Fastest modulation: laser diode vs LED

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fastest modulation: laser diode vs LED....

Hello Forum,

in the telecomm industry, the most common and economical form of modulation is direct modulation, acting on the current operating LEDs or diode lasers.

Using an external intensity modulator (OOK), like a super fast chopper, the process of modulation and source characteristics (carriers recombination) are independent.

Will LED pulses generated with an external intensity modulator be of poorer quality, after a certain limit chopping speed, compared to the laser diode pulses? If so, why? Is it due to the different coherence properties of the two sources, the laser and the LED?

Theoretically, It would seem that light pulses from either a laser or LED would be comparable (eye diagram), no matter how high the OOK speed, if we used an external modulator....

(LEDs have lower coherence properties, both spatial and temporal, due the broader linewidth, less directionality and power density)

thanks,
fisico30
 

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The non-coherence of LEDs means you have to have repeaters in the fiber cable far more often (probably less than a mile up compared to dozens or hundreds of miles). The dispersion effects is very strong.

That alone pretty much seals it for LEDs in telecom - you have to bring power into the cable to power the repeater circuitry.

Additionally there are techniques for amplifying laser light in the cable itself (Er-doped fibers) which wouldn't work for non-coherent sources. These Er-doped fibers have become pretty common because it further reduces the repeater complexity/count.

The other issue is cost - the technology to make modern LEDs and make Laser diodes is virtually identical - MOCVD, heterojunctions, III-V materials, etc. So there isn't so much cost advantage. Once you've spent the capital on such a fab you can do either without too much marginal investment (economics is a feature equal to any technical spec in engineering).

An external modulator vs. actually shutting off a laser diode. Depends. The latter is probably faster right now. Switching frequencies at GHz are pretty common. It's not clear that an external modulator would be faster.

Perhaps if it were spintronic based on plasmons. I think there's work in that area but it's a long ways from being ready for general use. But a bigger problem is nothing can feed/consume the kind of data rates that a spintronic switch would deliver (THz, yet).
 

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