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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: how the laser happened
Date: Thu, 27 Jun 2024 10:04:57 -0400
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On Wed, 26 Jun 2024 20:07:12 -0700, john larkin <jl@650pot.com> wrote:

>On Thu, 27 Jun 2024 02:49:37 -0000 (UTC), Phil Hobbs
><pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>>john larkin <jl@650pot.com> wrote:
>>> On Tue, 25 Jun 2024 17:43:56 -0400, Joe Gwinn <joegwinn@comcast.net>
>>> wrote:
>>> 
>>>> On Tue, 25 Jun 2024 08:19:03 -0700, john larkin <jl@650pot.com> wrote:
>>>> 
>>>>> On Tue, 25 Jun 2024 11:50:05 +0100, Martin Brown
>>>>> <'''newspam'''@nonad.co.uk> wrote:
>>>>> 
>>>>>> On 21/06/2024 14:05, john larkin wrote:
>>>>>>> There was a thread somewhere above about photon wave/particle duality.
>>>>>>> 
>>>>>>> 
>>>>>>> This is worth reading:
>>>>>>> 
>>>>>>>< https://www.amazon.com/How-Laser-Happened-Adventures-Scientist/dp/0195153766 >
>>>>>>> 
>>>>>>> Einstein, in one of his fits of genius, predicted in around 1916 that
>>>>>>> under the right conditions, a photon could pass by an excited atom and
>>>>>>> the atom would kick in another photon, or add to the wave amplitude,
>>>>>>> depending on how you feel about these things. He called it stimulated
>>>>>>> emission. He also declared that the laws of thermodynamics made this
>>>>>>> effect impossible to use in practical situations.
>>>>>>> 
>>>>>>> In 1951, Charles Townes invented a work-around trick and built the
>>>>>>> maser, a gaseous microwave oscillator. His superiors thought he was
>>>>>>> crazy to dispute Einstein and almost threw him out of grad school, but
>>>>>>> it worked.
>>>>>> 
>>>>>> More interesting still nature beat him to it.
>>>>>> 
>>>>>> The natural source W3(OH) dense molecular cloud which has hydroxyl 
>>>>>> masers pumped by UV bright young stars embedded in it.
>>>>>> 
>>>>>> Very bright ultra narrow band point sources on a fuzzy nebulous object.
>>>>>> 
>>>>>>< https://articles.adsabs.harvard.edu/pdf/1981MNRAS.194P..25S>
>>>>>> 
>>>>>>> What's interesting is that any decent neon sign shop could have built
>>>>>>> a HeNe laser in 1920.
>>>>>> 
>>>>>> They would have needed to make the mirror just cavity right though.
>>>>> 
>>>>> I know a guy who built a HeNe. It wasn't hard.
>>>>> 
>>>>>> 
>>>>>> A nitrogen gas UV pulsed laser is possible just by getting the pressure 
>>>>>> right and creating the  population inversion. Self starting - there was 
>>>>>> a (dangerous) experiment in SciAm Amateur Scientist column to do it 
>>>>>> sometime in the 1970's. June 1974 in fact - cover shows the BZ reaction.
>>>>>> 
>>>>>>< https://www.scientificamerican.com/article/the-amateur-scientist-1974-06/>
>>>>>> 
>>>>>> The failure to discover fullerenes in soot was a lot more surprising 
>>>>>> since they were there all the time since the invention of fire just 
>>>>>> waiting to be extracted by benzene. For a long time space dust had a 
>>>>>> spectrum that could not be reproduced on Earth by any known compound.
>>>>>> 
>>>>>> Much like Helium was in the sun but more pervasive.
>>>>> 
>>>>> Too many powerful old farts declare things to be impossible.
>>>> 
>>>> .<https://en.wikipedia.org/wiki/Planck%27s_principle>
>>>> 
>>>> This is often paraphrased as "Science progresses one funeral at a
>>>> time".
>>>> 
>>>> Joe Gwinn
>>> 
>>> I see the same thing in electronic design. People favor accepted
>>> practice, validated in textbooks, and apply all their intelligence to
>>> showing how new ideas won't work.
>>> 
>>> A recent case is deciding that the LC's at the output of a switching
>>> power supply are "a filter" so must follow  classical filter theory,
>>> pole-zeros and Butterworths and such. I tell them "It's just a power
>>> supply."
>>
>>Classical filter theory is very useful for designing a power supply , as
>>long as you don’t just wave some canned design over it like a dead chicken.
>>
>>
>>Controlling rolloff and ringing over a wide range of conditions is easier
>>with a bit of theory—you can estimate the overshoot via the Q of the
>>network, for instance. 
>>
>>Canned designs such as Butterworth, Chebyshev, and so on assume constant,
>>resistive source and load. While that’s a useful fiction in lots of
>>signal-level applications, it’s not remotely true in a power supply. 
>>
>>Cheers 
>>
>>Phil Hobbs 
>
>My switching power supply filters are usually dominated by the first
>inductor. It has to let some tolerable ripple current into the
>downstream caps, has to not saturate, and must not get too hot in the
>minimum expected air stream, from core loss and copper loss. And fit
>available space and not cost too much and be available for purchase.
>
>I'll often have a secondary high-current ferrite bead to reduce EMI
>spikes, typically maybe a per cent of the main inductance.
>
>None of that is classic filter theory.
>
>Only Spice can predict the power supply load response. It's too
>nonlinear for classic filter theory.

Yes, this is exactly how all the radar power engineers of my
acquaintance solve the problem.  LT Spice is their standard tool.


>There are cheap tricks to compensate the control loop, once the big
>power stuff is designed.

Yep.

Joe Gwinn
>
>