Jeff Hecht, “Helium neon lasers flourish in face of diode-laser competition”, Laser Focus World, Nov 1992, p. 99 – 108.
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.
sers 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
632.8-nm red line has been the dominant wavelength. Commercial lasers can generate tens of
milliwatts CW on the red line, although typical
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 la-
HeNe Laser
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Jeff Hecht, “Helium neon lasers flourish in face of diode-laser competition”, Laser Focus World, Nov 1992, p. 99 – 108.
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.
cial-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 1000-2000 V.
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.
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).
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 spe-
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 632.8 nm.
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Jeff Hecht, “Helium neon lasers flourish in face of diode-laser competition”, Laser Focus World, Nov 1992, p. 99 – 108.
1523.5 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.
WAVELENGTHS AND PROPERTIES
Red HeNe lasers operating on the 632.8-nm
line remain the least expensive and offer the
highest power levels. Ironically, the gain at 632.8
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 632.8-nm red line.
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.
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 0.5-10 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.
BANDWIDTH AND COHERENCE
Most HeNe lasers oscillate in a single transverse mode, producing a TEM00 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 0.0019 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.
Infrared HeNe lasers have long been available,
primarily at 1152.6 and 3392 nm, where powers
can reach tens of milliwatts. A weak line at
Mass-produced HeNe lasers with normal
linewidth of 1 part in 300 000 have coherence
lengths of 20 - 30 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.
HELIUM NEON WAVELENGTHS
AND POWER LEVELS
Wavelength
(nm)
Maximum
power
(mW)
Gain (relative
to 632.8 nm)
543.5
1.5
1/17
594.1
7.0
1/15
604
2.5
1/10
611.9
7.0
1/5
629
-
1/5
632.8
75
1
635
-
1/8
640.1
1.5
1/5
730.5
0.3
1/8
1152.6
17.5
4/5
1523.5
1.5
-
2396
0.5
-
3392
24
Figure 2. A triangular ring laser can be used to sense
rotation around the laser axis.
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Jeff Hecht, “Helium neon lasers flourish in face of diode-laser competition”, Laser Focus World, Nov 1992, p. 99 – 108.
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.
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.
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.
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.
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.
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
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.
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 (1.9 10-3 nm), mode spacing is
0.5 GHz (6.7 10-4 nm) and cavity FWHM is 1.2 MHz
(1.6 10-6 nm) when HeNe gain peaks at 4.738 1014 Hz
(632.8 nm).
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Writing and recording. A modulated laser
beam can be scanned across a light-sensitive sur-
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Jeff Hecht, “Helium neon lasers flourish in face of diode-laser competition”, Laser Focus World, Nov 1992, p. 99 – 108.
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.
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.
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 1-8002McGRAW.
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.
REFERENCE
1.
Robert G. Knollenberg, "Prospects for
the HeNe laser through the end of the century,"
in Design of Optical Systems Incorporating LowPower Lasers, SPIE Proc. Vol.741, Betlingham,
WA (1987).
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.
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