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FrozenGate by Avery

ECDL setup with custom controller

atomd

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In this multi post series, I'll describe step by step series how I built by ECDL setup. Let's start with physical construction.

Design​

In this build, I'll use diffraction grating to reduce the bandwidth. The grating will be operating in Littrov configuration. In this case, it's beneficial to choose a grating that has the lowest grove count while providing only single diffraction order to not lose power unnecessarily. Rearranging some equations, we find that 2nd order diffraction in grating working in Littrov mode happens at the angle arcsin(3/2*lambda*grove density). If this arcsin doesn't exist, because argument is bigger than one, then grating doesn't have higher orders. For 520 nm this critical grove density is around 1300/mm. On the other hand, we would like the grating to have the lowest possible grove density for highest dispersion and easier single mode operation. My supplier stocks only 1200 and 1800 groves/mm gratings, not 1300. And while 1200 would provide some loss in higher order diffraction it's hard to judge which one will have higher efficiency, while 1200 provides higher dispersion. For testing, I decided to order both.

Grating bandwidth​

Laser diode facet size is roughly 1 μm x 5 μm and the collimator used has an effective focal length of 3mm. This means that for the feedback power to reduce by half the returned beam has to move by 0.5 μm or 2.5 μm, depending on the axis. This implies that change of angle of returning beam by just 9.5 or 47.7 milidegrees is enough to reduce feedback by half. For 1200 groves / mm grating used in this project it gives 3dB bandwidth of just 0.27nm or 1.38nm respectively.

External cavity mode spacing​

When cavity round trip distance is 100 mm, then around 2e5 wave nodes fit in the resonator. Then mode spacing is 520 nm/2e5 or around 2.6 pm, much tighter than grating bandwidth. Clearly, many modes of external resonator can fit in gratings bandwidth. Fortunately, there's also an internal cavity that provides selectivity. Because the external cavity's length is defined by aluminum bracket holding parts together, it's expected to change by around 0.0023%/K. While it doesn't seem like much, it gives almost full mode hop for 1K of temperature change.

Internal cavity mode spacing​

A laser diode's structure is usually a few hundred um long. For simplicity, let's assume it's 500 um long and that index of refraction is 3.5, giving 7630 wave nodes inside the resonator for a mode spacing of 77.3 pm. Overlapping both cavities and grating bandwidth creates higher selectivity filter (see picture), additionally external cavity stabilizes small drifts due to thermal changes of internal cavity. It should be noted that because of big internal cavity mode spacing, small changes to temperature or injection current will hop external cavity modes one by one retuning the system. It's therefore critical to stabilize diode temperature and current. How precisely should temperature be stabilized depends on what semiconductor is used. For eg. red laser diodes, made from AlInGaP are very sensitive to temperature, drifting 120 pm / K while free running. To maintain single external cavity mode, temperature has to be regulated to with-in 21.7 mK. For GaN diodes the requirements aren't nearly as strict.

Physical construction​

Let's look from outside to inside. The entire system is placed on big slab of aluminum working as sturdy base and heatsink. On that there's Peltier module mounted, on which rests an inner baseplate. The inner baseplate holds the entire optical path. On one right there's New Port mount to which diffraction grating will be glued, on the other end there's another Peltier module, hosting brass laser diode + collimator mount. To protect the optics from dust and air currents, everything will be covered with 3d printed plastic cover with glass window positioned at brewster angle or AR coated window, depending what will be easier to source.
ECDL_adnotations.png


And as always more details are on my blog: https://sduc6.blogspot.com/2024/03/external-cavity-diode-laser-part-1.html
 





Can you use a volume Bragg grating? I make those in photopolymer material and could make something for you for free if it helps get me 100mW of SLM at 520nm. Wasatch Photonics sells a 1200 lpm Bragg grating for 830nm for about $400.
 
Very nice. I've been planning an ECDL for some time... your report will be helpful. Thanks for sharing!
 
Can you use a volume Bragg grating? I make those in photopolymer material and could make something for you for free if it helps get me 100mW of SLM at 520nm. Wasatch Photonics sells a 1200 lpm Bragg grating for 830nm for about $400.
I need reflective grating for ECDL. If you'd somehow managed to make a grating that reflects 20-60% of the beam in Littrow configuration, I'd be very interested to test them. Otherwise there's Edmund Optics ruled grating for €80 that seems to fit the bill.
 
I don't want to share all the details, but there'll be some synchronous pumping, some supercontinuum and a lot of fast photodiodes involved.

btw I can't keep myself and have to ask @kecked where did you obtain you 10 and 30 GHz photodiodes?
 
I don't have them handy anymore. They were part of a Keyence device we had that was used with a similar o'scope. WAY pricey stuff. Had THZ stuff with it too. Was an imaging system proposal for medical applications. I can dig up the papers it is not classified or anything at least anymore. The THZ stuff was an early explosives detector too. That tech is everywhere now. The rating on the photodetectors was bandwidth not frequency. Be very useful for your application actually. I see why that would peek your curiosity. 30ghz in photons modulated would be something.

I'm interested in anything supercont. IR by chance?
I updated the signature. I have not done that in a long time.
 
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I don't have them handy anymore. They were part of a Keyence device we had that was used with a similar o'scope. WAY pricey stuff. Had THZ stuff with it too. Was an imaging system proposal for medical applications. I can dig up the papers it is not classified or anything at least anymore. The THZ stuff was an early explosives detector too. That tech is everywhere now. The rating on the photodetectors was bandwidth not frequency. Be very useful for your application actually. I see why that would peek your curiosity. 30ghz in photons modulated would be something.

I'm interested in anything supercont. IR by chance?
I updated the signature. I have not done that in a long time.

Just for your own interest the word is actually "pique" not peek your curiosity. Thought you might like to know.
 
Just for your own interest the word is actually "pique" not peek your curiosity. Thought you might like to know.
This coming from a guy who constantly misspells "geez" with "geese". :rolleyes:
 
Hilarious! I have never minded being corrected. It's actually how one learns. Mine wasn't snarky though.

Edited for a typo
 
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Hilarious! I have never minded being corrected. It's actually how one learns. Mine wasn't snarky thought.
Amazing that despite your comment being previously edited you still didn't correct your mistake by saying "though" instead of "thought".
 
I don't have them handy anymore. They were part of a Keyence device we had that was used with a similar o'scope. WAY pricey stuff. Had THZ stuff with it too. Was an imaging system proposal for medical applications. I can dig up the papers it is not classified or anything at least anymore. The THZ stuff was an early explosives detector too. That tech is everywhere now. The rating on the photodetectors was bandwidth not frequency. Be very useful for your application actually. I see why that would peek your curiosity. 30ghz in photons modulated would be something.

I'm interested in anything supercont. IR by chance?
I updated the signature. I have not done that in a long time.
The roundtrip time of this resonator is around 250 ps so 10+ GHz photodiode would useful for analyzing pulse forming in external cavity (but that's foreshadowing future events 😊 ). I have access to 28 Ghz 4 channel oscilloscope, so that's not an issue. The best I could find that don't cost an arm and a leg have bandwidth of 3 Ghz max, so I may have to extend cavity length. This would be unfortunate for pulsed experiments though (more foreshadowing).

I can't dig up papers right now, but basically modulating a laser diode with RF at close to cavity round trip time can create spectrum similar to mode locked lasers.
 
I have heard of measuring the electric field with a small antenna mounted by the beam hitting a surface. Seems it induces a current. This could let you use a spec analyzer all be it an expensive one. I’m making the next part up. You could also try some form of interferometry and observe the fringes. That would been a pretty big arm to see that change. I know people measure the wavelength to a couple mhz all the time so it can be done. Just don’t know how they did it. Literature search time. Maybe modulate the temp to get something slower you can infer the result from? Use second know standard?

Leave paul alone. I am horrible at spelling. I didn’t proof read my text. I can see how you’d get bitched at by less mature folks paul. I’m not biting.
 
I have heard of measuring the electric field with a small antenna mounted by the beam hitting a surface. Seems it induces a current. This could let you use a spec analyzer all be it an expensive one. I’m making the next part up. You could also try some form of interferometry and observe the fringes. That would been a pretty big arm to see that change. I know people measure the wavelength to a couple mhz all the time so it can be done. Just don’t know how they did it. Literature search time. Maybe modulate the temp to get something slower you can infer the result from? Use second know standard?

Leave paul alone. I am horrible at spelling. I didn’t proof read my text. I can see how you’d get bitched at by less mature folks paul. I’m not biting.
This wasn't even meant only for you. Most people have heard the phrase "piqued my interest", but the word is so esoteric that unless one has seen it in print many don't even know about it. I'm not saying you are one of them. Just put it out there for the ones who don't. Thanks for the defense. ;)
 
Amazing that despite your comment being previously edited you still didn't correct your mistake by saying "though" instead of "thought".

Most people know a typographic error when they see one. You did. The edit was to add information and the typo was just that.
 
Most people know a typographic error when they see one. You did. The edit was to add information and the typo was just that.
You entirely missed the point that you criticized someone for an error but then proceeded to make them yourself.
 





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