Hewlett-Packard 5501A Helium-Neon Laser (Gallery)
HP manufactures what is called a "Two Frequency HeNe Laser" for use in
their interferometry based measurement and calibration systems. There
are several versions of these lasers including the 5501 and 5517 (shown
in the next section).
The tube below was used in at least some versions of the
HP 5501B laser head, part of the HP 5501A Interferometry Measurement System.
Position/distance resolution down to better than 10 nm (that's nanometer as
in 0.000000001 meter!) were possible with this equipment. Additional
information may be found in the Laser FAQ section:
Hewlett-Packard HeNe Lasers.
I acquired this tube for its curiosity value. Here are some observations
of its characteristics:
- This is a red HeNe laser with a power output of about 0.25 mW (based on a
comparison with a normal 0.5 mW HeNe laser). There is no model number on
the tube itself.
- The cylindrical central area contains some powerful toroidal magnets.
Their field appears to be oriented N-S along the axis as expected with poles
at each end. These would be for Zeeman splitting to generate the two
frequencies needed for the interferometer application.
- The beam output is from the left side (in the photo) via a beam expander
and collimator. The exit diameter is about 6 mm. There appear to be more
than just the two lenses expected for a simple beam expander and collimator.
In disassembling and already partially disassembled collimator from a 5517
laser, I found a couple of thin pieces of plastic, purpose unknown.
- The external optics are adjustable both via the knurled barrel that is
visible as well as via internal knurled rings accessible behind the cover
(attached by the single Philips head screw). By adjusting just the knurled
barrel, a very well collimated beam could be obtained. With a 6 mm diameter,
this would be quite low divergence - about .27 mR total. (The blue dots line
up when collimation is optimal.) By continuing to turn the barrel, I could
focus the beam to a really small dot (a singularity as far as appearance is
concerned) at 20 feet.
- With the optics assembly removed, the beam looks like the beam from any
other HeNe laser.
- Both the OC and HR mirrors are totally inside the glass envelope of the
tube and are about 7 mm thick and 15 mm in diameter. They appear to be
mounted solely by springs against the ground faces of the central bore. No
mirror adjustment seems possible. I wonder if this is a case of simply
manufacturing the raw tube to such precision that no adjustment is needed.
Perfection by design!
- The optical window for the beam (which is separate from the OC) is fused
to the rest of the glass envelope at a slight angle - perhaps 5 degrees. It
is not AR coated (though the outside surface of the OC itself IS AR coated
as expected) and thus there is a not-so-weak ghost beam a few degrees off
of the main beam. However, this does not affect anything being totally
blocked by the optics assembly.
- Unlike most ordinary HeNe lasers, even from initial power-up, there is
minimal or no mode cycling or beam intensity fluctuations (monitoring with
a simple photodiode based laser power meter).
- The beam is moderately polarized - maybe 10:1. The axis is oriented as
though there was a Brewster window angled horizontally. I don't know if this
is by design just an artifact of this particular tube - or the defect that
caused it to be rejected (which is apparently why I was able to get it).
- Power supply connections are via an anode terminal on the far
side near the output-end and a cathode terminal on the far side of the big
glass bulb on the right. (Neither terminal is visible in this photo.)
Most HeNe laser power supplies designed for 1 mW tubes should work. It
requires a 75K ballast resistor. Optimal current is about 3.5 to 4 mA
(based on maximizing beam intensity).
- There is a third terminal most likely for precise control of the cavity
length to adjust the longitudinal mode position relative to the gain curve.
It appears to attach to the something behind the HR mirror, most likely a
piezo-driver though I could not detect any significant capacitance
or piezo-electric effect (while tapping the tube) with my DMM though it did
seem to hold some voltage when measuring on DC. It reads open circuit
on the ohms range. Nor could I see any change in the beam when driving
it with a few dozen volts.
- A warning on the tube reads: "Do not disconnect power supply as damage to
laser may result". I don't know whether this refers to the tube, the power
supply, or the third electrode (whatever it is) specifically. I did attempt
accidentally power it initially via the third electrode which resulted in
some zapping but no apparent damage.
- The total length of the tube including optics is about 250 mm and it
weighs about 3 pounds (mostly due to the magnet assembly). The distance
between the mirrors is about 125 mm.
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View 01 - Overall view of strange HeNe tube.
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View 02 - End-view showing piezo electrode terminal.
Here are some views of a HP-5501A laser head with the skins off (not
the same one from which the tube, above, was removed):
Hewlett-Packard 5517 Helium-Neon Laser Head and Accessories (Gallery)
The first group of photos are of the laser head and a beam splitter from a
Hewlett-Packard Interferometric Measurement System.
See the Laser FAQ section: Hewlett-Packard
HeNe Lasers for more information and links describing this technology.
(The following 7 photos provided courtesy of: Alvin Lim
(alvin100@singnet.com.sg).)
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View 01 - Interior view of right side of HP 5517A.
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View 02 - Closeup of power supply, ballast resistor, and tube label.
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View 03 - Interior view of left side of HP 5517A showing electronics PCB.
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View 04 - Beam splitter used in the interferometer to direct the beam
to/from the target or reference.
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View 05 - Beam splitter with lenses removed.
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View 06 - Another view.
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View 07 - And another.
Next, we have the bare HeNe laser tube removed from a similar HP5517
laserhead.
(The following description and photos provided courtesy of: Mike Harrison
(mike@whitewing.co.uk).)
The description of the HP5517A laser head exactly matches the unit I have
down to the color of the wires and PSU part number! I think the small vent
mentioned is simply used for filling with the silicone encapsulating goop to
avoid air bubbles.
The tube initially looks fairly conventional in overall layout, but is
made of much thicker glass than normal. The capillary, with the
mirrors on each end, is mechanically separate from the outer glass
envelope, and has a translucent appearance (like silica) and is about
9 mm O.D, with a 1.5 mm bore. It's supported at each end by two springs
(presumably the spring pressure demands the thick glass envelope). The
mysterious second cathode connection goes to a coil of wire, wound in a
helical groove on the capillary, with a turn pitch of about 3 mm. It's
bifilar wound, so it can't be for magnetic purposes. This looks like
it's a heater used to fine-tune the cavity length by thermal expansion.
The beam output is much more divergent than conventional tubes - about
1/4 inch a foot away, hence the external optics in the laser head.
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View 08 - Overall plasma tube after being cut away from aluminum
shroud.
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View 09 - HR-end.
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View 10 - OC-end.
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View 11 - HR-end showing discharge path.
