From: Dr Bruce Griffiths <[EMAIL PROTECTED]> Subject: Re: [time-nuts] HP 10811A/B OCXO OP/SRV manual is onlineathparchive.com AND HP 5501B manual. Date: Wed, 08 Nov 2006 13:56:38 +1300 Message-ID: <[EMAIL PROTECTED]>
> Magnus Danielson wrote: > > From: Dr Bruce Griffiths <[EMAIL PROTECTED]> > > Subject: Re: [time-nuts] HP 10811A/B OCXO OP/SRV manual is > > onlineathparchive.com AND HP 5501B manual. > > Date: Wed, 08 Nov 2006 11:25:44 +1300 > > Message-ID: <[EMAIL PROTECTED]> > > > > > >> To measure the Allan variance you need at least 2 (preferably more) > >> stabilised lasers and a mixer (photodiode) plus a suitable amplifier to > >> produce a beat signal for analysis. > >> The beat frequency may be as high as 100 MHz with a pair of 5501s > >> (frequency/wavelength accuracy of 0.1ppm) so the photodiode and > >> associated amplifier will need adequate bandwidth. > >> > > > > It seems like three 5501s, optical splitters/joiners and a fairly normal > > counters should allow for a three-cornered hat and then should the Allan > > variance and friends be possible to measure. > > > > Cheers, > > Magnus > > > > > Things to watch out for when attempting to mix optical frequencies. > > Orthogonally linearly polarised beams incident on a photodiode (or other > photodetector) will not produce a beat note. Which is quite natural IF you think about it. > A polariser in front of the diode with its transmission axis aligned so > that the transmitted beam intensities are approximately equal for each > of the 2 orthogonally polarised beams will allow a beat note to be > produced. For identical incident beam intensities the polariser > transmission axis will be at 45 degrees to the plane of polarisation of > either beam. > > The angle (in radians) between the 2 beams has to be much smaller than > /l/d./ > where /l /is the wavelength and /d/ is the effective detector diameter. > e.g. when /d/ = 1mm and /l/ = 633nm > then the angle between the 2 beams must be << 2 arc minutes. Yes, but if they where being brought together in a fused optical fibre splitt/merge and one of the beam is adjusted through a mouse-ear assembly or programable polarizer (fancy mouse-ear), then that problem wouln't be that much of a problem? I think I have a bunch of suitable PIN diodes lying around doing nothing good. But then again, I don't have a suitable set of lasers to check, unless you count the DWDM lasers also lying around. :) Thanks for the heads-up. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
