# Hack fibre optic transceiver for wireless data link

I'm brainstorming ways to implement a high speed wireless laser data link without the exorbitant cost.

Would it be possible to take a standard SFP transceiver module and use that across free space rather than through a fibre? In other words, to point two transceivers at each other from a distance without any fiber optic cable between them. (It might be necessary to have the modules believe there are cables plugged in)

There would certainly be severe attenuation and noise issues to deal with. However, since some fiber optic modules have to transmit across 40km links @ 10Gb/s, the existing circuitry might have enough power and sensitivity to handle this.

There is also the issue of beam focus and divergence, although this could be cheaply mitigated by using a telescope.

Would this work for 10Gb/s with a 1 kilometer range?

Does anyone have any modules they could try across a room to begin with?

• There is also the issue of beam focus and divergence, but one thing at a time Sure, if you ignore that then it will probably work. The whole point of the fibre is to get the beam to the right place. If you do that some other way, it should still work. The challenge is to do that without using the fibre. – Bimpelrekkie Aug 13 '17 at 10:03
• You need to better describe your requirement. What range? What data rate? Is powering an issue (mains/battery/Alpha Centauri mission...) – Russell McMahon Aug 13 '17 at 10:14

Would it be possible to take a standard SFP transceiver module and use that across free space rather than through a fibre? In other words, to point two transceivers at each other from a distance without any fiber optic cable between them.

No. "Proof" to follow:

The point of the fiber is to get almost all energy from the transmitter into the receiver. You'd need a whole lot of very expensive lenses to achieve a focussing effect that would allow you to shine sufficient light from the transmitter to the receiver.

There is also the issue of beam focus and divergence, although this could be cheaply mitigated by using a telescope.

You're probably underestimating this. That emitter doesn't give you the parallel waves that a telescope can focus on a single point. The hard part is getting the beam shaped. The telescope only helps keeping a well-shaped beam narrow. Or not - it's actually not what a telescope does, but this leads to deeply into optics.

Would this work for 10Gb/s with a 1 kilometer range?

No way.

Think about it this way: since you're not using a fiber to conduct the light wave (which is an electromagnetic wave, just like radio waves), Friis' path loss formula applies, and that hates you; receive to transmit power is

$$\frac{P_r}{P_t} = G_t G_r \left(\frac{\lambda}{4\pi R}\right)^2$$

R is your 10 km, $\lambda=1270\,\text{nm}=1.27\cdot10^{-6}\,\text m$ the wavelength of your light (I used what 10GBase-LR (long range) transceivers use, here), and $G_t$ and $G_r$ are the directional gains of your transmit and receive optics here.

So, the wavelength- and distance term in the () amounts to

$$\left(\frac{1.27}{4\pi}10^{-10}\right)^2\approx \left(\frac{1.27}{12.7}\right)^2\cdot 10^{-20}=10^{-22}$$

So, that 220 dB (!) of attenuation. Let's go crazy and claim your receive and transmit optics have a gain of 50 dB (which requires incredibly fine alignment so that they see each other, and that requires incredibly stable buildings for these to sit upon – otherwise, wind blows, building flexes, beam hits nothing). So, that'd be an attenuation of 120 dB in total. Then we have atmospheric absorption – and that's mainly weather, and dust, dependent. So, since it's hard to guess, let's ignore it, and remember that things will break down with the slightest fog.

So, we have 120 dB less power at the receiver than what the LR transmitter used. Standard 10GBase-LR use a maximum of 0.5 dBm, so you get -119.5 dBm at the receiver.

The noise in a semiconductor receiver (hopefully) is still well enought described by the thermal noise with Boltzman's constant $k$, and the bandwidth $B=10^{10}\,\text{Hz}$ at temperature $T=300\,\text K$. It's well-memorized the noise power in 1 Hz at room temperature is -174 dBm, so 100 dBHz of that gives us a noise power in 10 GHz of -74 dBm.

That means we get an

$$\text{SNR}=\frac{-119.5\,\text{dBm}}{-74\,\text{dBm}}=(-119.5+74)\,\text{dB}=-45.5\,\text{dB}\approx 2.8\cdot10^{-5}\text.$$

Now, Shannon has given us a limit for how much info we can get through a noisy channel, and that limit is Shannon's Channel Capacity

\begin{align} C &= B\cdot \log_2 \left(\text{SNR}+1\right)\\ & \approx 406 \,\text{kbit/s} \end{align}

So, it's mathematically impossible to make this system work at 1Mb/s, let alone 10Gb/s.

• 0.4 Mbps is still pretty darn impressive for a fiber optic link with a missing fiber. Even if it only works in good weather and requires an extra widget to keep it aimed perfectly. That's better than a good chunk of free WiFis at a fraction of the distance. – John Dvorak Aug 13 '17 at 11:27
• "an extra widget" would probably need to transport that amount of data between transmit and receiver to keep the beams aligned :D no, seriously, it is darn impressive, and I'm still amazed that there are commercially viable laser beam links between buildings. Notice, also, that this is a capacity, ie. it only says "you can't go faster than that, ever", it doesn't say "do this and you'll go this fast!". Important distinction to make. – Marcus Müller Aug 13 '17 at 11:29
• Hmm... a ring of four detectors around the main receiver transmitting their data as a pair of floats with every frame over a custom link-layer protocol isn't that much overhead. Even if you go for UDP because it's easier to implement and transmit alignment every millisecond just to be sure, that's still just 30 kBps = 0.24 MBps. Surprisingly tight (so you better tone down the frequency), but there's still space for data. – John Dvorak Aug 13 '17 at 11:42
• Thanks for your reply. The telescope is used for the receiver. A large lens should mitigate some of the divergence. I took a quick look at the Wiki entry for the Friis transmission equation - doesn't this apply to isotropic rather than directive radiators? Also, according to goo.gl/TnOYb9, 10GBASE-ER can handle a drop of ~20dB. – Mikey Top Aug 13 '17 at 11:57
• @JohnDvorak, Now your challenge is to get a transceiver designed to work at 1 Gbps or higher to run at 0.4 Mbps. With Manchester code, it might work (now your actual data rate is 0.2 Mbps). Without a code that guarantees a very high transition rate, no way. – The Photon Aug 13 '17 at 15:25

I do not think this is at all feasible. Aiming the transceiver module at 1km range in and of itself is never going to work reliably - even if you could somehow get the light beam focused correctly.

The other significant factor to consider the incredible attenuation of signal over distance in the free space. You would never get this to work.

• Not to mention you'll need a straight line of sight. These things are hard to get in the kilometer range. – John Dvorak Aug 13 '17 at 11:20
• If I've understood correctly, 10GBASE-ER can handle a 20dB drop. goo.gl/TnOYb9 – Mikey Top Aug 13 '17 at 12:15
• that's laughably little compared to what you lose. – Marcus Müller Aug 13 '17 at 12:23
• According to goo.gl/DfQm57, attenuation on a clear day is 0.2-0.4dB/km – Mikey Top Aug 13 '17 at 12:39
• but you have a beam, which is always divergent. Please re-read my answer. Free-space path loss is not the same phenomenon as absorption. You have to add these. – Marcus Müller Aug 13 '17 at 12:47