Thin-film lithium niobate dual-parallel Mach-Zehnder modulator for a simple photonic system measuring Doppler frequency shift

In recent years,thin-film lithium niobate(TFLN)electro-optic(EO)modulators have developed rapidly and are the core solution for the next generation of microwave photonics(MWP)problems.We designed and fabricated a dual-parallel Mach-Zehnder modulator(DPMZM)based on TFLN,achieving a 3 dB electro-electro(EE)bandwidth of 29 GHz and a low drive voltage(Vπ=6 V).The device we manufactured is metal-encapsulated.It is noteworthy that we proposed a single-channel Doppler frequency shift(DFS)measurement system based on this device and conducted verification experiments.We coupled light from an external laser into the chip and passed it through each of the two sub-MZMs of the DPMZM.These lights were modulated by echo signals and reference signals.By measuring the frequency of the output signal,we can obtain a DFS value without directional ambiguity.The success of this experiment marks a key step in the practical application of TFLN modulators in MWP.

Tunable frequency-multiplying optoelectronic oscillator based on a dual-parallel Mach-Zehnder modulator incorporating a phase-shifted fiber Bragg grating

A tunable frequency-multiplying optoelectronic oscillator(OEO) based on a dual-parallel Mach-Zehnder modulator(DPMZM) is proposed and experimentally demonstrated. In the proposed system, the tunable fundamental microware signal is generated by a tunable optoelectronic oscillator incorporating a phase-shifted fiber Bragg grating(PS-FBG). By adjusting the DC bias of the DPMZM, the frequency-doubled microwave signal with a tunable frequency range from 11 GHz to 20 GHz and the frequency-quadrupled microwave signal with a tunable frequency range from 22.5 GHz to 26 GHz are generated. The phase noises of the fundamental, frequency-doubled and frequency-quadrupled signals at 10 k Hz offset frequency are-105.9 d Bc/Hz,-103.3 d Bc/Hz and-86.2 d Bc/Hz, respectively.

Compact temperature-insensitive modulator based on a silicon microring assistant Mach-Zehnder interferometer

On the silicon-on-insulator platform, an ultra compact temperature-insensitive modulator based on a cascaded microring assistant Mach-Zehnder interferometer is proposed and demonstrated with numerical simulation. According to the calculated results, the tolerated variation of ambient temperature can be as high as 134 ℃ while the footprint of such a silicon modulator is only 340 μm2.

High modulation efficiency thin-film lithium niobate modulator using a three-mode folded phase shifter[Invited]

In this study,we proposed and experimentally demonstrated an electro-optic modulator with a small footprint and high modulation efficiency,achieved through the utilization of a mode-folded phase shifter with lumped electrodes.The threemode phase shifter recycles the light three times with different waveguide modes while leading to a pronounced tightening of the optical field confinement.We experimentally obtained a 3.7-times improvement in the modulation efficiency.A low V_(π)L for thin-film lithium niobate(TFLN)Mach-Zehnder modulators of 1 V·cm is realized with a device footprint of 2.7 mm×0.6 mm(0.5 mm for the phase shifter).Even greater improvements in modulation efficiency can be expected through the incorporation of additional modes.

FSK Modulation Scheme for High-Speed Optical Transmission

In this paper, we describe the generation, detection, and performance of frequency-shift keying （FSK） for high-speed optical transmission and label switching. A non-return-to-zero （NRZ） FSK signal is generated by using two continuous-wave （CW） lasers, one Mach-Zehnder modulator （MZM）, and one Mach-Zehnder delay interferometer （MZDI）. An RZ-FSK signal is generated by cascading a dual-arm MZM, which is driven by a sinusoidal voltage at half the bit rate. Demodulation can be achieved on 1 bit rate through one MZDI or an array waveguide grating （AWG） demultiplexer with balanced detection. We perform numerical simulation on two types of frequency modulation schemes using MZM or PM, and we determine the effect of frequency tone spacing （FTS） on the generated FSK signal. In the proposed scheme, a novel frequency modulation format has transmission advantages compared with traditional modulation formats such as RZ and differential phase-shift keying （DPSK）, under varying dispersion management. The performance of an RZ-FSK signal in a 4 x 40 Gb/s WDM transmission system is discussed. We experiment on transparent wavelength conversion based on four-wave mixing （FWM） in a semiconductor optical amplifier （SOA） and in a highly nonlinear dispersion shifted fiber （HNDSF） for a 40 Gb/s RZ-FSK signal. The feasibility of all-optical signal processing of a high-speed RZ-FSK signal is confirmed. We also determine the receiver power penalty for the RZ-FSK signal after a 100 km standard single-mode fiber （SMF） transmission link with matching dispersion compensating fiber （DCF）, under the post-compensation management scheme. Because the frequency modulation format is orthogonal to intensity modulation and vector modulation （polarization shift keying）, it can be used in the context of the combined modulation format to decrease the data rate or enhance the symbol rate. It can also be used in orthogonal label-switching as the modulation format for the payload or the label. As an example, we propose a simple orthogonal optical label switching technique based on 40 Gb/s FSK payload and 2.5 Gb/s intensity modulated （IM） label.

12.5 Gb/s carrier-injection silicon Mach-Zehnder optical modulator

We demonstrate a 12.5 Gb/s carrier-injection silicon Mach-Zehnder optical modulator.Under a nonreturn -zero（NRZ） pre-emphasized electrical drive signal with voltage swing of 6.3 V and forward bias of 0.7 V, the eye is clearly opened with an extinction ratio of 8.4 dB.The device exhibits high modulation efficiency,with a figure of merit V_πL of 0.036 V·mm.

Dual-services generation using an integrated polarization multiplexing modulator

We proposed and demonstrated a new scheme for simultaneous generation of independent wired and wireless signals employing an integrated polarization multiplexing modulator. In the experimental system, a 10 Gb/s wired signal is imposed on the original optical carrier with one polarization, while a wireless signal with a bit rate larger than 4 Gb/s is carried on the generated millimeter wave of 76.44 GHz, which has polarization orthogonal to the wired signal. The dual services are successfully delivered over a 15 km standard single mode fiber;the power penalties of the wired and wireless signals are around 0.4 d B and 1.5 dB, respectively.

Nonlinearity-Compensation-Free Optical Frequency Domain Reflectometry Based on Electrically-Controlled Optical Frequency Sweep

A nonlinearity-compensation-free optical frequency domain reflectometry(OFDR)scheme is proposed and experimentally demonstrated based on the electrically-controlled optical frequency sweep.In the proposed scheme,the linear frequency sweep light is generated by propagating an ultra-narrow-linewidth continuous-wave(CW)light through an electro-optic frequency shifter which consists of a dual-parallel Mach-Zehnder modulator(DPMZM)and an electronic 90°hybrid,where the electro-optic frequency shifter is driven by a linear frequency modulated signal generated by a direct digital synthesizer(DDS).Experimental results show that the spatial resolution and signal-to-noise ratio(SNR)of the proposed OFDR scheme without the nonlinear phase compensation are comparable to those of OFDR employing a commercial tunable laser source(TLS),an auxiliary interferometer,and a software-based nonlinear phase compensation method.The proposed OFDR scheme is helpful to reduce the complexity of the optical structure and eliminate the difficulty of developing the nonlinear phase compensation algorithm.

Photonic Generation of Frequency 16-Tupling Millimeter-Wave Signal without Optical Filter

A generalized optical filterless approach to achieve photonic generation of frequency 16-tupling millimeter-wave (mm-wave) signal based on two cascaded dual-parallel Mach-Zehnder modulators (DPMZMs) is presented. A theoretical analysis leading to the operating conditions to achieve frequency 16-tupling is developed. Different modulation indices (MIs) can be implemented to achieve the frequency multiplication by adjusting the delay of tunable optical delay line (TODL). It is confirmed by simulation that the proposed scheme is effective, and the radio frequency spurious suppression ratio (RFSSR) of the generated frequency 16-tupling signal can be as high as 40 dB when the sub-MZMs have extinction ratios of 30 dB. Influencing factors such as extinction ratio, DC bias drift, phase shift deviation and RF voltage deviation on the performance of optical sideband suppression ratio (OSSR) and RFSSR are also investigated.

Mid-infrared all-optical modulators based on an acetylene-filled hollow-core fiber

We report all-optical mid-infrared phase and intensity modulators based on the photo-thermal effect in an acetylene-filled anti-resonant hollow-core fiber.Optical absorption of the control beam promotes the gas molecules to a higher energy level,which induces localized heating through non-radiative relaxation and modulates the refractive index of the gas material and hence the accumulated phase of the signal beam propagating through the hollow-core fiber.By modulating the intensity of the control beam,the phase of the signal beam is modulated accordingly.By use of a 1.53μm near-infrared control beam,all-optical phase modulation up to 2.2πrad is experimentally demonstrated at the signal wavelength of 3.35μm.With the phase modulator placed in one arm of a Mach-Zehnder interferometer,intensity modulation with on-off ratio of 25 dB is achieved.The gas-filled hollow-core-fiber modulators could operate over an ultra-broad wavelength band from near-to mid-infrared and have promising application in mid-infrared photonic systems.

Recent developments in graphene-based optical modulators

Graphene has shown promising perspectives in optical active components due to the large active-control of its permittivity-variation. This paper systematically reviews the recent developments ofgraphene-based optical modulators, including material property, different integration schemes, single-layer graphene-based modulator, multi-layer and few-layer graphene-based modulators, corresponding figure-of-merits, wavelength/temperature tolerance, and graphene-based fiber-optic modulator. The different treatments for graphene＇s isotropic and aniso- tropic property were also discussed. The results showed graphene is an excellent material for enhancing silicon＇s weak modulation capability after it is integrated into the silicon platform, and has great potentials for complemen- tary metal oxide semiconductor （CMOS） compatible optical devices, showing significant influence on optical interconnects in future integrated optoelectronic circuits.

Photonic Vector Signal Generation by Frequency Sextupling for Radio Over Fiber Systems

To generate high-frequency radio frequency(RF) vector signals, a vector signal generation method by optical frequency sextupling using a dual-parallel modulator is proposed. The method modulates vector signal on +3 rd order optical sideband and local oscillator(LO) signal on-3 rd order sideband using the intermodulation process in the DPMZM. After suppressing of the optical carrier and other sidebands through proper adjustment for modulator biases and modulation index, a frequency sextupled millimeter-wave vector signal can be generated after photodetection. The frequency sextupling will lower the bandwidth of the modulator, the local oscillator and the driving circuits. In addition, the phase of generated signal is not distorted after detection, and the power fading after fiber transmission can be avoided. In the simulation, a 500-MSym/s QPSK signal at 60 GHz is generated by 10-GHz drive signal. After travelling over fiber with length of 20/30/40-km, receiver power penalty keeps below 2.5 dB.

An Isotropic Electric-Field Sensing System Using Optical Probe

We design an isotropic electric-field sensing system using an optical probe. The optical probe consists of three electric-field sensors based on a Mach-Zehnder interferometer with the structure of segmented electrodes, which are perpendicular to each other. The measured results show that the ±5 dB baseband can reach 3 GHz, the linear dynamics range is 70dB, and the minimum detectable electric field is lower than 90 dBμ V/m （the spectrum analyser resolution is 100 Hz）. The directional characteristics are almost isotropic within deviations of ±1.5 dB. Therefore this sensing system can be used in the electromagnetic compatibility measurements.

An Integrated-Plasmonic Chip of Bragg Reflection and Mach-Zehnder Interference Based on Metal-Insulator-Metal Waveguide

In this paper,a Bragg reflector is proposed by placing periodic metallic gratings in the center of a metal-insulator-metal(MIM)waveguide.According to the effective refractive index modulation caused by different waveguide widths in a period,a reflection channel with a large bandwidth is firstly achieved.Besides,the Mach-Zehnder interference(MZI)effect arises by shifting the gratings away from the waveguide center.Owing to different optical paths with unequal indices on both sides of the grating,a narrow MZI band gap will be obtained.It is interesting to find out that the Bragg reflector and Mach-Zehnder interferometer are immune to each other,and their wavelengths can be manipulated by the period and the grating length,respectively.Additionally,we can obtain three MZI channels and one Bragg reflection channel by integrating three different gratings into a large period.The performances are investigated by finite-difference time-domain(FDTD)simulations.In the index range of 1.33–1.36,the maximum sensitivity for the structure is as high as 1500 nm/RIU,and it is believed that this proposed structure can find widely applications in the chip-scale optical communication and sensing areas.

Theory study on a photonic-assisted radio frequency phase shifter with direct current voltage control

A photonic-assisted radio frequency phase shifter with direct current voltage control is proposed using a polymer- based integrated Mach-Zehnder modulator. A closed-form expression of radio frequency （RF） signal power and phase is given. Theoretical calculation reveals that by carefully setting the bias voltages, RF signal power variation lower than 1-dB and phase accuracy less than 3° can be achieved and are not degraded by perturbation of modulation index once the bias voltage drift is kept within -3% -- 3%.

Colorless WDM-PON Using Single Side Band Carrier Suppressed Signals Mitigating Carrier Rayleigh Backscattering

A new scheme is offered for minimizing carrier Rayleigh backscattering(CRB) in single feeder fiber based wavelength division multiplexed passive optical network(WDM-PON). The proposed scheme is based on single side band carrier suppressed(SSBCS) signal, both at network and receiver sides, used for the first time at optical line terminal(OLT) and optical network unit(ONU) sides. We use dual-drive Mach-zehnder modulator(DD-MZM) for generating SSB-CS signals, which decreases the expense per bit in full transmission. SSB-CS mitigates CRB, both at OLT and ONU sides, because of having no chance of reflections from the carrier. Since no extra dedicated RF or laser source is used at ONU side, we thus achieve cost effective colorless WDM-PON system. Suppressed signals from four channels, each of 10 Gbps, are multiplexed before injecting into the fiber span of 25 km at OLT. At ONU side, half of the downlink power is used for re-modulating the data signal. The simulation results show an errorfree transmission. Moreover, the detailed power budget calculations show that the proposed scheme can be sought out for splitting ratio up to 128. Hence it offers enough system's margin for unseen losses.

Review on photonic method for generating optical triangular pulses

In this paper, several photonic generating methods for optical triangular pulses were reviewed. Four frontier research methods for generating optical triangular pulses were introduced, these four methods are respectively based on the frequency-to-time conversion, using normally dispersive fiber, by single-stage dual-drive Mach-Zehnder modulator （MZM）, and using dual-parallel MZM. These four methods can be classified into two categories in terms of the optical source employed, such as mode-lock laser （MLL） and continuous-wave （CW） respectively. Compared with the methods based on MLL, those based on CW have many advantages, such as simpler structure, lower price, higher stability, more flexible and wider tunability. Besides, the method using single-stage drive MZM can generate versatile waveform optical pulses, which has better performance than the first two methods in tunable capability of both repetition rate and center wavelength. With the same driving signal applied, the optical source using the dual-parallel MZM can generate signal with higher frequency than that of using the single-stage MZM.