Fastest modulation: laser diode vs LED

In summary, the conversation discusses the use of direct modulation in the telecomm industry, comparing the use of LEDs and laser diodes. It is noted that LEDs have lower coherence properties and require repeaters more frequently, making laser diodes a more common choice. The cost and speed of external modulators versus shutting off a laser diode are also considered. It is mentioned that spintronic switches may be a future option, but currently they are not ready for general use.
  • #1
fisico30
374
0
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
 
Engineering news on Phys.org
  • #2


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).
 

1. What is the difference between a laser diode and an LED?

A laser diode is a semiconductor device that produces coherent light through stimulated emission, whereas an LED (light-emitting diode) produces incoherent light through spontaneous emission. This means that the light produced by a laser diode is highly focused and has a narrow spectrum, while the light from an LED is more diffuse and has a broader spectrum.

2. Which one is faster - a laser diode or an LED?

Generally, a laser diode is faster than an LED. This is because the coherent light produced by a laser diode can be modulated more quickly than the incoherent light produced by an LED. However, the exact speed of modulation will depend on the specific properties of the diode or LED being used.

3. What factors affect the speed of modulation for laser diodes and LEDs?

The speed of modulation for both laser diodes and LEDs can be affected by several factors, including the material composition, size, and temperature of the device. Additionally, the type of modulation being used (such as amplitude, frequency, or phase modulation) can also impact the speed.

4. Is there a significant difference in speed between laser diodes and LEDs for practical applications?

In general, the difference in speed between laser diodes and LEDs is not significant for most practical applications. Both types of devices can be modulated at high speeds and are used in various applications, such as telecommunications, optical data storage, and medical equipment.

5. Are there any advantages of using a laser diode over an LED for fast modulation?

Yes, there are several advantages of using a laser diode over an LED for fast modulation. These include the ability to produce more focused and intense light, a narrower spectral bandwidth, and a higher signal-to-noise ratio. Additionally, laser diodes can be modulated at higher frequencies and are more efficient at converting electrical energy into light energy.

Similar threads

Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
5K
Back
Top