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HeNe laser RF emission and back-reflection
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Alexey Galakhov
Dec 18, 2012, 11:33:21 AM12/18/12
to
Hi,
There is an interesting effect in a HeNe laser. It emits signals at
radio frequencies around 0.5-1 MHz. Looks like they come from the
discharge itself, not from the PSU. No surprise since plasma it
oscillating, but I don't quite understand some of the underlying physics.
Why the RF emission is quite narrow band? I observed it using a
household AM receiver and it appears to be an AM signal with 3 kHz
modulation. FFT on an oscilloscope shows thai it has actually at least 3
mixed frequencies around 3 kHz that are very stable. I don't believe
they're from quantum effects and not from the power circuit interacting
with the discharge.
But this modulation is very sensitive to laser back-reflection. It
causes the modulation frequency to shift a little bit. But why? Some
strange plasma oscillations or just change in the discharge voltage due
to change in level population?
Are there any publications about it?
I'm asking because one of the pseudo-scientists based the whole "theory"
on this "unexplainable effect" and I was asked to investigate his snake
oil thoroughly.
Regards,
Alex
There is an interesting effect in a HeNe laser. It emits signals at
radio frequencies around 0.5-1 MHz. Looks like they come from the
discharge itself, not from the PSU. No surprise since plasma it
oscillating, but I don't quite understand some of the underlying physics.
Why the RF emission is quite narrow band? I observed it using a
household AM receiver and it appears to be an AM signal with 3 kHz
modulation. FFT on an oscilloscope shows thai it has actually at least 3
mixed frequencies around 3 kHz that are very stable. I don't believe
they're from quantum effects and not from the power circuit interacting
with the discharge.
But this modulation is very sensitive to laser back-reflection. It
causes the modulation frequency to shift a little bit. But why? Some
strange plasma oscillations or just change in the discharge voltage due
to change in level population?
Are there any publications about it?
I'm asking because one of the pseudo-scientists based the whole "theory"
on this "unexplainable effect" and I was asked to investigate his snake
oil thoroughly.
Regards,
Alex
Phil Hobbs
Dec 18, 2012, 12:48:01 PM12/18/12
to
can get intermodulation between one mode and a third-order
intermodulation product of two others, i.e.
(2*F2-F3) +- F1.
That shows up as baseband mode beats in the laser output--you can watch
the spurious product sweep around between about 100 kHz and 1 MHz. When
it gets below about 100 kHz the modes usually jump into lock and it goes
away. Cooling one end of the laser tube a bit with your hand will often
start and stop this.
I expect that the plasma is responding to the difference frequency as
well. Interesting!
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Alexey Galakhov
Dec 18, 2012, 1:27:02 PM12/18/12
to
> There's a bit of dispersion between adjacent modes in the laser, so you
> can get intermodulation between one mode and a third-order
> intermodulation product of two others, i.e.
>
> (2*F2-F3) +- F1.
>
> That shows up as baseband mode beats in the laser output--you can watch
> the spurious product sweep around between about 100 kHz and 1 MHz. When
> it gets below about 100 kHz the modes usually jump into lock and it goes
> away. Cooling one end of the laser tube a bit with your hand will often
> start and stop this.
>
> I expect that the plasma is responding to the difference frequency as
> well. Interesting!
Thank you! I thought about mode beats but they should be at relatively high frequency and shift more than 100 Hz. I don't believe my laser is SO stable. It also seems to be at the same ~3 kHz beat does repeat on different models of lasers, so I'm looking for the origin of these 3 kHz. (Yes, I analyzed the spectrum of the "snake oil" video soundtrack, it is noisy but still has the same 3 kHz peak).
> can get intermodulation between one mode and a third-order
> intermodulation product of two others, i.e.
>
> (2*F2-F3) +- F1.
>
> That shows up as baseband mode beats in the laser output--you can watch
> the spurious product sweep around between about 100 kHz and 1 MHz. When
> it gets below about 100 kHz the modes usually jump into lock and it goes
> away. Cooling one end of the laser tube a bit with your hand will often
> start and stop this.
>
> I expect that the plasma is responding to the difference frequency as
> well. Interesting!
The stability of frequency suggests that it has either pure electrical nature (the circuit is oscillating) or some kind of standing acoustic wave in plasma or even in glass. But why does it depend on back-reflection? Change of the power consumption due to different level occupations? Depletion of the metastable state in inactive parts of tube?
Regards,
Alex
Alexey Galakhov
Dec 18, 2012, 1:43:26 PM12/18/12
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Phil Hobbs
Dec 18, 2012, 5:23:35 PM12/18/12
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Alexey Galakhov
Dec 19, 2012, 5:41:51 AM12/19/12
to
On 12/19/2012 04:23 AM, Phil Hobbs wrote:
> The 3 kHz is probably the power supply ripple.
I thought so too but why does it depend on back-reflections? Also it is
> The 3 kHz is probably the power supply ripple.
unlikely that different power supply models all have the same ripple
frequency.
May it be that the tube forms a Pearson-Anson relaxation oscillator that
changes its frequency due to change of impedance of the discharge?
Regards,
Alex
Phil Hobbs
Dec 19, 2012, 10:07:20 AM12/19/12
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can find out by putting an open-circuited scope probe next to the power
supply.
Alexey Galakhov
Dec 19, 2012, 2:19:00 PM12/19/12
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> I think the manufacturers mostly farm out their power supply work. You
> can find out by putting an open-circuited scope probe next to the power
> supply.
The same 3 kHz frequency was observed on a USSR-made laser. Its power supply is definitely made from different components. It is rather old and most likely uses lower switch frequency like all USSR-made SMPS units. The "brick" runs directly from 220VAC. It was a stabilized laser and the stabilizing circuit outside the brick uses old-style parts and is definitely not a direct clone. I don't own such a laser. Before I repeated the effect I suspected that it is caused by stabilizing circuit malfunction due to extra light on PDs. It isn't.
Regards,
Alex
Phil Hobbs
Dec 19, 2012, 4:05:33 PM12/19/12
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found an old article that says that the sweet spot for efficient 632.8
nm operation occurs in an unstable plasma. http://tinyurl.com/cr4tg38
Salmon Egg
Dec 19, 2012, 6:44:13 PM12/19/12
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In article <kas5ld$9h$1...@speranza.aioe.org>,
It would help to know more about the laser itself. How is it constructed
and excited? Early HeNe's used rf excitation. It is hard for me to
believe that the sidebands from power supply ripple does not have
multiple sidebands. Are there any high resolution Fabry-Perot
spectrograms of the output?
--
Sam
Conservatives are against Darwinism but for natural selection.
Liberals are for Darwinism but totally against any selection.
and excited? Early HeNe's used rf excitation. It is hard for me to
believe that the sidebands from power supply ripple does not have
multiple sidebands. Are there any high resolution Fabry-Perot
spectrograms of the output?
--
Sam
Conservatives are against Darwinism but for natural selection.
Liberals are for Darwinism but totally against any selection.
Alexey Galakhov
Dec 20, 2012, 4:41:08 AM12/20/12
to
On 12/20/2012 05:44 AM, Salmon Egg wrote:
> It would help to know more about the laser itself. How is it constructed
> and excited? Early HeNe's used rf excitation. It is hard for me to
> believe that the sidebands from power supply ripple does not have
> multiple sidebands. Are there any high resolution Fabry-Perot
> spectrograms of the output?
Unfortunately my one and only working Fabry-Perot has not enough resolution.
> It would help to know more about the laser itself. How is it constructed
> and excited? Early HeNe's used rf excitation. It is hard for me to
> believe that the sidebands from power supply ripple does not have
> multiple sidebands. Are there any high resolution Fabry-Perot
> spectrograms of the output?
The effect was repeated on at least four different lasers. One is an
ordinary Melles Griot barcode scanner tube with a brick power supply.
This one is mine. Two others are Melles Griot 05-LHP-121 and 05-LHP-123,
I have no access to them. The fourth one is USSR-made stabilized
LGN-303, it was on the video where the effect was first demonstrated.
They all give similar beat frequencies.
I think it can easily be reproduced on any modern laser.
Alexey Galakhov
Dec 20, 2012, 4:44:10 AM12/20/12
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On 12/20/2012 03:05 AM, Phil Hobbs wrote:
> Interesting. Could be a plasma instability, then, as you suggest. I
> found an old article that says that the sweet spot for efficient 632.8
> nm operation occurs in an unstable plasma. http://tinyurl.com/cr4tg38
Thanks for the link. I'll continue measurements.
> Interesting. Could be a plasma instability, then, as you suggest. I
> found an old article that says that the sweet spot for efficient 632.8
> nm operation occurs in an unstable plasma. http://tinyurl.com/cr4tg38
Regards,
Alex
Samuel M. Goldwasser
Dec 20, 2012, 7:17:35 PM12/20/12
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1. Switching frequencies of HeNe switchmode power supplies are generaly
in the 20 to 50 kHz range, a few may be above this. But anything lower
is not from the power supply.
2. Plasma oscillations are very common and may be in the 100 kHZ to 1 MHz
range or below or above.
Test: How does the dominant frequency vary as a function of power supply
current? As the current is reduced, the tendancy for plasma oscillation
will increase until the tube drops out.
3. There is a definite effect of lasing on discharge impedance. This is
even measurable at DC by looking at the tube voltage using a constant
current power supply. One can demonstrate this either by "spoiling"
lasing from a long conventional internal HeNe laser (e.g., 1145P or
MG 928) or with an open cavity (Brewster window) HeNe blocking the
intracavity beam. So, back-reflections can definitely affect this.
However, it probably isn't changing the average output power very
much if at all (unless an external mirror is actually mounted
with precise alignment), but is causing massive amounts of mode hopping.
4. The MG -121 and -123 are probably almost identical lasers; the barcode
tube isn't all that different. The LGN-303 uses a tube of similar
length.
--
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Alexey Galakhov
Dec 21, 2012, 4:55:06 AM12/21/12
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On 12/21/2012 06:17 AM, Samuel M. Goldwasser wrote:
> 4. The MG -121 and -123 are probably almost identical lasers; the barcode
> tube isn't all that different. The LGN-303 uses a tube of similar
> length.
Yes, that was exactly what I thought. They have different electronics
> 4. The MG -121 and -123 are probably almost identical lasers; the barcode
> tube isn't all that different. The LGN-303 uses a tube of similar
> length.
but similar glasswork.
BTW, considering a standing one-dimension acoustic wave in He at such
pressure and such length one gets about 3 kHz frequency. I suspect that
He-Ne misture doesn't differ much.
Regards,
Alex
Samuel M. Goldwasser
Dec 21, 2012, 4:22:15 PM12/21/12
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