And this tube is peculiar in that it eventually reverts to normal behaviour once close to thermal equilibrium as shown below: Plot of “Flipper” Aerotech OEM1R He-Ne Laser Head at Transition to Normal Behaviour (Combined). Usually, modes when part of the way through the gain curve and then either flipped abruptly or oscillated between polarizations for a short time and then flipped. This was the case with the original He-Ne laser but is quite rare today given the design of internal mirror He-Ne tubes and the relative simplicity of the required DC power supply. The tube is a Melles Griot model 05-LHB-570. The mirrors may be perfectly flat (planar) or one or both may be spherical (concave with respect to the inside of the cavity) with a typical Radius of Curvature (RoC = 2 * the focal length) ranging from approximately the length of the cavity (L) to 2 or 3 times L. (Positive RoC means a concave mirror. Starting voltage of 5 to 12 kV at almost no current.In the case of a He-Ne tube, the initial breakdown voltage is much greater than the sustaining voltage. With only two active modes, effects of mode changes may be obvious even without anything more than Mark-I eyeballs and a polarizing filter but fancy equipment may be needed to fully characterize what’s going on. A wide bore tube would actually be cheaper to manufacture than one requiring a super straight narrow capillary. Lasers from Melles Griot, JDS Uniphase, and Siemens/LASOS generally have well behaved polarization. The U.S. Occupational Safety and Health Administration (OSHA) also have laser safety guidelines to help health care facilities keep their medical staff and patients safe. . However from a test using a Melles Griot plasma tube with two perpendicular windows in place of internal mirrors, the orientation of the external mirrors had no impact on the polarization axes and the modes were well behaved. CAUTION: While most modern He-Ne tubes use the mirror mounts for the high voltage connections, there are exceptions and older tubes may have unusual arrangements where the anode is just a wire fused into the glass and/or the cathode has a terminal separate from the mirror mount at that end of the tube. The beam on/off button on the side not surprisingly didn’t control the power supply but rather moved a sliding shutter. As the tube is used (many thousands of hours or from abuse), operating and starting voltages may be affected as well – generally increasing with the ultimate result being that a stable discharge cannot be initiated or maintained with the original power supply. The main advantage of gas lasers (eg: He-Ne lasers) over solid state lasers is that they are less prone to damage by overheating so they can be run continuously. The date of manufacture was 1978. Average several readings of both the laser and transmitted power to minimize the error introduced due to power variations from mode cycling. Don’t know where you’d find it, but the Laser Institute of America (LIA) might be a good start. However, there is anecdotal evidence to suggest that instant tube death may result from such cleaning attempts. Pressing on the mirror mount of my flipper tube in any direction would reduce the output power significantly due to changing mirror alignment. (These photos courtesy of Meredith Instruments.) External mirror He-Ne lasers almost always use Brewster windows and so are inherently linearly polarized. Some will also have some ‘wedge’ (where the outer surface is angled slightly with respect to the beam axis to minimize instability resulting from reflections directly back into the resonator.I have also come across He-Ne laser output mirrors with a slight *negative* RoC – they are convex rather than concave with respect to inside the cavity. In the absence of external influences like magnetic fields, the mode orientation in a laser will be determined by at least two factors: Since a transverse magnetic field can also introduce a polarization preference, it is possible to cause a well behaved He-Ne laser tube to exhibit flipper behaviour by the careful placement of s strong magnet near the tube. This is why a He-Ne life curve is usually pretty flat, then quickly degrading to nothing over about a 200 hour period. In the first He-Ne lasers (see the diagram below), exciting the gas atoms to the higher energy level was accomplished by coupling a radio frequency (RF) source (i.e., a radio transmitter) to the tube via external electrodes. The remainder were totally dead.). However, with this small amount of gas, it doesn’t take much contamination or leakage to ruin the tube. Note that there are some multi-mode (non-TEM00) He-Ne tubes with wider bores and a different mirror curvature that produce up to perhaps twice the power output for a given tube length. I would expect that it is more likely to be back-reflections from the highly curved outer surface of the output mirror but could it be some other aspect of their design? The positive output of the power supply is connected to a series ballast resistor and then to the anode (small) electrode of the He-Ne tube. Every He-Ne tube will have a nominal current rating. :-)Should the getter spot (if visible) turn to a milky white or red powdery appearance, it is exhausted and the tube is probably no longer functional.If you had grown up during the vacuum tube age, the getter would be familiar to you since nearly all radio and TV tubes had very visible silvery getters (and CRTs still do).The getter electrode can be seen in photos: The mirrors are not silvered or aluminized (metal coated) but are a type called ‘dielectric’. However, on a few (usually Hughes-style), the anode may be a wire fused into the glass with the mirror mount separate from it.The discharge at this end produces little heat or damage due to sputtering. As shown by Table 3, various lasers have been used for various ophthalmic applications including retinal photocoagulation using argon blue-green laser (488/514 nm), double-YAG green laser (at 532 nm), krypton laser (at 647 nm) and diode lasers (at 806-810 nm). 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