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Frequently Asked Questions

What are the axes on the interferometer graphs?

The horizontal (x) axis of the graph represents the chip number of the ccd line-array. The vertical (y) axis is the voltage of every single chip in milliVolt. The graph therefore is the direct reproduction of the interferences pattern focussed on the array. It is not the spectrum! But you can get some spectral information of a interferometer, e.g. if your laser is running single mode or if it has got sidebands.
  X and Y axes

What is the minimum & maximum power my device can manage?

Minimum Input Energy
The minimum input energy required differs for the several wavelength ranges. Please see the right graph to see sensitivity information of our products.

Maximum Input Energy / Damage Threshold
Three types of input limits have to be distinguished.

1. Damage threshold of the fiber patch cord and connector
Standard fiber optical patch cords and the FC/PC connectors can endure up to 300 µJ. Higher energies will most likely damage the fiber input.

2. Damage threshold of the optional multichannel switch
CW lasers: in the UV range the limit is 5mW, in the visible or IR range 20mW. HighFinesse
wavemeters usually only need a few μW, this should be more than sufficient, even when transmission losses through the multichannel switch are taken into account.
Pulsed lasers: the recommended maximum peak power is 2 W in the UV, up to 5 W in the IR range. High power versions of switches can be ordered. For ARAK switches the recommended maximum peak power is 20 W in the UV, up to 50 W in the IR range.

3. Damage threshold of a wavemeter
The actual damage threshold of a HighFinesse wavemeter is not relevant, because no free-beam is possible. Please be aware, that the CCD of a wavemeter will be overexposed long before the damage threshold is reached. This means that the necessary input energy for a good illuminated measurement will always lie far below the wavemeters threshold.
  Required input energy

What kind of pulsed measurement is possible and what is the accuracy?

The shorter a laser pulse is, the broader the spectral width of this laser. Laser pulses shorter than 100 ps can only be measured by the D option (Diffraction grating). The accuracy of the wavelength meter is determined by the length of the longest interferometer inside the optical unit. The longer the interferometer, the smaller the spectral width of the laser has to be; if the laser is broader than the "free spectral range" of the interferometer, the interferences vanishes and no signal can be analyzed.

The type of lasers that can be used for each model can be found in the product description and in the datasheet brochure. Look at each wavemeter type at the line "Fizeau-interferometers". 40% of the FSR is roughly the upper limit for the spectral width of lasers that can be measured for each interferometer. Please inquire with our service, in case your laser might exeed these limits. Also for each model (except WS-5) there is a so called "Wide Mode" available. In this measurement mode the longest interferometer will be ignored for the calculation. The WS-7 in Wide Mode for instance is able to measure lasers with a maximum spectral width of 8-12 GHz. In normal mode, lasers with a spectral width < 1.6 GHz may be measured.

One more important point, we can only measure at a maximum of a factor of 3-5 better than the laser width. So if the laser is 10 GHz broad, the best accuracy, you can expect is 2 GHz (Using an Ultimate by having only 10 GHz broad laser sources doesn't make sense).
  Pulse Width

What is the difference between good and bad illumination?

To ensure high accuracy of the measurements you have to adjust the optical setup well, so that the interferometers are illuminated evenly. Good illumination means the peaks have approx. 60 % of the screen height (at least 1000) and the height does not vary by a factor of more than 2.
  Illumination via Multi-mode Fiber

Does my software support the latest Windows OS version?

Current software supports all latest Windows OS versions. If your wavemeter has been delivered before the year 2011, then in some cases the drivers have to be updated. Please first install the software as usual and then download and install the HighFinesse Driver Update Package here: The file is a compressed RAR-file that can be uncompressed with programs like WinZip or WinRAR. Alternatively you can rename the file to "HFDrvUpdatePkg.exe". It will become a self-extracting archive that you can simply execute by double-clicking.

Please confirm to overwrite already existing files during installation. After the installation of software and Driver Update Package simply connect your wavemeter. You can then start the software and make your measurements.

What are LTR files?

HighFinesse offers help in the interpretation of interference patterns. For this we need a spectral film of the measurement of interest.

A Long Term Recording (LTR file) includes all measured values such as wavelength, pressure, and temperature time independently. It also contains some device information like interferogram data, used calibration and software settings.

No user information is stored.

Please read the next FAQ to know how a LTR-file is recorded.

How to generate a LTR file?

Please read chapter 2 of your printed manual, run a measurement and make sure everything is adjusted well and the exposure time allows a quite good signal so that half of the interferometer charts is illuminated.

Stop the measurement and start the recording by selecting "Operation | Start | Recording ..."
and choose a filename and location for the recording. Immediately after closing the dialog the recording will start and the measurements will be saved until you press "Stop".

It is not necessary to produce very long recordings as the filesize is growing strongly with the measurement count. Usually up to 100 measurement points are more than enough. You may choose to record for a longer time to depict the phenomenon at hand.

Send this file along with these additional files:

- all *.ini
- all *.stn
- history.serial_of_wavemeter
- and complete folder "calibrations"

You can find these files in the program folder of your device.

If you are using Windows Vista, Win7, Win8 or Win10, these files have been redirected to the virtual store. You can find this hidden folder usually at this place:
C:\Users\%USERNAME%\AppData\Local\VirtualStore\Program Files\HighFinesse\Wavelength Meter WS ...

Please copy and complete the folder path into the file explorer of your choice. Otherwise the Windows folder settings have to be changed to "Show hidden files, folders and drives" to see the hidden folders.
  Start a LTR Recording

Why does the multichannel switch show the same picture on all channels?

The multichannel switch does not switch.

1. Please check if all communication and power cables are connected. Some switches have an external power supply.

2. Check the software if the communication protocol settings are correct. Select SETTINGS | SWITCH SETTINGS | SWITCH PARAMETERS:

 TTL (5pin LEMO): choose WLM TTL S
 RS232: choose COM
 USB: choose USB COM U

For COM-port connected switches check the COM-port settings as well. Ensure everything is plugged in and you chose the correct COM-port and baudrate (usually 57600, older versions 9600).

What is the temperature and pressure stability of my Wavemeter?

Pressure Stabililty
In the diagram you can see a sample measurement of our WS-Ultimate series with an accuracy specification of ± 2 MHz. The measurement effectively is not influenced by the pressure change from ca. 560 to 800 mBar.

Temperature Stabililty
Before measuring the wavelength meter needs to be in thermal equilibrium with its environment. Usually it is enough when it is kept in the lab for 1-2 hours before the measurement.

As the temperature of the interferometer is measured and considered in the calculation, slow temperature changes do not influence the accuracy.

In the diagram you can see the temperature stability of our WS-7 (lower graph). The Wavemeter was kept in a climate chamber for several hours while changing the temperature drastically. As long as the change is below approx. 2 °C/h, the temperature drift is compensated quite well.
  Temperature of a WS-7 and pressure stability of a WS-U2

When do I need the external trigger option (TTL)?

Pulsed Mode (all wavemeters)
All wavemeters can measure cw and pulsed lasers. In case a pulsed laser is coupled to any HighFinesse wavemeter it automatically detects incoming pulses and calculates the wavelength. In case the repetition rate exceeds the maximum measurement rate of the wavemeter, it will handle the laser as quasi-cw, automatically.

External Trigger Option
As the name says, it allows to externally trigger the wavemeter for pulsed measurements. This can be useful in several situations, e.g. when only every other pulse needs to be measured, low duty cycles, strong background radiation fills up the array with noise, or single pulses (pulse groups) need to measured at specific time sets. Two trigger modes are available. Both modes allow to define start and stop point of the measurement in a very specific way.

In mode 1 the TTL signal starts the read-out of the wavemeter arrays. The measurement will only stop upon a second TTL signal.

In mode 2 the TTL signal starts the read-out of the wavemeter arrays, which lasts for a user pre-defined period of time.

Why is the (re-)calibration of my wavemeter so important?

HighFinesse wavemeters are known for their robust design due to no moving parts. It is still necessary to re-calibrate the device within a suggested calibration period (see datasheet). Most of the wavemeters have integrated calibration sources, that enable quick and easy calibration within a couple of seconds.

Over time temperature and pressure drifts influence the whole calibration curve. If left uncalibrated high accuracy wavemeter optics will possibly get shifted and tilted by temperature and pressure shifts. This can influence absolute and relative accuracy.

Those small anomalies can be detected and eliminated by storing calibration data of not only a single wavelength value, but also the whole inteference pattern of all interferometers.

The calibration data is saved in the installation folder of your PC. When the wavemeter is moved to a different PC a new installation and calibration will be necessary. A transfer of the calibration data from one PC to the other is not advised.

How to find the best parameters for the PID-controller?

To find the right parameters and to optimize the PID-controller to have the most accurate and fastest feedback control of your laser can be a real challenge sometimes. We hereby describe a quick procedure finding the parameters P, I, D and the time constant Ta for the I- and D-calculation. This procedure is based on our PID-Simulator which estimates these parameters according to your setup- and laser-conditions.

First, you need to setup your Wavelength Meter:
Set an appropriate exposure time (the smaller the better). (e.g. 5 ms)
Start the measurement and determine the repetition rate (simply type "RepetitionRate" in the "Start | Run..." dialog box). (e.g. 40 Hz)

Now, you need to know your laser:
Find a working range where the laser is operating mode hop free in a certain range. This working range must cover the wavelength range you want to regulate.
Apply 0 V on the LC-output of the wavemeter using the "Calibration"-tab in the laser control settings window and press "Send". Which wavelength does the wlm display? (e.g. 780,311 nm)
Increase the output voltage step by step and observe the max. mode hop free range and the amplification V/nm.
Decrease the output voltage step by step and observe the max. mode hop free range and the amplification V/nm.
Now calculate the amplification (e.g. 7 V/100 pm)

It might be also necessary to limit the maximum/minimum voltage of the LC-output in order not to cause the laser to jump during the regulation (mode hop). Therefor set these values in the "Bounds"-tab in the laser control settings window.

Note: The "Test"-output voltage might be not available in older software versions. Then you need to disconnect the LC-cable and apply the desired voltage by an external powersupply to find out the 0 V-wavelength and the amplification.

Run the » PID-Simulator:
You need to adjust the General- and Laser-settings to fit your experimental conditions:

General settings:
Time/Measurement = 1/Repetition Rate (e.g. 1/40 Hz = 25 ms = 0,025 s)
Response count = 2 (The PID gets an answer to its action after the next measurement shot, do not change this!)
Measurement count is only for displaying the function, it has no influence on the calculation (-speed).
V_out maximum = +/- 4096 mV (this is the max. range the wlm can give out)
V_out resolution. = 0,5 mV (14 bit resolution on max. range, do not change this!)

Laser settings:
Type in the amplification and 0 V-wavelength you determined above.
Noise and Sudden hops can be left \"0\" usually.

Formula: Use whatever you desire, within the mode hop free wavelength range (e.g. f(t) = 780,311+ 0,001 * (triangle(t/2)-0,5) to see how the laser reacts)
Ta, P, I and D, you can leave blank as these values are estimated automatically.
The sensitivity should be the found laser amplification.

Now you have entered all desired values. Press "Auto TPID" to start the iteration and wait until the "Auto TPID"-button is solid black again. The simulator now tries to find the best settings corresponding to your initial parameters. You can use these parameters in the laser control settings window and maybe adjust them a bit to optimize your feedback loop.
If the controller does not work properly with the found parameters, you might need to repeat the procedure with slightly different initial values. The better you know your conditions and the laser properties, the better are the estimated values.
If you change the formula in your laser control settings, you need to use the same formula in the simulator and run it again!
  PID Simulator v2