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The Mach-Zehnder modulator (MZM) is an interferometric structure made from a material with strong electro-optic effect (such as LiNbO3, GaAs, InP). Applying electric fields to the arms changes optical path lengths resulting in phase modulation. Combining two arms with different phase modulation converts phase modulation into intensity modulation.
The Mach-Zehnder modulator structure is shown in Figure 1.
The optical input Ein is split into the upper and lower modulator arms that are phase modulated with two phase shifters driven by the electrical signals V1 and V2 and then recombined into the optical output Eout. Before we analyze the above implementation in more details, let’s look at a couple of MZM structures that have attracted a lot of attention lately due to advancements in data center interconnect (DCI) technologies and photonic integration.
Figure 1: Mach-Zehnder modulator structure
The structure of Figure 1 is often referred to as “concentrated” or “lumped” modulator structure. In the “distributed” or “traveling-wave” structure of an MZM, the radio frequency (RF) electrical structure acts as transmission line rather than a concentrated capacitor, thereby increasing electrical and optical interaction length, and improving the tradeoff between the efficiency and bandwidth of the modulator. For more information, refer to [1].
TW-MZM schematic in OptSim Circuit
In SE-MZM, each arm of the MZM uses multiple phase shifter segments, as shown below.
TW-MZM schematic in OptSim Circuit
In SE-MZM, each arm of the MZM uses multiple phase shifter segments, as shown below.
Schematic of a PAM-4 transmitter using SE-MZM
The longer interaction length in conventional traveling-wave MZMs (TW-MZMs) helps reduce drive voltage. However, longer electrodes result in higher RF losses and mismatch in group velocities between RF and optical signals, which in turn impact modulation bandwidth. The segmented approach offers the advantage of longer interaction lengths without increased loss by shifting the velocity matching to electronic timing circuits to control timing of applied electrical signals to match the optical delay between segments. Such structures can also accomplish multi-level modulation without using electrical digital-to-analog converters (DAC) by using inherent DAC capabilities of segmented phase-shifters. For more information, please refer to [2].
The splitting imbalance represented by the k factor results in the appearance of the extinction ratio. The Mach-Zehnder modulator is never completely off and we can in fact define the extinction ratio coefficient directly from the k factor as:
Moreover, the imaginary j factor in front of the sine component represents a spurious phase shift that appears when the extinction ratio is not ideally infinite.
And rewrite the equation above:
There are now phase shift terms that depend on the average of the two electrical inputs, while the intensity modulation is controlled by the difference of the two electrical inputs.
In the reference article [1], the chirp factor is defined as the ratio between the modulator output frequency modulation and amplitude modulation:
In an ideal Mach-Zehnder modulator with perfect power split between arms and driven with opposite voltages, the chirp factor can be assumed equal to zero [3].
There might be other sources of chirp-like asymmetry of construction between the upper and lower arms [4].
The chirp factor as defined above is used in the industry to characterize spurious frequency modulation at the output of the modulator and can be measured directly, see for instance [5].
A modulator is one of the most expensive components in a high-speed transmitter. The bandwidth, footprint (i.e., packaging density) and energy consumption of a modulator – all of which are very critical in today’s mega datacenter deployments - depend on a number of design trade-offs. If the modulator design is sub-optimal, the rest of the system cost and complexity can dramatically increase, with only marginal gains in performance.
If you’re a datacenter or its transceiver supply chain, you want MZM behavior modeled in a system for the purpose of technology selection and for showing your clients that your products meet or exceed customer’s system performance specifications. OptSim fits the bill.
If you are a supply chain to the transceiver designers, you want an optimal device-level design of the MZM to meet with the packaging, bandwidth and energy consumption specifications from your customers.
To design an MZM device, use 草榴社区’ Sentaurus TCAD and BeamPROP BPM from RSoft Photonic Device Tools. This example demonstrates the simulation of a 3D Mach-Zehnder Modulator implemented on silicon-on-insulator.
To simulate behavior of an MZM, you need advanced tools to enhance and accelerate user-modeling capabilities and provide real field design scenarios using extensive industry specifications. Simulation tools from the 草榴社区 Photonic 草榴社区 portfolio and the Sentaurus TCAD product can be used for accurate modeling.
MZM Layout in OptSim
Application Notes for Further Study
The splitting imbalance represented by the k factor results in the appearance of the extinction ratio. The Mach-Zehnder modulator is never completely off and we can in fact define the extinction ratio coefficient directly from the k factor as:
References
[1] /photonic-solutions/product-applications/foundry-process-tw-mzm.html
[2] /photonic-solutions/whatsnew.html#pic
[3] F.Koyama and K. Iga, Frequency chirping in external modulators, J. Lightwave Technol., vol.6, no.1, pp.87-33, Feb.1988.
[4] Tetsuya Kawanishi et al., Direct measurement of chirp parameters of high-speed Mach-Zehnder-type optical modulators, Optics Communications, vol.195, Issues 5-6, 15 August 2001, Pages 399-404.
[5] F. Devaux, Y. Sorei, J.F. Kerdiles, Simple measurement of fiber dispersion and of chirp parameter of intensity modulated light emitter, J. Lightwave Technol. 11 (1993) 1937-1940.