A laser phase noise measurement apparatus and method

Abstract

The application discloses a laser phase noise measuring device and method, comprising: a light source to be measured, a first optical coupler, a second optical coupler, an acousto-optic frequency shifter, a first single-mode optical fiber, a third optical coupler, a first polarization controller, a second single-mode optical fiber, a second polarization controller, a fourth optical coupler, a fifth optical coupler, a first balanced photodetector, a second balanced photodetector and a signal acquisition and processing unit. The technical scheme of the application overcomes the mutual restriction problem between sensitivity and bandwidth in the traditional delay interference method, and improves the sensitivity and bandwidth comprehensive performance of the laser phase noise measurement.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical coherent measurement, and particularly relates to a laser phase noise measurement device and method. BACKGROUND

[0002] Optical coherent systems have the advantages of high coherence, energy concentration, high sensitivity, wide measurable objects, and anti-radiation and electromagnetic interference, and have been widely used in laser communication, measurement and optical fiber sensing and other application fields. The performance of the optical coherent system is affected by many factors, among which the spectral purity of the laser, also known as the laser linewidth, plays a crucial role in coherence. The phase noise of the laser is the main form of the laser coherence, and the main sources of the laser phase noise are the instability of atomic transition, spontaneous emission noise, environmental noise and pump noise when the optical cavity resonates, which exist widely in any laser signal and directly affect the laser coherence and stability, and are important factors that determine the performance of the optical coherent system, such as precision, accuracy and measurable distance. Therefore, in order to accurately characterize the influence of the laser coherence on the optical coherent system, it is important to accurately and effectively analyze and measure the laser coherence, i.e. the phase noise, for the performance evaluation of the optical coherent system. Generally speaking, according to different characterization methods, the laser linewidth or phase, frequency noise spectrum can be measured to evaluate the laser coherence. In particular, the phase noise spectrum can provide more frequency domain noise distribution information than the linewidth, and the laser linewidth can be analyzed by the phase noise spectrum. At present, typical phase noise spectrum measurement methods include double-beam heterodyne frequency mixing and delay self-coherence. The double-beam heterodyne frequency mixing method relies on a narrow linewidth laser as a reference, and the linewidth of the narrow laser is much better than that of the to-be-measured laser. The to-be-measured light source outputs light at a fixed frequency, and the phase noise of the to-be-measured laser is characterized on the intensity change by using a narrow linewidth reference laser. Specifically, a high-speed photodetector obtains the intensity of the beat frequency signal of the two lasers, and an electrical spectrum analyzer is used for analysis. Due to the narrow linewidth characteristic of the reference laser, its phase noise can be ignored, and we can approximately evaluate the phase noise of the to-be-measured laser by analyzing the beat frequency signal. However, this method requires high stability of the frequency and amplitude of the laser, which makes the experimental system complex. In view of the inconvenience of the double-beam heterodyne frequency mixing method requiring an additional high-stability light source, the delay self-coherence method as a single light source measurement method, which divides the to-be-measured light source into two paths with different time delays, uses the beat frequency signal obtained by the self-heterodyne characteristic to measure the laser linewidth and phase noise, avoids the need for an additional high-stability independent light source or an additional to-be-measured laser, and simplifies the requirements for the test conditions compared with the double-beam heterodyne frequency mixing method, and is suitable for a wider range of application scenarios.

[0003] The delay self-heterodyne method includes a long delay self-heterodyne method and a short delay self-heterodyne method. The long delay self-heterodyne technique is a non-coherent optical interferometry method. This method requires the delay length of the optical fiber to be much larger than the coherence time of the laser, specifically, at least six times the coherence length. After the laser passes through the long delay, it is no longer coherent with the local oscillator, and their phases are independent of each other, but the statistical characteristics are the same. After the laser signal passes through the long delay self-heterodyne to extract the phase noise of the beat frequency signal, the calculated phase noise spectrum divided by 2 is the phase noise spectrum of the laser. Limited by the non-coherent measurement mechanism of the long delay self-heterodyne method, the minimum Fourier frequency resolution of the frequency noise spectrum is limited by the inverse of the optical fiber delay, which means that frequency noise lower than the Fourier frequency resolution cannot be measured.

[0004] The short delay self-heterodyne method is a partially coherent optical interferometry method, and the ability of the method to extract laser phase noise is significantly affected by the delay difference of the interference loop, which is less than the coherence time of the laser. The delay difference not only defines the frequency range in which the phase noise can be effectively extracted, but also determines the gain level of the phase noise in the frequency range. When the Fourier frequency is much smaller than the frequency range, the gain is flat and proportional to the loop delay. However, once the Fourier frequency approaches the frequency range, the gain presents periodic attenuation and oscillation, so the phase noise cannot be effectively extracted and analyzed. Its effect is equivalent to the transfer function of the unbalanced Mach-Zehnder interferometer system, which helps us understand the extraction of phase noise by the system under different loop delay differences. Therefore, for phase noise measurement, a trade-off needs to be made between high-frequency phase noise measurement capability and maintaining the sensitivity of the system to phase noise. In order to ensure that high-frequency phase noise can be measured without losing any information, we need to choose a smaller loop delay difference. This can obtain a wider frequency range, so that the system can process higher frequency signals. However, reducing the loop delay difference, while being beneficial to expanding the frequency range, will also make the gain of the system (sensitivity to phase noise) smaller. This means that the phase noise is easily overwhelmed by other random noise in the system, affecting the accuracy of the extraction of the phase noise.

[0005] In summary, the current mainstream laser phase noise spectrum measurement method faces the contradiction between sensitivity and bandwidth, and it is difficult to accurately measure laser phase noise covering a larger frequency range SUMMARY

[0006] The technical problem solved by the present application is to provide a laser phase noise measurement device and method, which overcomes the mutual restriction of sensitivity and bandwidth in the traditional delay interference method, and improves the comprehensive performance such as sensitivity and bandwidth of the laser phase noise measurement.

[0007] To achieve the above object, the present application adopts the following technical scheme:

[0008] The laser phase noise measurement device comprises a to-be-measured light source, a first optical coupler, a second optical coupler, an acousto-optic frequency shifter, a first single-mode optical fiber, a third optical coupler, a first polarization controller, a second single-mode optical fiber, a second polarization controller, a fourth optical coupler, a fifth optical coupler, a first balanced photodetector, a second balanced photodetector and a signal acquisition and processing unit.

[0009] The to-be-measured light source is used for providing a light source for the system and is connected with the first optical coupler.

[0010] The first optical coupler is used for dividing the signal light into local light and probe light.

[0011] The second optical coupler is used for dividing the local light into first local light and second local light, which are respectively interfered with the first probe light and the second probe light.

[0012] The acousto-optic frequency shifter is used for shifting the frequency of the probe light to a high frequency.

[0013] The first single-mode optical fiber is used for providing a delay τ1 close to the coherence time of the to-be-measured light source, to form a non-equal-arm interferometer.

[0014] The third optical coupler is used for dividing the probe light into the first probe light and the second probe light, which respectively enter two delay self-coherence loops.

[0015] The first polarization controller is used for adjusting the polarization state of the first probe light and is connected with the fourth optical coupler.

[0016] The second single-mode optical fiber is used for providing an additional delay τ2 for the second probe light, and the length of the second single-mode optical fiber is much smaller than that of the first single-mode optical fiber.

[0017] The second polarization controller is used for adjusting the polarization state of the second probe light and is connected with the fifth optical coupler.

[0018] The fourth optical coupler is used for interfering the first local light with the first probe light to form a first delay self-coherence heterodyne signal, and is connected with the first balanced photodetector.

[0019] A fifth coupler is configured to interfere the second local light and the second probe light to form a second delayed self-coherent heterodyne signal, and is connected to the second balanced detector;

[0020] A first balanced detector is configured to convert the first delayed self-coherent heterodyne signal into an electrical signal, and is connected to the signal acquisition and processing module;

[0021] A second balanced detector is configured to convert the second delayed self-coherent heterodyne signal into an electrical signal, and is connected to the signal acquisition and processing unit;

[0022] A signal acquisition and processing unit is configured to analyze and process the signals, and is connected to the first balanced detector and the second balanced detector.

[0023] Preferably, the first optical coupler is a 90:10 optical coupler, 90% of which is used as the probe light, and 10% of which is used as the local light; the second optical coupler, the third optical coupler, the fourth optical coupler and the fifth optical coupler can be 50:50 optical couplers.

[0024] Preferably, the length of the first single-mode optical fiber should be close to the coherence length of the light source to be measured, and the length of the second single-mode optical fiber should be much smaller than that of the first single-mode optical fiber.

[0025] Preferably, the frequency shift Δf of the acousto-optic frequency shifter is 40 MHz.

[0026] The application also provides a laser phase noise measurement method, comprising the following steps:

[0027] Firstly, a first delayed self-coherent heterodyne signal and a second delayed self-coherent heterodyne signal are acquired;

[0028] Secondly, based on Hilbert transform and IQ demodulation, the phase of the first delayed self-coherent heterodyne signal is extracted and the phase of the second delayed self-coherent heterodyne signal is extracted

[0029] Thirdly, the obtained phases are de-skewed, i.e. the phase 2πΔft caused by the acousto-optic frequency shifter is eliminated, to obtain a first beat frequency phase noise item and a second beat frequency phase noise item wherein τ2 is much smaller than τ1; wherein, wherein ： is the phase noise to be measured;

[0030] Fourthly, the first beat frequency phase noise item and the second beat frequency phase noise item are added and subtracted to obtain a common-mode phase and a differential phase wherein,

[0031] Fifth, the combination of the above common mode phase and differential phase extraction phase noise

[0032] As preferred, when Fourier frequency f is much smaller than 1 / τ2,

[0033] As preferred, the integral term corresponds to the frequency domain, that is, a low-pass filter equivalent to the integrator, after Laplace transform, there is:

[0034]

[0035] Where, τ is the average of the delay of the two loops (2τ1+τ2) / 2;

[0036] The measured phase noise The bandwidth of the measured phase noise is extended from 1 / τ1 of the long delay self-coherent method to 1 / τ2.

[0037] Compared with the prior art, the present application has the following advantages and technical effects:

[0038] 1. The present application uses long optical fiber delay interference to measure laser phase noise, provides high gain for laser phase noise acquisition, reduces the influence of other noises on phase noise extraction, and ensures the extraction sensitivity of phase noise.

[0039] 2. The final calculation of laser phase noise in the present application is realized through the data of double-loop delay interference, which improves the measurable bandwidth of laser phase noise and realizes effective acquisition of high-frequency phase noise.

[0040] 3. The laser phase noise measurement method and device based on double-loop delay interference provided by the present application have higher precision and accuracy in phase noise measurement and extraction, and can provide technical support for optical coherent communication, coherent measurement, optical fiber sensing and other fields. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and those skilled in the art can obtain other drawings according to the provided drawings without creative labor.

[0042] Figure 1 The structure diagram of the laser phase noise measurement device of the present application embodiment is shown in the figure.

[0043] Figure 2This is a flowchart of the laser phase noise measurement method according to an embodiment of the present invention;

[0044] The components include: 1. Light source under test; 2. First optical coupler; 3. Second optical coupler; 4. Acousto-optic frequency shifter; 5. First single-mode fiber; 6. Third coupler; 7. First polarization controller; 8. Second single-mode fiber; 9. Second polarization controller; 10. Fourth optical coupler; 11. Fifth optical coupler; 12. First balanced photodetector; 13. Second balanced photodetector; and 14. Signal acquisition and processing unit. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] Example 1:

[0048] like Figure 1 As shown, this embodiment of the invention provides a laser phase noise measurement device, comprising: a light source under test, a first optical coupler, a second optical coupler, an acousto-optic frequency shifter, a first single-mode optical fiber, a third optical coupler, a first polarization controller, a second single-mode optical fiber, a second polarization controller, a fourth optical coupler, a fifth optical coupler, a first balanced photodetector, a second balanced photodetector, and a signal acquisition and processing unit; wherein,

[0049] The light source under test is used to provide light to the system and is connected to the first optical coupler;

[0050] The first optical coupler is used to split the signal light into local oscillator light and probe light;

[0051] The second optical coupler splits the local oscillator light into a first local oscillator light and a second local oscillator light, which interfere with the first probe light and the second probe light, respectively.

[0052] an acousto-optic frequency shifter is used to shift the probe light to a higher frequency;

[0053] The first single-mode fiber is used to provide a delay τ1 close to the coherence time of the light source under test, forming a non-equidistant interferometer;

[0054] The third optical coupler is used to split the probe light into a first probe light and a second probe light, which then enter two delayed autocoherent loops respectively.

[0055] The first polarization controller is used for adjusting the polarization state of the first probe light and is connected with the fourth optical coupler.

[0056] The second single-mode optical fiber is used for providing the second probe light with an additional delay τ2 and has a length far less than that of the first single-mode optical fiber.

[0057] The second polarization controller is used for adjusting the polarization state of the second probe light and is connected with the fifth optical coupler.

[0058] The fourth optical coupler is used for interfering the first local light and the first probe light to form a first delayed self-coherent heterodyne signal and is connected with the first balanced detector.

[0059] The fifth optical coupler is used for interfering the second local light and the second probe light to form a second delayed self-coherent heterodyne signal and is connected with the second balanced detector.

[0060] The first balanced detector is used for converting the first delayed self-coherent heterodyne signal into an electric signal and is connected with the signal acquisition and processing module.

[0061] The second balanced detector is used for converting the second delayed self-coherent heterodyne signal into an electric signal and is connected with the signal acquisition and processing module.

[0062] The signal acquisition and processing module is used for analyzing and processing signals and is connected with the first balanced detector and the second balanced detector.

[0063] As an embodiment of the present application, the signal light emitted by the light source to be measured is divided into local light and probe light through the first optical coupler. The local light is divided into first local light and second local light through the second optical coupler. The probe light is injected into the first single-mode optical fiber after passing through the acousto-optic frequency shifter, and the length of the first single-mode optical fiber is close to the coherence length of the light source to be measured, thereby forming a non-equal-arm interferometer. Then, the probe light is divided into first probe light and second probe light through the third optical coupler. The second probe light also needs to pass through the second single-mode optical fiber, which is used for providing the second probe light with an additional delay τ2 and has a length far less than that of the first single-mode optical fiber. The polarization controller is used for controlling the polarization state and matching the polarization states of the probe path and the local path. The second polarization controller is used for adjusting the polarization state of the second probe light and is connected with the fifth optical coupler. Then, the first local light and the first probe light are combined through the fourth optical coupler, and the second local light and the second probe light are combined through the fifth optical coupler. After the probe light and the local light interfere, the delayed self-coherent heterodyne signals of the two loops are converted into electric signals, and then the signal processor is connected for signal processing.

[0064] As an embodiment of the present application, the first optical coupler can be a 90:10 optical coupler, 90% of which is used as probe light and 10% of which is used as local light. The second optical coupler, the third optical coupler, the fourth optical coupler and the fifth optical coupler can be 50:50 optical couplers.

[0065] As an embodiment of the present application, the length of the first single-mode optical fiber should be close to the coherence length of the light source to be measured, and the length of the second single-mode optical fiber should be much smaller than that of the first single-mode optical fiber.

[0066] As an embodiment of the present application, the frequency shift Δf of the acousto-optic frequency shifter can be 40 MHz, so as to minimize the influence of other low-frequency noises.

[0067] Embodiment 2

[0068] As shown in Figure 2 the present application also provides a laser phase noise measurement method, comprising:

[0069] First, collect the time-domain signals of the two delay self-coherent loops, which are:

[0070]

[0071] wherein τ1 represents the time delay of the first single-mode optical fiber, τ2 represents the time delay of the second single-mode optical fiber, represents the first beat frequency phase noise; the second beat frequency phase noise term; and Δf is the frequency shift of the acousto-optic frequency shifter;

[0072] Second, based on Hilbert transform and IQ demodulation, extract the first delay self-coherent heterodyne signal phase and the second delay self-coherent heterodyne signal phase, which are:

[0073]

[0074] Third, remove the skew of the obtained phase, i.e. eliminate the phase 2πΔft caused by the acousto-optic frequency shifter, where Δf is the frequency shift of the acousto-optic frequency shifter, to obtain the first beat frequency phase noise term and the second beat frequency phase noise term wherein τ2 is much smaller than τ1; wherein:

[0075]

[0076] Fourth, add and subtract the first beat frequency phase noise term and the second beat frequency phase noise term to obtain the common-mode phase and the differential phase

[0077]

[0078] Fifth, combine the above common-mode phase and differential phase to extract the phase noise When the Fourier frequency f is much smaller than 1 / τ2, we have:

[0079]

[0080] Wherein, the integral term corresponds to the frequency domain, namely a low-pass filter equivalent to the integrator, after Laplace transform:

[0081]

[0082] Wherein, τ is the average value of the delay sum of two loops (2τ1+τ2) / 2.

[0083] Therefore, the measurable phase noise The bandwidth of the measurable phase noise is expanded from 1 / τ1 of the long delay self-coherent method to 1 / τ2.

[0084] The above-described embodiments are merely descriptions of the preferred modes of the present application, and are not intended to limit the scope of the present application. Various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application as defined by the claims.

Claims

1. A laser phase noise measurement device, characterized in that, include: The system includes a light source under test, a first optical coupler, a second optical coupler, an acousto-optic frequency shifter, a first single-mode fiber, a third optical coupler, a first polarization controller, a second single-mode fiber, a second polarization controller, a fourth optical coupler, a fifth optical coupler, a first balanced photodetector, a second balanced photodetector, and a signal acquisition and processing unit; among which, The light source under test is used to provide light to the system and is connected to the first optical coupler; The first optical coupler is used to split the signal light into local oscillator light and probe light; The second optical coupler splits the local oscillator light into a first local oscillator light and a second local oscillator light, which interfere with the first probe light and the second probe light, respectively. an acousto-optic frequency shifter is used to shift the probe light to a higher frequency; The first single-mode fiber is used to provide a delay τ1 close to the coherence time of the light source under test, forming a non-equidistant interferometer; The third optical coupler is used to split the probe light into a first probe light and a second probe light, which then enter two delayed autocoherent loops respectively. The first polarization controller, used to adjust the polarization state of the first probe light, is connected to the fourth optical coupler; The second single-mode fiber is used to provide an additional delay τ2 for the second probe light, and its length is much smaller than that of the first single-mode fiber. The second polarization controller, used to adjust the polarization state of the second probe light, is connected to the fifth optical coupler; The fourth optical coupler is used to interfere the first local oscillator light and the first probe light to form a first delayed autocoherent heterodyne signal, which is connected to the first balanced photodetector. The fifth optical coupler is used to interfere with the second local oscillator light and the second probe light to form a second delayed autocoherent heterodyne signal, which is then connected to the second balanced photodetector. The first balanced photodetector converts the first delayed autocoherent heterodyne signal into an electrical signal and connects it to the signal acquisition and processing module. The second balanced photodetector converts the second delayed autocoherent heterodyne signal into an electrical signal and connects it to the signal acquisition and processing unit. The signal acquisition and processing unit is used to analyze and process signals and is connected to the first balanced photodetector and the second balanced photodetector.

2. The laser phase noise measurement device as described in claim 1, characterized in that, The first optical coupler is a 90:10 optical coupler, with 90% of it used as probe light and 10% as local oscillator light; the second, third, fourth, and fifth optical couplers are 50:50 optical couplers.

3. The laser phase noise measurement device as described in claim 2, characterized in that, The length of the first single-mode fiber should be close to the coherence length of the light source under test.

4. The laser phase noise measurement device as described in claim 3, characterized in that, The frequency shift Δƒ of the acousto-optic frequency shifter is 40MHz.

5. A method for measuring laser phase noise, characterized in that, Includes the following steps: The first step is to acquire the first delayed self-coherent heterodyne signal and the second delayed self-coherent heterodyne signal; The second step involves extracting the phase of the first-delayed autocoherent heterodyne signal based on Hilbert transform and IQ demodulation. φ Phase 1 and the second delayed self-coherent heterodyne signal φ 2; The third step is to de-skew the obtained phase, that is, to eliminate the phase 2πΔƒ introduced by the acousto-optic frequency shifter. t Where Δƒ is the frequency shift of the acousto-optic frequency shifter, the first beat frequency phase noise term Δ is obtained. φ ( t , τ 1) and the second beat frequency phase noise term Δ φ ( t , τ 1+ τ 2), where, τ 2 is much smaller than τ 1; where Δ φ ( t , τ 1) = φ ( t )- φ ( t - τ 1), Δ φ ( t , τ 1+ τ 2) = φ ( t )- φ ( t - τ 1- τ 2), where: φ ( t ) represents the phase noise to be measured; τ1 represents the time delay of the first single-mode fiber, and τ2 represents the time delay of the second single-mode fiber; The fourth step involves adding and subtracting the first and second beat frequency phase noise terms to obtain the common-mode phase Δ. φ + (t) and differential phase Δ φ - (t); where Δ φ + (t)=Δ φ (t,τ1)+Δ φ (t, τ1+τ2), Δ φ - (t)=Δ φ (t,τ1)-Δ φ (t, τ1+τ2); The fifth step is to combine the common-mode phase and differential phase to extract phase noise. φ (t).

6. The laser phase noise measurement method as described in claim 5, characterized in that, When Fourier frequency f When much smaller than 1 / τ2, .

7. The laser phase noise measurement method as described in claim 6, characterized in that, The integral term corresponds to a low-pass filter in the frequency domain, which is equivalent to an integrator. After the Laplace transform, we have: ； in, τ The sum of the delays of the two loops is (2τ1+τ2) / 2; Measured phase noise φ ( t The bandwidth of the measurable phase noise bandwidth of the long-delay autocoherent method is 1 / τ 1 expanded to 1 / τ 2.

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