Divergence over Power

[ElektroFreak]

I have some Aixiz 50mW green labbies that all have divergence values below 1mrad. I also have a CNI 100mW green labby, a NewStar Lasers 150mW labby and an o-like 100mW labby that all have divergence values in excess of 1.5 mrad.. It seems like the higher the power, the worse the divergence. I believe this is due in part to the fact that they are all "microchip" lasers that use DPM crystal sets. These crystal sets have cavities that are 2-3mm long at best. Also, the optical arrangement of many Chinese lasers may not be 100% optimal. I generally don't complain about it because I use most of my labbies in scanners and the slightly higher divergence makes crowd scanning safer while at the same time preserving the razor thin laser beam look.. As long as the divergence is less than 2 mrad, I'm fine with it.

I'm not an optics expert, and things like focal length and lens selection give me a headache. I do know that sometimes, even if you use a collimating lens and focus the beam as well as possible, the beam can only be collimated so far. As an example, I attempted to refocus my NewStar Lasers 150mW to reduce the divergence, but no matter how much I adjusted, I could not achieve a divergence less than 1.8mrad. I'm not 100% sure why that is, but I'm hoping someone else out there does and can explain it for us.

 

[Dam]

I'm not 100% sure why that is, but I'm hoping someone else out there does and can explain it for us.

Thanks for input ElectroFreak. I also hope that someone wise explain it for us.

Unfortunately the people who care of divergence, range and beam purity are in wast minority to the people who care of burning match power only.

Maybe we should develop a new kind of beam focusability parameter:

Initial beam diameter x beam divergence. (Kind of beam expanding ratio)

It would tell us more about maximum laser range. Example, if we want to get no more than 180 mm spot at 1000 m distance we must apply a 10x beam expander to the X laser (from my post above). For Y laser, the cheaper 5x expander will be fine and we get 2x tighter beam at the close distance at the same divergence number.

 

[ElektroFreak]

The mRad value of divergence is calculated by measuring the beam diameter at the aperture and again at a given distance. Basically it is exactly the parameter you describe..

 

[Dam]

No, I mean divergence times beam diameter.

It gives you an idea of not only the beam expansion rate, but the beam 'parallelity' as well.

So, let's name this parameter 'parallelity'.

If we forget about units and consider only the values, laser X will have (1.5 [mm] x 1.8 [mrad])=2.7 parallelity
And laser Y=(1.5 [mm] x 0.9 [mrad])=1.35 parallelity

Y is lower, so it's better. It will give you smaller divergence at the same beam diameter.

Thinking on other way:
According to my two X and Y laser with beam expanders applied, 15 mm diameter beam with 0.15 mrad divergence is not equal to 7,5 mm diameter beam with the same 0.15 mrad becouse it will be thinner at the beginning, however expansion ratio (divergence) will be the same. Calculating my 'parallelity' parameter gives here the same results with or without the beam expander. The Y laser wll be work with 5x beam expander and X laser will need 10x beam expander to achieve similiar result at the far distance. Both virtually will produce same spot size far away, but I know that you will like Y laser's beam more becouse it will be thinner at the beginning and 5x beam expander will be cheaper.

Huh, that's the point.

I can't explain it briefly as English isn't my native language, I'm sorry...

 

[ElektroFreak]

I think that there is already a measurement for what you are describing, it's just that the term for it escapes me ATM..
I'm currently at work, so I didn't read the entire entry, but I'm fairly sure that this link describes what you are talking about..
http://www.rp-photonics.com/beam_parameter_product.html

Also this link is relevant:

http://www.rp-photonics.com/diameter_divergence_product.html

 

[Dam]

Thanks for the links above, I've read entire article from your link, ElectroFreak. BPP is the exactly what I described.

It would be a good measure of our lasers beam quality.

 

[steve001]

ElektroFreak;

Nice explanation.

I have been testing lens assemblies for several months now ...

and see large differences in divergence among them.

The Blu-Rays are, of course, the best I have tested with short FL lenses.

I am getting 1/8" beam at 14 feet.

By my rough calculations, that looks to be ~1 mrad.

I have some adjustable-focus green lasers on the way to test also.

LarryDFW

The measurement should be done at 10 meters or 32.8 ~33 feet.

 

[Emc2]

Hey dude the optotronics RPL 425 has a divergence of 0.8~1.0mrad as stated on his webpage. I also have the +150mW pointer with the 1.2mrad and the RPL 425 will be less. I just ordered one of these in the silver and I will receive it on Wednesday July 1st. Jacks RPL's have the lowest divergence in the industry even at that power. Take a look at these:

YouTube - Optotronics RPL-425 burning stuff Green laser burn

YouTube - Optotronics RPL-425 Burning Matches, Printer Paper etc.

This is the RPL 400 laghtig a match

YouTube - Optotronics RPL-400 4-meter matchlight


Some info from The Optotronics site:
The duty cycle

The adjustable RPL has the ability to tune the amount of IR diode current so that the optimum amount of IR goes to the crystal. If more than the optimum amount of IR hits the crystal, the crystal becomes over saturated and 532nm output no longer increases. Only more heat is produced and in fact this can cause output to decrease some. When the RPL laser is tested, the optimum current level setting is noted in the customer documentation and this is the same current level that the average output power tested and recorded for the customer. Most of the time, this current level is #4 or #5 and adjustable RPLs sold after 2/1/2007 will run until the battery dies at this level (no duty cycle). The reason we give a recommended duty cycle of 5 minutes on and two minutes off is that some users will buy the non-adjustable model and it's current cannot be tuned. It is safe to leave the RPL on until it shuts itself off, in fact that is what the battery protection circuit is for. The shut off is to protect the battery from excess discharge/low voltage as this can be dangerous for lithium-ion cells. The RPL diode is never ran at more than 80% of it's rated operating current, so as long as the ambient room temperature in the spec does not exceed 30C, the diode will be not be thermally damaged by extended use.

Beam Divergence

Laser Divergence
Why a small change in this specification greatly affects laser beam intensity.

Divergence is the increase in laser beam diameter with distance from the aperture from which the beam emerges in any plane that intersect the beam axis. What this means is that the diameter of the beam expands over distance at an angle (the divergence in milliradians or mrad ). As this angle increases, (even by small amounts) the diameter of the beam increases and the area of the beam (Area of a circle = π multiplied by the square of the radius) increases at an exponential rate. Optotronics handheld laser products have a divergence spec. of 1.2mrad orless (unless otherwise noted). Most other suppliers of handheld laser products have a divergence spec. of 1.5mrad. This means that for the same beam diameter and output power, the light intensity or irradiance (Output power / area of beam spot) is up to 56% greater with a divergence of 1.2mrad vs 1.5mrad. So with an Optotronics laser your not only getting more for your money, your getting a beam that's 14.8% more intense at 0.50 meters distance to over 55% more intense at distances of greater than 60 meters.

Below is a chart showing the divergence for a 260mW laser with a 1.5mm beam diameter and a distance of 3 meters vs. an Optotronics portable RPL with a divergence of less than 1.2 mrad.

(click the chart to download the Excel sheet to experiment with the divergence / irradiance calculators for various senarios).


Link for the chart

http://www.optotronics.com/laser-divergence.php



-Todd

 

[themandalorian]

I would love to do Opto but just don't have enough money for what I want, hopefully a 10X beam expander can get me where I want to be

 

[LarryDFW]

The measurement should be done at 10 meters or 32.8 ~33 feet.

Steve;

The Optotronics spreadsheet has inputs for different lengths to the measured spot size.

It was actually under 1 mrad.

LarryDFW

 

[steve001]

Steve;

The Optotronics spreadsheet has inputs for different lengths to the measured spot size.

It was actually under 1 mrad.

LarryDFW

Strictly speaking only about divergence.
Generally lasers speaking laser beam divergence is measure using the accepted standard of 10 meters. Because in most cases you are beyond the Rayleigh Length. Shorter distances won't give an accurate measure of beam divergence. It should be measured from that distance onward for greatest accuracy. Inside of the Rayleigh Length the beam expands 1.4 times its original diameter. I had this booked marked at one time and then deleted the link as it was dead, but it seems to be working
pseudonomen137's JScript mRad Calculator
Other link Section 2.2: Properties of Laser Beams

I am getting 1/8" beam at 14 feet.

You are probably right it being around 1mrd. For sh!ts and giggles I'd still measure it a 10 meters just to see what the diameter is.