The best-known and most widely used He-Ne laser operates at a wavelength of 632.8 nm, in the red part of the visible spectrum. It was developed at Bell Telephone Laboratories in 1962,[2][3] 18 months after the pioneering demonstration at the same laboratory of the first continuous infrared He-Ne gas laser in December 1960.[4]
The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in approximately a 10:1 ratio, contained at low pressure in a glass envelope. The gas mixture is mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states, responsible for non-laser lines.
A neon laser with no helium can be constructed, but it is much more difficult without this means of energy coupling. Therefore, a He-Ne laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will lose its laser functionality because the pumping efficiency will be too low.[5] The energy or pump source of the laser is provided by a high-voltage electrical discharge passed through the gas between electrodes (anode and cathode) within the tube. A DC current of 3 to 20 mA is typically required for CW operation. The optical cavity of the laser usually consists of two concave mirrors or one plane and one concave mirror: one having very high (typically 99.9%) reflectance, and the output coupler mirror allowing approximately 1% transmission.
Commercial He-Ne lasers are relatively small devices compared to other gas lasers, having cavity lengths usually ranging from 15 to 50 cm (but sometimes up to about 1 meter to achieve the highest powers), and optical output power levels ranging from 0.5 to 50 mW.
The precise wavelength of red He-Ne lasers is 632.991 nm in a vacuum, which is refracted to about 632.816 nm in air. The wavelengths of the stimulated emission modes lie within about 0.001 nm above or below this value, and the wavelengths of those modes shift within this range due to thermal expansion and contraction of the cavity. Frequency-stabilized versions enable the wavelength of a single mode to be specified to within 1 part in 108 by the technique of comparing the powers of two longitudinal modes in opposite polarizations.[6] Absolute stabilization of the laser's frequency (or wavelength) as fine as 2.5 parts in 1011 can be obtained through use of an iodine absorption cell.[7]
The remaining step in utilizing optical amplification to create an optical oscillator is to place highly reflecting mirrors at each end of the amplifying medium so that a wave in a particular spatial mode will reflect back upon itself, gaining more power in each pass than is lost due to transmission through the mirrors and diffraction. When these conditions are met for one or more longitudinal modes, then radiation in those modes will rapidly build up until gain saturation occurs, resulting in a stable continuous laser-beam output through the front (typically 99% reflecting) mirror.
The gain bandwidth of the He-Ne laser is dominated by Doppler broadening rather than pressure broadening due to the low gas pressure and is thus quite narrow: only about 1.5 GHz full width for the 633 nm transition.[6][9] With cavities having typical lengths of 15 to 50 cm, this allows about 2 to 8 longitudinal modes to oscillate simultaneously (however, single-longitudinal-mode units are available for special applications). The visible output of the red He-Ne laser, long coherence length, and its excellent spatial quality, makes this laser a useful source for holography and as a wavelength reference for spectroscopy. A stabilized He-Ne laser is also one of the benchmark systems for the definition of the meter.[7]
Prior to the invention of cheap, abundant diode lasers, red He-Ne lasers were widely used in barcode scanners at supermarket checkout counters. Laser gyroscopes have employed He-Ne lasers operating at 633 nm in a ring laser configuration. He-Ne lasers are generally present in educational and research optical laboratories.
Red He-Ne lasers have an enormous number of industrial and scientific uses. They are widely used in laboratory demonstrations in the field of optics because of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence (a single-mode Gaussian beam) and long coherence length (however, since about 1990 semiconductor lasers have offered a lower-cost alternative for many such applications).
Starting in 1978, HeNe tube lasers (manufactured by Toshiba and NEC) were used in Pioneer LaserDisc players. This continued until the 1984 model lineup, which contained infrared laser diodes instead. Pioneer continued to use laser diodes in all subsequent players until the format's discontinuation in 2009.
A Helium-Neon laser, typically called a HeNe laser, is a small gas laser with many industrial and scientific uses. These lasers are primarily used at 632.8 nm in the red portion of the visible spectrum. Thorlabs' line of red Helium-Neon gas lasers have stable output powers from 0.5 to 35 mW and a fundamental Gaussian beam. Depending on the model chosen, the output will be either linearly polarized or randomly polarized.
The gain medium of a HeNe laser is a mixture of helium and neon gases in a 5:1 to 20:1 ratio that is contained at low pressure in a sealed glass tube. The excitation source for these lasers is a high-voltage electrical discharge through an anode and cathode at each end of the glass tube. The optical cavity of the laser consists of a flat, high-reflecting mirror at one end of the laser tube and a concave output coupler mirror with approximately 1% transmission at the other end of the laser tube (see figure below). HeNe lasers tend to be small, with cavity lengths from around 15 cm to 0.5 m.
Randomly Polarized Beam
The output of a randomly polarized HeNe laser consists of a rapidly fluctuating, linearly polarized beam whose polarization orientation changes on a nanosecond time scale. Randomly polarized lasers are ideal for applications where there are no polarizing elements in the beam path. Depending on the time scale of an application, large power fluctuations are possible.
A red HeNe laser is 632.816 nm in air, although it is often reported as either 632 nm or 633 nm. The wavelength gain curve of a HeNe laser is actually made up of several longitudinal modes that fluctuate within the range due to thermal expansion of the cavity and other external factors.
The linewidth of a HeNe laser is specific to the application. The axial mode structure of the HeNe laser is characterized by the number of modes, the free spectral range (FSR), and the Doppler width (see figure below). The linewidth of individual axial modes is usually small (kHz) and is primarily determined by external factors and measurement timescales, rather than fundamental laser parameters. In most interferometric applications, the most relevant parameter is the coherence length, which is determined by the axial modes that are furthest apart. For a red HeNe laser, the coherence length is approximately 30 cm.
The laser process in a HeNe laser starts with the collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to a long-lived, metastable excited state. These excited helium atoms then collide with the ground-state neon atoms, producing excited neon atoms. The number of neon atoms entering the excited states builds up until population inversion is achieved. Spontaneous and stimulated emission between the states results in emission of 632.82 nm wavelength light, along with other emission wavelengths (see figure at right). From these states, the electrons quickly decay to the ground state. The HeNe laser's power output is limited because the neon upper level saturates with higher current, while the lower level varies linearly with current.
The laser cavity can be designed with the correct mirrors and length to promote other wavelengths of laser emission. There are infrared transitions at 3.39 µm and 1.15 µm wavelengths and a variety of visible transitions, including a green (543.365 nm), yellow (593.932 nm), yellow-orange (604.613 nm), and orange (611.802 nm) transition (see figure below). The typical red 632.8 nm wavelength output of a HeNe laser has a much lower gain compared to other wavelengths, such as the 1.15 µm and 3.39 µm lines.
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