Professor Mark Csele's Homebuilt Lasers Page

Neutral Neon (I) Lasers

Neon Laser Output

Not an average HeNe laser, the laser above is a helium-neon laser tube specially configured in this experiment to produce output at 614.3nm (this is not the usual orange HeNe output at 612nm either).


A helium-neon laser tube is modified for use with pure neon gas in order to lase a number of neutral-neon superradiant transitions. A pulsed laser, energy is supplied in the form of a fast discharge from several low-inductance ceramic capacitors. Although the green transition at 540.1nm was expected, an orange transition at 614.3nm was dominant. Highly asymmetric output was seen with laser output observed only from the 'live' end of the plasma tube. Optimal output was observed at a neon pressure of 0.12 torr.


It is well known that low pressure nitrogen gas excited by a massive electrical current in a travelling wave discharge [1][2] produces superradiant emission at 337.1nm. Neutral neon gas has a similar self-terminating transition at several wavelengths including 540.1nm in the green which can be excited in a similar manner [3][4]. Nitrogen can also be excited in a longitudinal tube arrangement [5] in which small-diameter plasma tubes are excited with a fast electrical discharge producing over 1kW.

It has been known for some time [6] that neutral neon has several high-gain transitions at 540.1nm, 594.5nm, and 614.3nm when a long, thin capillary filled with the gas is driven by a high-voltage discharge with pulses of 30kV. Low pressures (0.3 torr) favour the orange and yellow transitions while higher pressures (3 torr) favour the green transition. Other researchers [7][8][9][17] describe similar success in lasing these transitions

Normal HeNe Levels Consider the Helium-Neon laser for which the energy level diagram is seen to the left. In this laser neon is elevated to the upper-lasing level (denoted 1s22s22p55s1) via pumping from energetic helium atoms. From the upper levels (a cluster of four of them) neon decays to one of ten lower levels (denoted 1s22s22p53p1) producing one of the transitions shown here in colour. The lower levels have shorter lifetimes than the upper levels so population inversion is ensured. Lower levels depopulate to a metastable state by spontaneous emissions of approximately 600nm and finally collision with tube walls returns neon atoms to the ground state of 1s22s22p6.

Pure Neon Levels This particular laser operates on transitions between the lower-lasing levels of the helium-neon laser and the metastable state in the system. In the helium-neon system these transitions serve to rapidly depopulate the lower-lasing levels however here they become the upper-lasing levels.
The energy-level system, as described, is not conducive to CW laser action since the lifetime of the upper-lasing levels is much shorter than that of the metastable state which serves as the lower-lasing level [14]. In this case, like that of the nitrogen laser, the mechansm of the laser must pump the upper-level quickly to ensure an inversion exists. Shortly thereafter, laser action may ensue but the transition will be self terminating and lasing action will cease.

Without helium in the system electron collisions are necessary to pump neon atoms directly to the upper-lasing levels. The presence of helium would hinder, not help, laser action since it would pump the incorrect energy levels in neon for the desired transitions.
Upper-lasing levels are identified as 'Paschen 2p levels' on the diagram. Paschen notation renames the first excited energy level as '1s' regardless of the actual designation of the level. In this case the first excited state is 2s22p53s1 becomes '1s'.

Each set of levels ('2p' and '1s') consist of clusters of multiple individual levels. In Paschen notation these are labelled consecutively. When in the 2p level (with electron configuration 1s22s22p53p1), the angular momentum and spin of the neon atom (with five valence electrons still in the core) combine to form ten allowed energy levels denoted 2p1 through 2p10. In the 2s level (with electron configuration 1s22s22p53s1), four discrete energy states result labelled 3s2 through 3s5.

The '2p' levels are the same as those which serve as the lower-lasing levels (LLLs) of the 'normal' HeNe CW transitions: the 2p1 level serving as the LLL for the 730nm IR transition, the 2p4 level for the 632.8nm red transition, and the 2p6 level for the 612nm orange transition. In this particular experiment, the transitions of interest here are the 2p1 to 1s4 transition at 540.1nm, the 2p4 to 1s5 transition at 594.4nm, and the 2p6 to 1s5 transition at 614.3nm.

Neon Transitions from NIST (ref 11) While numerous transitions would apparently be possible (fourty, in all), not all are probable since the quantum mechanics rules dictate that certain changes in J (the L-S coupling term) are necessary for 'allowed' transitions [10]. In the case of the 540.1nm transition the actual transition is from a level designated 2p5(2P1/2)3p (with J=0) to a level designated 2p5(2P3/2)3s (with J=1) [11]. The change DJ = 1 is an allowed change. Similarly, the 614.3nm transition results from a transition between a state with J=2 to another state with J=2 - again, an allowed change. These levels are outlined to the left in an excerpt from [11] outlining the energy levels and J terms for each.

Experimental Setup

In one of the original reports of superradiant transitions [8], Heard mentions that two dielectric mirrors with peak reflectivity at 632.8nm were used in a plasma tube 40cm long and 1mm in diameter. Examining the use of a helium-neon laser tube for repumping experiments [12], which features a port allowing attachment to a turbomolecular pumping system, it is evident that such a tube should serve well to lase these transitions.

Neon Laser PCB
Figure 1: Experimental Setup

The entire laser was constructed on a piece of single-sided FR4 printed-circuit board approximately 30cm by 20cm in size as pictured in figure 1. Capacitors are mounted to the board via copper straps soldered to the board and the plasma tube itself is mounted on the board via two fuse clips. The spark gap consists of two solid brass ornamental nuts with smooth, rounded, ends approximately 5mm apart. Traces on the board are deisnged to be as wide as possible and the layout minimizes inductance of the discharge path.

Neon Laser Circuit
Figure 2: Circuit

Figure 2 details the circuit used for the laser. Three 460pF/30kV 'doorknob' capacitors in parallel are charged from a small neon sign transformer controlled with a variac via a voltage tripler through a 50K 10W wirewound resistor. When the capacitors charge to a suitable voltage the spark gap discharges through a peaking capacitor (with a function described in [5]) and through the plasma tube producing a fast discharge.

A Melles-Griot 05-LHR-099 laser tube was used in this experiment - this tube has the OC mounted on the anode end of the tube and is designed for CW red (632.8nm) emission. Optically, HeNe tube mirrors are peaked for reflection in the red however both mirrors have an estimated transmissivity of >50% at 540nm and >30% at 600nm. In this respect they should not impede the output of superradiant emission at any of the lines described and may, in fact, help improve the quality of the output beam.

It is hypothesized that the laser can be made to operate optimally in the green at 540.1nm and quite possibly in the orange at 614.3nm.


With the system configured as above the tube was evacuated to a pressure of better than 10-6 torr using a turbomolecular pump [13]. The tube was then filled with research grade neon to a pressure of 10 torr and the pressure gradually reduced as the laser was operated. Orange superradiant output was observed at pressures between 1.6 torr and 0.05 torr but the optimal pressure was found to be 0.116 torr. Various peaking capacitors were tried including no capacitor, 25pF, and 100pF. Weak output was observed with no peaking capacitor as well as when a 100pF capacitor was installed however optimal output occurred with the 25pF capacitor installed as per figure 2.

Output was observed from the 'hot' end of the tube only and never from the grounded end as per reference [6] (this is the OC end on the original tube in which the OC was located at the anode). At various pressures the output was analyzed with a grating spectroscope and found to be entirely monochromatic consisting of the 614.3nm line only - the expected green output at 540.1nm was never observed in this configuration.

Pure neon gas produced the best results and dilution with gases such as argon or helium decreased the power output until lasing ceased. With 25% of helium in the mixture the laser was found to cease operating entirely. Incidentally, with a helium-neon mixture the red line at 632.8nm was never observed.

At the low pressures involved the discharge in the plasma tube appears violet as per figure 3. It was also observed that repeated usage of the tube in this manner destroyed the OC optic, with the delicate dielectric coating visibly damaged. While this has apparently not impared 614.3nm output, the tube is now incapable of oscillating at 632nm when filled with an appropriate helium-neon gas mixture.

Neon Laser Single Shot
Figure 3: Plasma Tube Observations


Reliable output at 614.3nm was observed however the expected output on the green line was not observed. No attempt was made to measure the discharge current nor the time frame of the excitation pulse. The experiment (as well as others in the literature) seem to suggest that a faster discharge is required in order to favour the green 540.1nm transition - many successful experiments utilize segmented electrodes with very small capacitances for an extremely fast discharge which appears to be required.

A subsequent search of available literature [15][16] details that the 614.3nm line is favoured by shorter (30cm long, 3mm diameter bore) tubes operated at low pressures (approx. 1 torr), while the 540.1 nm line is favoured in longer tubes (100cm long, 3mm diameter bore) tubes operated at higher pressures (approx. 7 torr). With longer tubes, a pulse transformer or Marx-bank generator is required to supply the extraordinarily high voltages required (up to 200kV).


[1] An Unusual kind of gas laser that puts out pulses in the ultraviolet
Scientific American, Amateur Scientist Column, June 1974
A simple nitrogen laser easily constructed by an amateur. Based on James Small's design in Review Of Scientific Instruments.

[2] Travelling wave excitation of high power gas lasers
John Shipman, jr.
Applied Physics Letters, Vol. 10, No. 1, 1 Jan 1967
A long laser of Blumlein design. Includes data on using neon gas as a lasant.

[3]The 5401A pulsed neon laser
IEEE Journal of Quantum Electronics, March 1967, p.134
The use of neon in a crosse-field laser with multiple electrodes

[4]Observation of a super-radiant self-terminating green laser transition in neon
Leonard, et al
Applied Physics Letters, 15 Sept 1965, Vol 7, No. 6, p.175

[5]The poor man's nitrogen laser David Phillips and John West
American Journal Of Physics, Vol 38, No. 5, May 1970
A small, simple N2 laser which does not use a Blumlein design but rather a capillary tube.

[6]Asymmetric visible super-radiant emission from a pulsed neon discharge
D.M. Clunie et al, Physics Letters, Vol 14 No 1, Jan 1965
Output at 594.4nm and 614.3nm observed on 2P4-1S5 and 2P6-1S5 transitions

[7]Superradiant transitions in argon, krypton, and xenon
K.G. Erics, et al
IEEE Journal of Quantum Electronics, 1967, pp. 94

[8]Super-Radiant Yellow And Orange Laser Transitions in Pure Neon, H.G. Heard, Proceedings Of The IEEE, Oct. 1964, pp. 1258
Using a 40cm tube of 1mm bore filled with 0.2torr or Neon laser action is observed at 611.8nm and 594.0nm. Performance improves when dielectric mirrors centered at 632.8nm are included

[9]Laser-Ubergange und Superstrahlung bei 6143A und 5944A in einer Gepulsten Neon-Entladung, D. Rosenberger, Physics Letters, Vol 13 No 3, Dec 1964
Output at 594.4nm and 614.3nm observed in a tube 20cm long by 1mm dia. filled with 0.4 torr of pure neon. Entire article is in German.

[10]Section 3.12 in Fundamentals of Light Sources and Lasers (ISBN 0-471-47660-9), John Wiley & Sons, 2004

[11]Handbook of Basic Atomic Spectroscopic Data
Jean E. Sansonetti and W. C. Martin
NIST eBook

[12]Helium-Neon Lasers Page including repumping experiments.

[13]He-Ne reprocessing lab describing the vacuum system.

[14]Section 5.3 in Fundamentals of Light Sources and Lasers (ISBN 0-471-47660-9), John Wiley & Sons, 2004

[15]Shape and duration of superradiance pulses corresponding to neon lines, A.A. Isaev, et al
Soviet Journal of Quantum Electronics, Vol 2 No 1, July-Aug 1972

[16]High-power nitrogen and neon pulsed gas lasers, I.I. Magda, et al
Soviet Journal of Quantum Electronics, Vol 3 No 3, Nov-Dec 1973

[17]Pulsed Neon Laser with 350-psec Pulse Duration and Subnanosecond Jitter at 6143A, T.J. Davies and M.A. Nelson
Applied Optics, Vol 12, No 4, April 1973