Push-pull Mach-Zehnder Modulator Research Articles

Flat Optical Frequency Comb Generation Based on Monolithic Integrated LNOI Intensity and Phase Modulator

A flat optical frequency comb (OFC) is generated by using a monolithic integrated electro-optic intensity and phase modulator fabricated on lithium niobite on insulator (LNOI) platform. The LNOI-based modulation chip consists of a push–pull Mach–Zehnder modulator (MZM) and a U-shaped phase modulator (PM) connected by a curved optical waveguide. Microwave and optical packaging are implemented for the modulation chip, where the input and output pigtails of the packaged modulation device are polarization-maintaining fibers, with a core diameter of 6.5 μm. The packaged LNOI-based modulation device is featured by a fiber-to-fiber insertion loss as low as 6.97 dB. The half-wave voltages of the MZM and the PM are measured to be 3.6 V and 3.4 V at 5 GHz, respectively. By using the modulation device, an OFC with a tooth spacing of 5 GHz is generated, and the 13 comb teeth in the generated OFC are with a power flatness of 2.4 dB. The measured results of this device indicate that the tooth spacing of the generated OFC can be extended to tens of gigahertz by using a microwave source with a higher output frequency. In addition, the number of the comb teeth can be enhanced beyond 20 by increasing the power of the radio-frequency signal applied to the PM or by further reducing the half-wave voltage of the PM.

A New and Simple Frequency Quadrupling Millimeter-Wave Signal Generation Enabled by a Single Mach-Zehnder Modulator Without Optical Filter

In this work, we have designed and proposed a new optical method to realize the generating of frequency quadrupling millimeter-wave (mm-wave) only employing one Mach-Zehnder modulator (MZM), and no any optical filter is needed. Compared with the previously reported quadruple frequency scheme, this work greatly simplifies the structure of the system, and makes the performance better and the cost lower. In the scheme, the MZM is operated at peak bias point, which is applied to retain the even order optical sidebands and wipe off the odd order optical harmonics. By tuning-up the optical shifter and optical attenuator (OATT) to control the zeroth order optical carrier returns zero, only ±2nd order optical tones are generated. We demonstrated and discussed the presented scheme feasibility based on the detailed mathematical formula derivation and numerical simulation, the simulation results of which show that employing a sinusoidal radio frequency (RF) source with frequency at 10 GHz to drive a push-pull MZM, a 40 GHz millimeter-wave (mm-wave) signal can be obtained. In the work, the performance of the generated quadrupling signal is that the optical sideband suppression ratio (OSSR) is 34.8 dB and the radio frequency sideband suppression ratio (RFSSR) can reach 27.2 dB.

Photonic time-stretch based on phase modulation for sub-octave applications.

Photonic time-stretch (PTS) has been extensively studied due to its great potential in analog-to-digital converters. Here, we propose and demonstrate a PTS system based on phase modulation for sub-octave applications. Different from the PTS system using a Mach-Zehnder modulator (MZM), the PTS system, which uses a phase modulator (PM), has an operation bandwidth within an octave and is more suitable for preprocessing of sub-octave signals. Within the sub-octave band, the system is free of all second-order spurious signals. Because there is no direct current bias in a PM, the problem of bias drift, as well as the nonlinear distortion caused by it, can be thoroughly avoided. In addition, based on phase modulation and direct detection, the proposed PTS system has higher stability and a more simplified structure than that based on coherent detection. An exact analytical model has been established, and some compact expressions have been derived to fully characterize all frequency components of the PM-based PTS system. System properties, including the power transfer function, 3-dB bandwidth, and nonlinear distortion have been discussed, and numerical and experimental results on the performance of the PM-based PTS have been presented. In addition, a dual-channel PTS that employs a PM and a push-pull MZM has been proposed to extend the operation bandwidth to multi-octave.

D-Band mm-Wave SSB Vector Signal Generation Based on Cascaded Intensity Modulators

We proposed and experimentally demonstrated a novel and simple method to realize D-band millimeter-wave (mm-wave) single-sideband (SSB) vector signal generation using cascaded one single-drive Mach-Zehnder modulator (MZM) and one push-pull MZM. After the first MZM driven by a radio frequency (RF) signal of 20-GHz, an optical frequency comb (OFC) with six flat carriers was successfully generated. Using the subsequent push-pull MZM driven by 10-GHz SSB vector signals and a photodiode (PD) for detection, we finally generated D-band SSB vector mm-wave signals at frequencies of 130-GHz and 150-GHz, respectively. The experimental results are well consistent with theoretical and simulation analysis. Based on the proposed scheme, 4-Gbaud generated D-band quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16QAM) mm-wave signals were transmitted over 10-km/25-km single-mode-fiber (SMF) and 1-m wireless links. The bit-error-rate (BER) performance can reach less than 7% hard-decision forward-error-correction (FEC) threshold of 3.8 × 10−3.

Terahertz-Wave Generation Based on Optical Frequency Comb and Single Mach-Zehnder Modulator

A novel Terahertz-wave (THz-wave) generation on the strength of optical frequency comb and single push-pull Mach-Zehnder modulator (MZM) with a pair of transmitter and receiver in 0.4-THz band is proposed. The advantages of the optical frequency comb based on phase modulator (PM) 1 and PM2 in series includes the uncomplicated operation and good stability to produce numerous subcarriers at 25-GHz intervals. THz-wave is generated based on two comb lines derived from the same optical frequency comb. One comb line is used as an optical local oscillator (LO) and the other is served as signal carrier for data modulation for heterodyne mixing. We investigate the generation and transmission of a 0.4 THz THz-wave signal carrying 4 Gbaud quadrature-phase-shift-keying (QPSK) or 1 Gbaud 16 quadrature amplitude modulation (16-QAM) data over back-to-back (BTB) transmission or 10 km standard single-mode fiber (SSMF) transmission, with the bit-error-rate (BER) performance below the hard decision forward error correction (HD-FEC) threshold of 3.8e-3.

Electrical Probing Test for Characterizing Wideband Optical Transceiving Devices with Self-Reference and On-Chip Capability

An electrical probing test method is proposed based on serial or parallel modulation mixing scheme for characterizing wideband optical transceiving devices including high-speed semiconductor laser diodes, electro-absorption modulators, photodetectors, and Mach-Zehnder modulators. The method enables self-referenced frequency responses measurement, such as modulation index of semiconductor laser diodes, modulation index of electro-absorption modulators, responsivity of photodetectors, and modulation index and half-wave voltage of dual-parallel Mach-Zehnder modulators, dual-drive Mach-Zehnder modulators, and push-pull Mach-Zehnder modulators. In the demonstration, the experimental results are compared to those obtained with the conventional optical or electrical methods for accuracy. The full-electrical test nature is promising for microwave characterization of wafer level devices and circuits with fully integrated transceiving components on chip.

Low voltage 25Gbps silicon Mach-Zehnder modulator in the O-band.

In this work, a 25 Gb ps silicon push-pull Mach-Zehnder modulator operating in the O-Band (1260 nm - 1360 nm) of optical communications and fabricated on a 300 mm platform is presented. The measured modulation efficiency (VπLπ) was comprised between 0.95 V cm and 1.15 V cm, which is comparable to the state-of-the-art modulators in the C-Band, that enabled its use with a driving voltage of 3.3 Vpp, compatible with BiCMOS technology. An extinction ratio of 5 dB and an on-chip insertion losses of 3.6 dB were then demonstrated at 25 Gb ps.

Harmonics analysis of the photonic time stretch system.

Photonic time stretch (PTS) has been intensively investigated in recent decades due to its potential application to ultra-wideband analog-to-digital conversion. A high-speed analog signal can be captured by an electronic analog-to-digital converter (ADC) with the help of the PTS technique, which slows down the speed of signal in the photonic domain. Unfortunately, the process of the time stretch is not linear due to the nonlinear modulation of the electro-optic intensity modulator in the PTS system, which means the undesired harmonics distortion. In this paper, we present an exact analytical model to fully characterize the harmonics generation in the PTS systems for the first time, to the best of our knowledge. We obtain concise and closed-form expressions for all harmonics of the PTS system with either a single-arm Mach-Zehnder modulator (MZM) or a push-pull MZM. The presented model can largely simplify the PTS system design and the system parameters estimation, such as system bandwidth, harmonics power, time-bandwidth product, and dynamic range. The correctness of the mathematic model is verified by the numerical and experimental results.

RF Performance of Single Sideband Modulation Versus Dual Sideband Modulation in a Photonic Link

Single sideband optical modulation can be used in multiple RF photonic link applications. However, single sideband optical modulation is often thought to provide lower RF output power than traditional dual sideband modulation techniques. While true in one specific case, it is not true in all cases. We theoretically and experimentally analyze the RF performance of a photonic link utilizing a Z-cut dual-electrode Mach-Zehnder intensity modulator with a 90° RF hybrid that produces optical single sideband modulation. The single sideband performance is compared to double sideband modulation using the same Mach-Zehnder modulator in either a push-pull configuration using a 180° RF hybrid or a single-arm drive configuration. The optical sideband powers, RF output power and the output intercept power and spur-free dynamic range of the third-order intermodulation nonlinearity are compared both theoretically and experimentally for each of the three cases. The performance of a double sideband modulated link using an X-cut inherently push-pull Mach-Zehnder modulator is also compared theoretically, assuming the same V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> and insertion loss as the dual-electrode Mach-Zehnder modulator.

A Software-Based and Dither-Less Modulators Bias Control Technique

A software-based bias control system of push-pull Mach-Zehnder Modulators (MZMs) without dither tone is described in this paper. The slope of the average optical output power respect to the bias voltage is used to control the operating bias points in the automatic control system. The paper analyses the theory of automatic bias control intensively and verifies the effectiveness of the proposed technique which is a modulation-format-free method under the RF signal through testing and contrasting the BER (bit error rate) between the new technique and manual measure on the experimental platform.

UWB-over-Fiber transmission system using a dual-output Mach-Zehnder modulator

A novel scheme for ultra-wideband (UWB) monocycle pulse generation is proposed based on a dual-output push-pull Mach-Zehnder modulator (MZM). The MZM is driven by a non-return-to-zero (NRZ) data sequence and biased at the nonlinear point to generate edge-triggered pulses. To combine the complementary pulses forming a monocycle UWB pulse, two schemes are used to implement incoherent summation structure. A proof-of-concept experiment of UWB-over-fiber down-link system is set up. Further, by using digital signal processing (DSP) to calculate the bit error rate (BER), the transmission performance of two system configurations is studied.

Distortion Cancellation in Time-Stretch Analog-to-Digital Converter

We show that the symmetry properties of the two outputs from a push-pull Mach-Zehnder modulator can be exploited to reject dispersion-induced second-order distortions in the time-stretch analog-to-digital converter (TS-ADC). Using differential operation and signal processing in the digital domain, we experimentally demonstrate suppression of the signal harmonics generated due to system nonlinearities. Simulations predict that distortion-limited dynamic range improves from 34 to 57 dB. In addition, differential signaling improves the signal-to-noise ratio (SNR) by 3 dB and rejects an even-order nonlinear distortion added by an electronic digitizer in the TS-ADC. It is also shown that this approach is robust even in the presence of mismatches in the differential signal paths. In general, the approach can also be applied to any ultrawideband (multioctave) optical link to obtain a high dynamic-range system.