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Old 05-05-2014, 08:23 AM #1
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Default Laser simultaneous power + data transmission

Hi there!

I have started a project whose aim is to demonstrate the possibility of transmitting both power and data simultaneously using a laser link. The idea in the first place is just to show that it is possible, with limited amount of power and data rate. New in the field of lasers, I reviewed the literature in order to find out what would be the best set up to do that.

The idea is to modulate the output of a 5mW IR laser diode (already acquired: https://www.thorlabs.de/thorproduct....mber=ML925B11F) thanks to the following driver board (http://www.ichaus.de/upload/pdf/Wj_c1es.pdf page 11). On the receiver side, a photodiode is used to operate the conversion back to electrical energy, which is used to charge a gold-cap. Any byte (corresponding to commands) sent should be retrieved by a micro-processsor for further processing. I have several questions regarding this set-up:
  • The manufacturer of the driver advised me to use pulsed operation to modulate the digital signal that I want to send (does this corresponding to TTL modulation?) instead of analogue modulation. But since in my case I need the link to also transmit power, I am not sure that it is the best solution for me? Could it be possible to use analogue modulation here in order to have a DC level even when no data is being transmitted?
  • I am also new to the field of electronics. It was ok to figure out to use gold-capacitors to demonstrate power transmission but I am less confident when it comes to what would be required between the photodiode, the gold-caps and the micro-controller. It is easy to find example circuits for only power or data transmission but not for both at the same time! Do you have any suggestions?
  • Unfortunately, I am working in an institute who has never worked with laser before and therefore I am not allowed to use this 5 mW class 3 laser. A momentary solution would be to use a class 2 laser whose ouput can be modulated to start experimenting. Do you any advice on what kind of such laser I could use for this application?

Thanks in advance for your help!
Cheers,
Maxime


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Old 05-07-2014, 06:16 AM #2
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Default Re: Laser simultaneous power + data transmission

You don't need a DC level to transmit power. Powerlines transmit power, as do headphones, (ideally) without a DC component.

Likewise, a modulated laser beam can transmit power as well. The capacitor you're planning on using will filter the pulses and average out the response. Look up how PWM is used to generate an analog signal.

Overall, you're not going to produce much power with your laser. You can demonstrate that the photodiode produces some voltage or current, which could in theory be harvested, but you'll probably use more power amplifying the signal into something useful than you'll end up harvesting.

Good luck!
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Old 05-07-2014, 08:40 PM #3
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Default Re: Laser simultaneous power + data transmission

Hmm.. I'm not entirely sure here, but looking at the spec sheet, I think you may be able to drive the diode under spec moving it into class 2. I think it list minimum current at 8mA, and minimum output .2mW. If you can get access to a laser power meter you could set output to 1mW.

Or enclose the diode such to put it into class 2.
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Old 05-08-2014, 07:45 AM #4
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Default Re: Laser simultaneous power + data transmission

Hey Guys,

Thanks for your answers.

Quote:
The capacitor you're planning on using will filter the pulses and average out the response.
Quote:
you'll probably use more power amplifying the signal into something useful than you'll end up harvesting.
Maybe the best solution would then be to switch between two circuits, one only to harvest power with no signal amplification required and another to demodulate any signal sent. In this case what would be the best way to send power, CW or pulsed operations?

Any example of simple demodulation circuits for OOK without carrier frequency?

Cheers,
M.
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Old 05-08-2014, 10:46 AM #5
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Default Re: Laser simultaneous power + data transmission

Use a higher power laser, then use a solar cell to convert it back to power at the receiver.

You then modulate the laser intensity so that the voltage at the solar cell fluctuates - that's how you'd carry the signal.
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Old 05-18-2014, 01:30 AM #6
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Default Re: Laser simultaneous power + data transmission

With limited experience and equipment i'm affraid this will prove dificult.

The first thing to investigate is modulation: You may want to choose a modulation mode that keeps your laser output on a stable average level so it can power the receiver regardless of what data you are transmitting.

The easiest way to achieve that is to modulate the laser with constant amplitude signal, and using something else to do the actual modulation. Using AM to power the actual laser and FM for the signal seems logical.

As far as the laser power is concerned: limiting it to only 1 mW of light would probably problematic. If you consider an overal recovery of power efficiency of 10% that would deliver a mere 0.1 mW to the receiver in terms of electrical power. I doubt that would be sufficient to run any kind or receiver circuit.
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Old 05-19-2014, 01:13 PM #7
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Default Re: Laser simultaneous power + data transmission

Hi Benm,

Thanks for your reply!

I will use a 5mW laser but I realize it will also most probably not be enough to power anything. As time is running out, I'll have to use it anyway, and try to show that the power part of the link is working by charging a capacitor.

Regarding the modulation, I agree that keeping a constant amplitude signal while sending the data would be the best in terms of power transmission. However I am not sure how to implement a FM scheme, what kind of driver is required in this case?

To simplify the implementation, I am about to use a driver with a switching input connected to the UART output of a microcontroller, to implement a OOK scheme. I am aware that this means transmitting a digital 0 corresponds to laser off and therefore no power is being transmitted. But it seemed the simplest for a first experiment where succeding to send just one byte would be successful!
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Old 05-21-2014, 12:42 AM #8
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Default Re: Laser simultaneous power + data transmission

Well, if you have no other option for modulation than choosing a scheme where information is transmitted as amplitude, using buffering caps on the receiving end would be a solution of sorts.

I would still use some encoding scheme (trellis perhaps) that represents a one or zero has a phase change rather than an absolute on/off symbol though. If you do not, you would essentially run the risk that transmitting a long string of 00000000 causes your receiver to remain in the dark and lose power at some point.

Last edited by Benm; 05-21-2014 at 12:43 AM.
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