Abstract
Preliminaries
Blumlein and His Circuit
The Issue of Latency
Travelling-Wave Excitation
Issues Related to Scale
Power and Energy
Closing Remarks
Some Interesting Papers
Abstract
Many Do-It-Yourselfers have built nitrogen lasers, often
from a design published in the Amateur Scientist column
of Scientific American magazine. This page discusses the
text of that column in some detail, and shows several
ways in which the explanation of the design and how it
operates is faulty.
In the Amateur Scientist column, on page 122 of
the June, 1974 issue of Scientific American, there was a
design for a tabletop nitrogen laser. It was written by
someone named Jim Small, who was a student at MIT at the
time. The article was later republished in the
Scientific American book Light and Its Uses, and
is also on the CD of Amateur Scientist columns,
which you can get from
The Society for Amateur Scientists.
I have also found this CD available from
The Surplus Shed,
and from
American Science and Surplus.
The design isn’t bad at all: it’s easy to build,
easy to operate, and puts out enough energy to drive a small
dye laser. In fact, people are still building lasers from it
today. Unfortunately, there are serious problems with the
author’s explanation of how it works.
I’m not about to violate copyright by reproducing the
drawings from the article, and I don’t have time to
redraw them, so it will help you to have a copy in front of
you. If you don’t already own Light and Its Uses
or the collected Amateur Scientist columns on CD-ROM, you can
probably find the book or the magazine at your local public
library or the nearest college or university library of Physics,
Engineering, or Sciences. Alternatively, if it is still up on
the Web,
this page
has copies of the illustrations on it. They aren’t
very large, but you should be able to see enough to follow
what I have to say.
Mr. Small’s explanation of the general principles
of operation of the nitrogen laser appears, for the most
part, to be reasonable. For example, he identifies one
cause of the short pulses as bottlenecking in the lower
laser level: the lifetime of the upper laser level is on
the order of 40 nsec at low pressures, and is perhaps 20
or 30 nsec at the pressures ordinarily used in
low-pressure nitrogen lasers; the lifetime of the
lower level, on the other hand, is some tens of
μsec, literally about a thousand times as long.
Broadly speaking, this limits the pulsewidth to less
than the lifetime of the upper level.
That is certainly correct as far as it goes; but in
practice, the pulses from many nitrogen lasers
(including the Scientific American laser) are
considerably shorter, often in the 6 to 8 nanosecond
range. This is because most small-scale driver circuits
“run out of steam” — within a few
nanoseconds after lasing starts, they cease to be able
to give the electrons in the discharge enough energy to
pump nitrogen molecules to the upper laser level at a
sufficiently rapid rate, and the existing inversion is
then depopulated by the lasing process. Lasing ceases
long before there is time for a large lower-level
population to build up. [This is supported by the fact
that various high-power nitrogen lasers put out pulses
that are more than 15 nsec long.]
It is, of course, possible to get a very short pulse
from a nitrogen laser by pumping (and presumably lasing)
about half of the available nitrogen molecules. At that
point you have bottlenecking in the lower level
regardless of the duration of the output pulse. At any
reasonable pressure, however, doing this in just a few
nsec takes far more electrical input than the Scientific
American laser could possibly provide, and I have seen
only one or two reports of high-power nitrogen lasers
that appear to operate in this regime.
It is also possible, though not particularly common, to
create a resonant shortening of the laser pulse; this
also has to do with the design of the driver circuit,
but in a different way. See the Tsui, Silva, Couceiro,
Tavares Jr, and Massone reference, below, for more
information.
Please note: some references claim that the short
lifetime of the upper laser level limits the
pulsewidth. That’s idiotic nonsense. Lots of
organic laser dyes that have upper-level lifetimes of
just a few nanoseconds will happily run for 1
microsecond or longer under flashlamp pumping, and can
even be operated CW with laser pumping.
Let’s take a look at some of the claims in the
article, see what they mean, and find out how they
stack up against observable reality.
First of all, Small describes his laser as a Blumlein
circuit, and speaks of “the Blumlein phenomenon”.
The real Blumlein phenomenon is the fact that
Alan Dower Blumlein essentially invented stereophonic
sound. He even got a patent on it. Among audio engineers, he
is rightly famous. There are Web pages about this, and
someone has written a biography of him. Among electrical
engineers, however, he is also known for his work on
transmission lines. He came up with something called a
“Blumlein line” or “Blumlein circuit”,
or sometimes just “Blumlein”.
This circuit involves two matched transmission
lines, with a matched load between them that has
twice the impedance of either line. There’s an
explanation of it
Note that the two transmission lines do not have to be
of the same length; but they do have to have the same
characteristic impedance, and the load must be matched
to both of them. This implies that the load must also
have a specific impedance, which does not change. (The
Kentech page includes diagrams showing idealizations of
what happens when the impedance of the load is or is not
matched to the impedances of the transmission lines.)
It is important to note that this is a transmission
line circuit we’re talking about here, and that,
as such, it involves relatively well-behaved and
well-matched impedances. The impedance of a nitrogen
laser’s discharge channel changes constantly during
the discharge cycle, and is nontrivial even to define. It
is not really possible to match such an impedance with a
transmission line, which has fixed parameters. Various
articles (see, for example the Tsui et al. and
Persephonis references, below) have discussed this or
related issues.
The Blumlein circuit also requires extremely fast
switching, because otherwise the energy storage devices
behave as discrete capacitors, not as transmission
lines. This is crucial, and is a major point of failure
of Small’s explanation. (More about this shortly.)
If you read the references I list at the end of this
rant, you will find repeated statements to the effect
that the measured risetimes of these lasers are much too
long for any transmission-line behavior to occur, at
least on the switched side. (There is, however, some
chance of observing transmission-line behavior on the
unswitched side in a well-designed laser of this
type.) Note that I’m not talking about theory here
— these are actual measured risetimes of real
lasers, most of them a lot better than Small’s.
Some of them, in fact, put out several megawatts of
power, whereas Small’s design puts out perhaps 50
or 100 kilowatts. (I will provide a relevant diagram
later.)
In his article, Small states that “at the instant the
switch closes”, a discharge wave is initiated in the
circuitboard capacitor that presumably forms one of the
transmission lines of the device. Let’s think about
that for a moment.
First off, the word “instant” is not defined
in physics, electronics, or engineering, except when
people are discussing mathematical entities
(“...the instantaneous value of the second
derivative...”). It’s not appropriate here,
and in plain point of fact, it’s meaningless.
(That should serve as a pertinent warning about
any description of a physical device that
employs this word.)
Second, even if we were to pretend that “instant”
had a meaning, that it meant, say, “appreciably
less than 1 nanosecond,” there wouldn’t and
couldn’t be any such instant in any case. The
switch in question is an untriggered spark gap, designed
and constructed so that it includes a nice big one-turn
inductor. Even excellent spark gaps, well designed and
carefully triggered, take several nanoseconds to
initiate; and the free-running spark gap in this laser
is slower than even a reasonably good triggered one.
(2011.0510, afternoon and evening)
Let me show that to you.
Here are two oscilloscope traces. The first shows the
output of a TEA nitrogen laser that I built. It is here
to demonstrate that my scope [a Tektronix 7104, with a
7A19 vertical amplifier (600 MHz bandwidth)] and
photodetector [a Motorola MRD500 photodiode (risetime
specified at 1 nsec or less), in a commercial mounting]
are actually fast enough to support this measurement.
The pulse from a TEA nitrogen laser is much shorter than
the pulse from the SciAm laser. If you measure it at
half of its maximum amplitude, it is generally a bit
less than 1 nanosecond long. It is showing up in this
photo at just under 1.5 nsec, which is very reasonable
— the risetime of my setup is 1 nsec or a bit
more, and it is difficult to show a pulse that is
shorter than the risetime of your detector/scope
combination. Unless the risetime of the spark gap is
still shorter, however, it will show up.
The next photo is a trace of the light from the spark
gap of that same nitrogen laser. As you can see, the
0-100% risetime of the spark gap is about 18 nsec.
Here’s a photo of the gap, so you can see the
design for yourself:
The top electrode is mounted on a broad piece of brass
shim that comes off the top capacitor plate, so it
avoids part of the inductance of the large one-turn coil
that is inherent in Small’s design. In addition,
the gap has a starting capacitor across it [the small
brown cylinder just to the left of the gap], which
speeds it up even further. As a result, this gap is
at least as fast as the one in the SciAm laser,
and in fact it is probably significantly faster.
The first part of the bottom line is that I don’t
want to hear any idiocy about “the instant”
the spark gap switches, because there isn’t any
such animal.
Because much of the rest of Small’s explanation
depends upon the switch closing in an unrealistically
short time, it cannot possibly accurately reflect what
is actually going on inside the laser. There are,
however, other issues.
Light travels at a finite velocity, which is very roughly
300,000,000 meters per second in a vacuum or in air.
In materials with higher density (and higher refractive
index), it is correspondingly slower. As Small points
out, a discharge wave in a transmission line travels
at the speed of “light” too, but that speed
turns out to be related to the impedance of the line —
the electrical equivalent, if you will, of the refractive
index.
In a piece of circuitboard, the speed is on the order of
5 nanoseconds per meter (see the Schwab and
Hollinger reference). That’s roughly 8 inches
per nanosecond. If a discharge wave travels 8 inches
during 1 nsec, then it takes 125 picoseconds to go 1
inch, and 12.5 picoseconds to go 1/10 of an inch. Please
take a good look at the diagram of
“The Blumlein switching phenomenon”,
either on the Web or in a copy of the article. In this
diagram, edge of the discharge wave is shown as a
vertical wall, which is totally ridiculous.
Even if we take it to represent a 10-psec risetime,
there isn’t any such thing as a spark gap that
switches in 10 picoseconds, except perhaps in
a very carefully designed transmission line,
pressurized to about 1500 psi.
In fact, as you can see from the oscilloscope trace,
above, it takes literally hundreds of times as long as
that for even a good spark gap of the regular sort to
turn on at these voltages and currents. If such a switch
could make a discharge wave at all, the leading edge of
that wave would be several meters wide, not the
vertical wall that is shown in the diagram; and you
obviously can’t have a wave that is several meters
across in a device that is, itself, less than half a
meter wide.
As Schwab & Hollinger point out in their excellent
article, for a Blumlein generator that is built from
transmission lines with characteristic impedance of
0.160 Ω (a fairly reasonable value compared with
the effective impedance of a laser channel that is fully
conducting), it would take a spark gap with 0.2 nh
series inductance to create a risetime even as short as
2 nsec. That’s about the size of Small’s
entire laser.
10 psec? In a free-running gap that has a nice large
series inductor built into it? I don’t think so!
In addition, Small never addresses the fact that the
laser channel can’t be a well-matched load,
because its characteristics are constantly changing
during the electrical pulse. This makes it difficult to
get any such device to operate fully in transmission-line
mode, even if it is correctly designed and constructed.
(If you read the references, though, you will find that
it is possible to get some transmission-line
behavior in a circuit that is sufficiently well
designed, at least on the unswitched side. See the
Shipman reference, in particular, for a fine example.
There is also relevant information in the
Fitzsimmons et al.; Schwab & Hollinger; and
Iwasaki & Jitsuno references.)
It is interesting to note that Small says, “In
effect the assembly behaves as an adjacent pair of
interconnected capacitors.” It’s not just
“in effect”; his assembly is just
a pair of interconnected capacitors; it is not a
Blumlein circuit.
Unfortunately, instead of using the term
“Voltage-Doubler”, which would be at least
vaguely appropriate even though the circuit does not
actually double its voltage, he gives a distorted
version of what would happen in a Blumlein device,
including the claim that “As the charge rushes
through the spark gap a steep difference of potential
appears within the plate across a narrow boundary that
separates the charged and discharged regions of the
metal.” Well, no. Not with the design he’s
describing.
(Small also indicates “no voltage” in the
region of the “transmission line” where the
discharge wave has passed, which would not really be
correct even if the device were operating as a
transmission line; but the issue is not particularly
important to this discussion, and we don’t need to
get into it. If you want more and better information,
read the Kentech page referred (and linked) to above,
and a few of the articles cited at the end of this
page.)
Something Small never addresses (possibly because it had
not yet been examined or measured when he wrote his
article) is the fact that it takes time to pump enough
nitrogen molecules into the upper laser level to
establish a population inversion, which means that
lasing does not start as soon as the spark gap begins to
conduct. Here, for example, is a diagram that I have
adapted from one that appears in a published paper:
(Click the small image if you want a larger one.)
First, note that this is a charge-transfer laser, so the
voltage risetime is probably slower than that of a laser
like Small’s.
Second, note that the current in the laser channel
really starts to flow about 100 nanoseconds after the
voltage across the channel begins to rise. While it is
true that with Small’s design this time will be
shorter, it is certainly going to be measured in dozens
of nanoseconds.
Third, note that lasing does not begin until something
like 8 nanoseconds after the channel starts to
conduct; it takes time to create a population
inversion. (The nitrogen laser is pumped by direct
electron impact, so there can’t be a significant
amount of pumping going on until there is a significant
amount of current flowing in the channel.)
Fourth, note that lasing ceases while there is still
quite a bit of current flowing in the channel. This
laser has FWHM pulsewidth of 13 nanoseconds, which puts
it in the high-performance class and suggests that
lower-level bottlenecking is likely to be what
terminates the pulse. (The term FWHM refers to the Full
Width of the pulse at Half of its Maximum value.)
Another problem with Small’s explanation is that
he claims to have produced a travelling optical
wave. Let’s think about this, too, for a moment.
By Small’s own admission, the output pulse from his
laser is about as long as a broomstick, or a bit longer;
let’s say 6 feet, which is about 6 nanoseconds. If
we think about a time during which the entire laser channel
is above threshold, and is lasing, light that starts at
either end will be amplified by the discharge in the channel,
and will reach the other end just over 1 nanosecond later,
because the laser channel is a little over 1 foot long.
If we assume that one end of the channel reaches
threshold first, and then a hypothetical discharge wave
“walks along the channel” as Small proposes,
to create a travelling optical wave, what do we see from
the two ends of the laser? The “back” end,
where lasing starts first, should show a small amount of
output, which will increase as the leading edge of the
electrical discharge wave gets farther away. That is,
the back end will lase first, but not very strongly, and
the output from that end will increase during the first
nanosecond or so until the entire channel is above
threshold. (As pumping continues, the output from this
end will continue to increase until either too much of
the nitrogen in the channel has collected in the lower
laser level, or the capacitors can no longer push
enough power into the channel.)
After that first nanosecond, the discharge wave (and the
initial laser light from the back end) simultaneously
reach the front end, and lasing begins there. Thus, the
pulse from the front end should have a much sharper
leading edge than the pulse from the back end.
After that point, however, the entire channel is above
threshold, so for the rest of the pulse, which is to say
the next several nanoseconds, output from both ends will
be identical, or nearly so. Needless to say, this fails
to match Small’s description of the action; but
Small’s description fails to match his own
statements about the laser and what it does.
There are only a few ways in which such a laser, which
is only 1/6 as long as the pulse it emits, can produce
dramatically higher output from one end than the other,
and the primary ones involve something interfering with
the output at one end. In an ordinary TEA nitrogen
laser, this can be a disturbed discharge that
doesn’t have much gain, or it can be an arc or
spark. (It is, however, possible for an arc to form
after lasing has ceased, so an arc by itself is not a
reliable indicator.)
If anybody can show me such a laser that puts out a
large pulse from only one end of its channel
without any arc or spark formation and with no
obvious visual difference between the discharge at one
end of the channel and the discharge at the other, I
would very much like to see it.
A travelling optical wave can be produced when a
discharge wave intersects the laser channel at
approximately the speed of light. This causes the
excitation to “walk” down the laser channel
as fast as the laser pulse does. It thus produces much
more power in the forward direction than it does in the
reverse direction. This phenomenon is described by
Small, and it has, indeed, been observed in a discharge
laser: see the John D. Shipman reference.
Of course, the discharge wave has to be relatively
straight, and it has to be angled correctly with respect
to the channel, so that the region of intersection
advances down the channel at the speed of light. Shipman
was obliged to use a series of solid dielectric switches
driven by cables of precisely graduated lengths in order
to create this effect, and his laser had to be rebuilt
every time it was fired. It is extremely unlikely that a
circular discharge wave would be effective at producing
a travelling optical wave even if the Scientific
American laser were capable of producing such a wave,
which it clearly is not.
Alternatively, it appears to be possible to produce far
more output at one end of the laser by angling the
electrodes slightly so that one end of the channel is
wider than the other. The usual explanation is that the
narrow end of the channel begins to conduct first, which
has an effect somewhat like that of a traveling
discharge wave; but the length of the laser channel has
to be a substantial fraction of the length of the output
pulse for that effect to produce a dramatic asymmetry of
power from the two ends. It seems more likely that
wedging the electrodes simply produces a discharge that
is better at pumping the nitrogen at one end than the
other, and this is supported to some extent by visual
observations of TEA lasers.
If I may once again quote Schwab & Hollinger,
writing about discharge waves, “For
low-impedance Blumlein generators (as used with
N2 lasers), propagation time on the
transmission lines is on the order of 5 ns/m.
Considering rise times of about 25 ns, which are
inherent to many reported N2 lasers, the
traveling-wave concept becomes obsolete.... If the
advantages of true traveling-wave excitation shall be
utilized, either multiple spark gaps, solid-dielectric
spark gaps, or lines with high characteristic impedance
(e.g., 20 ohms) must be employed.”
The problem with high impedance is that such lines
deliver only a small fraction of the current that is
delivered by low-impedance lines, and must operate at
much higher voltage to produce the same power levels
(alternatively, you can use lots of them in parallel, as
in the lasers reported in the Woodward, Ehlers, and
Lineberger paper). The Schwab & Hollinger laser,
btw, which does not even attempt to provide
travelling-wave excitation, produces over 1 MW peak
output power at 12 kV charging voltage. It’s a
very decent design, though not as easy to construct as
some.
It is possible that Small inadvertently introduced some
angle between his electrodes; but as we have already
seen, that is not sufficient to produce a travelling
optical wave in a laser the size of his.
To give you an example of what a real travelling-wave
laser does, the laser described by Shipman emitted 2.5
megawatts in the forward direction and only 250 kilowatts in
the reverse direction. Not surprisingly, its channel was 183
cm long, just about the length of the pulse it produced.
Small claims, in his discussion of scaling, that
although a discharge path one meter long can develop an
output pulse of almost a million watts, “...there
is a trick to it. Because the laser turns itself off so
quickly, radiation does not have time to travel the full
length of the column before the gain automatically drops
to zero.” This is just plain stupid: he himself
states elsewhere in the article that the length of the
pulse from a low-pressure nitrogen laser is
“usually less than 10 nanoseconds”; 10
nanoseconds is more than three meters, so
he’s contradicting himself.
(As it happens, a nitrogen laser with a discharge path
one meter long has developed more than three million
watts, and did so without any “tricks”,
though Small didn’t know that when he was writing
his article because it hadn’t been accomplished yet.)
I should perhaps point out once again the fact that the
pulse can be terminated either by accumulation of
nitrogen molecules in the lower laser state, or by a
decrease in the electron temperature in the discharge,
so that the nitrogen is no longer being pumped
effectively. Most Do-It-Yourselfers are unaware of this,
even though it may be what is actually happening in
their lasers. It appears that at pressures on the order
of 30-60 Torr, a nitrogen laser that is pumped hard
enough can run for 15 nanoseconds or so, depending on
its design. (For further information, see the
“high power” references in the list at the
end of this page.)
Second, it is true that for a very long channel, in the
absence of travelling-wave excitation, the laser’s
output pulse would be created by only the part of the
channel that the light actually succeeded in travelling
through before amplification stopped. If you made the
channel still longer the output would change very
little, because the working length would remain the
same, and any extra you added would be wasted, as its
contribution would merely be absorbed at the ends after
lasing ceased. If you can create actual travelling-wave
excitation, of course, this ceases to be the case; but
for a low-pressure nitrogen laser, the channel would
have to be considerably more than two meters long before
this even began to become an issue. In fact, for a few
of the lasers mentioned in the papers I cite at the end
of this page, specifically those with pulsewidths on the
order of 20 nsec, the channel would have to be over
FIVE meters long before this problem could show up.
Small claims that the Scientific American laser puts out
50 to 100 kilowatts. I suspect that someone must have
measured the output of one of these machines, but just
at the moment I don’t recall ever seeing any
actual reported numbers. There are, fortunately, some
informal ways to estimate power output. I believe that
it takes only a few dozen kW to threshold a small dye
laser, and the Scientific American laser can certainly
do that.
There are other, somewhat informal ways to estimate peak
power, but they tend to require that the laser be
focused to a point, which is very difficult with most
nitrogen lasers because the beam is more or less a wide
ribbon shape. If you can focus down to a point, you will
probably find that it takes about 200 kW peak power to
produce a spark when the beam is focused onto a metal
surface (be very careful about possible
reflections!); see the Bergmann & Eberhardt
reference, below. It probably takes considerably more
than a megawatt to produce a spark in open air at 337
nm.
It is also possible to measure the output energy, and if
you can also measure the pulsewidth it’s easy to
derive the power. I have a Scientech power head, which we
got on eBay, and I’ve been able to do a rough
measurement of the pulse energy of a commercial TEA
nitrogen laser, also acquired on eBay. This laser, a
PRA LN-1000,
is rated to produce about 1.5 millijoules per pulse; I
measured it at just under 1.1, which is reasonable for
an older machine in less than perfect condition. It is
more than powerful enough to run small dye lasers.
If you don’t have a power head, you can make one
from a thermoelectric cooler held to a block of aluminum
with heatsink grease between them, and some flat black
paint. The advantage of the Scientech head is that it
has a built-in calibration heater; but it can’t be
too difficult to construct a homebrew head with such a
heater.
I have to confess that crap like Small’s explanation
drives me right up the wall, especially in the pages of
Scientific American (which used to be a real science magazine),
and double-especially when it comes out of MIT.
Anyone who bothers to perform any kind of decent testing
on a laser built to Small’s design can’t fail
to detect the huge bogosity quotient, and I can’t
believe that he didn’t have access to decent equipment,
MIT being, after all, one of the premier institutions for
this sort of thing in the entire world.
Moreover, just reading the Shipman article carefully is
enough to tell you that Small’s laser can’t
do what he claims it does, and that article was published
two years before Small’s Scientific American
piece, so presumably he had access to it. It’s
even moderately likely that he read it, though apparently
not carefully enough. Small certainly should have known
that there’s no such thing as an instant in physics,
and that even a one-turn coil (such as the one he builds
into his spark gap) has substantial inductance.
Argh.
In Small’s defense, I have to point out that there
was relatively little literature available to him;
almost all of the nitrogen laser articles I’ve got
were published after the Scientific American nitrogen
laser appeared. On the other hand, Shipman’s
article is quite clear about some of the conditions that
must be achieved in order to create a travelling
discharge wave. (Shipman even suggests the technique of
using multiple coaxial cables that later became the
basis of the Woodward, Ehlers, & Lineberger lasers;
and he suggests graduated lengths to create travelling-wave
excitation, which those lasers didn’t use, though
their article mentions the possibility.)
Unfortunately, the explanation given in the Scientific
American article is an example of the worst effects of
bad scholarship (and accidental memetic engineering):
its claims have been believed and repeated by a great
many people who failed to check or fully understand
them, and they have polluted a large sector of amateur
and even professional nitrogen laser work. I have seen
the words “At the instant the switch closes”
in articles from major journals, for example. Also,
although it isn’t just Small’s doing, the
term “Blumlein” has become hopelessly
polluted. Too many articles and Web pages refer to
voltage-doubler circuits as “Blumleins”. It
is likely that Shipman’s laser could, with some
justice, be described as a Blumlein circuit, and there
are one or two others, very similar to his, that were
reported; but those few are about it, as far as
I’m aware. Nothing else comes close. Not your
laser, not my laser, not anybody’s
laser. Throughout much of the professional research
literature, however, any simple voltage-doubling-circuit
laser is referred to as a Blumlein circuit.
Some people even talk about how “easy” it is
to build Blumlein circuits. See, for example, the comments
about Ben Franklin in
Sam’s Laser FAQ.
In fact, the subject of Blumleins is about the only
thing that bothers me about Sam’s otherwise
remarkably excellent set of pages. (If you didn’t
arrive at this page from his, you should definitely go
take a look. I cannot recommend Sam’s pages too
highly — they contain a huge amount of extremely
valuable and useful information for the DIY laser
builder and enthusiast, as well as lots of helpful
links.)
It also seems to me that Small never actually performed
rigorous measurements on his laser, else he
wouldn’t have made such grandiose claims for
it. As I said above, it’s a nice straightforward
design; the only two things that are really wrong with
it, aside from his ridiculous claims, are: first, the
fact that epoxy-fiberglass circuitboard is very lossy at
high frequencies, which makes it a mediocre candidate
for this service. It’s readily available, though,
and not very expensive, so that isn’t as bad as it
might be. (Just for reference, btw, I think the
dielectric constant of G-10 circuitboard is about 5.2 or
5.3.) Second, the spark gap could very easily have been
a lot better.
I might also point out that Small misuses the word
“superradiance”, but in fact almost everybody
does that. A better term for a laser that operates without
any mirrors is “superfluorescence”. (If I
remember correctly, R. H. Dicke proposed the term
“superradiance” to describe a specific
phenomenon he envisioned, something that is fairly
specialized, and is rarely observed.)
As to travelling-wave excitation (or any method for
creating TW optical behavior) in an amateur low-pressure
laser, if anyone cares to come forward with a convincing
demonstration of this I’ll be only too happy to
mention it here, and to link to it if there’s a Web
page for it. (I know of a few people who are working on
TW TEA nitrogen lasers, but I am not aware of anyone who
is trying it at low pressure.) Do be sure, however, that
you have adequate instrumentation and that you aren’t
just fooling yourself.
H. G. Heard
This article presents the discovery of the nitrogen laser.
-----------------------------------------------------
A. W. Ali
This is a fine article, written only a few years after
the nitrogen laser was discovered.
-----------------------------------------------------
John D. Shipman
(This is a real classic, and is central to my claims.)
Shipman’s laser, as far as I can tell, actually did
operate mostly in transmission-line mode, and was about
as close as any to being a real Blumlein. He also makes
several key points about circuitry and design.
-----------------------------------------------------
E. L. Patterson, J. B. Gerardo, and A. W. Johnson
I have not read this article, but I believe that it
reports generation of 24 MW, easily the highest peak
power that I have ever heard of in a believable nitrogen
laser. There is a paper in Comptes Rendus for
February, 1972 that claims 50 MW by direct electrical
excitation, but I did not find it to be particularly
believable.
“Réalisation d’un amplificateur laser
d’une puissance atteignant 50 MW à 3 371
Å dans l’azote moléculaire.”
[As of September, 2013, you can find this paper on the Web at
http://visualiseur.bnf.fr/ark:/12148/bpt6k5794353k ]
-----------------------------------------------------
Adolf J. Schwab and Fritz W. Hollinger
This makes the crucial point that for a transmission
line with impedance of 0.16 ohms, you can achieve a
2-nanosecond risetime only if your switch has inductance
of less than 0.2 nh, which the authors point out
“is unrealizable using a single spark gap.”
Real voltage-doubler lasers usually have risetimes more
like 25 nsec.
-----------------------------------------------------
W. A. Fitzsimmons, L. W. Anderson, C. E. Riedhauser, and
Jan M. Vrtilek
Even though their own Figure 7 clearly shows many
reflections on their “transmission line”
during a single pulse, which proves that the device is
operating mostly as a capacitor and not as a
transmission line matched to a load, they nonetheless
describe this version of their laser as a
“Blumlein”. That, however, is about the
only problem I have with the article, which is otherwise
excellent and thorough.
----------------------------------------------------
B. W. Woodward, V. J. Ehlers, and W. C. Lineberger
The lasers in this article use coaxial cables as
“peaker” caps; the cables operate at least partly
as transmission lines. The experimenters didn’t try
to create a travelling optical wave, but the design
should easily lend itself to that kind of effort, and
there is mention of one possible method (graduated cable
lengths).
A key quotation:
“Further, the impedance of the laser gas after
breakdown has started is very small, so that the
maximum coupling of power to the discharge will occur
when the impedance of the transmission line is
minimum (in the range available).”
-----------------------------------------------------
Seishiro Saikan and Fujio Shimizu
With a spark gap that goes through the circuit
board rather than around it, and a spacing of 0.1 mm,
these researchers measured a risetime of 1-2 nsec for
their switch. They do state that the actual risetime was
probably somewhat faster, because the driving circuitry
itself had a risetime of about 1 nsec. On the other
hand, they point out that a water-filled gap is a
dielectric switch under their conditions, and they note
that dielectric switches display fast switching
characteristics. This is supported by the work of
Shipman; see the reference to his article, above. It
also shows that Small’s laser cannot generate a discharge
wave, most especially one with a 10-picosecond risetime!
-----------------------------------------------------
Chigusa Iwasaki and Takahisa Jitsuno
These guys actually tried different spark gaps, and they
report the performances they got. Again, they refer to
their laser as a Blumlein, which it clearly isn’t; but
it is sufficiently similar to Jim Small’s design that it
is relevant. If I may quote,
“In a laser discharge device using a transmission line
as a discharge capacitor, the duration of the current pulse
is affected by the reflection of the voltage pulse at the
open end of the transmission line (so-called transmission
line effect), and therefore, the laser output may depend on
the roundtrip transit time in the transmission line when the
transit time is smaller than the lifetime of the upper state.
“.... However, no appreciable voltage wave arising from
the reflection in the transmission line has been observed in
the measured waveform of the anode voltage. This may be due to
the fact that the rise time of the voltage wave (~5 ns) is
much longer than the transit time, and therefore, it is
supposed that the transmission line effect does not play an
important role in this case....”
(Note that this is a TEA laser, where the lifetime of the
upper state is perhaps 2 nsec, and they calculated the
roundtrip transit time of their capacitor to be 1.3 nsec.)
The key thing here is the risetime of the voltage in their
“Blumlein”, which was 5 nsec. This is vastly
longer than the risetime that would be required to create
a voltage wave in a device the size of Small’s, and
is also vastly longer than the risetime he indicates in
his diagrams and text. It is, moreover, a measured
risetime, in a real device, and not just handwaving.
-----------------------------------------------------
J. I. Levatter and S. C. Lin
The authors of this (excellent) article built a truly
righteous laser; it developed three megawatts of output
power, and was for some time the most powerful purely
discharge-pumped nitrogen laser on record... but even
though they tried to design it to create a travelling
optical wave, they were unable to find any evidence of one.
-----------------------------------------------------
Riccardo Polloni
[This paper is cited by the Oliveira dos Santos et al.
paper, and is included here for completeness. I haven’t
read it yet.]
-----------------------------------------------------
B. Oliveira dos Santos, C. E. Fellows, J. B. de Oliveira e Souza,
and C. A. Massone
This is a strange and intriguing article that
illustrates an entirely different approach. Using a
coaxial capacitor of only 800 pf, driven by one of three
“dumper” caps (1.5, 10, or 20 nf), they
achieved up to 3 MW output power at efficiencies ranging
as high as 3%. Peculiarly, their pulsewidth decreased as
the amount of stored energy increased, which may suggest
that they are pumping a substantial fraction of the
nitrogen molecules in their laser. Well worth reading
and thinking over very carefully.
Unfortunately, an attempt by Dr. Vicente Aboites and
colleagues to reproduce the performance of this design
did not succeed:
V.J. Pinto, V. Aboites and J. de la Rosa,
(See below for other articles by Dr. Aboites.)
-----------------------------------------------------
K. H. Tsui, A. V. V. Silva, I. B. Couceiro, A. D. Tavares, Jr.,
and C. A. Massone
This article, though not necessarily easy to follow,
contains a valuable discussion of a topic that is seldom
discussed in the nitrogen laser literature. It may explain
(at least partly) the occasional high-performance laser
operating at relatively low pressure but producing
extremely short pulses, for example the Armandillo and
Kearsley laser (see below).
-----------------------------------------------------
A. D. Papadopoulos and A. A. Serafetinides
Note that this laser closely resembles the Scientific
American laser, but is much faster and produces
considerably higher output. Nonetheless, the authors
describe it as a doubling circuit, not as a Blumlein;
and they analyze it in terms of lumped constants, not
transmission lines. The oscilloscope traces of the
current and voltage waveforms in their laser and of the
laser output pulse support this approach.
-----------------------------------------------------
P. Persephonis
This early Persephonis article is good, despite the
misuse of the term “Blumlein”, but see
the next reference.
-----------------------------------------------------
P. Persephonis, B. Giannetas, J. Parthenios, C. Georgiades,
and A. Ioannou
This is a beautiful look at the optimum capacitances and
capacitance ratio for the doubler circuit nitrogen
laser. (Note that by 1993, Persephonis had ceased to
refer to these as “Blumlein-lines”.) The
findings in this article are somewhat surprising, in
that they obtain best results with relatively large
capacitances; but entirely expectable in that they
confirm the general wisdom, which is that the capacitors
in a doubling circuit should be of about equal value. To
say that this article is well worth reading would be an
understatement.
-----------------------------------------------------
Imre Sánta, László Kozma,
Béla Német, János Hebling,
and M. R. Gorbal
These people figured out how to angle the electrodes
in order to cause the discharge to form at one end
and walk down the cavity to the other. Because a TEA
nitrogen laser has an output pulse that is only about
600 psec long, it is possible to make a TW laser that
is only about a foot long, and they appear to have done
so. DiY folks take note.
-----------------------------------------------------
K. R. Rickwood and A. A. Serafetinides
A rather intriguing paper for its general premise; also
has some good information about optical cavity
considerations, and about the effects of adding helium
to the gas. Well worth a careful read.
----------------------------------------------------
E. Armandillo and A. J. Kearsley
This article covers the design considerations of
a nitrogen laser that delivered 5 MW (!), the highest
output power reported in a discharge-pumped nitrogen
laser up to the time of the article’s publication,
and probably still one of the highest power levels ever
achieved in N2. Oddly, their pulses were
only 4 nsec long, which is quite unusual for high-performance
nitrogen lasers. The article is good, if a bit brief.
Crucial points here include the dimensions of their
channel, which used electrodes a full 4 cm across,
spaced 25 mm apart; and the fact that the addition
of Helium, while it did not increase the output energy
or power of their laser, did give them better pulse-to-pulse
uniformity and a cleaner discharge. In addition, they
were able to operate their laser with enough He to
bring the total pressure up to more than 1 atmosphere.
I have taken advantage of that in at least one of my
own lasers: it allows you to operate without a vacuum
pump, which can be very convenient.
-----------------------------------------------------
F. Encinas Sanz and J. M. Guerra Perez
This article concerns a charge-transfer
(“dumper-peaker”) laser that developed
20.5 mJ in the UV (!). Because it had a relatively
long output pulse, however, the peak power was only
1.5 MW. One interesting thing about this article is
the fact that they found an optimum interelectrode
spacing of about 38 mm, much wider than is common
in circuitboard (or other) low-pressure nitrogen
lasers, but similar to the spacing in the high-energy
laser built by Rebhan et al., which is cited
below.
Another key point is that the article shows voltage,
current, and laser output traces taken from oscilloscope
photos. These clearly demonstrate the fact that their
laser didn’t reach threshold until about 10 nsec
after current began to flow in the laser channel, and
also the fact that current didn’t begin to flow
until dozens of nsec after voltage began to appear
across the channel. Granted, their design was a
charge-transfer circuit, not a doubler circuit, so the
voltage risetime was slower than you would expect in a
Small-type laser; still, there is definitely some nsec
delay between the onset of the discharge and the onset
of lasing.
-----------------------------------------------------
U. Rebhan, J. Hildebrandt, and G. Skopp
This is another of the best nitrogen lasers ever
constructed. With some SF6 in the gas mix, it delivered
30 mJ over 19 nsec, and even without any SF6 it
delivered 16 mJ over 14 nsec! It uses a
liquid-dielectric peaker cap of very ingenious
design. It describes the use of long electrodes to avoid
sparking at the ends, an important technique.
-----------------------------------------------------
Godard, Bruno
This is very likely Godard’s fairly infamous
article in which he claims to have derived 9 MW (!)
from a laser built out of kapton circuitboard. Inasmuch
as nobody has ever been able to repeat the result, there
is considerable skepticism. I’m not 100% sure
about the reference, btw; my copy of the article was
handed to me by Godard himself in either 1973 or 1974,
and is not from J-QE. It says on it...
-----------------------------------------------------
Ernest E. Bergmann and N. Eberhardt
Bergmann (not to be confused with H. M. von Bergmann,
a South African researcher who did pioneering work
with TEA nitrogen lasers) and Eberhardt note that
their laser's unfocused beam could pump several
dyes to superfluorescence, and that sparks could be
produce by focusing the beam on various metal surfaces.
This laser had 200 kW peak output power, so these
results provide a rough diagnostic.
-----------------------------------------------------
A. Vasquez Martinez and V. Aboites
This is another important paper, though the theoretical
investigation is not as thorough as in some others, and
also despite the fact that the authors speak of “the
instant the spark gap triggers”, which is not accurate.
Even so, there is some very interesting information here.
As mentioned above, Dr. Aboites has also published other
nitrogen laser articles, including a MOPA or MOPO design:
Jorge Sandoval Ch, Alejandro Apolinar, Victor J. Pinto, Vicente Aboites
-----------------------------------------------------
C. H. Brito Cruz, V. Loureiro, A. D. Tavares, and
A. Scalabrin
This is a small laser, used to investigate both
preionization and helium; mixing nitrogen and
helium 50-50 doubled their output power. With
preionization, they measured best output at E/p
of 87.
-----------------------------------------------------
Peter Schenck and Harold Metcalf
Bert Pool
used to have a copy of this fine article on his Web page,
but I don’t find it now. It is a nice easy design
that develops more than 100 kW peak power under optimum
conditions. I believe that it uses a thyratron as a switch,
but you could very easily build it with a spark gap instead.
I will, however, advise you to use a triggered spark
gap — they’re a lot faster than free-running
spark gaps, and speed is the reason why you would want to
use a spark gap rather than a thyratron in the first place.
If you want to build a nitrogen laser that puts out
considerably more power than Small’s, I have published
a design that delivers approximately 250 kW
and is capable of making sparks when the beam is
focused onto a metal surface. I am currently (late
2006) working on a laser that will be less expensive
to build and should put out at least 500 kW.
Finally, I need to point everyone at
a remarkable site
put together by Thomas Rapp, in Germany. He really knows
how to build lasers, including TEA nitrogen lasers. (You
can have Google translate his pages; it does a fair job,
considering, and although you’ll still have a lot
of figuring out and thinking to do, it’s
definitely worth doing.)
Email: a@b.com, where a is my first name (jon, only 3 letters,
no “h”), and “joss” replaces “b”
Phone: +1 240 604 4495.
Last modified: Tue May 9 12:17:29 EDT 2017
To Begin
1. The Renowned Blumlein Circuit
1A. Switch Closure Timing, a Central Issue
1B. Formation of a Discharge Wave
The Issue of Latency
3. The Travelling Wave
5. Scaling
Output Power
In Closing
Residual Fallout
References:
“Ultra-violet Gas Laser at Room Temperature”
Nature, v. 200 (1963), p. 667
“A Study of the Nitrogen Laser Power Density
and Some Design Considerations”
Applied Optics, v8n5, May 1969, pp 993-996
“Traveling Wave Excitation of High Power Gas Lasers”
Applied Physics Letters, volume 10 number 1, January, 1967, pages 3 & 4.
“Intense electron-beam excitation of the 3371 Å
N2 laser system”
Applied Physics Letters, volume 21 (September, 1972) pages 293-295.
Jean-Pierre Girardeau-Montaut, Michel Roumy, Joël
Hamelin, et Louis Avan, présentée par M.
Alfred Kastler
Comptes Rendus [Série B] 274(9) (28 Feb, 1972), pp. 607-610
“Compact High-Power N2 Laser: Circuit Theory and Design”
IEEE Journal of Quantum Electronics, volume QE-12,
number 10, October, 1976, pages 183-188
“Experimental and Theoretical Investigation of the
Nitrogen Laser”
IEEE Journal of Quantum Electronics, volume QE-12,
number 10, October, 1976, pages 624-633
“A reliable, repetitively pulsed, high power nitrogen
laser”
Review of Scientific Instruments, volume 44, 1973, pages
882-887
“Water spark gap for a nitrogen laser”
Review of Scientific Instruments, volume 46 number 12,
December, 1975, pages 1700 & 1701
“An Investigation of the Effects of the Discharge
Parameters on the Performance of a TEA N2 Laser”
IEEE Journal of Quantum Electronics, volume QE-18,
number 3, March, 1982, pages 423-427
“High-power generation from a parallel-plates-driven
pulsed nitrogen laser”
Applied Physics Letters, volume 26, pages 118-120, 1975
“Design of a reliable, high-power, nitrogen laser”
Optics and Quantum Electronics 8(6) (1976), p. 565-566
“A 3% Efficiency Nitrogen Laser”
Applied Physics B (Photophysics and Laser Chemistry)
41 (1986), pp. 241-244
"High Efficient N2 Laser"
Revista Mexicana de Fisica, 37(3) (1991), pp. 391-395
Resonant Narrowing of the Nitrogen Laser Pulse by Plasma Impedance Matching
IEEE Journal of Quantum Electronics, Vol. 27 No. 3 (March, 1991),
pages 448-453
“Characteristics of Doubling Circuits Used in Gas Laser
Excitation: Application to the N2 Laser”
IEEE Journal of Quantum Electronics, volume 26 number 1,
January 1990, pages 177 to 188
“Electrical behavior of a Blumlein-line N2 laser”
Journal of Applied Physics, volume 62, pages 2651-2656,
1987
“Capacitance Allocation and Its Role in the
Performance of Doubling-Circuit Pulsed Gas Lasers:
Its Application to the N2 Laser”
IEEE Journal of Quantum Electronics Vol. 29, No. 8,
August, 1993, pages 2371-2378
“Experimental and Theoretical Investigation of a
Traveling Wave Excited TEA Nitrogen Laser”
IEEE Journal of Quantum Electronics, vol. QE-22,
Number 11, (November, 1986), pages 2174-2180
“Semiconductor Preionized Nitrogen Laser”
Rev. Sci. Instr. 57(7), July 1986, pp 1299-1302
“High-power nitrogen laser”
Applied Physics Letters, volume 41 number 7,
(1 October, 1982), pages 611 through 613
“A High Power High Energy Pure N2 Laser
in the First and Second Positive Systems”
Applied Physics B, volume 52 (1991), pages
42 through 45
“A High Power N2 Laser of Long Pulse Duration”
Appl. Phys. 23, 341-344 (1980)
“A Simple High-Power Large Efficiency N2 Ultraviolet Laser”
IEEE J-QE vol QE-10 no 2, February 1974, pp. 147-153
“LABORATOIRES DE MARCOUSSIS
CENTRE DE RECHERCHES DE LA
COMPAGNIE GENERALE D’ELECTRICITE
DEPARTEMENT RECHERCHES PHYSIQUES DE BASE
Section Sources d’Ondes Cohérentes
91460 - MARCOUSSIS - FRANCE”
...and is dated “MAI 1973”. The title is also
slightly different; it begins “A VERY SIMPLE HIGH
POWER....”
A Short High-Power TE Nitrogen Laser
IEEE Journal of Quantum Electronics vol. 9 no 8, August, 1973,
pages 853-854
“High-Efficiency Low-Pressure Blumlein Nitrogen Laser”
IEEE J-QE vol QE-29 no 8, August, 1993, pp. 2364-2370
"Perfiles transversales de un sistema MOPO de N2"
Revista Mexicana de Fisica 43(3) (1997), p. 381
“Characteristics of a Wire Preionized
Nitrogen Laser with Helium as Buffer Gas”
Appl. Phys. B 35 (1984) pp. 131-133
“Low Cost Nitrogen Laser for Dye Laser Pumping”
Applied Optics, Vol. 12 # 2, February, 1973, starting on page 183
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