Helium- Neon Laser

 

Helium-Neon laser is the first gas laser developed by Ali-Javan and his co-workers at Bell Telephone Laboratories, USA. The lasing medium is a mixture of Helium and Neon at ten torr pressure (1 torr is 1 mm of Hg pressure). Neon atoms are the laser-active centers, and Helium plays a significant role in the excitation of Neon. Electrical pumping helps the excitation process.

A helium-neon laser demonstration. The glow running through the center of the tube is an electric discharge. This glowing plasma is the gain medium for the laser. 

Construction:

A mixture of He and Ne (10:1 ratio) at low pressure is the lasing medium in a glass tube of 80 cm in length and 1 cm in diameter. The ends of the glass tube are closed by oblique quartz windows, making an angle α with the axis satisfying the relation tan α=n, the refractive index of quartz.

It renders the output as plane polarized. Two spherical mirrors on both sides of the glass tube help the stable resonant cavity action. One mirror is fully reflecting, and the other is partially reflecting. For electrical pumping 1000 V DC, there is a cooling arrangement surrounding the tube (Figure 1).

Figure 1 Helium-Neon Laser

1 Laser medium                                       2 Quartz widows

3 Electrical pumping                                4 Totally reflecting mirror

5 Partially reflecting mirror                       6 Laser output

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 refracts 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 within 1 part in 108 by comparing the powers of two longitudinal modes in opposite polarizations. [2] Absolute stabilization of the laser's frequency (or wavelength) is 2.5 parts in 1011 through an iodine absorption cell. [3]

The mechanism producing population inversion and light amplification in a He-Ne laser plasma[1] originates with the collision of energetic electrons with ground-state helium atoms in the gas mixture. As shown in the energy level diagram- the collisions excite helium atoms from the ground to higher energy excited states. Collisions between these helium metastable atoms and ground-state neon atoms result in a selective and efficient energy transfer from helium to neon [4]

The gain bandwidth of the He-Ne laser is 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. [5][6] 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, its long coherence length, and its excellent spatial quality make this laser a useful source for holography and as a wavelength reference for spectroscopy. 

 He-Ne lasers were used in barcode scanners at supermarket checkout counters before the invention of cheap, abundant diode lasers. He-Ne lasers are generally present in educational and research optical laboratories. They are also unsurpassed for use in nano-positioning in such applications as semiconductor wafer Fabs. Precision Laser gyroscopes have employed He-Ne lasers operating at 633 nm in a ring laser configuration.

Laser action:

He-Ne laser is a four-level laser system (Figure 2). F1 is the ground level, and F2 and F3 are the excited levels of He. E1 is the ground level of Neon, and E2, E3, E4, E5, and E6 are excited levels. The excited levels of Helium, F2, and F3 coincide with E4 and E6 of Neon. This coincidence helps to achieve population inversion. In the Y axis, the wave number is there. We can calculate energy by multiplying wave number by hc. 

Figure 2 Energy level diagram of He and Ne 

The electric discharge through the gas mixture causes excitation. The excited Helium atom makes a resonant collision with ground-level Neon atoms and transfers them to the corresponding excited levels. Helium falls back to the ground level. Because of this resonant collision energy transfer, E4 and E6, the metastable levels become densely populated. 

Three lasing transitions take place:  

1) E6 to E5 emitting radiation at 3.39 µm

2) E4 to E3 emitting radiation at 1.15µm

3) E6 to E3 level, emitting radiation of wavelength 0.63µm at the red region of visible light.

We can collect the desired wavelength with the resonant cavity action.

Features of output:

• Plane polarized
• Continuous wave mode
• The high degree of coherence, monochromaticity, and directionality 
• Efficiency less than 0.5%
• Power from a fraction of milliwatt to 80 milliwatt.

Applications

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 and long coherence length (however, since about 1990 semiconductor lasers have offered a lower-cost alternative for many such applications).
From 1978 onwards, Pioneer Laser Disc players were using the He-Ne tube lasers. In 1984, infrared laser diodes replaced the He-Ne tube laser.
Pioneer continued to use laser diodes in all subsequent players until 2009.[4]

We can make the Helium-Neon lasers oscillate at over 160 different wavelengths by adjusting the cavity Q to peak at the desired wavelength.
It is possible by adjusting the spectral response of the mirrors or by using a dispersive element in the cavity. Units operating at 633 nm are in the laboratories in schools because of their low cost and near-perfect beam qualities. [4]

References

1 Javan, A.; Bennett, W.R.; Herriott, D.R. (1 February 1961). "Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture". Physical Review Letters. 6 (3). American Physical Society (APS): 106–110. Bibcode:1961PhRvL...6..106J. doi:10.1103/physrevlett.6.106. ISSN 0031-9007

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2^Jump up to:a b Niebauer, T.M.; Faller, James E.; Godwin, H.M.; Hall, John L.; Barger, R.L. (1988-04-01). "Frequency stability measurements on polarization-stabilized He–Ne lasers". Applied Optics. 27 (7). The Optical Society: 1285–1289. Bibcode:1988ApOpt..27.1285N. doi:10.1364/ao.27.001285. ISSN 0003-6935. PMID 20531556.

3^ Jump up to a b Iodine-stabilized helium-neon laser. National Institute of Standards and Technology (NIST). NIST Museum (Report). U.S. Department of Commerce. Archived from the original on 21 July 2006.

4 Wikipedia

5  "Sam's Laser FAQ – Helium-Ne Lasers". K3PGP.org

6  "Sam's Laser FAQ". RepairFAQ