Considerations in Evaluating Used or Rebuilt
Hewlett Packard/Agilent Metrology Lasers

Version 1.08 (4-Mar-10)

Sam's Laser FAQ, Copyright © 1994-2010
Samuel M. Goldwasser
--- All Rights Reserved ---

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Agilent Technologies [1] (formerly Hewlett Packard or HP) is perhaps the leading supplier of two-frequency HeNe metrology lasers. used in all areas of precision manufacturing. The most well known application is probably for sub-micron positioning in semiconductor wafer steppers. These lasers generally have a long life (50,000 hours typical) but when they do fail, replacement with a new laser at relatively high cost ($9,000 and up) has always been the low risk option for critical applications. However, these lasers are also available surplus (used or "preowned") often at very attractive prices but nearly always in unknown operating condition with equally unknown life expectancy. And, a few companies do claim to offer rebuild services or rebuilt lasers at greatly reduced cost compared to a new one. This note addresses the issues that might arise with a used or rebuilt laser, and their impact on measurement precision and service life.


HP/Agilent metrology lasers are Helium-Neon (HeNe) lasers that use an axial magnetic field to split a single longitudinal lasing mode into two modes that are orthogonally polarized and offset in optical frequency from each-other by several MHz. [2] One component called the "measurement beam" is sent to a remote "Test Arm" whose position is to be measured and returned via a mirror or retro-reflector, while the other component called the "reference beam" is returned locally from a fixed retroreflector. These are combined in a high speed photodiode producing a beat signal via heterodyning. When the Test Arm moves, it results in a Doppler shift changing this beat frequency. By comparing the phase of the beat signal (called MEAS) with a locally generated un-shifted version (called REF), the position of the Test Arm can be determined down to a resolution of 10 nanometers (nm) or better. And, through computation and/or special optics, velocity, angle, straightness, and other measurements can be made with similar precision.

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:


HP-5517B Laser Showing Tube Assembly on the Left and Analog Control PCB on the Right

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:

Tube Assembly from HP-5517B Laser

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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Used ("Previously Owned") HP/Agilent Metrology Lasers

For many applications, a very viable alternative is to purchase a used laser. Assuming such a laser hasn't been modified or tampered with (or rebuilt!), then most of the issues associated with rebuilt lasers will not exist. Only two parameters really change significantly with use and these are the laser output power (which declines, especially towards end-of-life) and to a lesser extent, the REF (split) frequency (which tends to increase slightly as a side effect of the decrease in output power). The principle remaining issue would be that the laser tube starts reasonably quickly and runs reliably without any dropout, sputtering, or flickering; and that it will continue to do so with an acceptable lifespan. I do not know if the newest Agilent lasers with the Digital Control PCBs have run-time meters, but those with Analog Control PCBs do NOT (and this includes all Hewlett Packard lasers). However, it's generally possible to get a good feel for the health and expected remaining life of any given unit through simple tests and comparison of output power and REF (split) frequency with the measured values when new, which can often be found on the label (if the seller hasn't conveniently removed it!). They may also be available from Agilent by referencing the serial number of the tube assembly, but Agilent probably would not release this information except to the original owner.

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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Rebuild Options

Note: I am not at liberty to divulge and/or am not even absolutely sure of the names of the companies whose rebuilt laser(s) I've tested, being acquired from a third party, so please do not ask. There aren't that many so a Web search should be able to locate them.

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:

Major Components of HP/Agilent 5517B/C/D or 5501B Tube Assembly

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:

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Potential Issues

The following are the main things to check either by testing (where possible) or getting data from the supplier or better yet, from their previous customers. A rigorous acceptance test procedure can identify many of the issues that can affect performance. However, specifications and the experience of others must be used to predict long term stability and life. Some of these will only apply to lasers with rebuilt tubes since most of the fundamental parameters affecting performance are unlikely to have changed on used lasers unless they have been tampered with or modified.

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Acceptance Testing Summary

With an arrangement similar to the one shown in the diagram above, "Two-Frequency Interferomter Laser Tester", along with a polarizer, laser power meter, and high speed photodiode, most of the following tests can be performed in under 10 minutes. However, it will be desirable to run the laser for a few hours or more to make sure everything remains stable, but this can be essentially unattended if the measurement display catches laser loss-of-lock or dropout errors as does the 5508A. (The only test that will require a somewhat more complex setup is the one for optical frequency, and performing that is probably not essential in most cases.)

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!

  1. Starting: Except for the 5501B, a new or rebuilt laser should produce an output beam almost immediately, though many used lasers will require a few seconds to a minute or more to come on (though they will typically restart instantly if power is interrupted). The 5501B doesn't turn the laser tube on until the internal mirror spacing rod has reached operating temperature, usually about 2 minutes. At that point, the yellow "Laser" LED signifies that the laser tube should be on. A slow start tube that doesn't light up within a few seconds may cause the controller to go through multiple attempts at locking, extending the lock time to much more than the typical 4 minutes.

    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.

  2. Running: Once the laser starts, it should remain on until power is removed. Any dropout, sputtering, or flickering is a cause to reject the laser unless corrective action is taken. But this is usually beyond the capabilities (or desires!) of the end-user. It's essential to monitor the laser status until at least when it locks (READY on solid) since marginal tubes may only start misbehaving after they warm up.

  3. Locking: Most of the 5517 lasers, the 5519A/B, 5519A, and 5501B typically lock (READY LED on solid) in about 4 minutes. A bit less or a bit more doesn't matter, but a very long lock time could indicate other underlying problems, and may result in a hard error from some measurement electronics. But it is normal for some versions of 5517 lasers to take 10 minutes or more to lock. A very short lock time could indicate that the internal temperature adjustment is not set properly, and the laser may lose lock after awhile.

    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.

  4. Locked output power: The optical output power from the front of the laser with the normal (large) aperture should exceed the minimum specification for the particular laser. For most of these, it's 180 µW, but may be slightly lower (120 µW) for some, like the 5517B and 5501A. For original HP/Agilent laser tubes, the output power will generally increase slightly after the laser locks and over the next hour or so, especially for used lasers with higher mileage tubes. However, it is normal for the precise output power to differ slightly each time the laser is powered on based on which precise mode order (actual spacing rod temperature) to which it locks.

    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.

  5. Beam profile: The normal beam profile for HP/Agilent lasers is along the lines of the center portion of a Gaussian with the sides cut off, not the typical full Gaussian TEM00 spatial mode of a common HeNe laser. The beam profile won't change in a used laser, though the variation from center to edge will tend to increase as the power declines. But a rebuilt laser with a conventional tube may not have matched optics, so almost anything is possible. To make it seem like there is more power, the rebuild company may used optics that pass more of the beam, resulting in a hot spot in the center. The easiest way to check is to compare with an original HP/Agilent laser with a similar size beam option. Two possible effects of a sub-optimal beam profile will be to decrease MEAS signal amplitude and make interferometer alignment more critical. For many applications, the exact beam profile may not matter, but for some equipment, there may be specific installation tests that will fail if the beam profile isn't close to the original from HP/Agilent.

  6. Mode alignment and purity: When a polarizer (sheet polarizer or polarizing beam splitter cube such as from an HP interferometer) is rotated in the output beam, the MEAS signal from an optical receiver should be present at all orientations except in a small angular range centered around the X and Y axes with respect to the laser baseplate.

    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.

  7. Mode balance: The output power in the X and Y polarized modes should be within about 10 percent of each-other. While a slightly larger difference won't really affect performance, it may be an indication of an electronics problem resulting in a significant optical frequency offset, and possible loss-of-lock after awhile.

  8. Mode purity: Using a polarizer, a fast photodiode, and DC-coupled oscilloscope, the check for a high modulation depth with the polarizer oriented at 45 degrees to the baseplate compared to the DC level along X or Y. Excessively low modulation depth could be an indication of less than optimal setup of the tube assembly's optics (waveplates), or rogue modes in the laser tube itself.

  9. REF signal: An optical receiver should produce a clean stable waveform (squarewave) with crisp sharply delineated tops and bottoms and rising and falling edges. Any fuzz here may indicate amplitude ripple of the optical output. This is most likely to be present in lasers with low output power, often as a result of plasma oscillations in the laser tube. Sometimes, these will disappear once the tube has fully warmed up and its output power has increased, but as with tubes that don't want to stay lit, corrective action may be needed.

  10. REF frequency: The actual frequency should be within the lower and upper limits for the particular laser model (e.g., 5517C) and options. If it is near or beyond the upper limit, this may still be acceptable though some equipment may be very fussy, or may simply be designed so close to the limit that the processing will fail. However, a high REF frequency in a used laser could also indicate that it is high mileage, as the REF frequency tends to increase as a result of the decrease in gain and thus output power.

  11. MEAS signal (stationary): This should be the same as the REF signal, above - clean and stable.

  12. MEAS signal (moving): The MEAS waveform from the optical receiver should be clean, just like REF. Movement of the "Target" will result in the period/frequency of the waveform changing, but at any instant, it should have no fuzz or other indication of instability. Misaligned, impure, or rogue modes can result in both amplitude and duty cycle changes, with the most obvious result being fuzzy rising and/or falling edges (depending on the scope triggering).

  13. Transient errors: The easiest way to check for these is to install the laser in an interferometer, or even simply with an optical receiver monitoring its output, and use the normal error detection capabilities of a measurement display like the HP 5508A to catch any loss of signal events over several hours. Alternatively, a data acquisition system can be used to monitor the H and V polarized output power, and possibly the REF frequency. Transient errors are relatively uncommon with these lasers, so testing for them is probably not worth the additional effort. Any failure should be caught early on by the measurement electronics in the intended application.

  14. Optical frequency: For a rebuilt laser, knowing the optical frequency is probably not very important as long as any machine calibration procedure takes the corresponding variation in wavelength into consideration. The optical frequency for rebuilt lasers may be quite different from the original, especially those using conventional tubes.

    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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Discussion and Conclusions

A used or rebuilt HP/Agilent metrology laser may represent a viable alternative to a high cost new laser or previously owned laser in uncertain condition. For a used laser, it's critical to measure key parameters to determine the likely condition of the laser. The most important would be the laser output power, followed by the REF frequency and optical frequency.

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 [6]. They manufacture several lasers that are similar to those from HP/Agilent. However, metrology lasers from Zygo Corporation [7] 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. [8] 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. [9]

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References and Links

  1. Agilent Technologies. Search for a specific model laser or system, or "metrology lasers". Their Web site has specifications for all current lasers and systems but little if any on older models like the 5501B that they consider obsolete and no longer support. There is also extensive technical information on all aspects of Agilent metrology systems and components.

  2. General information on HP/Agilent metrology lasers and systems: Sam's Laser FAQ chapter "Commercial HeNe Lasers", sections starting with: Hewlett-Packard/Agilent HeNe Lasers.

  3. Building or modifying two-frequency metrology lasers: Sam's Laser FAQ chapter "Home-Built Helium-Neon Laser", sections starting with Two-Frequency HeNe Lasers Based on Zeeman Splitting.

  4. Experiments in rebuilding HP/Agilent lasers: Sam's Laser FAQ chapter "Home-Built Helium-Neon Laser", section: Installing a Common HeNe Laser Tube in an HP 5517 or 5501B.

  5. In depth treatment of measurement anomolies due to rogue or off-axis modes: "An investigation of two unexplored periodic error sources in differential-path interferometry", Tony Schmitz and John Beckwith, Precision Engineering, volume 27, issue 3, July 2003, pages 311-322.

  6. Excel Precision. Very little technical information.

  7. Zygo Corporation. Go to: "Stage Position (OEM)" or search for "ZMI".

  8. General information on Zygo metrology lasers and systems: Sam's Laser FAQ chapter "C,ommercial HeNe Lasers", sections starting with: Zygo HeNe Lasers.

  9. Companion document: Considerations in Evaluating Used or Rebuilt Zygo Metrology Lasers.

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Sam's Laser FAQ, Copyright © 1994-2010, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the
Sci.Electronics.Repair FAQ Email Links Page.