technology  InP optoelectronics

one of the major weaknesses of all-optical packet switching is the omission of large optical random access memories, which are needed for buffering. Efforts have tended to focus on realizing buffering through various delay lines, such as slow-light waveguides, and re- circulating fiber loops. However, buffering times have to be relatively short if the signal is not to be degraded by attenuation or distortion. There are also concerns relating to the footprint and power consumption of delay-line- based buffers. This has led to the need to compromise between the different figures of merit. To make matters worse, other devices for optical switching and routing, such as wavelength converters, are usually based on one or more semiconductor optical amplifiers with a large power consumption and a substantial footprint. And to

junction Quantum Tunnel well Top metal BCB Bottommetal SiO2 SOI wg. Fiber Disk

Lateral contact

top it all, it is extremely difficult and unpractical to integrate several switches, delay line buffers, wavelength converters and gates together into practically useable photonic integrated circuits (PICs) for optical packet switching. However, despite these challenges, progress is being made, thanks in part to recent developments in heterogeneous integration of InP-based devices onto silicon-on-insulator (SOI ) passive circuits, and small, low- power lasers that can be achieved using this approach.

Our European team is capitalizing on this success and developing low-power, small-footprint PICs for all-optical packet switching through a project called HISTORIC – heterogeneous InP-on-silicon technology for optical routing and logic. This program, which kicked off in July

Grating coupler

(a)

Top metal

Bottom metal

Disk

1μm

SOI wg.

Microdisk laser

Figure 1. Microdisk lasers could provide a key building block for next-generation optical switches that set a new benchmark for speed, low-power consumption, and a small footprint. These tiny lasers are united with silicon-on- insulator waveguides, with coupling provided by a grating coupler (a). A scanning electron microscopy image reveals the various layers in the structure (b). Light-voltage curves show the two competing modes produced by the microdisk laser, which has a 7.5 µm diameter (c). Lasing spectrum for the CW mode at a bias of 3.8 mA (d) . All powers are calculated inside the SOI waveguide by taking into account the coupling efficiency of the grating coupler. (From: L. Liu, et al,, ‘An ultra-small, low power all-optical flip-flop memory on a silicon chip’, Nature Photonics, ISSN , 1749-4885, 4 182-187, March 2010)

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