Frequency measuring device based on optical frequency comb and cyclic frequency shift

Abstract

The invention provides a frequency measurement device based on an optical frequency comb and cyclic frequency shift. The frequency measurement device comprises a laser, an optical beam splitter, an electro-optic intensity modulator, an optical cyclic frequency shift loop, an optical frequency comb generation unit, an optical beam combiner, a multichannel optical filter, a multichannel photoelectric detector, a multichannel low-pass filter and a multichannel acquisition processing unit. According to the device, broadband high-precision frequency measurement is realized by adopting a method of combining cyclic frequency shift with Fourier transform, an optical frequency comb is adopted as a local oscillation light source, sub-band division of a radio frequency spectrum is realized by combining a multi-channel optical filter, parallel frequency measurement of each sub-band can be realized, the cycle index of a frequency shift loop is reduced, the cycle time of the loop is shortened, and the measurement efficiency is improved. Noise accumulation caused by multiple cycles is reduced, and the signal-to-noise ratio and the dynamic range of radio frequency signal frequency measurement detection are improved, so that the frequency measurement requirements of broadband high precision, high signal-to-noise ratio, high dynamic and quick response are met.

Description

Technical Field

[0001] The present invention relates to the technical field of frequency measuring equipment, and in particular to a frequency measuring device based on an optical frequency comb and cyclic frequency shift. Background Art

[0002] The current demand for microwave signal frequency measurement is gradually developing towards a larger bandwidth and higher measurement accuracy. Some traditional electronic methods for microwave frequency measurement have high power consumption, limited bandwidth, and are easily affected by measurement performance in complex electromagnetic environments. Taking advantage of the large bandwidth, strong anti-interference ability, and low power consumption of optical devices, photonics-based methods provide a new approach and unique advantages for the measurement of microwave signal frequency. The current microwave photon frequency measurement technology essentially converts microwave frequency information that is not easy to measure directly into other easily measured information, such as converting frequency information into power, pulse arrival time, or spatial distribution, but it is difficult to meet the requirements of multiple frequencies, high precision, and large bandwidth at the same time. The preliminary measurement of microwave frequency is achieved by cyclic frequency shifting, and the accuracy of frequency measurement is further improved by Fourier transform. It can measure multiple frequency components at the same time, and can meet the requirements of broadband high-precision frequency measurement. However, as the number of cycles of the frequency shift loop increases, the cyclic frequency shift scheme will inevitably accumulate loop noise, resulting in a decrease in the detection signal-to-noise ratio of the final frequency measurement. At the same time, the increase in the output noise floor will compress the dynamic range of the frequency measurement. Since there is a time delay in each cycle, a large number of cycles are required to achieve broadband frequency measurement, resulting in a long frequency measurement time. The frequency measurement scheme using cyclic frequency shifting has the disadvantages of long frequency measurement time, low high-frequency measurement signal-to-noise ratio, and small dynamics. Therefore, it is particularly important to propose a broadband, high signal-to-noise ratio, high dynamic, and fast response frequency measurement device. Summary of the invention

[0003] In order to solve the technical problems existing in the background technology, the present invention proposes a frequency measurement device based on optical frequency comb and cyclic frequency shift.

[0004] The frequency measuring device based on optical frequency comb and cyclic frequency shift proposed by the present invention comprises: a laser, an optical beam splitter, an electro-optical intensity modulator, an optical cyclic frequency shift loop, an optical frequency comb generating unit, an optical beam combiner, a multi-channel optical filter, a multi-channel photodetector, a multi-channel low-pass filter and a multi-channel acquisition processing unit;

[0005] The laser output is divided into two optical signals through an optical beam splitter, one of which is passed through an electro-optical intensity modulator to complete the electro-optical conversion of the RF input signal and output an optically-carried RF signal. The optically-carried RF signal is cyclically shifted through an optical cyclic frequency shifting loop and outputs a cyclically-shifted optically-carried RF signal. The other is passed through an optical frequency comb generating unit to generate an optical frequency comb signal. The cyclically-shifted optically-carried RF signal and the optical frequency comb signal are input into an optical beam combiner for beam combining. The combined optical signal is spectrally filtered through a multi-channel optical filter. The spectrally-filtered optical signal is photoelectrically detected through a multi-channel photoelectric detector. The output electrical signal of each photoelectric detection channel is low-pass filtered through a multi-channel low-pass filter. The output electrical signal of each low-pass filtering channel is signal collected and processed through a multi-channel acquisition and processing unit.

[0006] Preferably, the optical cyclic frequency shift loop performs cyclic frequency shift on the input optical signal, and the frequency shift amount per cycle is f A , the number of cycles is M.

[0007] Preferably, the laser frequency is f L0 The frequency laser passes through the optical frequency comb generation unit to generate an optical frequency comb with N frequency comb teeth. The frequencies of the optical frequency comb teeth are f L0 ,…,f L(N-1) , the frequency interval between all adjacent frequency comb teeth is M×f A .

[0008] Preferably, the multi-channel optical filter has N filtering channels, wherein the filtering starting frequency of the nth filtering channel is f L(n-1) , the bandwidth of all optical filtering channels is M×f A .

[0009] Preferably, the multi-channel photodetector has N photodetection channels, each of which performs photodetection on the output light signal of each filtering channel, and the bandwidth of all photodetection channels is greater than or equal to M×f A .

[0010] Preferably, the multi-channel low-pass filter has N low-pass filter channels, and the low-pass cutoff frequency of all low-pass filter channels is f A .

[0011] Preferably, the multi-channel acquisition and processing unit has N acquisition and processing channels, each acquisition and processing channel performs signal acquisition and Fourier transform on the output electrical signal of each low-pass filter channel respectively, and finally splicing and frequency measurement of the spectrum data of each acquisition and processing channel are performed in the multi-channel acquisition and processing unit.

[0012] Preferably, the optical cyclic frequency shift loop includes an optical switch, an optical fiber coupler, a semiconductor optical amplifier, an optical filter, an acousto-optic frequency shifter and a delay optical fiber; the optically-carried radio frequency signal output by the electro-optic intensity modulator is modulated by an optical switch to complete optical pulse modulation, and the modulated optical pulse is input into a frequency shift loop composed of an optical fiber coupler, a semiconductor optical amplifier, an optical filter, an acousto-optic frequency shifter and a delay optical fiber to complete cyclic frequency shifting.

[0013] Preferably, the opening time of the optical switch is less than or equal to the time required for the optical signal to circulate once in the subsequent circulation structure.

[0014] The present invention adopts a method of cyclic frequency shift combined with Fourier transform to achieve broadband high-precision frequency measurement, adopts an optical frequency comb as a local oscillator light source, and combines a multi-channel optical filter to achieve sub-band division of the radio frequency spectrum. Each sub-band can achieve parallel frequency measurement, reduce the number of frequency shift loop cycles, shorten the loop cycle time, reduce the noise accumulation caused by multiple cycles, and improve the signal-to-noise ratio and dynamic range of radio frequency signal frequency measurement detection, so as to meet the needs of broadband, high-precision, high-signal-to-noise ratio, high-dynamic and fast-response frequency measurement. Compared with the prior art, the present invention has the following advantages:

[0015] 1. The present invention realizes preliminary measurement of microwave frequency by cyclic frequency shifting through an optical cyclic frequency shifting loop, further improves the accuracy of frequency measurement by performing Fourier transform on the collected signal through a multi-channel acquisition processing unit, and can measure multiple frequency components at the same time, thereby achieving broadband high-precision frequency measurement requirements.

[0016] 2. The present invention adopts an optical frequency comb as a local oscillator light source and combines it with a multi-channel optical filter to perform spectrum division, thereby realizing sub-band division of the RF spectrum and measuring the frequency of each sub-band in parallel. This can effectively shorten the cycle time of the frequency shift loop, reduce the noise accumulation caused by multiple cycles, and improve the signal-to-noise ratio and dynamic range of RF signal frequency measurement and detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the structure of a frequency measurement device based on an optical frequency comb and cyclic frequency shift proposed by the present invention;

[0018] Figure 2 It is a structural schematic diagram of the optical cyclic frequency shift loop in the frequency measurement device based on optical frequency comb and cyclic frequency shift proposed by the present invention;

[0019] Figure 3 A spectrum diagram of the optical cyclic frequency shift loop in the frequency measurement device based on optical frequency comb and cyclic frequency shift proposed by the present invention;

[0020] Figure 4 This is a spectrum diagram of the multi-channel optical filter and optical frequency comb described in the frequency measurement device based on optical frequency comb and cyclic frequency shift proposed by the present invention. DETAILED DESCRIPTION

[0021] Reference Figure 1 The frequency measurement device based on optical frequency comb and cyclic frequency shift proposed in the present invention includes: a laser 1, an optical beam splitter 2, an electro-optical intensity modulator 3, an optical cyclic frequency shift loop 4, an optical frequency comb generating unit 5, an optical beam combiner 6, a multi-channel optical filter 7, a multi-channel photodetector 8, a multi-channel low-pass filter 9 and a multi-channel acquisition processing unit 10.

[0022] The laser output by the laser 1 is divided into two optical signals through the optical beam splitter 2, one of which is passed through the electro-optical intensity modulator 3 to complete the electro-optical conversion of the RF input signal, and output the optically-carried RF signal, which is passed through the optical cyclic frequency shift loop 4 to complete the cyclic frequency shift, and output the cyclically-shifted optically-carried RF signal; the other is passed through the optical frequency comb generating unit 5 to generate an optical frequency comb signal, and the cyclically-shifted optically-carried RF signal and the optical frequency comb signal are input into the optical beam combiner 6 for beam combining, and the optical signal after beam combining is spectrally filtered through the multi-channel optical filter 7; the optical signals of each filtering channel are passed through the multi-channel photoelectric detector 8 to complete the photoelectric detection; the output electrical signals of each photoelectric detection channel are passed through the multi-channel low-pass filter 9 to complete the low-pass filtering; the output electrical signals of each low-pass filtering channel are passed through the multi-channel acquisition processing unit 10 to complete the acquisition and frequency measurement processing.

[0023] The optical cyclic frequency shift loop 4 performs cyclic frequency shift on the input optical signal, and the frequency shift amount per cycle is f A , the number of cycles is M.

[0024] The frequency of the laser 1 is f L0 The frequency laser passes through the optical frequency comb generating unit 5 to generate an optical frequency comb with N frequency comb teeth, and the frequencies of the optical frequency comb teeth are f L0 ,…,f L(N-1) , the frequency interval between all adjacent frequency comb teeth is M×f A .

[0025] The multi-channel optical filter 7 has N filtering channels, wherein the filtering starting frequency of the nth filtering channel is f L(n-1) , the bandwidth of all optical filtering channels is M×f A .

[0026] The multi-channel photoelectric detector 8 has N photoelectric detection channels, each of which performs photoelectric detection on the output light signal of each filter channel, and the bandwidth of all photoelectric detection channels is greater than or equal to M×f A .

[0027] The multi-channel low-pass filter 9 has N low-pass filter channels, and the low-pass cut-off frequency of all low-pass filter channels is f A .

[0028] The multi-channel acquisition and processing unit 10 has N acquisition and processing channels, each of which performs signal acquisition and Fourier transform on the output electrical signal of each low-pass filter channel, and splicing and frequency measurement of spectrum data of each acquisition and processing channel are performed in the multi-channel acquisition and processing unit 10.

[0029] The specific working principle is as follows:

[0030] like Figure 1 As shown, the frequency of laser 1 is f L0 , when the frequency is f s1 After the RF signal enters the electro-optical intensity modulator 3, the optical carrier will be modulated to generate a frequency of f L0 ±f s1 The two sidebands of the modulated optical signal are as follows: Figure 3 As shown, the modulated optical signal enters the optical cyclic frequency shift loop 4 for cyclic frequency shift.

[0031] like Figure 2 As shown, the optical cyclic frequency shift loop 4 includes an optical switch 41, an optical fiber coupler 42, a semiconductor optical amplifier 43, an optical filter 44, an acousto-optic frequency shifter 45 and a delay optical fiber 46. The optically-carried radio frequency signal output by the electro-optic intensity modulator 3 is modulated by an optical switch 41, and the modulated optical pulse is input into the frequency shift loop composed of the optical fiber coupler 42, the semiconductor optical amplifier 43, the optical filter 44, the acousto-optic frequency shifter 45 and the delay optical fiber 46. Each time the frequency shift loop is cycled once, the frequency needs to be shifted once by the acousto-optic frequency shifter 45, and the frequency-shifted optical signal is output through the optical fiber coupler 42. The optical switch 41 in the optical cyclic frequency shift loop 4 can be an electro-optic switch, a magneto-optic switch, a MEMS optical switch or an acousto-optic switch. The function of the optical switch 41 is to act as an optical switch with a high extinction ratio. In order to avoid the optical signals in adjacent cycle periods from overlapping in time during the cyclic frequency shifting process, the opening time of the optical switch 41 should be less than or equal to the time required for the optical signal to cycle once in the subsequent cycle structure. The semiconductor optical amplifier 43 is used to amplify the optical signal in the loop and compensate for the loss of the optical signal in the loop. The optical filter 44 is used to filter out the noise outside the bandwidth of the optical radio frequency signal, which will accumulate in the loop. The acousto-optic frequency shifter 45 is used to shift the frequency of the optical radio frequency signal in each cycle. The delay fiber 46 is used to set the cyclic frequency shift time interval. The upper and lower sideband optical signals modulated by the electro-optic intensity modulator 3 have a frequency of f L0 ±f s1 , for a frequency of f L0 -f s1 The lower sideband light of the laser will be quickly shifted to a lower frequency far away from the laser 1 frequency in the optical frequency shift loop 4 and will not be detected at the back end. L0 +f s1The upper sideband of the optical frequency shift loop 4 is shifted once each time it passes through the acousto-optic frequency shifter 45. The frequency shift frequency of the acousto-optic frequency shifter 45 is f A After passing through the acousto-optic frequency shifter for the mth time, the frequency of the optical signal in the upper sideband changes to f m =f L0 +f s1 -mf A The output spectrum of the optical frequency shift loop 4 is as follows: Figure 3 shown.

[0032] The optical frequency comb generating unit 5 is composed of an electro-optical intensity modulator and an electro-optical phase modulator in cascade. The optical signal output by the laser 1 passes through the optical frequency comb generating unit 5 to generate an optical frequency comb with N frequency comb teeth. The frequencies of the optical frequency comb teeth are f L0 ,…,f L(N-1) , the frequency interval between adjacent comb teeth is M×f A The optical signal output by the optical fiber coupler 42 and the optical signal of the optical frequency comb are combined in the optical combiner 6, and the combined optical signal is sent to the multi-channel optical filter 7 for spectrum filtering. After filtering, the optical-to-electric conversion is completed by the multi-channel photodetector 8. The spectrum of the multi-channel optical filter 7 and the spectrum of the optical frequency comb are as shown in FIG. Figure 4 As shown, the cyclic frequency shifted optical signal filtered by the multi-channel optical filter 7 and the optical frequency comb signal beat each other in each photodetection of the multi-channel photodetector 8, wherein the frequency f of the output electrical signal of the nth channel in the multi-channel photodetector 8 is Fn for:

[0033] f Fn =f s1 -(n-1)Mf A -mf A

[0034] The multi-channel photodetector 8 is followed by a multi-channel low-pass filter 9, and the low-pass cut-off frequency of each low-pass filter channel is f A When the signal frequency output by the detector falls within the passband of the low-pass filter, it can be collected by the multi-channel acquisition processing unit 10. The multi-channel acquisition processing unit 10 uses the enable pulse of the optical switch 41 as a trigger source for acquisition. According to the arrival time of the acquisition signal, it can be known how many times the signal has been circulated. At the same time, the acquired signal is fast Fourier transformed to measure the specific frequency value of the signal after passing through the low-pass filter, that is, the frequency of the signal output after m times of frequency shift obtained by the fast Fourier transform of the nth channel of the multi-channel acquisition processing unit 10 is f Fn Thus, the signal frequency f measured by the nth channel of the multi-channel acquisition processing unit 10 can be Fn , calculate the actual input frequency f into the electro-optical intensity modulator 3 s1, obtained by the following formula:

[0035] f s1 =f Fn +(n-1)Mf A +mf A

[0036] Since the device performs frequency measurement based on the optical frequency comb and cyclic frequency shift, the optical frequency comb and the optical radio frequency signal after cyclic frequency shift are divided into N spectrum sub-bands by the multi-channel optical filter 7. The optical domain spectrum range corresponding to the nth spectrum sub-band is (f L0 +(n-1)Mf A , f L0 +nMf A ), the bandwidth of each subband is Mf A , N sub-bands can be measured in parallel, with a total measurement bandwidth of NMf A , the total number of cycles of the optical frequency shift loop 4 is M. Compared with using only the laser 1 frequency f L0 As a solution for local oscillator light, to achieve a bandwidth of NMf A Therefore, the device of the present invention can effectively reduce the number of cyclic frequency shifts, thereby shortening the cycle time of the frequency shift loop, reducing the noise accumulation caused by multiple cycles, and further improving the signal-to-noise ratio and dynamic range of the RF signal frequency measurement detection.

[0037] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A frequency measurement device based on an optical frequency comb and cyclic frequency shift, characterized in that: include: Laser (1), optical beam splitter (2), electro-optic intensity modulator (3), optical cyclic frequency shift loop (4), optical frequency comb generation unit (5), optical beam combiner (6), multi-channel optical filter (7), multi-channel photodetector (8), multi-channel low-pass filter (9) and multi-channel acquisition processing unit (10); The laser output of the laser (1) is divided into two optical signals through an optical beam splitter (2), one of which is passed through an electro-optical intensity modulator (3) to complete the electro-optical conversion of the radio frequency input signal and output a light-carrying radio frequency signal, the light-carrying radio frequency signal is passed through an optical cyclic frequency shift loop (4) to complete cyclic frequency shift, and outputs a cyclic frequency shifted light-carrying radio frequency signal; the other is passed through an optical frequency comb generating unit (5) to generate an optical frequency comb signal, the cyclic frequency shifted light-carrying radio frequency signal and the optical frequency comb signal are input into an optical beam combiner (6) for beam combining, the beam-combined optical signal is passed through a multi-channel optical filter (7) to complete spectrum filtering; the optical signal after spectrum filtering is passed through a multi-channel photoelectric detector (8) to complete photoelectric detection; the output electrical signal of each photoelectric detection channel is passed through a multi-channel low-pass filter (9) to complete low-pass filtering; the output electrical signal of each low-pass filtering channel is passed through a multi-channel acquisition processing unit (10) to complete signal acquisition and frequency measurement processing.

2. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 1, characterized in that: The optical cyclic frequency shifting loop (4) performs cyclic frequency shifting on the input optical signal, and the frequency shifting amount per cycle is f A , the number of cycles is M.

3. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 2, characterized in that: The frequency of the laser (1) is f L0 The frequency laser passes through the optical frequency comb generating unit (5) to generate an optical frequency comb with N frequency comb teeth, and the frequencies of the optical frequency comb teeth are f L0 ,…,f L(N-1) , the frequency interval between all adjacent frequency comb teeth is M×f A .

4. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 2, characterized in that: The multi-channel optical filter (7) has N filtering channels, wherein the filtering starting frequency of the nth filtering channel is f L(n-1) , the bandwidth of all optical filtering channels is M×f A .

5. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 2, characterized in that: The multi-channel photoelectric detector (8) has N photoelectric detection channels, each of which performs photoelectric detection on the output light signal of each filtering channel, and the bandwidth of all photoelectric detection channels is greater than or equal to M×f A .

6. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 2, characterized in that: The multi-channel low-pass filter (9) has N low-pass filter channels, and the low-pass cutoff frequency of all low-pass filter channels is f A .

7. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 1, characterized in that: The multi-channel acquisition processing unit (10) has N acquisition processing channels, each of which performs signal acquisition and Fourier transformation on the output electrical signal of each low-pass filter channel, and finally splicing and frequency measurement of spectrum data of each acquisition processing channel are performed in the multi-channel acquisition processing unit (10).

8. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to any one of claims 1 to 7, characterized in that: The optical cyclic frequency shift loop (4) comprises an optical switch (41), an optical fiber coupler (42), a semiconductor optical amplifier (43), an optical filter (44), an acousto-optic frequency shifter (45) and a time-delay optical fiber (46); the optically-carried radio frequency signal output by the electro-optic intensity modulator (3) is modulated by an optical switch (41) to complete optical pulse modulation, and the modulated optical pulse is input into the frequency shift loop composed of the optical fiber coupler (42), the semiconductor optical amplifier (43), the optical filter (44), the acousto-optic frequency shifter (45) and the time-delay optical fiber (46) to complete cyclic frequency shift.

9. The frequency measurement device based on optical frequency comb and cyclic frequency shift according to claim 8, characterized in that: The opening time of the optical switch (41) is less than or equal to the time required for the optical signal to circulate once in the subsequent circulation structure.

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