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Considerations in Evaluating Rebuilt Hewlett Packard/Agilent Metrology Lasers


Agilent (formerly Hewlett Packard) 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) or a previously owned similar laser (with some risk) have generally been the only options. However, a few companies do claim to offer rebuild services at greatly reduced cost. This note addresses the issues that might arise with a 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. 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 optical receivers, and associated digital processing channels.

The key attributes that make these lasers ideal for metrology applications is 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. A 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 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.

Used HP/Agilent metrology lasers are also widely available. But many of them are already unusable due to low output power or other problems. 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. The working environment claim may not be inaccurate, it's just that the laser is dead! :) However, if the seller has tested the laser and offers a warranty, 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, where the tube is bad, if it were possible to rebuild it, then this opens up a third possibility with performance potentially equal to that of a new laser at a fraction of the cost. 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:

Rebuild Options

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, I would estimate it to be between 1/4 and 1/2 the price of a new laser. However, 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. Since a semiconductor wafer stepper (a typical application 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, it's not clear at what point the modest savings can be justified against the substantial risk. Nonetheless, some large semiconductor companies are known to have seriously considered, going this route to save a few bucks.

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 HeNe laser tube, which is mounted within the magnet using a rubbery potting compound.

Rebuilding an HP/Agilent tube assembly can take two forms:

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 issues that can affect performance. However, specifications an the experience of others must be used for long term stability and life.


A rebuilt HP/Agilent metrology laser may represent a viable alternative to a high cost new laser or previously owned laser in uncertain condition. However, depending on the technique and quality of the rebuild, 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. And 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, which manufactures several models that are similar to those from HP/Agilent. However, metrology lasers from Zygo are based on different technology and already use HeNe laser tubes of conventional design, though built by or for Zygo. 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.

Sam's Laser FAQ, Copyright © 1994-2021, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the
Sci.Electronics.Repair FAQ Email Links Page.