Xref: utzoo comp.arch:17488 comp.lsi:1121 sci.electronics:13305 sci.physics:13877 Path: utzoo!attcan!uunet!cs.utexas.edu!samsung!sdd.hp.com!uakari.primate.wisc.edu!aplcen!haven!udel!berryh From: berryh@udel.edu (John Berryhill) Newsgroups: comp.arch,comp.lsi,sci.electronics,sci.physics Subject: Re: Electro-optic bus Message-ID: <26481@nigel.udel.EDU> Date: 2 Aug 90 19:31:50 GMT References: <1965@trlluna.trl.oz> <7914@tekgvs.LABS.TEK.COM> Sender: usenet@ee.udel.EDU Reply-To: berryh@udel.edu (John Berryhill) Followup-To: sci.electronics Organization: University of Delaware Lines: 74 In article <7914@tekgvs.LABS.TEK.COM> arnief@tekgvs.LABS.TEK.COM (Arnie Frisch) writes: >Wrong! LEDs exhibit generally poor bandwidth or rise-time of the >optical output. In addition, because of their broad linewidth, they >are highly subject to dispersion when transmitting fast pulses over >moderate distances. Define "moderate." I assume from the title that we're talking about distances less than a few meters. If you can detect significant phase dispersion for signals of a couple of hundred megabits per second over that distance, I tip my hat to you. It's true that LED's aren't going to run at several gigabits, but neither are the chips that you want to get the data into and out of. It is excruciatingly difficult to built a computer that uses internal data rates approaching 200 MBPS. Granted, lasers can run faster than greased owlshit, but the frequencies where designers are getting into trouble aren't all that high. I'm not going to get into the sort of argument here where we shout "Wrong!" at one another, but the problem of getting many signals around within a digital system confined to several meters is distinctly different from the problem of sending multiplexed signals through a single fiber halfway around the planet. Replacing laser diodes in a system where you've got just one fiber is no big deal. In repeaters for underwater cables, it's not uncommon to have several lasers packaged in such a way that when one fails, another can be used. But in a system where you've got lots of elements talking to one another, the low mean time to failure of lasers relative to LEDs is going to be a major headache (aside from thermal instability). You've also got to deal with rotators and polarizers in order to prevent optical feedback from the fiber from disturbing the laser. That's not a big deal for, say, a LAN or similar applications, but I'm assuming that this discussion concerns using optical interconnections internal to the box in the corner of your room with the IBM nameplate on it (or DEC or whatever). >Laser delay is a function of several factors, type of laser, modulator >rise time, even output load, but 100 to 200psec is probably typical. I >haven't measured any LEDS because I never considered them suitable for >the reason cited above. The fastest commercial LED's that I've seen have risetimes of about 2 ns. I've made surface emitters with a rise time of 560 ps and Wolfgang Harth has made ones that are < 300 ps. The question, however, is not so much how long it takes for the device to get to its equilibrium power level, but how long it takes for the optical power to cross the decision threshold of your receiver circuit and THAT figure can be mighty low. As was mentioned earlier, lasers don't work so well if you're talking about turning them on and off. That means that your receiver is going to have to be somewhat more sophisticated than, say, a simple photodiode which could be fabricated on the same chip as the digital circuit that you want to talk to. The reflexive answer than lasers are "right" and LEDs are "wrong" is reminiscent of gallium arsenide having been the "material of the future" for the last thirty years. As long as improved performance can be squeezed out of silicon, or lately out of Si-Ge alloy BJTs, then it's going to be cheaper and easier to deal with than GaAs. LEDs will always be more reliable and cheaper than lasers since they don't have to contain such a high internal optical power density or the high fields associated with maintaining population inversion. Part of the problem here, I guess, is that we've all got our pet application in mind when we argue the merits of this vs. that. Building ECL circuits is a bitch. Replacing some of the coax cable, twisted pairs, and multilevel circuit boards with optical fibers may make ECL design a somewhat more manageable bitch, but the care and feeding that lasers would require might not be worth the trouble. If we're talking about GaAs HEMT LSI chips running at a zillion bits per second with active cooling all over the place, then lasers are probably called for. I may not be right, but I don't think I'm "Wrong!" -- John Berryhill 143 King William, Newark DE 19711