T.E.A. Nitrogen Laser

When I began to study lasers way back in early 1990, one of the first desires was to construct a working laser in which I designed and built.

From the Mid 1970s, Scientific American published a series of articles, "The Amateur Scientist" featuring several home-built laser projects. I recall I was in awe of the ability to being able to build a laser from scratch.

The series was also published as a book, "Light and Its Uses: Making and Using Lasers, Interferometers and Instruments of Dispersion (Readings from Scientific American)" by C.L Strong.

One Laser that has been on the backburner since the early days has been the TEA Nitrogen Laser. Perhaps one of the main reasons, up to this point that I have not built one, was due to the fact of its output (337nm in the ultraviolet) which at the time saw little application over a visible laser, and that most common designs are quite large overall.

In 1996/1997, I came on what is one of the best resources on lasers, Sam's Laser FAQs , to this day it remains one of the best resources that covers many aspects of lasers and optics.

Fast Forward to March 2021, I recently began following the work of Jay Bowels of Plasma Channel (YouTube). Jay had published a video on a Nitrogen laser he had built, however the design is significantly smaller in size and as such, resparked my interest to investigate finally building a Nitrogen Laser.

T.E.A refers to the operation of the laser using Transverse Excitation at Atmospheric pressure, such a laser operates in the pulse regime.

Having watched Jay,s Video, I also happened to notice in the suggested video list, a high-performance home-made TEA Nitrogen Laser, this video published by Leslie Wright (YouTube Channel Les's Lab).

Stumbling on Leslie's channel, I have to say his lasers and work is very inspirational. His home-built TEA Nitrogen laser is one of the best examples that I have come across and I owe my thanks to Leslie for producing a series of excellent videos on the subject, as this was the catalyst for embarking on my Nitrogen laser Project with the goal to also finally experiment with dye lasers.

Over the next several weeks and many late evenings I began to research various Nitrogen lasers, power supplies, capacitors, designs and laser physics. Much of this work also included research into Laser Dyes as well as the construction of plate capacitors.

LC Inversion Circuit Laser

On the 7th April 2021, I commenced application of theory to building of plate capacitors using foil and Polypropylene plastic sheets for the purpose of measuring the capacitance against calculated models I had setup in Excel.

The target value for the capacitor(s) is 60pF per centimetre of laser channel, therefore the models and calculations based on the known length of 210mm laser channel, the dielectric and a final area to arrive at a total capacitance of around 1260pF.

Following several evenings of building capacitors, testing of the dielectric thicknesses and dimensions, the next task was to set about seeing if able to power the capacitors.

On 13th April 2021, I assembled the first LC Inversion (more commonly and incorrectly referred to as a "Blumlein circuit"), TEA Nitrogen Laser.

I had built the capacitors for a 150mm Laser channel, a total capacitance of 900pF.

In the first test, I used some small brass hex rods that I had laying around, however only providing a channel length of 63mm. The power supplied was a simple ignition coil and the charging inductor, I used a 30uH coil that I also had laying around.

The result of the first test, was a very weak output, the channel filling with many hot arcs (as a result of the larger capacitance than needed).

For the second test, I upgraded to 150mm long x 8mm Hex rods, and keeping the rest the same. The resultant output observed a much stronger output at 337.1nm.

Nitrogen laser, 150mm Channel

Nitrogen laser Test, 150mm Channel

Following the successful operation of the first Laser, the next few days I spent in testing out different thicknesses of foil (35um vs 10um), dielectric thickness (210um vs 160um).

Experimenting with the laser, I also improved the spark gap from the very basic folded aluminium point which I had to more rounded aluminium electrodes. This modification improved performance with greater output observed.

Improved Spark Gap

Improved Spark Gap

The next task was scaling to the 210mm laser channel (which would be the length of channel for the proposed final laser). I also updated the charging inductor coil and replaced the spark gap with brass acorn nuts and a cover.

Following these changes, the laser successfully ran at around 8 Hz repetition with excellent uniform glow discharge within the channel.

TEA Nitrogen Laser with visible output and uniform glow discharge. 16th April 2021.

Uniform glow discharge & Output.
16th April 2021

Charge Transfer Circuit

On the 18th April 2021, I commenced work on the Charge Transfer circuit test bed, which is also the basis for the proposed laser.

The charge transfer circuit differs somewhat, where the supply charges a larger value "Dumping" capacitor to high-voltage at which point the spark gap closes, this charges the smaller value "Peak" capacitor where it discharges its energy in the laser channel in a very fast high rise time pulse.

Design Sketch of Charge Transfer TEA Laser. 18th April 2021.

Design Sketch: CT Circuit TEA Laser.
18th April 2021

Based on the optimal capacitance of 60pF / cm of Laser channel, the peaking capacitor value will be around 900pF for a 150mm channel, while the dump capacitor will be set at around twice the peaking capacitor (around 1.8nF).

Charge Transfer TEA Laser Test Bed. 18th April 2021.

Charge Transfer TEA Laser Test Bed.
18th April 2021

Charge Transfer TEA Laser Firing. 18th April 2021.

Charge Transfer TEA Laser Firing.
18th April 2021

Observation of more hot arcs within the laser channel, however successful operation and output achieved.

Introduction of Flowing Nitrogen Gas

The next day, experimentation commenced with flooding the channel with flowing Nitrogen gas. Both variants of the laser test beds tested with observed increase in optical output.

LC Inversion (top) and Charge Transfer (Bottom) TEA Lasers. 19th April 2021.

LC Inversion (top) & CT (Bottom).
19th April 2021

Operating Laser with N2 Flooded Channel

Operating Laser with N2 Flooded Channel

Design And Build of Charge-Transfer TEA Nitrogen Laser

Following considerable research and experimentation, the next step was to take all the knowledge and information and apply this to the design and construction of the "production" Laser.

Specifications for the base plate and laser channel completed on 20th April 2021.

The base is 6mm thick Aluminium plate measuring 120mm x 300mm, this serving to support the laser channel, capacitors, mirror mount and needle valve.

The plate also servers as the common (ground) for the capacitors and forms one "plate" of the peaking capacitor.

The laser channel is formed by two 8mm (between flats) brass hex rod, one rod is fixed to the base plate while the other is mounted to the acrylic channel enclosure and constructed to allow for adjustment of width of the channel.

Pressurised Spark Gap

The first part of the build started with the construction of the pressurised spark gap.

The advantages of a pressurised spark gap are switching of high-energy currents from capacitors with low loss and inductance, as well as offering a higher hold-off voltage and very fast switching times (in the order of nanoseconds).

Spark Gap Components
Internal Port

Pressurised Spark Gap

Pressurised Spark Gap

With thanks and credit to Leslie Wright (YouTube Channel: Les's Lab) for the design of the Spark Gap;

I set about obtaining the parts (which I found somewhat of a challenge to find the PVC fitting here in Australia). The gap is constructed from brass end-caps, brass acorn and brass bolt.

The 3mm acrylic disks are laser cut by a local supplier which are used for additional insulation.

Once assembled, the actual gap is approximately 2mm. When the gap is pressurised, this increases its hold-off voltage.

Operation of the gap is very reliable, by adjusting its pressure, can also adjust the hold-off voltage and subsequently, its repetition rate.

Engineering And Assembly

On 13th May 2021, I received the cut aluminium base plate. Over the course of the next two weeks, the work consisted of marking out, drilling and tapping of holes in the base plate, acrylic work for the channel enclosure, drilling out of the diode assembly for the mirror, and assembly of the components.

For the capacitors, the Dumper capacitor consists of two Murata N4700, 40kV 1300pF doorknob capacitors for a total of 2600pF.

The peaking capacitor is constructed using 10um thick aluminium foil of 210mm x 60mm, and a sheet of 160um thick Polypropylene for the dielectric. The measured capacitance of 1368pF.

Laser Assembly

Base Plate & Channel (underside)

Laser Assembly

Base Plate, Dumper & Channel

The laser was finally completed on 29th May 2021, and ready for initial testing.

For the high-reflector, I have purchased an Enhanced UV aluminium mirror through our Local Thor Labs supplier, Lastek. The mirror would be installed later following several tests to become familiar with the laser's performance.

The connection of the dumping capacitors and spark gap was constructed with a brass buss bar and 0.75mm thick copper sheet. This assembly was also pre-stressed so when screwed together would apply an even downward clamping force on the spark gap and the plate of the peaking capacitor.

Nitrogen gas feed to the laser is supplied by a Nitrogen bottle. The Spark gap is pressurised to around 1.5 to 2 bar and the channel is flooded with Nitrogen via the needle valve.

Completed Laser

Completed Nitrogen Laser

First Test Fire

To power the laser, I used a commercial Nitrogen Laser power supply that I had on hand (more on this at the end).

For the first few runs, I did not have a bleed resistor installed to discharge the peaking capacitor between shots, later I found that once I installed the resistor, this eliminated the hot arcs visible in the channel, resulting in a much more uniform glow discharge.

First Test Fire of Laser

First Test Fire of Laser

On the 7th June 2021, I installed the 120k Ohm resistor and prepared to conduct some experiments with some Coumarin 1 (7-Diethylamino-4-methylcoumarin) Dye in Ethanol in a 10mm path length quartz cuvette.

This first experiment would become the first time that I have successfully pumped a dye laser using the 337.1nm excitation from the Nitrogen Laser.

I was able to achieve successful laser output from the dye cell without a resonant cavity of approximately 440nm.

N2 Pumping of Coumarin 1 Dye

N2 Pumping of Coumarin 1 Dye

N2 Pumping of Coumarin 1 Dye

N2 Pumping of Coumarin 1 Dye

Dedicated Power Supply

As part of the project, a dedicated power supply will be completed to run the laser.

The power supply consists of a ZVS driver, Flyback transformer and a voltage multiplier. The multiplier board also has 6.6M Ohm series resistance to limit the current.

At the time of writing, the power supply is in the process of being housed.

The completed supply will feature an analogue voltage meter, the output adjustable up to 30kV.

Laser Power Supply

Laser Power Supply

Multiplier Board

Multiplier Board

Update: 20th July, 2021:

The power supply was completed on 1st July, 2021 with a further addition of an AC output to the front panel and kapton tape around multiplier board.

For more details on the power supply build, please check out this project in the High Voltage section.

Mirror Install And Dye Laser Experiment

One of the main motivating goals for building the Nitrogen laser is to experiment with pumping of a dye laser.

Throughout the time of researching and building the laser, I have placed orders for some Laser dyes, including 7-Hydroxy-4-methylcoumarin (Coumarin 4, 456), 7-Diethylamino-4-methylcoumarin; (Coumarin 1, 47, 460) and Rhodamine 6G. I also ordered a quantity of HPLC grade Ethanol as the solvent.

Some references I have come across have referred to the use of highlighter inks as a fluorescent medium.

Having available a Quartz cuvette, I decided to look at testing the theory of possibly thresholding the dye used in a yellow highlighter.

On 22nd June 2021, I commenced the experiment by suspending yellow highlighter dye in distilled water.

To bring the quantity to correct concentration, I fired the Nitrogen laser and observed the absorption of the laser beam in the dye to the point the laser was absorbed within the first 2mm from the quartz window.

At this point, the cell was observed to fluoresce vibrantly with yellow/green light, however, I was not able to achieve threshold and observed output as with the Coumarin 1 dye.

The test step was testing the cell in my LSI dye laser module which I had realigned the previous week with the Coumarin dye.

Adjusting the micrometer, I was able to bring the dye cell to threshold and produce a strong green output.

In this first experiment, I had not yet installed the Enhanced UV HR mirror in the Nitrogen laser.

Dye Laser Output

Dye Laser Ouput (No HR Mirror)

A final task to complete on the laser was the installation of the Enhanced UV aluminium mirror within the mirror mount of the Nitrogen laser, and complete final alignment.

Immediately following alignment, a significant increase in the observed output from the Nitrogen Laser.

Conducting the above experiment with the Dye Laser, A significant increase in output also observed from the Dye Laser.

The wavelength of the dye laser checked optically with a spectroscope against a reference 532nm laser, being between 500-520nm.

Dye Laser Output

Dye Laser Ouput (Mirror Installed)

Dye Lasers

In Late July 2021, I dusted off a spectrometer module that I have and undertook re-alignment of the optical bench and calibration of spectromer.

For details on the Spectromer: B&WTek BTC110-S Spectrometer.

Further study on Dye Lasers, please check out the next section on Dye Lasers.

Discussion Point

N2 Pumping of Coumarin 1 Dye

A topic that seems to come up is the correct term for the operation of a Laser without a resonant cavity, that is, there is enough gain within the medium to produce significant output.

In most texts and papers, the term used to describe the observation of laser operation without a resonant cavity is "Superradiance", however, at the quantum level, this may not be accurate.

The more accurate term may be "Superfluorescence".

Let's explore this a little further;

Superfluorescence is similar to Superradiance, however there is a small difference at the quantum level, superradiance exhibits macroscopic dipole moment.

First described by Robert H. Dicke in 1954; Superradiance is defined as: "Collective emission of an ensemble of atoms or ions after coherent excitation".

Superfluorescence is defined as: "Collective emission of radiation by an ensemble of excited atoms or ions".

RP Photonoics: Superradiance
RP Photonoics: Superfluorescence

Commecial Nitrogen lasers

Throughout the process, I also invested in acquiring two commercial Nitrogen lasers.

The first purchase was for a Laser Science Inc. VSL-337960 with 337121 Dye Laser Module. The purchased was made for the Dye Laser Module.

One interesting aspect of this model is that internal to the larger encloser, the laser is the portable 12V VSL-337000 which uses the smaller, less common 337900 plasma cartridge. The laser supply does work, and the plasma cartridge does fire, however output is no longer evident due to its age.

The second laser is the larger LSI VSL-337-ND utilising the more common 337290 Plasma. This laser does still produce an output.

VSL-377 Laser

VSL-377 Laser

- Flavio Spedalieri -
Written: 25th June 2021

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