# Laser Power vs Efficiency

Is there some kind of graph or relation between laser power and efficiency. What I'm trying to find out is whether the efficiency of a laser decreases with increase in power or is it that there is no definite relation.

For example, which would be better; using 10 nos 100 mW laser diodes concentrated to one point or a single 1 W laser diode.

Found these two laser diodes:

Here are the links to the datasheets of the same, ML520G73 ML562G84

So which will be more efficient using one 2.5 W laser diode or five 0.5 W laser diode (the five are set up to point to the same location). And what would be the advantages or disadvantages of both setups?

Thinking of making a laser engraver, but had this idea of using multiple low powered laser diodes instead of a single high powered one. But in a dilemma of what's a better choice. I don't feel complicated in using a multiple lasers instead of a single one.

• What is the part number(s) for the laser diode? Can you please include the datasheets in the question? – Mahendra Gunawardena May 6 '20 at 17:57
• This is more of a general question. Not being specific about anything here. Like a generic 100 mW and 1 W laser diode with the same wavelength and all. – Manu G May 6 '20 at 19:16
• As it is question is too broad, and it is difficult for the community to provide a good answer in this platform. I suggest doing some research on components that exhibit characteristics and update the post. – Mahendra Gunawardena May 6 '20 at 20:24
• Only saw the edit after posting my answer, the question, while also not too specific, seems to be meant less general than I understood it. I'll still leave it here if someone is interested in what happens inside a laser :P To get an answer to your specific question, you need to provide more info about what you want to achieve with these laser diodes. – OpticalResonator May 6 '20 at 21:14
• Thank you. The questions has good chance of receiving multiple answers. – Mahendra Gunawardena May 6 '20 at 23:37

As I already mentioned in my comment, I would recommend to take a single laser source rather than five seperate laser sources, for multiple reasons:

1. The effort to align the beams of the five diodes on the same spot is out of proportion, when you can instead just get a single beam out of one diode.
2. The 2.5 W diode that you have been looking at has just as much of a linear current-power behaviour as the 0.5 W diodes, so you can also adjust the power there, if needed.
3. In case you want less power than the minimum threshold power of the 2.5 W diode: try to avoid reducing the power at all. For pulsed laser manufacturing processes (engraving is one of them), you run the process at 100 % power and influence the power-per-processing-length via the axis speed. That way the laser always operates at its optimal pulse form. The pulse form and peak power are only specified for 100 % power by the manufacturer (that's a chapter for itself, feel free to make a new question if you're interested).

Finally, a disclaimer for this kind of questions about lasers: The laser sources you picked here are class 4 lasers. I assume from your question (I may be wrong) that you don't have much experience with lasers. Be extremely careful with what you are doing, as they can cause permanent damage, not only to your eyes but also your skin, or fire and explosion hazard. Follow proper procedures for laser safety and make sure that you don't put yourself or anyone else in danger.

I keep my old answer for the mechanism behind power-efficiency relations here in case it still helps someone:

TL-DR: There are saturation effects in the laser-active medium that can lead to the laser process running more inefficient the more power we try to "suck out" of the laser-active medium.

Long version: This is a pretty interesting question, maybe on first glance one might think that, as long as we can cool the losses, there would be no decrease in efficiency with increasing the power. However, what might come the closest to what you ask is the saturation effect of the laser-active medium, especially in the case of optically pumped lasers. To understand what is happening here, we need to look a bit deeper into the mechanisms behind the laser process.

For simplification, consider a rod laser with cross sectional area $$A$$ and length $$L_L$$, the equations also work for disk and fiber lasers, as they are just extreme cases of rod lasers. The maximum power that we can extract from this laser rod is given by $$P_{max \ extract.}=g_{kl}(v)I_sAL_L$$ This equation can be derived from the equations describing the population inversion of the laser levels. For your question, the two remaining variables in the equation are important, $$g_{kl}(v)$$ is the small-signal gain coefficient and $$I_s$$ is the saturation intensity. The saturation intensity, again, is described by $$I_s = \frac{hv}{\sigma_{ou}\tau_o},$$ so it is a function of the energy of the emitted photon $$hv$$ (-> output wavelength of our laser), the cross section $$\sigma_{ou}$$ (-> probability that a photon is emitted) and the mean life time $$\tau_o$$ (-> average time until a particle in the upper laser level spontanously emits a photon) and thus depends on values that are inherent to the laser-active medium.

To return now to your question, there is a saturation mechanism in our laser-active medium that occurs when the intensity of the laser beam that gets amplified in the laser-active medium reaches the saturation intensity. In this case, the gain coefficient $$g$$ falls down to only half of the small-signal gain coefficient $$g_{kl}$$. What a small gain coefficient means, is, that the population inversion, an absolutely necessary condition for the laser process, gets less and we can not amplify our laser beam as much as we would like to. If the intensity increases even more, the gain coefficient gets even lower. At this point, we see an effect that gets pretty close to your question, a lowered overall efficiency due to higher power.

A graph to illustrate this behaviour is shown below:

Source: Graf, Thomas; Laser - Grundlagen der Laserstrahlerzeugung (Fundamentals of laser beam generation, translation by me); Springer 2015

• What I want to know is kinda simple, which is better. Using five 0.5 W laser diodes or one 2.5 W laser diode. Been curious about this for a while. Based on the datasheets provided what do you think is a better choice; – Manu G May 6 '20 at 21:44
• Or take it like this. In a hypothetical situation, I have 5 dollars and a 0.5 W laser diode costs 1 dollar whereas a 2.5 W laser diode costs 5 dollars. Which would be a better choice for me to generate 2.5 W of laser output power, under the circumstance that the five 0.5 W laser diode setup is made to concentrate to the same spot. Ignore the resources required for the setups. Under ideal conditions, which would be a better choice. – Manu G May 6 '20 at 21:48
• Yes, I understand the problem. In this specific situation, take the single high power laser. The losses from not actually aligning all 5 on the same spot (you never manage 100 %) will be way higher than any efficiency issue. I understood the initial question more as "How much power can we tickle out of a high-power laser", not whether 5 mini diodes are better than 1 slightly less mini diode :) – OpticalResonator May 6 '20 at 21:54
• But with multiple mini diodes instead of the slightly less mini diode it's easier to control the required power (if needed). Or is it that the slightly less mini diode can do it much better. – Manu G May 6 '20 at 22:03
• If you need help picking a laser source for an application, then you need to describe the application better, it's a bit difficult if the requirements trickle in comment by comment, especially under a somewhat unrelated answer. Please edit your question to contain everything that matters for your choice of laser source. – OpticalResonator May 6 '20 at 22:13