Device for characteristic analysis of light fields in real time
Patent Information
- Application Number
- PCT/EP2026/055227
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-02-28
- Filing Date
- 2026-02-26
- Publication Date
- 2026-09-03
Smart Images
Figure EP2026055227_03092026_PF_FP_ABST
Abstract
Description
[0001] Device for the characteristic analysis of light fields in real time
[0002] The invention relates to a device for the characteristic analysis of light fields, in particular for the reconstruction of a quantum state of an optical signal in real time.
[0003] Systematic investigations of light signals have become an indispensable tool for gaining insights into light-matter interactions in both fundamental and applied research. Photon correlation spectroscopy in dynamic light scattering has become the standard method for measuring the size of particles and molecules. Second-order photon correlations are the method of choice for characterizing the performance of single-photon sources and lasers.
[0004] For industrial applications, such measurements require higher throughput and real-time measurement capability. Therefore, determining the statistical properties of light, which describe the coherent, stochastic, and quantum mechanical properties of light field states via density matrices, correlation functions, or phase space distributions in real time with low latency, is crucial.
[0005] Traditionally, characterizations of photon statistics and quantum state tomography are only carried out with a delay of several minutes to, at best, seconds between the measurement of the raw data and the availability of the evaluated results.
[0006] Quantum state tomography is a method for reconstructing the quantum state of a system, such as a photon, an atom, or another quantum object. The goal of quantum state tomography is to obtain a complete description of the state of a quantum object, encompassing all relevant information about the amplitudes and phases that characterize that state. Quantum state tomography makes it possible to measure and reconstruct the complete probability distribution of the system. This is a central method in quantum mechanics, as it enables the precise analysis and understanding of quantum phenomena such as entanglement and interference.
[0007] The process of quantum state tomography is based on repeated measurements of a quantum object in different basis states. In these measurements, the system is examined using different projected basis states to capture various aspects of the quantum state. The measured data are then used to reconstruct the system's state. This is done using mathematical algorithms that allow the determination of the system's density operator. The density operator fully describes the state of a quantum system and includes all the necessary information to characterize it.
[0008] The reconstructed representation of a quantum state can take various forms, such as a Bloch vector for a single qubit, where the state is visualized as a point on the Bloch sphere. For higher dimensions and more complex systems, such as quantum computers with multiple qubits, the reconstruction becomes considerably more challenging, as the number of measurements required increases with the system's dimensions. Quantum state tomography is therefore a statistical method that relies on the repeatability and precision of measurements. A complete picture of a system often requires thousands or even millions of measurements, which, depending on the system size and measurement methods, presents significant practical challenges.
[0009] Quantum state tomography finds application in a wide variety of fields, particularly in quantum computing, quantum communication, and quantum metrology. In quantum computing, it is used to verify the quality and error-proneness of quantum operations, for example, by reconstructing the state of qubits. This is crucial for the development of quantum computers, where precise control over the state of the qubits is a prerequisite for performing error-free quantum operations. In quantum communication, such as in quantum cryptography, quantum state tomography is necessary to ensure the integrity and security of quantum communication channels. Here, it is used to examine how quantum information behaves during transmission and whether any interference or eavesdropping attempts have altered the signal's state.Quantum state tomography also plays an important role in quantum metrology, where extremely precise measurements are required – for example, in gravitational wave research or for determining fundamental physical constants. It is also a central component of quantum process tomography, which represents the most precise physically possible determination of the properties of measuring instruments.
[0010] Despite its wide-ranging applications, quantum state tomography presents practical challenges. The accuracy of the reconstruction depends on the number of measurements performed and the choice of measurement basis. The larger the quantum object under investigation and the more complex its state, the more computationally expensive the tomography process becomes. For systems with many qubits or high dimensions, the reconstruction effort grows exponentially, highlighting the need for new, more efficient algorithms to improve measurement accuracy and reconstruction speed. Conventional quantum state tomographs cannot operate continuously. This means that a recorded dataset, for example, 5 GB in size, must be temporarily stored by a processing unit, such as a computer, using its RAM. The dataset can then be analyzed.This results in high latencies during measurement and parallel evaluation, because the intermediate storage and subsequent evaluation are slower than the measurement itself.
[0011] The devices or methods known from the prior art do not yet allow the delay in quantum state tomography to be reduced to such an extent that the measurements are suitable for commercial use.
[0012] Based on this, the object of the invention is to provide a device for quantum tomography in real time and with reduced complexity.
[0013] This problem is solved by the subject matter of claim 1. Preferred embodiments are found in the dependent claims. According to the invention, a device for the characteristic analysis of light fields, in particular for the reconstruction of a quantum state of an optical signal in real time, is thus provided. The device comprises
[0014] a first light source to generate an optical reference signal with a first polarization,
[0015] a second light source for generating an optical test signal, a delay line for generating a relative phase shift between the optical reference signal and the optical test signal,
[0016] a first polarizing beam splitter connected to the first light source and the second light source for combining the optical reference signal with the optical test signal in such a way that both optical signals have the same spatial mode,
[0017] a half-wave plate downstream of the first polarizing beam splitter for shifting the polarization of the optical reference signal and the optical test signal at least partially by 45 degrees each,
[0018] a second polarizing beam splitter downstream of the half-wave plate for splitting the combined optical signal into a first optical partial signal and a second optical partial signal,
[0019] a detector downstream of the second polarizing beam splitter for detecting the first and second optical partial signals interfering at the detector, a filter downstream of the detector for filtering out the reference signal from the interfered signal and generating a measurement signal,
[0020] an evaluation unit downstream of the filter for evaluating the filtered measurement signal,
[0021] a photodiode arranged in such a way that part of the optical reference signal is directed to the photodiode to activate the evaluation unit by means of a third beam splitter.
[0022] In this context, "downstream" means in particular that the respective component is located behind it in the direction of the beam.
[0023] When the term "test signal" is used here, it preferably refers to a signal to be tested or measured. "Same spatial mode" specifically means that both signals are identical with respect to their spatial propagation, but may have different polarizations.
[0024] The first light source comprises, in particular, a laser source and generates a coherent reference signal. More preferably, the first light source comprises a local oscillator, most preferably a pulsed local oscillator. The frequency and phase of the reference signal are predetermined. "Predetermined" means, in particular, that the frequency and phase are known and can be deliberately selected.
[0025] A small portion of the laser power from the first light source, or reference signal, is directed to a photodiode to generate a trigger signal for the evaluation unit. This allows the evaluation unit to be activated before the first measurement data can be recorded. Furthermore, the reference signal can be synchronized with the electronic evaluation unit. Here, the pulsed reference laser is used as the activation signal for the evaluation unit, specifically for the analog-to-digital converter. This has the advantage that the digitized signals have a fixed timing relationship, so that, for example, an activation signal always occurs at time "zero." Thus, all recorded measurement data has a fixed temporal relationship to each other. If the analog-to-digital converter is not synchronized with the pulsed laser, this can result in jitter.This means that the digitized values are not always at exactly the same point in time. This disadvantage can be overcome using the activation signal.
[0026] The delay line serves to generate a relative phase shift between the optical reference signal and the optical test signal. This ensures analysis across the entire phase space (2 / 360 degrees) of the test signal. The polarization of the light source can be orthogonal to that of the reference signal.
[0027] A second light source serves as the source for the test signal. This second light source is, in particular, a continuous-wave light source. The polarization of the test signal and the reference signal are then rotated using a half-wave plate, specifically by 45° each. The signals can then interfere at the detector, preferably a photodiode. The interference patterns depend on the relative phase alignment of the test signal and the reference signal. The test signal and the reference signal interfere at the detector, and the detected luminous flux is a function of the relative phase between the two optical signals. This interference pattern contains information about the amplitude and phase of the test signal.
[0028] The detector measures the intensity of the light beam resulting from the interference between the test signal and the reference signal. This intensity depends on the relative phase between the test signal and the reference signal. The measured intensity can be considered a combination of two components: one for the in-phase and one for the quadrature component of the signal. To obtain complete information about the test signal, two measurements are performed. One measurement is taken in the in-phase direction. This means that either the phase relationship is randomized by rapidly moving the delay line, resulting in a communication of the quadratures across all phases, or the delay line is moved slowly, resulting in a communication of the quadratures corresponding to each phase shift (pairs of values).
[0029] This homodyne detection process generates a large amount of data, which can be evaluated in real time by the parallel evaluation unit.
[0030] A key aspect of the invention is that the data can be evaluated in real time in parallel with the measurement. Complexity is reduced by continuously monitoring the photon count and variance of the light field during real-time operation in order to dynamically adjust the dimensions of the Hilbert space considered in the jib base.
[0031] In this context, "real-time" is understood to mean, in particular, that the time for data acquisition is greater than or equal to the time for data analysis. Preferably, in-situ control and readjustment via feedback loops are provided. By rotating the polarizations of the test signal and the reference signal at the first polarizing beam splitter, both signals exhibit p-polarized and s-polarized components. S-polarized light refers to the polarization component of an electromagnetic wave that is perpendicular to the plane defined by the direction of propagation and the normal to the reflecting surface (the plane of incidence). When light strikes a surface and is s-polarized, the electric field oscillates in a direction perpendicular to the plane of incidence, and no component of this field lies in this plane.
[0032] When the term "polarizing beam splitter" is used here, it refers specifically to both a polarizing beam splitter that splits and simultaneously polarizes the input light within a single component, and to a combination of several components, namely a "normal" non-polarizing beam splitter and at least one downstream polarizer. In other words, a "polarizing beam splitter" is preferably understood to be any combination of components suitable for splitting and polarizing the incoming signal. For example, with free-standing light, a polarizing beam splitter can be used as a single component, while with fiber optics, a combination of a "normal" beam splitter followed by a polarizer can be used.
[0033] In contrast, p-polarized light refers to the polarization component that runs parallel to the plane of incidence. This means that when p-polarized light strikes a surface, the electric field lies entirely in the plane of incidence and oscillates in a direction parallel to that plane. This orientation is orthogonal to s-polarization, where the electric field runs perpendicular to the plane of incidence.
[0034] This splits the optical signals into equal parts at the second polarizing beam splitter, which is connected downstream of the first polarizing beam splitter. The more precise the modification of the half-wave plate, the more accurately the two signals are divided and subsequently interfered. According to a preferred embodiment of the invention, the evaluation unit comprises an analog-to-digital converter, the analog-to-digital converter being configured to ensure a transmission rate of at least 1 gigasample per second (GS / s). The analog-to-digital converter is therefore, in particular, a fast analog-to-digital converter with a high bandwidth, for example 5 GHz, and / or a high resolution, for example up to 12 bits, and / or a high sampling rate, for example up to 5 GS / s.At a transfer rate of 1 GS / s, approximately 1.5 GB of data can preferably be converted per second, since it has been shown that a data point requires approximately 1.5 bytes of storage space.
[0035] According to a preferred embodiment of the invention, the evaluation unit comprises a microprocessor, a field-programmable gate array (FPGA), or a tensor processing unit (TPU). A field-programmable gate array is a reconfigurable digital circuit consisting of programmable logic units and interconnects. The FPGA offers the possibility of flexibly defining the hardware structure itself. This is achieved using hardware description languages such as VHDL or Verilog, which allows specific logic circuits to be implemented directly at the hardware level.
[0036] A microprocessor, on the other hand, is a general-purpose computing unit with a fixed instruction set, typically controlled by software. Microprocessors are ideal for general-purpose computing and offer a simple development environment through programming languages such as C or Python. The structure of a microprocessor is fixed and cannot be changed.
[0037] According to a preferred embodiment of the invention, the microprocessor includes a graphics processing unit (GPU). The use of a GPU offers significant advantages in real-time data analysis because it is optimized for parallel processing. Unlike conventional CPUs, which are specialized for serial processing, GPUs consist of thousands of cores that can perform calculations simultaneously. This massive parallelization makes it possible to efficiently analyze large amounts of data by applying the same calculations to many data points at the same time. According to a preferred embodiment of the invention, the evaluation unit is configured to digitize the incoming optical signal at the analog-to-digital converter and transmit it directly to the microprocessor, FPGA, or TPU. In the present context, "directly" is understood to mean direct transmission without any intermediate steps.This is possible because the evaluation unit is integrated on-chip. Traditionally, data is first temporarily stored in a computer's general RAM and only then sent to a processor. Direct data transfer from the analog-to-digital converter (ADC) to the microprocessor, FPGA, or TPU is not a standard feature. It requires an ADC card that provides a dedicated operating mode for this purpose. Preferably, the ADC card includes a direct data streaming option that enables direct transfer to the microprocessor or FPGA. Preferably, dedicated FPGA firmware is provided on the ADC card for this purpose. It has been shown that data transfer rates of preferably at least 1.5 GB / s can be achieved with this.
[0038] The integrated (on-chip) and directly controllable evaluation unit, in particular a microprocessor, FPGA, or TPU, enables continuous operation during data acquisition. This means that the acquired data does not need to be temporarily stored but is forwarded and evaluated directly. Direct processing without intermediate storage ensures very low latency.
[0039] According to a preferred embodiment of the invention, the first light source for generating the reference signal comprises a titanium-sapphire pulsed laser, in particular with a pulse repetition rate of at least 80 MHz. The reference signal comprises a power of at least 10 mW so that the noise of the reference signal is dominant over the electronic noise of the detector.
[0040] According to a preferred embodiment of the invention, the detector comprises a balanced photodiode. A balanced photodiode (BPD) is a special arrangement of two photodiodes that is particularly suitable for use in optical systems, especially in optical communication and measurement technology. This arrangement improves the signal-to-noise ratio and reduces disturbances such as background noise or electrical interference. In the balanced photodiode, two photodiodes are connected in such a way that their electrical outputs are compared. The two optical partial signals are directed to the two photodiodes. The current generated by the photodiodes is proportional to the intensity of the incident light. The circuit subtracts the output currents of the two photodiodes from each other, thereby determining the difference between the two input signals.
[0041] According to a preferred embodiment of the invention, the filter comprises a notch filter, particularly in the 80 MHz and 160 MHz range. Specifically, the 80 MHz and 160 MHz frequencies are attenuated because they correspond to the pulse repetition rate or frequency of the first light source or the local oscillator and its higher harmonics. Consequently, the frequency resulting from the pulse repetition rate of the pulsed laser is filtered out. With imperfect subtraction of the signals from the two photodiodes, undesired signal components appear at these frequencies, the amplitude of which changes slowly. A notch filter (also called a notch filter) is an electronic or optical filter that strongly attenuates a narrow frequency or wavelength bandwidth, while frequencies outside this range can pass through almost unimpeded. In particular, the influence of the reference signal is filtered out, preventing it from generating an interference signal on the photodiode.This results in significantly improved sensitivity, allowing the setup to analyze photon counts as low as 1, or very faint light, on average. Furthermore, a low-pass filter follows the notch filter, particularly in the passband below 100 MHz. This allows the higher harmonics (3rd, 4th, etc.) to be filtered out.
[0042] According to a preferred embodiment of the invention, an amplifier is arranged between the filter and the evaluation unit. In particular, a fast amplifier, especially one with a frequency of at least 300 MHz, is used.
[0043] According to a preferred embodiment of the invention, the device further comprises a fourth beam splitter for splitting the optical test signal into two parallel optical test signals, a fifth beam splitter for splitting the optical reference signal into two parallel optical reference signals, a first parallel polarizing beam splitter downstream of the fourth beam splitter for combining the optical reference signal with the optical test signal such that both optical signals have the same spatial mode, a parallel half-wave plate downstream of the first parallel polarizing beam splitter for changing the polarization of the optical reference signal and the optical test signal at least partially by 45 degrees each, and a second parallel polarizing beam splitter downstream of the parallel half-wave plate for splitting the combined optical signal into a first optical partial signal and a second optical partial signal.A parallel detector is connected downstream of the second parallel polarizing beam splitter to detect the first and second optical partial signals interfering at the parallel detector. A parallel filter is connected downstream of the parallel detector to filter out the reference signal from the interfered signal and generate a parallel measurement signal. In this way, two detector units are arranged in parallel. However, the optical reference signal A is only delayed for one of the parallel paths. The advantage of this heterodyne detection with two balanced photodiodes is that the so-called Husimi function can be reconstructed in real time. This is achieved in particular as follows: First, the delay path is set so that...There is a 90-degree phase shift between the signals of the two parallel detector loops. Both measurement signals are digitized, and a pair of orthogonal quadrature values is provided for each. If these are then plotted on a graph, for example, the value of the first diode would be the x-intercept and the value of the second diode the y-intercept of the data point. The 2D histogram of all these data points is the Husimi function. The measured quantities available in real time differ from those obtained with homodyne detection. In homodyne detection, one obtains, in particular, the second correlation function, which describes the correlation of the intensities of the two parallel signals. Specifically, one obtains the amplitude and phase of the light field, as well as, and most importantly, the blurring of both components in real time. This data can preferably be provided directly, i.e., without the need for additional tomography.
[0044] The invention will now be explained in more detail with reference to the drawings and a preferred embodiment.
[0045] In the drawings, Fig. 1 shows a device for the characteristic analysis of light fields in a schematic representation according to a preferred embodiment of the invention.
[0046] Fig. 2 shows a device for the characteristic analysis of light fields in a schematic representation according to a further preferred embodiment of the invention.
[0047] Figure 1 schematically shows a device for the characteristic analysis of light fields 1 or a quantum state tomograph according to a preferred embodiment of the invention. First, a first light source 2 or a laser source generates an optical reference signal A. The characteristics, in particular the polarization, of the reference signal A are known and can be selected or adapted. A small portion of the reference signal A is directed as an activation signal D to a photodiode 11 via a beam splitter 15. The photodiode 11 is coupled to an evaluation unit 10, so that the evaluation unit 10 can be activated by the activation signal D that reaches the photodiode 11. In this way, the reference laser, which is preferably pulsed, is used as the activation signal D for the evaluation unit 10, specifically for the analog-to-digital converter 12.This has the advantage that the digitized signals have a fixed temporal relationship, so that, for example, an activation signal D always occurs at time "zero". Thus, all recorded measurement data have a fixed temporal relationship to each other.
[0048] A second light source 4 generates an optical test signal B. This signal first strikes a half-wave plate 6 and is then superimposed with the reference signal A using a first polarizing beam splitter 5. The half-wave plate 6 changes the polarization of the test signal C. Since the subsequent beam splitter 5 is polarizing, the proportion of the test signal C's intensity that is actually superimposed with the reference signal can be adjusted. Both optical signals, i.e., the reference signal A and the test signal B, thus exhibit the same spatial mode. An additional lens 17 is placed in the beam path to focus the signals onto a detector 8. This lens is not required when using fiber optics.By means of a further half-wave plate 6, the polarization of the optical reference signal A and the optical test signal B is shifted at least partially by 45 degrees each, so that they can subsequently be split equally into a first partial signal TI and a second partial signal T2 at a second polarizing beam splitter 7. Both partial signals TI and T2 then strike the detector 8 in the form of a balanced photodiode 14, so that the difference between the two partial signals TI and T2 can be determined.
[0049] The reference signal is then filtered out from the measurement signal C forwarded by the detector using a filter 9 and made available for further processing. The measurement signal C is amplified by amplifier 16 before being forwarded to the evaluation unit 10.
[0050] The evaluation unit 10 comprises an analog-to-digital converter 12 and a microprocessor 13 or FPGA. The analog-to-digital converter 12 can ensure particularly high transmission rates. The microprocessor 13 preferably includes a graphics processor optimized for parallel data processing and thus enables real-time data evaluation.
[0051] As shown in Fig. 1, the measurement signal C is forwarded directly from the analog-to-digital converter 12 to the microprocessor 13 for evaluation. Only the processed data is then forwarded to a computer 18. In this way, data processing and evaluation are performed directly, so that no latency occurs. The analog-to-digital converter 12 and the microprocessor 13 (or graphics processor or FPGA) must be specifically configured for direct data transmission and processing.
[0052] Fig. 2 shows essentially the setup from Fig. 1, with the addition of a second detector unit. The optical test signal B is split by means of an additional beam splitter, so that one part strikes the original first polarizing beam splitter 5 and a second part strikes the parallel first polarizing beam splitter 5'. This results in a further detector unit consisting of a first polarizing beam splitter 5', lens 17', half-wave plate 6', second polarizing beam splitter 7', detector 8', balanced photodiode 14', filter 9', and amplifier 16'. Both parallel measurement signals C and C' are forwarded to the evaluation unit 10 for analysis.
[0053] 1 Device for the characteristic analysis of light fields 2 First light source
[0054] 3 V delay section
[0055] 4 second light source
[0056] 5, 5' first polarizing beam splitter
[0057] 6, 6' half-wave plate
[0058] 7, 7' second polarizing beam splitter
[0059] 8, 8' Detector
[0060] 9, 9' Filter
[0061] 10 evaluation units
[0062] 11 Photodiode
[0063] 12 analogue-digital converters
[0064] 13 Microprocessor
[0065] 14, 14' balanced photodiode
[0066] 15 third beam splitter
[0067] 16, 16' Amplifier
[0068] 17, 17' lens
[0069] 18 computers
[0070] 19 fourth beam splitter
[0071] 20 fifth beam splitter
[0072] A, A' optical reference signal
[0073] B, B' optical test signal
[0074] C, C' Measurement signal
[0075] D Activation signal
[0076] TL, IT first optical partial signal
[0077] T2, T2' second optical partial signal
Claims
Patent claims 1. Device (1) for characteristic analysis of light fields, in particular for the reconstruction of a quantum state of an optical signal in real time, comprising a first light source (2) for generating an optical reference signal (A) with a first polarization, a second light source (4) for generating an optical test signal (B), a delay line (3) for generating a relative phase shift between the optical reference signal (A) and the optical test signal (B), a first polarizing beam splitter (5) connected downstream of the first light source (2) and the second light source (4) for combining the optical reference signal (A) with the optical test signal (B) such that both optical signals (A, B) have the same spatial mode, a half-wave plate (6) downstream of the first polarizing beam splitter (5) for rotating the polarization of the optical reference signal (A) and the optical test signal (B) at least partially by 45 degrees each, a second polarizing beam splitter (7) downstream of the half-wave plate (6) for splitting the combined optical signal (A, B) into a first optical partial signal (TI) and a second optical partial signal (T2), a detector (8) downstream of the second polarizing beam splitter (7) for detecting the first and second optical partial signals (TI, T2) interfering at the detector (8), a filter (9) downstream of the detector (8) for filtering out the reference signal (A) from the interfered signal (A, B) and generating a measurement signal (C), an evaluation unit (10) downstream of the filter (9) for evaluating the filtered measurement signal (C), 1. A photodiode (11) arranged such that a portion of the optical reference signal (A) is directed to the photodiode (11) by means of a third beam splitter (15) to activate the evaluation unit (10).
2. Device (1) according to claim 1, wherein the evaluation unit (10) comprises an analog-to-digital converter (12), wherein the analog-to-digital converter (12) is configured to ensure a transmission rate of at least 1 gigasample per second.
3. Device (1) according to claim 1 or 2, wherein the evaluation unit (10) comprises a microprocessor (13) or a field-programmable gate array or a tensor processing unit.
4. Device (1) according to claim 3, wherein the microprocessor (13) comprises a graphics processor.
5. Device (1) according to one of the preceding claims, wherein the evaluation unit (10) is configured to digitize an optical measurement signal (C) received at the evaluation unit (10) at the analog-to-digital converter (12) and to transmit it directly to the microprocessor (13) or FPGA.
6. Device (1) according to one of the preceding claims, wherein the first light source for generating the reference signal (2) comprises a titanium-sapphire pulsed laser, in particular with a pulse repetition rate of at least 80 MHz.
7. Device (1) according to one of the preceding claims, wherein the detector (8) comprises a balanced photodiode (14).
8. Device (1) according to one of the preceding claims, wherein the filter (9) comprises a notch filter, in particular in the 80 MHz and 160 MHz range.
9. Device (1) according to one of the preceding claims, wherein an amplifier (16) is arranged between the filter (9) and the evaluation unit (10).
10. Device according to one of the preceding claims, further comprising a fourth beam splitter (19) for splitting the optical test signal into two parallel optical test signals (B, B'), a fifth beam splitter (20) for splitting the optical reference signal into two parallel optical reference signals (A, A'), a first parallel polarizing beam splitter (5') downstream of the fourth beam splitter (19) for combining the optical reference signal (A') with the optical test signal (B') such that both optical signals (A', B') have the same spatial mode, a parallel half-wave plate (6') downstream of the first parallel polarizing beam splitter (5') for rotating the polarization of the optical reference signal (A') and the optical test signal (B') at least partially by 45 degrees each, a second parallel polarizing beam splitter (7') downstream of the parallel half-wave plate (6') for splitting the combined optical signal (A', B') into a first optical part signal (TI ') and a second optical part signal (T2'), a parallel detector (8') downstream of the second parallel polarizing beam splitter (7') for detecting the first and second optical part signals (T1', T2') interfering at the parallel detector (8'), a parallel filter (9') connected downstream of the parallel detector (8') to filter out the reference signal (A') from the interfered signal (A', B') and generate a parallel measurement signal (C').