The Test Arm may be a tool in a CNC milling machine, a stage in a semiconductor wafer stepper, a voice coil positioner in a hard drive servo writer, or any number of other precision devices. A single laser can be used with many independent measurement axes through the use of beamsplitters, separate interferometer optics and optical receivers, and associated digital processing channels.
The key attributes that make these lasers ideal for metrology applications are that they produce two frequency components a few MHz apart that are linearly polarized, orthogonal, and oriented along the X and Y axes (horizontal and vertical) relative to the laser baseplate. The optical frequencies are highly stable and the corresponding wavelength (the actual "yard stick") thus should be as well. And, they remain stable for the life of the laser without any maintenance. (However, environmental factors like temperature, pressure, and humidity must to be taken into consideration as they have a significant effect on wavelength.) Since the two frequencies originate from the same laser cavity as a TEM00 (single spatial) mode beam, they are inherently perfectly aligned with each-other, something not necessarily true of alternative techniques using other means such as an Acousto-Optic Modulator (AOM) to generate the second frequency component.
A typical 5517B laser with its covers removed is shown below:
The heart of HP/Agilent two-frequency lasers is the custom HeNe laser tube assembly, which represents most of the cost of the laser. When the heart degrades to the point of making the laser unusable or dead, the choices are either to obtain a replacement laser, or to do a heart transplant, or a rebuild. The transplant is simple, quick, and low risk: Find a good tube assembly in a laser that is broken for some other reason and pop it into a chassis with good electronics. Only one trivial adjustment is required. Problem solved. However, there's one slight difficulty with this approach: HP/Agilent lasers don't often fail for reasons other than the tube, and when they do, repair is generally very straightforward. So, there isn't a huge availability of good tubes in bad lasers.
Used HP/Agilent metrology lasers are also widely available. But many of these are already unusable due to low output power or other problems with the tube. Thus, finding one with both acceptable performance and adequate life expectancy requires a knowledge of what to look for and what tests to perform.
These lasers are often run 24/7 from the day they are installed until the day they die or fail preventive maintenence checks. Such lasers invariably find their way to eBay and unscrupulous sellers will either claim the "came from a working environment" or an inability to test them. The working environment claim may not be inaccurate, it's just that the laser was pulled because it was dead, not that the line was shut down! :) However, if a seller is reputable, has performed a few basic tests, and offers a warranty (even a relatively short one giving the buyer an opportunity to more fully test it), then a previously owned unit may be perfectly acceptable with low risk. Even where it has failed for other reasons like a bad HeNe laser power supply, a broken laser with a good tube may be easily repaired.
However, if it were possible to rebuild a bad tube, then this opens up a third possibility with performance potentially equal to that of a new laser at a fraction of the cost. When done properly, the laser would perform essentially like new and have a decent life expectancy (though possibly not as long as that of the original long-lived custom HP/Agilent tube).
A photo of a typical tube assembly removed from an HP laser is shown below:
And a diagram of the internal structure of a typical tube assembly is shown below:
The tube assemblies in all other 5517 lasers except the 5517A are similar. This is also true of the 5501B. The 5517A, as well as the 5518A/B and 5519A tube assemblies have a single-piece cast-metal casing with keying pegs to mount in their enclosures with no alignment. But the glass laser tube, magnet, and optical components in these are similar to those in the other 5517s. The very old 5501A, and even older 5500A/C lasers used tube assemblies that were quite different with PieZo Transducer (PZT) tuning rather than thermal tuning, rebuild options for these would likely be limited to regasing since the PZT tuning can't easily be replicated with a common commercial HeNe laser tube. However, most of the functional issues dealt with below would also apply to them as well. And in most applications, the 5501B is a drop-in replacement for the 5501A, so it's unlikely that anyone will consider rebuilding the older lasers commercially. Very few 5500A/Cs are still in use providing even less justification for rebuilding them. However used 5500A/Cs and 5501As may be desirable for people wanting to keep legacy systems running with minimal effort and cost.
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When semiconductor fab lines shut down, the lasers often become available at various stages of their life. They appear on eBay and from many surplus dealers at costs ranging from $25 or less to several thousand dollars. However, almost any of these may be less than the cost of a rebuilt laser. 5501Bs, 5517As, 5517Bs, and some 5517Cs are widely available. The 5517Ds are less common in working condition possibly because they are the highest performance common HP/Agilent laser still in use on state-of-the art fab lines, so they tend to become available only when pulled from service due to low power or associated (tube) failure..
If it were possible to have confidence in the operating condition and life expectancy of a previously owned laser, it would represent a low risk alternative to either a new or rebuilt one.
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There are a few companies who will rebuild the tube assembly or sell you a laser with a rebuilt tube assembly. Although I have yet to see a typical cost, the amount of labor involved (more below) would suggest that it is a substantial fraction of the cost of a new laser. And there is some risk since depending on the quality and type of rebuild, the laser may not perform to spec or have a short life. A semiconductor wafer stepper (one of the most common applications of these lasers) is a very expensive piece of equipment often run around the clock. Downtime is costly, and errors in fabrication only found after wafers have been completed are even more costly. So it's not clear at what point the modest savings of a rebuilt laser installed once or twice over the entire life of the machine can be justified against the risk. Nonetheless, some large semiconductor companies are known to have seriously considered going this route and may be using rebuilt lasers in production.
The HP/Agilent tube assembly consists of the actual glass HeNe laser tube, a permanent magnet, beam expander, and adjustable waveplates. The part that goes bad is the glass HeNe laser tube, which is mounted within the magnet using a rubbery potting compound. Rebuilding the laser tube will require that it be removed from the tube assembly as shown below:
This tube assembly was also from a 5517B, but there are some very minor physical differences, which are mostly of little consequence. However, unlike the beam expander in the photo of the complete tube assembly, the one in the disassembled unit is not easily adjustable, and this may make it more difficult to match up with a non-HP/Agilent laser tube. More below.
Rebuilding an HP/Agilent tube assembly can take two forms:
This glass laser tube is normally not considered a separate replaceable component, only the complete magnet/optics assembly (with the tube) at nearly the price of a new laser.
The second view shows the glass nipple at the bottom that would need to be carefully cut in a dust-free environment and attached to the vacuum/gas handling system.
Removing the glass tube intact from the tube assembly is the sort of thing that anyone in their right mind would only attempt once unless appropriate noxious chemicals are available to dissolve the potting compound without damaging anything else in the assembly (except that they likely rot internal organs). Mechanical removal using piano wire, saws, knives, and other instruments of torture is possible, though extremely tedious and time consuming for all the lasers with tubes that conform to the diagram, above. While the glass tube is physically the same for the 5517A, 5518A, and 5519A/B, it will be much more difficult - if not impossible - to remove intact since the outer casting is a single piece. There are no front and back sections that can be separated, which makes access at lesat a little bit easier. But, if the glass tube is to be reused as with regasing, the extraction must be done with great care to avoid damage to the glass envelope or internal components. Otherwise, a hammer and chisel will suffice. :-)
The benefit of regasing is that most of the vital original parameters of the special HeNe laser tube are preserved, especially if it is replaced in the same magnet/optics assembly at the same orientation so that no adjustments of the waveplates are required. Thus, the beam diameter and divergence, and the stability of the internal heater should be unchanged. However, without knowing the original isotope ratio and gas fill pressure, there could still be differences in optical frequency, split (Zeeman, REF) frequency, and other parameters. While the cavity loss probably has the dominant effect on the REF frequency and may be the primary means of setting it for lasers up through the 5517C (3.0 MHz max), and probably even the 5517D (4.0 MHz max), both the isotope ratio and gas fill pressure have a dramatic impact on the optical frequency of the center of the neon gain curve. And either of them (though especially the isotope mix) could affect the effective width of the neon gain curve, and this could impact the mode pulling that gives rise to the Zeeman split of the central longitudinal mode, and thus the REF frequency.
A typical conventional 6 inch HeNe laser tube is shown below:
Then, a heater with electrical characteristics similar to those of the heater in the original tube must be installed on the exterior of the replacement tube:
The tube above was actually mounted in the magnet assembly from an HP-5501A laser (because that was available) and installed in a 5517D laser chassis (because that was available). It locked and produced a usable output, though at a REF frequency of only 1.2 MHz. Normally, a 5501A would be between 1.5 and 2 MHz, but due to the use of the standard tube, the REF frequency was lower.
When a similar tube was installed in the magnet from a 5517B laser which normally has a REF frequency of 1.9 to 2.4 MHz, it produced a beat frequency of only about 1.3 MHz max. But these two tubes had been selected for high output power. When the rear mirror was deliberately misaligned to reduce the output power, it was possible to obtain REF frequencies from 1.5 to 2.4 MHz at somewhat lower, but still very decent output power. This would exceed the HP/Agilent minimum power specification by a factor of 2 or 3 when set for the 5517A or 5517B REF frequency, and still be comfortably above it when set for the 5517C REF frequency. It might be possible to reach the range for the 5517D (3.4 to 4.0 MHz), though that my be marginal. So, in a pinch, a standard tube might work, but would be a less than optimal solution due to the much lower life expectancy of the typical small HeNe laser tube - 12,000 to 16,000 hours.
A tube with a mirror adjuster added was then installed in the magnet assembly from the 5517B laser shown above, and that was installed in the 5517B body as shown below:
After adjustment of the waveplates, it produced a good MEAS signal, in fact actually cleaner than that of the commercially rebuilt laser described below.
But these were only lab experiments, never intended to find their way into commercial products or applications. Nonetheless, it's quite possible that the rebuilt laser I tested was implemented in a similar way. (More on its characteristics below.)
The benefit of using a conventional HeNe laser tube is that it doesn't involve anything that is totally custom or require high vacuum equipment, ultra pure gases, glass working, and other sophisticated processing by the laser rebuilder. When done on a small quantity basis, these can require substantial investment in high tech equipment and the skills to go with it. Even if the laser tube supplier has to modify their "recipe" starting with the mirror parameters (reflectance and radius of curvature), but possibly gas-fill (He:Ne ratio and pressure) as well, that should be much less expensive and risky than developing the capability to rebuild tubes in-house. And a company that produces HeNe laser tubes routinely, will have the experience to get the processing correct. And since there's no need to save the original tube, it can be removed from the magnet assembly quickly using any method that works. :)
But many parameters of the laser can change, and some of these may impact measurement performance and lifetime. Two key issues with a laser like this that I have tested are the beam profile and jitter or "fuzz" in the MEAS signal, traced to the existence of "rogue" longitudinal modes in the laser output (modes that shouldn't be there) not oriented with the X and Y axes. And life expectancy is a total unknown.
The Zygo 7705 laser which has specifications similar to those of a 5517D is based on a very short tube of conventional design with a total length of around 10 cm, so perhaps 9 cm between mirrors. It is a custom long life tube made by Zygo and is not available for non-Zygo uses. The closest commercial HeNe laser tube is the Spectra-Physics 007 (same as the Melles Griot 05-LHR-007) with a cavity length of about 11 cm. These are almost certainly no longer available, and even if they were, would have a relatively short life compared to the Zygo tube. It's not known whether the Zygo tube has isotopically pure gases, but it probably does not and thus has to be very short. The HP/Agilent tubes can have a much longer cavity length of 5 inches, most likely because they do have isotopically pure gases.
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The beam profile on a used laser should not have changed unless someone attempted to adjust it or swap optics to convert from one beam diameter to another (e.g., from 6 mm to 9 mm). Swapping standard HP/Agilent beam expanders doesn't usually cause problems.
The near field beam profile of a laser rebuilt using a conventional HeNe laser tube was found to be noticeably different than that of an original HP/Agilent laser with the 9 mm beam diameter option. There is a distinct hot spot in the center, very obvious by eye. While it should be possible to make this laser work in its intended application when new, it may be more sensitive to changes in interferometer alignment or may degrade more rapidly compared to the normal 9 mm beam profile. The profile looks more like a 6 mm beam where the edges are not cut off, as is normally the case with the HP/Agilent optics, rather than a true 9 mm beam where a higher power diverging lens is substituted in the beam expander and the center portion is enlarged by 50 percent. So, this may just be a 6 mm beam expander with a larger output aperture. Thus, the behavior will be somewhere between that of the normal 6 mm and 9 mm optics. In principle, it should be possible to correct the beam profile by decreasing the focal length of the expanding lens in proportion to the ratio of beam diameters, assuming the collimating lens has a large enough diameter and appropriate aperture.
None of these is likely to change in a significant way over the useful life of the laser. However, when the output power declines to well below the HP/Agilent spec'd minimum, amplitude ripple in the output power due to laser current ripple from plasma oscillations and/or the HeNe laser power supply itself, can interfere with a clean beat frequency.
The basic setup used to evaluate HP/Agilent lasers is shown below:
Any HP/Agilent 5501A/B or 5517A/B/C/D/E/F/G laser can be very quickly evaluated for basic functionality. The oscilloscope display was what initially called attention to the rogue mode issue.
The effect of rogue modes that aren't aligned with the X and Y axes will be interference or baseband fringes that will result in low frequency level shifts of the signal in the optical receiver as the tool or stage is moved. Essentially the same thing will happen if the normal Zeeman (F1 and F2) modes are not pure or are not aligned with the X and Y axes. (Or the entire laser isn't aligned with its baseplate parallel to the X or Y axis.) This may cause small transient errors in position while the tool or stage is in motion. However, the end-points may be accurate if the DC level shift resulting from the periodic static errors is ignored by the AC-coupled optical receiver circuitry.
The MEAS signal may become fuzzy from duty cycle variations but only when the position is changing. It will appear normal when everything is stationary. While the exact effect on accuracy is not known, theory suggests that there could be errors of up to 10s of nanometers while the tool or stage is in motion, but no error at the end-points, though the settling time may be larger. However, there could be ripples in the velocity (or other) function that needs to be feedback controlled between end-points.
The symptoms are as follows: The MEAS signal out of the optical receiver should be a clean square-wave with approximately a 50 percent duty cycle. As the remote mirror or cube corner moves, the period/frequency of the square-wave gets smaller or larger due to Doppler shift but the duty cycle remains the same. The system determines position by accumulating the relative phase difference between MEAS and REF, which is a square-wave of fixed frequency, the unmodified Zeeman beat output of the laser
These two photos show an example of a normal MEAS signal with clean rising
and falling edges, and a MEAS signal where there is a small amount of movement
of the remote mirror. In fact, it's likely that both the rising and falling
edges are moving back and forth, but since the oscilloscope only triggers on
the rising edge, that one appears sharp. (The jaggedness in the falling
edge is an artifact of the wiring termination.)
The photos below made using a Scanning Fabry-Perot Interferometer (SFPI) show the longitudinal modes of a normal 5517 laser on the left with those of a 5517 laser rebuilt using a conventional HeNe laser tube that had rogue modes on the right:
The large peaks consist of both F1 and F2, but the SFPI: is unable to resolve the difference in frequency of only a few MHz. These are the normal split longitudinal mode of the Zeeman laser. As can be seen, there is no evidence of any other modes in the left photo display of the normal 5517 laser. In fact, a genuine HP/Agilent laser will have at most 2 modes present as warmup when the modes are sweeping though the split neon gain curve, but supports only a single (split) mode when it is locked and centered. The display of the laser on the right with the rogue modes has a pair of smaller peaks on either side of the main peak for a total of 3 modes.
The FSR of the SFPI is 2 GHz. (The FSR or Free Spectral Range is essentially the longitudinal mode spacing of the SFPI cavity, and is thus the maximum extent in optical frequency over which there is no aliasing in the display - it is unique.) So, a 2 GHz FSR means that the longitudinal mode separation of the laser is either around 667 MHz or 1.33 GHz (aliasing due to the 2 GHz FSR). A mode spacing of 667 MHz cannot be correct as that would imply a distance between laser tube cavity mirrors of almost 9 inches - more than the available space! So, the smallest peak to the left of the large peak of the Zeeman modes actually goes with the previous large peak. And the other not quite as small peak to the right of the large peak of the Zeeman modes actually goes with the next large peak. Thus, the spacing of the rogue modes with respect to the F1/F2 modes are approximately +/-1.3 GHz corresponding to a laser tube cavity length of about 11.3 cm or 4.4 inches.
I did eventually confirm by visual inspection after removing the back cover and beam expander that the tube is indeed slightly less than 5 inches in overall length, which would be consistent with that cavity length after accounting for the thickness of the mirrors. I also sent the raw beam across the room and confirmed that it is TEM00 and no higher order spatial modes were evident. And, with very very careful adjustment of the SPFI alignment which causes its FSR to effectively double to 4 GHz, the display was as expected with one large central mode (F1/F2 displayed as a single mode due to the limited resolution of the SFPI) and two weak modes on either side around 1.3 GHz away.
The diagrams below show why rogue modes can be present. If the tube is too long and its longitudinal mode spacing (the FSR of the tube equal to c/2*L) is thus small enough, an adjacent longitudinal mode on each split neon gain curve may be above threshold and thus oscillate. One will be approximately 1 FSR above and the other will be approximately 1 FSR below the Zeeman modes. (The distance is approximate due to mode pulling effects.)
Normally, with the main lasing mode centered on the gain curve as it is here, there would be no chance of any other longitudinal modes having enough gain to lase in a tube with a cavity length of less than about 7 inches. (The difference between the optical frequencies of the F1 and F2 modes is greatly exaggerated in this diagram. If drawn to scale, they would almost merge into a single line and can thus be considered to be centered.) However, Zeeman splitting effectively widens the gain bandwidth for lasing with one part moving higher and the other part moving lower in optical frequency as shown above. So even a very short tube can produce rogue modes under some conditions. The use of isotopically pure neon may be required to suppress rogue modes since it results in a much narrower gain curve and thus a narrower Zeeman split gain curve. Thus, HP/Agilent tubes with their 5 inch cavity length are rogue-mode-free. The cavity losses, most notably due to mirror reflectivity, also determine whether modes that are on the tales of the gain curve will oscillate. So, a long tube with lower reflectivity mirrors could still be free of rogue modes, at the expense of output power in the Zeeman modes. This is all a balancing act and unfortunately for this tube, it fell over. :( :)
By rotating a polarizer in front of the laser while observing the SFPI display, it was possible to determine the orientation of the rogue modes. They were orthogonal to each-other and at approximately a 30 degree angle with respect to X and Y. At this point, such a result was expected (actually hoped for since it was the only explanation that made sense!) but still surprising, as all the longitudinal modes of a laser tube are normally oriented along a single axis, or orthogonal to it, and these were off by 30 degrees. However, this could mean that the rogue modes are elliptically polarized (before entering the waveplates) rather than being (nearly pure) circularly polarized like the Zeeman modes.
The reason that the rogue modes at 30 degrees cause signal degradation is that some portion of each rogue mode passes through both the reference arm and measurement arm of the interferometer, and thus they interfere with themselves at the optical receiver, resulting in the baseband fringes. These change the level of the signal envelope, and confuse the receiver electronics, essentially by shifting the threshold that is used to convert to the optical receiver's digital output. This effect is similar to that of misaligned or impure normal (F1/F2) modes.
The signal jitter using a 10780A or 10780C optical receiver is probably a worst case. The higher performance E1708A or E109A may be less susceptible to signal envelope level fluctuations.
Note that even if rogue modes are present, if they are aligned with the X and Y axes, the only effect will be to slightly decrease the signal level relative to output power. It's possible that careful adjustment of the waveplates could eliminate the effects of the rogue modes by aligning them with the X and Y axes. It's also possible that rogue modes could exist in genuine HP/Agilent lasers but go undetected if aligned in this way. Testing with an SFPI is not something that would be done routinely. However, all tests I've made with HP/Agilent as well as Excel two-frequency lasers show that there is NO evidence of rogue modes when the Zeeman-split mode is centered and locked. Zero, zip, null. :)
By adjusting the waveplates, it was possible to produce a MEAS signal with little or no jitter. In fact, by eye, the signal quality was such that on any other laser, it wouldn't warrant a second glance. However, when set this way, there was significant crosstalk between F1 and F2 indicating that their output was probably elliptically polarized rather than purely linearly polarized. The signal should nearly disappear when a polarizer is aligned with the X or Y axis, but with this laser, it's amplitude was still 20 or 30 percent of the peak with the polarizer oriented at 45 degrees.
The good news is that since the two rogue modes are already low level, they may actually disappear after the laser is used awhile and the overall gain (and output power) decreases!
Rogue modes (at least those that are not aligned with the X and Y axes) should not be present in used lasers.
For a much more in-depth treatment of this topic, see reference .
However, due to the change in thermal characteristics of a laser rebuilt with a conventional tube, a loss of lock after READY has come on solid may not be unusual but the laser will eventually settle down and behave.
Where a new conventional (but possibly modified) HeNe laser tube has been installed, the lifetime will depend on its construction and quality. Typical commercial HeNe laser tubes have an expected life of 10,000 to 20,000 hours with some going to 40,000 hours. But since a very small tube must be used in the the HP/Agilent lasers (due both to space limitations and the required longitudinal mode structure), the life could be much shorter. The cathode area is small and the gas reservoir is small. Such tubes may have a life of only 5,000 hours, though there are exceptions - Small Zygo tubes last 20,000 hours.
This should not be an issue with a used laser unless the tube assembly has been removed and reinstalled, or transferred to a different laser body, and then only horizontal alignment can really change unless something is preventing proper seating of the tube assembly feet. Then it's just a matter of loosening the 4 mounting screws and readjusting it horizontally. (On lasers with a pair of flat-head tube mounting screws, even changes in horizontal alignment are not likely assuming the two flat-head screws were carefully tightened before the other two screws.)
For a rebuilt laser, a large pointing error most likely indicates poor attention to detail during the final alignment. Depending on the design of the beam expander and access to its adjustments, it may be necessary to remove the tube assembly and perform the alignment with the aid of a jig that precisely positions the tube assembly with respect to a pair of apertures through which the beam should be adjusted to pass cleanly. However, since the tube will likely already be potted and fixed in the magnet assembly, it probably won't be possible to select the beam height and vertical angle independently so some compromise will be needed.
A rebuilt laser using a conventional HeNe laser tube required about 7 minutes before the READY LED started flashing, and over 9 minutes for the READY LED to come on solid. It just barely got in under the deadline.
A used laser with adequate output power should lock in the normal 4 minutes if it has the Analog Control PCB. Only when the output power is very low and well below HP/Agilent specs, will locking time be affected. However, lasers with the newest Digital Control PCB tend to take longer to lock - 6 minutes or more.
So, a rebuilt laser should be checked to make sure the REF frequency is acceptable, either based on the model and options of the original laser, or based on those of the machine in which it is to be installed. A higher REF frequency translates into a higher maximum velocity capability for the measurement system, so a rebuilt laser with a higher REF frequency should be acceptable if the data processing electronics can handle it without producing measurement errors, or aborting due to detecting that it is out of range. But the expected increase in REF frequency due to aging should be factored into the determination of whether it will continue to work correctly later. A rebuilt laser with a short useful life will be no bargain. However, if the REF frequency is below the spec'd minimum, performance may be unacceptable in speed-critical applications, or the machine may generate an error during self test and refuse to work at all.
The actual REF frequency of a used laser should not be an issue except to the extent it has increased with use, which may be a tip-off to its future life expectancy. Even if the output power is well above the spec'd minimum, an excessively high REF frequency may indicate that the original output power was much greater, and the laser is probably near end-of-life.
A rebuilt laser using a conventional HeNe laser tube was tested to be about 75 MHz above a healthy HP/Agilent laser. This is still only about 0.15 ppm difference, though it does represent a much larger deviation than even an end-of-life original HP/Agilent laser.
The test setup is shown below:
A rebuilt laser using a conventional HeNe laser tube had a frequency variation over 8 hours of less than +/-2 MHz, and this is very conservative. It could be much less.
The optical frequency tends to decrease with use. For a used laser, it's never likely to be so low as to not meet HP/Agilent specs for optical frequency, but this, too, can be used as a guide to the health of the laser.
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Confirm that the +/-15 VDC power supplies have the required specifications (voltage accuracy and current ratings). It would also be desirable to run everything on a power conditioner and/or constant voltage (e.g., Sola) transformer to rule out incorrect or noisy power as a cause of unexpected behavior, should any occur. And, of course, if the cabling isn't idiot-proof, double check the connections BEFORE applying power!
A laser that takes a long time to start but runs reliably may be perfectly acceptable, especially in applications where the laser is then run continuously for days or longer. However, starting can be hard on the HeNe laser power supply, and some measurement display electronics will give up after a fixed amount of time like 10 minutes, and produce a non-recoverable hard error.
The 5500A/C and 5501A should lock in 10 to 20 seconds as they use PZT rather than thermal tuning, which is much faster.
A healthy used laser should lock in about the same time as a new laser. However, a rebuilt laser with a conventional HeNe laser tube may take much longer to lock. And a used laser with marginal output power may go through a few locking cycles before the power has climbed high enough to be acceptable.
Sometimes, the locked output power when new is printed on a label on the backplate of the laser. Depending on the specific model and options and particular sample, this can range from around 250 µW to over 600 µW. How far the output power has declined relative to the printed value is good indication of the laser's usage, though this is not necessarily a linear function. However, being near the minimum acceptable value would be a cause for rejection.
For a rebuilt laser with a conventional tube, the output power may go down slightly as the mirror alignment changes slightly with temperature. This doesn't generally happen with HP/Agilent tubes which use a rigid (glass or Zerodur) mirror spacing rod for alignment.
Note that a measurement of output power should only be considered accurate once the laser has locked and READY is on solid. Before then, it may vary by 25 percent or more due to mode sweep, especially for a high mileage tube whose output power has declined significantly compared to its value when new.
Neither of these should change in a used laser. But in a rebuilt (or new!) laser, alignment being off by more than a degree or so or would be an indication of bad quality control during final adjustment of the waveplates. Lack of null points when the polarizer is aligned with the X or Y axes indicates crosstalk between the F1 and F2 optical frequencies and may also be the result of errors in waveplate adjustment, or that the waveplates were set up to eliminate the effects of rogue modes in a rebuilt laser at the expense of mode purity. These problems should be corrected by the supplier, and if not possible, the laser should be rejected as both can introduce periodic static or dynamic measurement errors.
For a used laser, the optical frequency relative to a similar known laser that is new, has seen little use, or has been run for a known amount of time, can provide another indication of how much use it has seen. The difference in optical frequencies for a healthy laser will typically be only a few MHz, while one near end-of-life may be lower by 15 MHz or more.
It's not clear that knowing the absolute optical frequency is of much added value for any of these lasers, except possibly to use it as a reference in testing other similar lasers in the future. So, comparing with a low mileage HP/Agilent laser is as good as comparing with an iodine stabilized HeNe laser.
(Although the HP/Agilent specification for laser wavelength changed by -0.000018 nm, an amount corresponding to an optical frequency increase of +14 MHz between the 5517B and 5517C, there is no evidence that an actual change was made in the design of the laser. Testing shows no obvious difference in the optical frequencies of the 5517A, 5517B, 5517C, 5517D, or 5517E, or the 5501B.)
However, since this test does require another similar HP/Agilent laser in known working condition, and a somewhat more complex setup to perform, the time, effort, and expense may not be justified if the other tests indicate good health.
Note that the above list does not include the temperature set-point adjustment as described in the HP/Agilent manual. This should be valid for new, used, or rebuilt lasers with HP/Agilent tubes. But, it may need to be modified for a rebuilt laser with a conventional tube.
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However, depending on the technique and quality of the work in rebuilding a laser, a new set of isseus can arise, requiring careful acceptance testing and periodic checks of performance. So far, there is very limited data on these lasers. I have tested a 5517D that had a conventional tube installed in place of the original HP tube, and a 5501B that I believe had its tube regased (at the very least). (I'm not entirely sure because I have not been able to confirm with the supplier. But the glass tube had obviously been removed and replaced.) The first of these lasers (used as the example above) had very obvious beam profile and rogue wavelength issues. The latter appears to be normal in all respects, but its expected life is unable to be predicted.
Even the laser with the beam profile and rogue mode issues could probably be made to work in perhaps all but the most critical applications. However, there have been reports of an inability to follow the manufacturers alignment procedure due to some aspect of this laser, so this would need to be modified. It's possible that readjustment of the waveplates could align the rogue modes to the X and Y axes, and thus greatly reduce or eliminate their effects on performance. And of course, with any laser that has been modified without HP/Agilent's strict quality control, there would be some risk, so rigorous adherence to a weekly or monthly test and calibration regiment would be essential in identifying and tracking any changes in performance over time.
These same issues could occur with other Zeeman lasers such as those from Excel Precision . They manufacture several lasers that are similar to those from HP/Agilent. However, metrology lasers from Zygo Corporation  are based on different technology and already use HeNe laser tubes of conventional design, though at present, custom built by or for Zygo to achieve long life.  There would still be some areas that could change, but these would be limited to slight changes in beam profile, and a likely shorter life. 
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References and Links
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