Laser phase noise measuring device and method

Abstract

The invention 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 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 photoelectric detector, a second balanced photoelectric detector and a signal acquisition and processing unit. By adopting the technical scheme of the invention, the problem of mutual restriction of sensitivity and bandwidth in a traditional delay interference method is overcome, and the sensitivity and bandwidth comprehensive performance of laser phase noise measurement is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical coherence measurement, and in particular relates to a laser phase noise measurement device and method. Background Art

[0002] Optical coherent systems have the advantages of high coherence, concentrated energy, high sensitivity, a wide range of measurable objects, and resistance to radiation and electromagnetic interference. They have been widely used in many application fields such as laser communication, measurement, and fiber optic sensing. The performance of optical coherent systems is affected by many factors, among which the spectral purity of the laser, also known as the laser linewidth, plays a crucial role in its coherence. Laser phase noise is the main manifestation of laser coherence. The main sources of laser phase noise are the instability of atomic transitions during cavity resonance, spontaneous radiation noise, environmental noise, and pump noise. It is widely present in any laser signal and directly affects the laser coherence and stability. It is an important factor that determines the performance of optical coherent systems, such as key indicators such as precision, accuracy, and measurable distance. Therefore, in order to accurately characterize the impact of laser coherence performance on optical coherent systems, it is crucial to accurately and effectively analyze and measure laser coherence, that is, its phase noise, for the performance evaluation of optical coherent systems. Generally speaking, depending on the different characterization methods, it can be evaluated by measuring the laser linewidth or phase and frequency noise spectrum. In particular, the phase noise spectrum can provide more frequency domain noise distribution information than the linewidth, and the laser linewidth can also be obtained through phase noise spectrum analysis. At present, typical phase noise spectrum measurement methods include: dual-beam heterodyne beat frequency and delayed self-coherence. The dual-beam heterodyne beat frequency method relies on a narrow linewidth laser as a reference, and the linewidth of this narrow laser is much better than the laser to be tested. The light source to be tested outputs light of a fixed frequency, and a narrow linewidth reference laser is used to characterize the phase noise of the laser to be tested in terms of intensity changes. Specifically, a high-speed photodetector obtains the intensity of the beat frequency signal of the two lasers and analyzes it using an electrical spectrometer. Thanks to the narrow linewidth characteristics of the reference laser, its phase noise can be ignored, and we can approximately evaluate the phase noise of the laser to be tested by analyzing this beat frequency signal. However, this method places extremely high demands on the frequency and amplitude stability of the laser, which makes the experimental system complicated. In view of the inconvenience of the heterodyne beat frequency method requiring an additional highly stable light source, the delayed self-coherence method is a single light source measurement method. It interferes with the two paths of light with different delays generated by the splitting of the light source to be measured, and uses the beat frequency signal obtained from the self-heterodyne characteristics to measure the laser line width and phase noise, avoiding the need for an additional highly stable independent light source or an additional laser to be measured. Compared with the heterodyne beat frequency method, it simplifies the requirements for test conditions and is suitable for a wider range of application scenarios.

[0003] The delayed self-coherence method includes the long-delay self-coherence method and the short-delay self-coherence method. The long-delay self-coherence technology is an incoherent light interference measurement method. This method requires that the delay length of the optical fiber is much longer than the coherence time of the laser. Specifically, it needs to be at least six times the coherence length. Since the laser is no longer coherent with the local oscillator after a long delay, their phases are independent of each other, but their statistical characteristics are the same. After the laser signal is subjected to long-delay self-coherence to extract the phase noise of the beat signal, the calculated phase noise spectrum is divided by 2 to obtain the phase noise spectrum of the laser. Due to the incoherent measurement mechanism of the long-delay self-coherence method, the minimum Fourier frequency resolution of its frequency noise spectrum is limited by the inverse of the fiber delay, which means that frequency noise lower than the Fourier frequency resolution cannot be measured.

[0004] The short-delay self-coherence method is a partially coherent optical interferometry method. The ability of this 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. This delay difference not only defines the frequency range in which phase noise can be effectively extracted, but also determines the gain level of phase noise in this frequency range. When the Fourier frequency is much smaller than this frequency range, its gain is flat and proportional to the loop delay. However, once the Fourier frequency approaches this frequency range, its gain shows periodic decay and oscillation, so it is impossible to effectively extract and analyze the phase noise. Its impact 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 the measurement of phase noise, it is necessary to make a trade-off between the ability to measure high-frequency phase noise and maintain the system's sensitivity 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. Doing so can obtain a wider frequency range, allowing the system to process higher frequency signals. However, while reducing the loop delay difference is beneficial to expanding the frequency range, it will also reduce the system gain (sensitivity to phase noise), which means that the phase noise can be easily submerged by other random noises in the system, thus affecting the accuracy of phase noise extraction.

[0005] In summary, the current mainstream laser phase noise spectrum measurement method faces the contradiction of difficulty in achieving both sensitivity and bandwidth, and it is difficult to accurately measure the laser phase noise covering a larger frequency range. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a laser phase noise measurement device and method, which overcomes the problem of mutual restriction between sensitivity and bandwidth in the traditional delay interference method, and improves the comprehensive performance of laser phase noise measurement such as sensitivity and bandwidth. The laser phase noise measurement method and device based on dual-loop delay interference proposed in the present invention have higher accuracy in the measurement of laser phase noise, and can provide effective technical support for multiple fields such as optical coherent communication, coherent measurement and optical fiber sensing.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] A laser phase noise measurement device comprises: 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 processing unit; wherein,

[0009] A light source to be tested, used to provide light source for the system and connected to the first optical coupler;

[0010] A first optical coupler, used for dividing the signal light into a local oscillator light and a detection light;

[0011] A second optical coupler is used to separate the local oscillation light into a first local oscillation light and a second local oscillation light, which interfere with the first detection light and the second detection light respectively;

[0012] an acousto-optic frequency shifter, used to shift the frequency of the probe light to a high frequency;

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

[0014] A third optical coupler is used to divide the detection light into a first detection light and a second detection light, and respectively enter two delayed self-coherent loops;

[0015] a first polarization controller, used for adjusting the polarization state of the first detection light, connected to the fourth optical coupler;

[0016] A second single-mode optical fiber is used to provide an additional delay τ2 for the second detection light, and its length is much shorter than that of the first single-mode optical fiber;

[0017] a second polarization controller, used for adjusting the polarization state of the second detection light, connected to the fifth optical coupler;

[0018] a fourth optical coupler, used for interfering the first local oscillator light and the first detection light to form a first delayed self-coherent heterodyne signal, and connected to the first balanced detector;

[0019] a fifth coupler, used for interfering the second local oscillator light and the second detection light to form a second delayed self-coherent heterodyne signal, and connected to the second balanced detector;

[0020] A first balanced detector, which converts 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 converts the second delayed self-coherent heterodyne signal into an electrical signal and is connected to the signal acquisition and processing unit;

[0022] The signal acquisition and processing unit is used for analyzing and processing the signal 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, with 90% of the light being used as detection light and 10% of the light being used as local oscillator light; the second optical coupler, the third optical coupler, the fourth optical coupler and the fifth optical coupler may 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 shorter than that of the first single-mode optical fiber.

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

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

[0027] The first step is to acquire a first delayed self-coherent heterodyne signal and a second delayed self-coherent heterodyne signal;

[0028] The second step is to extract the phase of the first delayed self-coherent heterodyne signal based on Hilbert transform and IQ demodulation. and the second delayed self-coherent heterodyne signal phase

[0029] The third step is to de-skew the obtained phase, that is, to eliminate the phase 2πΔft caused by the acousto-optic frequency shifter, where Δf is the frequency shift of the acousto-optic frequency shifter, and obtain the first beat frequency phase noise term and the second beat frequency phase noise term Among them, τ2 is much smaller than τ1; among them, in: is the phase noise to be measured;

[0030] In the fourth step, the first beat frequency phase noise term and the second beat frequency phase noise term are added and subtracted to obtain the common mode phase and differential phase in,

[0031] Step 5: Combine the common-mode phase and differential phase to extract phase noise

[0032] Preferably, when the Fourier frequency f is much smaller than 1 / τ2,

[0033] As a preferred embodiment, the integral term corresponds to a low-pass filter equivalent to an integrator in the frequency domain. After Laplace transformation, there is:

[0034]

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

[0036] Measured Phase Noise The bandwidth is extended from the measurable phase noise bandwidth 1 / τ1 of the long delay autocorrelation method to 1 / τ2.

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

[0038] 1. The present invention uses long fiber delay interferometry to measure laser phase noise, which provides a higher gain for obtaining laser phase noise, can reduce the influence of other noises on phase noise extraction, and ensure the extraction sensitivity of phase noise.

[0039] 2. The final calculation of the laser phase noise of the present invention is realized through the data of dual-loop delay interference, which increases the measurable bandwidth of the laser phase noise and can achieve effective acquisition of high-frequency phase noise.

[0040] 3. The laser phase noise measurement method and device based on dual-loop delay interferometry proposed in the present invention have higher precision and accuracy in phase noise measurement and extraction, and can provide technical support for multiple fields such as optical coherent communication, coherent measurement, and fiber optic sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0042] Figure 1 This is a schematic diagram of the structure of a laser phase noise measurement device according to an embodiment of the present invention;

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

[0044] Wherein: 1 is a light source to be measured, 2 is a first optical coupler, 3 is a second optical coupler, 4 is an acousto-optic frequency shifter, 5 is a first single-mode optical fiber, 6 is a third coupler, 7 is a first polarization controller, 8 is a second single-mode optical fiber, 9 is a second polarization controller, 10 is a fourth optical coupler, 11 is a fifth optical coupler, 12 is a first balanced photodetector, 13 is a second balanced photodetector and 14 is a signal acquisition and processing unit. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] Embodiment 1:

[0048] like Figure 1 As shown, an embodiment of the present invention provides a laser phase noise measurement device, 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 processing unit; wherein,

[0049] A light source to be tested, used to provide light source for the system and connected to the first optical coupler;

[0050] A first optical coupler, used for dividing the signal light into a local oscillator light and a detection light;

[0051] A second optical coupler is used to separate the local oscillation light into a first local oscillation light and a second local oscillation light, which interfere with the first detection light and the second detection light respectively;

[0052] an acousto-optic frequency shifter, used to shift the frequency of the probe light to a high frequency;

[0053] The first single-mode optical fiber is used to provide a delay τ1 close to the coherence time of the light source to be measured, so as to form a non-equal-arm interferometer;

[0054] A third optical coupler is used to divide the detection light into a first detection light and a second detection light, and respectively enter two delayed self-coherent loops;

[0055] a first polarization controller, used for adjusting the polarization state of the first detection light, connected to the fourth optical coupler;

[0056] A second single-mode optical fiber is used to provide an additional delay τ2 for the second detection light, and its length is much shorter than that of the first single-mode optical fiber;

[0057] a second polarization controller, used for adjusting the polarization state of the second detection light, connected to the fifth optical coupler;

[0058] a fourth optical coupler, used for interfering the first local oscillator light and the first detection light to form a first delayed self-coherent heterodyne signal, and connected to the first balanced detector;

[0059] a fifth coupler, used for interfering the second local oscillator light and the second detection light to form a second delayed self-coherent heterodyne signal, and connected to the second balanced detector;

[0060] A first balanced detector, which converts the first delayed self-coherent heterodyne signal into an electrical signal and is connected to the signal acquisition and processing module;

[0061] A second balanced detector converts the second delayed self-coherent heterodyne signal into an electrical signal and is connected to the signal acquisition and processing unit;

[0062] The signal acquisition and processing unit is used for analyzing and processing the signal and is connected to the first balanced detector and the second balanced detector.

[0063] As an implementation method of an embodiment of the present invention, the light source to be measured emits a signal light, which is divided into a local oscillator light and a detection light through a first optical coupler. The local oscillator light is divided into a first local oscillator light and a second local oscillator light through a second optical coupler. The detection light is injected into the first single-mode optical fiber after passing through an acousto-optic frequency shifter, and its length is close to the coherence length of the light source to be measured, forming a non-equal arm interferometer. Then the detection light passes through a third optical coupler to divide the detection light into a first detection light and a second detection light. The second detection light also needs to pass through a second single-mode optical fiber to provide an additional delay to the second detection light, and its length is much shorter than the first single-mode optical fiber. The polarization controller is used to control the polarization state and match the polarization state of the detection path and the local oscillator path. The second polarization controller is used to adjust the polarization state of the second detection light and is connected to the fifth optical coupler. Then the first local oscillator light and the first detection light are combined after passing through the fourth optical coupler, and the second local oscillator light and the second detection light are combined after passing through the fifth optical coupler. After the interference of the detection light and the local oscillator light, the delayed self-coherent heterodyne signals of the two loops are converted into electrical signals, which are then connected to the data acquisition and processor for signal processing.

[0064] As an implementation of the embodiment of the present invention, the first optical coupler may be a 90:10 optical coupler, 90% of which is used as detection light and 10% of which is used as local oscillator light; the second optical coupler, the third optical coupler, the fourth optical coupler and the fifth optical coupler may be 50:50 optical couplers;

[0065] As an implementation manner of the embodiment of the present invention, 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 shorter than the first single-mode optical fiber.

[0066] As an implementation of the embodiment of the present invention, the frequency shift Δf of the acousto-optic frequency shifter may be 40 MHz to minimize the influence of other low-frequency noise.

[0067] Embodiment 2:

[0068] like Figure 2 As shown, an embodiment of the present invention further provides a laser phase noise measurement method, comprising:

[0069] The first step is to acquire the time domain signals of two delayed 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; Δf is the frequency shifting frequency of the acousto-optic frequency shifter;

[0072] In the second step, based on Hilbert transform and IQ demodulation, the phase of the first delayed self-coherent heterodyne signal and the phase of the second delayed self-coherent heterodyne signal are extracted as follows:

[0073]

[0074] The third step is to de-skew the obtained phase, that is, to eliminate the phase 2πΔft caused by the acousto-optic frequency shifter, where Δf is the frequency shift of the acousto-optic frequency shifter, and obtain the first beat frequency phase noise term and the second beat frequency phase noise term Among them, τ2 is much smaller than τ1; among them:

[0075]

[0076] In the fourth step, the first beat frequency phase noise term and the second beat frequency phase noise term are added and subtracted to obtain the common mode phase and differential phase

[0077]

[0078] Step 5: Combine the common-mode phase and differential phase to extract phase noise When the Fourier frequency f is much smaller than 1 / τ2, we have:

[0079]

[0080] Among them, the integral term corresponds to a low-pass filter equivalent to an integrator in the frequency domain, and after Laplace transform, it is:

[0081]

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

[0083] Therefore, the measurable phase noise The bandwidth is extended from the measurable phase noise bandwidth 1 / τ1 of the long delay autocorrelation method to 1 / τ2.

[0084] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A laser phase noise measurement device, characterized in that: include: 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 processing unit; wherein, A light source to be tested, used to provide light source for the system and connected to the first optical coupler; A first optical coupler, used for dividing the signal light into a local oscillator light and a detection light; A second optical coupler is used to separate the local oscillation light into a first local oscillation light and a second local oscillation light, which interfere with the first detection light and the second detection light respectively; an acousto-optic frequency shifter, used to shift the frequency of the probe light to a high frequency; The first single-mode optical fiber is used to provide a delay τ1 close to the coherence time of the light source to be measured, so as to form a non-equal-arm interferometer; A third optical coupler is used to divide the detection light into a first detection light and a second detection light, and respectively enter two delayed self-coherent loops; a first polarization controller, used for adjusting the polarization state of the first detection light, connected to the fourth optical coupler; A second single-mode optical fiber is used to provide an additional delay τ2 for the second detection light, and its length is much shorter than that of the first single-mode optical fiber; a second polarization controller, used for adjusting the polarization state of the second detection light, connected to the fifth optical coupler; a fourth optical coupler, used for interfering the first local oscillator light and the first detection light to form a first delayed self-coherent heterodyne signal, and connected to the first balanced detector; a fifth coupler, used for interfering the second local oscillator light with the second detection light to form a second delayed self-coherent heterodyne signal, and connected to the second balanced detector; A first balanced detector, which converts the first delayed self-coherent heterodyne signal into an electrical signal and is connected to the signal acquisition and processing module; A second balanced detector converts the second delayed self-coherent heterodyne signal into an electrical signal and is connected to the signal acquisition and processing unit; The signal acquisition and processing unit is used for analyzing and processing the signal and is connected to the first balanced detector and the second balanced detector.

2. The laser phase noise measurement device according to claim 1, characterized in that: The first optical coupler is a 90:10 optical coupler, with 90% of the light being used as detection light and 10% of the light being used as local oscillation light; the second optical coupler, the third optical coupler, the fourth optical coupler and the fifth optical coupler may be 50:50 optical couplers.

3. The laser phase noise measurement device according to claim 2, characterized in that: 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 shorter than the first single-mode optical fiber.

4. The laser phase noise measurement device according to claim 3, characterized in that: The shift frequency Δf of the acousto-optic frequency shifter is 40 MHz.

5. A laser phase noise measurement method, characterized in that: The following steps are involved: The first step is to acquire a first delayed self-coherent heterodyne signal and a second delayed self-coherent heterodyne signal; The second step is to extract the phase of the first delayed self-coherent heterodyne signal based on Hilbert transform and IQ demodulation. and the second delayed self-coherent heterodyne signal phase The third step is to de-skew the obtained phase, that is, to eliminate the phase 2πΔft caused by the acousto-optic frequency shifter, where Δf is the frequency shift of the acousto-optic frequency shifter, and obtain the first beat frequency phase noise term and the second beat frequency phase noise term Among them, τ2 is much smaller than τ1; among them, in: is the phase noise to be measured; In the fourth step, the first beat frequency phase noise term and the second beat frequency phase noise term are added and subtracted to obtain the common mode phase and differential phase in, Step 5: Combine the common-mode phase and differential phase to extract phase noise 6. The laser phase noise measurement method according to claim 5, characterized in that: When the Fourier frequency f is much smaller than 1 / τ2, 7. The laser phase noise measurement method according to claim 6, characterized in that: The integral term corresponds to a low-pass filter equivalent to an integrator in the frequency domain. After Laplace transform, we have: Where τ is the average value of the delay sum of the two loops (2τ1+τ2) / 2; Measured Phase Noise The bandwidth is extended from the measurable phase noise bandwidth 1 / τ1 of the long delay autocorrelation method to 1 / τ2.

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