Why and How to Use HeNe Lasers for Industrial and Scientific Applications
Contributed By Digi-Key's North American Editors
Lasers are now an indispensable part of the industrial system designer’s toolkit, as they enable applications ranging from micro-level measurements and sensing to large-scale industrial functions. One of the most widely used lasers for industrial and scientific applications is the helium-neon (HeNe) gas laser, and for many good reasons, including high performance, small size, stability, and high-quality optical output. However, designers must match the laser tube to a suitable high-voltage power supply for effective laser start-up, continuous operation, and long life.
This article discusses lasers and laser options before looking more closely at the HeNe laser, and why it is so widely used. It then looks at factors to consider for a successful application of this class of laser example devices from Excelitas Technologies’ REO family of HeNe lasers and suitable power supplies.
What are lasers?
Laser is short for “light amplification through stimulated emission of radiation”. The unique properties of laser beam output is that the electromagnetic energy and output waves are monochromatic, coherent, and aligned with each other in phase, time, and space. This is true whether the laser output is in the visible or invisible part of the optical spectrum. Most lasers have a fixed output wavelength (λ), but some can be set to one of several discrete wavelength values.
The first laser was demonstrated by Theodore H. Maiman, a physicist at Hughes Research Laboratories in Malibu, CA, in May of 1960. He used ruby (CrAlO3) and photographic flashlamps as the laser’s “pump” source to produce a red light beam at a wavelength of 694 nanometers (nm). The issue of who should get scientific credit for the laser’s conception as well as royalty rights was the subject of a 30-year patent dispute among three physicists.
How lasers work
Lasers have three basic building blocks:
- The lasing material itself, which can be a solid, liquid, gas, or semiconductor, and can emit light in all directions
- A pump source that adds energy to the lasing material such as a flash lamp, an electrical current to cause electron collisions, or radiation from another laser
- An optical cavity consisting of reflectors—one fully reflecting and the other partially reflecting—which provide the positive feedback mechanism for light amplification
For the lasing action to take place, it is necessary to excite a majority of the electrons inside the cavity to a higher energy level, known as population inversion. This is an unstable condition for the electrons, so they stay in this state for a short time and then decay back to their original energy state in two ways:
- First, there is spontaneous decay, as the electrons simply fall back to their ground state while emitting randomly directed photons
- Second, there is stimulated decay where the photons from spontaneous decaying electrons strike other excited electrons, which causes them to fall to their ground state
This stimulated transition will release energy in the form of photons, which travel in phase and at the same wavelength and in the same direction as the incident photon. The emitted photons travel back and forth in the optical cavity, through the lasing material between the totally reflecting mirror and the partially reflecting mirror. This light energy is amplified until sufficient energy is built up for a burst of laser light to be transmitted through the partially reflecting mirror.
The four major types of lasers
While the first optical laser was based on a ruby crystal, there are now four major laser types and materials in use: semiconductor diode, gas, liquid, and solid. In brief, and with considerable simplification, they work as follows:
1: The laser diode: This is a light emitting diode (LED) that uses an optical cavity in solid-state material to amplify the light emitted from the energy band gap that exists in semiconductors. The laser diode can be tuned to different wavelengths by varying the applied current, temperature, or magnetic field, and the output can be a continuous wave (CW) or pulsed.
2) Gas lasers: These use a gas-filled tube for the cavity. A voltage (called the external pump source) is applied to the tube to excite the atoms in the gas to the population inversion in which electrons move from one energy state to a higher one and back. The photons bounce back and forth between the ends of the cavity due to the mirrors, and their numbers build up in an oscillating action. The light emitted from this type of laser is normally CW.
3) Liquid or dye lasers: These use an active material in a liquid suspension in a dye cell as the lasing medium. These lasers are popular because they may be tuned to one of several wavelengths by changing the chemical composition of the dye.
4) Solid free-electron laser: This uses an electron beam traveling along an optical cavity that is immersed in a serpentine external magnetic field. The change in direction of the electrons due to the magnetic field causes them to emit photons. This laser can generate wavelengths from the microwave to the X-ray region.
Of course, the details of operation involve advanced quantum physics, materials science, electromagnetic energy principles, power supplies, and pump sources. The specific wavelength emitted is a function of laser type, materials, and how the laser is excited, or pumped (Table 1).
Table 1: A summary of the various laser types shows the specific wavelength of the light produced by each lasing material. (Table source: Federation of American Scientists)
For laser-based systems designers, the underlying principles are of interest in so far as they contribute toward acquiring an understanding of the related parameters, their implications, and their limitations.
Critical laser parameters for designers
As with all components, there are some top-tier parameters that define basic selection and performance, along with many second and third-tier parameters. For lasers, the parameters looked at first are output wavelength, output power, beam diameter, and beam divergence (spread). Also important are output type (pulse or CW), efficiency, output beam cross-section shape (profile), lifetime, controllability, and ease of use.
Note that laser output power can range from milliwatts (mW) to kilowatts (kW), depending on wavelength and laser type. Many laser applications such as small-scale test and measurement instrumentation only need a few milliwatts, while kilowatt lasers are used for metal cutting and directed-energy weapons.
As with all optical power measurements, quantifying laser output power and doing so accurately is complicated, and technologists at the National Institute of Standards and Technology (NIST) have devoted considerable effort to the challenge. The measurement is affected by the characteristics of the optical energy: wavelength, power level, CW or pulse, and what parameter is being measured, such as average power, peak power, spectrum, and dispersion) (Table 2).
Table 2: Measuring laser optical power is a major challenge, and different sensors and techniques are required depending on the wavelength and output period. (Table source: Coherent Inc.)
Also note almost anything to do with lasers, output power, and wavelength is subject to many safety restrictions to prevent eye, skin, and material damage. These complicated restrictions and the associated laser classes are defined by regulatory agencies in various countries and regions of the world. This is another good reason to use the lowest possible laser power for the project and why vendors offer lasers with spaced output power levels. For example, the REO family includes similar HeNe lasers with 0.8, 1.0, 1.5, 2.0, 3.0, 5.0, 10, 12, 15, and 25 mW output—a range of over 25:1.
HeNe laser applications, features, and operation
As with all component choices, there is no single “best” laser unit, as applications need different wavelengths, power levels, and other specifications, generally defined by the physics of the situation. The HeNe laser is often a good fit for many industrial and test projects such as Raman spectroscopy, a non-destructive optical inspection technique that does not require direct physical contact with the sample.
This spectroscopy is used for fast and accurate chemical analysis of solids, powders, liquids and gases in material analysis, microscopy, pharmaceutical, forensics, food fraud identification, chemical process monitoring, and various homeland security functions. Among the attractive attributes of the HeNe laser for these applications are its stable output wavelength and power, extremely monochromatic red output at λ = 632.8 nm (often simplified to 633 nm), narrow beam, low divergence, and good output coherence and stability over distance and time.
The HeNe laser is built around a hollow glass tube with inward-facing mirrors and filled with 85-90% helium gas and 10-15% neon gas (the actual lasing medium) at a pressure of about 1 Torr (0.02 pounds per square inch (lb/in2)). The tube also has two inward-facing mirrors. One is a flat, high-reflecting mirror at one end, the other a concave output coupler mirror with approximately 1% transmission at the other end (Figure 1).
Figure 1: The heart of the HeNe laser is a glass tube filled mostly with helium, with a small percentage of neon; the tube has a fully reflective internal mirror at its back end and a 1% transmission mirror for output coupling at the beam-exit end. (Image source: Wikipedia)
During the pumping process, an electrical discharge through the gas mixture is initiated by a high-voltage pulse (approximately 1000 volts to 1500 volts DC, at 10 to 20 milliamps (mA)). The actual lasing comes from de-excitation of carriers between electron orbital energy levels (such as 3s to 2p) of Ne atoms. This 3s to 2p transition produces the primary 632.8 nm output. Other energy level transitions also occur, producing outputs at 543 nm, 594 nm, 612 nm and 1523 nm, but the 632.8 nm output is the most useful.
HeNe lasers now catalog items
In the early days of lasers, units were often hand crafted as was the power supply. Now, lasers—especially widely used ones such as HeNe gas lasers—are available as immediate “off-the-shelf” components with power ratings spanning a wide range, as demonstrated by two lasers in the REO family from Excelitas Technologies.
The first example, the Model 31007, is at the low end of the power scale, capable of delivering 0.8 mW (minimum) with a beam diameter of 0.57 millimeters (mm) and beam divergence of 1.41 milliradians (mrad) (Figure 2). It requires 1500 volts at 5.25 mA during operation for the laser tube, which is about 178 mm long and 44.5 mm in diameter; it has a Center for Devices and Radiological Health (CDRH)/CE safety rating of IIIa/3R.
Figure 2: The Model 31007 low-power HeNe laser can deliver at least 0.8 mW with a beam diameter of 0.57 mm and beam divergence of 1.41 mrad. (Image source: Excelitas Technologies)
At the higher end of the REO power range is the 30995, a 17 mW (typical), 25 mW (maximum) laser that requires 3500 volts at 7 mA. Its tube length is about 660 mm, beam width is 0.92 mm, and divergence is 0.82 mrad. It has a more restrictive IIIb/3B CDRH/CE safety rating.
There are many reasons to select the lowest power laser that can do the job. Lower power means reduced safety concerns and regulatory mandates, along with smaller tube size, lower cost, and a smaller power supply.
Power supply: critical to HeNe lasers
The power supply is critical to laser component performance. For HeNe lasers, the tube first needs about 10 kV DC (breakdown voltage) to initiate the excitation process. In addition, it requires a steady-state sustaining voltage in the 1 to 3 kV DC range, along with current below 10 mA. Although the power level is modest—just 20 to 30 watts—few engineers are equipped, trained, or have the time to design a proper supply for this voltage, particularly given the safety and regulatory requirements and certifications for factors such as creepage and clearance, in addition to basic electrical and electromagnetic (EMI) performance.
Why the need for the higher initiation voltage compared to the sustaining voltage? The HeNe laser is a “negative resistance” device so the voltage across the tube decreases as the current increases. The same issue occurs with the simple neon bulb, such as the legendary but now largely obsolete NE-2 “glow lamp” bulb. Its breakdown or “striking” voltage is at around 90 volts (AC or DC), after which the operating voltage drops to about 60 volts. One way designers provided the higher initiation voltage, followed by a lower operating voltage, was to use a series ballast resistor of around 220 kilohms (kΩ) (Figure 3).
Figure 3: Negative-resistance devices such as HeNe laser tubes and neon lamps (such as the NE-2 pictured here) need a ballast resistor function to accommodate their higher voltage/lower current initiation phase, followed by their lower voltage/higher current sustaining phase. (Image source: Lewis Loflin/Bristol Watch)
However, this simple solution isn’t appropriate for an HeNe laser tube in a commercial application. First, there are the safety and regulatory mandates. Second, the supply must be properly matched to the tube for optimal performance and the initiation voltage must be maintained within tolerance. Third, the stability of the supply’s output voltage and current sourcing is critical to maintaining laser stability.
For these reasons, Excelitas Technologies offers plug-in supplies that meet technical and regulatory requirements for lower power HeNe lasers. For example, the 39783 power supply operates from 100 to 130 volts AC and 200 to 260 volts AC (50 to 400 hertz (Hz)), and delivers 1500 to 2400 volts with a starting voltage above 10 kV DC, and an operating current of 5.25 mA (Figure 4). Tight current regulation is important for stable HeNe tube performance, so the 39783 keeps it to ±0.05 mA. The power supply has a modest footprint of 241 x 133 mm and a height of 54 mm. It also comes with a physical keylock for security and safety.
Figure 4: The 39783 power supply for HeNe lasers provides a stable, controlled voltage and current for both the initiation and sustained operating phases of the HeNe tube, while meeting stringent regulatory requirements for kilovolt-class supplies. (Image source: Excelitas Technologies)
For larger HeNe tubes, Excelitas has the 39786 supply in the same package size. This unit has a higher output of 3200 to 3800 volts, a starting voltage above 12.5 kV, and supplies DC current up to 7.0 mA.
Lasers come in many forms for many applications. For industrial system designers looking for stable monochromatic output at reasonable power levels, the HeNe gas laser is an attractive option. However, as shown, the lasers must be combined with the right power supply to meet performance, regulatory, safety, and security requirements.
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