A time-delay self-photofrequency method and device for measuring laser linewidth

Abstract

The application relates to a time-delay self-heterodyne method and device for measuring laser linewidth, the time-delay self-heterodyne method comprising the following steps: dividing the laser to be measured into first local light, second local light and first signal light; inputting the first signal light into a phase compensation delay fiber system, so that the phase compensation delay fiber system extracts the additional phase noise introduced by the delay fiber by using the first local light, and actively compensates the phase noise of the first signal light by using an active locking technology; optically mixing the second local light and the compensated first signal light; coherently detecting the mixed light of the first signal light and the second local light by using a balanced detector, and outputting the mixed light to a spectrum analysis device, and calculating the laser linewidth according to the spectrum diagram given by the spectrum analysis device. The application can actively compensate the phase noise of the first signal light by using the active locking technology, and reduces the influence of environmental temperature, vibration and other factors on the laser linewidth measurement.

Description

Technical Field

[0001] This application relates to the field of laser technology, specifically to a method and apparatus for measuring the delay Selfie frequency of laser linewidth. Background Technology

[0002] Currently, narrow-linewidth lasers possess extremely low phase noise and excellent coherence length, making them crucial for applications in scientific research and engineering; for example, high-precision spectroscopy, optical clocks, and coherent communication. The linewidth of a laser plays a decisive role in the noise performance, measurement range, accuracy, and sensitivity of these systems, making high-precision measurement of the linewidth of ultra-narrow lasers essential.

[0003] In related technologies, there are two main methods for measuring the linewidth of narrow lasers in the kHz range: the beat frequency method and the time-delay beat frequency method. The traditional beat frequency method offers high measurement accuracy, but it is demanding, often requiring a secondary, narrower, and more stable light source as a reference, and the frequency difference between these two independent light sources must be within the detector's measurement bandwidth. The time-delay beat frequency method only requires the laser itself to perform heterodyne beat frequency measurement, eliminating the need for an additional laser. It is a more ideal method for measuring ultra-narrow linewidths, requiring only one light source, resulting in a simple structure, low cost, and suitability for engineering applications. In 1980, Japanese scholar T. Okoshi first proposed using a non-equilateral interferometer to measure laser linewidth. The delay time of the optical fiber must be at least six times greater than the laser coherence time for the beat frequency signal to accurately reflect the linewidth of the laser under test. Therefore, for narrow-linewidth lasers in the kHz range, the required optical fiber delay line needs to be hundreds of kilometers long, increasing the additional noise of the entire testing system and limiting its application in measuring the linewidth of kHz narrow lasers.

[0004] Therefore, it is necessary to design a new method and device for measuring the time-delay Selfie frequency of laser linewidth to overcome the above problems. Summary of the Invention

[0005] This application provides a time-delay Selfie method and apparatus for measuring laser linewidth, which can solve the technical problem in related technologies where the large additional noise of the entire test system limits its application in kHz narrow laser linewidth measurement.

[0006] In a first aspect, embodiments of this application provide a method for measuring the time-delayed Selfie frequency of a laser linewidth, comprising the following steps: The laser beam to be tested is divided into a first local oscillator beam, a second local oscillator beam, and a first signal beam; The first signal light is input into the phase compensation delay fiber system, which uses the first local oscillator light to extract the additional phase noise introduced by the delay fiber, and uses active locking technology to actively compensate for the phase noise of the first signal light. The second local oscillator light is optically mixed with the compensated first signal light; A balanced detector is used to perform coherent detection on the mixed light of the first signal light and the second local oscillator light, and the output is sent to a spectrum analysis device. The laser linewidth is calculated based on the spectrum diagram given by the spectrum analysis device.

[0007] In conjunction with the first aspect, in one embodiment, the step of inputting the first signal light into the phase-compensated delay fiber system, enabling the phase-compensated delay fiber system to extract the additional phase noise introduced by the delay fiber using the first local oscillator light, and using active locking technology to actively compensate for the phase noise of the first signal light, includes: The first signal light is passed through an acousto-optic frequency shifter to shift the laser frequency; The frequency-shifted first signal light is input into the delay fiber for the first delay; The first signal light after the first delay is divided into a first part and a second part. The first part is reflected and then delayed a second time through the delay fiber. The second part is optically mixed with the first local oscillator light to obtain the additional phase noise introduced by the delay fiber. The phase noise of the first signal light is actively compensated based on the additional phase noise introduced by the time-delayed fiber.

[0008] In conjunction with the first aspect, in one embodiment, the step of dividing the first signal light after the first delay into a first part and a second part, the first part being reflected and then delayed a second time via a delay fiber, and the second part being optically mixed with the first local oscillator light to obtain additional phase noise introduced by the delay fiber, includes: The first signal light after the first delay is split into a first part and a second part by a beam splitter. The first part is reflected by a Bragg reflector and re-enters the delay fiber for a second delay. The second part is optically mixed with the first local oscillator light. The difference frequency between the first local oscillator and the laser output from the delayed fiber is measured using a high-speed detector to obtain the difference frequency electrical signal. The difference frequency electrical signal is input into the frequency and phase detector and mixed with the single-frequency microwave signal output from the fixed-frequency microwave source to obtain an additional phase noise signal. The center frequency of the single-frequency microwave signal is the same as that of the difference frequency electrical signal.

[0009] In conjunction with the first aspect, in one embodiment, the step of dividing the first signal light after the first delay into a first part and a second part, the first part being reflected and then delayed a second time via a delay fiber, and the second part being optically mixed with the first local oscillator light to obtain additional phase noise introduced by the delay fiber, includes: The first signal light after the first delay is divided into a first part and a second part by a partial reflector in the optical fiber. The first part is directly reflected and re-enters the delay fiber for a second delay. The second part is optically mixed with the first local oscillator light. The difference frequency between the first local oscillator and the laser output from the delayed fiber is measured using a high-speed detector to obtain the difference frequency electrical signal. The difference frequency electrical signal is input into the frequency and phase detector and mixed with the single-frequency microwave signal output from the fixed-frequency microwave source to obtain an additional phase noise signal. The center frequency of the single-frequency microwave signal is the same as that of the difference frequency electrical signal.

[0010] In conjunction with the first aspect, in one implementation, active compensation for the phase noise of the first signal light based on the additional phase noise introduced by the delay fiber includes: After the additional phase noise signal is input into the servo controller, it is subjected to proportional-integral-differential calculation. The output signal of the servo controller is fed back to the voltage-controlled microwave source, so that the voltage-controlled microwave source acts on the acousto-optic frequency shifter to actively compensate for the phase noise of the first signal light.

[0011] In conjunction with the first aspect, in one implementation, active compensation for the phase noise of the first signal light based on the additional phase noise introduced by the delay fiber includes: After the additional phase noise signal is input into the servo controller, it is subjected to proportional-integral-differential calculation. The output signal of the servo controller is fed back to the fiber stretcher, which changes the length of the fiber in the fiber stretcher and actively compensates for the phase noise of the first signal light.

[0012] Secondly, embodiments of this application provide a time-delay Selfie device for measuring laser linewidth, comprising: An interferometer body, the interferometer body including a first beam splitter, a circulator and a second beam splitter, the first beam splitter being connected to the circulator and the circulator being connected to the second beam splitter; A phase-compensated delay fiber optic system and a spectrum analysis device, wherein the phase-compensated delay fiber optic system is connected to the first beam splitter and the circulator; The first beam splitter is used to split the laser beam under test into a first local oscillator beam input to the phase-compensated delay fiber system, a second local oscillator beam input to the second beam splitter, and a first signal beam input to the circulator. The phase-compensated delay fiber system is used to extract the additional phase noise introduced by the delay fiber using the first local oscillator light, and to actively compensate for the phase noise of the first signal light using active locking technology. The second beam splitter is used to optically mix the second local oscillator light with the first signal light output from the circulator; The interferometer body also includes a balanced detector, which is connected to the second beam splitter and the spectrum analysis device for coherent detection of the mixed light of the first signal light and the second local oscillator light.

[0013] In conjunction with the second aspect, in one embodiment, the phase compensation delay fiber optic system includes a delay fiber optic system, a noise monitoring unit, and a compensation unit, wherein the delay fiber optic system connects the circulator to the noise monitoring unit, and the delay fiber optic system is connected to the compensation unit. The delay fiber system is used to input the first signal light back and forth into the delay fiber. The noise monitoring unit is used to extract the additional phase noise introduced by the delay fiber using the first local oscillator light. The compensation unit is used to actively compensate for the phase noise of the first signal light.

[0014] In conjunction with the second aspect, in one embodiment, the time-delay fiber system includes an acousto-optic frequency shifter, a time-delay fiber, a third beam splitter, a fourth beam splitter, and a Bragg reflector; the noise monitoring unit includes a high-speed detector, a frequency and phase detector, and a fixed-frequency microwave source; and the compensation unit includes a servo controller and a voltage-controlled microwave source. The acousto-optic frequency shifter connects the delay fiber to the circulator, the third beam splitter connects to the delay fiber, and the third beam splitter connects to the fourth beam splitter and the Bragg reflector. The fourth beam splitter connects to the first beam splitter and the high-speed detector. The high-speed detector is also connected to the frequency and phase detector, which is connected to the fixed-frequency microwave source and the servo controller. The servo controller is connected to the acousto-optic frequency shifter through the voltage-controlled microwave source.

[0015] In conjunction with the second aspect, in one embodiment, the time-delay fiber system includes an acousto-optic frequency shifter, a time-delay fiber, a fiber partial reflector, and a fourth beam splitter; the noise monitoring unit includes a high-speed detector, a frequency and phase detector, and a fixed-frequency microwave source; and the compensation unit includes a servo controller and a fiber stretcher. The acousto-optic frequency shifter connects the fiber stretcher and the circulator; the delay fiber connects the fiber stretcher and the fiber partial reflector; the fourth beam splitter connects the fiber partial reflector and the first beam splitter; the fourth beam splitter also connects to the high-speed detector; the high-speed detector connects to the frequency and phase detector; the frequency and phase detector is connected to the acousto-optic frequency shifter via the fixed-frequency microwave source; and the frequency and phase detector is also connected to the fiber stretcher via the servo controller.

[0016] The beneficial effects of the technical solutions provided in this application include: By dividing the laser under test into a first local oscillator beam, a second local oscillator beam, and a first signal beam, and inputting the first signal beam into a phase-compensated delay fiber system, the phase-compensated delay fiber system can extract the additional phase noise introduced by the delay fiber using the first local oscillator beam and use active locking technology to actively compensate for the phase noise of the first signal beam. This can reduce the influence of environmental temperature, vibration and other factors on laser linewidth measurement, and solve the technical problem that the large additional noise of the entire test system in related technologies limits its application in kHz narrow laser linewidth measurement. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating a delayed Selfie method for measuring laser linewidth, provided in an embodiment of this application; Figure 2 A flowchart of another delayed Selfie method for measuring laser linewidth provided in this application embodiment; Figure 3 A flowchart for actively suppressing additional noise in delayed optical fibers, provided as an embodiment of this application; Figure 4 A schematic diagram of a first time-delay Selfie device for measuring laser linewidth provided in an embodiment of this application; Figure 5 A schematic diagram of a second time-delay Selfie device for measuring laser linewidth provided in an embodiment of this application; Figure 6 A schematic diagram of a third type of time-delay Selfie device for measuring laser linewidth provided in an embodiment of this application; Figure 7 A schematic diagram of a fourth time-delay Selfie device for measuring laser linewidth provided in this application embodiment; Figure 8 The output image of the spectrum analysis device provided in the embodiments of this application.

[0019] In the picture: 10. Interferometer body; 1001. Online isolator; 1002. First beam splitter; 1003. Second beam splitter; 1004. Circulator; 1005. Balance detector; 20. Phase-compensated delay fiber optic system; 2001. Acousto-optic frequency shifter; 2002. Delay fiber; 2003. Third beam splitter; 2004. Fourth beam splitter; 2005. High-speed detector; 2006. Bragg reflector; 2007. Voltage-controlled microwave source; 2008. Servo controller; 2009. Frequency and phase detector; 2010. Fixed-frequency microwave source; 2011. Fiber optic partial reflector; 2012. Fiber optic stretcher; 30. Spectrum analysis device. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0021] Several methods exist for measuring laser linewidth, but current methods fail to address the additional noise generated by the delay fiber itself under environmental factors such as temperature and vibration. This noise is superimposed on the phase noise of the laser under test, leading to errors in the system's laser linewidth measurement. Current methods primarily focus on passive stabilization, such as wrapping or isolating the delay fiber, but these methods cannot fundamentally solve the problem of additional phase noise introduced by the delay fiber.

[0022] This application provides a method and apparatus for measuring the delay Selfie frequency of laser linewidth, which solves the technical problem in related technologies where the large additional noise of the entire test system limits its application in kHz narrow laser linewidth measurement.

[0023] See Figure 1 and Figure 2 As shown, this application provides a method for measuring the time-delay Selfie frequency of a laser linewidth, which includes the following steps: S1: The laser to be tested is divided into the first local oscillator light, the second local oscillator light, and the first signal light.

[0024] S2: Input the first signal light into the phase compensation delay fiber system 20, so that the phase compensation delay fiber system 20 can use the first local oscillator light to extract the additional phase noise introduced by the delay fiber 2002, and use active locking technology to actively compensate for the phase noise of the first signal light.

[0025] S3: Perform optical mixing between the second local oscillator light and the compensated first signal light.

[0026] S4: Use the balanced detector 1005 to perform coherent detection on the mixed light of the first signal light and the second local oscillator light, and output it to the spectrum analysis device 30. Calculate the laser linewidth based on the spectrum diagram given by the spectrum analysis device 30.

[0027] In step S1, a beam splitter can be used to split the laser beam under test into a first local oscillator beam, a second local oscillator beam, and a first signal beam. The time-delay Selfie method provided in this embodiment can be implemented using the time-delay Selfie device provided in any of the following embodiments, and achieve the corresponding functions.

[0028] This embodiment divides the laser under test into a first local oscillator beam, a second local oscillator beam, and a first signal beam. The first signal beam is input into a phase compensation delay fiber system 20. The phase compensation delay fiber system 20 can use the first local oscillator beam to extract the additional phase noise introduced by the delay fiber 2002 and use active locking technology to actively compensate for the phase noise of the first signal beam. This can reduce the influence of environmental temperature, vibration and other factors on laser linewidth measurement, and solve the technical problem that the large additional noise of the entire test system in related technologies limits its application in kHz narrow laser linewidth measurement.

[0029] Further, in one embodiment, the step of inputting the first signal light into the phase-compensated delay fiber system 20, enabling the phase-compensated delay fiber system 20 to extract the additional phase noise introduced by the delay fiber 2002 using the first local oscillator light, and using active locking technology to actively compensate for the phase noise of the first signal light, may include: S21: The first signal light is shifted by the acousto-optic frequency shifter 2001 to change the laser frequency.

[0030] S22: Input the frequency-shifted first signal light into the delay fiber 2002 for the first delay.

[0031] S23: The first signal light after the first delay is divided into a first part and a second part. The first part is reflected and then delayed a second time through the delay fiber 2002. The second part is optically mixed with the first local oscillator light to obtain the additional phase noise introduced by the delay fiber 2002. Preferably, the proportion of the first part is 90% and the proportion of the second part is 10%.

[0032] S24: Actively compensate for the phase noise of the first signal light based on the additional phase noise introduced by the delay fiber 2002.

[0033] Furthermore, in one embodiment, see... Figure 3 , Figure 4 and Figure 7As shown, the process of dividing the first signal light after the first delay into a first part and a second part, reflecting the first part and then delaying it a second time through the delay fiber 2002, and optically mixing the second part with the first local oscillator light to obtain additional phase noise introduced by the delay fiber 2002 can include: dividing the first signal light after the first delay into a first part and a second part through a beam splitter; reflecting the first part through the Bragg reflector 2006 and then entering the delay fiber 2002 for a second delay; optically mixing the second part with the first local oscillator light; then using a high-speed detector 2005 to measure the difference frequency between the first local oscillator light and the laser output from the delay fiber 2002 to obtain a difference frequency electrical signal; then inputting the difference frequency electrical signal into a frequency and phase discriminator 2009 and mixing it with a single-frequency microwave signal output from a fixed-frequency microwave source 2010 to obtain an additional phase noise signal, wherein the center frequency of the single-frequency microwave signal is the same as that of the difference frequency electrical signal.

[0034] Furthermore, in one embodiment, see... Figure 5 and Figure 6 As shown, the process of dividing the first signal light after the first delay into a first part and a second part, reflecting the first part and then delaying it a second time through the delay fiber 2002, and optically mixing the second part with the first local oscillator light to obtain additional phase noise introduced by the delay fiber 2002 can include: dividing the first signal light after the first delay into a first part and a second part through the fiber optic partial reflector 2011; directly reflecting the first part and then entering the delay fiber 2002 again for a second delay; optically mixing the second part with the first local oscillator light; then using a high-speed detector 2005 to measure the difference frequency between the first local oscillator light and the laser output from the delay fiber 2002 to obtain a difference frequency electrical signal; then inputting the difference frequency electrical signal into a frequency and phase discriminator 2009 and mixing it with a single-frequency microwave signal output from a fixed-frequency microwave source 2010 to obtain an additional phase noise signal, wherein the center frequency of the single-frequency microwave signal is the same as that of the difference frequency electrical signal.

[0035] Furthermore, in one embodiment, see... Figure 4 and Figure 6 As shown, the active compensation for the phase noise of the first signal light based on the additional phase noise introduced by the delay fiber 2002 includes: inputting the additional phase noise signal into the servo controller 2008 and performing proportional-integral-differential operations; the output signal of the servo controller 2008 is fed back to the voltage-controlled microwave source 2007, causing the voltage-controlled microwave source 2007 to act on the acousto-optic frequency shifter 2001 to actively compensate for the phase noise of the first signal light. The compensation mechanism in this embodiment uses electronic control, resulting in a fast response speed.

[0036] Furthermore, in one embodiment, see... Figure 5 and Figure 7As shown, active compensation for the phase noise of the first signal light based on the additional phase noise introduced by the delay fiber 2002 includes: inputting the additional phase noise signal into the servo controller 2008 and performing proportional-integral-differential operations; the output signal of the servo controller 2008 is fed back to the fiber stretcher 2012, causing a change in the length of the fiber in the fiber stretcher 2012, thereby actively compensating for the phase noise of the first signal light. In this embodiment, the fiber stretcher 2012 is used instead of the voltage-controlled microwave source 2007, changing the compensation mechanism from an electronic closed loop to a mechanical closed loop, resulting in a simple structure and low cost.

[0037] Combination Figure 3 and Figure 4 As shown, exemplarily, in one optional embodiment, the first signal light first passes through an acousto-optic frequency shifter 2001 to shift the laser frequency by f1. This frequency signal can originate from a voltage-controlled microwave source 2007. Then, the first signal light enters a delay fiber 2002, where additional phase noise δφ is generated due to temperature, vibration, and other factors. It then passes through a third beam splitter 2003, splitting into 10% and 90% portions. The 90% portion is reflected by a Bragg reflector 2006 and re-enters the delay fiber 2002. The 10% portion undergoes optical mixing with the first local oscillator light at a fourth beam splitter 2004. High-speed detector 2005 measures the frequency difference between the first local oscillator and the laser output from delay fiber 2002, obtaining a first electrical signal with a center frequency of f1 and additional phase noise δφ. This first electrical signal enters frequency and phase discriminator 2009 and is mixed with a single-frequency microwave signal with the same center frequency f1 output from fixed-frequency microwave source 2010, generating an additional phase noise signal. This additional phase noise signal is input to servo controller 2008 for proportional-integral-differential calculation. The output signal of servo controller 2008 is fed back to voltage-controlled microwave source 2007 to compensate its output signal. The laser is reflected back to delay fiber 2002 by Bragg reflector and then shifted again by f1 by acousto-optic frequency shifter 2001. Since the RF signal input to acousto-optic frequency shifter 2001 cancels out the additional phase noise generated by delay fiber 2002, the reflected laser is not affected by the additional phase noise of delay fiber 2002, and only has the effects of delay and two frequency shifts of f1.

[0038] See Figure 5As shown, exemplarily, in another alternative approach, the first signal light first passes through an acousto-optic frequency shifter 2001 to shift the laser frequency by f1. The radio frequency signal of the acousto-optic frequency shifter 2001 comes from a fixed-frequency microwave source 2010. The frequency-shifted first signal light enters a delay fiber 2002 for a first delay, resulting in additional phase noise δφ due to temperature, vibration, and other factors. After the first delay, the first signal light is divided into two parts, a larger part and a smaller part, by a partial reflector 2011. The larger part is directly reflected and re-enters the delay fiber 2002 for a second delay, while the smaller part is optically mixed with the first local oscillator light on the fourth beam splitter 2004. Preferably, the larger part accounts for 90% and the smaller part accounts for 10%. Then, the high-speed detector 2005 is used to measure the difference frequency between the laser output from the first local oscillator and the delay fiber 2002, obtaining the first electrical signal with a center frequency of f1 and an additional phase noise δφ. The first electrical signal enters the frequency and phase discriminator 2009 and is mixed with a single-frequency microwave signal with the same center frequency f1 output from the fixed-frequency microwave source 2010 to obtain the second electrical signal, which is the additional phase noise signal. The second electrical signal is input to the servo controller 2008 and performs proportional-integral-differential calculations. The output signal of the servo controller 2008 is fed back to the fiber stretcher 2012, changing the length of the fiber in the fiber stretcher 2012, thereby compensating for the change in the length of the delay fiber 2002.

[0039] See Figure 4 As shown in the illustration, this application also provides a time-delay self-timer frequency measurement device for measuring laser linewidth, which may include: an interferometer body 10, the interferometer body 10 including a first beam splitter 1002, a circulator 1004 and a second beam splitter 1003, the first beam splitter 1002 being connected to the circulator 1004, and the circulator 1004 being connected to the second beam splitter 1003; a phase-compensated time-delay fiber optic system 20 and a spectrum analysis device 30, the phase-compensated time-delay fiber optic system 20 being connected to the first beam splitter 1002 and the circulator 1004; the first beam splitter 1002 is used to split the laser to be measured into a first local oscillator beam input to the phase-compensated time-delay fiber optic system 20 and a second local oscillator beam input to the phase-compensated time-delay fiber optic system 20. The second local oscillator light input to the second beam splitter 1003 and the first signal light input to the circulator 1004 are used. The phase-compensated delay fiber system 20 is used to extract the additional phase noise introduced by the delay fiber 2002 using the first local oscillator light and to actively compensate for the phase noise of the first signal light using active locking technology. The second beam splitter 1003 is used to perform optical mixing between the second local oscillator light and the first signal light output from the circulator 1004. The interferometer body 10 also includes a balanced detector 1005, which is connected to the second beam splitter 1003 and the spectrum analysis device 30, and is used to perform coherent detection on the mixed light of the first signal light and the second local oscillator light.

[0040] In this embodiment, the laser under test is split into three parts after passing through the first beam splitter 1002: a first local oscillator beam, a second local oscillator beam, and a first signal beam. The first local oscillator beam is used to measure the phase drift of the delay fiber 2002, the second local oscillator beam is used for Selfie frequency measurement, and the first signal beam is injected into the circulator 1004 and enters the phase-compensated delay fiber system 20. Finally, it is output from the circulator 1004 and optically mixed with the second local oscillator beam at the second beam splitter 1003. Coherent detection is performed using a balanced detector 1005 to suppress the common-mode noise signal of the laser intensity before outputting it to the spectrum analysis device 30 for analysis. By adopting a balanced detection scheme, the common-mode noise of the laser under test is suppressed, reducing the impact of laser intensity noise on linewidth measurement.

[0041] The time-delay Selfie frequency device of this embodiment is a device for actively suppressing the additional phase noise generated by the time-delay fiber 2002 due to factors such as ambient temperature changes and vibration. It features insensitivity to optical path polarization and high measurement accuracy. By splitting the laser under test into a first local oscillator beam, a second local oscillator beam, and a first signal beam, the first signal beam runs in the time-delay fiber 2002 of the phase-compensated time-delay fiber system 20. The additional phase noise introduced by the time-delay fiber 2002 is extracted using the first local oscillator beam, and active locking technology is used to actively compensate for the additional phase noise generated by the first signal beam on the time-delay fiber 2002. This reduces the influence of ambient temperature, vibration, and other factors on laser linewidth measurement, solving the technical problem of large additional noise in the entire test system in related technologies, which limits its application in kHz narrow laser linewidth measurement. The time-delay self-timer frequency device of this embodiment adopts the long-delay decoherent self-heterodyne method, solving the problem of lack of low-frequency noise measurement in short fiber subcoherent methods. Furthermore, the optical system adopts all-fiber devices, with each fiber element fused together, which has advantages such as strong anti-interference ability, low cost, and easy integration.

[0042] Further, in one embodiment, the phase-compensated delay fiber system 20 includes a delay fiber system, a noise monitoring unit, and a compensation unit. The delay fiber system connects the circulator 1004 and the noise monitoring unit, and is also connected to the compensation unit. The delay fiber system is used to input the first signal light back and forth into the delay fiber 2002. The noise monitoring unit is used to extract the additional phase noise introduced by the delay fiber 2002 using the first local oscillator light. The compensation unit is used to actively compensate for the phase noise of the first signal light. In this embodiment, by setting up the delay fiber system, the first signal light can pass back and forth through the delay fiber 2002. By setting up the noise monitoring unit, an additional phase noise signal can be obtained. By setting up the compensation unit, the phase noise of the first signal light can be actively compensated. This embodiment uses the method of laser passing back and forth through the delay fiber 2002 to reduce the polarization drift problem in the delay fiber 2002. Single-mode fiber can be used instead of polarization-maintaining fiber as the delay fiber 2002, greatly reducing the system cost.

[0043] See Figure 4 and Figure 7 As shown, in one embodiment, the time-delay fiber system includes an acousto-optic frequency shifter 2001, a time-delay fiber 2002, a third beam splitter 2003, a fourth beam splitter 2004, and a Bragg reflector 2006. The acousto-optic frequency shifter 2001 connects the time-delay fiber 2002 to the circulator 1004. The third beam splitter 2003 connects to the time-delay fiber 2002 and also connects the fourth beam splitter 2004 to the Bragg reflector 2006. The fourth beam splitter 2004 connects the first beam splitter 1002 to the noise monitoring unit. In this embodiment, the acousto-optic frequency shifter 2001 is directly connected to the circulator 1004, and the acousto-optic frequency shifter 2001 is sequentially connected to the time-delay fiber 2002, the third beam splitter 2003, and the fourth beam splitter 2004. The first signal light is output from the circulator 1004 and enters the delay fiber system. First, it passes through the acousto-optic frequency shifter 2001 to shift the laser frequency by f1. The frequency-shifted first signal light then enters the delay fiber 2002 for the first delay. Due to temperature, vibration, and other factors, additional phase noise δφ is generated. After the first delay, the first signal light is split into two parts, a larger and a smaller portion, by the third beam splitter 2003. The larger portion is reflected by the Bragg reflector 2006 and re-enters the delay fiber 2002 for the second delay. The smaller portion undergoes optical mixing with the first local oscillator light at the fourth beam splitter 2004, and then enters the noise monitoring unit for noise monitoring, which yields the additional phase noise signal. Preferably, the larger portion accounts for 90%, and the smaller portion accounts for 10%.

[0044] In an alternative embodiment, a fiber grating with a certain transmittance can be used to replace the third beam splitter 2003 and the Bragg reflector 2006, or an fiber stretcher 2012 can be added as an action mechanism.

[0045] For example, the noise monitoring unit may include a high-speed detector 2005, a frequency and phase detector 2009, and a fixed-frequency microwave source 2010. The high-speed detector 2005 is connected to the fourth beam splitter 2004 and the frequency and phase detector 2009. The frequency and phase detector 2009 is connected to the fixed-frequency microwave source 2010 and the compensation unit. In this embodiment, after a small portion of the laser and the first local oscillator light are optically mixed on the fourth beam splitter 2004, the high-speed detector 2005 measures the difference frequency between the first local oscillator light and the laser output from the delay fiber 2002 to obtain a first electrical signal with a center frequency of f1 and additional phase noise δφ. This first electrical signal enters the frequency and phase detector 2009 and is mixed with a single-frequency microwave signal with the same center frequency f1 output from the fixed-frequency microwave source 2010 to obtain a second electrical signal, which is the additional phase noise signal.

[0046] For example, see Figure 4 and Figure 6 As shown, the compensation unit may include a servo controller 2008 and a voltage-controlled microwave source 2007. The servo controller 2008 is connected to the frequency and phase detector 2009 and the voltage-controlled microwave source 2007, and the voltage-controlled microwave source 2007 is connected to the acousto-optic frequency shifter 2001. In this embodiment, after the second electrical signal is input to the servo controller 2008, proportional-integral-differential calculations are performed. The output signal of the servo controller 2008 is fed back to the voltage-controlled microwave source 2007 to compensate its output signal; the radio frequency signal of the acousto-optic frequency shifter 2001 comes from the voltage-controlled microwave source 2007. The compensation mechanism in this embodiment uses electronic control, resulting in a faster response speed.

[0047] In this embodiment, a large portion of the first signal light is reflected back to the delay fiber 2002 by the Bragg reflector, and then shifted again by f1 by the acousto-optic frequency shifter 2001. Since the input radio frequency signal of the acousto-optic frequency shifter 2001 comes from the voltage-controlled microwave source 2007, the phase change generated under the feedback compensation effect cancels out the additional phase noise generated by the delay fiber 2002. Therefore, the reflected laser is not affected by the additional phase noise of the delay fiber 2002, and only has the effects of delay and two frequency shifts by f1.

[0048] Furthermore, in some alternative embodiments, see Figure 5As shown, the time-delay fiber system may include an acousto-optic frequency shifter 2001, a time-delay fiber 2002, a fiber optic partial reflector 2011, and a fourth beam splitter 2004. The noise monitoring unit includes a high-speed detector 2005, a frequency and phase discriminator 2009, and a fixed-frequency microwave source 2010. The compensation unit includes a servo controller 2008 and a fiber stretcher 2012. The acousto-optic frequency shifter 2001 connects the fiber stretcher 2012 to the circulator 1004, and the time-delay fiber 2002 connects to the fiber stretcher 2012. The fourth beam splitter 2004 connects the fiber optic partial reflector 2011 and the first beam splitter 1002. The fourth beam splitter 2004 is also connected to the high-speed detector 2005. The high-speed detector 2005 is connected to the frequency and phase detector 2009. The frequency and phase detector 2009 is connected to the acousto-optic frequency shifter 2001 through the fixed-frequency microwave source 2010. The frequency and phase detector 2009 is also connected to the fiber stretcher 2012 through the servo controller 2008.

[0049] See Figure 5 As shown, in this embodiment, the time-delay fiber system uses a fiber optic partial reflector 2011 to replace the third beam splitter 2003 and the Bragg reflector 2006, and a fiber optic stretcher 2012 to replace the voltage-controlled microwave source 2007 in the compensation unit, changing the compensation mechanism from an electronic closed loop to a mechanical closed loop, resulting in a simple structure and low cost. Specifically, the first signal light is output from the circulator 1004 and enters the time-delay fiber system. It first passes through the acousto-optic frequency shifter 2001, which shifts the laser frequency by f1. The frequency-shifted first signal light enters the time-delay fiber 2002 for the first delay, during which additional phase noise δφ is caused by temperature, vibration, etc. After the first delay, the first signal light is split into two parts, a larger and a smaller part, by the fiber optic partial reflector 2011. The larger part is directly reflected and re-enters the time-delay fiber 2002 for the second delay, while the smaller part is optically mixed with the first local oscillator light at the fourth beam splitter 2004. A high-speed detector 200 is used. 5. The difference frequency between the first local oscillator and the laser output from the delay fiber 2002 is measured to obtain a difference frequency electrical signal (i.e., the first electrical signal), with a center frequency of f1 and an additional phase noise δφ. This difference frequency electrical signal enters the frequency and phase detector 2009 and is mixed with a single-frequency microwave signal with the same center frequency f1 output from the fixed-frequency microwave source 2010 to obtain a second electrical signal, i.e., the additional phase noise signal. This additional phase noise signal is input to the servo controller 2008 and subjected to proportional-integral-differential operations. The output signal of the servo controller 2008 is fed back to the fiber stretcher 2012 to compensate its output signal. The radio frequency signal of the acousto-optic frequency shifter 2001 comes from the fixed-frequency microwave source 2010. Preferably, the larger portion accounts for 90%, and the smaller portion accounts for 10%.

[0050] In this embodiment, a large portion of the first signal light is reflected back to the delay fiber 2002 by the fiber optic partial reflector 2011 for a second delay, and then shifted again by f1 by the acousto-optic frequency shifter 2001. The phase change generated by the fiber stretcher 2012 under the feedback compensation effect cancels out the additional phase noise generated by the delay fiber 2002. Therefore, the reflected laser is not affected by the additional phase noise of the delay fiber 2002, and only has the effects of delay and two frequency shifts f1.

[0051] After a significant portion of the first signal light is output through circulator 1004, it undergoes optical mixing with the second local oscillator light at the second beam splitter 1003. The frequency difference between the two is 2f1, and after a long fiber delay, they are decoherent, without introducing additional phase noise. Therefore, delay Selfie frequency measurement can be performed to obtain laser linewidth information. The spectrum analysis device 30 displays a spectrum with a width of -20 dB. It is approximately 1 / 20. Since the returning first signal light travels back and forth in the delay fiber 2002, the polarization drift effect caused by the delay fiber 2002 is canceled out after passing through the fiber twice.

[0052] The above embodiments provide two delay fiber optic systems and two compensation units. These two delay fiber optic systems and two compensation units can be freely combined to obtain four schemes, namely... Figures 4 to 7 These are the four options. Figure 4 The scheme shown is a time-delay fiber system with a third beam splitter 2003 and a Bragg reflector 2006 used in conjunction with a compensation unit with a voltage-controlled microwave source 2007. Figure 5 The scheme shown is a time-delay fiber system with a fiber optic partial reflector 2011 used in conjunction with a compensation unit with a fiber optic stretcher 2012. Figure 6 The scheme shown is a time-delay fiber system with fiber optic partial reflector 2011 used in conjunction with a compensation unit with voltage-controlled microwave source 2007; Figure 7 The scheme shown is a time-delay fiber system with a third beam splitter 2003 and a Bragg reflector 2006 used in conjunction with a compensation unit with a fiber stretcher 2012.

[0053] For example, see Figure 4As shown, the time-delay fiber system includes an acousto-optic frequency shifter 2001, a time-delay fiber 2002, a third beam splitter 2003, a fourth beam splitter 2004, and a Bragg reflector 2006; the noise monitoring unit includes a high-speed detector 2005, a frequency and phase discriminator 2009, and a fixed-frequency microwave source 2010; the compensation unit includes a servo controller 2008 and a voltage-controlled microwave source 2007; the acousto-optic frequency shifter 2001 connects the time-delay fiber 2002 to the circulator 1004, and the third beam splitter 2003 connects to the time-delay fiber 2002, and... The third beam splitter 2003 is connected to the fourth beam splitter 2004 and the Bragg reflector 2006. The fourth beam splitter 2004 is connected to the first beam splitter 1002 and the high-speed detector 2005. The high-speed detector 2005 is also connected to the frequency and phase detector 2009. The frequency and phase detector 2009 is connected to the fixed-frequency microwave source 2010 and the servo controller 2008. The servo controller 2008 is connected to the acousto-optic frequency shifter 2001 through the voltage-controlled microwave source 2007.

[0054] For example, see Figure 5 As shown, the time-delay fiber system includes an acousto-optic frequency shifter 2001, a time-delay fiber 2002, a fiber optic partial reflector 2011, and a fourth beam splitter 2004. The noise monitoring unit includes a high-speed detector 2005, a frequency and phase discriminator 2009, and a fixed-frequency microwave source 2010. The compensation unit includes a servo controller 2008 and a fiber stretcher 2012. The acousto-optic frequency shifter 2001 connects the fiber stretcher 2012 to the circulator 1004, and the time-delay fiber 2002 connects the fiber stretcher 2012 to... The fiber optic partial reflector 2011 is connected to the first beam splitter 1002 via the fourth beam splitter 2004. The fourth beam splitter 2004 is also connected to the high-speed detector 2005. The high-speed detector 2005 is connected to the frequency and phase detector 2009. The frequency and phase detector 2009 is connected to the acousto-optic frequency shifter 2001 via the fixed-frequency microwave source 2010. The frequency and phase detector 2009 is also connected to the fiber stretcher 2012 via the servo controller 2008.

[0055] Furthermore, in one embodiment, the interferometer body 10 further includes an online isolator 1001, which is connected to the first beam splitter 1002. See also Figure 4 As shown, the laser to be tested first passes through the online isolator 1001 and then enters the first beam splitter 1002. The online isolator 1001 can isolate the reflected light and prevent reflection interference.

[0056] In this embodiment, during measurement, the laser under test first passes through an online isolator 1001, and then is split into three parts—a first local oscillator beam, a second local oscillator beam, and a first signal beam—by a first beam splitter 1002. The first signal beam is injected into the first port of a circulator 1004, output from the second port of the circulator 1004, and then input into a phase-compensated delay fiber optic system 20. The first signal beam reflected from the phase-compensated delay fiber optic system 20 is input from the second port of the circulator 1004 and output from the third port to the second beam splitter 1003. The phase-compensated delay fiber optic system 20 uses active locking technology for active noise compensation. After two delays, the first signal beam and the second local oscillator beam are optically mixed at the second beam splitter 1003. A balanced detector 1005 is used to perform coherent detection on the mixed light of the first signal beam and the second local oscillator beam. After suppressing the common-mode noise signal of the laser intensity, the mixed light is output to a spectrum analysis device 30. The laser linewidth is calculated based on the spectrum diagram provided by the spectrum analysis device 30.

[0057] See Figure 8 The image shown is the output of the spectrum analysis device 30 in this embodiment. Its -20dB full width is approximately 183.4kHz, corresponding to a laser linewidth of 9.17kHz.

[0058] The time-delay Selfie frequency measurement device for measuring laser linewidth provided in this embodiment uses an unequal-arm Mach-Zehnder interferometer to measure the laser Selfie frequency. The time-delay arm adopts a single-fiber round-trip delay structure, which effectively avoids the polarization drift caused by excessively long optical fibers. In addition, active phase compensation technology is adopted to suppress the influence of the additional phase noise of the time-delay fiber 2002 on the laser linewidth measurement.

[0059] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0060] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0061] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A time-delay Selfie method for measuring laser linewidth, characterized in that, It includes the following steps: The laser beam to be tested is divided into a first local oscillator beam, a second local oscillator beam, and a first signal beam; The first signal light is input into the phase compensation delay fiber system (20), so that the phase compensation delay fiber system (20) uses the first local oscillator light to extract the additional phase noise introduced by the delay fiber (2002), and uses active locking technology to actively compensate for the phase noise of the first signal light. The second local oscillator light is optically mixed with the compensated first signal light; The mixed light of the first signal light and the second local oscillator light is coherently detected using a balanced detector (1005) and output to a spectrum analysis device (30). The laser linewidth is calculated based on the spectrum diagram given by the spectrum analysis device (30). The first signal light is input into the phase-compensated delay fiber system (20), which uses the first local oscillator light to extract the additional phase noise introduced by the delay fiber (2002) and uses active locking technology to actively compensate for the phase noise of the first signal light, including: The first signal light is passed through an acousto-optic frequency shifter (2001) to shift the laser frequency; The frequency-shifted first signal light is input into the delay fiber (2002) for the first delay; The first signal light after the first delay is divided into a first part and a second part. The first part is reflected and then delayed a second time through the delay fiber (2002). The second part is optically mixed with the first local oscillator light to obtain the additional phase noise introduced by the delay fiber (2002). Active compensation is performed on the phase noise of the first signal light based on the additional phase noise introduced by the time-delay fiber (2002).

2. The time-delay Selfie method for measuring laser linewidth as described in claim 1, characterized in that, The process of dividing the first signal light after the first delay into a first part and a second part, the first part being reflected and then delayed a second time via a delay fiber (2002), and the second part being optically mixed with the first local oscillator light to obtain additional phase noise introduced by the delay fiber (2002), includes: The first signal light after the first delay is split into a first part and a second part by a beam splitter. The first part is reflected by a Bragg reflector (2006) and re-enters the delay fiber (2002) for a second delay. The second part is optically mixed with the first local oscillator light. The difference frequency between the laser output from the first local oscillator and the delayed fiber (2002) was measured using a high-speed detector (2005) to obtain the difference frequency electrical signal; An additional phase noise signal is obtained by mixing the difference frequency electrical signal input to a frequency and phase detector (2009) with the single-frequency microwave signal output from a fixed-frequency microwave source (2010), wherein the center frequency of the single-frequency microwave signal is the same as that of the difference frequency electrical signal.

3. The time-delay Selfie method for measuring laser linewidth as described in claim 1, characterized in that, The process of dividing the first signal light after the first delay into a first part and a second part, the first part being reflected and then delayed a second time via a delay fiber (2002), and the second part being optically mixed with the first local oscillator light to obtain additional phase noise introduced by the delay fiber (2002), includes: The first signal light after the first delay is divided into a first part and a second part by a fiber optic partial reflector (2011). The first part is directly reflected and re-enters the delay fiber (2002) for a second delay. The second part is optically mixed with the first local oscillator light. The difference frequency between the laser output from the first local oscillator and the delayed fiber (2002) was measured using a high-speed detector (2005) to obtain the difference frequency electrical signal; An additional phase noise signal is obtained by mixing the difference frequency electrical signal input to a frequency and phase detector (2009) with the single-frequency microwave signal output from a fixed-frequency microwave source (2010), wherein the center frequency of the single-frequency microwave signal is the same as that of the difference frequency electrical signal.

4. The time-delay Selfie method for measuring laser linewidth as described in claim 1, characterized in that, Active compensation for the phase noise of the first signal light based on the additional phase noise introduced by the time-delay fiber (2002) includes: The additional phase noise signal is input into the servo controller (2008) and then subjected to proportional-integral-differential calculation. The output signal of the servo controller (2008) is fed back to the voltage-controlled microwave source (2007), so that the voltage-controlled microwave source (2007) acts on the acousto-optic frequency shifter (2001) to actively compensate for the phase noise of the first signal light.

5. The time-delay Selfie method for measuring laser linewidth as described in claim 1, characterized in that, Active compensation for the phase noise of the first signal light based on the additional phase noise introduced by the time-delay fiber (2002) includes: The additional phase noise signal is input into the servo controller (2008) and then subjected to proportional-integral-differential calculation. The output signal of the servo controller (2008) is fed back to the fiber stretcher (2012), causing the length of the fiber in the fiber stretcher (2012) to change, thereby actively compensating for the phase noise of the first signal light.

6. A time-delay Selfie device for measuring laser linewidth, characterized in that, It includes: Interferometer body (10), the interferometer body (10) includes a first beam splitter (1002), a circulator (1004) and a second beam splitter (1003), the first beam splitter (1002) is connected to the circulator (1004) and the circulator (1004) is connected to the second beam splitter (1003); A phase-compensated delay fiber optic system (20) and a spectrum analysis device (30), wherein the phase-compensated delay fiber optic system (20) is connected to the first beam splitter (1002) and the circulator (1004). The first beam splitter (1002) is used to split the laser under test into a first local oscillator beam input to the phase-compensated delay fiber system (20), a second local oscillator beam input to the second beam splitter (1003), and a first signal beam input to the circulator (1004). The phase-compensated delay fiber system (20) is used to extract the additional phase noise introduced by the delay fiber (2002) using the first local oscillator light, and to actively compensate for the phase noise of the first signal light using active locking technology. The second beam splitter (1003) is used to optically mix the second local oscillator light with the first signal light output by the circulator (1004); The interferometer body (10) also includes a balanced detector (1005), which is connected to the second beam splitter (1003) and the spectrum analysis device (30) for coherent detection of the mixed light of the first signal light and the second local oscillator light.

7. The time-delay Selfie device for measuring laser linewidth as described in claim 6, characterized in that, The phase compensation delay fiber optic system (20) includes a delay fiber optic system, a noise monitoring unit, and a compensation unit. The delay fiber optic system connects the circulator (1004) and the noise monitoring unit, and the delay fiber optic system is also connected to the compensation unit. The delay fiber system is used to input the first signal light back and forth into the delay fiber (2002), the noise monitoring unit is used to extract the additional phase noise introduced by the delay fiber (2002) using the first local oscillator light, and the compensation unit is used to actively compensate for the phase noise of the first signal light.

8. The time-delay Selfie device for measuring laser linewidth as described in claim 7, characterized in that, The time-delay fiber system includes an acousto-optic frequency shifter (2001), a time-delay fiber (2002), a third beam splitter (2003), a fourth beam splitter (2004), and a Bragg reflector (2006); the noise monitoring unit includes a high-speed detector (2005), a frequency and phase discriminator (2009), and a fixed-frequency microwave source (2010); the compensation unit includes a servo controller (2008) and a voltage-controlled microwave source (2007). The acousto-optic frequency shifter (2001) connects the delay fiber (2002) to the circulator (1004), the third beam splitter (2003) connects to the delay fiber (2002), and the third beam splitter (2003) connects to the fourth beam splitter (2004) and the Bragg reflector (2006). The fourth beam splitter (2004) connects to the first beam splitter (1002) and the high-speed detector (2005). The high-speed detector (2005) is also connected to the frequency and phase detector (2009), which is connected to the fixed-frequency microwave source (2010). The frequency and phase detector (2009) is also connected to the servo controller (2008), which is connected to the acousto-optic frequency shifter (2001) via the voltage-controlled microwave source (2007).

9. The time-delay Selfie device for measuring laser linewidth as described in claim 7, characterized in that, The time-delay fiber system includes an acousto-optic frequency shifter (2001), a time-delay fiber (2002), a fiber partial reflector (2011), and a fourth beam splitter (2004). The noise monitoring unit includes a high-speed detector (2005), a frequency and phase discriminator (2009), and a fixed-frequency microwave source (2010). The compensation unit includes a servo controller (2008) and a fiber stretcher (2012). The acousto-optic frequency shifter (2001) is connected to the fiber stretcher (2012) and the circulator (1004). The delay fiber (2002) is connected to the fiber stretcher (2012) and the fiber partial reflector (2011). The fourth beam splitter (2004) is connected to the fiber partial reflector (2011) and the first beam splitter (1002). The fourth beam splitter (2004) is also connected to the high-speed detector (2005). The high-speed detector (2005) is connected to the frequency and phase discriminator (2009). The frequency and phase discriminator (2009) is connected to the acousto-optic frequency shifter (2001) through the fixed-frequency microwave source (2010). The frequency and phase discriminator (2009) is also connected to the fiber stretcher (2012) through the servo controller (2008).

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