Segmented Modulator Optical DAC

• Introduction

  Optical domain digital-to-analog conversion (oDAC) has been shown to have great potential for efficient transmission of pulse amplitude modulated (PAM) and quadrature amplitude modulated (QAM) signals [1,2,3]. Compared to a standard approach employing an electrical DAC, a linear driver, and an electro-optical modulator, the oDAC allows for a simpler transmitter architecture and a significantly lower power consumption without compromising performance.

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• Description

  This setup illustrates how oDAC for M-PAM generation can be implemented and properly parameterized in VPIphotonics Design Suite (see Figure 1). Segmented Mach-Zehnder modulators (SEMZMs) and arrayed driver ICs are the key components that enable digital-to-analog conversion in the optical domain. The digital resolution of the oDAC is dependent on the number of high-frequency drivers and phase shifters included in the electronic and photonic ICs respectively. The variable number of components is modeled with a single PhotonicsTLM block with a parameterized section number.

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• Typical Results

  Among the several investigations that can be performed with this setup, binary-weighted and thermometer driving schemes are analyzed here. The multilevel optical eye diagrams for an SEMZM with three active sections are plotted in Figure 2 and Figure 3 respectively. In the first case, the amplitudes of the voltage applied on the i-th modulator active segment is V = 1/2^(i-1). Such a scheme maximizes the oDAC resolution, with a number of optical intensities equal to 2^(the number of segments), that is, 8 for this example.
  Thermometer drive reduces the design complexity, as the voltages applied to each modulator section are identical. However, the number of reachable optical intensities is reduced to (the number of segments + 1), that is, 4 using 3 modulator segments.
  The effect of the SEMZM nonlinear transfer function, that is, the compression of the levels about the dynamic range extremities, is observed by increasing the modulation depth (see Figure 4).

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• Further Information

  Keywords: Optical DAC, Segmented Mach-Zehnder Modulator, Pulse Amplitude Modulation, Photonic Integrated Circuits, PICs
  
  Similar demonstrations are available in VPIComponentMaker Photonic Circuits and on the VPIphotonics Forum.
  
  [1] T. Kato et al., "10-Gb/s - 80-km operation of full C-band InP MZ modulator with linear-accelerator-type tiny in-line centipede electrode structure directly driven by logic IC of 90-nm CMOS process," in OFC, 2011.
  [2] A. Shastri et al., "Ultra-Low-Power Single-Polarization QAM-16 Generation Without DAC Using a CMOS Photonics Based Segmented Modulator," IEEE JLT, Vol. 33, No. 6, 2015.
  [3] A. Aimone et al., "DAC-free Ultra-Low-Power Dual-Polarization 64-QAM Transmission with InP IQ Segmented MZM Module," in OFC, 2016.

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