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| Working Group 5.42 |
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Multiwavelength Interferometry for Geodetic Lengths
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Development
Running and completed projects
Running projects
Normally, form- and diameter measurements are performed by mechanical probing. On thin cylindrical parts
such as thin wires or optical fibres, the required probing forces may cause a deformation of the test piece.
In the project applied for, a new procedure with non-contact optical probing shall be developed in the case
of which the test piece is not deformed. In addition, the very complex rotary guide of commercial roundness
measuring instruments is not required. In the case of the procedure to be developed, the cylindrical sample
serves as the measuring reflector of an interferometer. Core piece of the procedure is an internal cone mirror
which deflects the light in such a way that a plane mirror serves as a second reflector (reference reflector)
of the interferometer. The surface profile of the sample is measured by means of phase-shifting interferometry.
By using two diode lasers as light sources, the sample diameter can, on the one hand, be determined with the
aid of the synthetic wavelength. On the other hand, samples with a poor optical surface quality can be measured.
The two diode lasers used will be stabilized automatically to Doppler-broadened absorption lines of iodine vapour
to compensate the disadvantages of the diode laser with respect to its stability and the knowledge of its wavelength.
The interferometer shall measure the form and diameter (range 0.1 mm to 2.5 mm) of cylindrical samples with an
uncertainty of less than 0.1 μm.
Contact: Dr. Karl Meiners-Hagen,
Dr. Otto Jusko, Dr. Ahmed Abou-Zeid,
Alexander Höink
It is the aim of the research project to obtain as simple a construction of an interferometer for absolute
length measurements as possible which combines the methods of length measurement by means of (i) continuous
tuning of the emission frequency of a laser as well as (ii) multi-wavelength interferometry. With this
arrangement, a measurement uncertainty of 1 μm/m is aimed at in the range of up to 20 meters. This meets the
requirements of a great number of practical applications, for example for the positioning of components in
vehicle or aircraft construction. The optical measurement of distances can alternatively be performed with
laser distance measuring instruments based on running-time measurements. For short distances (a few meters),
the resolution therefore amounts to at least 100 micrometers. Conventionally counting interferometers typically
reach a measurement uncertainty which is better by several orders of magnitude. However, only relative displacements
of the measuring reflector are measured which are avoided in the case of absolute interferometers. In spite of these
advantages compared to other procedures, absolute interferometers are not yet widely used in practical applications,
which is probably due to the complexity of their implementation.
The arrangement shown in Figure 1 is used to determine the absolute length, i.e. the length difference between the
two interferometer arms, by modulating the emission frequency of diode lasers 2 (in Littmann configuration) and
simultaneously measuring the interference phase. Except for pre-factors, the length is determined by the ratio of
the changes of phase and emission frequency. The latter is measured by means of a Fabry Pérot resonator. For this
measurement, the measurement uncertainty is mainly determined by vibrations. To correct their influence on the measured
length, they are measured with laser 1. This laser is installed in Littrow configuration, stabilized to the rubidium
D1 line at 795 nm and also coupled into the interferometer.
In a second step, laser 2 is stabilized to the rubidium D2 line at 780 nm. Thus, the two lasers provide a synthetic
wavelength Λsynth of approx 42 μm.
By measurement of the respective synthetic phase, the result of the first measuring step can be further interpolated
within a span of half the synthetic wavelength. The measuring principle is demonstrated in Figure 2. At a position of approx.
1.20 m, the measuring reflector in an interferometer arm is displaced by 100 μm in steps of 5 μm.
Fig. 2(a) shows how the integer interference order is determined for Λsynth
from the result of the frequency modulation. Further interpolation of the result is shown in Fig. 2(b).
If the absolute distance z obtained in this way is compared with a counting interferometer, a deviation of less than ±1 μm
is obtained.
This work is supported by the Deutsche Forschungsgemeinschaft within the scope of the ME 2691/1-1 project.
Fig. 1: Experimental setup of the absolute distance interferometer. Laser 2 is optionally
employed in the stabilised or in the modulated mode.
Fig. 2: An example of length measurement for a shift of the measuring reflector of 100µm in steps of 5μm
at 1.20 m.
(a) Lmod obtained from absolute distance interferometry determines the integer order of interference
for Lsynth and the corresponding length Labs.
(b) Employing the two stabilised lasers in counting interferometers, the synthetic phase ΔΦs
and the length Lsynth = ΔΦs/2π · Λs/2 can be determinend.
The shift of the measuring reflector is monitored by an additional Heidenhain IK 121counter card.
Contact: Dr. Lutz Hartmann,
Dr. Karl Meiners-Hagen, Dr. Ahmed Abou-Zeid
Completed projects
Diode lasers are offered in large varieties, both regarding the wavelength and on other parameters of the module,
depending upon intended use. The demand for laser diodes with particularly characteristics optimized for length
measuring purposes is however so small that such modules are comparatively expensively or not available. The feasibility
of absolute interferometers, i.e. of interferometers, with which distances without shift of the measuring reflector can
be measured, depends strongly on the availability on diode lasers with very special characteristics. The investigations
accomplished within the project should supply criteria and methods, according to which laser diodes, developed primarily
for applications in the communications technology, can be selected for absolute interferometry, and the influence of their
parameters on the uncertainty of an absolute interferometric length measurement should be estimated. Investigations on DFB-,
VCSEL- and Littrow lasers were carried out. In addition, a laser module suitable for absolute interferometry based on the
studies conducted should be sketched.
As example a characterisation carried out for a VCSEL laser is here briefly described. The characterisation permits statements
how the used power source has an influence on the emission bandwidth and thus the coherence length of the laser. Due to the
dependence of the emission frequency on the operating current (approx. 200 MHz/µA), particularly strong with the examined
type of diode, this characterisation is crucial for planning an absolute interferometer working with this type of diode.
Some commercially available constant current sources offer 0.1 µA current noise at a detection bandwidth of 100 kHz.
Thus theoretically an emission bandwidth of 20 MHz results with exclusive consideration
of the influence of the current noise. Even if particularly low-noise power sources are used, the emission bandwidth of the examined diode will
actually be clearly larger.
Figure 1 shows that the emission bandwidth rises linear with inverse power output. This relation is typical for
quantum-limited noise. If the spectral density of noise can be described by a Lorentz shaped distribution additionally, then the minimum emission
bandwidth results as the ordinate intersection of linear fit to 36 MHz.
This research was carried out in co-operation with the Helios company.
Fig. 1:Emission bandwidth of a VCSEL laser against the inverse power output
Contact: Dr. Karl Meiners-Hagen,
Dr. Ahmed Abou-Zeid
For many atmospheric research fields flight-based measurements of the ambient air humidity
are of special interest. At present there exists a large number of humidity sensors, which
are based on different operational principles. However none of these sensors fulfils the
requirements of the flight measuring technique like dynamics, measuring range, spatial resolution,
insensitivity to condensed water, weight and design. In the research project a new humidity
measuring system with a wide tunable diode laser will be used. A hygrometer, which determine
the humidity from the strength of the water vapour absorption lines in the IR spectral region,
will be developed and tested, and optimized on real flight conditions. Compared with conventional
humidity sensors, the new sensor is to be characterised by high dynamics (kHz range), wide
measuring range with high resolution, miniaturization (weight and volume), simple calibration
as well as small drift and hysteresis features. Concerning the sensor design for flight test both
instrumentation (minimization of the measurement uncertainty) and aerodynamic criteria
(minimization of the influence of the hygrometer on the air current) must be considered.
This requires detailed investigations for the optimization of the sensor, in order to achieve
a relatively high accuracy also in the flight test.
Abb. 1: Schematic setup of the ECDL based absorption hygrometer and a water vapour absorption
line at 1383.9 nm, taken up with the new sensor.
Contact: Dipl.-Phys. Hans-Jürgen Altmeyer,
Dr. Ahmed Abou-Zeid
The aim of the project was to develop an optical profilometer which allows surfaces to be precisely measured.
It was planned to obtain a resolution in the nanometer range, with a measuring range of approx. 100 μm.
For this purpose, an interferometer working with diode lasers has been developed. In the measurement of
surface profiles, one disadvantage of a conventional interferometer is that the interferometer phase runs
periodically with half the laser. For height changes of more than half a wavelength it is necessary to count
the periods of the interferometer phase in order to obtain an unambiguous result. If the surface contains steps
with a height exceeding half a wavelength, on which the phase change can no longer be counted, the height
information gets lost. A resort to this problem is to use one or more lasers with different wavelengths.
The difference of the interferometer phases of two wavelengths acts like the phase of a so-called "synthetic"
wavelength which is longer than the used optical wavelengths. The period of this difference amounts to half
the synthetic wavelength, by which the range of unambiguous of the measurement is increased. When several
wavelengths are used, the range of unambiguousness can, theoretically, be increased almost arbitrarily.
In practical applications, the required effort increases, however, in a disproportional way.
To keep the effort as low as possible, the developed profilometer works with three relatively low-priced
diode lasers whose wavelengths are stabilized only via the parameters operating current (injection current)
and operating temperature. The light of the lasers is coupled jointly into the interferometer via
a single-mode glass fibre, which considerably facilitates the adjustment. For dynamic measurements it is
necessary to measure the signals of all wavelengths simultaneously. The separation of the signals
of several wavelengths at the output of an interferometer is usually done optically with the aid of coloured
filters or grids, by means of different polarizations or electronically via different heterodyne frequencies.
Each one of these procedures has its specific disadvantages. For three wavelengths, a polarisation-optical
separation is difficult. A separation of proximate wavelengths (difference less than 3 nm), which is required
for the multi-wave procedure, would require very good interference colour filters or, in the case of a separation
via a grid, relatively long ways. The heterodyne procedure, in contrast, requires relatively expensive
acousto-optical modulators. This is why another approach, in the case of which the operating current of
the diode lasers is modulated with different frequencies, was selected for the developed profilometer.
The resulting wavelength modulation leads to corresponding modulations of the interferometer signal.
Like in the case of the heterodyne procedure, it thus it becomes possible to measure the signals of all
three lasers simultaneously with only one photo detector. The separation and measurement of the interferometer
phases is performed with lock-in-amplifiers developed for this purpose.
Measurements of groove depths and step heights allowed the efficiency of the procedure to be demonstrated
on plane surfaces.
The apparatus reaches its limits in the case of rough surfaces where the interference contrast decreases.
Comparison measurements on so-called "supra-fine roughness standards" showed that it is -
within specific roughness limits - nevertheless possible to
determine the technical roughness parameters in good agreement with the calibration data. In conclusion,
measurements of the roundness deviations were performed on different samples. Comparison measurements
with a probe showed that on well-reflecting surfaces such as polished steel or nickel, the deviations
between the two procedures were below 1 µm. Here, the measurement uncertainty of the interferometer
is limited by vibrations of the air-cushioned rotary disk of the roundness measuring instrument used.
A larger deviation of up to a few micrometers is observed on aluminium surfaces. After measurements
with the probe, traces can be found on the surface to which the deviations might be attributed.
This is why the non-contact measurement can be of advantage also in the case of a larger measurement
uncertainty.
Abb. 1: Schematischer Aufbau des Profilometers
Abb. 2: Interferometer mit x-y-Verstelleinheit
Abb. 3: Oberflächenprofil eines Rauheitsnormals:
Das obere Profil zeigt eine Messung mit einem Tastschnittgerät,
das untere wurde mit dem Diodenlaser-Profilometer aufgenommen.
Contact: Dr. Karl Meiners-Hagen,
Dr. Ahmed Abou-Zeid
Development of a transportable calibration device for simple measuring
instruments
In the course of an EU project a transportable calibration device for simple length measuring instruments was developed.
The calibration device is equipped with a diode laser interferometer as the measuring system. In practical applications,
the uncertainty of an interferometric length measurement is mainly influenced by the temperature of the object
to be measured and by the refractive index of the air.
In this device the air wavelength is stabilized on the
length of a resonator made of steel, which is fastened to the rail laterally. Thus the influences of the temperature
of the object to be measured and the air refractive index compensate themselves as far as possible.
The calibration device has a measurement range of 300 mm. the user can adjust the device for the calibration of the
following hand-held devices: vernier callipers, external micrometers, dial indicators for linear measurement,
dial gauges, dial test indicators (lever type), vernier for depth measurement, internal micrometers for two and
three point measurement. Depending on the measurement the uncertainty of measurement amounts up to
1 µm + 1.10-5 L.
Fig. 1: Plan of the diode laser with stabilization on a resonator
Fig. 2: Calibration device for hand-held measuring devices
Contact: Dr. Karl Meiners-Hagen,
Dr. Ahmed Abou-Zeid
Jod-stabilisierter Diodenlaser für die Präzisions-
interferometrie und Spektroskopie
Beschreibung des Forschungsvorhabens folgt
Ansprechpartner: Dr. Karl Meiners-Hagen,
Dr. Ahmed Abou-Zeid
Wave meter for measuring laser wavelengths
The wave meter works with a diode laser as a wavelength reference. The laser is stabilised on a Doppler
broadened atom absorption line (D2) of Rubidium
at ~ 780.24 nm. The wave meter is constructed as a homodyne interferometer with
a range of about 20 cm movement of the measuring arm. The working range of the polarising
beam splitter is between 620 nm and 850 nm.
By varying the range of movement and the number of measurements to be averaged,
a short measuring time or a lower measuring uncertainty can be chosen.
For wavelengths different from 780 nm a correction of the influence of the refractive index
of air is necessary.
The device is used only for internal calibrations of laser wave lengths in the department.
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