Backward-wave cancellation in distributed traveling-wave photodetectors

Abstract

This paper describes the design, fabrication, and measurement of backward-wave-cancelled distributed traveling-wave photodetectors. One of the fundamental issues in traveling-wave photodetectors is the generation of backward-waves, which reduces bandwidth or, in the case of matched input termination, reduces their radio-frequency (RF) efficiencies by up to 6 dB. We report a traveling-wave photodetector with multisection coplanar strip transmission lines. The reflections at the discontinuities of the transmission line cancel the backward propagating waves exactly. The bandwidth reduction due to backward-waves is eliminated without sacrificing the RF efficiency. We have demonstrated a broadband backward-wave-cancelled traveling-wave photodetector with three discrete photodiodes. The photodetector is realized in InGaAs/InGaAsP/InP material systems and operates at 1.55 m. A 3-dB bandwidth of 38 GHz and a linear RF output of 1 dBm at 40 GHz have been achieved. The experimental results agree very well with the theoretical calculations. Index Terms-Backward-wave cancellation, coplanar transmission line, distributed photodetectors, high-power photodetectors, microwave photonics, traveling-wave photodetectors, velocity matched distributed photodetectors. I. INTRODUCTION T RAVELING-WAVE photodetectors (TWPDs) have attracted much attention recently [1]-[12]. The speed of conventional lumped element photodetectors is limited by RC time and carrier transit time. By embedding the photodetector in a microwave transmission line, the capacitance of the photodiode becomes part of the distributed capacitance of the transmission line, and the RC time associated with the line impedance and the diode capacitance is eliminated. This enables us to build photodetectors with higher speed, or, perhaps more importantly, high-speed photodetectors with large absorption volume for high power operation. There are two main applications of TWPDs: the first is generation of widely tunable millimeter and submillimeter waves or even Manuscript received

High‐Power Distributed Photodetectors for RF Photonic Applications

“…This point is illustrated experimentally in Figure 3.14 for a TWPD with and without a 50 W input termination [33]. With a 50 W input termination, the RF response drops by 6 dB, while the bandwidth broadens by a few gigahertz.…”

“…For the case of equal photocurrent contributions from each photodiode, the equation simplifies to Z n ¼ Z 1 =n, where Z 1 and Z n are the line impedance of the input and n-th sections, respectively [33]. Initially, we demonstrated BWC for unequally distributed photocurrents [31], but later employed photodiode arrays with uniform photocurrent distribution [33]. Figure 3.16 compares the frequency response of a standard distributed TWDP (SS-TWDP) with a continuous CPS and MS-TWDP.…”

“…Consequently, the responsivity dropped by 30 %, but the MS-TWDP remained operational. With thicker metals and improved heat sinking, much higher linear RF power is achievable [33,34].…”

“…The impedances needed for BWC at each section are indicated on the respective transmission line segments[33] (Ó 2003 IEEE)…”

Bandwidth enhancement of traveling wave photodetector with inductive gain peaking

“…An advisable solution is split the incident optical power into multiple PDs and then integrating them by electrode configuration, including lumped electrode [4] and traveling-wave (TW) electrode [5][6][7][8]. Although the TW electrode is favorable for high bandwidth, it usually needs a matching impedance at the input terminal to avoid the detrimental RF reflection [9]. However, the matching impedance shunts half of total photocurrent.…”