[time-nuts] HP 10811A/B OCXO OP/SRV manual is onlineathparchive.com AND HP 5501B manual.

[Dr Bruce Griffiths]

Jack Hudler wrote:
> Hmm. I think I have all the parts for this. Except the programmable polarizer.
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf
> Of Magnus Danielson
> Sent: Tuesday, November 07, 2006 7:11 PM
> To: bruce.griffiths at xtra.co.nz
> Cc: time-nuts at febo.com
> Subject: Re: [time-nuts] HP 10811A/B OCXO OP/SRV manual is onlineathparchive.com
> AND HP 5501B manual.
>
> From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> 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: <45512B46.8090307 at xtra.co.nz>
>
>   
>> Magnus Danielson wrote:
>>     
>>> From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>>> 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: <455107E8.1020201 at xtra.co.nz>
>>>
>>>   
>>>       
>>>> 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
>
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>   
Ordinary fibres don't tend to preserve the polarisation state of the light.
This may or may not be an advantage.

The easiest way around this is to select the (with a piece of polaroid) 
required beam polarisations (and hence frequencies) before coupling into 
a fibre, then you don't need any polarisation controller and there is 
some advantage in using a non polarisation preserving fibre.

Its probably much easier to do the experiment with free space beams.
The alignment requirement of less than 1 arc minute or so is very easy 
to accomplish with a couple of mirrors in suitable kinematic mounts.

It is not necessary to use expensive polarising beamsplitters unless you 
happen to have some.
However they may be very convenient.
If necessary adequate suppression of the unwanted polarisation can be 
achieved with a piece of polaroid.

Either a a non polarising beamsplitter or a polarising beamsplitter can 
be used to superimpose the 2 beams.

A programmable polariser is just a 1/2 wave plate between a pair of 1/4 
wave plates all of which can be rotated about the beam passing through them.
Equivalent 1/2 and 1/4 wave plates can be constructed by winding 
suitable lengths of fibre around a cylinder.

However using a polarisation controller to match the input beam 
polarisation to an eigen polarisation state that is preserved by a fibre 
is unecessarily complex.
Even non polarisation  preserving fibres will preserve a particular 
polarisation state (usually elliptical) as it propagates in the fibre. 
However the particular eigen polarisation is sensitive to the bending 
and othere stress in the fibre.
Bruce