Laser Linewidth - measurement and explanation

[@yourfavouriteta]

Note: At 10:20, I incorrectly assert that the linewidth measured on the ESA is 5MHz and that the inherent linewidth of the laser therefore must by 2.5MHz. The mistake is that the 5MHz is only the distance from the peak to where the spectral power drops by 3dB. When measuring linewidth, we actually care about the full 3dB width, which in this case would be 10 MHz. Therefore, the actual inherent linewidth of the laser is 5MHz.

[@bubleedas9610]

Thanks! I was wondering about that! At 4:35, you wrote the same expression for the linewidth of both the AOM and 100km spool output, but are they supposed to be the same? Is the linewidth of the ESA is twice the linewidth of the two (just the sum of the linewidths)?

[@mathewyu7494]

I think the first linewidth near 200MHz is more due to environment noise so they are similar and is actually Gaussian shape, more porper way might be to use the sidewings to fit for Lorentizian shape to get actual linewidth. Very nice demonstration!

[@yourfavouriteta]

Interesting idea! Makes sense that if one diode is more sensitive to external noise, "smearing" could occur in the spectrum.

[@shuch2744]

Thank you for useful video. Is this measurement setup configured by PM fiber better than non-PM? I expect PM-setup gets higher signal intensity at the photodiode.

[@yourfavouriteta]

Glad that you like it! Using PM fiber does not provide a significant advantage. To ensure maximum interference between the reference signal and the signal from the AOM, they just need to have the same polarization at the photodiode, which can easily be ensured by using a regular polarization controller placed in one of the "arms" as shown at 6:08. Since the exact SOP does not matter as long as it's the same for both signals, the paddles can be adjusted until maximum interference is achieved.

[@shuch2744]

@@yourfavouriteta Thank you for your reply! I understand to use polarization controller.

[@bharatgroverkuuni171290]

Really nice informative video! Thank you! However, how would you stabilize the laser frequency over time for some high precision spectroscopy measurement?

[@yourfavouriteta]

I am glad you found it helpful! Laser diodes with linewidths below 1MHz are commercially available for around $1.000. More expensive models in the low kHz range are also available. Depending on your application and budget, these might be sufficient. Alternatively, you can stabilize a laser to an external cavity. The idea is that two highly reflective mirrors with high mechanical and thermal stability create a Fabry-Peroy cavity with extremely narrow lines. Let's assume that this FP cavity line width is 0.1kHz. If you shine a 1MHz laser centered on one of the FP lines into this cavity, the light that comes out will be filtered and have a 0.1kHz linewidth. If this light is sent back into the source of the 1MHz laser light, it will be amplified and sent back to the FP cavity for another "round trip". Eventually, the filtered light inside the 0.1kHz range will "steal" all the available power in the laser source, effectively changing it from 1MHz to 0.1kHz. One detriment is that the central frequency of the laser source must be aligned with one of the FP lines for this trick to work, which makes the system less tunable.

[@hemantyadav4220]

Thanku for the info. Can we use multi mode fiber for this setup?

[@yourfavouriteta]

You're welcome, glad you found it useful. I don't think using a MMF would be advantageous. If two (or more) transverse modes propagating with different speeds are present in the system, I think both of the signals they produce will contribute to the measurement simultaneously, making the result more difficult to interpret.

[@KunalSingh-my5nd]

Thank you so much.

[@yourfavouriteta]

You're welcome!

[@zhafu8862]

Good. when I tested, there were many peaks near the peak. what was the reason

[@yourfavouriteta]

Hmm, could be many different things. If possible, please tell me the following: 1) What is the driving frequency of the AOM and what is the frequency spacing from the main peak to the many peaks you mention? Can you post a link to the spec sheet of the AOM and to the function generator you use for driving it? 2) What type of laser are you testing? Can you post a link to the spec sheet? Maybe it emits light at more than one frequency? This can also be checked with a high resolution optical spectrum analyzer if its resolution is higher than the spacing between the peaks you are observing. 3) What current are you driving the laser with? If driven with too high current, the electric field in the gain medium may become so strong that four-wave-mixing occurs, where new frequency components are generated. If FWM is the problem you can check it with an optical spectrum analyzer and remove the problem by reducing the driving current. Note that changing the current will alter the gain of the medium and therefore change the linewidth a bit. 4) What photodiode are you using? What is its bandwidth (maximum operational frequency) and saturation power? If a very intense sinusoidal optical signal hits a photodiode so the saturation level is exceeded, the generated electrical signal may get "clipped", which causes spurious frequency components to appear. For example, a clipped 200MHz signal will cause extra frequency components at 400MHz, 600MHz and so on. You can check this by reducing the incident power. 5) Try running the linewidth measurement setup with the laser turned off. The electrical spectrum analyzer may be experiencing interference from other devices in your laboratory.

[@jelenabaturina2889]

Could You use "Three-Cornered-Hat" approach with three independent lasers?

[@yourfavouriteta]

I was not previously familiar with that method, but after looking into it, I think the answer is yes, provided that the three lasers have approximately the same linewidth: www.wriley.com/3-CornHat.htm

[@zhoumin6927]

Nice try. The 2nd laser may suffer from more 1/f noise than the 1st one.