Nitrogen laser

Nitrogen laser, a gas laser can only operate in pulse mode. The excitation of the nitrogen gas carried out by a high-voltage discharge transversely to the direction of propagation of the laser beam. Noteworthy on the nitrogen laser is its simple design without laser resonator ("super light" ) and the possible operation with atmospheric air, so it can be built with simple means by hobbyists.

Typical data:

• Gas pressure: a few millibars to several bar
• Energy: microjoules to millijoule.
• Power: peak power few kilowatts to a megawatt average power few milliwatts.
• Wavelength: 337.1 nm strongest line at (ultraviolet).
• Pulse duration: a few hundred picoseconds to a few tens of nanoseconds.

Operation

Nitrogen (as well as hydrogen and neon ) can be brought by a very short (around 1-10 ns) and intense electric gas discharge for lasers. The resonator is useless for this laser, since the residence time of electrons in the upper laser level of the nitrogen for the " pump " is shorter than the duration of the light from one mirror to the other. The reflected light would only come back when the population inversion after the excitation has ended. But then there are no high-energy atoms longer there which can be energetically " milked ". In this case, the intensity of light is increased per millimeter ( the gain ), but is sufficiently large that there is a spontaneous creation laser. If laser operation takes place without a resonator is required, it is called a super radiator.

Laser operation of nitrogen bar is possible even at normal pressure of 1. Such lasers are called TEA laser ( transverse electric excited atmospheric pressure laser), they are also available for CO2 as a laser medium for the wavelength of 10.6 microns.

The lower the pressure of nitrogen, the rare collide atoms and the longer the lifetime of the upper laser level, the requirements on the intensity and brevity, the pump discharge are then lower.

The measures necessary for excitation of short and intense electric pulses, as, inter alia, Satyendra Nath Bose found out are generated by spark gaps and a Blümleingenerator. Suitable for this purpose switching spark gaps have to work very quickly and therefore run partially in inert gas under high pressure. The circuit essentially consists of a plane parallel to a strip conductor formed as a switching spark gap, the capacitor 1 and the laser discharge path, to which a further capacitor is 2. The first voltage is on capacitor C 1 and the spark gap of the spark gap breaks down and the voltage is in the following briefly to the laser discharge gap. This breaks through immediately, a current flows through the circuit and both capacitors discharge.

Construction with easy traveling wave

The form of the capacitor plates results from the traveling wave to be generated:

As a nitrogen laser usually is longer than the distance traveled by the light within 1 ns, one has to use an electric strip conductor, to direct the pulse of the spark gap to the side electrode of the nitrogen discharge. Especially with long nitrogen lasers, the discharge gap during the discharge of very low resistance ( R <10 ohms), so that a corresponding adaptations of the stripline must be very flat and wide to high inductances between spark discharge and stripline be avoided by the spark gap is placed directly in the strip conductor ( shown in the figure as a ring ). The electric pulse front obliquely strikes the electrodes of the discharge of nitrogen to make the entire length of the laser pulse passing through each of the right time.

The spark gap is s powered by a separate high voltage source within 0.1-10 (depending on power ) together with the strip line structure, only to be discharged within 1 ns by spontaneous ignition of the spark gap. The electrodes of the nitrogen discharge is only during this short pulse, leading to the discharge voltage - only by the ions of the nitrogen discharge can reach the necessary inversion for lasing, since the upper laser level is emptied very quickly. Also, the discharge becomes homogeneous - she has no time to constrict to individual channels.

Prolonged discharge ( "removed" is less than about a light - nanosecond ) through a capacitor to the second wall of the waveguide near the spark-gap prevents He also discharges and the voltage difference between the electrodes is zero. This capacitor also be said of the short pulses as a stripline. In order to lose as little energy in this, its impedance must be as poorly adapted to the actual waveguide, that is, it must still be significantly lower resistance. To refine by about 1 ns to delete the radio link again, its capacity must be less than 10 nF be ().

Construction with full traveling wave for large laser

The waveguide structure shown in the images and thus prevents radiation power loss or interference with other devices. Only a single high-impedance, high-induction pipe leads from the rear of the condenser to the outside. This interacts with the 10 nF capacitor than 10 Hz low- pass, protects the power supply against short circuit and usually needs to be cooled.

Wherein an internal pressure differing from atmospheric pressure is needed lateral window for coupling out the laser pulse. With suitable design is given almost in one direction only laser radiation.

In the drawing shown in red, the electrodes above and below the laser channel. The spark gap is shown in blue.

The concave end surfaces of the dielectric around the laser channel around care for so-called pre-discharges and thus to the preionization of the laser channel by their UV emission. Thus, a homogeneous discharge is achieved.

A homogeneous discharge

You need a corona discharge and no spark discharge. This is hampered by the short pulse time (the discharge has no time to constrict to spark) and reaches the other by a preionization of the discharge channel. The pre-ionization is often achieved by the separate weak discharge ( predischarges or specially produced discharges ), by their emission of ultraviolet - ionize the gas in the laser channel.

Applications

Nitrogen lasers have mainly scientific importance. A long time they were the only available laser in the ultraviolet. The short, intense laser pulses are used, inter alia, for the pumping of dye lasers, for the study of fluorescent dyes.