Frequently asked questions (FAQ) related to SIOS interferometers


Why does SIOS GmbH use He-Ne lasers instead of laser diodes as light sources?

He-Ne-lasers are the traditional laser sources for precision interferometry. In interferometry the laser wavelength is the measurement reference. The wavelength of a SIOS He-Ne laser is traceable to an international standard and is calibrated against a Iodine stabilized He-Ne laser. This certified traceablity to a national calibration standard is essential in the application of laser interferomety for use as a calibration tool.

Furthermore, the SIOS He-Ne lasers have good wavelength stability with nearly no aging effects. At the moment, in the visible light range, similar traceablity and aging stability cannot be reached with laser diodes.

What is the safety class for the interferometer? What should be considered?

Regarding the international laser saferty standard DIN EN 60825-1, the SIOS interferometer is classified in the laser safety class 2M. The automatic reflex of the eye to close with exposure to bright light sources provides sufficient protection in this classification so special safety glasses are not normaly required.

The stand-alone lasers SL02/SL03/SL04 (available without the interferometer and only as a laser light source) have slightly higher optical power and belong to the laser safety class 3R. A direct view with the eye into the laser beam may be dangerous, so special laser protection glasses for 632nm are required; other precautions may also be required (mechanical interlocks).

For more information contact your national standard agency.


What are the advantages of fiber optical coupling the sensor heads?

The light from the He-Ne-laser is fed to the sensor head via a special optical fiber. This has the following main advantages:

  • Due to optical fiber coupling, the laser and controller can be located away from the measurement object, preventing heating of the measurement object by the laser. This also reduces potential sources of vibration affecting the head.
  • The fiber coupled sensor head is much smaller than a laser. The head is therefore much better suited for measurements in limited space and can be easily placed in any orientation.
  • The adjustment of the sensor head is much easier and faster because only two components (fiber coupled sensor head and a reflector) have to be aligned, instead of three components (laser, interferometer, reflector).

Which materials are available for the sensor head?

The sensor head can be constructed from aluminum (standard), stainless steel and invar. For normal calibration applications aluminum is preferred because long term stability (> hours) doesn’t play a key role in most applications. Aluminum is also lightweight, making the sensor easier to mount.

For measurements where long term stability is very important or for integration of the sensor head into machine assemblies, the thermal expansion coefficient of the sensor material should match the base material to avoid stresses due to different mechanical expansion. Sensor heads constructed in stainless steel match steel and granite assemblies. Sensor heads made of invar match best to Zerodur, ceramics and quartz assemblies. For more details please contact SIOS.

Which interfaces are supported?

The following interfaces are available:

  • USB, incl. SIOS-software for displacement, angle and vibration measurements
  • RS-232
  • AquadB: digital incremental quadrature signals with a configurable resolution from 1 nm to 10 µm via RS422-level, (for up to four channels available). This interface is often used as feedback sensor in position control systems or with motion controllers and other closed loop systems.
  • Analog quadrature signal with 1 Vss amplitude (Heidenhain encoder standard).
  • Parallel digital interface with 36 bit: Outputs the measurement values as a 36 bit digital data word. This interface is used if high data rates are required. This requires a PC digital-I/O card, for instance from National Instruments, to interface with this output.
  • Analog displacement signal: The measured displacement (length value) is provided as an analog voltage (0-1V), with several selectable sensivities (Vper nm). This option is often used in vibrometry because traditional vibrometry systems offer also these analog interfaces. For a true length measurement, the analog voltage is normally not sufficient due to its limited resolution.

The USB interface is available in nearly all devices as a standard. The other interfaces may be ordered as optional components.

Which software is available for the interferometer?

We offer the following standard software for our interferometers or vibrometers:

  • InfasNTC: Software for displacement or length measurement with graphics, data saving and data export,
  • InfasVibro: Software for vibration measurement, incl. real time, FFT-analysis
  • SIIMS-Calibration: Software for calibrating different types of axis, tooling machines, coordinate measurement machines (CMM) and similar equipment

Some tasks require the creation of application specific software. We have a huge library of typical solutions and extensive experiences in measurement applications. So we are able to offer customer specific software at competitive rates. Please contact SIOS for more details.

May I use the interferometers with my own written software?

Yes. You can access the USB- and RS232-interfaces. We offer a comprehensive software support:

  • a Win32/Win64 DLL to embed the interferometer in Windows software, incl. a description of the API and some easy to understand C-language examples. The DLL can be used in most of the programming languages available for Windows
  • a .so library for Linux for most of the architectures (i386, amd64, armhf …)
  • Examples for the usage in LabView
  • Examples for Mathlab and Python

Can I synchronize the interferometer measurements with other devices?

The interferometer controller has trigger inputs for triggering the measurement of single values and to trigger the start of a measurement. It enables the synchronization with other measurement equipment with a common TTL type pulse. The trigger has low latency to external events like activations of index switches and incremental encoder pulses.

Optional: A trigger generator card is available which can produce a software controlled common trigger pulse for synchronize different measurement devices.

How accurate is the SIOS interferometer?

Calibrations of SIOS interferometers at the German national institute PTB have shown mean systematic deviations in the range around 0.12 ppm which is equivalent to 120nm/m.

The measurement accuracy of a laser interferometer in air is mainly influenced by the determination of the air refraction along the laser beam. To determine the air refraction environmental parameters like temperature, air pressure and humidity are measured with high precision.

In practice the limiting factor is the assembly in which the interferometer is integrated into, because it must guarantee a sufficient stability (thermally and vibrationally) and allow representative measurements of the environmental parameters to be measured. Please ask us for more support.

Independently from the measurement accuracy, the measurement resolution of the signal processing is around 5 pm. This resolution is particulary visible in ultra stable assemblies (especially, for example, in vacuum) and with vibrometry.

Are measurements with a SIOS interferometer traceable?

Yes. Traceable measurements are essential for calibration applications and our interferometers are developed with the prerequisite to facilitate traceable measurements.

For the calibration of our laser interferometers the responsibility of national standards institutes and a list of related offers can be found on the website of the BIPM. In Germany the PTB in Braunschweig makes these calibrations for SIOS.

As an alternative to a calibration of the whole interferometer at a national institute, SIOS can provide separate test reports or DAKKS calibrations for laser frequency and environmental sensors (temperature, air pressure, humidity).

Can the devices be used in vacuum?

SIOS has extensive experiences with measurements in vacuum. Implementing a sensor head in vacuum, requires special materials for sensor head, cables, connectors and vacuum feed throughs. Vacuum compatible systems are available on request. Please specify your application and the specification of the vacuum in mbar.



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