On Sat, 9 Jun 2018 23:28:29 +0800 mimitech mimitech <mimit...@gmail.com> wrote:
> Just do a little bit research on 1560nm telecom laser transceiver. The > closest wavelength is 1560.61nm (DWDM channel 21) as defined by ITU. > Usually telecom laser transceiver module uses DFB (distributed feedback) > type laser diode instead of VCSEL laser. However, the DFB LD should be > capable of direct modulation, so there maybe not much difference with VCSEL > in terms of usage. For the modulation bandwidth, I guess a 2.5Gbps module > should be OK for 1.71GHz analog modulation. A 2.5Gbps module should be good even for 3.4GHz modulation. The 3dB frequency of should be higher than the baud rate used. Besides, even if the 3dB frequency is below 3.4GHz, that does not matter, it just means that your sidebands will be damped (second order low pass, IIRC), but you don't need that much power anyways. > So, imagine I modulated 1.71GHz microwave signal onto 1560nm laser wave, > then used a KPT non-linear optical crystal to double the frequency, > hopefully I could get 780nm laser with +/-3.42GHz sideband spectrum, that > should be suitable to shine on a Rubidium vapor cell and trigger CPT > effect. I don't know if I understand this process correctly, many details > must be missed. Using a non-linear element will give you lots of intermodulation products. Ie, your sidebands will be +/-1.71GHz, +/-3.42GHz, +/-6.84GHz,... So you will have to be a bit carefull with the laser tuning in order to get the right harmonics. An alternative approach would be to use an EOM after the doubler to get the sidebands. These have bandwidths in the order of 10-20GHz, so applying 3.42GHz shouldn't be a problem. BTW: a lot of the frequency doubler architectures for high stability lasers use resonant cavities, to increase efficiency and decrease noise. You will either have to tune the free spectral range of the cavity to a divisor of 6.84GHz or use a non-resonant one. > Another question is, after passing through the KPT doubler crystal, whether > the light remains single mode and linear polarization mode or not? If > someone could provide any relevant information and suggestion, thanks very > much. If I understood the optical process correctly (disclaimer: I'm not a physicst and have never worked with optical systems), then the multiple modes are a problem of the laser source, not of the doubler. As multi-modes are a problem for high speed communication, I expect telecom lasers to be quite clean. You will have to ensure that you are not operating it close to a mode jump, though. A side note on the doubler approach: The group at UniNE around Gaetano Mileti and Christoph Affolderbach focuses only on double-resonant Rb standards. That means they do not modulate the laser. As they have gotten the whole standard to the SNR limit, they are now focusing on getting the shifts due to laser (frequency and intensity shift) and cavity (low Q cavity to avoid pulling) down. The current state of affairs is quite nicely documented in their two papers at 8FSM[1,2,3]. Attila Kinali [1] Proceedings of the 8th Symposium on Frequency Standards and Metrology http://conferenceseries.iop.org/conferenceseries/issue/1742-6596/723/1 [2] "High performance vapour-cell frequency standards", Gharavipour, Affolderbach, Kang, Bandi, Buret, Pellaton, Mileti, 2015 http://iopscience.iop.org/1742-6596/723/1/012006/pdf/1742-6596_723_1_012006.pdf [3] "Pulsed Optically Pumped Rb clock", Micalizio, Levi, Godone, Calosso, François, Boudot, Affolderbach, Kang, Gharavipour, Gruet and Mileti, 2015 http://iopscience.iop.org/1742-6596/723/1/012015/pdf/1742-6596_723_1_012015.pdf -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.