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Natural Science Forum / Physics / Optics / December 2004noise reduction in laser
Hi, I want to reduce noise in my fiber laser. The RIN is around -85dB/Hz and i want to reduce it. I have read about negative feedback loop to reduce noise. What i didn't understand in that is how do we determine how much light to split to put it in the splitter. I mean whether to use 50:50 coumpler or 90:10 coupler? Also, what kind of amplifier can we put in the feedback loop? thanks The optimum amount of light to be put on the detector (for the feedback) depends on the following factors: - You want to use as little as possible if you are short of output power. - For the detector, you ideally want something in the order of 1 mW so as to get the best signal-to-noise ratio of your error signal. This may require some compromise, unless your laser anyway produces way more than 1 mW. The feedback loop must be carefully designed if you want to achieve optimum noise suppression. The feedback amplifier could of PID type, ideally with adjustable P, I and I parameters. You may find a commercial device to do this job. If your feedback is supposed to use the drive current of the pump diode as a control, you will also have to take into account the low-pass filtering which is introduced by the finite upper-state lifetime of your gain medium. For erbium, e.g., this may limit your control bandwidth to something in the order of a few hundred Hertz. --------------------------- Dr. Rüdiger Paschotta RP Photonics Consulting GmbH www.rp-photonics.com > The optimum amount of light to be put on the detector (for the > feedback) depends on the following factors: [quoted text clipped - 13 lines] > your gain medium. For erbium, e.g., this may limit your control > bandwidth to something in the order of a few hundred Hertz. We do this with Nd:YVO4 microchip lasers. A photodiode gets about 10 percent of the beam power and the feedback loop varies the pump diode current. Although a PID scheme may be optimal, a very simple circuit will get something like 20 dB of reduction in noise. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. > --------------------------- > Dr. Rüdiger Paschotta > RP Photonics Consulting GmbH > www.rp-photonics.com |
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