Helium neon lasers flourish in face of diode-laser competition

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1 Helium neon lasers flourish in face of diode-laser competition The venerable HeNe laser still has a place in applications that need coherence, visible wavelengths, and good beam quality. Jeff Hecht, Contributing Editor Competition from semiconductor lasers has forced the helium neon (HeNe) laser to evolve. A diode laser can generate a 1-mW beam more cheaply and more efficiently, from a much smaller package, and without the need for high voltage. HeNe lasers survive because they have other advantages, including better coherence, better beam quality, and shorter wavelengths. These advantages combine to let HeNe lasers do some jobs that diode lasers cannot, such as recording holograms or generating milliwatt powers of green, yellow, or orange light. Red HeNe lasers remain cost-effective for other applications, such as high-speed laser printers and certain displays, because the human eye and many materials are more sensitive to the 633-nm HeNe line than to the 650- to 680-nm output of current commercial red diode lasers. Diode lasers are encroaching on many traditional HeNe-laser markets, including the largest one - barcode scanners. But the HeNe laser is not dead yet. HeNe-laser manufacturers are reducing production costs, improving lifetimes, and finetuning performance, which has helped HeNe lasers retain a healthy share of many markets, including fixed barcode scanners at supermarket checkouts, holography, and diagnostic medical systems. In addition, continuing production of equipment designed with HeNe lasers and a large installed base of existing HeNe-laser systems ensure a continuing market for new and replacement tubes. And before you scoff at the HeNe laser as just another vacuum tube, remember that electronics suppliers still stock some vacuum tubes 40 years after Sony built the first transistor radio. BASICS OF HENE-LASER OPERATION The first gas laser demonstrated the HeNe laser was constructed in 1961 by Ali Javan, Donald Herriott, and William Bennett at AT&T Bell Laboratories (Murray Hill, NJ). The original HeNe laser emitted at 1153 nm in the infrared (IR); lasing on the visible red line was demonstrated the following year. Since then, that strong nm red line has been the dominant wavelength. Commercial lasers can generate tens of milliwatts CW on the red line, although typical HeNe Laser - 1 (5) -

2 power levels are in the milliwatt range. Millions of milliwatt range red HeNe lasers have been sold since they came on the market in the 1960s, and they are the usual types used to demonstrate laser operation in schools and museums. Infrared, green, yellow, and orange HeNe lasers are also available, but for most purposes the basic HeNe laser remains a small red-emitting laser. The standard HeNe laser is a sausage-shaped glass tube filled with gas. An electric discharge passing between electrodes at opposite ends of the tube excites the gas, producing a population inversion. Light resonates between mirrors at opposite ends of the tube; one is totally reflecting, the other transmits about 1% of the incident light, which emerges as the beam. Cavity lengths range from about 10 cm to 2 m long. Over the years, the internal structure has become complex (see figure on the previous page). The discharge passes from the cathode to the anode through a capillary bore that is about 1 mm in diameter, which concentrates the discharge current to improve overall efficiency. The smalldiameter bore also controls transverse mode structure, beam diameter, and beam divergence. Overall tube diameter is much larger, typically about 3 cm, to provide a gas reservoir. Direct bonding of the mirrors to metal end plates on the tube reduces helium leakage to rates low enough that standard mass-produced tubes have operating lifetimes of 20,000 to 25,000 h. (Some special-purpose HeNe lasers are made with Brewster-angle windows and external-cavity mirrors; these typically are for multiwavelength or highpower operation.) The active medium in a HeNe laser is a mixture of helium and neon with total pressure of a fraction of a torr to several torr, depending on tube diameter. Typical mixtures contain 5 to 12 times more helium than neon. After an ignition pulse of 10 kv breaks down the gas to start laser operation, a current of a few milliamperes passes through the tube at V. Electrons in the discharge raise both helium and neon atoms to excited states. The moreabundant helium atoms collect most of the energy, then transfer it to neon atoms, which have excited states at about the same energy. The excited neon atoms then drop to lower metastable levels. Lasing is possible on several transitions, with the wavelength selected by the choice of optics and operating conditions (see Fig. 1). Although standard HeNe lasers are linear tubes, one unusual variation the ring laser-has found unusual applications. Ring laser resonators have three or four mirrors that define a "ring" path inside the laser cavity (see Fig. 2). Light oscillates around the ring in both directions. Slight differences between the two counterpropagating beams can detect rotation around the central axis in ring-laser gyroscopes used in commercial and military aerospace systems. Figure 1. Many HeNe-laser transitions are possible (solid lines with arrows). The expanded view at right shows how transitions to different sublevels produce the family of visible HeNe-laser lines. The strongest transition is at nm. - 2 (5) -

3 WAVELENGTHS AND PROPERTIES Red HeNe lasers operating on the nm line remain the least expensive and offer the highest power levels. Ironically, the gain at nm (0.5 db/m) is much lower than a gain at 3.39 μm (22 db/m). 1 A lack of applications for the strong IR line, however, led developers to concentrate on the nm red line. As the table shows, gain on the red line is 5 to 17 times higher than gain on other visible lines, and the highest visible powers are in the red. The other visible lines probably would not have been developed if HeNe-laser technology had not already been established for red lasers. Typical red powers are mw, with the most powerful commercial models rated at 75 mw. The 543 nm green line is the weakest, with highest powers only about 1.5 mw, but there is strong interest in that wavelength. The yellow and orange lines can deliver more power, but fall far short of the 633 nm red line. Some HeNe lasers can be tuned to emit on several visible and near-ir lines from 543 to 730 nm or to emit simultaneously on several visible lines. Infrared HeNe lasers have long been available, primarily at and 3392 nm, where powers can reach tens of milliwatts. A weak line at HELIUM NEON WAVELENGTHS AND POWER LEVELS Wavelength (nm) Maximum power (mw) Gain (relative to nm) / / / / / / / / / / nm, generating up to 1.5 mw, has attracted interest because it falls into the window of minimum loss in silica-glass optical fibers, making it useful for testing. Limited production and the need for special optics combine to make prices higher for HeNe lasers operating at wavelengths other than the standard red line. Sales of mass-produced red HeNe lasers remain much larger than those of HeNe lasers operating at other wavelengths, but the number of those specialty lasers may grow as diode lasers displace red HeNe lasers in many traditional uses. BANDWIDTH AND COHERENCE Most HeNe lasers oscillate in a single transverse mode, producing a TEM 00 beam with classical Gaussian-intensity distribution, but some models emit multiple transverse modes. The Doppler-broadened gain curve of mass-produced HeNe lasers typically is about 1.4 GHz, the equivalent of nm in the red. This spans several narrow longitudinal cavity modes (the number is dependent on the length of the cavity), which determines mode spacing (see Fig. 3). The shorter the laser cavity, the fewer modes fall under the gain curve. Mass-produced HeNe lasers with normal linewidth of 1 part in have coherence lengths of cm, adequate for holography of small objects if care is taken. If narrower linewidth is required, optics can be added to restrict oscillation to a single cavity mode, which also greatly extends coherence length. Figure 2. A triangular ring laser can be used to sense rotation around the laser axis. - 3 (5) -

4 APPLICATIONS Sales of HeNe lasers rose steadily through the 1980s, driven largely by the growth of barcode scanning-first in supermarkets, then in other inventory-control applications. HeNe lasers have long dominated that market, but other light sources recently captured a large share. The reasons are partly historical. Barcode developers wrote their initial standards for the Universal Product Code (UPC) in the 1970s, at a time when the HeNe laser was the only low-cost, mass-produced laser. Those specifications included the assumption that UPC symbols would be read at the red HeNe-laser wavelength. That specification allows visible inspection of barcode quality during printing and lets packagers use colors other than black and white, but it forces all scanners to use red light. The high beam quality of HeNe lasers is essential for supermarket scanners. The beam repetitively scans a well-defined pattern, and variations in the scattered light are decoded to read the symbols. In theory, the scanner should be able to read barcodes regardless of their orientation if they come reasonably close to the scanning window in the checkout counter. Reading barcodes without regard to their orientation requires a large depth of focus, which is possible only with a good-quality beam, because the window is about 0.5 m from the laser head Figure 3. Doppler-broadened gain curve of a HeNe laser shows the much narrower cavity resonances that lie within the curve. Doppler FWHM (full width at half maximum) is 1.4 GHz ( nm), mode spacing is 0.5 GHz ( nm) and cavity FWHM is 1.2 MHz ( nm) when HeNe gain peaks at Hz (632.8 nm). under the checkout counter. That depth of focus is not needed for hand-held wand scanners used in lower-volume retail operations in which clerks pass the scanner directly over the barcode. At this writing, HeNe lasers remain the standard choice for stationary checkout scanners, but red diode lasers and LEDs are preferred for scanners that do not require great depth of focus. Helium neon lasers are well established in a broad range of applications. While some of these applications may be challenged by visible diode lasers, the better coherence, beam quality, and wavelength of HeNe lasers will hold other applications. Holography. The coherence, low cost, and visible output of red HeNe lasers have made them the standard choice for recording holograms of stationary objects since the first laser holograms were made in the early 1960s. They are likely to remain so because their output is more coherent than standard diode lasers. Demonstrations and displays. Red HeNe lasers have been the standard choice for school and museum laser demonstrations because of their good beam quality, coherence, and modest cost. Similarly, they are used in laser displays in which their red color and milliwatt power levels will suffice. Other visible HeNe-laser lines can be used in displays, taking advantage of the human eye's greater sensitivity at shorter wavelengths. Red diode lasers have captured much of the market for laser pointers, because diode lasers can be made much smaller and can operate from batteries for a reasonable time. Alignment and positioning. An early commercial use of HeNe lasers was to draw straight lines to aid in aligning or positioning using human eyes or electronic detectors. Construction workers use scanning beams to define a plane or line while building walls and hanging ceilings. Visible lasers draw straight lines to keep sewer pipes and tunnels on straight courses. Laser beams are used to align grading equipment for construction and agricultural irrigation. Lasers define straight paths for ma-chine tools and help medical personnel position patients in x-ray imaging systems. Infrared diode lasers have replaced HeNe lasers in some systems with electronic sensors; the use of visible diode lasers is spreading. HeNe lasers offer better beam quality and visibility than visible diode lasers, but their larger power requirement is a disadvantage. Writing and recording. A modulated laser beam can be scanned across a light-sensitive sur- - 4 (5) -

5 face to record information. In laser printers, the beam scans a photoconductive drum, discharging the electrostatic charge held by the surface at points where the beam on, producing a pattern that is printed by a copier-like process. Lasers also can encode data as a series of dots on light-sensitive disks for computer data storage. Initially, HeNe lasers were used for these applications, but today most systems use semiconductor lasers, except for high-speed printing. HeNe lasers also are used in some printing and publishing applications such as color separation and reprographics. Medicine. Red HeNe lasers have been used in several types of medical therapy, primarily unconventional treatments such as laser acupuncture, biostimulation, wound-healing stimulation, and pain alleviation, but they are being replaced by visible diode lasers in many of these treatments. The orthodox medical establishment uses HeNe lasers as pointers for IR surgical lasers. HeNe lasers are also used in diagnostic instruments that sort cells and perform biological measurement such as light scattering. The green and yellow lines can excite fluorescent dyes and also are used in instrumentation for cell sorting and counting. Measarement. Good coherence and beam quality make red HeNe lasers the most common choice for interferometric measurements of surface contours. They also are used to measure light scattering, as are some HeNe lasers operating at other visible wavelengths. Ring lasers are used as rotation sensors or laser gyroscopes for aircraft navigation; they are standard on Boeing 757 and 767 airliners. Research and development. HeNe lasers provide inexpensive sources of tightly collimated, coherent beams for many types of research. Although red HeNe lasers are common, those operating at other wavelengths also are widely used in research. For example, the 3.39 μm line is absorbed strongly by carbon hydrogen bonds in hydrocarbons, which is important for spectroscopy. Single-frequency lasers may be used in laboratory measurements of time and frequency. ACKNOWLEDGMENT This material was adapted with permission from The Laser Guidebook, 2nd. ed., J. Hecht (McGraw-Hill, New York, NY, 1992). Ordering information can be obtained by calling McGRAW. REFERENCE 1. Robert G. Knollenberg, "Prospects for the HeNe laser through the end of the century," in Design of Optical Systems Incorporating Low- Power Lasers, SPIE Proc. Vol.741, Betlingham, WA (1987). - 5 (5) -