Understanding Laser Coherence Lengths and Improving Coherence

[major_tom]

Hi all

I am hoping that somebody here will be able to tell me why that different types of lasers eg He-Ne or Nd:YAG have different coherence lengths and how the coherence of a laser can be improved.

I am thinking it is to do with the different gain mediums and the emission process involved but am unsure exactly.

Thanks in advance

Tom

 

[mgb_phys]

Different laser technologies have different coherence properties (diodes are much worse than gas lasers) within say gas lasers it cna be controlled by controlling the cavity length and gas temperature/pressure.
You can also do tricks like having two lasers feeding into each other so they keep relocking each other, not sure of the details but the guys in the lab downstairs said they had two CO2 lasers locked like this with a coherence length of >250,000km

 

[major_tom]

Thanks for the reply, what is it which actually effects the coherence properties? Also would it be possible that you could explain what relocking is please.

Thanks

Tom

 

[mgb_phys]

You need to keep the condictions inside the cavity stable (particularly length) so the same number of wavelengths fit.
Gas lasers often take a small part of the output (in He:Ne you typically take one polarization) and use this as a feedback to control the cavity length with heaters on the rods holding the mirrors apart.

A laser basically makes copies of the first photon in the emission, by having two lasers if one starts to drift to a new wavelength then a copy of the original photon form the second laser triggers the emission of more copies in the first - and vice versa.
Sorry not really an expert since we could just buy super-stable lasers from HP.

 

[Andy Resnick]

Hi all

I am hoping that somebody here will be able to tell me why that different types of lasers eg He-Ne or Nd:YAG have different coherence lengths and how the coherence of a laser can be improved.

I am thinking it is to do with the different gain mediums and the emission process involved but am unsure exactly.

Thanks in advance

Tom

The coherence length is given by the frequency spread of the source. In a laser, it arises from the resonant interaction of the atoms and the radiation field. Spontaneous processes always occur, but the stimulated emission process is the amplified one in a laser cavity. There are several ways to generate a long coherence length:

1) long residence time in the excited state (decreased bandwidth of the resonant interaction). Nd:YAGs are good examples of this.

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

2) Spectral filtering the output- again, there are lots of ways to do this, including distributed feedback laser diodes.

http://www.toptica.com/products/laser_diodes/distributed_feedback_laser_diodes.html

3) Size of the laser cavity- the coherence length is (absent other considerations) about twice the length of the cavity. Gas lasers (He-Ne, Ar, Kr) are good examples. NIST has a great example of a stabilized high-finesse cavity:

http://tf.nist.gov/timefreq/general/pdf/2237.pdf

CO2 lasers, as mgb_phys mentioned, have a very long upper state lifetime (3s), but collisions reduce this to 10^-5 or 10^-7 s.

 

[mgb_phys]

CO2 lasers, as mgb_phys mentioned, have a very long upper state lifetime (3s), but collisions reduce this to 10^-5 or 10^-7 s.

Never knew that - I thought the group used CO2 because they enjoyed burning through things on open days.

 

[Andy Resnick]

Never knew that - I thought the group used CO2 because they enjoyed burning through things on open days.

heh... they are good for that, too!

 

[major_tom]

thanks so far, it has been very helpful and interesting. One last question (I hope).

From what has been said i assume this means that as the laser heats up and expands the coherence length will reduce due to the changes in wavelength and frequency?

Thanks

 

[Andy Resnick]

Interesting question... I suspect that if the laser heats up that much it will cease to function properly. At least, I'm guessing that because of all the thermal management that takes place in a good laser design.

Changing the length of the cavity like that will most likely only shift the center frequency, leaving the frequency spread (relatively) unchanged.

You could control the length of a cavity with a piezo device- I think that's how the NIST cavity is tuned. Distributed feedback diodes can be tuned by rotating the Bragg reflector.