CN115685235B - 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 rapid time-varying signal, which comprises a first laser, a first signal generator, a first phase modulator, a first beam splitter, a first balance detector, a second phase modulator, a second laser, a second beam splitter, a third phase modulator, a second balance detector, an additional optical path and a phase measuring 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, foundation or space-based gravitational wave observation and the like. In the field of high-precision measurement, the 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 uncertain principle of Hessenberg in quantum mechanics, and the aim of the field is to complete measurement approaching the standard quantum limit and even measurement exceeding the standard quantum limit.
Previous phase estimation experiments have tracked a relatively slow rate of change signal, however, in reality there are many fast signal capture requirements, such as fine tracking of aircraft trajectories, high-speed photography, etc. The signal change speed is mainly relative to a phase locked loop (Phase Locked Loop, PLL), and when the signal change speed is too fast, the phase locked loop cannot work normally beyond the response time. For example, in monitoring real-time automobile movement or animal movement, it is necessary for a phase-locked loop to lock the phase of signal light and the phase of local light at pi/2 positions all the time in order to achieve a limit accuracy under quantum confinement. When the target object moves too fast to cause phase mismatch, the rapidly changing target object cannot be tracked, resulting in a decrease in signal estimation accuracy.
Disclosure of Invention
The invention aims to: aiming at the problems existing in the prior art, the invention provides an optical phase tracking system for measuring a rapid time-varying signal, which has higher signal estimation precision.
The technical scheme is as follows: the optical phase tracking system for measuring the rapid time-varying signal comprises a first laser, a first signal generator, a first phase modulator, a first beam splitter, a first balance detector, a second phase modulator, a second laser, a second beam splitter, a third phase modulator, a second balance detector, an additional optical path and a phase measuring module, wherein the first phase modulator loads a random signal generated by the signal generator into laser emitted by the first laser to form a modulated light beam, the first beam splitter divides the modulated light beam into a first modulated light beam and a second modulated light beam, the second beam splitter divides the laser 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 measuring 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 measuring module generates an interference signal required by the phase signal received by the first balance detector according to the first balance detector, the phase difference signal is estimated by the second phase measuring module after being coherent with the second phase measuring module, and the phase signal is fed back by the second phase measuring module after being coherent with the second phase measuring module.
Further, the phase measurement module specifically includes a phase discriminator, a kalman filter and a server which are connected in sequence, the phase discriminator measures an interference phase difference signal of the third modulated light beam and the first modulated 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 the filtered signal.
Further, the first signal generator is configured to generate a random signal having a speed of 10 7 rad/s.
Further, the first balance detector is composed of two photodiodes with the same gain response, and is used for outputting after subtracting photocurrent according to the received optical power of each photodiode.
Further, the second laser light frequency is consistent with the first laser light frequency.
Further, the length of the additional optical path is equal to the propagation optical path of the first modulated light beam through the first balance detector, the signal measuring module and the signal modulator.
Further, the second balance detector has the same detection performance as the first balance detector.
The beneficial effects are that: compared with the prior art, the invention has the remarkable advantages that: the invention can realize the tracking of the rapidly-changed target object 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 the measurement of homodyne detection and time delay detection with the deviation of the operating point;
FIG. 3 is a schematic diagram of measurement variance as a function of photon number ratio at different angles of departure;
FIG. 4 is a graph of random displacements generated;
FIG. 5 is a comparison of the accuracy of measuring direct phase lock feedback and post-filter feedback;
fig. 6 is a comparison of classical direct probe measurement, delayed probe measurement and kalman filter measurement results.
Detailed Description
The present embodiment provides an optical phase tracking system for tracking a fast time-varying signal, as shown in fig. 1, which includes a first laser 1, a first phase modulator 2, a first beam splitter 3, a first balance detector 4, a second phase modulator 5, a second beam splitter 6, a second laser 7, a third phase modulator 8, an additional optical path 9, a second balance detector 10, a first signal generator 11, and a phase measurement module, wherein the phase measurement module specifically includes a phase discriminator 12, a kalman filter 13, and a server 14 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.
The first laser 1 outputs free space continuous wave narrow linewidth laser, the first signal generator 11 provides a random displacement signal with the speed of 10 7 rad/s, the random displacement signal is applied to the first phase modulator 2 as a modulation signal, the first phase modulator 2 modulates the phase of the laser signal according to the time-varying random signal, so that the time-varying random signal is loaded on the laser phase, a phase modulation light beam carrying the time-varying random signal is output, the phase modulation of a light field is realized, the further modulation light beam is incident into the first light splitting sheet 3, and the first light splitting sheet 3 splits the modulation light beam into a first modulation light beam with the light field amplitude of alpha 1 and a second modulation light beam with the light field amplitude of alpha 2. The intensity ratio of the two paths of light can be adjusted by the light splitting sheet, and the accuracy of the system can be further improved by properly improving the light field amplitude alpha 2.
The first balance detector 4, the phase detector 12, the Kalman filter 13, the server 14 and the second phase modulator 5 together form an optical phase-locked loop for tracking measurement of the time-varying random phase. The second phase modulator 5 is an actuator of a phase-locked loop, which adjusts the phase of the second laser according to the signal fed back by the optical phase-locked loop, and locks the phase difference between the 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 by the second beam splitter 6, the first laser beam enters the optical phase-locked loop, the second laser beam enters the third phase modulator 8, and the two balance detectors measure the same local light source, so that the system is stable. The second phase modulator 5 loads the phase signal fed back by the server 14 into the first laser beam to form a third modulated beam, the third modulated beam is received by the first balance detector 4 after being coherent with the first modulated beam, the phase discriminator 12 measures an interference phase difference signal of the third modulated beam and the first modulated beam, the kalman filter 13 filters the interference phase difference signal, and the server 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 server 14 into the second laser beam to form a fourth modulated beam, the second modulated beam is coherent with the fourth modulated beam through the additional optical path 9 and then received by the second balance detector 10, and the light modulated by the third phase modulator 8 can have a sufficiently high space-time resolution and coherent with the second modulated beam.
The first balance detector 4 and the second balance detector 10 are used for detecting laser interference signals, and are composed of two photodiodes with the same gain response, and output after the photocurrent is subtracted according to the optical power received by each photodiode, so that common mode noise of a photoelectric system is subtracted, and detection accuracy is improved.
Two modulated light beams after the first beam splitter 3, one of which enters the optical phase locked loop and the other of which enters the second balanced detector 10 via an additional optical path 9. The time for the laser to pass through the extra optical path 9 just meets the feedback time of the optical phase-locked loop. Since the first balanced detector 4 performs the first measurement, knowing the signal phase in advance, 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, and can be basically defaulted to an optimal measurement point always at pi/2 phase difference. Since the bandwidth of the optical phase-locked loop is limited by the phase detector, 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, the laser light after passing through the third phase modulator 8 has relatively high space-time resolution, and the high sampling requirement is met.
The system of this embodiment is analyzed and validated as follows.
The difference between traditional detection and time delay detection under different deviation angles under the condition that the photon numbers are the same and |alpha| 2＝0.5×106 is met is analyzed first. As shown in fig. 2, the spectral ratio of the two measurements is 50/50, and the results of the two measurements are identical when there is no deviation in the signal measurement points. However, as the angle of departure increases, fig. 2 can see that the effect of the delay structure appears. This is desirable, i.e. to track a fast time varying signal.
The present example also analyzed the effect of different spectral ratios on the overall system, as shown in fig. 3. The figures show that with 30 degrees, 45 degrees and 60 degrees of deviation, respectively, the measurement error gradually approaches the theoretical accuracy of classical quantum limit as the number of photons occupied by the first measurement decreases.
In the embodiment, discrete signals are adopted for simulation, and 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 40MHz, the bandwidth of the third phase modulator 8 is 1GHz, and the feedback delay of the optical phase locked loop is 25ns. Fig. 4 is a graph of the random displacements generated. The speed of the working phase is approximately at the level of 10 7 rad/s, which in the previous literature was only 10 5 rad/s. The phase information is then tracked using the delay system (50/50 split ratio) of fig. 1 and a conventional optical phase tracking system.
Here, the feedback part of the phase locked loop is estimated using a kalman filter and the advantage of the kalman filter in phase tracking is demonstrated as shown in fig. 5. After the use of the kalman filter, the measured error fluctuations are reduced, with the total photon flux |α| 2＝0.5×106 and the signal noise q=10 -6. Here, errorIs the phase and x is the measured or filtered value. Through simulation comparison, the mean square error obtained through direct measurement is 1.57×10 -6, and the mean square error after Kalman filtering is 1.06×10 -6. Thus, the application of the Kalman filter can improve the real-time random phase estimation accuracy by 1.7dB.
Finally, this embodiment simulates and compares the performance of an optical phase tracking system of a time delay structure with that of a conventional optical phase tracking system. As shown in fig. 6, when the tracking signal speed is about 10 7 rad/s, the mean square error under the conventional homodyne detection is 7.03x10 -7, and the mean square error of the delay measurement is 5.29 x 10 -7, so that the measurement accuracy is improved by 2.4dB. Furthermore, when the total photon flux |α| 2＝0.5×106, the tracking accuracy of the classical shot noise limit is 5×10 -7, and thus the optical phase tracking system of the time delay structure approaches the classical limit.
The above disclosure is illustrative of a preferred embodiment of the present invention and is not to be construed as limiting the scope of the invention, which is defined by the appended claims.
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 beam splitter, a first balance detector, a second phase modulator, a second laser, a second beam splitter, a third phase modulator, a second balance detector, an additional optical path and a phase measurement module, wherein the first phase modulator loads random signals generated by the signal generator into laser emitted by the first laser to form a modulated light beam, the first beam splitter divides the modulated light beam into a first modulated light beam and a second modulated light beam, the second beam splitter divides the laser emitted by the second laser into a first laser beam and a second laser beam, the second phase modulator loads phase signals 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 estimated phase signals required by generating interference phase difference signals according to signals received by the first balance detector, the second phase modulator feeds back the phase signals into the fourth phase modulator after being coherent with the second phase modulator, and the fourth phase modulator receives the phase signals.
2. The optical phase tracking system for measuring a rapidly time varying signal of claim 1, wherein: the phase measurement module specifically comprises a phase discriminator, a Kalman filter and a server which are connected in sequence, wherein the phase discriminator measures an interference phase difference signal of the third modulated light beam and the first modulated 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 the filtered signal.
3. The optical phase tracking system for measuring a rapidly time varying signal of claim 1, wherein: the first signal generator is configured to generate a random signal having a speed of 10 7 rad/s.
4. The optical phase tracking system for measuring a rapidly time varying signal of claim 1, wherein: the first balance detector is composed of two photodiodes with the same gain response, and is used for carrying out photocurrent subtraction according to the optical power received by each photodiode and outputting the optical power.
5. The optical phase tracking system for measuring a rapidly time varying signal of claim 1, wherein: the second laser light frequency is consistent with the first laser light frequency.
6. The optical phase tracking system for measuring a rapidly time varying signal of claim 1, wherein: the length of the additional optical path is equal to the propagation optical path of the first modulated light beam through the first balance detector, the signal measuring module and the signal modulator.
7. The optical phase tracking system for measuring a rapidly time varying signal of claim 1, wherein: the second balance detector has the same detection performance as the first balance detector.
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