Helium-neon laser
The helium-neon laser is a gas laser that usually emits red light. It was developed in 1960 by the Iranian physicist Ali Javan together with William R. Bennett and Donald R. Herriott . It was the first laser to generate light in continuous operation . The first laser from Maiman , a ruby laser , which was realized seven months earlier, however, generated pulsed laser light .
Structure and functionality
It consists (also substantially from a thin glass tube, capillary , diameter approximately 1 mm, length some dm), in which a helium - neon - gas mixture is located.
The gas mixture is under a pressure of approx. 100 Pa , with a ratio of the partial pressures of helium / neon of approx. 10/1 (for 1152 nm) or 5/1 (for 633 nm). At the ends there are mostly so-called Brewster windows or often the resonator mirrors directly. The optional Brewster windows are plane-parallel plates that allow light with a certain polarization direction to pass through without losses due to reflection . H. there is only one transmitted, no reflected beam of this polarization direction. Light with a perpendicular polarization is partially reflected. Since a laser always selects the operating state with the lowest losses, the "wrong" polarization is suppressed. In principle, a He-Ne laser with Brewster windows emits linearly polarized light; in the case of a design with resonator mirrors that are in direct contact with the discharge tube, the polarization is arbitrary. In the case of the construction with Brewster windows, this is located between two mirrors arranged outside the tube, which form the resonator (see schematic diagram, the lower glass tube in the second picture).
The voltage supply for the gas discharge must meet the following requirements:
- Provision of ignition voltage at the beginning (10–15 kV)
- Current limitation of the discharge current flowing after ignition
The discharge voltage after ignition is typically 1–2 kV, the current 1–30 mA.
He-Ne laser tubes have a prescribed polarity of the operating voltage: the cathode usually consists of a large, external metal cylinder, while the small anode is located between the capillary and the beam outlet.
With a helium-neon laser, the helium is required for pumping , the neon is the laser medium . There are also two electrodes in the glass tube , between which a gas discharge takes place. This gas discharge brings the helium atoms into a comparatively long-lived (approx. 10 −3 s) excited state. The helium atoms now transfer their energy to the neon atoms through collisions of the second kind , where they generate a population inversion between energetically high states and low states. Laser operation is now possible on transitions between the energetic states of the neon, as shown in the following diagram.
The states and the helium are metastable . The emission of photons in the neon atom takes place through stimulated emission ; the return from the lower laser level to the basic state through spontaneous emission and recombinations on the capillary wall. Due to the latter fact, it does not make sense to choose the diameter of the glass tube larger than 1.5 mm.
The standard helium-neon laser emits light with wavelengths of 632.816 nm (the red laser light, realized in 1963), 1152.3 nm (infrared, realized in 1960) and 3392.2 nm (infrared). The laser levels are split by spin-orbit coupling. The output power of a red helium-neon laser is in the range of a few milliwatts, in rare cases up to around 100 mW.
Possible laser wavelengths of the He-Ne laser
The following (incomplete) table shows typical emission lines of the He-Ne laser based on the indication of the energy transfer. The wavelength is the wavelength in air (not in a vacuum). The description of the energy transfer in the Ne atom was carried out in the so-called Paschen notation. It is an alternative to the Racah notation, which is also used in the literature.
| Wavelength (nm) |
Energy transfer in the Ne atom | Color impression |
|||
|---|---|---|---|---|---|
| 3392.2 | 3s 2 - 3p 4 | (infrared) | |||
| 1523.1 | 2s 2 - 2p 1 | ||||
| 1198.8 | 2s 3 - 2p 2 | ||||
| 1177.0 | 2s 2 - 2p 2 | ||||
| 1161.7 | 2s 3 - 2p 5 | ||||
| 1160.5 | 2s 2 - 2p 3 | ||||
| 1152.3 | 2s 2 - 2p 4 | ||||
| 1141.2 | 2s 2 - 2p 5 | ||||
| 1084.7 | 2s 2 - 2p 6 | ||||
| 1080.1 | 2s 3 - 2p 7 | ||||
| 1062.3 | 2s 2 - 2p 7 | ||||
| 1029.8 | 2s 2 - 2p 8 | ||||
| 886.5 | 2s 2 - 2p 10 | ||||
| 730.5 | 3s 2 - 2p 1 | red | |||
| 640.1 | 3s 2 - 2p 2 | ||||
| 635.2 | 3s 2 - 2p 3 | ||||
| 632,816 | 3s 2 - 2p 4 | ||||
| 629.4 | 3s 2 - 2p 5 | ||||
| 611,802 | 3s 2 - 2p 6 | ||||
| 604,613 | 3s 2 - 2p 7 | ||||
| 593,932 | 3s 2 - 2p 8 | orange | |||
| 543,365 | 3s 2 - 2p 10 | green | |||
coherence
Another distinctive feature of helium-neon lasers is their long coherence length . Even with simple models ( multimode laser ) it is in the range of the resonator length, i.e. usually between 20 cm and 30 cm. The reason is the extremely narrow gain bandwidth of the neon laser transition of around 1.5 GHz, so that only a few longitudinal modes can oscillate. Thermally stabilized frequency-selective resonators of commercially available He-Ne lasers enable a stability of a few megahertz and a corresponding coherence length of more than 100 m. In addition, there are frequency-stabilized helium-neon lasers with a coherence length of several kilometers.
Applications
The comparatively low price and the long service life make the helium-neon laser interesting for many fields of application. It used to be found in the barcode scanners of cash registers or laser printers , for example , but has now been almost completely displaced by the diode laser .
It still plays a major role when there are special requirements for beam quality and coherence , for example in interferometers or when calibrating spectrometers . Helium-neon lasers are also well suited for holography , even if mass production has also switched to more powerful and shorter-wave lasers ( argon-ion laser , helium-cadmium laser ).
swell
- ↑ Biography of Ali Javan (English)
- ^ A. Javan, WR Bennett, DR Herriott: Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing a He-Ne Mixture . In: Physical Review Letters . tape 6 , no. 3 , February 1961, p. 106-110 , doi : 10.1103 / PhysRevLett.6.106 .
- ↑ Archived copy ( Memento of the original dated December 6, 2008 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Cross section through a laser tube
- ↑ Fundamentals of Light Sources and Lasers - Chapter 3 Notes ( Memento from June 18, 2012 in the Internet Archive ) Representation of the Paschen notation (Eng.)
- ↑ Archived copy ( Memento of the original dated December 6, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Resonator modes
Web links
- Stefanie Wiedigen: LP - The Helium-Neon-Laser (incl. Sketches, photos and references) and a. also measurement of the divergence
- Michael Hartwig: How does a laser work? The He-Ne laser , u. a. with description of the energy level scheme (PDF file; 202 kB)