Most often, He–Ne lasers emit red light at 632.8 nm at a power level of a few milliwatts and with excellent beam quality. The gain medium is a mixture of mostly helium and some neon gas in a glass tube, which normally has a length of the order of 15–50 cm.

After initial cleaning, white reference panel remains slightly contaminated. Water still absorbs to a majority of the panel. Additional cleaning still required.

6. If the water beads up on the surface of the white reference panel and no longer appears to absorb into the panel, the surface should be sufficiently clean. Examples of heavily soiled and cleaned panels appear below.

Non Lambertian effects

Due to the narrow gain bandwidth (≈1.5 GHz, determined by Doppler broadening), He–Ne lasers typically exhibit few-mode oscillation, or for short laser tubes even stable single-frequency operation, even though mode hopping is possible in some temperature ranges where two longitudinal resonator modes have similar gain.

5. Gently move the white reference panel in a figure-eight motion on the sandpaper. A thin layer of the white reference will be removed and washed away as you continue the cleaning process. To avoid a potentially painful injury, be very careful that you do not allow your fingertips to contact the sandpaper.

If your panel is lightly soiled, it may be airbrushed with a jet of clean dry air or nitrogen. Be sure not to use FreonTM or a Freon containing compressed air canister. Our newly designed 3.62” Spectralon round carrying case should aid in keeping the panel cleaner and minimizing contaminant when the panel is not in use.

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In the gas discharge, helium atoms are excited into metastable states (23S1 and 21S0). During collisions, the helium atoms can efficiently transfer energy to neon atoms, which have excited states with quite similar excitation energies (4s2 and 5s2). Neon atoms have a number of energy levels below that pump level, so that there are several possible laser transitions. The transition at 632.8 nm (5s2 → 3p) is the most common, but other transitions allow the operation of such lasers at 1.15 μm, 543.5 nm (green), 594 nm (yellow), 612 nm (orange), or 3.39 μm. The emission wavelength is selected by using resonator mirrors which introduce high enough losses at the wavelengths of all competing transitions.

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Every attempt should be made to keep the panel as clean as possible and it is particularly important not to allow any oily substances to contact the panel. This includes oils from fingers and hands. Contamination from oils will be difficult or impossible to remove and they can contribute their spectral signature to the panel’s reflectance signature. Significant contamination from dust or dirt will interfere with the near-perfect reflectance qualities of the panel and may contribute a false spectral signature if the contamination is extensive enough. The best practice is to keep the panel as clean as possible.

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Some He–Ne lasers are serving in optical frequency standards. For example, there are methane-stabilized 3.39-μm He–Ne lasers, and 633-nm iodine-stabilized versions.

Lambertian surface

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Helium–neon (He–Ne) lasers are a frequently used type of continuously operating gas lasers, which is also the first demonstrated gas laser (already in 1961 [1]).

Calibrated panels will retain the specified reflectance properties as part of the panel’s composition, however, there is a specific surface finishing process used with calibrated panels. If you decide to clean your panel by using the procedures below, the calibration may change slightly. This is not a concern for many users but if you require absolute accuracy, you should consider having the panel cleaned and re-calibrated by LabSphere.

Spectralonwhite paint

The lower laser level for 632.8-nm operation is still highly excited and is partially depopulated by spontaneous emission. Therefore, one obtains some fluorescence at wavelengths between 0.54 μm and 0.73 μm. This leads to the metastable 3s state; depopulation in that can be made fast enough by using a small diameter laser tube, so that the neon atoms can dissipate energy in collisions with the tube walls. (One also often uses a smaller laser bore tube within a larger glass envelope.) That requirement prohibits simple power scaling via the tube diameter.

2. Attach 220-240 grade wet/dry sandpaper to the cutting board, using tape or binder clips. After wetting the cutting board and sandpaper, they may adhere to each other sufficiently such that binder clips and tape are not required.

Spectralonpaint

Authored by Don Campbell. Don is an Applications Chemist at Malvern Panalytical, supporting the ASD brand of spectroradiometers. He is focused on pre and post-sale customer support, including feasibility studies and quantitative calibration development for new industrial opportunities, spectrometer installation, and customer training for chemometrics and remote sensing applications. Don has 13+ years of experience with ASD products and accessories and looks forward to helping you obtain the greatest value from your near-infrared (NIR) data.

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The lifetime of a helium–neon laser can be far beyond 10,000 hours, since the glow discharge with quite moderate current density is associated with quite moderate operation conditions, e.g. with little erosion of the electrodes. A limiting factor can be leakage of helium. However, the tube may break when being subject to mechanical shock.

As you head out into the field to pursue your remote sensing research, it is important to use a reference panel to perform real-time reflectance measurements and optimize your ASD spectroradiometer’s response. We recommend the use of Spectralon® Diffuse Reflectance Material for these tasks due to its highly diffuse reflectance properties and suitability for field use.

Helium–neon lasers, particularly the standard devices emitting at 632.8 nm, are still used for alignment and in interferometers. However, they are more and more replaced with laser diodes, which are much cheaper, more compact and efficient. Remaining advantages of the He–Ne laser can be the smaller emission linewidth (particularly in the case of single-mode emission), which is associated with a long coherence length, and the high beam quality.

Some He–Ne lasers have a tube with internal resonator mirrors, which can not be exchanged. Brewster windows are then not required.

It is quite possible that the same tube would work with other gas mixtures, if you also exchange the resonator mirrors according to the used laser transition.

A DC current, which is applied via two electrodes with a voltage of the order of 1 kV (but higher during ignition), maintains an electric glow discharge with a moderate current density. In the simplest case, a ballast resistor stabilizes the electric current. The current is e.g. 10 mA, leading to an electrical power of the order of 10 W. The glass tube as shown in Figure 1 has Brewster windows, and the laser mirrors must form a laser resonator with a small round-trip loss of typically below 1%. Due to the polarization-dependent loss at the Brewster windows, a stable linear polarization is obtained.

If your panel is heavily soiled, you can still restore your Spectralon panel to its original or near original condition by following the “Spectralon Material Care and Handling Guidelines,” provided by LabSphere online, or by using the recommendations stated below.

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The 3.39-μm transition involves the same upper laser level manifold as the 632.8-nm transition and exhibits a rather high laser gain, while the 632.8-nm transition. Therefore, 632.8-nm operation is only possible if parasitic lasing on the 3.39-μm line is suppressed by introducing high power losses at that wavelength.

4. Turn on faucet at low flow rate to provide a steady stream of clean water to the white reference and sand paper during cleaning. (Optional) Attach Tygon® Tubing to faucet head to reduce splashing and allow for better direction of water flow if necessary.