Stability: Less is more?

[donprado]

In the rare laser reviews which contain a graph of the laser power for some minutes, I realized that the output of most (if not all) portable lasers has quite large fluctuations, depending on many factors like temperature, batteries,... but since it's a review of one laser, there is no information about output stability depending on laser power.

When you are looking for stable and reliable output, is a less powerful laser probably better? :thinking:

For example, when you compare an Optotronics RPL 250 and a RPL 450 - can you expect a more stable output from the 250?


The next question is, where these fluctuations come from - is it linked with small changes in the mode of the laser? Is the pump diode unstable? Is there a problem with oversaturation of the KTP crystal? Or does the alignment slightly change with temperature?


MANY questions - I would be very grateful for some explanation from experienced members. Thanks.

 

[anselm]

For example, when you compare an Optotronics RPL 250 and a RPL 450 - can you expect a more stable output from the 250?

I guess when you're talking about that company, you can expect both
of those models to be stable.

When you go for cheap companies, stability will be a matter of luck
getting a good unit, the quality usually varies a LOT, regardless of power.
Hey, I have got 5mw pens off ebay, some are stable, some are not.

Is there a problem with oversaturation of the KTP crystal? Or does the alignment slightly change with temperature?

You might be onto something there
Some greens go brighter as they warm up, some go dimmer.
So those DPSS crystals are rather temperature-sensitive.
In exactly what way that works I don't know...

 

[RA_pierce]

A lot of factors determine what happens in the laser cavity.

Temperature and current input of the diode, temperature of the KTP and Nd:YVO4, alignment, crystal quality, heat dispersing properties of the heatsink, etc.

Generally, with lower power it will be easier to manage beam modes and heat, so will be more stable.

 

[ElektroFreak]

In the rare laser reviews which contain a graph of the laser power for some minutes, I realized that the output of most (if not all) portable lasers has quite large fluctuations, depending on many factors like temperature, batteries,... but since it's a review of one laser, there is no information about output stability depending on laser power.

When you are looking for stable and reliable output, is a less powerful laser probably better? :thinking:

For example, when you compare an Optotronics RPL 250 and a RPL 450 - can you expect a more stable output from the 250?


The next question is, where these fluctuations come from - is it linked with small changes in the mode of the laser? Is the pump diode unstable? Is there a problem with oversaturation of the KTP crystal? Or does the alignment slightly change with temperature?


MANY questions - I would be very grateful for some explanation from experienced members. Thanks.

You're actually very much on the right track with some of your speculation.

In DPSS lasers, the power fluctuations are due to a couple different factors:

1) Pump diode temperature stability. If the diode is not kept thermally stable, it's wavelength will change with temperature, which in turn affects the crystals ability to absorb the incoming pump energy. Also, if the power supplied to the diode is unstable, the diode's output will be similarly unstable.

2) Crystal temperature stability. Almost all laser crystals (both lasing mediums and non-linear crystals) have thermal sweet spots at which they behave at peak efficiency. If the crystal is not kept thermally stable, then it's efficiency will be compromised, and output will drop off.
Also, there are short-term thermal effects. When the energy from the pump diode is focused into the laser crystal, the crystal's temperature changes due to the heat generated by the incoming energy. This causes minute changes in the dimensions of the crystal, and thus the laser cavity itself, resulting in rapid fluctuations in the laser's output.

In most portable lasers and pointers there is little or no means of providing any real thermal stability. Typically in larger lab-style systems various methods of heating/cooling are applied to the various internal parts of the laser, thus greatly improving stability.. this means that portables and pointers will exhibit much bigger fluctuations in output than lab/industrial lasers. With lower powered lasers, the amount of energy in the cavity is less, leading to reduced thermal effects.

 

[donprado]

Thanks for your explanations - I really appreciate your help.

So, if I want a very stable laser, I have to go for a lab laser, where Nd:xxx and KTP crystals are separately temperature controlled.

Are there portable lasers with temperature control for the crystals?

And if there is no temperature control, is the alignment of portable lasers optimized to have a good peak power in the first seconds (because good for burning...) or for the output after the laser reached a thermal equilibrium?

 

[ElektroFreak]

Thanks for your explanations - I really appreciate your help.

So, if I want a very stable laser, I have to go for a lab laser, where Nd:xxx and KTP crystals are separately temperature controlled.

Yes

Are there portable lasers with temperature control for the crystals?

None that I am aware of.

And if there is no temperature control, is the alignment of portable lasers optimized to have a good peak power in the first seconds (because good for burning...) or for the output after the laser reached a thermal equilibrium?

That depends on the quality of the laser. Many cheapies are just thrown together with minimal attention to alignment, but most reputable manufacturers try to align so that the system is stable after it reaches equilibrium.