Forward bias operation of silicon photonic Mach Zehnder modulators for RF applications

Abstract

In this paper, we demonstrate that forward bias (+0.9V) of a high-speed silicon (Si) optical Mach-Zehnder modulator (MZM) increases the radio-frequency (RF) link gain by 30 dB when compared to reverse bias operation (-8V). RF applications require tunable, narrowband electro-optic conversion with high gain to mitigate noise of the optical receiver and realize high RF spur-free dynamic range. Compared to reverse bias, the forward bias gain rolls off more rapidly but offers higher RF link gain improvement of more than 13.2 dB at 20 GHz. Furthermore, forward bias is shown to result in comparable spurious-free dynamic range (SFDR: 104.5 dB.Hz). We demonstrate through an analytical dc transfer curve the existence of simultaneous high gain and OIP3 and verify the theoretical results with measurement under forward bias at a bias point of around +0.9 V.

Three-Port Optical Phase-Shifters and Modulators With Ultra-High Modulation Efficiency, Positive RF-Linking Gain, and Low Residual Amplitude Modulation

“…It can be clearly seen that under 3-port operation the device can not only provide us with a smaller RAM (0.18 vs. 1.6 dB) but also a smaller V π (0.18 vs. 0.22 V). The achieved RAM number (0.18 dB) is also much smaller than that reported for Si-photonic MZI under forward bias operation, where the RAM is usually as high as 2 dB [4], [5]. The smaller RAM of our device can be mainly attributed to the corresponding sweeping current (I peak − I valley ) for V π being as small as 2 mA.…”

“…linking gain is difficult on the SiP platform due to the fact that the required driving voltage for the π phase-shift at the reverse bias regime of Silicon based modulators is usually large (nearly ten volts) [3]- [5]. Increasing the p-type doping density in the active volume is one of the most effective ways to reduce the driving-voltage, but this leads to a significant optical insertion loss [3], [4].…”

“…Thermo-optics resistive phase-shifters are popular because of their simple structure and small footprint [7]- [12], but device performance has been constrained by large power consumption (>12mW) and longer than µs scale switching/response time. On the other hand, the more conventional Mach-Zehnder interferometers (MZI) require high driving voltages (>6V; under reverse bias) for the π phase shift [4], [5], [13]. This impedes their application in optical phase arrays (OPA), which usually need to have hundreds of optical phase-shifters inside with a lowdriving voltage for >2π phase shifting [10]- [12].…”

“…This has been the most effective method to reduce the 2π driving-voltage but at the expense of lower modulation speed and higher power consumption [4], [5] as compared to MZIs operated under a reverse bias. Although the forward bias gain rolls off more rapidly with the increase of operating frequency, it offers a higher RF link gain improvement of more than 10 dB from nearly DC to 25 GHz [5]. Furthermore, forward bias operation is shown to result in a comparable spurious-free dynamic range [5].…”

“…Although the forward bias gain rolls off more rapidly with the increase of operating frequency, it offers a higher RF link gain improvement of more than 10 dB from nearly DC to 25 GHz [5]. Furthermore, forward bias operation is shown to result in a comparable spurious-free dynamic range [5]. Nevertheless, large (>2 dB) residual-amplitude-modulation (RAM), which leads to undesirable AM noise during phase modulation, usually happens in these forward-bias based p-n junction phase-shifters due to the free-carrier absorption loss induced by the high level injection carriers [4], [5].…”