DE4325758A1 - Phase-modulated interferometer III - Google Patents

Abstract

The invention relates to a phase-modulated interferometer for evaluating phase shifts on the basis of variations in optical wavelengths in the measuring arm of the interferometer. The object of realising a phase-modulated interferometer which generates the sin-cos signals which are usual in length measuring systems and makes them available as conventional interface, is achieved according to the invention in conjunction with sinusoidal driving of the phase modulator (11) by admixing with the split superimposed signal from the reference arm (12) and the measuring arm (13) in a separate and mutually parallel fashion in each case sinusoidal signals of the form sin [(2n-1) omega t] and cos (2m omega t) which are coupled to the drive signal of the phase modulator in a phase-locked and frequency-locked manner. After filtering by means of two low-pass filters (51, 52), the conventional quadrature signals are available at an interface (5). The invention is preferably used for precision length measuring systems based on a heterodyne evaluation. <IMAGE>

Description

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 Use evaluation.

Precision length measuring systems based on interferometers have been in existence since the introduction known of the laser. In principle, there is a distinction between homodyne and heterodyne Differentiated evaluation methods. Heterodyne processes are generally preferred due to their possibility of counting up and down and the due to vanishing constant light component high interpolation. Is currently only the single sideband detection is used for evaluation. For generation a sideband or for spatial separation of the sidebands Zeemann splitting or Bragga deflection used. With integrated optical In addition to beam splitting and recombination, heterodyne interferometers can also frequency or phase modulation can be carried out. Because of the stability and the elaborate image of single-mode strip waveguides Layer waveguides and vice versa with the help of tapers, lenses or gratings interferometers with continuous strip waveguides are aimed for. Thereby However, the acousto-optical Bragg deflection for spatial separation of the Sidebands. Based on the electro-optical effect, the Strip waveguides implement phase modulation. With exactly defined electrical control of the modulator can suppress sideband can be achieved. VOGES describes a. 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 achieve one Sideband suppression of 40 dB.

However, the generation of such control signals is complex and requires a great deal high control effort.

In addition, all length measuring systems on the market use, as well Interferometer, sin-cos outputs as a standard interface for a subsequent one Forward / backward detection. This interface has changed in recent years Enforced users, the sin-cos signals as rectangular pulses or as Analog signals are made available. The heterodyne evaluation of Interferometers already include the forward / backward detection and delivers  no sin-cos signals, which means that the above-mentioned Interface not provided can be.

The invention is therefore based on the object of a phase-modulated Realize interferometer that generates the conventional sin-cos signals and provides this as a common interface.

The object is achieved according to the invention in a phase-modulated interferometer with measuring arm and reference arm, in which a phase modulator for phase modulation of the optical radiation is arranged in at least one of the interferometer arms and in which a detector for recording an optical superposition signal from the measuring and reference arm is present, the detector An electronic evaluation unit for determining the phase shift of the measuring arm signal is arranged downstream, in that means for parallel multiplication with sinusoidal signals are present in the signal path of the superimposed signal, these sinusoidal signals being phase and frequency-locked with a likewise sinusoidal drive signal of the phase modulator ( 11 ) and separated into two split signal paths, mixed in parallel with the beat signal, that the sinusoidal signals have the general form

S₁ = sin [(2n-1) ωt] and
S₂ = cos (2mωt) with m, n = 1, 2, 3. . .

have, where ω is the frequency of the sinusoidal drive signal at the phase modulator ( 11 ), and that the means for parallel multiplication are each followed by a low-pass filter ( 51 , 52 ), the outputs of which form uniform light components

S _gl1 = J _2n-1 (2Φ) sin 2kx and
S _gl2 = J _2m (2Φ) cos 2kx m, n = 1, 2, 3. . .

deliver, with the J _i - B-th function of the i-th order, geeignet the suitably set modulation depth on the phase modulator, k the wave number and x the location coordinate and the constant light components represent conventional quadrature signals that can be analyzed in a known manner for phase shift and for a common sin cos Interface.

In order to obtain the desired usual quadrature signals at the sin-cos interface, the phase modulator is expediently set so that it has its operating point by choosing the modulation depth at J _2n-1 (2Φ) = J _2m (2Φ), where m and n agree with the values from the equations above.

The means for parallel multiplication are preferably connected so that the one connected to a sine wave generator and the other together  the phase modulator with a frequency divider with π / 2 phase shift the same sine generator is connected.

The multiplication means are preferably as follows for the detector Multiplier, with each multiplier being followed by a low-pass filter. However, optical modulators and Phase shifter in the split light path of the beat signal before two separate detectors, the optical modulators being arranged Addition of the sine and cosine signals to the beat signal of the Take over interferometers.

The basic idea of the invention is based on a pure consideration Sinus modulation of the phase modulator of an interferometer in the usual way evaluable sin-cos signals. For this, the sine control is carried out with a signal of form

ϕ (t) = Φωt,

where ϕ (t) the modulation signal, Φ the phase shift (the amplitude), ω the Modulation frequency and t mean time. This control leads to Excitation of sidebands in the spectrum of the detector

A direct evaluation of these sidebands to determine the phase position is not possible, since the signal with a Spatial frequency is modulated and its amplitude becomes zero at certain locations. However, multiplying the spectrum of the detector by the signals

S₁ = sin [(2n-1) ωt] and S₂ = cos (2mωt) m, n = 1, 2, 3. . .

then contain the products

S (t) · sin [(2n-1) ωt] ∼ sin 2kx · sin² [(2n-1) ωt]
S (t) · cos (2mωt) ∼ cos 2kx · cos² (2mωt)

Uniform light components, the shape after appropriate low-pass filtering

S _sin ∼ J _2n-1 (2Φ) sin 2kx and
S _cos ∼ J _2m (2Φ) cos 2kx

to have. These two signals can now be used by the user as a sin-cos interface Will be provided.

With the phase-modulated interferometer according to the invention, it is possible to obtain a provide sin-cos interface and the phase evaluation as well as to the Interface subsequent direction detection in for conventional Length measuring systems to perform in the usual way.

The invention will be explained in more detail below using an exemplary embodiment become. The drawing shows:

Fig. 1 shows an embodiment of the interferometer according to the invention.

The arrangement according to the invention - as can be seen in FIG. 1 - consists in its basic structure of a phase-modulated interferometer module, which is preferably an integrated optical chip 1 with a phase modulator 11 using the electro-optical effect, a light source in the form of a laser 2 , a detector 3 , Means for parallel multiplication 4 of the superposition signal from the interferometer with sine and cosine signals, which are phase and frequency locked to the control signal of the phase modulator 11 , and a sin-cos interface 5 .

Referring to FIG. 1 coming from the laser 2 is split by the suitably shaped strip waveguide of the chip 1 in the reference arm 12 and measuring arm. 13 The light beams which are reflected at the indicated reflectors and brought together on chip 1 to form the superimposed signal then arrive, for. B. via an optical fiber - on the detector 3rd As a result of the sine modulation of the phase modulator 11, the detector 3 takes an interferometer signal with the sidebands

J _2n-1 (2Φ) sin 2kx sin [(2n-1) ωt] and
J _2m (2Φ) sin 2kx cos (2m ωt) with m, n = 1, 2, 3. . .

where k is the wave number ( 2 π / λ with λ as the light wavelength of the laser 2 ), x is the change in path length, J _i (2Φ) is the I-th order Bessel function and t is time. For reasons of expediency, the first and second harmonics (m = n = 1) are selected in this example.

The multiplication of these sidebands by means of a multiplier 41 with a phase and frequency-fixed sine signal sin ωt and a subsequent low-pass filtering by means of the low-pass filter 51 provide a constant light component of the interferometer signal in the form

J₁ (2Φ) sin 2kx.

The elements multiplier 42 and low-pass filter 52 arranged in the second signal path after the detector 3 cause the interferometer signal to be multiplied by a phase and frequency-fixed cosine signal cos 2ωt and the constant light component to be filtered out

J₂ (2Φ) cos 2kx.

With an appropriate choice of the operating point of the phase modulator in the form of Modulation depth Φ (amplitude) becomes equality

J₁ (2Φ) = J₂ (2Φ)

set so that the classic quadrature signals sin 2kx and cos 2kx are offered at the interface 5 following the low-pass filters 51 and 52 .

Fig. 1 indicates the following possibility for the phase and frequency-rigid coupling of the drive signal and the multiplication signals. A sine generator 6 with a fundamental frequency of 2ω outputs its signal on the one hand to a frequency divider 7 with a 90 ° phase shift, the frequency divider 7 being connected both to the phase modulator 11 and to the multiplier 41 , both of which receive the signal sin ωt. On the other hand, the sine generator 6 is directly connected to the multiplier 42 and supplies the signal cos 2ωt. The signals are thus coupled with the correct phase and frequency.

An embodiment of the interferometer according to the invention is also possible when using an integrated optical chip 1 by optically adding the sinusoidal signals to the superposition signal of the interferometer. The control principles remain the same. Compared to FIG. 1, the multiplication means 4 are advanced into the optical signal path of the beat signal in front of the detector 3 . For this purpose, two separate detectors 3 are necessary because of the required splitting of the signal, which can advantageously be implemented on chip 1 . The means for multiplication then take the form of two further phase modulators, which work with the sinusoidal signals

sin (2n-1) ωt and cos 2m ωt with m, n = 1, 2, 3

can be controlled. Because of the decreasing intensity of the sidebands higher Order is also useful in this case with m = n = 1 to use the first and second harmonics from the beat signal of the interferometer worked.

The phase and frequency-rigid coupling of the control signals can thus take place as in FIG. 1. However, it is also possible to integrate phase rotation and frequency division or frequency doubling on the chip 1 , as a result of which the external circuitry of the chip is reduced and the interferometer module thus represents a compact unit.

Because of the high cost of such a special integrated optical chip, however, the electronic sine signal admixture according to FIG. 1 will initially be of greater practical importance.

The solution according to the invention thus becomes a user-friendly, modern one phase-modulated interferometer (with the possibility of a heterodyne Evaluation) meet the conventional interface requirements.

Reference list

1 chip
11 phase modulator
12 reference arm
13 measuring arm
2 lasers
3 detector
4 means for parallel multiplication
41 , 42 multipliers
5 sin-cos interface
51 , 52 low pass filter
6 sine generator
7 frequency dividers

Claims (4)

1. Phase-modulated interferometer with measuring arm and reference arm, in which a phase modulator for phase modulation of the optical radiation is arranged in at least one of the interferometer arms and in which a detector for recording an optical superimposition signal from the measuring and reference arm is present, the detector being equipped with evaluation electronics for determination is arranged downstream of the phase shift of the measuring arm signal, characterized in that

• a) means for parallel multiplication with sinusoidal signals are present in the signal path of the heterodyne signal, these sinusoidal signals being phase-locked and frequency-rigidly coupled to a likewise sinusoidal control signal of the phase modulator ( 11 ) and separated in two split signal paths, mixed in parallel with the heterodyne signal,
• b) the sine signals the general form S₁ = sin [(2n-1) ωt] and
  S₂ = cos (2mωt) with m, n = 1, 2, 3. . . have, where ω is the frequency of the sinusoidal drive signal at the phase modulator ( 11 ), and
• c) the means for parallel multiplication are each arranged below low-pass filters ( 51 , 52 ), the outputs of which are uniform light _{components of} the form S _gl1 = J _2n-1 (2Φ) sin 2kx and
  S _gl2 = J _2m (2Φ) cos 2kx m, n = 1, 2, 3. . . deliver, where the J _i - Bessel function of the i-th order, Φ the suitably set modulation depth on the phase modulator ( 11 ), k the wave number and x the location coordinate and the constant light components represent conventional quadrature signals that can be analyzed for phase shift in a known manner and on a common sin-cos interface can be performed.
• 2. Arrangement according to claim 1, characterized in that the phase modulator ( 11 ) is set to receive the classic quadrature signals so that it has its operating point when choosing the modulation depth according to the equation J _2n-1 (2Φ) = J _2m (2Φ) m, n = 1, 2,. . .Has.

3. Arrangement according to claim 1, characterized in that one of the means for parallel multiplication with a sine generator ( 6 ) is connected, said sine generator ( 6 ) on the other hand via a frequency divider ( 7 ) with π / 2 phase shift with the phase modulator ( 11 ) and the other of the means for parallel multiplication is coupled. 4. Arrangement according to claim 1, characterized in that the means for parallel multiplication of the detector ( 3 ) are separate multipliers ( 41 , 42 ), each multiplier ( 41 , 42 ) being followed by a low-pass filter ( 51 , 52 ). 5. Arrangement according to claim 1, characterized in that the means for parallel multiplication in the light path before separate Detectors as parallel optical modulators and phase shifters are integrated optically.