[time-nuts] Current state of optical clocks and the definition of the second

[Stéphane Rey]

Hi,

 

I've took the time to read carefully your long and detailed message Magnus and this was very interesting. I've learned many things that have enabled me to investigate further. Ah yes, you're right saying that the more you fall into these things, the more you discover that you have to learn. Recently I've worked a lot on PLLs and I've actually learned a lot on special care to ensure low noise.... Very interesting. By the way I'm still working on this topic to improve again the noise (currently on a 3 GHz LO)

 

 

Here are some experiment results : http://www.ptp-images.com/affiche-directement-l-image-kccsz71c9a.html

 

1.       Setup #1 (blue plot)

HP5370A

standard input from HP GPSDO

EXT input not connected, internal Arming 0.4s rate

START input from 10 MHz distribution unit RacalDana 9478 Rubidium 

 

2.       Setup #2 (pink plot)

HP5370A 

standard input from HP GPSDO

EXT input not connected, internal Arming 0.4s rate

START input from DUT (10 MHz homemade GPSDO)

 

I'm not sure this is the proper way to connect everything... but this is the setup providing the lowest ADEV... which is between 1E-10 and 1E-13. But is the truth ?

I feel strange the two plots having the same decreasing path along a linear slope (I mean linear on the log-log plot) ... I'm not sure of what I'm measuring ? Could this be the system measurement floor ? By the way how to measure the ADEV floor of a system other than having a source greater than the measurement system ?

What could be these oscillations on the homemade (not by me) GPSDO  ?

 

 

I've tried to downmix the DUT 10 MHz to few kHz using a SR DDS generator and a double balanced mixer from minicircuit via a low pass filter tuned at 100 kHz, but the level wasn't high enough for the counter (which I found strange as it was already nearly 200mV). I hadn't anything in hands to make a squarer quickly so I've just added a Minicircuits RF amplifier. The level was good but the ADEV has jumped to 1E-6. The signal was noisy already on the oscilloscope which I know is for sure the cause. 

I need to make a squarer. I was hesitating between several methods : using a CMOS gate, but this will increase the flicker noise from what I've read, using an amplifier and clamping diodes or a fast comparator which might create some noise around the trigger point... Any recommendation there ? 

I'll try to make this squarer next week to continue my investigations

 

Cheers 

Stephane

 

 

 

-----Message d'origine-----
De : time-nuts [mailto:time-nuts-bounces at febo.com] De la part de Magnus Danielson
Envoyé : mercredi 14 janvier 2015 06:05
À : time-nuts at febo.com
Cc : magnus at rubidium.se
Objet : Re: [time-nuts] Current state of optical clocks and the definition of the second

 

 

 

On 01/13/2015 11:41 AM, Attila Kinali wrote:

> On Mon, 12 Jan 2015 20:09:45 +0000

> Gregory Maxwell < <mailto:gmaxwell at gmail.com> gmaxwell at gmail.com> wrote:

> 

>> On Mon, Jan 12, 2015 at 12:34 PM, Attila Kinali < <mailto:attila at kinali.ch> attila at kinali.ch> wrote:

>> Seems that the state of the art in stabilized lasers has improved a 

>> lot lately, e.g. there are commercial available 1550nm devices which 

>> have a <=3Hz line-width:  <http://stablelasers.com/products.html> http://stablelasers.com/products.html (well 

>> on a short term basis, the medium term performance is not so

>> impressive)

> 

> Laser stabilization, especially for quantum metrology is still an 

> actively researched field. Current state of the art is IIRC 0.3Hz 

> linewidth (sorry, cannot find the reference at the moment).

> Mid- and long term stability depends highly on the reference used. 

> Current research is fucused mainly on special, low vibration 

> structures made out of low expansion glass or silicon. And these 

> cavities are usually put into a temperature controlled chamber in 

> vacuum.

 

Well, guess what I found standing around in a lab with an optical comb? :)

 

With optical line-widths in sub-Hz range and optical combs you have a nice way of comparing the frequency of that free-running and un-steerable but stable oscillator. However, as you mix it down the noise of the optical comb will dominate, but you can know which multiple of the optical comb and offset it is.

 

>> Considering the rarity and extreme cost of H-masers, or just really 

>> exceptional quarts oscillators; might it be the case that optical LOs 

>> start looking interesting for applications which just need stability 

>> (or being steered by other sources; e.g. GPSDL)?

> 

> Well, an 8607 costs more than a Rb-standard. Yes, the 8607 has lower 

> close in phase noise and up to several 1000s it rivals the Rb, but 

> handling it is much more difficult than handling an Rb.

> Also, if you want to buy one of those exceptionally low noise/high 

> stable 8607's (those that go down into the 10^-14 range) you'd have to sell your car.

> 

> But, if you buy a H-maser from SpectraTime, you get a 8607 for free 

> ;-)

 

That is also the only way to get the 8607 now, as Oscilloquartz is closing down that business.

 

> There used to be quite some literature on how to build low noise 

> quartz oscillators. Most of those books are out of print today.

> With two notable exceptions:

> 

> "Discrete Oscillator Design: Linear, Nonlinear, Transient, and Noise Domains"

> by Randall Rhea, 2010

> 

> and

> 

> "Understanding Quartz Crystals and Oscillators", by Ramon Cerda, 2014

> 

> I had a look at the book by Rhea, it looks quite well written and 

> contains a lot of real world information, but is a bit weak on the 

> more theoretical part (description of oscillation, noise sources,...) 

> and thus on the on the why things are done that way.

> I didn't had the chance to buy Cerdas book yet.

 

An interesting book in that context is Enrico Rubiolas book on phase-noise, which among other things goes into explain the Leeson model of oscillators and it's real life design aspects.

 

> The UFFC has some of the older books online. You need to be registered 

> to access them, though.

> 

> There is also a lot of knowledge on quartz crystalls hidden in old 

> papers, but going trough them is some serious work.

> 

> On the topic of opto-electronic oscillators, those are technologically 

> nice, but they are rather bulky. That's why they are mostly used in 

> research projects for atomic clocks. Also getting them to do low phase 

> noise is not that easy, and unlike quartz oscillators, there is not 

> much literature about that.

 

It's a serious bulk of glas in there, but the laser-technology as well as temperature-stabilization of it isn't rocket science,

 

>> (They can be down-converted to microwave frequencies using an optical 

>> comb; a mode-locked laser whos pulses are phase locked to an incoming

>> beam.)

> 

> That is actually the current trend. There was a paper by NIST last 

> year on downconverting the beat frequency of an optical comb down to 

> RF using a frequency divider chain. They managed to get noise measures 

> that rival that of a good quartz oscillator at 5MHz. Ie at higher 

> frequencies, it is actually better than what a quartz oscillator can deliver.

> (for some reason i have not archived that paper and google fails me)

 

The NIST T&F archive is where you should go. Nice folks doing that work.

 

>> Certainly just the local oscillator is _closer_ to something a 

>> time-nut might experiment with than a complete optical atomic 

>> standard (if still not quite in reach).

> 

> Well, building a CPT based Rb vapor cell frequency standard should be feasible.

> Yes, it's not a primary standard, but should do the job for most :-)

> 

>  From what i've read, using one of the MOT cells like those of Sachser 

> Laser [1] one might even be able to build a primary standard.

> But my understanding of MOT is relatively weak and i cannot say how 

> difficult it actually would be. But it would be definitly a fun 

> project to try :-)

 

Building a MOT setup is relatively easy know, I have not yet seen it go sub 100.000 USD, but it seems like modern setups could do that if you let it. I've been playing with such a system and when setup, tuning it up and locking it to do cooling is so trivial that I've done it.

 

> But i agree, building one of those ion or neutral atomic standards is 

> pretty much out of question on a hobby budget. Heck, even an optical 

> frequency comb is difficult to build, at best. And buying them.. i 

> think buying a good Rb is still cheaper.

 

The technology of such traps is pretty established knowledge, but an engineering feat to be done is to have those drop in price, and doing so could have very interesting implications.

 

Once you have your atoms in some form of (glas) cell, is more about reducing cost of the optical bench needed. Would be interesting to see what some intense "research" could be done in that regard.

 

Laser cooling depends on the lockable frequency of the lasers as well as the detector cavities to make it easy to detect where in the range the laser is, once locked to that cavity, a relatively small optical bench is needed and well, the magnetic field is just two coils and a current-source supply, which ends up not being that critical in setting.

The optical effect for these things seems to be relative benign too. 

Just haven't had the time to learn what (cheap) lasers is available to steer, and how the steering is actually done.

 

Rubidium actually laser-cools very well, better than cesium, because of the lower cross-section area of rubidium than cesium atoms, which reduces the collision effects of the atoms in the super-cooled ball.

This is just the same rubidium that we have in our rubidium clocks. The non-primary aspect of gas-cells is due to wall-shift and gas-shift of the buffert-gas to compensate for wall-shift. The temperature of the cloud also contribute, but that is way down in the list.

 

Building a modernized laser-cooled rubidium cell atomic clock would be doable. It might not be the cool research project maybe, but an interesting step-stone mix of technologies.

 

Cheers,

Magnus

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