Integrated Electro-Optics Modulator

*Yufeng Tao*

## **Abstract**

Electro-optic modulation (EOM) is an essentially important optical manipulation for on-chip photonics, optical communication and optical sensing. With emerging demands on efficient, broadband electro-optic modulation, the highperformance, integrated electro-optic modulation becomes indispensable. By manipulating phase or amplitude of optical field, optical information will be coded/modulated for communication or modulation. Through advanced micro/ nano fabrication technique, the electro-optics crystal could be cut into the required volume/shape as specific, integrated modulator, waveguide or meta-surface for nano-photonic applications, paving a solid way for the imminent nano-photonic devices. Herein, the basic electro-optics effects, opto-electronic applications, methods of fabrication/integration, and future prospect of lithium niobate crystal are discussed or introduced. Demonstrations of box-sealed EOM, in-fiber EOM and the fabricated lithium niobate waveguides on substrate will be found here.

**Keywords:** electro-optics modulator, lithium niobate, micro/nano fabrication, nanometric interferometric sensing, on-chip photonics

### **1. Introduction**

The emerging microwave photonics, optical communication network, photonic quantum computation and optical sensing require advanced electro-optics modulators with wide electrical modulation bandwidth (over 100 GHz), ultra-low optical insertion loss (<3 dB), scalable size, low half-wave drive voltage, ultra-fine signal quality, massively-producible fabrication ability and easy integration on various of insulator platforms [1–3]. Therefore, electro-optic modulators (EOMs) based on nonlinear optical materials such as lithium niobate (LiNbO3), lithium tantalite (LiTaO3), potassium titanium oxygenic phosphate (KTiOP4), liquid crystal on silicon (LCOS) or 2D layered material modulator are often utilized to convert electronic signals to the optical domain via generating optical phase carrier or highorder harmonics, which are critical components in modern telecommunication networks and microwave-photonic systems [2, 4].

By inducing the known phase carrier via EOMs to specific optical field, the useful optical information will be coded/manipulated, and frequency character will be re-distributed, after receiving optical field by photodetectors or spectrometers, the delivered optical information could be reconstructed through algorithms [5–10]. In practical opto-electronic applications, the extensively-studied and applied LiNbO3 often employs high driven voltage (>100 V) for relatively-low frequency band (DC to 1 GHz) in bulky size, while the resonant LiNbO3 allows a radio-frequency slewing rate (>1 GHz) at low driven voltages (<50 V, compatible to the CMOS devices). In addition of constant direct voltage, the external-applied electrical waveform could be sinusoidal, triangle, saw-tooth, trapezoid or other artificial waveforms.

Methods of self-growth of original bulky lithium niobate crystal is mature now, the real challenging mostly focuses on: (1) How to micro-miniaturize EOMs, massively-integrate them on various substrates [11–14]. (2) How to give large phase or amplitude variation in a tiny size EOMs, where short optical path inside EOM is helpful to decrease optical propagation loss [12, 15–17]. (3) How to improve their electrooptical efficiencies/group velocity matching/optical damage threshold [14, 17]. The solution of above challenges is highly-valued in optical communication networks/ microwave photonics, quantum photonics and non-reciprocal optics [18].

Of great interest to broad application, the technological advance of micro/ nanofabrication, high-power ultrafast laser processing makes the stiff, transparent crystal be artificially fabricated with ultrafine spatial resolution on substrate via high instantaneous power nanosecond or picosecond laser, which provides the key to integrate EOMs. Predictably, the imminent era of on-chip photonic devices via integrated EOMs will revolve optics, microwave, telecommunication, large-volume memory by providing tremendous new photonic applications.
