HP (now Agilent) 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:
I later discovered that the high polarization ratio was due to the particular cavity length that just happened to be present with no voltage on the PZT. And the defect is apparently that the PZT somehow doesn't work - there is no change in the modes regardless of PZT voltage from 0 V to 1.5 kV. That is, unless, my incorrect connections somehow ruined the PZT. (But everything else about the tube seems normal so a dud PZT makes the most sense for the defect.)
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):
The HP-5501B is functionally similar to the 5501A but uses a different HeNe laser tube with thermal instead of PZT tuning, and thus different electronics. The tube is similar to the one in the HP-5517A (below) and other later model HP/Agilent lasers.
(These 4 photos provided courtesy of: John Cincotta (johncincotta@comcast.net).)
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).)
Next, we have the bare HeNe laser tube removed from a similar HP-5517A laserhead. Essentially the same bare tube is used in the HP-5501B, HP-5517A, B, C, and D, and other HP/Agilent metrology lasers other than the earliest ones like the 5501A and 5500C.
(The following description and photos provided courtesy of: Mike Harrison (mike@whitewing.co.uk).)
The description of the HP-5517A 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. This is the heater used to fine-tune the cavity length by thermal expansion. It is bifilar wound to minimize magnetic fields. By placing the heater inside the tube, the time from power-on to a locked condition is reduced from 10 or 20 minutes, to under 5 minutes.
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.
The HP/Agilent 5519A and 5519B lasers are very similar to the 5517A in terms of case style, but include an optical receiver for the return beam from an interferometer. This allows the 5519A/B to be used stand-alone without any other optical receiver, only the measurement electronics. In fact, the 5519A/B are the only of these lasers to include a DC power supply internal to the laser head so they plug into a normal AC outlet instead of requiring a separate DC power supply.
