Need some help for a FM LIFI communication

[Zter]

Good morning Miss, Mister.
I am a student in engineering, and I like to develop sensors and other devices myself for specific purposes.

I got the idea to make high data rate wireless underwater communications using lasers and photodiodes, in the same way a LIFI system works.
The first major issue is that (sea)water diffuses light a lot, and with the ambient light, which becomes noise in our case, interpreting directly the output of the photodiode, like every lifi project I see at first glance on the Internet, becomes impossible due to the level of noise.
I decided to circumvent this issue by modulating the laser's imput with FM, thus the signal I would look for would be in a specified bandwidth, and could be demodulated with PLL.
I bought some CD4046 PLL ICs, some photodiodes but I have no idea if the PLL working will be affected by the signal not-fixed amplitude (depending on tge angle of the light hitting the photodiode, she becomes less responsive, with increased distance, the laser beam spreads more, the surfacic energy decreases and the output of the photodiode accordingly).

If you have any suggestions about circuit I could use for modulation (a simple good quality VCO easy to power would be ideal), and demodulation, I am all ears.

Thanks in advance
PS I would like to especially have an estimate of the frequency to differenciate between the frequencies corresponding to a 0 and a 1, also I would like to have a measurement of the amplitude of the signal I am looking for as to use one photodiode or another, in a case were several photodiodes with their demodulators are present in different orientations to use the most effective one

 

[rjenkinsgb]

Most of the circuit would be using well known RF principles, in effect an FM transmitter and receiver with the antennas replaced by the laser modulation switch for transmit and the photodiode output for receive.

The major limiting factor for speed will be the response times of the laser modulator, and the receive photodiodes.

The maximum data rate would be some fraction of the modulating carrier frequency.

For initial experimentation, I'd suggest using 455KHz or 10.7MHz as the RF carrier, as you can use generic IF strips from old AM or FM radios for most of the receiver & demodulator sections.
Most of the receive gain in those is in the IF strip.

You can use either varicap modulation on a crystal oscillator, or phase modulation after the frequency source.

I did some experimenting with modulated lasers a few years back & at the time at even working with a 10MHz carrier was quite difficult and expensive! Most cheap lasers will not switch at anything like that rate, and I needed PIN diodes with a high voltage bias for receive.

Also, underwater, the higher the modulation frequency the more sensitive it will be to multiple path effects - at 10MHz the carrier wavelength is around 30 metres, so a reflected signal with a 15m different path length would null the signal. You would likely need several detectors & front ends using a diversity system to minimise such effects.