# Maximum bandwidth of laser direct modulation

Can you practically send data using laser and direct modulation (by this I mean simply turning laser on and off)? If yes what is maximum bandwidth you can achieve?

That is exactly how fiber-optic communications work. Your answer will depend on transmission medium, distance, power levels, the percentage of bandwidth allocated to error correction, and many other factors.

If you have more specific conditions we can probably figure it out for those conditions. Or you can, using the Nyquist-Shannon sampling theorem.

You can do it without fiber, too, just through the air:
https://en.wikipedia.org/wiki/Free-space_optical_communication

Yes, of course you can and historically, this has been the most common way to use a laser for optical communication. As others have said, the bandwidth depends on many things. For example, the laser itself will only support modulation over a certain bandwidth, but that can certainly be engineered to be multiple tens of Gigahertz. Any propagation medium will have some level of dispersion, which will limit the bandwidth due to things like laser chirp (wavelength changes with the modulation current). Also, the receiver sensitivity naturally degrades with increasing bandwidth, so longer links will not be able to support as much bandwidth as short ones. The list of effects goes on and one. Modern system often use more sophisticated techniques, including coherent modulation and detection, as well as wavelength division multiplexing. Ultimately, one is limited by the optical bandwidth of the transmission medium.

I don’t have the textbook handy, but the maximum theoretical bandwidth is a gigantic number. I vaguely recall it’s terrabits.

Why?

OP: it always has to be modulated, regardless of the medium.

You can operate a laser in pulsed operation versus CW (continous wave) which gives you the advantage of not having to modulate over the CW (as much at least). These gives you very fast response times, in the femtoseconds range. To be real simple, we’ll assume everything is perfect at all times, everything. 8 pulses at a femto second each is 1 byte, devide that by 1 billion, and we have 125,000 gigabytes per second. I think.

Reality will be different, probably half that after you factor in dealing with a less than ideal transmission medium, and then, the control overhead to pass data (routing, error correction, etc).

Current optical systems easily (sort of) do 100 Gb/s. Systems in development do 400 Gb/s. Check some company websites such as www.alcatel-lucent.com, look for 1830PSS (Photonic Service Switch)

Disclaimer: I’m employed by Alcatel-Lucent as a support engineer for DWDM and PSS services.

Yes, those system use wavelength division multiplexing, so multiple lasers, to get to those enormous bit rates. They use coherent techniques to get as much 100 Gb/s with one laser. For example, they use polarization division multiplexing to get a factor of two and QPSK (quadrature phase shift keying) to get another factor of two relative to the simple on/off keying that the OP asked about.

Those numbers are telling you the optical bandwidth of the fiber transmission medium. The frequency of the infrared light used is about 300 THz. The range of frequency over which the fiber has good transmission characteristics can be many tens of THz.

Using fancy transmission schemes involving multiple lasers at different wavelengths, multiple bits per symbol, multiple fiber modes, and polarization division multiplexing, modern technology allows transmission rates well exceeding 1 bit per second per hertz. For example, NEC has demonstrated transmission over many kilometers at 101 Tb/s. That is a lot of data!

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That is amazing. I thought the record was still down in the tens of gigabits.
It amazes me that a piece of hardware could generate data at 100 Tb/s in order for it to be transmitted…

It’s not one piece of hardware : it’s a whole rack of separate cards working in parallel, and various optical devices are used to combine all the different beams into a single optical fiber. Conceptually it is no different than 10 radio stations all broadcasting in parallel, and the bandwidth available in a piece of optical fiber exceeds the bandwidth of all RF frequencies combined, or so I have read.