the experimental demonstration, two silicon photonic integrated OEOs using two MDRs with different micro-heaters are studied. The first OEO has a high-resistivity metallic micro-heater placed on top of the MDR, and the second OEO has a p-type doped silicon heater in the MDR. The two thermally tunable MDRs are characterized, and the performance of the MPFs based on the two MDRs is evaluated. The use of the two MPFs to implement two OEOs is performed, and their performance is evaluated in terms of frequency tunable range, phase noise, and power consumption.

Photonic UWB pulse generation and OOK, PPM, BPSK modulation for WDM systems based on twin-core fiber

We propose and demonstrate an approach for optical ultra-wideband (UWB) monocycle pulses generation by employing a phase modulator (PM) and twin-core fiber (TCF). Based on phase modulation to intensity modulation (PM-IM) conversion principle, multi-wavelength and polarity-reversed ultra-wideband (UWB) monocycle pulses are generated simply by making the optical carrier wavelength located at the positive or negative sloop of the periodic comb-transmission spectrum of TCF. The generated UWB monocycles with different optical carrier wavelengths are studied and analyzed. In addition, multiple modulation formats such as on-off keying (OOK), pulse position modulation (PPM) and binary phase shift keying (BPSK) are implemented. Simulation results show that the proposed scheme can offer a realistic solution to photonic UWB pulse generation and modulation for WDM systems.

Spurious Noises Analysis of Microwave Photonic Filters Based on Phase Modulation to Intensity Modulation Conversion Using an Optical Notch Filter

The spurious noises of a narrow-bandpass microwave photonic filter (MPF) using phase modulation to intensity modulation (PM-IM) conversion and an optical notch filter are analyzed in theory and demonstrated experimentally. Due to the difference between the nonlinear filtering effect of PM-IM conversion and the linear one of finite impulse response or infinite impulse response, additional spurious noises will be generated in the former filter. Analysis models with two input microwave tones are developed for revealing the spurious noises in the MPF based on PM-IM conversion under small-signal and large-signal modulations, which are then verified by simulations and experiments, respectively. The results show that, besides the target microwave signal to be selected, the inter-modulation noise and the harmonic noises also coexist at the output of the MPF, wherein the 2nd-order inter-modulation noises and the 2nd-order harmonic noise of the target signal are the dominant spurious ones. Therefore, an excellent agreement among the results from analysis models, the simulations, and the experiments is obtained.

Microwave Photonic Filter With Two Independently Tunable Passbands Using a Phase Modulator and an Equivalent Phase-Shifted Fiber Bragg Grating

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 ±1st 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 1st and 2nd passband is achieved. The frequency tunable ranges of the 1st and 2nd passbands are 5.4 and 7.4 GHz, respectively, with the magnitude variations of about ±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.

Wavelength Reuse in a UWB Over Fiber System Based on Phase-Modulation to Intensity-Modulation Conversion and Destructive Interferencing

A novel wavelength reuse scheme based on phase-modulation to intensity-modulation (PM-IM) conversion and destructive interferencing using a polarization modulator (PolM) and a fiber Bragg grating (FBG) in an ultra-wideband (UWB) over fiber (UWBoF) system is proposed and experimentally demonstrated. In the proposed scheme, a PolM and an FBG in the central station (CS) are used to generate intensity-modulated impulse UWB signal based on PM-IM conversion. The downstream UWB signal is sent to a base station (BS) over an optical fiber. At the BS, when the principle axis of the polarizer is aligned with one of the two orthogonal states of polarization (SOPs), the downstream UWB signal is received and detected. When the principle axis of the polarizer is oriented at a special angle to one of the two orthogonal SOPs, a clear optical carrier without intensity modulation is generated, which is then reused for upstream UWB data transmission. A theoretical analysis is performed which is verified by an experiment. A bidirectional point-to-point transmission of 1.25 Gb/s UWB signal over 25-km single-mode fiber (SMF) using a single light source from the CS is demonstrated. The receiver sensitivity and the BER performance for both downstream and upstream transmissions are measured.

Multitap Microwave Photonic Filter With Negative Coefficients Based on the Inherent Birefringence in a $\hbox{LiNbO}_{3}$ Phase Modulator

A novel technique to implement a multitap microwave photonic filter with positive and negative coefficients based on the inherent birefringence in a LiNbO3 phase modulator is proposed and demonstrated. In the proposed filter, a microwave signal is applied to the phase modulator and an optical polarizer is connected at the output of the phase modulator to perform polarization interference and phase-modulation to intensity-modulation (PM-IM) conversion. Thanks to the inherent birefringence in the LiNbO3 crystal, a π phase shift is obtained by adjusting the wavelength spacing between two adjacent wavelengths, which leads to the generation of a positive coefficient and a negative coefficient. An equivalent experiment is performed. Four-tap and six-tap microwave photonic filters with positive and negative coefficients are experimentally demonstrated. The reconfigurability of the four-tap and six-tap microwave photonic filters is also investigated.

Wavelength reuse in a bidirectional UWB over fiber system

Wavelength reuse in a bidirectional UWB over fiber system using a polarization modulator (PolM) and an electro-absorption modulator (EAM) is proposed and experimentally demonstrated. Since the PolM functions as a special phase modulator that supports phase modulation along the two principal axes with opposite modulation indices and the EAM is a polarization-independent component, the signals due to the phase-modulation to intensity-modulation (PM-IM) conversion along the two orthogonal directions in the upstream link will be complementary and cancelled out, thus the impact of the downstream signal to the upstream transmission due to the PM-IM conversion is fully eliminated. Error-free bidirectional transmission of a 1.25-Gbps UWB signal over 17 km single-mode fiber (SMF) is demonstrated. A power penalty due to the wavelength reuse for upstream transmission is measured to be as low as 0.2 dB.

Photonic generation multiband UWB impulse radio signal

A novel method to photonic generation multiband ultra-wide band (MB-UWB) impulse radio signal based on the summation of multiple doublet pulses with different weights and proper time delays is proposed. Microwave sinusoidal signal is used to amplitude modulate a tunable laser to get multi-wavelength source. The optical signal is then phase modulation with Gaussian pulse and fed into a section of single-mode fiber (SMF). The SMF services as phase modulation to intensity modulation (PM-IM) conversion to generate Gaussian doublet pulse, and offers proper time delays to Gaussian doublet pulses with different weights. MB-UWB pulses with sidelobe suppression levels over 25 dB, central frequency from 5 to 8 GHz, 10 dB bandwidth from 2.9 to 3.8 GHz are obtained in experiment.

An Economic Phase-Mdulation to Intensity-Modulation Converter

A novel phase-modulation to intensity-modulation (PM-to-IM) converter is proposed and experimentally demonstrated basing on a vertical-cavity surface-emitting laser (VCSEL). Comparing with the published schemes, in which the employed DI, FBG, OBPF or dispersion devices can only process a certain phase-modulated RF signal, the proposed scheme can achieve the conversion process in an economic and resourceful manner. To be the first one of achieving such PM-to-IM conversion process by a VCSEL, the feasibility of the proposal is experimentally demonstrated with an open eye diagram when it is employed to convert phase-modulated RF signal.

High-frequency characterization of an optical phase modulator with phase modulation-to-intensity modulation conversion in dispersive fibers

We propose a new method for characterizing optical phase modulators based on phase modulation-to-intensity modulation (PM-to-IM) conversion in dispersive fibers. The fiber dispersion spectrally alters the relative phasing of the phase-modulated signal and leads to the PM-to-IM conversion, which is extended to measure the modulation efficiency of optical phase modulators. In the demonstration, the frequency-dependent modulation index and half-wave voltage are experimentally measured for a commercial phase modulator. Compared with conventional methods, the proposed method works without the restriction of small-signal operations, and allows swept-frequency measurement with high resolution and accuracy by using a vector network analyzer.

All-optical binary phase-coded UWB signal generation for multi-user UWB communications

An all-optical incoherent scheme for generation of binary phase-coded ultra-wideband (UWB) signals is proposed and experimentally demonstrated. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PM-IM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating the phase-modulated light waves at the positive and negative slopes of the DI transmission spectra, binary phase encoded UWB codes (0 and π) are generated. We also experimentally demonstrate a bipolar UWB coding system with a code length of 4, operating at 1.25 Gb/s. And the decoding is analyzed as well. Our proposed system has potential application in future high-speed UWB impulse radio over optical fiber access networks.

Approach to all-optical bipolar direct-sequence ultrawideband coding

An approach to all-optical bipolar direct-sequence ultrawideband (UWB) encoding for multiple access communications is proposed and demonstrated. The bipolar coding is performed based on electro-optic phase modulation and phase modulation to intensity modulation (PM-IM) conversion in a fiber Bragg grating (FBG) array that serves as a multichannel frequency discriminator. The chip number and the chip period of the code are determined by the number of FBGs and their physical separation. By locating the optical carriers that carry a Gaussian pulse at the left or right slopes of the FBG reflection spectra, bipolar direct-sequence UWB codes are generated. A bipolar UWB coding system with a code length of 4 is experimentally demonstrated.