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Testing at High Currents in Motion

krazer

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Hi all, I am the owner of the website linked in the first post.

Upon closer inspection of my initial measurements, I did make a computational error and have since corrected it on my website. In the projector the diodes are run at 1.7W for 3.2ms, followed by a brief pulse of 1.4W for 1.3ms, followed by on off period at ~20ma for 3.8ms. I believe the 1.7W pulse corresponds to the period while the projector is making green light using the phosphor wheel, the 1.4W pulse is when the diodes are being used to make blue light directly, and the 3.8ms off period is obviously when the red LED is on.

Furthermore, I have concluded 2A is NOT safe current to run the diode at if you want it to last more than a few hours. So far my test at 2A of current has caused the diode power to drop 15% in the last 20 hours, and now it is back at the same power as running at 1.5A. So if you want your diode to live for more than 10 hours best keep current below 1.5A!

To those who are wondering, the diodes I am testing are from an A140 projector. If someone would like to send me a diode from an A130 projector I would be happy to put it through the same tests to see how different they really are.
 



ossumguywill

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good to know krazer. I wonder why they didn't just use a phosphor wheel to make the red too?
 

Uberstrobe

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Good to see another member hailing from 4hv.

... has a very good reputation.

I still have that 200mW 808nm diode and the ZVS driver you put into the migratory box. :p
 

Meatball

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Krazer, good to hear from you! We all appreciate your willingness to put these things through some harder paces. It sounds like 2A is a good way to degrade the things.

I wonder why it didn't simply go by COD at such a high current?

Have they really improved the facets that much??
 

jmgallego

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Hey krazer,

Welcome!!

I been following your experiment closely, very intersting :D

Jose
 
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pullbangdead

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Krazer, good to hear from you! We all appreciate your willingness to put these things through some harder paces. It sounds like 2A is a good way to degrade the things.

I wonder why it didn't simply go by COD at such a high current?

Have they really improved the facets that much??
COD is about optical power density at the facet...these lasers, being multi-mode like they are, likely have a larger cross-section at the facet, so the larger power may spread over a larger area on the facet, giving a lower power density at any given point. It has been shown that by increasing the size of the mode in the laser, ie lowering the light confinement, you lose some efficiency because the light isn't as "concentrated" in the active region, but you can gain overall output power because you're lowering power density at the facet, increasing the total optical power output at which COD occurs.

That, and yes, facets are improving all the time.
 
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ossumguywill

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Isn't the facet just the partially mirrored surface on the business end of the diode? And how do you increase the size of the mode, I though the mode referred to the reflective patterns generated by the resonance cavity and its respective shapes at different temperatures?
 

pullbangdead

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Isn't the facet just the partially mirrored surface on the business end of the diode? And how do you increase the size of the mode, I though the mode referred to the reflective patterns generated by the resonance cavity and its respective shapes at different temperatures?
Yes, the facet is the partially-mirrored surface at the "end" of the laser diode, one on each end. But it can be engineered. It is pretty much guaranteed that there are dielectric coatings on the facets, for several reasons. 1) protect the facet, it has been shown that lasers with uncoated facets die faster than equivalent lasers with coated facets. 2) to tailor the reflectivities. For some applications, it is advantageous to put high-reflectivity coatings on both facets of the laser. Sometimes it is advantageous to have the facets have equal high reflectivities (say for a built-in photodiode behind the diode), sometimes advantageous to have different reflectivities on each facet. For this application, which is high power, it is most likely that the back facet has a very high reflectivity and the front output facet has an anti-reflective coating on it. This insures as little light as possible goes out the back, and as much as possible comes out the front. But the mirror reflectivities are undoubtedly well-engineered using dielectric thin films.

As far as the mode, think of the volume of light contained in the cavity, the shape where there is light. It has a specific shape, defined by the waveguide that forms the cavity. The 2 ends of the waveguide (longitudinal) are the facets. The top and bottom (transverse) of the waveguide are formed by layers of materials grown epitaxially, such as cladding layers, or an SCH (separate confinement heterostructure). The sides of the waveguide (lateral) are likely to be index-guided, like if you put a different material on the sidewalls of a ridge; or they could be gain/loss-guided, in which case there's no physical guiding element.

So the shape of the mode is determined by the construction of the diode. Possible explanation for these lasers: it looks like these lasers have multiple lateral modes (the "wings" that are in the direction perpendicular to the "fast axis"), it looks as if there may be a wide lateral waveguide, which allows a wider optical mode in that direction, indeed wide enough to allow multiple modes. That change in shape allows more power over a wider area, so it increases overall power at the facet while not increasing power density at the facet (ie power per unit facet area). Since the power is spread over a wider area, there is not a small area with a high power density, which can lead to COD. But I don't know the internals of the diode, and you can't tell everything with one short look at the far-field pattern, which is all I've done, so this part is all a guess.
 
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krazer

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To echo what has been said earlier, the reason these diodes don't just up and explode is that they have a much larger facet (no one has had a chance to measure it directly, but it is at least 10 times the area of the singlemode diodes extracted from cd-rom drives) so the power density is not nearly as high.

I am not sure exactly what the mechanism that is causing the diodes to fail is (as it turns out, failure analysis in laser diodes is tricky, who would have thought ;) ) but the current theory is that at high currents parts of the active region are damaged and stop emitting light. This process is irreversible, but also a slow decay, as soon as you take the excessive current off the diode will never be as efficient, but when the current is reduced back to a safe level it doesn't degrade any faster than it would normally (at least for short term, <100 hour time periods I can test).

This is concluded from my tests, where running the diode at 2A caused the power to drop quite rapidly, but after allowing the diode to run at 2A for 30 hours (which caused the power to drop from 1.7 to 1.0w), the power remains constant (at 1.0W instead of 1.4W when the diode was new) when running at 1.5A. Furthermore, increasing the current past a certain point (about 1.7A for this diode) does not cause the power to rise significantly, which further suggests that the power is going into damaging the device somehow.
 

jmgallego

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Krazer,

If I remember correctly, you mentioned that from a 140 projector. Is that correct?
 

Seoul_lasers

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This is all quite interesting. Not only are these laser diodes capable of 1W they can be pushed given proper cooling to +2W. Amazing.
Durable little diode, indeed!
 




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