Laser interferometer of new generation

Laser interferometer for length measurements ranging from a nanometre up to a few metres

The compact SP-NG interferometer for a diagonal measurement in a machining centre
The compact SP-NG interferometer for a diagonal measurement in a machining centre

Increasing demands on the accuracies of coordinate measuring machines, machine tools and positioning devices with movement ranges over several metres pose a challenge to measurement systems when it comes to their acceptance and calibration. In this context, new generation laser interferometers offer unique properties that combine a large length measuring range with an extraordinarily high resolution. By using the wavelength of He-Ne lasers as a long-term stable measuring standard, the measuring systems can be traced back to national standards and are, therefore, ideally suited for calibrations and tasks in metrology. The basis for the interferometers is SIOS Messtechnik GmbH‘s single-beam concept, which combines excellent linearity over the entire measuring range with simple calibration.

The universal interferometer of the SP NG series

The basic system of the SP-NG series is based on the proven concept of the compact single-beam laser interferometer from SIOS Messtechnik GmbH. These interferometers are distinguished by the fact that only one measuring beam, which is reflected by the measuring reflector back into itself, is used for the interferometric length measurement. This results in a defined sensing point on the measurement object. Therefore, it is possible to design the metrological arrangement so that the laser beam is exactly aligned with the measuring axis. This minimizes the Abbé error that is a typical source of error in all length measurements. With a small reflector that can tilt up to ±12.5°, measurement set-ups can be quickly and easily calibrated. The measuring principle of the interferometer also allows the use of a simple plane mirror as a reflector if there is considerable transverse displacement of the measurement object along the beam direction in the set-up. With the environmentally corrected light wavelength of a stabilized He-Ne laser as a highly stable natural measuring standard, these sensors have nanometre accuracy and excellent linearity. The light source is located outside the sensor in the evaluation electronics and the light is generally supplied via fibre-optic cables. As a result, the very compact size of the sensor head is determined only by optical elements.

The excellent options for length measurement with an interferometer are provided by the correlation between the measuring resolution in the sub-nanometre range and large measuring lengths of a few metres. This correlation is possible because the device concept of the measuring systems includes several metrologically relevant options.

Length measurements over longer distances are primarily characterised by the requirements for exact measurement of the environmental parameters in the air, the wavelength stability and the material temperature. Therefore, the SP-NG interferometers are equipped with a highly accurate environmental compensation, which is crucial for the measurement deviation.

For short measuring distances, frequently neglected and non-measuring-equipment-related alignment errors can significantly influence the measurement uncertainty. If the measuring direction does not match the direction of movement of an object in a short guide, these errors can dominate the measurement result. The standard version of the SP-NG interferometer has built-in alignment optics, which are indispensable in these applications. The reflected measuring beam is evaluated and a deviation between the measuring direction and the object movement is visualised. The geometric alignment of the system can be done without errors.

The following table summarises requirements for length measuring systems and options of an SP-NG interferometer, required for accurate length measurement. The specified measurement tasks are only shown as examples and are derived from empirical values.

Measurement task Measuring system requirements Realization in the SP-NG interferometer
Length measurement and calibration over a few metres, > 0.5 m – High accuracy of the measuring standard – Long-term, frequency-stabilised lasers
– Highly accurate detection of air temperature, material temperature and air pressure
– Single-beam interferometer, measuring beam defines the measuring axis and Abbé errors can be minimised.
– Sturdy housing
Length measurements and calibration in the range from 100 mm to 500 mm – High accuracy of the measuring standard
– Alignment of the system
– Long-term, frequency-stabilised lasers
– Highly accurate detection of air temperature, material temperature and air pressure
– Integrated alignment optics
– Abbé error minimisation through a beam-defined measurement site
Length measurement and calibration up to 100 mm – High system stability
– High resolution and linearity
– Measuring axis and movement axis are aligned
– Abbé-error-free measurement
– Integrated alignment optics
– Low-noise signal evaluation with sub-nanometre resolution
– Stable mounting of the sensor head
– Material selection for the sensor is possible
– Michelson interferometer principle and a interferometer dead path defined by it

The measurement and calibration process with the SP-NG interferometer can also be synchronised with the positioning control of the system. Extensive options for electrically isolated triggering of the system such as:

  • start/stop triggering
  • triggering of individual measured values
  • substitution of the sampling frequency

allow control of the measured value recording. As a result, measurements over longer distances can be carried out “on-the-fly” and thus, save time. The measured values ​​are transmitted to a laptop or computer over a fast USB interface.

The concept for long distances

The long-range interferometer for measuring ranges up to 80 m
The long-range interferometer for measuring ranges up to 80 m

A lens option and a special design of the SP-NG sensor head allow measurements up to 80 m.

Various optical reflectors can be used for this long-range interferometer. A special reflector, in which no glass medium is trapped between the reflective surfaces is used for standard measurements over longer distances. This reflector consists of three highly accurate mirrors, which are arranged at an angle of 90° to each other. This prevents any additional falsification of the measured values ​​due to a transition between the air and the reflecting medium for the measuring beam. The maximum angle of inclination of the reflector around the centre of the reflector can be up to ±22.5°. The use of such reflectors with high tilt invariance greatly simplifies the calibration and set-up of the interferometer. The permissible range of motion of the long-range reflector transversely to the measuring beam is up to ±1.5 mm. The measurement of environmental parameters is given high importance for longer distances. Therefore, the systems can optionally be equipped with several wired or also wireless temperature sensors to record the temperature distribution in large spaces.

Applications of long-range interferometers include laser interferometric measurements on guides, the calibration of high-precision axes on measuring machines and machine tools as well as coordinate measuring machines for double or multi-coordinate measurements.

The OEM concept

SP-NG laser interferometer systems as built-in version
SP-NG laser interferometer systems as built-in version

The concept of the SP-NG measuring system enables a sensor design with the option for long-term, stable calibration of the measuring beam. This solution is applied if the interferometers are used as an OEM component in a custom set-up. Figure 3 shows two aligned systems, whose beam adjustment is ensured by special optics.

The measuring systems in this version are aligned in an arrangement during installation and the calibration remains temperature and stress-resistant for long periods. In the OEM segment, a version made of the materials, adapted to a measurement set-up is possible to obtain a low-drift measurement set-up.

Maximum accuracy for large measuring ranges

The new generation laser interferometers are a valuable tool for all applications, where accurate length measurements are required. The systems are characterised by high precision and resolution as well as high immunity to interference. The fibre-optic coupler for transmitting the laser light prevents heat sources in the sensor head and allows its flexible arrangement in the room as well as quick and easy adjustment. A high tilt invariance and low sensitivity to a lateral offset of the measuring reflector make these interferometers a versatile tool for the commissioning, adjustment and calibration of precision guides, coordinate measuring machines and machine tools. These measuring systems are extremely versatile and can be easily adapted to a wide variety of measuring tasks.

Compact calibration interferometer up to 6 m

Calibration interferometers for measuring of straightness, angle and  position

Two calibration interferometers were developed for measuring lengths up to 6 m especially for narrow spaces. The high-precision straightness measurement makes them also suitable for the alignment of machine components.

Calibration interferometers are used, wherever highly accurate detection of rotational and translational guidance deviations of linear positioning axes, machine tools and coordinate measuring machines is required. In addition to the high measurement resolution and linearity achievable, the traceability to the metre specification is a decisive advantage of the interferometric measurement method. This is ensured by the frequency connection of the laser source used as a benchmark.

Calibration interferometer SP 5000 C5 2D, compared to the proven SP 15000 C5 system

The high-precision requirements of a system can only be achieved if the existing deviations are known. These can be documented, for example, in the form of acceptance reports. Nevertheless, what is much more important is the fact that the accuracy can be increased by a compensation of the existing deviations – be it linear axis, coordinate measuring machine or machine tool.

As with all interferometers of SIOS Messtechnik GmbH, the basic concept of the calibration interferometers presented here is the use of a fibre-coupled sensor that avoids any heat generation at the measurement site. The measuring reflectors are completely passive, which means they are electrically and magnetically neutral. Another important advantage is the simultaneous acquisition of several measured variables, a principle that SIOS has been following for more than 10 years, since multicomponent measurements allow a mutual assignment (also random proportions) of guidance deviations and also drastically reduce the required measuring times.

The SP 15000 C5 laser Interferometer has already proven itself on the market as a system that records the linear position up to a distance of 50 m, the pitch and yaw angle as well as a straightness component by laser interferometry in a single measurement process. Special advantages of the system are the long range in the high angular resolution as well as the large measuring ranges for the angle and straightness measurement. This and the integrated alignment aids make system alignment child’s play.

However, in a number of applications, the space available in the machine is limited, or the permissible mass of the measuring reflector is limited due to the system design or measurement task (dynamic analysis). In addition, the attachment of the components of the measuring system can be difficult especially in machine tools.

Since the required measuring range is less than 6 m in many cases, two new calibration interferometers with a compact sensor head and a compact measuring reflector for measuring lengths up to 6 m have been developed on the basis of the tried-and-tested SP 15000 C5.

The position resolution and the resolution of the interferometric straightness measurement remain the same as with the SP 15000 C5, the angular resolution is very high at 0.001 arc seconds. Since the systems are to be used even in narrow spaces, focus was on easy handling of the compact system components. Thus, appropriate alignment aids are also included here.

Calibration interferometer with interferometric straightness measurement

Compact sensor head and measuring reflector of the calibration interferometer SP 5000 C5

With the new SP 5000 C5 calibration interferometers, all measured variables are recorded by interferometric method. The set-up and functioning are analogous to the large SP 15000 C5. The significantly more compact sensor head detects the position, the pitch and yaw angles as well as the horizontal and vertical straightness.

With dimensions of 50 mm x 50 mm x 50 mm and a weight of 240 g, the measuring reflector used is small and light.

The straightness components are measured in two passes without realignment of the system. The measurement resolution of the interferometric length measurement is 5 pm over a measuring range of up to 6 m. The system is characterised by very high angular and straightness measuring ranges and accuracies.

Calibration interferometer with 2D straightness measurement

The SP 5000 C5 2D calibration interferometers (Figure 2) are designed to further simplify handling and reduce measuring times. The straightness mirror is eliminated, so that the calibration is made easier and functionality is significantly improved. The compact, fully passive measuring reflector allows simultaneous measurement of position, pitch and yaw angle as well as straightness components in a single pass. Due to the absolute, micrometre-accurate straightness measurement, the SP 5000 C5 2D is also ideal for the alignment of machine components.

High-performance software solutions for standardised machine acceptance, volumetric correction of machine tools for all common machine control systems, dynamic component analysis and measuring system-supported alignment of machine components exist for all measuring systems.

Use of C5 and C5 2D series multicomponent interferometers significantly reduces the time required for acceptance inspections, calibrations, system analyses and correction range adjustments. This allows cost reduction, while increasing accuracy.

Tidal earth crust deformation measurements

Long-term measurements of earth crust deformation

Deformation of the earth crust mainly results from the tidal forces of sun and moon acting on the Earth, but also comes from seismic wave propagation or regional and local sources. Strainmeters allow the observation of crustal deformation with a resolution better than 1 nm. At the Geodynamic Observatory Moxa in Thuringia/Germany an assembly of strainmeters of different types records the deformation. The analysis of the strainmeter data shows the comparability of the data from the different instruments as well as the good data quality connected to the very low noise level at the Geodynamic Observatory Moxa. The strainmeter systems of SIOS Meßtechnik GmbH described in this paper are long range laser interferometers of Michelson type. These interferometers with corner cube reflectors are precision length measurement instruments and coupled by optical fibers. They are specially designed for long range operation under difficult environmental conditions. The interferometers are installed in the Geodynamic Observatory in a gallery dug horizontally to the adjacent slope. The measuring ranges are 26 m and 38 m. The resolution of the systems is about 1 nm. All interferometers are designed for long-term measurements over several weeks.
To minimize the influence of temperature and air pressure changes on the interference fringes due to the dependence of the refractivity index of air on these values, the horizontal borehole for the diagonal 38m strain is sealed at both ends with a special glass and the 26m strains use the hermetic metal tubes for the laser beam protection.
All data are sampled every 10 s. In addition, several environmental sensors measure variations of temperature, air pressure and humidity inside the gallery. At the laser strainmeter, another air pressure sensor is installed and five temperature sensors are placed at different points along the laser beam. Outside the observatory building, a meteorological station records environmental  parameters.

Fiber-coupled homodyne interferometer

Displacement measuring homodyne interferometers are interferometers which use only one frequency light source and compare the measuring displacement with a light wavelength. The devices allow ultra precise measurements with nanometer-scale resolutions. A metrological analysis of this measuring method shows the opportunities they afford and the metrological limits of laser-interferometric systems. For that purpose, consider a Michelson interferometer, the basic type of interferometer for displacement measurements, and assume that the laser light source employed emits plane waves that are split into a pair of coherent, partial waves and interfere through superposition.
The homodyne interferometers presented here are Michelson interferometers. The figure below shows schematically such interferometer, configured in the form interferometer with cube corner reflector.

Fully fiber-coupled homodyne interferometer sensor
1. Fully fiber-coupled homodyne interferometer sensor

Both the laser light source and power-supply/signalprocessing unit are separated from the measurement head. Light from the frequency-stabilized He-Ne laser is transmitted to themeasurement head on a single-mode fiberoptic lightguide, which allows keeping heat sources well away from the location where measurements are conducted. The advantage of the metrological method presented here is based on transmitting just a single beam that is reflected by the moving retroreflector per measurement axis, which allows configuring the metrological setup such that there will be a well-defined point of contact with the object being measured and that the laser beam will remain accurately aligned on the measurement axis, which, in turn, means that the configuration of an Abbé comparator will be maintained. Abbé errors, a typical error source, will thus be minimized or totally eliminated. Such interferometers have nanometer precisions and excellent linearity over displacements in meter range. The presented interferometer type is used in the application for the earth crust deformation measurement, because
of the low heat generation and simple design.

Interferometer application for tidal crust deformation measurements

The Geodynamic Observatory Moxa consists of a three-part building located at the foot of a hill, and a gallery dug horizontally into the adjacent slope. The gallery is separated from the building by several doors to keep environmental conditions, especially temperature, as stable as possible. Its instrumentation consists of different types of measurement systems, including besides strainmetersalso tiltmeters, seismometers, a superconducting gravimeter as well as a spring gravimeter. Additionally, outside the observatory and in the surrounding area a large number of sensors record environmental parameters like air pressure, temperature, wind, or soil moisture. All strainmeters are installed inside the gallery. The 26 m long quartz tube strainmeters and the 38 m long laserstrainmeter are connected to the ground by steel pillars whereas the borehole instrument is installed at about 10 m depth and the borehole filled up with concrete. The quartz tube strainmeters are oriented in east-west and north-south direction, respectively, along the two arms of the gallery. Distance changes along the two quartz tube strainmeters are measured by inductive sensors. The laser diagonal instrument connects the far ends of the quartz tube strainmeters through a horizontal borehole, and the borehole strainmeter is installed at the gallery elbow. The new laser based strainmeters have been installed paralle to quartz strainmeters, so they build now the completely triangular system for crust deformation monitoring.

2. Assembly of laser strainmeters at the Geodynamic Observatory Moxa.

The Figure 2 shows an assembly of the laser-interferometric strainmeters in the observatory.

The interferometers used as laser strainmeters are modified fiber-coupled homodyne interferometers with cube corner reflector (Figure 1). The high humidity at the measuring location of about 100% and the long stand-off distance of the reflector cause main problems for the
interferometer design. To minimize the influence of temperature and air pressure changes on the interference fringes due to the dependence of the refractive index of air on these values, the horizontal bore 38m hole is sealed
at both ends with a special glass and the 26m strains use the hermetic metal tubes for the laser beam protection.

 

Figure 3 presents the measuring principle of the laserinterferometric strainmeters.

3. Measuring principle of the laser-interferometric strainmeter

The light source for the interferometer is a modified fiber coupled SL-03 He-Ne laser of SIOS Meßtechnik GmbH, which is placed outside the device and outside the gallery behind the styropor door. The light comes to the interferometer via singlemode PM-fiber. The measuring beam of the interferometer goes through the tube with two glass windows. The tube is intended to be hermetic sealed. The air pressure sensor is installed on the tube and four temperature sensors are placed at different points along the laser beam. In addition, the one temperature sensors measure the temperature inside the gallery. This interferometer setup is used for both N-S and E-W strains. Also diagonal laserinterferometric strainmeter has similar setup.

Figure 4 shows the interferometers of the strains in gallery.

4. Thermally isolated north-south strain(left) and interferometer base for N-S and E-W strains (right)

Measurement results

The usual correlations of the interferometer measuring results in air to the temperature and to the air pressure are very high. The natural temperature stability in the gallery is only some tens of Kelvin over the year. But the air pressure changes are directly influences by the external natural effects. Therefore the variations of the air pressure along the measuring beam plays the main role for the stability of the measuring results.

Stability of the temperature of N-S strain (left) and air pressure of N-S and E-W strains
5. Stability of the temperature of N-S strain (left) and air pressure of N-S and E-W strains

The Figure5 presents the stability of temperature of N-S strain over 8 h and the stability of the air pressure over 12 d. The natural standard deviation of the air temperature in the gallery over 8 h is about 5mK. This temperature influences only short part of the interferometer measuring beam before and after the hermetic tube. The hermetic tube is additionally isolated from the environment by styropor material. Four other temperature sensors are connected to the tube and measure the temperature variations along the measuring beam. These sensors are used for monitoringpurposes only. As is presented in Figure 5 the standard deviation of the temperature along the beam is in the range of 1.5 mK.

The variation of the air pressure for N-S and E-W strains is presented in Figure 5 (right). It is measured inside the hermetic tube along the measuring beam of interferometers. For the air pressure measurements the high-precision resonance pressure sensor RPT201 is used. The sensor is temperature compensated and provides the air pressure dependent frequency output. In order to minimize the power consumption of the electronics these sensor digital outputs are converted in the data pre-processing box into the analogue harmonic signals and transferred over more than 66 m to the main data processing station. As is presented in the Figure 5, the high stability of the air pressure in the tube was achieved. Over the 12 d both sensors show the standard deviations of variations of the air pressure in the range of 1.6 Pa. Therefore the influence of these parameters to the measuring result of the interferometer can be neglected.

The high stable temperature and air pressure conditions of the measurements allow the direct monitoring of the earth crust deformation. Figure 6 presents the representative measuring curve of the N-S laser-interferometric strainmeter.

Typical earth deformation curve over 24 h
6. Typical earth deformation curve over 24 h

The amplitude of the curve in Figure 6 is about 1.2 μm. For the stand-off distance of 26m it is a relative value of 4.6 × 10−8. Such values are usually not achievable for the interferometric displacement measurements in air. The measuring curve is not noisy and is not drift affected. Even the earthquake effects, which are monitored in the observatory by different instruments can be detected in the measuring results. The data is saved every 10 s continuously over 24 h.

The data processing of the measured earth crust deformation and comparison to the other measuring instruments is a complex mathematical task.

Summary

The article presented an application of the fiber coupled homodyne interferometers for the tide caused earth crust deformation measurements. Three laser-interferometric strainmeters measure the crust deformation continuously over 24 h. The interferometers for this application were designed for long-range application under the high humidity conditions. The achieved high stable condition of the measurements specially in respect to the air pressure and advanced instrumentation for measurement of the environmental parameters provided the background for successful installation of the system.

This joint research project was realized between the FSU Jena, Institute of Geosciences in Jena and SIOS Meßtechnik GmbH.