Helium-neon lasers

Details Helium-neon lasers Helium-neon lasers

1989-11-03

1991-01-22

Scott, Jr., Leon

Coherent light generators

Particular beam control device

Modulation

372 37, 372 34, 372 32, 372 27, H01S 310

Patent

active

049875744

DESCRIPTION:

BRIEF SUMMARY
The present invention relates particularly to helium-neon lasers of the type emitting colors other than red.
Lasers have been known for many years and have many uses. The gain medium may be gaseous, and one of the most commonly used gas lasers is the helium-neon laser which commonly emits a red light.
In the simplest form of helium-neon laser the frequency of the emitted light changes as the laser tube heats up. This is because the frequency emitted is dependent on the length of the laser tube which expands as it heats. In addition, most lasers emit light in more than one mode so that the output is not a pure single frequency, but is composed of two or more optical frequencies (cavity modes).
Many uses of a laser require the emitted light to be of a known and constant single frequency. Consequently many helium-neon lasers use a frequency stabilisation system. One such system controls the length of the laser such that the frequencies of its two emitted modes are on either side of the centre frequency of the neon transition. These two modes are typically in orthogonal polarisation states. The intensities of these modes are compared to provide a correction signal. If the modes change frequency, the correction signal also changes and may be used in a feedback circuit so as to prevent or reduce such frequency changes. The correction signal is applied by means such as a heater or a piezo-electric element, that can alter the length of the laser.
The heater may take many different forms, such as heater wires, thermofoils or a thin film heater coated on the laser tube, the last being described in an article by K. Seta and S. Iwasaki entitled `Frequency Stabilisation of the He-Ne Laser using a thin film heater coated on the laser tube`, Optics Communications, Volume 55, No. 5, October 1985, Elsevier Science Publishers B V (Amsterdam, NL).
Another problem that occurs with lasers operating at high temperatures is that the tube bends and becomes misaligned. A laser alignment system is described in U.S. Pat. No. 4,010,363 which describes a rather complicated system comprising a plurality of heaters along the tube. By use of control circuits, selective portions of the tube are heated if the laser becomes misaligned. The system is complex and requires positional misalignment data collection and feedback signals.
Another form of stabilisation system, which is applicable particularly to a laser that emits in only one mode, makes use of the Zeeman effect. A magnetic field is applied so that the mode is split into two orthogonally polarised components. In the stabilisation system, these components are controlled to be on either side of the neon frequency. The correction signal may be derived either from the relative intensities of the two components, or from their frequency separation.
Such a stabilisation system is described in U.S. Pat. No. 3,649,930 where the Zeeman effect is used in conjunction with the Doppler effect in a stabilisation system for frequency stabilisation of a single-mode gas laser.
It has recently been found possible to manufacture helium-neon lasers which emit light of a color other than red. This can only be done by very careful control of the helium to neon gas mix ratio and of the mirror reflectivities. There are many potential uses for non-red helium-neon lasers in applications where non-red light is required. For example a non-red helium-neon laser can be used in conjunction with a red helium-neon laser in multi-wavelength interferometry.
However in multi-wavelength interferometry, and in some other areas where non-red helium-neon lasers might be used, it is important that the emitted frequency be stable. Unfortunately it has been found that the system which are so effective in stabilising red helium-neon lasers are not directly applicable to non-red lasers. Reasons include laser emission in more than two cavity modes (frequencies), the tendency of such modes to change their polarisation states when the laser length changes very slightly, and the extreme sensitivity of the laser to misalignment by b

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Optics Communications, vol. 55, No. 5, Oct. 1985, Elsevier Science Pub. B (Amsterdam, NL), K. Seta et al., "Frequency Stabilization of A HeNe (cont.) laser Using a Thin Film Heater Coated on the Laser Tube", pp. 367-369.

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