Nitrogen laser

The nitrogen laser is a 3-level gas laser that can only work in pulse mode . The nitrogen gas is excited by a high voltage discharge perpendicular to the direction of propagation of the laser beam. What is remarkable about the nitrogen laser is its simple construction without a laser resonator (" super emitter ") and the possibility of operating it with atmospheric air , so that it can also be built by hobbyists with simple means.

Construction of an N ₂ laser; on the left the spark gap made of two cap nuts

Typical data:

Gas pressure : a few milli bar to several bar.
Energy : micro joules to millijoules.
Output : peak output from a few kilowatts to one megawatt, average output a few milliwatts.
Wavelength : strongest line at 337.1 nm ( ultraviolet ).
Pulse duration: a few hundred pico seconds to a few tens of nanoseconds.

Mode of action

Nitrogen (like hydrogen and neon) can be made to laser with a very brief (around 1–10 ns) and intense electrical gas discharge. With this laser, the resonator is worthless because the time the electron stays in the upper laser level of the nitrogen after “pumping” is shorter than the transit time of the light from one mirror to the other. The reflected light would only come back when the population inversion has already ended after the excitation. But then there are no more energy-rich atoms that can be energetically "milked". In this case, however, the increase in the intensity of the light per millimeter (the amplification) is sufficiently large for the laser to develop spontaneously. The N ₂ laser thus has an extremely high radiation gain. When laser operation takes place without the need for a resonator, it is called a super emitter.

The service life of the upper laser level ( spectroscopic notation :) is several orders of magnitude shorter than that of the lower laser level ( ) with . This prevents cw operation ( continuous wave ) and limits the N ₂ laser to pulse operation, since the upper laser level empties faster than the lower level . So the initial population inversion is destroyed by filling in. This interruption of the laser activity is called "self-termination" in the English-language literature. ${\ displaystyle \ tau _ {C}}$ ${\ displaystyle C ~ ^ {3} \ Pi _ {u}}$ ${\ displaystyle 40 ~ ns}$ ${\ displaystyle B ~ ^ {3} \ Pi _ {g}}$ ${\ displaystyle \ tau _ {B} = 10 ~ ms}$ ${\ displaystyle C}$ ${\ displaystyle B}$ ${\ displaystyle B}$

Laser operation of nitrogen is also possible at normal pressure of 1 bar. Such lasers are called TEA lasers (transversely electrically excited atmospheric pressure lasers); they are also available for CO ₂ as a laser medium for a wavelength of 10.6 µm.

The lower the nitrogen pressure, the less often atoms collide and the longer the service life of the upper laser level; the demands on the intensity and brevity of the pump discharge are then lower.

The short and intense electrical pulses required for excitation can, as u. a. Satyendra Nath Bose found out, can be generated by spark gaps and a Blümlein generator. Switching spark gaps suitable for this must work particularly quickly and therefore run partially in noble gas and under high pressure. The circuit consists essentially of a switching spark gap, which is parallel to a capacitor 1 designed as a strip conductor, and the laser discharge gap, on which a further capacitor 2 is located. The voltage is first applied to capacitor 1 and the spark gap, the spark gap breaks down and the voltage is then applied to the laser discharge path for a short time. This breaks down immediately, a current flows through the circuit and both capacitors discharge.

Structure with a simple moving field

Conductor structures of a nitrogen laser with a simple traveling field, top view

The shape of the capacitor plates results from the traveling field to be generated:

Since a nitrogen laser is usually longer than the distance that light travels within 1 ns, you have to use an electrical strip conductor to guide the pulse from the spark gap to the side of the nitrogen discharge electrodes. In the case of long nitrogen lasers in particular, the discharge path has a very low resistance during the discharge (R <10 Ohm), so that a strip line adapted to it must be very flat and wide. Excessive inductance between the spark discharge and the strip conductor is avoided by accommodating the spark gap directly in the strip conductor (shown as a ring in the picture). The electrical pulse front hits the electrodes of the nitrogen discharge at an angle in order to hit the laser pulse at the right time over the entire length.

Structure of a nitrogen laser with a simple traveling field, cross section

The spark gap is fed with a separate high voltage source within 0.1-10 s (depending on the power) together with the stripline structure, in order then to discharge within 1 ns through spontaneous ignition of the spark gap. The electrodes of the nitrogen discharge only receive the voltage leading to the discharge during this short pulse - this is the only way for the ions of the nitrogen discharge to achieve the inversion required for lasing, because the upper laser level empties very quickly. This also makes the discharge more homogeneous - it has no time to constrict into individual channels.

A longer discharge is prevented by a capacitor on the second wall of the waveguide near (less than about a light nanosecond "away") the spark gap: It also discharges and the voltage difference between the electrodes becomes zero. This capacitor also behaves like a stripline for the short pulses. In order to lose as little energy as possible in this, its impedance must be matched as poorly as possible to the actual waveguide, i. i.e. it has to be significantly lower resistance. In order to extinguish the spark gap after approx. 1 ns, its capacitance must be less than 10 nF ( ). ${\ displaystyle R \ cdot C = t}$

Construction with complete traveling field for large lasers

Structure of a nitrogen laser with complete traveling field, top view

Dynamics. Left: spark gap, right: laser spark gap. Blue = U = voltage, black = I = current, red = ions = ion concentration

The waveguide structure shown in the pictures prevents radiation and thus loss of power or interference with other devices. Only a single high-resistance, highly inductive line leads from the rear of the capacitor to the outside. Together with the 10 nF capacitor, this acts as a 10 Hz low pass , protects the power supply unit from short circuits and usually has to be cooled.

Completely embedded capacitor plates for a structure with a complete traveling field, cross-section

If the internal pressure deviates from normal pressure, side windows are required to decouple the laser pulse. With a suitable construction, laser radiation is emitted in almost only one direction.

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

The concave end surfaces of the dielectric around the laser channel ensure what are known as pre-discharges and thus pre- ionization of the laser channel through its UV emission. This results in a more homogeneous discharge.

Achieving a homogeneous discharge

A corona discharge is required, not a spark discharge . This is achieved on the one hand by the short pulse time (the discharge does not have time to constrict to form sparks) and on the other hand by pre-ionizing the discharge channel. Pre-ionization is often achieved by separate weak discharges (pre-discharges or specially generated discharges) which ionize the gas in the laser channel through their ultraviolet emission.

Applications

Nitrogen lasers are mainly of scientific importance. For a long time they were the only available lasers in the ultraviolet . The short, intense laser pulses are u. a. used for pumping dye lasers , for studying fluorescent dyes .

Individual evidence

↑ Markus Werner Sigrist: Laser: Theory, Types and Applications . 8th edition. Springer Spectrum, Berlin 2018, ISBN 978-3-662-57514-7 , p. 244-248 .
↑ ^a ^b Hans Joachim Eichler, Jürgen Eichler: Laser - Structures, Beam Guidance, Applications . 8th edition. Springer, Berlin, Heidelberg 2015, ISBN 978-3-642-41437-4 , pp. 115-118 .
↑ J. Michael Hollas: Modern Spectroscopy . 4th edition. Wiley, Chichester 2004, ISBN 0-470-84416-7 , pp. 355-356 .

Web links

Wiktionary: nitrogen laser - explanations of meanings, word origins, synonyms, translations

[Sigrist-1] Markus Werner Sigrist: Laser: Theory, Types and Applications . 8th edition. Springer Spectrum, Berlin 2018, ISBN 978-3-662-57514-7 , p. 244-248 .

[Eichler-2] Hans Joachim Eichler, Jürgen Eichler: Laser - Structures, Beam Guidance, Applications . 8th edition. Springer, Berlin, Heidelberg 2015, ISBN 978-3-642-41437-4 , pp. 115-118 .

[Hollas-3] J. Michael Hollas: Modern Spectroscopy . 4th edition. Wiley, Chichester 2004, ISBN 0-470-84416-7 , pp. 355-356 .