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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

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Threshold current of 2 mA at room temperature cw operation is realized in a vertical distributed feedback surface-emitting laser diode with lateral buried heterostructure (LBH). In this LBH structure, the vertical distributed feedback active region (AlGaAs/GaAs multilayer) is entirely surrounded with n- and p-type AlGaAs cladding layers for minority-carrier confinement. The far-field angle is 7°. The beam shape is nearly circular. However, the lasing spectrum is broad (2–3 nm) compared with the conventional edge-emitting laser. Major differences between the surface-emitting laser diode presented here and the conventional edge-emitting laser diode are discussed.

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