Breaking voltage-bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes (2011.13422v1)

Abstract: Electro-optic modulators with low voltage and large bandwidth are crucial for both analog and digital communications. Recently, thin-film lithium niobate modulators have enable dramatic performance improvements by reducing the required modulation voltage while maintaining high bandwidths. However, the reduced electrode gaps in such modulators leads to significantly higher microwave losses, which limit electro-optic performance at high frequencies. Here we overcome this limitation and achieve a record combination of low RF half-wave voltage of 1.3 V while maintaining electro-optic response with 1.8-dB roll-off at 50 GHz. This demonstration represents a significant improvement in voltage-bandwidth limit, one that is comparable to that achieved when switching from legacy bulk to thin-film lithium niobate modulators. Leveraging the low-loss electrode geometry, we show that sub-volt modulators with $>$ 100 GHz bandwidth can be enabled.

Breaking Voltage-Bandwidth Limits in Integrated Lithium Niobate Modulators Using Micro-Structured Electrodes

The manuscript presented addresses the advancement of electro-optic (EO) modulators integrated with lithium niobate (LN) platforms. In particular, it focuses on improving the modulator's performance by mitigating microwave losses at high frequencies via the implementation of micro-structured electrodes. Integrated LN modulators are pivotal to both radiofrequency (RF) analog and digital optical communications, but they traditionally require high drive voltages at microwave frequencies, limiting their efficiency and facilitating increased power consumption.

Technical Improvements

The primary innovation in this research is breaking the traditional voltage-bandwidth trade-off in LN modulators by adopting micro-structured electrode designs rather than conventional rectangular ones. This structured configuration reduces ohmic loss by effectively redistributing the current across extended electrode segments, thereby facilitating greater modulation efficiency at reduced voltages. Electro-optic modulators with LN substrates have historically functioned at half-wave voltage requirements ($V_\pi$) of around 3.5 V at lower frequencies, significantly increasing at frequencies over 50 GHz due to microwave losses and gap reduction constraints.

Key Numerical Results

The paper reports an outstanding RF half-wave voltage of 1.3 V maintained with an electro-optic response exhibiting only 1.8 dB roll-off at 50 GHz, a noteworthy improvement over legacy and existing thin-film LN modulators. The segmented electrodes achieve a simulated 3-dB bandwidth of 228 GHz which surpasses conventional designs illustrating a proportional enhancement in modulator efficiency. Furthermore, it highlights experimental validation, including a reduction in RF loss from 7 dB/cm to 2 dB/cm for micro-structured electrodes at 50 GHz, confirming theoretical predictions.

Implications and Future Directions

This research has significant implications, both practically and theoretically. It suggests that sub-volt modulators with greater than 100 GHz bandwidth are feasible, leading to potential reductions in power requirements for high-speed optical networks. The enhanced performance metrics signify a shift in design paradigms for EO modulators utilizing segmented electrodes on low permittivity platforms, fundamentally altering their interaction with conventional CMOS drivers and RF signal processing units.

Future developments might focus on refining substrate designs and exploring lower permittivity materials to achieve higher phase index matching and further reduce microwave losses. Moreover, the adoption of these advanced modulator designs in industry could reshape communications technology by lessening dependency on high-gain, high-speed electronic amplifiers and improving the energy efficiency and sensitivity of optical data transmission systems.

In conclusion, the research demonstrates a critical stride in overcoming existing bandwidth and voltage limitations inherent in LN modulators. Its projections for sub-volt operations at unprecedented frequencies suggest broader adoption across technological applications demanding efficient electro-optic modulation, marking considerable advancements in the field of integrated photonics.