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FrozenGate by Avery

Some facts about mode-hopping.

Looks like a mode-hop to me. It's probably just caused by everything stabilizing after the initial powerup. Almost all common DPSS lasers exhibit wild power fluctuations during the first few moments after being turned on. Sometimes they mode-hop as the diode comes up to a stable temperature. Probobly nothing to worry about, that's for sure. Just as long as it returns to and stays at TEM00 once everything has stabilized.
 





Don't forget to mention, that multiple modes can exist simultaneously. For most greens, it is usually just one mode or the other, but laser diodes often have multiple modes at once, where the relative ratios of the modes can shift with temperature and wavelength. I even saw a scientific paper once where was explained that you can actually have a beam profile that resembles TEM00 to within one percent and still consists of 0% TEM00 and like 6 different other TEM modes.

Here's my video of a nice mode "shift" in a PHR violet diode:
[media]http://www.youtube.com/v/qsbWE1gYrGw[/media]
 
Nice video! One thing about transverse modes is that, to me, they can be very striking, even beautiful. Never seen anything like that from a blu-ray though, very cool!
 
does it really look like rings of purple/blue to the eye when it does that?
might be worth it just to un focus it and leave it like that if its that cool haha
 
sad_sagi said:
does it really look like rings of purple/blue to the eye when it does that?
might be worth it just to un focus it and leave it like that if its that cool haha
Click to expand...
I think what you mean is the effect of the camera CCD overloading.


Here's another mode shifting vid from another diode

[media]http://www.youtube.com/watch?v=EGdUY84EVfE[/media]
 
so i just got a 50mw greeny and had mode hopping problems, it went away but now 9out of 10 times i turn it on with fully charged batteries i see it go from full power to about half.
ive determined this isnt due to warmup because if i keep turning it on and off it eventually will output at full power but then if i let go and push it again i get the same problem.
any know if mode hopping is the cause of this?
 
^good advice..

The biggest problem with cheap greenie laser pointers is that most of the time their behavior is random. No two are alike, and it's not at all uncommon to see large power fluctuations during normal use. It's just the nature of simple DPSS construction and it's completely normal. The only way to solve the issue if it's pronounced enough that it bothers you is to either RMA your laser or buy a different one.
 
You can also try adjusting the current a few mA up or down. Sometimes a slight adjustment is needed as they break in.

Ive had greens that started misbehaving after months of use some in minutes.

At the rate that they are pumped out on the assembly line, they most likely just get it working and package it. I cant see any real testing or TLC going into China lasers.

I always put at least an hour of run-time on my green modules before installing them into something permanent.


If this mode issue is only a problem with fresh batteries, it may be as simple as too much current going through the LD.


No better way to find out than to experiment.
 
wannaburn said:
You can also try adjusting the current a few mA up or down. Sometimes a slight adjustment is needed as they break in.

Ive had greens that started misbehaving after months of use some in minutes.

At the rate that they are pumped out on the assembly line, they most likely just get it working and package it. I cant see any real testing or TLC going into China lasers.

I always put at least an hour of run-time on my green modules before installing them into something permanent.


If this mode issue is only a problem with fresh batteries, it may be as simple as too much current going through the LD.


No better way to find out than to experiment.
Click to expand...

Yep, I had a dx 5mW that i was using in my RGV pointer and it was mod-hoppong right out out of the box. I ran the diode off a DDL driver and tried turning the current down. That didn't work, so i turned the current up and it worked great.

--Hydro15
 
Here's my video of mode hopping in Chinese 30mW module (not pointer).



And by the way, I wonder if green pointers uses intra-cavity freq. doubling. Does anyone know something about it?
 
It is intra-cavity, yes. The OC mirror on a bonded crystal set is on the outer face of the KTP crystal, which places the KTP crystal in between the mirrors which is what defines "intra-cavity" as opposed to extra-cavity which would place the SHG crystal outside the OC. With intra-cavity doubling, the incident light passes through the crystal many times, with more and more photons being doubled with each pass. This is the most efficient method of SHG. Almost ALL linear cavity DPSS lasers use intra-cavity SHG.
 
OK, thanks for explaining. So cheap Chinese modules but with separate YVO and KTP (that is strange in cheap modules but I think I got one like this) are intra-cavity doubled too, right? That means no coating on YVO's output surface and KTP's input surface? Or maybe they're AR coated?
 
Those faces are AR coated, yes. In systems with separate crystals the OC comes after the KTP crystal, so they too are intra-cavity doubled. Really the only extra-cavity SHG lasers I can think of are very high-powered pulsed systems where the sheer power involved causes the SHG process to work. In some systems like this the 1064nm light can be so powerful that multiple passes through the KTP can cause damage.
 
So crystals "bounce" light back and forth like a gas laser does? Interesting, I did not know that, I thought it was a "straight shot" through
 
Several basic things can be done to prevent modehopping in DPSS laser systems.

Generally, DPSS systems will use an Intra-Cavity aperture. This forces the laser to only run in a single mode. This can either be a physical pinhole, or a pump aperture. A Pump aperture is done by pumping a small cone in the lasing medium to prevent other modes from getting gain. These can also be used together to further force single transverse mode.

If you have a cavity without any aperture, it can lase single mode with the correct mirror configuration. On most small DPSS lasers, one or both of the mirrors are coated onto the crystals. This presents a problem for two reasons:

First, the mirrors are both flat meaning you have an unstable resonator, allowing any mode to lase.

Secondly, you get an effect that is called thermal lensing. This happens because the active area of the crystals expands due to the heat. The result is that you will get different cavity shapes, some of which only support non-TEM00 modes.

The combination of these in small crystals with unregulated temperatures can cause severe mode hopping problems.
 
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