Evanescently coupled photodiodes integrating a double-stage taper for 40-Gb/s applications-compared performance with side-illuminated photodiodes

The design, fabrication, and performance of double-stage taper photodiodes (DSTPs) are reported. The objective of this work is to develop devices compatible with 40-Gb/s applications. Such devices require high efficiency, ultrawide band, high optical power handling capability, and compatibility with low-cost module fabrication. The integration of mode size converters improves both the coupling efficiency and the responsivity with a large fiber mode diameter. Responsivity of 0.6 A/W and 0.45 A/W are achieved with a 6-/spl mu/m fiber mode diameter and cleaved fiber, respectively, providing relaxed alignment tolerances (/spl plusmn/1.6 /spl mu/m and /spl plusmn/2 /spl mu/m, respectively), compatible with cost-effective packaging techniques. DSTPs also offer a wide bandwidth greater than 40 GHz and transverse-electric/transverse-magnetic polarization dependence lower than 0.2 dB. Furthermore, a DSTP saturation current as high as 11 mA results in optical power handling greater than +10 dBm and a high output voltage of 0.8 V. These capabilities allow the photodiode to drive the decision circuit without the need of a broad-band electrical amplifier. The DSTP devices presented here demonstrate higher responsivities with large fiber mode diameter and better optical power handling capabilities and are compared with classical side-illuminated photodiodes.

I. Introduction

Photodetectors are key components for future optical communication systems and require high-speed, high-efficiency features [1]. In particular, to meet the new challenges of 40-Gb/s applications, photodetectors must possess several characteristics. 1

They must operate at high power levels. The use of an erbium-doped-fiber optical preamplifier allows the decision circuit to be driven from the photodiode [2], [3]. As the usual broad-band electrical amplifier can be eliminated, the signal-to-noise ratio can be significantly improved for 40-Gb/s transmission [4]. In this case, the photodiode must handle optical power well above +10 dBm, corresponding to about a 10-mA saturation current for high output voltage.

2

They must provide high-efficiency photoreceiver modules at affordable prices [5]. The cost of a module is mainly linked to the tight fiber alignment tolerances in front of the photodetector, which requires a sophisticated, costly, and time-consuming pigtailing process.