Hello Vladimir,

I try to answer your questions:

a) You can also find out, how the laser is polarized by putting a linear polarization filter into the beam. (the polarizator surface must be perpendicular to the beam) Behind the filter you put a laser power detector. Now rotate the polarisator around the laser axis and look at the power detector. If you cannot observe any change in the power, than the laser is circular polarized. If you have different power with different angles of the polarizator, than you have elliptical or even linear polarized light.

b) the coherence lenght you can check with the simple setup of a Michelson interferometer. (http://en.wikipedia.org/wiki/Michelson_interferometer) The difference in the path length until no interference takes place gives you the maxiumum coherence length. Everytime you can detect an interference pattern, the beam is coherent :-)

c.) What kind of "modes" do you want to measure? There are longitudinal modes ( different wave length of the laser) and transverse modes (spatial distruibution of beam intensity transverse to the beam direction. For the first one, you need some kind of spectrometer ( prism, grating fabri perot) for the second one you need a light detector with spatial resolution (ccd camera...).

I hope I could help you!

Best regards,

Alan

hello Vladimir,

kindly contact Prof G Ravindra kumar (my PhD classmate at IITK) currently at TIFR, Mumbai on any thing related to Lasers.  You can reach him on [email protected]

Measuring the coherence length of the laser using an interferometer could be difficult if the laser has a long coherence length (you would need a big interferometer).

The other means of doing this would be to use a spectrometer to measure the bandwidth of the laser, and use this to calculate the coherence length. 

http://en.wikipedia.org/wiki/Coherence_length

One of the most attractive properties of vertical-cavity surface-emitting lasers (VCSELs) is a circular polarization, simply can be checked, focus the laser beam by using objective lens and put IR viewing card in front of it, then you will see a circular beam

Regarding the coherence length, I think there are two kinds of coherence length, one is spatial coherence length and another is temporal coherence length. If you want to measure the spatial coherence length, using an spectrometer is a good way, but using this way you can only calculate the Fourier limited length and if you want to measure the spatial coherence length , the interferometer could be a suitable way.

Hi,

I wanted to add some notes about polarization, when you put linear polarizer filter between the laser beam and a powermeter then you rotate the polarizer you should not see variations of the measured power if your laser is circularly polarized, but this not a sufficient proof because you will see the same thing using unpolarized laser (random polarization). The best way to see if the laser is circularly polarised and even measure the degree of circular polarization is to add quarter-wave plate between the beam and the linear polarizer filter, it transforms circular polarization to linear one oriented at 45° from the wave-plate EO axis. So you will detect a maximum power when you align the linear polarizer with this axis and theoretically "0" power along the perpendicular axis, but for not perfect circular polarization (this is usually the case with real systems) you may measure some power along this perpendicular axis, this gives a measurement of the degree of polarization called S3 parameter (Stokes parameter) (=+/-1 for right-handed/left-handed circular polar).

To end here are some references that could help you:

- For Stokes parameter and degree of polarization see: http://en.wikipedia.org/wiki/Stokes_parameters

- For polarization states representation and transformation trough optical elements : see Photonics textbook by Amnon Yariv & Pochi Yeh chapter 1.6 (polarization states and representations)

I have few additional questions about lasers:

1)Suppose that we change the distance between laser resonator mirrors for same material inside. On which properties of the laser it will affect?

2) What are the disadvantages of 3 level laser model?

3) What are the disadvantages of 4 level laser model?