The general Electro-Optic modulators which employ lumped electrode structures face the limitation that the bandwidth of the device is constrained by the RC constant and a higher operation speed requires a shorter device length, which is also restricted by the RC-lump limitation. There is a significant advantage to employ a traveling-wave configuration of the electrodes type in order to eliminate limitations imposed by a lumped electrode design. In this section, the modulator employs traveling wave electrode structure is introduced and characterized. To simulate the distribution of the charge carriers, a self-consistent simulation of the charge and electrostatic potential is performed using CHARGE. MODE will then take the carrier density information and calculate the corresponding changes in the real and imaginary parts of the refractive index of the material. These parameters are then exported to INTERCONNECT, including the voltage dependent junction capacitance. INTERCONNECT element library offers the flexibility required for the design and simulation of traveling wave modulators. For more information of the simulation procedure, please see the Traveling Wave Modulator on device level.
Background
In a traveling wave electrode configuration, the reflections at the output end of the waveguide is significantly reduced by terminating the microwave signal with a matching load. Therefore the configuration overcomes the RC constant limitations suffered by lump devices. The device could be made longer and still can achieve the speed requirement as for the lump devices. By carefully controlling the index mismatch and impedance mismatch, the desired modulator can be achieved.
Literature review
In this section, we compared the simulation results of our Traveling Wave Electrode with the results in several published papers, and the results we reproduced are in good agreement with the published ones.
Modulation intensity versus microwave frequency for a traveling wave modulator
In Ref 2, a series of modulation intensity versus microwave frequency for a traveling wave modulator with various percentages of velocity mismatch between light wave and microwave is studied, we reproduced the results by simulating with our Traveling Wave Electrode element. The following figures show the modulation intensity simulation results with the modulator velocity mismatch varies from 5% to 50%. In each figure, the microwave loss varies from 1dB/(sqrt(GHz) cm) to 5dB/(sqrt(GHz) cm) .
Modulation intensity versus microwave frequency for a traveling wave modulator with various percentages of velocity mismatch between light wave and microwave
Modulation frequency response for different characteristic impedance and microwave losses
In Ref 3, the modulation frequency response for different characteristic impedance and microwave losses is studied; we reproduced the results by using our Traveling Wave Electrode for simulation taking electrode characteristic impedance as a parameter. The following plots show the simulation results with all the parameters indicated in the figure.
Modulation frequency response for different characteristic impedances and microwave losses
Modulation frequency response for different phase shift lengths
In Ref 4, several modulation frequency response for different phase shift lengths measurements are studied. Following are the figures we reproduced in our simulation using the Traveling Wave Electrode element. The phase shifter lengths for the two measurements are 1 mm and 2 mm, respectively, with the modulator bias voltage at 0 V and -3 V.
Modulation frequency response for different phase shift lengths
Modulation frequency response for different terminating impedance
In Ref 5, two measurements are performed. One is the frequency response with terminating resistor taking as a parameter, the other is the measurement of the normalized average voltage. The figures below show the results with all the parameters indicated.
Modulation frequency response for different terminating impedance
Modulation frequency response for 4 nm and 8 nm modulator
Using the model in Ref 3, the modulation intensity with 4 nm and 8 nm modulator bandwidth can be predicted by the following figure [6]. In the figure we reproduced, the blue and green curves measure the bandwidth for the 4 nm and 8 nm modulator, respectively.
All the simulations with the Traveling Wave Electrode element show a good agreement with the results in published literature, which proves the accuracy of the element. For more information, please see the application examples System Modeling Instruction & Results.
System modeling Instruction
In this section, the system modeling instruction of two traveling wave modulators are provided and the simulation results are discussed.
To illustrate the traveling-wave modulator, we built up two modeling systems, one modulator is driven by an external traveling wave electrode and the other modulator is directly driven by normal electrical signals but with build-in traveling-wave electrode.
In the file [[TWM_waveguide_electrodes.icp]], the optical modulator is driven by the NRZ electrical signal which goes through a traveling wave electrode waveguide. The optical modulator electrode type is set to be "lumped". The traveling wave electrode waveguide adds a filtering effect on the electrical signal. Following is the system modeling:
TW modulator waveguide electrode model
In the file [[TWM_modeling_electrodes.icp]], the optical modulator is directly driven by the NRZ electrical signal, however, the optical modulator itself has the electrode type set to be "traveling wave", Following is the system modeling:
TW modulator system model
System modeling results
TW Modulator Waveguide Electrodes System
For the TW modulator waveguide electrode system, with the element TW_1 disabled, the driving electrical signal and the eye diagram of the system are shown below:
Driving signal with traveling wave electrode disabled
Eye diagram with traveling wave electrode disabled
With the traveling wave electrode enabled, the electrical signal waveform after the waveguide has a filtering effect, hence the eye diagram of the system is degraded with time jitter and noise effect. The index mismatch for the traveling wave electrode waveguide is delta_n=0.1 and the microwave loss is 1080 dB/m. The standard parameters setting for the traveling wave electrode is:
The driving electrical signal and the eye diagram of the system with the traveling wave electrode enabled are shown below:
Driving signal with traveling wave electrode enabled (Δn=0.1, microwave loss=1080 dB/m)
Eye diagram with traveling-wave electrode enabled (Δn=0.1, microwave loss=1080 dB/m)
When the "microwave loss" is set to be 0 dB/m, with a 0.1 index mismatch, the waveform after the waveguide and the eye diagram of the system will only have slight differences compare with when the traveling wave electrode disabled.
Driving signal with traveling wave electrode enabled (Δn=0.1, microwave loss=0 dB/m)
Eye diagram with traveling-wave electrode enabled (Δn=0.1, microwave loss=0 dB/m)
TW Modulator Modeling Electrodes System
For the TW modulator modeling electrodes system, the working principle is the same as the traveling wave waveguide. With the electrode type of the modulator set to be "traveling wave" and the following parameter setting, the system has the same trend of waveform and eye diagram generated. The index mismatch in this example is delta_n=1 and the microwave loss is 0 dB/m.
Optical modulator enhanced setting
The received waveform and the eye diagram for the system are shown below:
Original and received signal (Δn=1, microwave loss=0 dB/m)
Eye diagram (Δn=1, microwave loss=0 dB/m)
The modulator with a larger index mismatch has a larger filtering effect thus the degradation of the signal and the eye diagram are more obvious.
Related publications
- Baehr-Jones, Tom, et al. "Ultralow drive voltage silicon traveling-wave modulator." Optics Express 20.11 (2012): 12014-12020.
- Kim, Inho, Michael RT Tan, and Shih-Yuan Wang. "Analysis of a new microwave low-loss and velocity-matched III-V transmission line for traveling-wave electro-optic modulators." Lightwave Technology, Journal of 8 (1990): 728-738.
- Kubota, K. A. T. S. U. T. O. S. H. I., J. U. N. I. C. H. I. Noda, and Osamu Mikami. "Traveling wave optical modulator using a directional coupler LiNbO 3 waveguide." Quantum Electronics, IEEE Journal of 16.7 (1980): 754-760.
- Xu, Hao, et al. "Demonstration and Characterization of High-speed Silicon Depletion-mode Mach-Zehnder Modulators." (2014): 1-1.
- Lin, S. H., and Shih-Yuan Wang. "High-throughput GaAs PIN electrooptic modulator with a 3-dB bandwidth of 9.6 GHz at 1.3 µm." Applied Optics 26.9 (1987): 1696-1700.
- Wang, S. Y., and S. H. Lin. "High speed III-V electrooptic waveguide modulators at λ-1.3 μm." Lightwave Technology, Journal of 6.6 (1988): 758-771.
- Chiu, Yijen, et al. "High-speed traveling-wave electro-absorption modulators."International Symposium on Optical Science and Technology. International Society for Optics and Photonics, (2001).