Old 10-21-2014, 11:59 PM #1
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Default Why is stability so important?

Hey guys,

I have another question about lasers I would like answered if you all don't mind!

I was curious as to why lab lasers have different stability levels & why they cost more for being more stable? Take for example 532nm(as shown below) what is the difference between 10% & 1% stability? Isn't it the same wavelength? Isn't 532nm already a stable wavelength??



-Alex


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Old 10-22-2014, 12:56 AM #2
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Default Re: Why is stability so important?

In scientific applications instability of output, in any form (amplitude, phase, polarization, frequency, spectral purity, beam cross section, divergence, etc) will impose a noise signal on the actual data signal. Depending on what you're measuring the noise signal may totally swamp the data signal, leaving you no usable at all.

For example: a simple amplitude modulated audio transmission setup is employed, to vary the intensity of the laser beam to match the input audio signal. You have a laser rated for 10% output stability, which when measured varies it's output by 10% up and down in the pattern of a sine wave 1000 times a second, perhaps as a result of voltage ripple across the output of the driver from an insufficient filter stage in the power supply and an overly loose feedback loop in the driver. The result is a loud 1KHz tone in the output.

Pertaining to solid state lasers, where modulation is in the driver:
Additionally, since the laser's output never leaves a certain range, you have to set the drive current bias point on the modulator based on the expected input levels. If your laser peaks at 10mW and your detector is set up to expect between 500uW and 10mW, and you want maximum modulation, you swing the output from 100% to say 5% (because the receiver throws an error when no signal at all is received), for a 95% modulation index. If your peak output keeps changing from instability, you can no longer use a set modulation index or a set level of gain on the receiver, one or both of them have to be adaptive to correct for the varying efficiency of the diode. If you don't correct for this, what happens in an example is at 100mA you would get 10mW at 25C, but at 30C you only get 9mW, your laser diode warms up and you aren't modulating the laser, but the detector thinks you are because the signal dropped to 9mW when the control electronics are saying "full steam ahead!" by supplying the 100mA. Again, noise is imposed on the received signal that shouldn't be there. In this example not only is a false signal received, but a signal of 100% state will NEVER be received, because the diode now only tops off at 9mW instead of 10mW.

Most systems that rely on lasers for communication don't use amplitude modulation to avoid this, but the principle is still valid.


Edit: also in reference to wavelength stability;
Sensors don't read equally for all wavelengths. So a sensor that puts out 1V for 1mW at 520nm may only put out 0.8V for 1mW at 530nm. If your diode's frequency shifts, now the measured output shifts because the sensor is nonlinear. 532nm is relatively stable in terms of frequency, but not in terms of amplitude, or phase (because of thermal transitions in the crystal set).
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Old 10-22-2014, 01:00 AM #3
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Default Re: Why is stability so important?

I think that's actually for power not wavelength. DPSSL's are know for power instability, especially in portables.

With lab lasers, the temperature, optics alignment, ect., can be optimized for max stability. Just like with a power increase, the extra effort to "tune" the system for extra stability adds to the price.

Edit: Wow, i was slow on this one! My TL; DR version took longer than Sigs thorough reply...
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Old 10-22-2014, 01:01 AM #4
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Default Re: Why is stability so important?

Am surprised you can't guess--common sense will tell you some of it---read up on lasers gain some functional knowledge of what goes into making them. Is way too complicated to explain everything in a post.

Why, depends upon the nature of the application--the need for a laser at all.

The beam pointing stability of commercial laser products is often quantitatively specified. Unfortunately, such specifications are often not precise or even meaningless. If a laser beam is sent through some optical setup, this will in general modify the magnitude and type of beam pointing fluctuations, even if the optical components are absolutely stable. There are several kinds of stability.

Have a look at Transverse mode - Wikipedia, the free encyclopedia

See: Encyclopedia of Laser Physics and Technology - beam pointing fluctuations, pointing stability

stability---1% or 10% stability depends on percise control of parameters-- the more percise the control and environment that creates a laser the more costly to do--the electronics and physical components---quality and design factors.

See: The World's Most Stable Laser

Google: laser stability --- it is all available information --hope that helps a little bit.

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Old 10-22-2014, 01:01 AM #5
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Default Re: Why is stability so important?

Thank you everyone for the answers!

@Encap I will definitely read up on it more since it's pretty interesting

-Alex
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Last edited by Hap; 10-22-2014 at 01:02 AM.
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