Mach-Zehnder Intensity Modulator Research Articles

Performance Analysis of Six Electro-Optical Crystals in a High-Bandwidth Traveling Wave Mach-Zehnder Light Modulator

In this study, a traveling wave Mach-Zehnder intensity modulator (TW-MZM) was designed and optimized for six different electro-optical (EO) crystals: lithium niobate (LNB), potassium niobate (KNB), lithium titanate (LTO), beta barium borate (BBO), cadmium telluride (CdTe), and indium phosphide (InP). The performance of each EO crystal, including optical and radio frequency (RF) loss, applied voltage, and modulation bandwidth, was estimated and compared. The results suggest that, in theory, KNB, LTO, BBO, and CdTe have the potential to outperform LNB. However, it should be noted that the loss associated with KNB and LTO is comparable to that of LNB. The findings demonstrated that BBO and CdTe exhibit a modulation bandwidth exceeding 100 GHz and demonstrate the lowest loss among the considered crystals based on the assumed geometry.

10 Hz Stable Mode-locked Er-Fiber Laser

Abstract: In this paper, a stable output at the reduction repetition rate from 80 MHz mode-locked Er-doped fiber laser (EDFL) was demonstrated by adjusting the operating point for external LiNbO3 Mach-Zehnder‎ electro-optic intensity modulator to suppress harmonic mode-locked pulses. More than 70% of the laser beam was launched into the external pigtail Mach-Zehnder intensity modulator using single mode fiber with collimating lens and fiber (ferrule connector) FC adapter. The optimum values of Mach-Zehnder modulator bias voltages V0 and Vπ (half wave voltage) which resulted in maximum and minimum laser output power to leave the modulator were found at 6.5 and 0.29 V, respectively. The best modulation was achieved for the RF modulating signal with an opening time of about 8 ns. For driving voltage equal to Vπ, a clean mode-locked pulse train was generated with a pulse shape and energy similar to the input pulse, albeit with a low repetition rate. The minimum repetition rate reached as low as 10 Hz, resulting in pulse energy and peak power of 0.32 nJ and 1.5 kW, respectively. Second-harmonic generation of the low repetition rate pulse train was also produced by using a PPLN crystal. This opens up the potential for using this laser in single photon detection experiments and as a seed laser for many applications. Keywords: Er-fiber laser, Passively mode-locked fiber laser, Nonlinear polarization rotation, Mach-Zehnder modulator, Fast electro-optic modulator. PACS: Fiber lasers, 42.55.Wd, Mode locking, 42.60.Fc.

Arbitrary bias control of LiNbO3 based Mach-Zehnder intensity modulators for QKD system

Quantum key distribution (QKD) can help distant agents to share unconditional secret keys, and the achievable secret key rate can be enhanced with the help of decoy-state protocol. To implement QKD experimentally, the agents are supposed to accurately transmit a number of different intensity pulses with the LiNbO3 based Mach-Zehnder (LNMZ) intensity modulator. However, the bias drift of LNMZ intensity modulator may affect the performance of a QKD system. In this letter, we reveal a simple RC circuit model to demonstrate the bias drift in the LNMZ intensity modulator. And based on the model, we propose a multi-step bias stable scheme to control the bias working point. Experimental result shows that our scheme can eliminate the bias drift of at arbitrary working point within a long time range. Besides, there is no need of any feedback mechanisms in the scheme. This means our scheme will not lead to any increasement in system complexity, making it more suitable for a QKD system.

Reconfigurable flexible PON downlink by using a wavelength selective switch and single intensity modulator

A reconfigurable passive optical network (PON) downlink has been experimentally demonstrated. A multi-carrier signal is generated in the digital domain and converted to an optical signal by a Mach-Zehnder intensity modulator. A wavelength selective switch (WSS) demultiplexes the sub-carriers for a distribution network. Each sub-carrier signal is a duo-binary filtered binary phase shift keying and directly detected at the receiver side. By changing the sub-carrier allocation and WSS port setting, the PON network can be easily reconfigured without changing the hardware configuration. The maximum capacity of 75-Gb/s with 10-km transmission and 128-way split has been achieved.

Integration of Raspberry Pi and antennas for multiplexing digital signals over a fiber optical communication system

A Fiber Optical Communication System (FOCS) is used to experimentally demonstrate the transmission of multi-channel information of two High Definition (HD) video signals and 18 digital TV channels. These signals are supplied using commercial Raspberry Pi boards and digital TV Antennas, respectively. Both digital signals and lightwaves of a couple of Distributed Feedback (DFB) lasers with wavelengths located at 1547.5 and 1552.5 nm interact with two Mach-Zehnder Intensity Modulators (MZ-IM). The resulting modulated optical signals from each modulator are combined by using an optical coupler. The combined optical field is amplified and transmitted through 30 km of single-mode optical fiber. To demultiplex the combined optical signals, a bandpass optical filter is used, then a PIN-photodetector is connected to recover HD video signals and the 18 digital TV channels. The measured electrical spectrum of the received signals shows that the transmission is performed without interference and the multi-channel information is successfully received in an HD display. The use of wavelengths with linewidths of 10 MHz and an optical fiber with a chromatic dispersion of 17 ps/nm-km allows for a maximum bit rate of approximately 11 Gbps to be achieved numerically. This result guarantees that HD video signals can be experimentally transmitted successfully with Bit Error Rate (BER) values between 10-6 and 10-15 when the optical power of lasers is tuned between 7 and 9 dBm.

Mach–Zehnder Modulator Output in Time and Frequency Domain—Calculation and Experimental Confirmation

The Mach–Zehnder intensity Modulator (MZM), named after Ludwig Mach and Ludwig Zehnder, is based on the corresponding interferometer. It splits light into two counter-rotating partial beams, which are later recombined with a controlled phase difference. The output of the MZM depends on the phase difference of the interferometer paths. This phase difference is usually adjusted by an electrical voltage applied to a Phase Shifter (PS) placed in one of the interferometer arms. For MZM applications in which the wavelength is changing, the applied voltage must be adjusted accordingly. We derived the equations describing the MZM output in the frequency domain for the case of a triangular PS voltage (necessary for a sinusoidal output) and compared the analytical results with measurements. Our setup uses an Optical Parametric Oscillator (OPO) with a tunable wavelength from 3.2–3.5 μm as the light source and a Lithium Tantalate (LT)-PS for the MZM’s phase modulation. The novel insights enable new control methods for MZMs particularly suited for spectroscopic applications where the wavelength is scanned or otherwise altered.

A full duplex FSO transmission system using RZ-4PAM downlink signals with 67% duty cycle and AMI uplink signals

To improve the transceiver quality of Free Space Optical (FSO) system and ensure that user access is feasible the network, a high-speed full-duplex 100 meters FSO transmission scheme using 67% duty cycle RZ-4PAM downlink and alternative mark inversion (AMI) uplink signals is designed system. In the downlink, one PAM sequence generator and one Mary pulse generator (MPG) module are used to generate 67% RZ-4PAM optical signals at 10Gbit/s through a Mach-Zehnder intensity modulator (MZM). The upstream AMI signals generator by the pseudo-random binary sequence (PRBS) generator and the precoding process. We have measured optical spectrums, time-domain waveforms and eye diagrams before and after signals transmission by system simulation. The transceiver performance of the downstream RZ-4PAM with 67% duty cycle and upstream AMI optical signals the in the 100m FSO transmission system is analyzed.

Photodetector optic power optimization to increase the gain on sub-octave microwave photonic link / Optimização da potência óptica do fotodetector para aumentar o ganho na ligação fotónica de microondas sub-octave

An investigation of the optic spectral components beating at the photodetector to recover the RF signal at the output of a sub-octave microwave photonic link is presented, by a theoretical and experimental approach. It is demonstrated the best efficiency is achieved when the carrier to sideband ratio (CSR) is 0 dB at a low bias voltage condition on Mach-Zehnder (MZM) intensity modulator. A RF power link gain improvement of 9.7 dB is demonstrated for a same photodetector incident optic power, compared with a link operating at quadrature bias voltage condition on the MZM.

Signal recovery based on optoelectronic reservoir computing for high speed optical fiber communication system

Signal recovery effects of a channel equalization method based on optoelectronic reservoir computing (RC) are investigated for actual high speed optical fiber communication systems. The RC is implemented numerically by using a Mach–Zehnder intensity modulator with an optoelectronic delay feedback loop. Two optical communication systems with different transmission distances and receivers based on 25G-class optics are built in-lab. 50-Gb/s non-return-to-zero (NRZ) and four-level pulse amplitude modulation (PAM4) signals transmitted by the two systems are recovered by using optoelectronic RC and different combinations of feed-forward equalizer (FFE) with decision feedback equalizer (DFE) and maximum likelihood sequence estimation (MLSE), respectively. Under the same conditions, optoelectronic RC is better than FFE&DFE, and far superior to FFE&MLSE in the equalization performances. When the receiver is a positive–intrinsic–negative or avalanche photodiode, for the NRZ signal, compared with 45-tap FFE and 9-tap DFE, the sensitivity is increased by ∼3.2/3.4 dB; for the PAM4 signal, the sensitivity is improved by ∼4.9/4.6 dB compared with 123-tap FFE and 9-tap DFE. These experimental results show that the optoelectronic RC has excellent performances for signal recovery of different modulation formats and transmission systems.

Simultaneous measurement of gas absorption and path length based on the dual-sideband heterodyne phase-sensitive detection of dispersion spectroscopy

We present a novel approach based on dual-sideband heterodyne phase-sensitive detection of dispersion spectroscopy to realize simultaneous measurement of the gas absorption signal and corresponding path length. The details of heterodyne phase-sensitive detection of dispersion spectroscopy are derived. A standard Mach-Zehnder intensity modulator (MZM) is adopted to generate a spectrum of a carrier and two sidebands. Phase shift of the beatnote signal generated by the two sidebands is detected to retrieve the path length as well as the gas absorption signal. The measurement range of the path length can be adapted by changing the modulation frequency. Proof-of-principle experiments are conducted with methane (CH4) as the absorber which is filled into a gas cell with a variable path length. We also utilize this approach to evaluate the path length of a White cell and meanwhile calibrate the experimental system with different concentrations of methane. The proposed method has a great potential for detecting the path length and gas absorption in multipass cells and the open path environment.

Investigation of optical short pulse generation using intensity modulation for LiDAR and free-space communications

A simple optical pulse generation scheme for pulsed light detection and ranging (LiDAR) and free-space communications is proposed and experimentally and numerically demonstrated, in which continuous-wave light emitted by a distributed feedback-laser diode (DFB-LD) is modulated by a lithium-niobate Mach–Zehnder intensity modulator to generate optical short pulses, and an Er-doped fiber amplifier (EDFA) is used to boost transmitting light power. Possibilities of intensity modulators for high speed communications being used to generate optical short pulses for low speed pulsed LiDAR are investigated. The influences of bias voltage of intensity modulator and bit rate of modulation signal on the generated optical pulses are discussed. The pulse width obtained by using the return to zero signal with 33% duty cycle on bit rate of 2.5 and 5 Gbit/s is respectively 128.0 and 63.2 ps, and the corresponding nominal accuracy of pulsed LiDAR is respectively 19.2 and 9.5 mm, and low repetition frequency required by pulsed LiDAR is achieved by coding to high-speed modulation signal. The system performance for free-space communications and pulsed LiDAR is evaluated, respectively. We believe that the proposed scheme is suitable for the integrated system of pulsed LiDAR and free-space communications.

Use of a differential evolution algorithm for determining input driving signals in optical frequency combs

In this work, a genetic algorithm based on differential evolution is proposed to identify sets of input driving signals used in the generation of optical frequency combs with Mach-Zehnder intensity modulators. Simulation results show that the algorithm is a powerful technique for solving a multiparameter optimization problem concerning the design of combs with a desired number of optical frequency lines under a given flatness. Results from experiments corroborate that combs with the desired number of lines can be achieved according to the driving voltages provided by the algorithm.

Transmission characteristics of 24.5 Gb/s atmospheric laser communication based on optical frequency comb

Through cascading Mach–Zehnder intensity modulator (MZIM) and phase modulator (PM), an optical frequency comb (OFC) with 7 flat comb teeth is produced. The frequency interval of the OFC is 25 GHz and the flatness is less than 1.5 dB. The 7 channels of the OFC are used as the carrier source of the one-to-many free-space optical (FSO) communication system, and the atmospheric turbulence simulation pool is used to simulate the turbulence intensity in the real atmosphere to verify its feasibility for FSO communication. In the experiment, the bit error rate (BER) and eye diagram are measured under the back-to-back (BTB) conditions, and the temperature difference of atmospheric turbulence simulation pool are 80 °C, 150 °C and 220 °C. Each channel of the system can stably transmit the on–off keying (OOK) signal of 3.5 Gb/s, and the 7 channels can stably transmit the OOK signal of 24.5 Gb/s. Thereby, the OFC can be used in place of multiple lasers or laser arrays as a carrier source for a one-to-many FSO communication system.

Switchable transverse-mode operation of an actively mode-locked EDF laser based on low-modal-crosstalk mode MUX/DEMUX

In this paper, we report an all-fiber actively mode-locked erbium-doped fiber laser based on the mode multiplexer/demultiplexer (MUX/DEMUX). Higher-order mode and mode-locked pulse outputs can be realized simultaneously. The mode-locking is achieved by using a LiNbO3 Mach–Zehnder intensity modulator. Multiple linearly polarized (LP) modes in the ring fiber laser are generated by mode MUX/DEMUX. For different spatial modes (LP01, LP11, LP21, hybrid modes), 17.32 ns, 20.40 ns, 23.00 ns and 17.24 ns mode-locked pulse trains are successfully obtained, respectively. The output laser operates at the fundamental frequency of 8.064 MHz with center wavelengths of 1528.71 nm for LP01 mode, 1528.30 nm for LP11 mode, 1530.45 nm for LP21 mode and 1530.05 nm for hybrid mode. Moreover, we also measure the slope efficiency, spectrum and intensity distribution. The results show that the proposed fiber laser has potential applications in mode-division-multiplexed systems, optical sensing and optical fiber communication.

Coupled Dual-Loop Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

A tunable optoelectronic oscillator (TOEO) with coupled dual-loop (CDL) based on stimulated Brillouin scattering (SBS) is proposed. In our scheme, the CDL is constructed by cascading a dual-output Mach–Zehnder intensity modulator (DOMZM) and a single output Mach–Zehnder intensity modulator (MZM). One optical output of the DOMZM is directly injected to the MZM, and the other optical output of the DOMZM is transformed into microwave signals to modulate the MZM. The narrow gain spectrum of SBS is used to select the oscillation frequency and realize frequency tunability. By joining the CDL, we can not only increase side mode suppression ratio (SMSR) and decrease phase noise, but also improve stability of the TOEO. Experimental results show that microwave signals from 2 GHz to 18 GHz with the SMSR as high as 60 dB and the phase noise as low as −95 dBc/Hz at 10 kHz frequency offset are achieved. Furthermore, the frequency drift is less than 0.3 ppm and the power drift is lower than 0.2 dB at 10 GHz within 30 min in lab condition.

Generation of noise-like pulse and dissipative soliton pair in actively mode-locked fiber laser

This paper reports on the experimental observation of a stable single dissipative soliton, a noise-like pulse, and a dissipative soliton pair in an actively mode-locked fiber laser within an all-normal dispersion cavity. The laser consists of fully polarization-maintaining fibers and components, providing a linear output polarization state. The full width at half maximum of the pulse is measured to be 25.3 ps and the output pulse energy after the gain fiber can reach 1.07 nJ. By shifting the direct current (DC) bias voltage of the modulator to Vπ, the spectral width increased from 6.3 nm to 7.7 nm. Under the same pumping power of 246 mW, the single dissipative soliton and the pair can exchange with each other by adjusting the DC voltage of the Mach–Zehnder intensity modulator.

Terahertz imaging using an optical frequency comb source

Optical frequency combs, which are generated by the cascade of a phase modulator and a Mach–Zehnder intensity modulator, are used as a polychromatic signal source in the terahertz imaging system to improve imaging quality. The interference effect caused by the monochromatic wave has been greatly suppressed. The required optical power in the presented system is as low as ∼30% of that in the system using the Er-doped fiber amplifier as a source, which can reduce cost and protect photodiodes from damage. This work provides an effective, low power consumption, low cost, and easy way to realize terahertz imaging with high quality and can be used in future security inspections.

Generation and characteristics of shock optical pulses based on a fiber-loop time-lens system

<sec>The shock ignition scheme has the advantages of low ignition energy threshold, high gain, and good hydrodynamic stability, which has become one of the key schemes for the potentially successful ignition of inertial confinement fusion. The crucial element of shock ignition is how to achieve a highly efficient shock laser pulse. We propose a new scheme based on a time-lens system combining the fiber-loop phase modulation and the grating-pair compression to generate a highly controllable shock pulse. Based on the asymmetric phase modulation in time-domain followed by linear dispersion compensation in frequency domain, the shock pulse can be actively controlled with high precision in both pulse duration and pulse contrast (peak power ratio of the compression part to the shock part of the pulse). We construct a theoretical model based on the nonlinear Schrödinger equation to simulate the evolution of the spectrum and temporal shape of the shock laser pulse. The influences of various key parameters of the proposed system on the characteristics of the generated shock pulse are analyzed in depth. </sec><sec>The time lens system consists of three parts, i.e. the seed pulse carving part, the phase modulation loop, and the chirp-compensating grating pair. The operation principle of this system for generating shock pulse is as follows. First, a single-mode continuous wave 1053 nm distributed feedback seed laser is chopped into pulses with a Mach-Zehnder intensity modulator. Then the pulses enter into a fiber-loop for phase modulation. Owing to different modulation frequencies exerted on the left and right side of the pulse, the amount of spectral broadening of these two sides of the spectrum are also different after phase modulation. The spectrally broadened pulses are linearly chirped when the phase-modulation function has a parabolic shape. Finally, the pulse transits through a grating pair system for chirp compensating. Just like an anomalous dispersion delay line, the grating pair applies an anomalous group velocity dispersion to the passing optical pulse. When the chirp is compensated for appropriately, the pulse will be compressed. What the target pulse can be finally shaped into is dependent on the combined optimization of all the above processes.</sec><sec>The simulation results show that by systematically designing the parameters such as chopping function, phase modulation function, modulation depth, modulation frequency, and chirp compensating, the target shock pulse can be actively controlled with high-precision in the pulse width, pulse rising edge, and peak-power contrast. In addition, we can also tune only one parameter (such as the pulse width) of the pulse, with the other parameters kept unchanged. This new design idea and the proposed system can actively and independently adjust the two key parameters (the peak power contrast and the pulse width) of the generated shock pulse, which is not only helpful in deepening our understanding of the principle of laser-pulse shaping, but also significant for the subsequent practical implement of shock ignition of inertial confinement fusion.</sec>

Photonic Integrated Fully Tunable Comb Generator Cascading Optical Modulators

We report the first monolithic InP photonic integrated comb generator made by cascading optical modulators. The device is extremely compact and tunable both in repetition frequency and wavelength, making real-world applications possible. A distributed Bragg reflector laser, a Mach–Zehnder intensity modulator, and two phase modulators are monolithically integrated on a 4.5 × 2.5 mm2 chip that also includes a booster semiconductor optical amplifier at the output. Modulators integrated on the circuit have a 3 dB bandwidth of 7 GHz and can generate up to 28 comb lines within a 5 dB power range when properly electrically driven (here demonstrated in the range 4–5 GHz). Operation up to 10 GHz is also reported.

Mode-locked all-fiber laser generating optical vortex pulses with tunable repetition rate

We propose and demonstrate an all-fiber actively mode-locked laser producing optical vortex pulses with high efficiency and a tunable repetition rate. Both vectorial and scalar optical vortices, i.e., cylindrical vector beams and orbital angular momentum beams, have been generated. Highly efficient mode conversion is realized by introducing a two-mode long-period fiber grating into the laser cavity as a mode converter with low insertion loss. A two-mode fiber Bragg grating is employed as a mode selector, a spectrum filter and an output mirror. A LiNbO3 Mach–Zehnder intensity modulator served as a mode-locker to achieve active harmonic mode-locking. The slope efficiency increases from 10.24% to 12.61% with the repetition rate of the pulse train flexibly tuned from 15.65 MHz at the fundamental mode-locking to 626 MHz at the 40th order harmonic mode-locking with superior stability. Switching between vectorial and scalar optical vortices at different repetition rates is realized through the intra-cavity state of polarization control.