I am currently working on a home build THz time domain spectrometer the gerenration is done with PCA and detection is by EO sampling. I am using manual rotion mount to align crystal, lambda/4 and Wollaston prism. The crystal and prism are ok but quarter wave plate shows unusual behavior after each scan or after 5 min we need to align the quarter wave plate again and again otherwise we don't get THz signal. Initially I thought that, the problem is with mount so i used another rotation mount but the problem remains same. When I align the lambda/4 and then run the programme I get signal but after few min. I need to align it again all other optics and electronics are ok just quarter waveplate need to align again and again. Can some one explain me the reason behind it. I also try by locking the rotation of manual rotation mount but the reading of Balance photo diode still reaches it max. after few minutes which is minimum after alignment.
Thanks for the help in advance.
Tam dalga yarım dalgaya, yarım dalga çeyrek dalgaya dönüştürülebilir. Başlangıçtaki çeyrek dalga ile sondaki çeyrek dalga birbirini yarım dalgaya tamamlar. Yarım dalga daha küçük tam dalgaya dönüştürülerek yapılır.
Malcolm White thanks for the answer, I think either it is problem with quarter wave plate or with the balance photo detector. Both are from thorlabs, when I kept power of laser low around 1 mW it take more time but eventually the photodiode saturetes then I need to rotate quarter wave plate again to make A-B ~ 0. When the power is high approx 4-5 mW the diode saturates faster but on rotation of lambda/4 every thing become normal. The threshold for diode as on website is 20mW we are umder the threshold value. Also we are usinf 780nm fmtosecond laser with oscillator repition rate in MHz for generation of THz. If you may know the possible reason and solution please let me know.
I am used to systems where the phase of a component may be temperature dependent, so changes in the temperature of the quarter-wave plate might change its phase, so it needs re-adjusting (or temperature control). I don't think I have any other ideas that might help.
The solution for this is: The photodiode saturates when incident power is more then 1mW even the thershold is 20mW. So check the manuel of the photodiode and keep the incident power less then the saturation power you can use ND filter to do this.
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