Multiple control frequency phase modulator in phase modulated interferometer precision distance measuring system

A phase-modulated interferometer has improved control and signal processing. Superimposition signals capable of evaluation in a phase-modulated interferometer without a complicated sawtooth control of the phase modulator are attained, in that two sinusoidal control signals which have modulation frequencies (.omega..sub.1, .omega..sub.2) and are rigidly coupled with respect to phase and frequency are applied to the known phase modulator and a cosine signal which is used in a conventional manner for evaluating the phase displacement is filtered of the superimposition signal generated in the interferometer by an electronic bandpass filter. At the filter frequency (.omega..sub.F) of the bandpass filter, an odd-number harmonic and an even-number harmonic of the two modulation frequencies (.omega..sub.1, .omega..sub.2) have the same frequency, when the amplitudes (.phi..sub.1, .phi..sub.2) of the control signals satisfy the condition for the suitable operating point of the phase modulator. The improvement has application to phase-modulated interferometers, in particular, for precision distance measuring devices, preferably by the heterodyne evaluating method.

BACKGROUND OF THE INVENTION 
a) Field of the Invention 
The invention is directed to a phase-modulated interferometer for 
evaluating phase displacements due to changes in optical path length in 
the measurement arm of the interferometer. It is used in particular for 
precision distance measuring systems which preferably make use of the 
heterodyne method for evaluating. 
b) Background Art 
Precision distance measuring systems based on interferometers have been 
known since lasers were first introduced. There is a basic distinction 
between homodyne and heterodyne evaluating methods. Heterodyne methods are 
generally preferred due to the possibility of counting forward and 
backward and the high interpolation due to the dwindling constant or 
direct light component. At present, single-sideband detection is used 
exclusively for evaluation. Zeemann splitting or Bragg deflection are used 
to generate a sideband or spatially separate the sidebands. In 
integrated-optical heterodyne interferometers, a frequency or phase 
modulation can also be effected in addition to the splitting and 
recombination of the beam. For reasons of stability and the difficulty of 
forming single-mode strip waveguides on layer waveguides, and vice versa, 
with the aid of tapers, lenses or grids, interferometers with continuous 
strip waveguides are desirable. However, this excludes the acousto-optical 
Bragg deflection for spatial separation of the sidebands. A phase 
modulation can be realized in the strip waveguide on the basis of the 
electro-optical effect. A sideband suppression can be achieved with a 
precisely defined electrical controlling of the modulator. For example, in 
IEEE Journ. Quant. Electr. QE-18 (1982), pages 124-129, Voges et al. 
describe a defined electrical control of the modulator by sawtooth pulses 
with defined flyback and accordingly achieve a sideband suppression of 40 
dB. However, production of such control signals is complex and requires a 
very high outlay in regulating means. 
OBJECT AND SUMMARY OF THE INVENTION 
The primary object of the present invention is to realize a phase-modulated 
interferometer which succeeds in providing superimposition [beating or 
heterodyning] signals capable of evaluation from the measuring and 
reference arm of the interferometer without complicated control of the 
phase modulator. 
In a phase-modulated interferometer with a measuring arm and a reference 
arm in which a phase modulator is arranged in one of the two arms of the 
interferometer for phase modulation of the optical beam and in which there 
is a detector for picking up an optical superimposition signal from the 
measuring and reference arm, evaluating electronics are arranged 
subsequent to the detector for determining the phase displacement of the 
signal. In such arrangement, the primary object is met, according to the 
invention, in that two sinusoidal control signals which have different 
modulation frequencies and amplitudes and are rigidly coupled with respect 
to phase and frequency are applied to the phase modulator and a bandpass 
filter is connected subsequent to the detector and filters a filter 
frequency from the frequency spectrum of the superimposition signal, which 
filter frequency satisfies the following condition: 
EQU .omega..sub.F =(2m-1).omega..sub.1 =2n.omega..sub.2, 
where m, n=1, 2, 3 . . . and .omega..sub.1 &gt;.omega..sub.2, so that suitable 
selection of an operating point dependent on the amplitudes results in a 
signal having the structure 
EQU S=const.multidot.cos (.omega..sub.F t-kx), 
which signal is evaluated with conventional methods for determining the 
phase displacement, where t designates time, k designates wave number, and 
x designates the distance to be measured. 
The phase modulator is advantageously adjusted in such a way that the 
amplitudes of the control signals satisfy the equation J.sub.0 (2 
.phi..sub.2) J.sub.2n (2 .phi..sub.2)=J.sub.0 (2 .phi..sub.1) J.sub.2m-1 
(2 .phi..sub.1), where J.sub.i is the i-th Bessel function and m and n 
represent positive whole numbers, so that even-number indices of the 
Bessel function are on one side of the equation and odd-number indices are 
on the other side. One or more signals of different frequencies are 
advisedly filtered out of the superimposition signal to regulate the 
control signals. 
In a three-arm interferometer having two reference arms to compensate for 
the wavelength drifts, it is advantageous to arrange a phase modulator in 
each of the reference arms so that only one of the two different 
sinusoidal control signals is modulated in each reference arm. In a 
three-arm interferometer with two measuring arms, preferably for achieving 
a distance measurement in two coordinate directions, it has proven 
advantageous to arrange a phase modulator in each of the measuring arms, 
the two different modulation frequencies being supplied in turn to each of 
these modulators. In the three-arm interferometer in particular, it is 
advantageous to carry out the splitting and recombination of the beams in 
measuring and reference arms and to realize the electro-optical phase 
modulators in integrated-optics. For certain applications, it may be 
advantageous to control the phase modulator or phase modulators with more 
than two sinusoidal control signals. 
The basic idea of the invention consists in achieving, by means of a simple 
sinusoidal control of the phase modulator, a signal structure of the 
superimposition signal from the measuring and reference arm signal which 
can be evaluated in a known manner with respect to the phase displacements 
in the measuring arm. This is achieved, according to the invention, by 
effecting a modulation with two phase-coupled frequency-stable sine 
signals and effecting a narrow-band filtering of a frequency from the 
superimposition signal, which frequency corresponds to an odd-number 
harmonic of the first modulation frequency as well as to an even-number 
harmonic of the second modulation frequency. By selecting a suitable 
operating point of the phase modulator, the filtering results in a cosine 
signal which can be analyzed in a conventional manner for phase 
displacement. 
Without using the complicated sawtooth control, the phase-modulated 
interferometer according to the invention makes it possible to arrive at 
the same signal structure which allows the evaluation of the phase 
displacement and accordingly the desired distance measurement. The simple 
sine control has the additional advantage that suitable electro-optical 
modulators can be realized in integrated-optical chips (IOC) and 
accordingly an integrated-optical phase-modulated interferometer, in 
particular a heterodyne interferometer, can be produced commercially for 
various technical applications. 
The invention is explained in more detail in the following with reference 
to an embodiment example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is shown in FIG. 1, the interferometer according to the invention is an 
interferometer arrangement which is preferably constructed as an 
integrated-optical chip 1 and contains a phase modulator 2 in one of its 
interferometer arms. The interferometer arrangement is conventionally 
coupled with a laser source, preferably in the form of a laser diode 3, a 
detector 4, and a measuring length 5 determining the measuring arm of the 
interferometer. FIG. 1 also shows the complete interferometer including 
beam splitter 12, reference arm 13, and superimposition signal 15. 
According to the invention, the phase modulator 2 is controlled with two 
control signals at modulation frequencies .omega..sub.1 and .omega..sub.2. 
The superimposition signal combined from the measuring arm and reference 
arm contains fundamental and harmonic waves of both modulation frequencies 
.omega..sub.1 and .omega..sub.2, from which a signal with filter frequency 
.omega..sub.F is filtered according to the invention by the bandpass 
filter 8. This signal is subjected to a threshold criterion in a 
comparator 9, examined in a directional discriminator 10 with respect to 
the direction of the phase displacement, and evaluated quantitatively in 
an evaluator computer 11. In addition, the control signals with modulation 
frequencies .omega..sub.1 and .omega..sub.2, which control signals are 
necessarily coupled rigidly with respect to frequency and phase, are 
advisedly preset by means of a generator 6, whose fundamental frequency 
f.sub.0 is divided by a frequency divider 7. In the simplest case, the 
fundamental frequency f.sub.0 is halved via the frequency divider 7, 
resulting in the modulation frequencies .omega..sub.1 =f.sub.0 and 
.omega..sub.2 =1/2f.sub.0 and the phase modulator 2 is controlled with the 
control signals S.sub.1 and S.sub.2 
EQU S.sub.1 (t)+S.sub.2 (t)=.phi..sub.1 sin (f.sub.0 t)+.phi..sub.2 sin 
(1/2f.sub.0 t), 
where .phi..sub.1 and .phi..sub.2 are the amplitudes of the control signal 
components. When there is a fundamental frequency f.sub.0 =10 MHz of the 
generator 6 and the receiver signal is filtered at this frequency 
.omega..sub.F =10 MHz, the second harmonic of the control signal S.sub.2 
=.phi..sub.2 sin (1/2f.sub.0 t) and the first harmonic of the control 
signal S.sub.1 =.phi..sub.1 sin (f.sub.0 t) are obtained. Appropriate 
selection of the operating point of the phase modulator at 
EQU J.sub.0 (2 .phi..sub.1)J.sub.1 (2 .phi..sub.1)=J.sub.0 (2 .phi..sub.2) 
J.sub.2 (2 .phi..sub.2) 
results in a signal 
EQU S(t)=const cos (f.sub.0 t-kx), 
from which the phase displacement relative to the fundamental frequency 
f.sub.0 can be determined in a known manner. 
A second example for frequency selection is provided in order to make clear 
the general condition for selecting the filter frequency .omega..sub.F. 
When the phase modulator 2 is controlled at the modulation frequencies 
.omega..sub.1 =10 MHz and .omega..sub.2 =7.5 MHz, the third harmonic is 
filtered out of the control signal S.sub.1 and the fourth harmonic is 
filtered out of the control signal S.sub.2 at .omega..sub.F =30 MHz 
(corresponding to the formula for the filter frequency .omega..sub.F in 
claim 1) so that the aforementioned cosine signal which is capable of 
being evaluated results at the operating point 
EQU J.sub.0 (2 .phi..sub.1) J.sub.3 (2 .phi..sub.1)=J.sub.0 (2 .phi..sub.2) 
J.sub.4 (2 .phi..sub.2). 
Moreover, at an operating point of 2.phi..sub.1 =3.06 and 2.phi..sub.2 
=4.27, fluctuations in amplitudes .phi..sub.1 and .phi..sub.2 only lead to 
minimum changes in the signal, since 
##EQU1## 
The selection of the modulation frequencies .omega..sub.1 and 
.omega..sub.2 is optional in principle provided the filter frequency 
.omega..sub.F satisfies the condition mentioned above. Due to the 
weakening of the signal of the higher harmonic, however, it is preferable 
that .omega..sub.1 =f.sub.0, .omega..sub.2 =1/2f.sub.0 and .omega..sub.F 
=f.sub.0. Accordingly, this has been assumed in the graphic representation 
in FIG. 1, but in no way limits the generality of the teaching according 
to the invention disclosed here. 
FIG. 2 shows an integrated-optical chip 1 for a three-arm interferometer 
with two reference arms. As is shown schematically in the drawing, the 
sequence of the measuring (element 5) and reference arms is reference 
arm--measuring arm 13--reference arm, and the measuring mirror moves in 
the interval between the two reference mirror positions. Accordingly, a 
distance measurement can be made independent of wavelength drifts of the 
laser diode 3 and changes in the optical characteristics along the 
measuring length 5. To be precise, this three-arm interferometer is a 
double interferometer with a common laser diode 3 and a common measuring 
length 5, because the invention requires an excitation of the phase 
modulator 2 with two modulation frequencies .omega..sub.1 and 
.omega..sub.2 in each interferometer in order to determine the occurring 
phase displacement between the measuring arm and a reference arm via the 
detector 4 and the bandpass filter 8 according to FIG. 1 and the prior 
art. 
The modulation frequencies .omega..sub.1 and .omega..sub.2 for the two 
phase modulators 2 need not necessarily be identical. It is also possible 
to use an individual phase modulator 2 in the measuring arm before the 
start of the measuring length 5. 
A three-arm interferometer with two measuring arms, preferably for coupled 
two-coordinate distance measurement, is constructed in principle in a 
manner analogous to the interferometer according to FIG. 2 and is shown in 
FIG. 3. In this case, the reference arm is centrally located on the chip 
1. It still qualifies as a double interferometer with separate 
superimposition and evaluation. The reference arm is 13, the measurement 
arms 14 and the superimposition signals 15. 
While the foregoing description and drawings represent the preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the true spirit and scope of the present invention.