Thorlabs' Stabilized Helium Neon Laser allows for either frequency or intensity stabilization. In frequency-stabilized mode, the laser will keep its lasing frequency, or wavelength, constant. In intensity-stabilized mode, the laser will keep its output power constant. Stabilized HeNe lasers are necessary for many spectroscopy, interferometry, and wavemeter applications.

The plots below show the key parameters of the HeNe when the laser is in the frequency-stabilized mode. The left-most plot shows the optical spectrum of the laser; the data for this graph was collected using Thorlabs' OSA201 Fourier Transform Optical Spectrum Analyzer.The center plot shows the power fluctuations over a five hour period. As seen in this graph, the HRS015 laser's power stabilizes after about 45 minutes. Finally, the plot to the far right shows the fractional change in wavelength over five hours of operation.

The laser is housed in a cylindrical housing, which can be conveniently mounted in a V-Clamp mount such as the C1503 featured below. The Ø1.77" tube is compatible with our HCM2 cage system HeNe mount as shown above. For details on our assortment of HeNe accessories, please see the bottom of this page. The front bezel of this stabilized laser is SM1(1.035"-40)-threaded and is compatible with any of Thorlabs' SM1-threaded components.

The polarization axis is marked by a laser-engraved line on the laser's front face. The front face also includes an integrated beam stop and a standard 4-40 tapped hole pattern. Please note that back reflections into the laser aperture will impair the ability of the control loop to stabilize the frequency or intensity of the laser. For instances where back reflections cannot be avoided, Thorlabs recommends using an optical isolator such as IO-2D-633-VLP.

This laser is supplied with a power supply with a universal voltage input.

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Stabilized HeNe Lasers

Stabilized HeNe lasers, in addition to the features discussed on the HeNe Tutorial tab, offer the ability to change between two modes of operation, frequency and intensity stabilization.

Stabilization Feedback Mechanism Schematic

Frequency Stabilization Mode
The frequency stabilization mode will balance the intensity of two modes under the gain curve in order to keep the frequency of the laser stable. The tube length is specifically chosen to only allow two cavity modes at the output. The polarization states of the modes are orthogonal (i.e. one will be s-polarized and the other will be p-polarized). Using a polarizing beamsplitter, one of the modes is directed to a photodetector, while the remaining mode passes through a second beamsplitter where 5% of the output is reflected to a second photodetector, as shown in the schematic above. An error signal generated by the two photodetectors is used to control a heater wrapped around the glass tube. the heater causes the tube to expand and contract as necessary to stabilize the frequency. The frequency stabilization mode also delivers some intensity stability.

Since our stabilized HeNe features a single mode output, the coherence length is increased to hundreds of meters.

Intensity Stabilization Mode
The intensity stabilization mode will stabilize the intensity of the output beam. This mode operates on the same principle as frequency stabilization; however, only one of the photodectectors is used to generate the feedback error signal to control the heater. The intensity stabilization mode also delivers some frequency stability.

On Thorlabs' HRS015 Stabilized HeNe, either mode can be quickly selected by adjusting the toggle switch into the desired position.

This tab gives a general introduction into HeNe lasers. For details on the operation of the stabilization mechanism, please see the Stabilized HeNe tab.

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 air), which is 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 (unpolarized).

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 an output coupler mirror with approximately 1% transmission at the other end (see figure below). HeNe lasers tend to be small, with cavity lengths from around 15 cm to 0.5 m and optical output powers ranging from 1 mW to 100 mW. Thorlabs offers standard HeNe lasers with output powers up to 22.5 mW, while our stabilized HeNe offers powers greater than 1.2 mW.

Optical Resonator Cavity
(1) Laser Bore, (2) Intracavity Beam, (3) Collimating Lens, (4) Output Beam

HeNe Polarization

Unpolarized (Randomly Polarized) Beam
Most common HeNe laser tubes are randomly polarized since for many applications the polarization of the beam does not matter. The term "random" here does not mean that the polarization is changing to totally arbitrary orientations or at high frequency. In fact, such behavior would be rather unusual. It just means that nothing special is done to control the polarization. The typical random polarized HeNe laser will lase on several longitudinal modes (how many will depend on the tube length of the resonator), with adjacent modes having polarizations orthogonal to each other, and the polarization axes fixed for the life of the tube. Each of the modes will change their relative intensities periodically over time, most notably during warmup as the cavity length changes due to thermal expansion.

Random polarized lasers are acceptable for applications where there are no polarizing elements in the beam path. Depending on the time scale of an application, polarizing elements could result in large power fluctuations. Reorienting the laser head (about its axis) may help to minimize these, but the use of a linearly polarized laser is the best solution.

Polarized Beam
The state of polarization in a polarized HeNe laser beam is linear, making these lasers ideal for polarization-sensitive applications. The output of the stabilized HeNe laser will be linearly polarized and aligned with the engraving on the front of the unit.

HeNe Linewidth

A red HeNe laser outputs at 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 gain curve due to thermal expansion of the cavity and other external factors.

The linewidth of a HeNe laser is specific to the application. The longitudinal 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 longitudinal 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 longitudinal modes that are farthest apart. For a red HeNe laser, the coherence length is approximately 30 cm. For our stabilized HeNe, one of the two cavity modes present is suppressed and used for the feedback mechanism, while the other is stabilized as the output. This increases the coherence length to hundreds of meters.

HeNe Gain Curve

HeNe Energy Levels

HeNe Energy Levels

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. Collision of excited helium atoms with ground-state neon atoms results in excited neon electrons.

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 at 632.82 nm, 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.

Environment

Environment is an important factor in achieving optimum 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. This line of HeNe lasers is ill-suited to any application or experiment where single frequency or long coherence length is required. For questions about the suitability of a HeNe laser to a particular application, please contact Technical Support.

Laser Safety and Classification

Safe practices and proper usage of safety equipment should be taken into consideration when operating lasers. The eye is susceptible to injury, even from very low levels of laser light. Thorlabs offers a range of laser safety accessories that can be used to reduce the risk of accidents or injuries. Laser emission in the visible and near infrared spectral ranges has the greatest potential for retinal injury, as the cornea and lens are transparent to those wavelengths, and the lens can focus the laser energy onto the retina. 

Safe Practices and Light Safety Accessories

• Thorlabs recommends the use of safety eyewear whenever working with laser beams with non-negligible powers (i.e., > Class 1) since metallic tools such as screwdrivers can accidentally redirect a beam.
• Laser goggles designed for specific wavelengths should be clearly available near laser setups to protect the wearer from unintentional laser reflections.
• Goggles are marked with the wavelength range over which protection is afforded and the minimum optical density within that range
• Laser Barriers and Blackout Materials can prevent direct or reflected light from leaving the experimental setup area.
• Thorlabs' Enclosure Systems can be used to contain optical setups to isolate or minimize laser hazards.
• All beams should be terminated at the edge of the table, and laboratory doors should be closed whenever a laser is in use.
• Do not place laser beams at eye level.
• Carry out experiments on an optical table such that all laser beams travel horizontally.
• Remove unnecessary reflective items such as reflective jewelry (e.g., rings, watches, etc.) while working near the beam path.
• Be aware that lenses and other optical devices may reflect a portion of the incident beam from the front or rear surface.
• Operate a laser at the minimum power necessary for any operation.
• If possible, reduce the output power of a laser during alignment procedures.
• Use beam shutters and filters to reduce the beam power.
• Post appropriate warning signs or labels near laser setups or rooms.
• Use laser sign lightboxes if operating Class 3R or 4 lasers (i.e., lasers requiring the use of a safety interlock).
• Do not use Laser Viewing Cards in place of a proper Laser Barrier or Beam Trap.

Laser Classification

Lasers are categorized into different classes according to their ability to cause eye and other damage. The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. The IEC document 60825-1 outlines the safety of laser products. A description of each class of laser is given below:

Class	Description	Warning Label
1	This class of laser is safe under all conditions of normal use, including use with optical instruments for intrabeam viewing. Lasers in this class do not emit radiation at levels that may cause injury during normal operation, and therefore the maximum permissible exposure (MPE) cannot be exceeded. Class 1 lasers can also include enclosed, high-power lasers where exposure to the radiation is not possible without opening or shutting down the laser.
1M	Class 1M lasers are safe except when used in conjunction with optical components such as telescopes and microscopes. Lasers belonging to this class emit large-diameter or divergent beams, and the MPE cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. However, if the beam is refocused, the hazard may be increased and the class may be changed accordingly.
2	Class 2 lasers, which are limited to 1 mW of visible continuous-wave radiation, are safe because the blink reflex will limit the exposure in the eye to 0.25 seconds. This category only applies to visible radiation (400 - 700 nm).
2M	Because of the blink reflex, this class of laser is classified as safe as long as the beam is not viewed through optical instruments. This laser class also applies to larger-diameter or diverging laser beams.
3R	Lasers in this class are considered safe as long as they are handled with restricted beam viewing. The MPE can be exceeded with this class of laser, however, this presents a low risk level to injury. Visible, continuous-wave lasers are limited to 5 mW of output power in this class.
3B	Class 3B lasers are hazardous to the eye if exposed directly. However, diffuse reflections are not harmful. Safe handling of devices in this class includes wearing protective eyewear where direct viewing of the laser beam may occur. In addition, laser safety signs lightboxes should be used with lasers that require a safety interlock so that the laser cannot be used without the safety light turning on. Class-3B lasers must be equipped with a key switch and a safety interlock.
4	This class of laser may cause damage to the skin, and also to the eye, even from the viewing of diffuse reflections. These hazards may also apply to indirect or non-specular reflections of the beam, even from apparently matte surfaces. Great care must be taken when handling these lasers. They also represent a fire risk, because they may ignite combustible material. Class 4 lasers must be equipped with a key switch and a safety interlock.
All class 2 lasers (and higher) must display, in addition to the corresponding sign above, this triangular warning sign

FiberPort and Thread Adapters for Standard Cylindrical HeNe Lasers*
	The SM05AHN Thread Adapter allows SM05-threaded components to be attached directly to the front of a HeNe laser and is ideal for enclosing a HeNe beam path using SM05 Lens Tubes. The HCL FiberPort Adapter allows a FiberPort coupler to be attached directly to the front of a HeNe laser. Both adapters can be attached to the laser via counterbored slots that fit industry-standard M3 and 4-40 four-bolt patterns. The HCL can also be mounted via the internal C-Mount-Threaded (1.00"-32) central bore.
HCM2 Cage Mount		Large V-Clamp Mounts
	The HCM2 Cage Mount enables integration of a standard Ø1.75" cylindrical HeNe laser into a 60 mm cage system or SM2 (2.035"-40) lens tube system. The HCM2 provides ±1.0 mm of coarse X and Y adjustment, and is compatible with Ø1/2" and Ø1" posts.		The C1502(/M) and C1503(/M) are designed specifically for fastening Ø0.56" (14 mm) to Ø2" (50 mm) tube lasers to Thorlabs' rigid Ø1.5" Posts. One PM4(/M) Clamping Arm is included with each unit and additional clamping arms can be purchased as needed here.

*Note that these adapters are not compatible with the HNL008 Series of Cylindrical HeNe Lasers.