Considerations in Evaluating Used or Rebuilt
Zygo Metrology Lasers
Version 1.11 (18-Apr-21)
Zygo Corporation  is one of the leading suppliers 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 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.
Zygo "ZMI" metrology lasers are Helium-Neon (HeNe) lasers that use
an Acousto-Optic Modulator (AOM) to split a single longitudinal lasing
mode into two modes that are orthogonally polarized and offset in optical
frequency from each-other by 20 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 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 20 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.)
Two views of a typical Zygo ZMI laser with its cover removed are shown below:
Left Side View of Interior of Zygo-7702C/E Laser
Right Side View of Interior of Zygo-7702C/E Laser
The heart of Zygo two-frequency lasers is a 3 to 4 mW HeNe laser tube
that is presently made by or for Zygo. A bifilar-wound or thin-film heater
is added to the laser tube, which is then mounted inside the gray metal
enclosure visible below the HeNe laser power supply brick. The output beam
exits toward the left, passing through the black beam sampler assembly to the
AOM, reflects off of two turning mirrors to a spatial filter and then
out to the right via beam expander telescope (hidden from view).
Most Zygo lasers including the 7701, 7702, 7712/14, and 7722/24 use
the same HeNe laser tube, though the optics may differ.
The custom HeNe laser tubes used by Zygo since the early to mid-1990s have
an expected life of 50,000 hours. Before then Zygo used standard
commercially available tubes from Aerotech (a former manufacturer of
HeNe lasers) and possibly others. While the present Zygo tube is not
available at every corner store :-), it is simply a random polarized
HeNe laser tube that has been designed for long life and mode-flip-free
behavior. A typical Zygo tube is shown below:
Zygo HeNe Laser Tube Used In 77XX Series ZMI Lasers
Its construction is essentially the same as a vanilla
flavored HeNe laser tube used in countless other applications. There
are no magnets, waveplates, or internal heaters to make it a truly
special tube. Unlike HP/Agilent lasers [3,4], failures
other than the tube reaching end-of-life are not that unusual and thus
Zygo lasers may be pulled from service due to a problem elsewhere, usually on
the controller PCB. Repair of the electronics is possible, but detailed
documentation and schematics may not be available to any non-Zygo service
organizations, rendering the newer digital control PCBs essentially
unrepairable. There is a similar lack of information on the older
analog control PCBs, but these at least could be reverse engineered
in a straightforward, but very tedious way.
So, where there is an electronic problem, it may be possilble to repair
the control PCB, but it may be easier to simply swap in a good control PCB
from a similar ZMI laser with a bad tube. This eliminates any issues of
optical alignment, but means that the settings may need to be adjusted on an
analog control PCB, or that whatever the settings were on a digital
control PCB will have to be close enough.
But where the tube is weak or dead, and the HeNe laser power supply and
optical alignment have been ruled out as possible causes, then either a
replacement tube needs to be installed, or the (good) electronics can be
swapped into another chassis with a good tube but bad controller.
Commercially available standard tubes can be used since although the
Zygo tube is custom, there is nothing really special about it as far
as lasing is concerned. The main thing that makes it custom is the
special thin film cathode deposited on the inside of the glass envelope
which is the primary means of achieving the 50,000 hour life. The
lasing parameters of a commercial HeNe laser tube with similar beam
diameter, divergence, output power, and similar length will be close
enough to permit such a tube to be installed in place of the original tube
without requiring major optical adjustments except alignment. Suitable
tubes are available from HeNe laser companies like
Pacific Lasertec (PLT, formerly
the HeNe division of Melles Griot):.
Used Zygo ZMI metrology lasers are also widely available. But many of
these are already unusable due to low output power or bad electronics.
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 replace a bad
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 Zygo tube).
Back to Top
Used ("Previously Owned") Zygo ZMI 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 one parameter really changes significantly with use and that is
the laser output power (which declines, especially towards end-of-life).
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.
All Zygo lasers have a run-time meter of some kind. The older ones with
analog control PCBs have a "mercury" column indicator inside the case
near the HeNe laser power supply. The digital control PCB keeps track
of run time in nonvolatile memory (NVRAM) which can be interrogated via
the RS232 port. Assuming that the tube is original, and in the latter
case that the control PCB hasn't been swapped, checking thes can give
an idea of the expected remaining life of the tube.
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.
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.
Back to Top
Note: I am not at liberty to divulge and/or am not even absolutely sure of
the name or names of the companies whose rebuilt laser(s) I've tested,
being acquired from a third party, so plesae 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 Zygo ZMI lasers by
installing a new (but probably non-Zygo) 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 Zygo tube assembly consists of the actual glass HeNe laser tube and
a bifilar-wound heater glued to its exterior.
Zygo Tube Assembly with Bifilar-Wound Heater
Note the very precise tube mount/centering technique - blobs of black
A thin-film heater is also used in some Zygo lasers as shown below:
Zygo Tube Assembly with Thin-Film Heater
However, I while I have seen late model 7702s using a thin film heater along
with a temperature sensor (not found with the wirewound heaters), I
do not know if it is now standard. Some versions of the control PCB appear
to be compatible with both types. The labor is certainly lower for a thin-film
heater, though the cost of the heater itself is much greater. The one above
is almost certainly not from a 7702 laser as the heater wires exit the wrong
end and it lacks the temperature sensor.
Rebuilding a Zygo tube assembly can take two forms: regasing the original
or replacement with a standard HeNe laser tube. Regasing is generally
not an option both for technical reasons and due to the high cost
compared to a replacement tube. So, we only concentrate on the
The physical process of tube replacement is straightforward: Find a
suitable random polarized HeNe laser tube about 9 to 10 inches long like
the one shown below:
Spectra-Physics Random Polarized HeNe Laser Tube
However, note that these are not the typical barcode scanner tubes that
may have similar size and appearance. They must have an output power
of 3 or 4 mW, not the ~1 mW typically used in barcode scanners.
Replacing the tube is not nearly as easy as swapping the entire tube
assembly in HP/Agilent lasers. But the glass tube itself can be extracted
without using extraordinary means to remove any potting compound
as there is none. It simply clamps within the gray enclosure using the
two sets of brackets visible in the photo and those blobs of RTV-Silicone.
Removal then requires taking off the front panel and beam sampler block,
disconnecting the heater cable and unsoldering the anode wire.
Prepare the new tube by adding a heater with similar electrical
characteristis to that of the original, which can be
wire-wound or a thin-film type. Or, if the
original was the latter, it might be possible to simply peel the
old one off and transfer it to the new tube. Then install the replacement
tube assembly in the Zygo chassis in reverse order from removal.
Next, the output must be precisely
aligned to pass the un-shifted mode through the output optics and
the AOM must be adjusted to pass the shifted mode as well. If the
tube is carefully centered when installed in its enclosure, this
process will be simpler. Finally, the
control PCB may need to be adjusted to match the amplitude of the
sampled modes for stabilization, or alternatively, their intensity
can be adjusted with a filter inside the beam sampler.
However, where a Zygo chassis is available with a good tube but bad
control PCB, swapping in good electronics is even easier and quicker.
Back to Top
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 replacement 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.
- Beam profile: The normal Zygo beam profile is probably
described best as a Gaussian that is cut off. The 3 mm and 9 mm beams
have the same shape as the 6 mm beam, but 1/2 and 3 times the diameter,
respectively. The easiest thing to do is to compare the beam profile of
the candidate laser with a genuine one. For many applications, the exact
beam profile is not that important and was simply selected based on the
desired interferometer optics. But where long measurement distances
are involved, for example, getting the maximum benefit from a larger
diameter beam may be required.
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 3 mm).
Since the beam originates internally from a pinhole, a rebuilt Zygo
should also have a similar beam profile to the original.
- Mode alignment, balance, and purity:. Test the alignment by
rotating a polarizer in front of the laser and confirming that the
beat signal from a high speed photodiode or optical receiver
(virtually) totally disappears when aligned with the X or Y axis.
Test the mode balance by comparing the optical power through the
polarizer when oriented with the X and Y axes. The two values
should be within 20 percent of each other for a 7701 or 5 percent
of each other for a 7702. For a rebuilt laser, errors in mode alignment
depend on how accurately the HeNe laser tube is oriented when it is
was originally mounted, and this may be related to mode purity. Mode
balance can also change since that is a function of the RF drive to
the AOM and its alignment. Unequal mode balance will reduce the
available optical MEAS signal level relative to laser output power
but have no other effect. Errors in mode alignment
or purity will result in interference between F1 and itself, and F2 and
itself in the interferometer optics. The result will be low
frequency level shifts of the envelope
of the detected signal from the photodiode in the optical receiver.
This may result in jitter in the MEAS signal.
For a much more in-depth treatment of issues relating to off-axis modes,
see reference .
None of these is likely to change in a significant way over the useful life of
the laser except possibly mode balance, which is affected by
the RF level to the AOM and thus electronic component drift.
However, when the output power declines to well below the
Zygo spec'd minimum, amplitude ripple in the output power due
to laser current ripple from the HeNe laser power supply can
interfere with a clean beat frequency signal.
- Rogue modes: Unwanted longitudinal modes are extremely unlikely
to be present. They won't develop in a used laser and it simply is not
possible for them to be present in a rebulit laser that has locked
correctly since there isn't enough space for a long enough tube to
support additional longitudinal modes!
- Mode flipping: In HP/Agilent metrology lasers, only
a single Zeeman-split longitudinal mode is lasing. However, in
Zygo lasers, there are a pair of orthogonally polarized longitudinal
modes lasing, parked approximately equidistant on either side of the neon
gain curve. However, which one is on the high side and which one is on the
low side is not a result of anything fundamental and during warmup they
basically march through the gain curve as the tube expands. Once the
feedback is enabled for locking, a specific polarization will be forced
to the high side, and the orthogonal polarization will be on the low side.
Only one of these modes is used in 7701/2/12/14, and only one at a time
in 7722/24 lasers.
Under some conditions, the two modes can flip polarization instantaneously.
They will then be on the wrong side of the neon gain curve.
In a well behaved tube, this won't happen spontaneously, but may occur
due to back-reflections. But in some cases, they can flip with
no obvious cause and such events may happen hours apart.
However, should one occur when the laser is in use, the optical
frequency will jump up or down by about 750 MHz (~1.5 ppm) and
the feedback will then cause the modes to drift back to their
normal position over a few seconds. This will result in either
(hopefully) a loss of signal error or (worse) an error in the measurement
data which could go undetected.
The HeNe laser tube must be thoroughly tested prior to installation
to assure that spontaneous mode flipping does not occur, but this
affliction can even be found in some Zygo lasers.
- Stick-slip noise: Zygo takes great pains to make sure that
as parts of the HeNe laser tube expand with temperature, the cavity
length won't suddenly change as a result of something being restrained
by friction, and then suddenly breaking loose changing the distance
between the mirrors. Both parts inside the tube, and its mounting,
can contribute to stick-slip noise. If a slip were to happen while
being used for a measurement, there would be a transient error that
might not even be caught by the electronics. Once fully warmed up,
this is not likely to occur, so it may not be an issue unless the
laser it put into service soon after it has locked.
Rebuilt lasers are more likely to suffer from this problem due to both
the use of laser tubes that haven't been designed to eliminate it, and
due to the possible use of rigid adhesives or other factors in the
physical mounting of the tube in the Zygo enclosure.
- Laser output glitches: Aside from electronic problems like bad
solder joints in the HeNe laser power supply or wiring,
this is not likely to happen with a replacement (non-Zygo)
tube. However, after having been run for 10s of thousands of hours (or
with some, much less) original Zygo tubes may exhibit momentary
dropouts and restarts of the laser tube resulting in a corresponding
momentary loss of beam. The cause is believed to be imperfect electrical
contact between the thin-film cathode coated on the inside of the glass
and the cathode end-bell. Successive events may occur within minutes - or
days. The duration is order of milliseconds and my result in a
mode flip after the glitch as well as described above. But the glitch
alone will result in measurement errors if not flagged as an error.
Unlike stick-slip noise, these generally do not go away after warmup.
Since these dropout occur infrequently, and are virtually imperceptible
by eye anyhow, the only way to test for them is to run the laser while
monitoring the beam or laser tube current using a data acquisition
system with a sample rate of at least 50 s/s. But again, this really
only applies to tubes manufactured by Zygo, usually after 10s of thousands
of hours of use. Dropouts are not likely
with the types of tubes used to rebuild Zygo lasers, nor the Aerotech
tubes found in really old Zygo lasers.
- Life expectancy: The original Zygo HeNe laser tubes are
specially processed to result in an expected life of 50,000 hours.
Where a new conventional 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.
- Beam alignment: Beam alignment in Zygo lasers is determined
by the position of the internal pinhole and beam expander telescope,
neither of which should change either in a used or rebuilt laser.
- Time to lock (READY LED on solid): This is typically 10 to 15
minutes for a healthy Zygo laser and should not change significantly.
- Beat (REF) frequency: Since this is determined by a crystal
oscillator, it should be unchanged.
- Absolute optical frequency/wavelength: It is not known if
Zygo laser tubes use isotopically pure He and Ne, which along with fill
pressure are what mostly determine the optical frequency. However,
normal machine calibration procedures should factor in any change.
- Short term optical frequency/wavelength stability: With a
replacment tube running at the same tube current as the original, the
stability should be similar.
The optical frequency tends to decrease with use. For a used laser, it's
never likely to be so low as to not meet Zygo specs for optical
frequency, but this, too, can be used as a guide to the health of the
- Long term optical frequency/wavelength stability: All HeNe lasers
drift in optical frequency as they age, mainly due to the decrease in
gas pressure inside the tube. A rebuilt laser should be similar to
an original in this regard as well.
Back to Top
Acceptance Testing Summary
With an arrangement similar to the one shown in the diagram above,
"Two-Frequency Interferometer Laser Tester", along with a polarizer,
laser power meter, and high speed photodiode, most of the following
tests can be performed in under 20 minutes (10 to 15 of which are
the time to lock). 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!
- 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).
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 the Zygo controller and/or measurement
display electronics may give up after a fixed amount of time, and produce a
non-recoverable hard error.
- 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 ("Stable" LEd on) since
marginal tubes may only start misbehaving after they warm up.
- Locking: Most of the Zygo lasers typically in 10 to 15
minutes (Stable LED on solid). 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
A healthy used or rebuilt laser should lock in about the same time as a
- 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 400 µW.
For original Zygo laser tubes, the output power may increase or decrease
slightly after the laser locks and over the next hour or so, especially
for used lasers with higher mileage tubes. Much of this change is due
to a change in mirror alignment due to thermal effects. Output power
from 7701s and 7702s with new tubes will typically be over 600 uW. For
7712/14s, 2 mW.
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.
- Beam profile: The normal beam profile for Zygo 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, though the most likely result is
no visible difference compared to the genuine Zygo laser.
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 Zygo.
- Mode alignment: 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 an angular range centered around
the X and Y axes with respect to the laser baseplate.
- Mode balance: The output power in the X and Y polarized modes
should be within about 20 percent of each-other for a 7701; 5 precent
for a 7702 or 7712/14 after the laser has been on for several hours.
(They drift somewhat during warmup.) While a modestly larger
difference won't really affect performance beyond slighlty
decreasing the useful MEAS signal level, it may be an indication of
sloppy adjustments (or lack of knowledge with respect to adjustments!)
during a rebuild. Also use a polarizer to check the appearance of the H
and V polarized components of the beam. Both should be fairly symmetric,
though perhaps not quite perfect.
- Mode purity: If the mode alignment and balance are acceptable,
mode purity should be decent.
- REF signal: An optical receiver should produce a clean stable
waveform (squarewave) with crisp sharply delineated tops and bottoms and
rising and falling edges.
- REF frequency: This should be very close to 20 MHz.
- MEAS signal (stationary): This should be the same as the REF
signal, above - clean and stable.
- 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).
- 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, and to
track changes as the tube ages. So, comparing
with a low mileage Zygo laser is as good as comparing with an
iodine stabilized HeNe laser.
- Mode flipping and other transient errors: Unless very severe,
this will require running the laser with some means of monitoring its output
over many hours. It may be possible to use the measurement electronics of the
equipment to do this, but they may not even catch some glitches. So,
it is probably best to use a stand-alone data acquisition system to
monitor the optical power of the H and V components of the laser
output at a rate of at least a few samples per second. If possible, have the
software flag any sudden changes that occur. A similar anomaly in both H and
V is due to something happening to the beam the laser tube produces.
An anomaly in one polarized component, with a partial opposite effect in the
other, is due to a problem in the AOM or its driver. In addition to mode
flipping, stick-slip effects may produce similar errors. And, of course,
electronic problems like bad solder connections or cables can also
result in similar symptoms. In some cases, the laser will turn on
the "Service" LED, but probably not for a momentary error that is
Back to Top
Discussion and Conclusions
A used or rebuilt Zygo 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. There are far fewer potential issues with used and
especially with rebuilt Zygo lasers compared to those from HP/Agilent
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 checked one 7712 that supposedly had been rebuilt.
It passed all basic tests and appeared indistinguishable from an original 7712
but apparently failed a few weeks later for unknown reasons.
And of course, with any laser that has been modified without Zygo'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.
Back to Top
References and Links
- Zygo Corporation. Go to:
"Stage Position (OEM)" or search for "ZMI".
- General information on Zygo metrology lasers and systems:
Sam's Laser FAQ chapter "Commercial HeNe Lasers", sections starting with:
Zygo HeNe Lasers.
- 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.
- 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.
- 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.
- Companion document: Considerations
in Evaluating Used or Rebuilt Hewlett Packard/Agilent Metrology Lasers.
Back to Top
Sam's Laser FAQ,
Copyright © 1994-2021,
Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the
Email Links Page.