Some facts about mode-hopping.

[ElektroFreak]

Hi

I've been reading posts on this forum for some time, and I've noticed that there seems to be some confusion about what mode hopping is, and how it happens

I created this thread to (hopefully) shed some light on the concept of mode-hopping and transverse emission modes in general.

Mode-hopping occurs due to many complex factors which cannot be adequately described here. The two main causes of mode-hopping are temperature and unstable power sources (like low batteries or using HiMH/NiCd rechargeable batteries in a laser pointer). With lab-style lasers these effects are countered by using a well-designed power supply and utilizing themoelectric (TEC) cooling in the laser head itself.

Laser pointers obviously lack room for these types of control mechanisms, however mode-hopping generally occurs in cheap higher-powered green laser pointers (>10mW and up). The reason for this is that in most cheap laser pointers the manufacturers use modules rated for 5mW or less and just turn them up (internally) to whatever power level they want. While this is a horrible practice for many reasons, the main two are that it drastically shortens the lifetime of the laser and it causes mode-hopping.

To elaborate a bit on laser emission modes: There are several different types of beam modes which can be produced by a DPSS laser. Typically, you want to see a clean beam with a single spot. Whenever another mode takes over (hops), then the beam can change to a large beam with 2 or more spots where it is projected. Also it can make the beam appear blurry and distended.

Some examples of various emission modes can be seen on Wikipedia here.

For the figures in the Wikipedia article picture a beam shining on a wall. The spot(s) produced are what the figure is showing.

I would hesitate to say that a mode-hopping laser is defective since these modes occur naturally in every DPSS laser. Whether or not it hops frequently is how I would determine if it is defective. A laser pointer that hops due to low batteries is not defective, it just needs new batteries. A laser that mode-hops when it is warm just needs to be operated on a duty cycle (a requirement that manufacturers rarely state).

To make sure that the dominant mode is TEM00, or the clean beam I just described, the manufacturer must ensure that the internal parts of the laser are precisely aligned and properly temperature regulated. This is usually not done with cheap laser pointers since it requires a great deal of time and expense. More expensive laser pointers are usually aligned better and built with better quality components, so it is less common (but not impossible) to see mode-hopping in these.

Hope this helps people better understand mode-hopping.

For loads more information on lasers of all types,
See: "Sam's Laser FAQ"http://www.repairfaq.org/sam/laserfaq.htm


Here's some examples of transverse modes:

 

[rocketparrotlet]

Excellent post, thanks very much!

Sticky?

-Mark

 

[Event_Horizon]

It should also be noted that TEM00 is the weakest powered mode there is. Generally speaking, the more separate blobs of light there is in your beam, the more of the laser cavity your laser is making use of, which generally means increased power output. If your intent is to focus the beam down to melt, burn or otherwise destroy, then a higher order mode is usually preferred. This is especially true on gas lasers. I have seen a CO2 laser, spec'd at a maximum of 30 watts, output over 56 watts of power using only really weird looking high-order modes.

 

[sad_sagi]

my "new wish" 5mw greenie does an unusual thing on startup, regarding beam shape. un focused(no front lens) it will make a fuzzy bean shaped spot and move, as if organically, in an odd movement, while not varying far from a bean ish shape. i'll have to get a vid of that...
i guess it can be described as going from tem00 to 01 or something... its very interesting to see happen in something like a laser.

 

[Event_Horizon]

That sounds interesting, I would like to see a video of that.

 

[Switch]

What is mode hopping actually caused by? I mean what actually determines the mode? Heat could cause the crystals to have a strange behaviour, but low batteries has no effect on crystals directly, they just affect the way the diode runs. :-/

So what is it?

 

[ElektroFreak]

Switch:

In truth, the amount of math involved in truly understanding and predicting transverse mode operation is pretty staggering and can't even begin to be stated here, but I'll try to elaborate a bit.

Low batteries (or other unstable power sources) can affect the frequency at which the pump diode emits light. In a DPSS laser using an Nd:YVO4 crystal (like most green laser pointers), the crystal's peak absorption band is at 808nm, +- a tiny amount. Even a fractional change in frequency of the 808nm pump diode will change how the crystal reacts to the input light, thereby changing the characteristics of the output. This can be manefested in several ways including drastic changes in output power and mode hopping. Temperature changes also affect the output frequency of the diode thereby causing the same effects.

Try not to think of the crystals as something that the pump light just travels through. Crystals do not filter the input light into something else, instead they create their own light within the crystal. The crystals use the light coming into them (the pump light) as stimulus for creating their OWN light at a different frequency. Typically these modes are formed within the gain medium, in this case the Nd:YVO4 crystal.

Also, Nd:YVO4 is very polarization sensitive. What this means is that if not aligned extremely precisely, the Nd:YVO4 crystal will not function at it's highest efficiency.

Sometimes higher-order modes are desirable as they do allow for more output power, but in applications where beam quality and coherence length are important, TEM00 is what you need.

 

[JLSE]

I dont understand one thing you are speaking of. If higher power can be obtained with higher order modes, why does the power cut in half when a shift occours? To get a higher ammount of power in this sense, do you mean by the alignment between the pump and the medium?

From what I have experianced with DPSS, once the crystals are aligned and the highest potential is hit, the modes will 'hop' if there is insufficient heatsinking to the medium. Sereve switching was an issue when too much power was put through a medium which could not shake off the heat. I have also seen this just before burning up a crystal set.

I have experianced drastic switching with different positioning in relation to the pump, but have never measured the output when it was anything other than 00. :-/

I would imagine to obtain a higher ammount of energy from the medium would be to cover more area where the pump light enters? As far as I can remember, the setups I have used had the medium as close as possible, where the 808 entered at its finest point.

My best DPSS so far has been a 1watt pump, and 300mW of 532. 300 was the absoulute max as far as output, but I didnt try testing when the beam was anything other than 00.

 

[ElektroFreak]

wannaburn:

To be a bit more precise, a higher-order mode structure will only allow more power output if the pump input is increased so as to allow the crystal to absorb more pump energy. Since crystals have distinct parameters regarding how much pump energy can be absorbed without damage, if you were designing a multiple transverse mode laser you would need to do calculations to determine the proper power input and crystal alignment to obtain the mode structure you want.

In other words, lasers that use higher-order modes to allow for higher output are designed from the start for that purpose, and many calculations would have had to be done to determine the correct structure of the laser cavity.

With mode-hopping, if your laser starts off at TEM00 and shifts to TEM01, then the original power at TEM00 would be split between two beams at TEM01. Each beam is half the strength of the original TEM00 beam.

Also, 300mW of green from a 1-watt pump is EXCELLENT overall efficiency, right at the top of the efficiency range for Nd:YVO4 + KTP. I'd say it would be nearly impossible for you to top that. That's a very efficient DPSS system.

 

[randomlugia]

I've got a question: My 5mW Ebay green broke last month. Now it puts out a very small amount of green light, and it looks like two connected dots with a faint ring around it. You can barely see it, but I looked with night vision and it's putting out more IR than it first did green! It this an allignment problem or mode hopping? Fresh batteries don't make a difference. :-/

 

[Event_Horizon]

Sounds like mode hopping as a result of an alignment problem.

My previous statement regarding higher modes resulting in higher power output does not apply so much to DPSS lasers like green laser pointers. Because the laser cavities involved are so damn small they generally use most of their lasing medium in the lasing process already. It applies much better to large cavity lasers, particularly gas lasers.

 

[randomlugia]

Oh well, sounds like it can't be fixed. :'( Mabye I'll start saving up for a Lyra.

 

[Event_Horizon]

It is pretty much never worth it to try and fix a 5mW greenie. If you want a replacement, I've had great experiences with the dealextreme ones.

 

[randomlugia]

Dealextreme doesn't ship lasers to America. Plus they're usually overpowered and I want one I can shine in peoples eyes accidentaly (flashing it around with mirrors, I usually get a small reflection in my eye all the time).

 

[ElektroFreak]

Here's my advice:

Lasers under 5mW generally do not cause eye damage even if directly exposed for several seconds, HOWEVER DPSS lasers also generate IR output that cannot be seen.

For your safety and others, DO NOT buy a laser pointer from DealExtreme if you feel that it might hit someone's eyes on accident or on purpose.

Save up your money and buy a low powered green laser pointer from a reputable company (not DX or WL). As long as you are buying from a reputable dealer, your pointer should have an IR filter installed. I don't feel comfortable recommending a company to you since I don't own a laser pointer, and have never bought anything except lab style DPSS modules.

Doing some research on this forum should lead you to a dealer that sells high qualty, IR filtered green laser pointers.

Also, it might be wise to obtain some safety goggles if you don't already own some.

You've only got one pair of eyes. Protect them.

 

[sad_sagi]

on the topic of ir and dx, my cr2 powered 5mw can easily be fitted with a filter if i got one. the pen styles are another story.

here is the blob i was talking about on said laser. this is without the front lens, and only the lens in front of the crystal

not sure how to imbed videos from photobucket...