Phase modulated interferometer.

The invention relates to a phase-modulated interferometer for evaluating phase shifts due to changes in optical path lengths in the measuring arm of the interferometer. In particular, it is used for precision length measuring systems, which preferably use the heterodyne method for evaluation.

Precision length measuring systems based on interferometers have been known since the introduction of the laser. In principle, a distinction is made between homodyne and heterodyne evaluation methods. Heterodyne methods are generally preferred due to their possibility of counting up and down and the high interpolation due to the vanishing constant light component. Currently, only the single sideband detection is used for evaluation. To generate a sideband or Zeemann splitting or Bragga deflection are used to spatially separate the sidebands. With integrated optical heterodyne interferometers, in addition to beam splitting and recombination, frequency or phase modulation can also be carried out. Because of the stability and the complex imaging of monomode strip waveguides on layer waveguides and vice versa with the help of tapers, lenses or gratings, interferometers with continuous strip waveguides are sought. However, this eliminates the acousto-optical Bragg deflection for spatial separation of the side bands. Based on the electro-optical effect, phase modulation can be implemented in the strip waveguide. With precisely defined electrical control of the modulator, sideband suppression can be achieved. This is how VOGES describes in IEEE Journ. Quant. Electr. QE-18 (1982), pp. 124-129 a defined electrical control of the modulator by sawtooth pulses with a defined return and thus achieves a sideband suppression of 40dB. However, the generation of such control signals is complex and requires a very high level of control.

The invention is based on the object of realizing a phase-modulated interferometer which, without complicated control of the phase modulator, obtains superimposed signals which can be evaluated from the measuring and reference arm of the interferometer.

According to the invention, the object is in a phase-modulated interferometer with a measuring arm and a reference arm, in which a phase modulator for phase modulation of the optical radiation is arranged in one of the two interferometer arms and in which a detector for recording an optical superposition signal from the measuring and reference arm is present, wherein An evaluation electronics for determining the phase shift of the signal is arranged downstream of the detector, thereby solved, that two sinusoidal control signals with different modulation frequencies and amplitudes are present at the phase modulator, which are phase and frequency-locked coupled, and the detector is followed by a bandpass filter that filters out a filter frequency from the frequency spectrum of the beat signal that meets the condition<maths id="math0001" num=""><math display="inline"><mrow><msub><mrow><mtext>ω</mtext></mrow><mrow><mtext>F</mtext></mrow></msub><mtext>= (2m -1) ω₁ = 2n ω₂ with m, n = 1,2,3 ...</mtext></mrow></math><img file="EP0612976A2_D0001.tif" /></maths><maths id="math0002" num=""><math display="inline"><mrow><mtext>and ω₁&gt; ω₂</mtext></mrow></math><img file="EP0612976A2_D0002.tif" /></maths> is sufficient so that with a suitable choice of an operating point dependent on the amplitudes, a signal of the structure<maths id="math0003" num=""><math display="inline"><mrow><msub><mrow><mtext>S = constcos (ω</mtext></mrow><mrow><mtext>F</mtext></mrow></msub><mtext> t - kx)</mtext></mrow></math><img file="EP0612976A2_D0003.tif" /></maths> arises, which is evaluated using conventional methods for determining the phase shift and in which t denotes the time, k the wave number and x the distance to be measured.

The phase modulator is advantageously set such that the amplitudes of the drive signals satisfy the equation <maths id="math0004" num=""><math display="inline"><mrow><msub><mrow><mtext>J</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><msub><mrow><mtext> (2 φ₂) J</mtext></mrow><mrow><mtext>2n</mtext></mrow></msub><msub><mrow><mtext> (2 φ₂) = J</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><msub><mrow><mtext> (2 φ₁) J</mtext></mrow><mrow><mtext>2m -1</mtext></mrow></msub><mtext> (2 φ₁)</mtext></mrow></math><img file="EP0612976A2_D0004.tif" /></maths> fulfill, where J_i is the i-th Bessel function. Here, m and n are positive integers, so that in the equation there are even and odd indices of the Bessel function on the one hand. One or more signals of different frequencies for regulating the control signals are expediently filtered out from the superimposed signal. In a three-armed interferometer, which has two reference arms to compensate for 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. For a three-armed interferometer with two measuring arms, preferably for achieving a distance measurement in two coordinate directions, it proves to be advantageous to arrange a phase modulator in each of the measuring arms, the two different modulation frequencies being fed to each of these modulators.

In the case of the three-arm interferometers in particular, it is advantageous to carry out the beam splitting in measuring and reference arms and their recombination as well as the implementation of the electro-optical phase modulators in an integrated-optical manner. For certain applications it proves advantageous to control the phase modulator (s) with more than two sinusoidal control signals.

The basic idea of ​​the invention lies in the idea of ​​using a simple sine control of the phase modulator to achieve a signal structure of the superimposed signal from the measuring and reference arm signals, which can be evaluated in a known manner with regard to phase shifts in the measuring arm. This is achieved according to the invention by modulating with two phase-coupled frequency-stable sinusoidal signals and filtering out a narrow-band frequency from the local signal, which corresponds to both an odd harmonic of the first modulation frequency and an even harmonic of the second modulation frequency. With the selection of a suitable operating point of the phase modulator, the result of the filtering is a cosine signal which can be analyzed in the usual way for phase shift. With the phase-modulated interferometer according to the invention, it is possible, instead of the complicated sawtooth control, to achieve the same signal structure with a double sine control, which permits the evaluation of the phase shift and thus the desired path measurement. The simple sine control also has the advantage that corresponding electro-optical modulators can be implemented on integrated-optical chips (IOC) and thus an integrated-optical phase-modulated interferometer, in particular heterodyne interferometer, can be manufactured industrially for various technical applications.

The invention will be explained in more detail below using an exemplary embodiment. The drawings show.<dl id="dl0001"><dt>Fig. 1</dt><dd>a block diagram of the arrangement according to the invention</dd><dt>Fig. 2</dt><dd>a design of a three-armed interferometer as an integrated optical double interferometer to compensate for wavelength drifts during distance measurement</dd></dl> As shown in FIG. 1, the interferometer according to the invention consists of an interferometer arrangement, which is preferably designed as an integrated optical chip 1 and contains a phase modulator 2 in one of its interferometer arms. The interferometer arrangement is usually coupled to a laser source, preferably in the form of a laser diode 3, a detector 4 and a measuring path 5 which determines the measuring arm of the interferometer. According to the invention, the phase modulator 2 is driven with two control signals with the modulation frequencies ω₁ and ω₂. The combined from the measuring arm and reference arm signal contains fundamental and harmonics of both modulation frequencies ω₁ and ω₂, of which, according to the invention, a signal with the filter frequency ω_F filtered out by the bandpass filter 8, subjected to a threshold criterion in a comparator 9, examined in a direction discriminator 10 with respect to the direction of the phase shift and evaluated in terms of amount in an evaluation computer 11. For this purpose, the control signals with the modulation frequencies ω₁ and ω₂, which are necessarily frequency and phase-locked, are expediently specified by means of a generator 6, the fundamental frequency f_O is divided via a frequency divider 7. The easiest way is the fundamental frequency f_O halved via the frequency divider 7, so that the modulation frequencies <maths id="math0005" num=""><math display="inline"><mrow><msub><mrow><mtext>ω₁ = f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub></mrow></math><img file="EP0612976A2_D0005.tif" /></maths> and <maths id="math0006" num=""><math display="inline"><mrow><msub><mrow><mtext>ω₂ = ½ f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub></mrow></math><img file="EP0612976A2_D0006.tif" /></maths> arise and the phase modulator 2 with the control signals S₁ and S₂<maths id="math0007" num=""><math display="inline"><mrow><msub><mrow><mtext>S₁ (t) + S₂ (t) = φ₁ sin (f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><msub><mrow><mtext> t) + φ₂ sin (½f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><mtext> t)</mtext></mrow></math><img file="EP0612976A2_D0007.tif" /></maths> is controlled. Here are φ₁ and φ₂ the amplitudes of the drive signal components. If you work with a fundamental frequency f_O = 10 MHz of the generator 6 and filters the receiver signal at this frequency ω_F = 10 MHz, the second harmonic of the control signal is obtained <maths id="math0008" num=""><math display="inline"><mrow><msub><mrow><mtext>S₂ = φ₂ sin (½f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><mtext> t)</mtext></mrow></math><img file="EP0612976A2_D0008.tif" /></maths> and the first harmonic of the drive signal <maths id="math0009" num=""><math display="inline"><mrow><msub><mrow><mtext>S₁ = φ₁ sin (f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><mtext>t)</mtext></mrow></math><img file="EP0612976A2_D0009.tif" /></maths> . With the selection of the suitable operating point of the phase modulator at<maths id="math0010" num=""><math display="inline"><mrow><msub><mrow><mtext>J</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><msub><mrow><mtext>(2 φ₁) J₁ (2 φ₁) = J</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><mtext> (2 φ₂) = J₂ (2 φ₂)</mtext></mrow></math><img file="EP0612976A2_D0010.tif" /></maths> then a signal is generated<maths id="math0011" num=""><math display="inline"><mrow><msub><mrow><mtext>S (t) = const cos (f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><mtext> t - kx),</mtext></mrow></math><img file="EP0612976A2_D0011.tif" /></maths> from which, in a known manner, the determination of the phase shift with respect to the fundamental frequency f_O can be done. To the general condition for the choice of filter frequency ω_F To make it clear, a second example of frequency selection is attached. If one drives the phase modulator 2 with the modulation frequencies ω₁ = 10 MHz and ω₂ = 7.5 MHz, one reaches (according to the formula given in claim 1 for the filter frequency ω_F) at ω_F = 30 MHz the filtering of the 3rd harmonic from the control signal S₁ and the 4th harmonic of the control signal S₂, so that with the operating point<maths id="math0012" num=""><math display="inline"><mrow><msub><mrow><mtext>J</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><msub><mrow><mtext>(2 φ₁) J₃ (2 φ₁) = J</mtext></mrow><mrow><mtext>O</mtext></mrow></msub><mtext> (2 φ₂) J₄ (2 φ₂)</mtext></mrow></math><img file="EP0612976A2_D0012.tif" /></maths> gives the evaluable cosine signal given above. At a working point of 2 φ₁ = 3.06 and 2 φ₂ = 4.27 also lead to fluctuations in the amplitudes φ₁ and φ₂ only minimal signal changes because<maths id="math0013" num=""><img file="EP0612976A2_D0013.tif" /></maths> applies. The choice of the modulation frequencies ω₁ and ω₂ is in principle arbitrary as long as the filter frequency ω_F the specified condition is sufficient. However, because of the signal attenuation of the higher harmonics, it is appropriate to drop the case<maths id="math0014" num=""><math display="inline"><mrow><msub><mrow><mtext>ω₁ = f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub></mrow></math><img file="EP0612976A2_D0014.tif" /></maths> , <maths id="math0015" num=""><math display="inline"><mrow><msub><mrow><mtext>ω₂ = ½ f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub></mrow></math><img file="EP0612976A2_D0015.tif" /></maths> and <maths id="math0016" num=""><math display="inline"><mrow><msub><mrow><mtext>ω</mtext></mrow><mrow><mtext>F</mtext></mrow></msub><msub><mrow><mtext> = f</mtext></mrow><mrow><mtext>O</mtext></mrow></msub></mrow></math><img file="EP0612976A2_D0016.tif" /></maths> to prefer. It was therefore included in the pictorial representation of FIG. 1, but is in no way intended to restrict the generality of the teaching according to the invention disclosed here.

2 shows an integrated optical chip 1 for a three-armed interferometer with two reference arms. If the measuring and reference arms - as indicated schematically there - are arranged in the following reference arm - measuring arm - reference arm and if the measuring mirror moves in the interval between the two reference mirror positions, then a distance measurement of wavelength drifts of the laser diode 3 and changes in the optical properties can be carried out make the measuring section 5 independent. Strictly speaking, this three-arm interferometer is a double interferometer with a common laser diode 3 and a common measuring section 5, because in each partial interferometer an excitation according to the invention of the phase modulator 2 with two modulation frequencies ω₁ and ω₂ is required in order to adjust the respective phase shift between the measuring arm and a reference arm via the detector 4 and to determine the bandpass filter 8 according to FIG. 1 and the prior art. The modulation frequencies ω₁ and ω₂ for the two phase modulators 2 need not necessarily be chosen to be identical. It is also conceivable to use a single phase modulator 2 in the measuring arm before the measuring section 5 begins. A three-armed interferometer (not shown) with two measuring arms, preferably for coupled two-coordinate displacement measurement, is in principle constructed analogously to the interferometer according to FIG. 2, the reference arm then being located centrally on the chip 1. The classification as a double interferometer with separate superimposition and evaluation remains the same.