sir, can you tell if the path length exceeds the coherence length then why fringes become visible after every 22 cm distance?

@@yashsingh616 Hello, Thanks for your message. This is something called coherence revival. The laser used in this experiment has quite a few longitudinal modes. Those longitudinal modes are separated by c/2nL, where L is the cavity length, c is the speed of light in vacuum, n is the refractive index. Let's consider the simplest and ideal case which means each mode has an infinitesimal linewidth. Mathematically, one can write the spectrum function as a product of the laser gain bandwidth G and a comb function which has a separation of c/2nL. The fringe visibility is the Fourier transform (FT) of the spectrum function. According to the convolution theorem, the result is the convolution of the FFT of G and FFT of that comb function. The FFT of comb function is a comb function, however, the separation is the reciprocal of the original comb function. Therefore, the fringes will appear again at a multiple of laser cavity length L.

@@xhu113 can u suggest any reference for this part (comb function and etc.)?

" a harmonic frequency of the cavity length."what does that mean

@@ARCSTREAMS ARCSTREAMS If you have a rope with a standing wave on it, say 2 feet long, a harmonic of that would be that the nodes fit exactly to the length of the rope. en.wikipedia.org/wiki/Harmonic For example if you send a wave down the rope and it returns to your hand, a one node wave on the rope would be 2 feet between nodes. 2 nodes on the rope would be 1 foot apart. 3 nodes on the rope 2/3 of a foot apart, etc. So the smaller laser has a cavity length of 22 cm, so the output frequency or frequencies when multiplied by a certain whole number will all give you the cavity length of 22 cm. So at about 6:00 when he goes from a single frequency to two frequencies from the 22 cm laser, the fringe patterns will come into harmony with the fringes at 22 cm changes of the one arm length of the interferometer. Essentially there is a standing wave in the cavity that has to end at the mirrors at the end of the cavity, if there are not in harmony, the laser output will not be very strong. When the nodes at the waves end exactly at the mirrors, the wave is in harmony and the output frequency at the output mirror will be strong. You can have more than one harmonic wave oscillating in the cavity that all come out strong at the output mirror. All strong output frequencies will be a harmonic of the laser cavity length.

@@ARCSTREAMS It would actually be more correct to state that a number of wavelengths of the output frequency will add up to be the cavity length.

@@JoeDeglman joe first of all thank you for the quick reply,,i kinda get the idea that there is obvious correlation that all have to do with the length of the cavity but my problem is i dont understand all the definitions of some of the words you are using like "nodes" ,"standing wave" etc ,i dont have that strong of a background in science or physics even though i am very interested,,,i am always putting experiments together or wondering what if or how certain things work ,atm i am trying to create a collimated beam of white light using an led ,a pin hole apperature and a 6" spherical lens which should be aspheric lol ,i been looking into simple equations about calculating divergence and tryign to understand the limits that i can go with my collimating attempt and i love lasers and always tried to build one somehow,, i am wondering is it possible to chat with you if i got questions? like maybe on webcam if possible,,anyways its late right now but i like to get back to you on this stuff,,thank you

@@ARCSTREAMS I do not have video conference capabilities. The concept for laser cavity harmonics is the same as for air columns. Mainstream says there is no medium for light, I Think there is enough evidence to show that there is. Regardless, the concept is the same and you can learn from air harmonics, the same principles as a laser cavity. www.physicsclassroom.com/class/sound/Lesson-5/Closed-End-Air-Columns Here is a guy who is working with trying to obtain a columnated beam of white light. Lasing doesn't work well with white light as it doesn't have a coherent frequency or harmonic frequency. White light has several colors of light to it and it cannot be oscillated well at a harmonic frequency at just about any length of laser cavity, they frequencies interfere with each other too much. The band width of light is a continuous spectrum of colors at different frequencies. www.researchgate.net/post/Collimating_white_light_from_a_tungsten_bulb The best way seams to be to used a hyperbolic mirror like in a telescope, a flashlight, or headlamp on a car, that uses a hyperbolic mirror to send all the light hitting it in one direction. or perhaps a pinhole like used to observe solar eclipse. The laser cavity length is usually picked based upon the general resonant frequency of the liquid or gas, and now semiconductor solids cavities. Then they try to oscillate the medium in the laser cavity with electricity at a certain frequency and fine tune the oscillation with a laser cavity and output optics, that gives the laser a very narrow bandwidth.. By bouncing the energy wave between the mirrors, creating a harmonic wave, just like sound in an air column. When you see video of people freezing or heating the laser and the output frequency changes, they are just changing the laser cavity length and thus the harmonic output frequency of the laser.