Dangerous Project
Please be advised this laser is a very
dangerous device. The power output of this laser will BLIND YOU!! Be extremely
careful! The light output is in the UV range, which means it is invisible to
the human eye. The UV light can also cause burns on the skin similar to the
burns from long exposure to intense sunlight. The hazards of high power lasers
can not be overemphasized. Use the laser in a closed environment and wear
protective gear and eye protection when in use.
Construction Article by Peter Schenck and Harold Metcalf,
"Low Cost Nitrogen Laser for Dye Laser Pumping", Applied Optics, v12
n2,
p183, Feb 1973. (thanks to Jon Singer for providing the reference!)
This article is for more advance
experimenters and caution should be taken dealing with use of high voltage.
Also the dyes specified in this article are highly toxic and should be used in
well ventilated areas. Avoid prolonged exposure to them.
A pulsed cross-field molecular nitrogen
laser (3371-a) has been simply constructed from two plate glass strips 5 cm by
1.2 m. During operation, twenty 500-pf capacitors evenly spaced along the
channel are charged from a 0.012 ufd storage capacitor by a thyratron at up to
30 hz. This distributed charge is available for discharge when the nitrogen gas
breakdown starts.
The flowing gas laser produces a 160-kw, 10
nano-second (full width half maximum) light pulse when operated at 15 kv and at
a pressure of 20 Torr. The laser has successfully pumped a tunable dye laser.
Introduction
A pulsed molecular nitrogen laser has been
constructed at relatively low cost. The design incorporates a bandsaw blade as
a multiple electrode structure to ensure an even transverse discharge in
flowing nitrogen and produces superradiant emission at 3371-a.
The nitrogen laser was constructed as a pump
for a tunable dye laser for use in spectroscopy. Other workers have built and
studied the performance and theory of both longitudinal and crossfield nitrogen
lasers.
General Description
The laser channel consists basically of two
plates of glass supported on an aluminum base, which serves both for support
and electrical ground. The high voltage edge of the laser channel consists of a
sandwich of a copper bus bar, bandsaw blade electrode and copper spacer.
The copper bus bar serves both for distribution
of the storage capacitors charge to the dumping capacitors and as mechanical
support for the dumping capacitors.
The principle of operation of the pulsed
molecular nitrogen laser is illustrated in Figure 2. Twenty dumping capacitors
are mounted in parallel along the laser channel. These are connected by the bus
bar to a low-inductance storage capacitor.
The storage capacitor is charged to 10-15 kV
through a current limiting resistor. When the thyratron switch closes,
grounding the positive side of the floating storage capacitor, the other side
is established at a high negative voltage. Its charge, therefore, flows on the
bus bar to the dumping capacitors, which provide the energy for the discharge
in the nitrogen.
This results in direct electron impact
excitation of the triplet states of molecular nitrogen’s second positive band
and superradiant lasing at 3371-a.
Mechanical Construction of Laser Channel
The base of the laser channel is a 1.25-m
length of aluminum stock 1.27 cm by 5 cm fig 1. Two parallel grooves are milled
the length of the bar to accommodate the plate glass strips that form the walls
of the laser channel.
We used double strength tempered plate glass
6 mm thick. The 7.0 mm by 7.0 mm grooves uses General Electric silicon seal RTV.
The silicon sealant alleviates mechanical problems because it is easily applied
and stays flexible when cured. Good vacuum is not important because the
operating pressure of the flowing nitrogen in the laser is 10-20 Torr, so this
material also provides adequate vacuum seal.
The dumping capacitors are mounted along the
laser channel in tapped holes spaced 6 cm apart. Two 3 mm holes centered in the
ridge are drilled 8 cm from each end of the base to admit and withdraw
nitrogen.
The bandsaw blade is about 1 mm thick, 15 mm
wide, with very little set and teeth about every 2 mm. It was cut just short of
the length of the glass walls. Electrical connection to the dumping capacitors
is through a bus bar made from 1-mm (# 16 B&S) copper stock.
The blade is sandwiched between this and
another bent piece of copper to complete the top of the channel. The three
pieces were glued in place with the silicon seal as indicated in Figure 1. The
blade is positioned so that the teeth are just below the copper spacer and bus
bar. They are about 3.81 cm from the aluminum base.
The ends of the channel are sealed with
optical grade quartz windows 3.17 mm thick. They are attached to the ends of
the glass walled channel with silicon seal.
The laser channel was mounted on a section
of 12.7 cm wide aluminum support channel. This provides support for leveling
feet, electrical components, and an adjustable front silvered mirror at one end
that used to increase the output power and decrease the divergence of the
output beam.
The nitrogen flow system uses a standard
fore-pump to pump dry tank nitrogen through the laser channel. The pressure is
monitored by a thermo-couple gauge and regulated by the needle valve in the
input line.
Electrical Construction of the Laser
The circuitry of the pulsed molecular
nitrogen laser is shown in Figure 3.
The circuit incorporates a 2N4852
unijunction oscillator and a plastic thyristor 2N4443 to fire the thyratron
through a trigger transformer. The thyratron is a hydrogen filled Amperex type
6279/5C22 that will switch the high voltage when provided with a 100 volt pulse
to the grid. To prevent high voltage grid flashback, which will destroy the
triggering circuitry, a negative voltage must be applied to the grid to shut
off the thyratron. Therefore, a pi-filter is provided between the grid and
triggering transformer so that the back voltage produced by its reactance turns
off the thyratron. As an added protection, a buffer amplifier and thyrector
protect the unijunction and thyristor.
The storage capacitor has low inductance,
since the transfer of charge must be accomplished early in the breakdown of the
nitrogen. The storage capacitor is mounted on the bus bar for both mechanical
support and electrical connection to the dumping capacitors.
The bulk of the circuitry is mounted in a
box near the laser channel, but the current limiting resistor is mounted on the
aluminum support channel. The thyratron was mounted in the box in our earlier
design; however, in our current design the thyratron is mounted on top of the
support channel. This minimizes the path to ground and results in increased
power output and increased efficiency.
The high voltage comes from an unregulated
15-kv supply. The laser requires about 5 ma when operated at 30 Hz with the
above circuitry. The oscillator is limited to 30 Hz in our design by the power
dissipation capability of the thyratron.
The laser channel is un-cooled. The current
limiting resistor in the circuit is chosen to limit the current of the high
voltage supply during its grounding by the thyratron. The charging and
discharging of the dumping capacitors has been observed with a Tektronix 6015
fast high voltage probe.
The small capacitors charge for about 100
nsec from the low inductance storage capacitor. Then these dumping capacitors
discharge in less than 20 nsec, from 100 percent to 10 percent, a time
comparable to the lifetime of the lasing levels of the second positive band in
molecular nitrogen.
The use of the bandsaw blade as a multiple
electrode structure resulted in a very uniform discharge in the nitrogen in the
laser cavity over a pressure range of 10-20 Torr. The pulse to pulse uniformity
of the laser is better than 1 percent.
At the 10-kv threshold of the laser’s
operation the superradiant laser action occurs with an output power of about 25
kw measured by a suitably attenuated and calibrated silicon photo-diode. At 10
Torr and 13 kv the power output is about 80 kw. Since our best measure of the
pulse duration is about 10 nsec; the energy is 0.8 mj.
The efficiency is 0.08 percent, since the
energy stored in the 0.012 ufd capacitor is about 1 j. At 20 Torr and 15 kv the
power output is about 160 kw increased from 120 kw by mounting the thyratron on
the support channel.
The power decreases by 50 percent if the
first surface mirror is removed or misaligned. The mirror need not be special;
we used a microscope slide with an evaporated aluminum coating. Removal of the
rear mirror increases the beam divergence from 1 mrad to several milliradians
and produces a very inhomogeneous beam.
The wavelength was a composite of lines in a
1-a band at about 3371-a. The laser will operate with air flowing through the
laser channel, but at reduced output power and reduced pulse uniformity.
The reliability of the laser is excellent.
The bandsaw blade electrode structure has survived almost 1000 hours of
operation without visible signs of deterioration. The windows had to be removed
to clean off deposits built up from excessive arcing at the end of the bandsaw
blade. This problem has been cured by inserting Teflon caps on the last 1 cm of
teeth.
The cost of the nitrogen laser is greatly
reduced if a fore-pump, N2 regulator, and a 10-15-kv power supply are
available. The last may readily be constructed from a neon sign transformer.
The cost of the low inductance capacitor is the largest part of the estimated
500.00 dollar cost of the laser, exclusive of the above three items. The rest
of the electronics including the thyratron cost about 250.00 dollars. The cost
may double or triple if the fore-pump, etc., are not available.
The main reason for construction of the
nitrogen laser was to pump a dye laser for a fast, powerful, tunable source for
spectroscopy. Our nitrogen laser has been used to pump a simple dye laser from
the near ultraviolet to 6200-a using PBD, coumarin, sodium fluorescein, and
rhodamine 6G.
Other dyes may be pumped by nitrogen lasers
to fill in the gaps left by the above dyes. We use an intra-cavity 30 times
telescope and an 1800 line/mm grating, so that the dye laser may be tuned with
a width of less than 0.5 a. Narrower lines may be obtained by other means. This
nitrogen laser design has proved more than adequate for pumping dye lasers.
References
1. A good list of reference appears at the
end of the design described by D.T. Phillips and J.
West, Am. J. Phys. 38, 655 (1970).
2. D. Leonard, App. Phys. Lett 7,4 (1965)
3. Motorola application notes numbers AN227
and AN913.
4, A Capacitor Specialists Inc., capacitor,
number 20A017, was used in this design. The damping
capacitors are Sprague 500-pf 20 kv doorknob
capacitors number 20DK-TS.
5. Hipotronics Inc transformer (65 ma at 15
kv).
6. J. Parks, D. Rao, and A. Javan, App.
Phys. Lett. 13, 142 (1968),
7. G. Capelle and D. Phillips, App.. Opt. 9,
2742 (1970).
8. J. Warden and I. Gough, App.. Phys. Lett.
19, 345 (1971).
9. T. Hansch, App.. Opt. 12, 895 (1972).
