A study on automatic bias control for arbitrary optical signal generation by dual-parallel Mach-Zehnder modulator

The radio frequency (RF) signal generation method using an external modulator is widely used in microwave photonics applications because it has the advantage of being able to generate coherent and stable RF signals with a higher resolution performance compared to the conventional method. A Mach–Zehnder modulator is widely used as an external modulator due to its high electro-optic coefficients and low attenuation characteristics but has a critical problem in that its electrical characteristics are changed by external environments such as temperature. In this paper, we considered the stabilization configuration to overcome this problem and propose an automatic bias control technique based on the simulated annealing algorithm of a dual-parallel Mach–Zehnder modulator (DPMZM) for quadruple signal generation. The proposed technique searches for the bias voltages of the modulator in real-time through the temperature test. In addition, the output of the quadrupled signal of the DPMZM is constantly controlled throughout the temperature range. Finally, it is confirmed that signals of a 10 GHz and 22 GHz frequency are generated using the intermediate frequency signals of a 2.5 GHz and 5.5 GHz frequency with the proposed automatic bias control technique, respectively.

Flat optical frequency comb generation by using one DPMZM and superposed harmonics

We proposed an optical 4-ary frequency shift keying (FSK) modulation scheme applying dual-parallel Mach-Zehnder (MZ) modulator. The 4-ary FSK based on the single-side-band modulation scheme can greatly lower the transmission speed in each sub-carriers and increase the transmission performance, comparing with the 2-FSK signal. The transmission performance of the 4-ary FSK was demonstrated after a 50 km single mode fiber. The results showed that the 4-ary FSK can realize error-free transmission. Moreover, we analyzed the influence of factors (such as disperse compensation and demodulation bandwidth) on the transmission performance in this paper. The analysis of the influenced factors can provide a theoretical basic for experiment.

A tunable ultraflat optical frequency comb generator based on the optoelectronic oscillator (OEO) using a dual-parallel Mach–Zehnder modulator (DPMZM) is proposed and experimentally demonstrated. By incorporating a tunable DPMZM-OEO, five comb lines were generated with frequency spacing from 5 to 12 GHz under a wide central wavelength tuning from 1530 to 1560 nm, and a comb flatness of 0.3 dB is obtained. The corresponding signal generated by the DPMZM-OEO is also measured, and the phase noise of the frequency tunable signals is as low as −125 dBc/Hz at 10-kHz frequency offset.

A new scheme for highly flexible optical Nyquist pulses generation is proposed by using a dual-parallel Mach–Zehnder modulator (DPMZM) and an intensity modulator (IM) without any external optical filter. The optical Nyquist pulses generator includes two stages. At the first stage, 2-, 3-, or 5-carrier light sources with two different frequency spacing are generated by utilizing a DPMZM. At the second stage, the light source used as seed light is then sent to IM, and two forms of optical frequency combs with spectral line flatness of less than 0.7 dB can be finally obtained by simply adjusting the DC bias and amplitude of radio frequency signal applied to IM. Ideal sinc-shaped Nyquist pulses are also generated in the time domain with bandwidth from 75 to 225 GHz and repetition rate from 40 to 80 ps. Simulation results show the high flexibility, effectiveness, and practicability of the proposed scheme, which can be applied to future high-capacity optical transmission systems.

A tunable high-order single-sideband signal generator with optical carrier suppression (SSB-OCS generator) is proposed, which is based on frequency multiplication operation using dual-parallel Mach–Zehnder modulator (DPMZM) and Mach–Zehnder interferometer (MZI). By changing all the modulators at maximum or minimum transmission point simultaneously, the amplitude of input radio frequency (RF) signals and the phase difference between input RF signals, two high-order even or odd signals with opposite amplitude of one sideband, are generated by upper DPMZM and lower DPMZM, respectively. Then, two optical signals are coupled to reserve one sideband, and the coupled SSB-OCS signal is injected into an MZI to suppress high-order intermodulation components. The simulated results show that the first-, second-, and third-order SSB-OCS signals can be achieved, and the suppression ratio is over 30 dB, which agrees well with the theoretical prediction. This scheme is featured by the capability to generate tunable high-order SSB-OCS signal with high suppression ratio, and the operation is simple and flexible. Finally, an experiment was carried out to demonstrate the feasibility of the proposed scheme, and a first- and second-order SSB-OCS signal were obtained, respectively, and the suppression ratio is over 24 dB.

In this paper, dual-chirped arbitrary microwave waveform has been generated through photonics, incorporated with single dual parallel mach-zehnder modulator (DPMZM) inbuilt mach zehnder interferometer (MZI) structure. We have taken two cases of chirping, i.e., linear and nonlinear chirps. A case of linear chirping has been explored previously. However, to the best of the authors' knowledge effect of nonlinear chirping in this paper is evaluated for the first time. Other photonics approaches are also available, such as spectra shaping and wavelength to time mapping. But due to fixed spectral response of spectral shaper, center frequency of linear chirp generated waveform is fixed. To get large center frequency again we have to use large number of spectral shapers which will increase the system complexity. DPMZM avoids such difficulties. These MZMs are biased at the minimum transmission point to get carrier suppressed modulation. Product modulator (PM) is cascaded to the lower arm of DPMZM. Here by using DPMZM we get two advantages. First we have two complimentarily chirped microwave waveforms and second up conversion of the frequency of microwave carrier. A dual-chirped microwave waveform with centre frequency 6 GHz with bandwidth 200 MHz and 2 GHz is generated. The paper gives specific details about various performance parameters such as input signal frequency and power, output signal parameters viz output frequency, chirp rate, chirp bandwidth, time bandwidth product (TBW), etc. The overall model and its performance parameters are computed through MATLAB simulation.

We have proposed a scheme of radio-over-fiber (RoF) system employing a dual-parallel Mach–Zehnder modulator (DP-MZM) based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). In this scheme, the pump and the signal are generated by properly adjusting the direct current bias, modulation index of the DP-MZM, and the phase difference between the sub-MZMs. Because of the pump and the signal deriving from the same optical wave, the polarization states of the two lightwaves are copolarized. The single-pump FWM is polarization insensitive. After FWM and optical filtering, the optical millimeter-wave with octuple frequency is generated. About 40-GHz RoF system with a 2.5-Gbit / s signal is implemented by numerical simulation; the result shows that it has a good performance after the signal is transmitted over 40-km single-mode fiber. Then, the effects of the SOA’s injection current and the carrier-to-sideband ratio on the system performance are discussed by simulation, and the optimum value for the system is obtained.

A reconfigurable microwave photonic frequency upconverter with local oscillator (LO) doubling or LO quadrupling is proposed based on two cascaded dual-parallel Mach–Zehnder modulators (DPMZMs) and a fiber Bragg grating (FBG) with a wide bandwidth (0.4 nm). The first DPMZM is used to generate the ±1st-order or ±2nd-order sidebands of the LO signal. The FBG is inserted to separate the positive and negative sidebands. Only the transmitted LO sideband is sent to the second DPMZM, and the carrier-suppressed single-sideband (CS-SSB) modulation of the intermediate frequency (IF) signal is realized by using an electrical 90 deg hybrid coupler. The reflected LO sideband via FBG and the −1st-order IF sideband are combined. Then, the upconverted signal with the frequency of 2ωLO + ωIF or 4ωLO + ωIF can be generated by a photodetector. The experimental results show that the spur suppression ratios of the upconverter with LO doubling and quadrupling are 21.3 and 11.3 dB, respectively. The proposed approach greatly reduces the frequency requirement of the LO signal. The purity of electrical spectrum is largely improved in both the LO doubling and LO quadrupling upconversion benefiting from the CS-SSB modulation. Furthermore, these two upconversion modes can be easily switched by tuning the DC biases of the first DPMZM.

Pre-distortion compensation for optical-based broadband LFM signal generation system

A novel automatic bias control (ABC) method which could lock an optical IQ modulator at arbitrary bias points is proposed and experimentally demonstrated for the first time. In the proposed method, the sequences and parameters of the used dither signals and the switch of the applied bias voltages are specially designed to minimize the influence of the power coupling between three MZMs in optical IQ modulator and the nonideal extinction ratios. By this way, precise electro-optical characteristic parameters of these three MZMs can be obtained by calculating the relevancy of the produced dither signals and the optical output signal. Based on these obtained parameters, any-bias-locking operation could be achieved via a feedback control loop. The accuracy and stability of the proposed ABC scheme are experimentally verified by using 1 GHz sine signals and 6 Gbaud single-carrier QPSK signals, and the accuracy of the proposed ABC algorithm is about 0.4° due to the limit of DAC resolution. Moreover, thanks to this ABC scheme, a simple and stable 29.2 GHz millimeter wave generation with SNR of 30 dB by optical frequency quadrupling technique could be realized without any optical or electrical filter.

We present a photonic approach to generating binary phase-coded microwave signals with large frequency tunability using a dual-parallel Mach-Zehnder modulator (DPMZM). The DPMZM consists of a pair of sub-MZMs embedded in the two arms of a parent MZM. In our scheme, one of the sub-MZMs is fed by a sinusoidal microwave signal to be phase-coded, and the other sub-MZM is driven by a rectangular coding signal. The optical signals from the two sub-MZMs are destructively interfered by adjusting the dc bias of the parent MZM. As a result, the optical carrier is binary phase coded. A binary phase-coded microwave signal is generated by beating between the optical carrier and the sidebands. The carrier frequency of the phase-coded microwave signal is widely tunable. Phase-coded microwave signals with two different frequencies at 10 and 20 GHz are experimentally generated, respectively.

A scheme to enlarge the spurious free dynamic range (SFDR) of the microwave photonic link is proposed based on a dual-parallel Mach–Zehnder modulator (DPMZM). By properly adjusting the phase of the RF signals and the bias voltages of the DPMZM, the second-order spurious components in the optical carrier band (OCB) of the two sub-MZMs can be canceled out completely, and the third-order and fifth-order spurious components in the first-order upper sideband (1-USB) produced by one sub-MZM have equal amplitude but $$180{^{\circ }}$$ phase difference with the other sub-MZM. Therefore, as the two optical beams are combined at the output of the DPMZM and the OCB and the 1-USB are abstracted by a bandpass filter to generate the transmitted signal, all the major optical spurious components that contribute to the third-order intermodulation distortion (IMD3) are canceled out. Theoretical analysis and simulation results show that the proposed scheme, without digital linearization and other optical processor, can suppress IMD3 approximately 30 dB and improve the SFDR by $$18~\hbox {dB}\,\hbox {Hz}^{2/3}$$ compared with the conventional quadrature biased MZM system.

A widely tunable microwave photonic (MWP) notch filter with high linearity based on a dual-parallel Mach—Zehnder modulator (DPMZM) is proposed and experimentally demonstrated. The motivation of this work lies in the fact that the MWP notch filter previously reported has nonlinear distortions generally induced by the nonlinear property of electro-optic devices. In this scheme, even-order modulated sidebands at the DPMZM output are effectively suppressed by properly adjusting bias voltages of the DPMZM, which are the dominant factor to induce the nonlinear distortions into the MWP notch filter. The proposed scheme is theoretically analyzed and experimentally verified.

We present some optical signal processing methods for optical time-division multiplexing, based on electro-optic modulators. Dual-parallel Mach–Zehnder modulator and time-lens-based signal processing for optical frequency comb generation, optical Nyquist pulse generation and time-domain demultiplexing are realized. Multifunctional optoelectronic oscillators which are capable of performing simultaneous electrical clock recovery, optical clock recovery and demultiplexing are demonstrated with modulators embedded as the key components.