Laser not that bright, even when driven with half an amp

[WarFreak131]

Hello guys, I'm new to these forums. I built a laser a few days ago using an LPC 826 in an aixiz module from hightechdealz.com. I made the DDL driver circuit to test it out, and it was working okay. I hate using a breadboard to do testing, so I decided to use a professional current source to simply select the current I wanted instead of using the 1.25V/R method.

I set the current to 100mA and the beam spot showed up on the wall and I focused it down to the tightest spot I could. Then I slowly increased it, and the spot got brighter up to about 200mA. After that, an increase in current doesn't produce any significant jump in brightness, even up to 400/500 mA.

Now what I'm mainly concerned about is not the brightness of the beam at the end of the path, but the brightness of the path itself. I want to use this for astronomy. It helps so much to be able to point to a star or planet with the beam apparently traveling on to infinity.

But even in the dark, there's no beam path, just a spot on the wall. I understand the chromatic response of the human eye, that the sensitivity of the eye to green is much greater than that of red or violet, but it seems odd that a laser driven to 300mW using 500mA appears dimmer than a 5mW green laser, and lacks a beam path. It seems like a lot of amateur laser builders are also concerned with heat sinking the diode, but even when being driven at a high current, the diode never got more than lukewarm, the same thing goes for the LM317. Also, even when being driven at .5A, it wouldn't light a match.

I'd appreciate any feedback you guys have.

 

[Bionic-Badger]

You should post some pictures of how it looks. Maybe it got burned out? Even so those diodes usually only go up to 300mW, and that wavelength isn't that bright for the eye anyway.

If you want a brighter red beam, you should look into a one of those 638nm lasers. Not only can they go up to higher powers (some to 1W), but the wavelength is also far more visible to the eye. The only problem is that the beam isn't perfectly round, but it's more than adequate for star-pointing.

The above were taken at Burning Man on a dusty night (especially the latter); so it shouldn't be considered typical viewing conditions. Still, it is quite bright, and will be brighter than any LPC 650nm laser.

 

[WarFreak131]

Thanks for the info. Are certain wavelengths capable of more power because of the material they are made of? Can you recommend any good inexpensive diodes?

 

[Bionic-Badger]

Usually it's more about whether a company wants to invest money into developing higher powered laser diodes at a particular wavelength, and whether the beam quality is up to that specific need. Higher powered lasers usually require multi-mode resonator cavities, and that affect beam-quality. For example, the 445nm laser diodes have very high power, but poor beam characteristics because they're usually used for (projector) lighting; their dot looks rectangular. High-powered 638nm lasers also have poor quality beam shapes (a wide bar) instead of a nice single-mode output. 405nm laser diodes in Blu-Ray writers and 655nm laser diodes in DVD writers, on the other hand, have more emphasis placed on maintaining good quality beams at high enough powers for their application (writing optical media); they could probably be manufactured in higher powers if needed for lighting purposes.

If you want red laser diodes, you should check out the 638nm laser diodes DTR sells. Even the 110mW laser would be brighter than your 655nm laser, especially in scotopic (dark adapted) viewing conditions; 638nm is about 2-3x as bright to the eye at a given mW as 655nm.

I would probably go for the 300mW 638nm laser, as it is the least expensive of the three offered and will do what you need. Try driving it at ~300mW (~330mA). If you drive it higher, the output wavelength may actually shift to longer wavelengths, which could actually reduce the perceived brightness.

 

[WarFreak131]

I re assembled my laser, and after shutting the lights in my room, I was able to get a decent thickness beam at ~200-250mA. I don't know what changed this time.

Also, I did some research, and the maximum sensitivity for scotopic vision is at just above 500nm. The green curve in the picture below is the response for scotopic vision.

And according to http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/Sample-Chapter.pdf, the max sensitivity for scotopic is at 505 nm. Are any diodes made at this wavelength or close to it?

 

[WarFreak131]

Nichia makes an 80mW green laser at 510-520 nm which would be perfect, but as far as I can tell it's not for sale.

 

[pschlosser]

Ocular sensitivity has been tested a great deal in the last century. I understand photopic efficacy (sensitivity to daylight using cones, not rods) peaks out at 555nm. This site summarizes the ideas quite well, IMO.

Luminous Efficacy

But with this said, you mentioned scotopic efficacy, which is night vision using rods, not cones. Even though the lights are out in your test area, because what you're viewing is a light source, aren't we talking about photopic (daylight) vision using cones?

It is for this reason, we can see red laser light pretty decently as night, because we are seeing it with our day vision receptors.

With that said, in *very* low-light conditions, we will see 507nm light way, way better than red (>600nm) light, which is far from the 507nm peak for night vision.

The reason red light preserves our night vision so well, is that our night vision (rods) are nearly blind to red light. Thus, when using red light at night, we are seeing with our daylight vision (cones) but not saturating our rods, so they are standing ready to help us see (in the dark) when we need them.

If you are seeking a LIGHT (be it viewed day or night) we are most sensitive to, then we are talking photopic-daylight-cone vision, and that frequency is 555nm. AFAIK, the 532nm green diodes that are popular, right now, are the most abundant and lowest cost solutions for efficient light

 

[WarFreak131]

Thanks for the response. So is what you're saying that we want to maximize our cone usage, while minimizing our rod usage?

In that case if you go with a 570nm laser, you'll be getting 95% converted photopic light with only 20% converted scotopic?

 

[Bionic-Badger]

If you want the most visible beam with common wavelengths, get a 532nm green laser. Under scotopic conditions even a 5mW laser will far more visible than a 1W 638nm.

The reason you choose red light for scotopic conditions is that it doesn't affect your night vision the way other wavelengths may. Still, it might be cheaper and easier just to get a lower powered 532nm laser for star-pointing (5-15mW will do), as long as you don't, for example, illuminate stuff close by which can ruin your night vision. You could also use 445nm laser, but that might be overkill and be dangerous, even if it looks cool.

 

[Cyparagon]

The rest of the astronomy community uses ~5mW green pointers for good reason: they're cheap, effective, and (relatively) safe. Your red diode setup is cumbersome, dim, and much higher power.