December 11, 2019, 12:04:09 PM
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Topic: Laser  (Read 627 times)

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Offline labeled

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Laser
« on: October 31, 2019, 03:13:32 PM »
Let's say I had a solar panel, battery, and laser diode.
The diode is turned on and off to transmit data.

So:

1. The solar panel feeds the battery
2. The battery feeds the laser, and
3. The laser turns electricity into light

Q1. Can sunlight directly augment the light put out by the diode somehow?
If so, can it increase the range and/or data transmission rate?
(I know it sounds weird, but I'm wondering)

Q2. Another case, if I go straight from a solar panel in full sunlight to the diode,
or panel -> capacitor -> diode, and focused it further somehow (cutting head),
would it be powerful enough to cut acrylic sheets?
If so, how many watts/volts are we talking about in terms of the solar panel?

Sorry if these things sound contradictory, I am very new to physics.

Offline Corribus

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Re: Laser
« Reply #1 on: October 31, 2019, 07:53:09 PM »
Q1. Can sunlight directly augment the light put out by the diode somehow?
If so, can it increase the range and/or data transmission rate?
(I know it sounds weird, but I'm wondering)
What do you mean by "augment", "range" and "data transmission rate"?
In the basic scheme you describe, the solar power and the laser aren't directly related, other than making a certain amount of stored power available.

Quote
Q2. Another case, if I go straight from a solar panel in full sunlight to the diode,
or panel -> capacitor -> diode, and focused it further somehow (cutting head),
would it be powerful enough to cut acrylic sheets?
If so, how many watts/volts are we talking about in terms of the solar panel?
Power in diode lasers is limited by destruction of the semiconducting material when too much light or heat is reabsorbed during the lasing process. This produces defects that ultimately degrade performance.  There are certainly very high powered diode lasers available, I guess in the 100s of W is possible now, and efficiencies can be quite high, in excess of 50%(?). I don't think solar generation would be limiting to generating the necessary power to drive these kinds of lasers, but that's a question for an electrical engineer, not a physicist per se.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Enthalpy

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Re: Laser
« Reply #2 on: November 01, 2019, 11:02:44 AM »
Some lasers can and are pumped by sunlight without the electricity step. It's done with YAG. Concentrated sunlight is necessary to create the inversion, and more dopants in YAG help absorb more light. A team in Japan did it.

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A laser diode needs a damn power density to operate, essentially because the lasing particles (electrons and holes) are very concentrated. In a YAG, the colour centres (Nd doping) are dilute so exciting most of them takes less power density.

Since a laser diode operates already near its destruction, any pumping scheme that is less efficient then electricity may be immediately fatal. The present current density in metals nearly zaps them. The power density uses to be almost the burning value. But if the pumping light is filtered and gets efficiently absorbed, it would be conceivable.

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An other obstacle is that the pumping method must not de-excite the lasing things. Though, if a pumping photon brings an electron from low to high energy, it also does the opposite action with equal ease, so you can't achieve >50% of excited electrons that way, and you get no lasing effect. Optical pumping demands an energy diagram with more levels, where for instance pumping light puts an electron in a first high energy level, from which it falls quickly to a less high one where it stays for long, waiting for a photon to lase and amplify the light.

An other condition is that the emitted light must not be absorbed too strongly. One way to achieve this is when the lasing transition ends on an energy level that the electron leaves quickly, so it can't absorb the emitted light by the inverse transition of the lasing one.

Semiconductors give flexibility to design the lasing medium, and I suppose laser diodes pumped by concentrated filtered sunlight would be feasible, but they would rather differ from current-pumped laser diodes.

I suggested sunlight-pumped lasers (more probably YAG in a near future) for optical datacommunications from space probes and satellites, to save the costly and inefficient solar panels. They would also be nice to analyse from orbit the soil of a celestial body.
https://www.scienceforums.net/topic/76627-solar-thermal-rocket/page/2/?tab=comments#comment-807522 and next

Offline Enthalpy

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Re: Laser
« Reply #3 on: November 01, 2019, 11:31:04 AM »
Other means to increase the range of a light datacomm include:
- Use a fibre
- Have good optics at the transmitter and receiver
- Protect the receiver from ambient light
- Improve the electronics, modulations, error correction

Apparently you have no optics at all up to now, this would bring more than a power increase. Don't touch the laser diode nor receiving diode. Add lenses or concave mirrors.

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Capacitors are good for high power and small energy, hence short duration. Batteries are good for high energy and reasonable power. Supercapacitors are in between.

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The voltage is no big concern. Electronics convert voltages efficiently. Only the power must match.

Offline Enthalpy

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Re: Laser
« Reply #4 on: November 01, 2019, 11:57:02 AM »
Laser diodes exist with at least 80W light output and >50% power efficiency. Modules exist for 9600W light output (lT-6500 at laser2000.co.uk). That power would easily cut polymers and thick steel,

BUT

Laser diodes are not the preferred choice to cut anything, because
  • Their light is difficult to concentrate. It is little coherent, so even perfect optics won't achieve a narrow spot.
  • Their light is continuous. Short pulses are better to evaporate the target material before the deposited heat spreads.

What does exist and is standard technology for cutting machines now is a YAG laser (or sometimes Ti:sapphire) pumped by arrays of laser diodes. The global conversion, like 50%*50%, is excellent. The YAG gives pulsed coherent light. Light from diode arrays is good enough to concentrate on the YAG rod as pumping light.

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Specifically about acrylic: the target material must not be transparent to the cutting light. (Nor can it reflect too much).

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While 5kW cutting light cuts steel quickly, probably mean 200W pulsed would suffice for plastic and with more patience. But this isn't trivial. It may demand 1kW electricity. Terrestrial cheap solar panels may offer 15% efficiency, and at an excellent location at the best time, you may receive 600W/m2 sunlight, so you need 11m2 panels.

If sunlight shall power that, I'd try instead a concentrating mirror and cut the acrylic directly at the focus. Something like a receiving antenna for satellite TV covered with aluminium thin film.

And if really heat shall cut the acrylic (saws are a fantastic invention) then I'd rather have a wire heated by electricity.

Offline Enthalpy

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Re: Laser
« Reply #5 on: November 07, 2019, 09:56:32 AM »
One limit to sunlight-powered laser diodes is that sunlight can't be concentrated at will. Imagine that a good location receives 500W/m2 and the concentrator has L/D=1, the diameter of the Sun's image at the focus provides "only" 6.6MW/m2. Of that, only a faction has a usable wavelength, leaving maybe 1 or 1.5MW/m2.

As opposed, a power laser diode emits some 2W from each 100┬Ám*1mm stripe, so at 50% power efficiency it needs 40MW/m2 pumping power.

This might be a very serious obstacle. Laser diodes can't work at low power density. Their "normal" operating power, which is nearly destructive in a short term, is very few times over the threshold power where lasing appears. Improving that needed a huge and long effort.

So: better a YAG for direct solar pumping.

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