Building FSO system for lab

[MohMaya]

Hello all,
I am trying to build a free-space optical communication system for my research lab. For the project I have, I want to transmit data up to 1 km using lasers. The operating wavelength is 1550 nm and the data rate I want is around 1 Gbps. The laser I have selected is L1550P5DFB from Thorlabs. Can you all please tell me what more things will I need? And is this laser the right choice or not?

 

[atomd]

Quality beam expander and collimators are your biggest problem. Next is precise aiming and automatic tracking system. Further some set of high speed detectors are needed. We could help you with many things but we need to know more details. And to be sure that the quesiton is serious please state your budget.

 

[MohMaya]

Hello, thank you so much. I have an initial budget of 10,000 Euros. It is for a research project. We want to achieve the data rate in Gbps and the distance is less than 1 km. The operating wavelength is 1550 nm. We have 2 used case scenarios. In one case both the transceivers are on the ground (located in a wind turbine park) and in another scenario, one transceiver will be in on water. I am using the transceiver term as we are planning to have a full duplex communication.

 

[atomd]

Full duplex, FSO system, with datarates of 1Gbps? Let's just call it a good joke
Let's start with obvious expenses (pries excl. tax):
2x photodiode - DET025AFC/M - 610e
2x beam expander - GBE20-C - 1400e
1x HeNe for alignment - 1000e
1x expander for HeNe - 600e
laser diode you selected is much underpowered for any experimentation, you need considerably more power for alignment - 2x L1550G1 - 640e
2x laser diode mount - LDM56F/M -1360e
2x piezo actuated mirror mounts - POLARIS-K1S3P - 2300e
that's already almost 8ke and I'm yet to include 2 breadboards, additional beam steering mounts, mirrors for directing light, lenses to capture light on photodiode, additional slow photodiodes to detect beam displacements, ..... I'd start by calculating required beam size and divergence, required positioning precision, atmosphere influences, etc. Then you can try thinking how exactly do you want to tackle the problems one by one.


BTW why did you choose 1550nm telecom wavelength for testing?

 

[MohMaya]

Hey thanks for the reply. Can you tell me why 1 Gbps is not possible? There are research papers that have demonstrated experiments achieving data rate upto 10 Gbps. I chose 1550 nm as it is the safe operating wavelength for human eye. We will be deploying our project in real used case scenarios. I am sorry if my questions are stupid as I know this technology theoretically but this is the first time we are developing something practically. Can you please tell me what is HeNe? Is it Helium Neon laser ? Also, if you can suggest me the laser. I am planning for ML925B45F from Thorlabs.

 

[atomd]

I'm not saying it's not possible. I'm saying it's not possible with your budget. BER grows with speed unless you compensate with power or better alignment.
Yes, HeNe is helium neon laser.

I already linked you another laser. The one you select is simply underpowered. With 5mW and 1Gbps you have 5pJ per bit. Detector noise is 2fW/sqrt(Hz) with 1GHz bandwidth, it gives you 63pW of noise. That gives you theoretical SNR of 10**8. To get BER of 10**-12 you need SNR of 200. In theory, you have high margin. In practice, many things deteriorate ideal SNR.

Starting from transmission efficiency. If you focus it with typical M9 collimator (C105TMD-C, which btw is also not included in my price list) you'll get 3mm beam. With x20 expander I suggested you'll get 60mm wide beam. At this width divergence at the receiving end will be negligible, so assuming you get another beam expander you can compress it back to 3mm beam with little loss. Both beam expanders have transmittance of just 97%, giving you another 6% light loss. The detector has 0.8mm aperture, hence you'll only be able to detect 7% of original light unless you use additional optics. Pointing at something km away with mm accuracy is impossible without active stabilization, and here's probably the best place to add it. Around your main detector, you add 3/4 additional photodiode that detect which way the beam shifted and send it to the transmitter. This is a complicated system to build and program, so for starters it's probably a much better idea to spread the beam to a much wider cone and hope it hits the receiver somehow. This already reduces your SNR to around 6*10**6.

Next thing to tackle is atmosphere. At 2-4dB/km in slight haze, it attenuates your beam in roughly half. But more importantly, due to unevenness, atmosphere lensing spreads and distorts your beam considerably. In "professional" systems, this is compensated with adaptive optics beam forming, but here it's just a loss that has to be estimated and accommodated.

Another thing that reduces your SNR considerably is electronics. You need a wideband, low noise laser driver. This is a hard thing to achieve. You also need optimal receiver with adaptive threshold, low noise preamplifier, some filters to compensate pulse distortion and clock recovery system.

Till now, calculations only included thermal noise of the detector. Sunlight has a lot of power in NIR. To cope with that, you need a narrowband filter on the detector. The narrower the band of the filter, the better rejection of sun's interference. At the same time, the narrower the filter the higher losses it introduces and requires better and better stabilization of source wavelength. At some point in time, thermal stabilization of the diode may become important.

Obviously, you're not going to put just optical table outside, facing environment. You need some enclosure that'll pass 1550nm, hide system from straight light. The window is going to have some losses, but more importantly it'll get dusty inducing further losses.

In general, you can skip some steps by taking 1550nm fiber optics communication system and trying to modify it for FOS but it still requires a lot more knowledge than you seem to possess.