CN115685235A - Optical phase tracking system for measuring fast time-varying signals - Google Patents
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Abstract
The invention discloses an optical phase tracking system for measuring a fast time-varying signal, which comprises a first laser, a first signal generator, a first phase modulator, a first light splitter, a first balanced detector, a second phase modulator, a second laser, a second light splitter, a third phase modulator, a second balanced detector, an extra optical path and a phase measurement module.
Description
Technical Field
The present invention relates to optical phase tracking technology, and more particularly, to an optical phase tracking system for measuring a fast time-varying signal.
Background
Optical phase estimation is one of the main tools for high-precision measurement such as coherent optical communication, optical frequency measurement and ground or space-based gravitational wave observation. In the field of high-precision measurement, measurement of a plurality of unknown physical quantities can be converted into optical phase difference measurement, so that an optical interferometer becomes a common equipment device in precision measurement and plays a vital role in engineering practice and scientific research. The limit of optical phase measurement is determined by the Heisenberg uncertain principle in quantum mechanics, and the aim in the field is to complete measurement approaching the standard quantum limit and even exceeding the standard quantum limit.
In the past, phase estimation experiments track a signal with a relatively slow change rate, but in reality, a lot of fast signal capturing requirements exist, such as fine tracking of an aircraft track, high-speed photography and the like. The signal change speed is mainly relative to a Phase Locked Loop (PLL), and when the signal change speed is too fast and exceeds the response time of the PLL, the PLL cannot work normally. For example, when monitoring real automobile motion or animal motion, a phase-locked loop is required to be able to lock the phase of signal light and the phase of local light at a position of pi/2 at all times in order to achieve the ultimate accuracy under quantum confinement. When the target object moves too fast to cause phase mismatch, the target object which changes rapidly cannot be tracked, resulting in a decrease in signal estimation accuracy.
Disclosure of Invention
The invention aims to: the invention provides an optical phase tracking system for measuring a rapid time-varying signal, which has higher signal estimation precision aiming at the problems in the prior art.
The technical scheme is as follows: the optical phase tracking system for measuring a time-varying fast signal comprises a first laser, a first signal generator, a first phase modulator, a first light splitter, a first balance detector, a second phase modulator, a second laser, a second light splitter, a third phase modulator, a second balance detector, an additional optical path and a phase measurement module, wherein the first phase modulator loads a random signal generated by the signal generator into laser light emitted by the first laser to form a modulated light beam, the first light splitter splits the modulated light beam into a first modulated light beam and a second modulated light beam, the second light splitter splits laser light emitted by the second laser into a first laser beam and a second laser beam, the second phase modulator loads a phase signal fed back by the phase measurement module into the first laser beam, the formed third modulated light beam is received by the first balance detector after being coherent with the first modulated light beam, the phase measurement module estimates an estimated phase difference signal required for generating an interference signal according to a signal received by the first balance detector, the third phase modulator feeds back the phase signal into the fourth modulated light beam after being coherent with the first modulated light beam, and the fourth phase measurement module receives the fourth modulated light beam after being received by the coherent laser beam.
Further, the phase measurement module specifically includes phase discriminator, kalman filter and the server that connects gradually, the phase discriminator measures the interference phase difference signal of third modulation light beam and first modulation light beam, kalman filter filters interference phase difference signal line, the server is used for producing required estimation phase signal according to the filtering signal.
Further, the first signal generator is used for generating the speed of 10 7 rad/s random signal.
Further, the first balanced detector is composed of two photodiodes with the same gain response, and is used for performing photocurrent subtraction according to the light power received by each photodiode and outputting the light power after the photocurrent subtraction.
Further, the optical frequency of the second laser is consistent with the optical frequency of the first laser.
Furthermore, the length of the additional optical path is equal to the propagation optical path of the first modulated light beam through the first balanced detector, the signal measurement module and the signal modulator.
Further, the second balanced detector and the first balanced detector have the same detection performance.
Has the beneficial effects that: compared with the prior art, the invention has the following remarkable advantages: the invention can realize the tracking of the target object with rapid change and has higher signal estimation precision.
Drawings
FIG. 1 is a system block diagram of the present invention;
FIG. 2 is a graph comparing the accuracy of measurement of homodyne detection and delay detection with the deviation of an operating point;
FIG. 3 is a schematic representation of the measured variance as a function of the ratio of photon counts at different angles of departure;
FIG. 4 is a graph of the random displacements generated;
FIG. 5 is a comparison of accuracy of measured direct phase-locked feedback and filtered feedback;
FIG. 6 is a comparison graph of classical direct probe measurements, delayed probe measurements, and Kalman filter measurements.
Detailed Description
The present embodiment provides an optical phase tracking system for tracking a fast time-varying signal, as shown in fig. 1, including a first laser 1, a first phase modulator 2, a first light splitting plate 3, a first balanced detector 4, a second phase modulator 5, a second light splitting plate 6, a second laser 7, a third phase modulator 8, an extra optical path 9, a second balanced detector 10, a first signal generator 11, and a phase measurement module, where the phase measurement module specifically includes a phase discriminator 12, a kalman filter 13, and a servo 14, which are connected in sequence. The laser frequencies of the first laser 1 and the second laser 7 are consistent, and the performances of the first balance detector 4 and the second balance detector 10 are consistent.
Wherein the first laser 1 outputs free space type continuous wave narrow linewidth laser, and the first signal generator 11 provides speed 10 7 A rad/s random displacement signal applied to the first phase modulator 2 as a modulation signal, the first phase modulator 2 modulating the phase of the laser signal according to the time-varying random signal, thereby loading the time-varying random signal onto the laser phase, outputting a phase modulation beam carrying the time-varying random signal, realizing the phase modulation of the light field, further modulating the light beam to be incident into the first light splitter 3, the first light splitter 3 splitting the modulated light beam into light fields with an amplitude of alpha 1 And the amplitude of the light field is alpha 2 The second modulated light beam of (2). The beam splitter can adjust the intensity ratio of two paths of light and properly improve the amplitude alpha of the light field 2 The accuracy of the system can be further improved.
The first balanced detector 4, the phase discriminator 12, the Kalman filter 13, the servo 14 and the second phase modulator 5 jointly form an optical phase-locked loop for tracking measurement of time-varying random phases. The second phase modulator 5 is an executive device of a phase-locked loop, and adjusts the phase of the second laser according to a signal fed back by the optical phase-locked loop to lock the phase difference of two arms of the interferometer to pi/2. The laser emitted by the second laser 7 is divided into a first laser beam and a second laser beam through the second light splitter 6, the first laser beam enters the optical phase-locked loop, the second laser beam enters the third phase modulator 8, and the same local light source is used for the measurement of the two-time balance detector, so that the stability of the system is facilitated. The second phase modulator 5 loads a phase signal fed back by the servo 14 into the first laser beam to form a third modulated light beam, the third modulated light beam is received by the first balanced detector 4 after being coherent with the first modulated light beam, the phase discriminator 12 measures an interference phase difference signal of the third modulated light beam and the first modulated light beam, the kalman filter 13 filters the interference phase difference signal, and the servo 14 is used for generating a required estimated phase signal according to the filtered signal. The third phase modulator 8 loads the phase signal fed back by the servo 14 into the second laser beam to form a fourth modulated light beam, the second modulated light beam is coherent with the fourth modulated light beam after passing through the extra optical path 9 and then received by the second balanced detector 10, and the light modulated by the third phase modulator 8 can have sufficiently high spatial and temporal resolution and is coherent with the second modulated light beam.
The first balanced detector 4 and the second balanced detector 10 are used for detecting laser interference signals, and are composed of two photodiodes with the same gain response, and photocurrent subtraction is performed according to the light power received by each photodiode and then output, so that common mode noise of a photoelectric system is deducted, and the detection precision is improved.
Two modulated beams after the first beam splitter 3 enter the optical pll, one enters the second balanced detector 10 via an extra optical path 9. Where the laser passes through the extra optical path 9 just enough to meet the optical phase lock loop feedback time. Since the first measurement is performed by the first balanced detector 4, the signal phase is known in advance, and the laser light after passing through the extra optical path 9 and the laser light after passing through the third phase modulator 8 are measured at the second balanced detector 10, which can be basically defaulted to an optimal measurement point always at a phase difference of pi/2. Because the bandwidth of the optical phase-locked loop is limited by the phase discriminator, in this case, the bandwidths of the first balanced detector 4, the second balanced detector 10, the phase modulator 2 and the third phase modulator 8 are relatively wide, so that the laser after passing through the third phase modulator 8 has relatively high space-time resolution, and the high sampling requirement is met.
The system of the present embodiment is analyzed and verified as follows.
First, it is analyzed that the number of photons is the same and | α - 2 ＝0.5×10 6 In the case of (2), the difference between conventional detection and delay detection at different deviation angles. As shown in FIG. 2, the two measurements have a splitting ratio of 50/50, and the results are the same when there is no deviation of the signal measurement point. But as the angle of departure increases, the effect of the time delay structure is seen in fig. 2. This is desirable, i.e., to track a fast time-varying signal.
This example also analyzes the effect of different spectral ratios on the overall system, as shown in fig. 3. The figure shows that the measurement error gradually approaches the theoretical accuracy of classical quantum limit as the number of photons occupied by the first measurement decreases at 30, 45 and 60 degrees deviations, respectively.
In the embodiment, discrete signals are adopted for simulation, and the bandwidths of the photoelectric detector and the phase-locked loop are set. Here, the bandwidth of the photodetector is set to 1GHz, the bandwidth of the second phase modulator 5 is set to 40MHz, the bandwidth of the third phase modulator 8 is set to 1GHz, and the feedback delay of the optical phase-locked loop is set to 25ns. Fig. 4 is a graph of the generated random displacements. The speed of the working phase is about 10 7 At the level of rad/s, this is only 10 in the literature 5 rad/s. The time delay system (50/50 split ratio) of fig. 1 and a conventional optical phase tracking system are then used to track the phase information.
Here, the feedback part of the phase locked loop is estimated using a kalman filter and demonstrates the advantage of the kalman filter under phase tracking, as shown in fig. 5. After using the Kalman filter, the error fluctuation of the measurement is reduced, wherein the total photon flux | α |, is reduced 2 ＝0.5×10 6 Sum signal noise Q =10 -6 . Here, error
Is the phase and x is the measured or filtered value. Through simulation comparison, the mean square error obtained by direct measurement is 1.57 in a significant amount10 -6 And the mean square error after Kalman filtering is 1.06 multiplied by 10 -6 . Therefore, the application of the kalman filter can improve the real-time random phase estimation accuracy by 1.7dB.
Finally, the present embodiment simulates and compares the performance of the optical phase tracking system of the time delay structure and the conventional optical phase tracking system. As shown in fig. 6, when the tracking signal speed is about 10 7 The mean square error under the traditional homodyne detection is 7.03 multiplied by 10 when rad/s -7 Mean square error of the delay measurement is 5.29 × 10 -7 Therefore, the measurement accuracy is improved by 2.4dB. In addition, when the total photon flux | α $ 2 ＝0.5×10 6 Its classical shot noise limit tracking precision is 5X 10 -7 Thus, the optical phase tracking system of the time delay structure approaches the classical limit.
The above disclosure is only for the purpose of illustrating the preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention.
Claims (7)
1. An optical phase tracking system for measuring a rapidly time-varying signal, characterized by: the phase measurement device comprises a first laser, a first signal generator, a first phase modulator, a first light splitter, a first balance detector, a second phase modulator, a second laser, a second light splitter, a third phase modulator, a second balance detector, an extra optical path and a phase measurement module, wherein the first phase modulator loads random signals generated by the signal generator into laser light emitted by the first laser to form a modulated light beam, the first light splitter divides the modulated light beam into a first modulated light beam and a second modulated light beam, the second light splitter divides the laser light emitted by the second laser into a first laser beam and a second laser beam, the second phase modulator loads a phase signal fed back by the phase measurement module into the first laser beam, the formed third modulated light beam is received by the first balance detector after being coherent with the first modulated light beam, the phase measurement module estimates an estimated phase signal required for generating an interference signal according to the signal received by the first balance detector, the third phase modulator loads a phase signal fed back by the phase measurement module into the second phase modulator, the fourth modulated light beam is received by the second balance detector after being coherent with the second light beam, and the fourth light beam is received by the phase measurement module after being received by the second balanced detector.
2. An optical phase tracking system for measuring a rapidly time-varying signal as defined in claim 1, wherein: the phase measurement module specifically comprises a phase discriminator, a Kalman filter and a server which are sequentially connected, the phase discriminator measures an interference phase difference signal of a third modulation light beam and a first modulation light beam, the Kalman filter filters the interference phase difference signal, and the server is used for generating a required estimated phase signal according to a filtering signal.
3. An optical phase tracking system for measuring a rapidly time-varying signal as defined in claim 1, wherein: the first signal generator is used for generating a speed of 10 7 rad/s random signal.
4. An optical phase tracking system for measuring a rapidly time-varying signal as defined in claim 1, wherein: the first balanced detector is composed of two photodiodes with the same gain response, and is used for outputting light current subtraction according to the light power received by each photodiode.
5. An optical phase tracking system for measuring a rapidly time-varying signal as defined in claim 1, wherein: the second laser optical frequency is consistent with the first laser optical frequency.
6. An optical phase tracking system for measuring a rapidly time-varying signal as defined in claim 1, wherein: the length of the additional optical path is equal to the propagation optical path of the first modulated light beam passing through the first balanced detector, the signal measurement module and the signal modulator.
7. An optical phase tracking system for measuring a rapidly time-varying signal as defined in claim 1, wherein: the second balanced detector and the first balanced detector have the same detection performance.
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