Helium-Neon Laser Tutorial

Applications

• Metrology
• Interferometry
• DNA Sequencing
• Food Sorting
• Flow Cytometry
• Confocal Microscopy
• Imaging and Medical Equipment
• Opacity Monitoring
• Alignment
• Maritime Visual Guidance Systems
• Hematology
• Semiconductor Inspection
• Polarization Experiments

Overview

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 22.5 mW and a fundamental Gaussian beam. Depending on the model chosen, the output will be either linearly polarized or the polarization state will fluctuate over time.

The gain medium of a HeNe laser is a mixture of helium and neon gases typically in a ~10:1 ratio (ratios from 5:1 to 20:1 are common) 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 opposite ends 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 (Figure 49A). HeNe lasers tend to be small, with cavity lengths from around 0.15 to 1 m. Table 49B lists typical values for various HeNe laser parameters.

Figure 49A  HeNe laser cavity and output beam. HR: high reflector, OC: output coupler, L: cavity length, D: beam waist diameter, and α: full angle beam divergence.

HeNe Wavelength and Linewidth

The wavelength of 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 of the gain curve due to thermal expansion/contraction of the cavity and other external factors.

The exact linewidth of a HeNe laser is determined by the specific properties of the mirrors and the physical cavity. The axial mode structure of the HeNe laser is characterized by the number of modes, the free spectral range (FSR), and the Doppler width (Figure 49C). The linewidth of individual axial modes is usually small (~kHz), while the gain curve is relatively wide (~GHz). The longitudinal modes that lase are those contained within the wide gain curve, which has a typical width of ~1.5 GHz. This ~1.5 GHz Doppler width defines the linewidth of the HeNe laser. In most interferometric applications, the most relevant parameter is the coherence length, which is determined by the axial modes that are furthest apart (which itself is mainly determined by the width of the gain curve). The coherence length formula for a Lorentzian distributed (in wavelength space) source is L=c/piΔf, where c is the speed of light and Δf is width of the source in wavelength space. Using a Doppler width of ~1.5 GHz for the gain curve we find that the coherence length for a red HeNe laser is approximately 6 cm.

Figure 49C  HeNe laser gain curve with longitudinal modes shown for two optical cavity lengths. L: cavity length, P: output power, and FSR: free spectral range.

HeNe Polarization

For a typical HeNe laser, without any polarization optics in the cavity, each longitudinal mode is linearly polarized along one of two orthogonal axes, with adjacent (in wavelength space) modes typically having orthogonal polarizations. As the cavity length changes due to thermal effects, individual longitudinal modes change frequencies, and as a result the "frequency comb" of individual modes "sweeps" through the gain profile.

For short lasers, such as the L=0.15 m, P=0.5 mW example depicted in Figure 49C, one or two modes will be lasing at any given time. If only one mode is lasing, the beam will be linearly polarized. If two modes are lasing, the beam will have power in both orthogonal linear polarizations. Note that the two axial modes have different polarizations and so the instantaneous polarization state of the beam will change as the relative phase between the two modes changes with time. This polarization effect can be seen by placing a polarizer, aligned to one of the mode polarizations, in the beam path while the laser is sweeping through modes. A detector placed before the polarizer will show a steady reading, while one placed after the polarizer will show the power fluctuating between zero and the total beam power.

For long lasers, such as the L=0.5 m, P=17 mW example depicted in Figure 49C, there are always multiple lasing modes within the gain profile, and the beam always contains power in both orthogonal polarizations. It should be noted that in such lasers, subsequent modes do not always alternate perfectly in polarization. These lasers also exhibit more instability in terms of the lasing modes, with power fluctuating between modes. However, the total power is relatively stable in all cases.

Click to Enlarge
Figure 49D  HeNe laser energy levels. Note the sublevels of the 3p neon shell are explicitly shown for reference to the available visible transitions. The sublevels of the other neon shells are omitted from the diagram for simplicity. The energy of the 4s neon shell is 19.78 eV.

During laser start-up, the modes sweep rapidly through the gain curve as the cavity reaches its operation temperature. During normal operation, mode sweep will still occur, but over longer time scales, which can be on the order of seconds to hours. As a result, HeNe lasers without polarization compensation/stablization should not be used for polarization-sensitive applications, as the power in each polarization state will fluctuate over time.

Polarized HeNe Lasers
In order to avoid the problems outlined above, some HeNe lasers are manufactured with a polarization optic inside the cavity, such as a Brewster window. By preferentially attenuating one polarization, all of the modes can be made to have the same linear polarization. Polarized HeNe lasers are typically marked with the polarization axis, and are ideal for polarization-sensitive applications.

HeNe Energy Levels

An energy level diagram outlining the relevant levels and transitions for a HeNe laser is shown in Figure 49D. The lasing process begins with the collision of electrons from the high-voltage electrical discharge with the atoms of the gas (He and Ne). The more numerous helium atoms (recall a typical He:Ne ratio of ~10:1) are preferentially excited to higher energy states by the collisions. A collision between an excited helium atom and a neon atom will then excite the neon atom to a metastable excited state, which has nearly identical energy to that of the excited helium atoms. The excited neon atoms then return to the 2p ground state via the 3p and 3s levels. Stimulated emission of 632.816 nm light is induced from the 5s to 3p transition once a population inversion is achieved (red arrow in Figure 49D). Other wavelengths of light are also emitted (see Figure 49D, and the following paragraph). The HeNe laser's power output is limited by the saturation of the neon upper level at higher current, while the lower level varies linearly with current.

The mirrors and length of the laser cavity can be chosen in order to promote other wavelengths of laser emission. As seen in Figure 49D, there are infrared transitions at 3.39 µm and 1.15 µm. The former occuring from the 5s to the 4p shell, and the latter from the 4s to the 3p shell. The transitions from the 5s level to the various sublevels in the 3p shell can also be promoted. These transitions allow for stimulated emission in the visible wavelength range, including green (543.365 nm), yellow (593.932 nm),
yellow-orange (604.613 nm), and orange (611.802 nm) transitions. These transitions are represented by the colored levels in the 3p shell in Figure 49D. 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.

Environment

Environment is an important factor in achieving optimal laser performance. In dirty environments, the optics can become contaminated, which causes the power output to drop below expected levels. Unstable output beams can be caused by noisy environments with large sources of vibrations. Proper mounting on an optical table can reduce the effects of ambient vibrations. If the environment where the laser is being used fluctuates in temperature, the output power can experience a large amplitude change. While a HeNe laser is less sensitive to variations caused by back reflections, large retro-reflections into the laser can cause unpredictable power changes. A free-space isolator can be used to reduce or eliminate these effects. HeNe lasers are not generally well-suited to applications or experiments where single-frequency operation or long coherence length are required.