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 nitrogenís 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 laserís 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).

 

 

 

 

 

 

 

 

 






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