Helium–neon laser
The helium -neon laser is a gas laser that emits red light most. 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, which generates light in a continuous operation. Seven months earlier, realized first laser by Maiman, a ruby laser, on the other hand generated pulsed laser light.
Design and operation
It consists essentially of a thin glass tube ( capillary tube and diameter approximately 1 mm, length several dm ), in which a helium-neon gas mixture is located.
The gas mixture is under a pressure of about 100 Pa, with a ratio of partial pressures of helium / neon of about 10 /1 ( of 1152 nm) or 5/ 1 (for 633 nm). Are located at the ends usually still called Brewster window or often even directly the resonator mirrors. With optional Brewster windows are plane-parallel plates, which transmit light with a certain polarization direction without losses by reflection, that is, there is only a transmitted, not reflected beam of polarization direction. Light with polarization perpendicular thereto is partially reflected. Since a laser always selects the operating state of the least losses, the "wrong" polarization is suppressed. A He -Ne laser with Brewster windows emitted thus in principle linearly polarized light on design with directly contacted on the discharge tube resonator, the polarization is arbitrary. In the case of the structure with Brewster windows this outside of the tube located between two mirrors that form the cavity is (see schematic diagram in the second image, the lower the glass tube ).
The supply voltage of the gas discharge must meet the following requirements:
- Providing the 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-scale, external metal cylinder, while there is a small anode between capillary and jet exit.
Wherein a helium -neon laser for pumping the helium is required, the neon lasing medium. In the glass tube also include two electrodes between which there is a gas discharge. These gas discharge now brings the helium atoms in a relatively long-lived (about 10-3 s ) excited state. The helium atoms by collisions of the second kind are now transferring their energy to the neon atoms, where they produce a population inversion between energy states of high and low states. On transitions between the energy states of neon is now the laser operation, as shown in the following scheme, possible.
The states and helium are metastable. The emission of photons in the neon atom is carried out by stimulated emission; the return from the lower laser level to the ground state by spontaneous emission and recombination at the capillary wall. Due to the latter fact, it is not useful to the diameter of the glass tube to choose greater than 1.5 mm.
The helium -neon laser emits light of the standard wavelength 632.816 nm ( the well-known red laser light implemented 1963), 1152.3 nm (infrared realized 1960) and 3392.2 nm ( infrared). Due to spin -orbit coupling the laser levels are split. Other wavelengths can be obtained by introduction of prisms and narrowband mirrors produce in the resonator (for example, in the green region at 543.5 nm, 594.1 nm in the yellow and the orange at 611.9 nm). You can also use filters that are brought into the resonator. The output power of a red helium-neon laser is up to about 100 mW in the range of a few milliwatts, in rare cases.
Another striking feature of helium - neon lasers is their high coherence length. Even with simple models (multimode laser ) in the range of the resonator length, that is generally between 20 cm and 30 cm. The reason is the extremely narrow gain bandwidth of the neon laser transition of about 1.5 GHz, so that only a few longitudinal modes can oscillate. Thermally stabilized frequency selective resonators commercially available He-Ne laser enable a stability of a few megahertz, and a respective coherence length of more than 100 m. Furthermore, there are frequency-stabilized He-Ne laser, the coherence length can be several kilometers.
Applications
The relatively low price and the long life make the helium -neon laser for many application fields of interest. Previously, he was found, for example in the barcode scanners from cash registers or laser printers, now he has been there almost completely displaced from the diode laser.
For special requirements on beam quality and consistency, for example, in interferometers or in the calibration of spectrometers, he still plays a major role. Also, helium -neon lasers are good for holography, even if one has switched there in mass production is also more powerful and shorter wavelength lasers ( argon ion laser, helium -cadmium laser).