Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!cbfsb!cbnewsb!mpope From: mpope@ATT.COM (michael.t.pope) Newsgroups: comp.arch Subject: Re: Optical Interconnect Message-ID: Date: 12 Apr 91 22:05:20 GMT References: <12618@pt.cs.cmu.edu> <510001@hpsciz.sc.hp.com> Sender: news@cbfsb.att.com Organization: AT&T Bell Laboratories Lines: 51 In-Reply-To: peng@hpsciz.sc.hp.com's message of 10 Apr 91 01:17:55 GMT Originator: mpope@cbnewsb.cb.att.com [submitted on behalf of the author] One advantage of free space optical techniques over fiber is that you're not creating a "wire" for each connection. Instead you're generating an array of "spots", each only a couple of microns across and then transferring that as an image "en mass" to a destination receiver array. Bulk optical elements such as beam splitters, lenses and mirrors do the directing. The "LEGO Block" idea was proposed by Jurgen Jahns and Alan Huang in the Digital Optics Research department here at Bell Labs [1]. The lenses and mirrors are created by etching diffractive optical elements on the surface of a glass substrate. The light propagates inside the substrate as if it were free space, reflecting off the parallel surfaces and the optical elements etched on them. This is nice because everything is lithographically defined, giving great accuracy and ease of manufacture. LCD modulators are cheap, but have a problem for high speed applications: they can only modulate at the rate of 100's of KHz. We are using Self Electro-Optic Effect Devices (SEEDs) as optical modulators, or Surface Emitting Microlasers (SELs) as directly modulated light sources. Receivers can be made out of regular digital CMOS if you're careful. Speeds of 100's MHz to GHz are possible with these devices, and beam spacings are around 20 microns. Using optical wavelengths that make silicon transparent is appealing, but (amongst other things) detection is hard. We're working towards manufacturable electro-optical systems where the regular, high density, high speed connections are made optically through the substrate, and slower, less regular connections are made with wires on the surface of the substrate. One might imagine for example a 1024 channel system bus connecting cache and main memory - this could reduce the miss penalty, allowing the use of very wide cache blocks. One of the main problems remains the cost-effective integration of GaAs based optical sources/modulators with silicon technology. I'd be happy to provide further references. [1] J. Jahns and A. Huang, "Planar integration of free-space optical components," Applied Optics, vol 28, page 1602, May 1989. Alex Dickinson alex@vax135.att.com Digital Optics Research AT&T Bell Labs, Holmdel, NJ