Hollow-core optical fiber axial light intensity uniformity evaluation method based on microwave Raman spectrum

Abstract

The invention relates to the technical field of light intensity measurement, in particular to a hollow-core optical fiber axial light intensity uniformity evaluation method based on a microwave Raman spectrum, and the method comprises the steps: conveying a free space cold atomic group into a hollow-core optical fiber based on first guide light and second guide light which are transmitted in the hollow-core optical fiber; closing any beam of the first guide light or the second guide light, and applying a microwave Pi pulse to the cold atomic group in the optical fiber by adopting a microwave gain antenna; sequentially sending three detection light pulses into the hollow-core optical fiber, and acquiring optical power voltage values of the three detection light pulses by an avalanche photodiode to obtain a normalized population; establishing a microwave Raman spectrum based on each sweep frequency and the corresponding normalized population, and obtaining a differential spectral frequency shift based on the microwave Raman spectrum; acquiring light intensity based on the differential light frequency shift; obtaining a standard deviation and a light intensity space gradient of the axial light intensity distribution of the hollow-core optical fiber based on the light intensity of each position; according to the method, the axial light intensity uniformity in the hollow-core optical fiber can be quantitatively characterized.

Description

Technical Field

[0001] This invention relates to the field of light intensity measurement technology, specifically to a method for evaluating the axial light intensity uniformity of hollow optical fibers based on microwave Raman spectroscopy. Background Technology

[0002] Hollow-core fiber (HCF), a novel microstructured optical fiber with air as the main light guiding medium, offers a new technological path for realizing long-distance strong light-atomic interactions due to its large core diameter, ultra-low transmission loss, and excellent mode modulation capabilities. It also strongly promotes the development of cutting-edge fields such as fiber-guided cold atom interferometers, optical quantum storage, quantum simulation, ultra-fine spectral measurement, and Rydberg state atom preparation.

[0003] In applications such as fiber-guided cold atom interferometers, the light intensity distribution within hollow-core optical fibers has a decisive influence on atomic dynamics. Specifically, light intensity loss and spatial modulation caused by higher-order mode excitation lead to non-uniform axial light intensity distribution, which in turn exacerbates atomic heating effects, introduces additional phase shifts, and ultimately degrades the interferometer's measurement performance. Therefore, accurately characterizing the axial light intensity uniformity within hollow-core optical fibers is crucial for guiding the structural design of hollow-core optical fibers, optimizing atomic guidance conditions, and suppressing spin coherence attenuation. It is an important prerequisite for improving the overall performance of such interferometers. However, current assessments of axial light intensity uniformity within hollow-core optical fibers mainly rely on indirect methods such as transmission loss and higher-order mode suppression ratio. These methods typically require additional test systems and even irreversible operations such as cutting and end-face treatment of fiber samples, making it difficult to perform non-destructive in-situ measurements while the system is running, and even more difficult to accurately reflect the distribution characteristics of light intensity along the fiber axis. Currently, there is a lack of effective methods for quantitatively characterizing the axial light intensity uniformity within hollow-core optical fibers.

[0004] In summary, quantitative evaluation of the axial distribution of hollow optical fibers can accurately quantify the suppression capability of higher-order modes in optical fibers, reveal the influence of light intensity uniformity on atomic evolution, and is of great significance to the development of the field of strong interaction between light and atoms in optical fibers. Summary of the Invention

[0005] In view of the above problems, the present invention provides a method for evaluating the axial light intensity uniformity of hollow optical fibers based on microwave Raman spectroscopy, which solves the technical problem that the existing technology cannot quantitatively characterize the axial light intensity uniformity in hollow optical fibers.

[0006] This invention provides a method for evaluating the axial intensity uniformity of hollow optical fibers based on microwave Raman spectroscopy, comprising the following steps: Step S1: Prepare free-space cold atomic clusters; Step S2: Release the free space cold atom cluster. Based on the first and second guiding light propagating in the hollow fiber, transport the free space cold atom cluster to the first position in the hollow fiber and turn it into a cold atom cluster inside the fiber. Step S3: Apply a bias magnetic field to the region where the hollow fiber is located; use pump light incident into the hollow fiber to pump atoms in the cold atom clusters inside the fiber to a magnetically insensitive state. Step S4: Turn off either the first or second guiding light beam; obtain the current sweep frequency, and use a microwave gain antenna to apply a microwave π pulse of the current sweep frequency to the cold atom cluster in the optical fiber to drive the atoms to perform magnetic dipole transitions; Step S5: Send three probe light pulses into the hollow fiber in succession. The avalanche photodiode obtains the optical power voltage value of the three probe light pulses after passing through the hollow fiber, and obtains the normalized population of the atom in the target transition state under the microwave π pulse at the current sweep frequency. Step S6: Update the sweep frequency and return to step S1 until the upper limit of the sweep frequency is reached; establish a microwave Raman spectrum based on each sweep frequency and the corresponding normalized population, obtain the differential optical frequency shift based on the microwave Raman spectrum, and obtain the light intensity based on the differential optical frequency shift. Step S7: Update the first position and return to step S1 until the position upper limit is reached; obtain the standard deviation of the axial light intensity distribution of the hollow fiber and the spatial gradient of light intensity based on the light intensity at each position.

[0007] Preferably, in step S2, the first guide light and the second guide light are emitted to the evaluation device, which includes a microwave gain antenna 2, a lens 3, a short-pass dichroic mirror 4, and an avalanche photodiode 5. The first guide light 6 is transmitted and emitted in the hollow fiber, passing through the lens 3 and the short-pass dichroic mirror 4 in sequence to reach the avalanche photodiode 5. The second guide light 7 is incident on the short-pass dichroic mirror 4 and reflected, then passes through the lens 3 and enters the hollow fiber.

[0008] Preferably, in step S4, the microwave gain antenna is disposed outside the hollow-core optical fiber 1; the microwave... The direction of the magnetic field component of the microwave field of the pulse is parallel to the direction of the bias magnetic field. The microwave π pulse refers to satisfying microwave pulses, in which, Indicates the rabbi frequency. Indicates the pulse width of the microwave pulse. Pi; The specific steps of obtaining the current frequency sweep include: determining the angular frequency of the currently transmitted microwave pulse. .

[0009] In step S5, the three probe light pulses are used to measure the optical power under the following three conditions: Light power after absorption by the target transition state atom The light power after absorption by all atoms and the light power without atomic absorption ; , and The voltage values ​​after conversion by the avalanche photodiode are respectively , and ; Based on voltage value , and The normalized population of the target transition state is obtained by the following expression:

[0010] in, This represents the normalized population in the target transition state. and These represent the optical depths corresponding to the first two probe light pulses, respectively.

[0011] Preferably, in step S5, the probe light 8 is transmitted and emitted in the hollow optical fiber along the same direction as the first guiding light 6, and passes through the lens 3 and the short-pass dichroic mirror 4 in sequence to reach the avalanche photodiode 5; specifically, the probe light 8 is directed perpendicularly to the center of the lens 3, collimated by the lens 3 and directed at the center of the short-pass dichroic mirror 4 at an incident angle of 45°, and the probe light 8 is transmitted through the short-pass dichroic mirror 4 to reach the avalanche photodiode 5.

[0012] Preferably, in step S6, the step of updating the sweep frequency specifically includes: The frequency sweep frequency is updated sequentially by increasing or decreasing the starting value of the frequency sweep frequency according to a preset sweep frequency step size. In step S6, the step of establishing the microwave Raman spectrum based on each sweep frequency and the corresponding normalized population specifically includes: Using the sweep frequency as the horizontal axis and the normalized population in the target transition state as the vertical axis, a microwave Raman spectrum is established. In step S6, the step of obtaining the differential optical frequency shift based on microwave Raman spectroscopy specifically includes: Obtain the frequency corresponding to the maximum normalized population value of the target transition state in the microwave Raman spectrum. ,Will and The difference is used as the differential optical frequency shift ,in This represents the angular frequency corresponding to the splitting of the hyperfine ground state energy level.

[0013] Preferably, in step S6, the step of obtaining light intensity based on differential optical frequency shift specifically includes: Determine the differential optical frequency shift With guiding light intensity Relational expressions between the two:

[0014] in, Indicates differential optical frequency shift, The speed of light in a vacuum. The natural linewidth of an atom, To reduce Planck's constant, The resonant transition frequency of the atom. Indicates the guide light relative to The amount of detuning, Indicates the intensity of the guiding light. Indicates the intensity of the guiding light With differential optical frequency shift The scaling factor between; According to the differential optical frequency shift With guiding light intensity The relationship expression is derived from differential optical frequency shift. The light intensity at the current position of the hollow fiber is obtained by inverse solving.

[0015] Preferably, in step S7, the step of updating the first position specifically includes: The first position is updated sequentially by increasing or decreasing the initial value of the first position according to the preset position update step size; In step S7, the step of obtaining the standard deviation and intensity gradient of the axial light intensity distribution of the hollow fiber based on the light intensity at each location specifically includes: The uniformity of the distribution is quantitatively assessed using standard deviation and gradient, expressed as follows:

[0016]

[0017] in, The standard deviation of light intensity distribution This indicates the total number of measurement points for the axial position of the hollow fiber. Indicates measurement point Light intensity, This represents the average light intensity at all measurement points. Represents the spatial gradient of light intensity. denoted by , and z represents the axial coordinate of the hollow fiber.

[0018] Preferably, the atoms in the free-space cold atom cluster are 87Rb atoms; The target transition state is state, among which, Represents the quantum number of angular momentum. It represents the magnetic quantum number.

[0019] Compared with the prior art, the present invention has at least the following beneficial effects: (1) Quantitatively assess the uniformity of axial light intensity in hollow optical fiber. Based on the response characteristics of differential light frequency shift to light intensity, this invention achieves quantitative measurement of the uniformity of axial light intensity distribution in hollow optical fiber by frequency shift of microwave Raman spectrum, breaking through the limitations of traditional methods.

[0020] (2) Non-destructive in-situ measurement: Compared with measurement methods that rely on optical parameters such as fiber loss and high-order mode suppression ratio, this invention does not require disassembly, movement or cutting of the fiber sample. It can perform in-situ measurement of the axial light intensity inside the fiber, providing effective technical support for system optimization and long-term stability control.

[0021] (3) The measurement scheme is simple. The present invention can directly use the laser coupling optical path of the fiber-guided cold atom interferometer system, which is simple to operate and does not change the original structure of the system. Attached Figure Description

[0022] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0023] Figure 1 The flowchart shows the method for evaluating the axial light intensity uniformity of hollow optical fibers based on microwave Raman spectroscopy provided by this invention.

[0024] Figure 2 This is a schematic diagram of the evaluation device scheme provided by the present invention.

[0025] Figure 3 This is a schematic diagram of the optical lattice configuration and single-beam configuration of the guiding light provided by the present invention.

[0026] Figure labels: 1-Hollow-core optical fiber, 2-Microwave gain antenna, 3-Lens, 4-Short-pass dichroic mirror, 5-Avalanche photodiode, 6-First guiding light, 7-Second guiding light, 8-Probe light, 9-Free-space cold atom cluster, 10-Cold atom cluster inside the optical fiber. Detailed Implementation

[0027] To better understand the above-described objectives, features, and advantages of the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other. Furthermore, the present invention can be implemented in other ways different from those described herein; therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0028] This invention measures the microwave Raman spectrum at different locations within an optical fiber to obtain the corresponding differential optical frequency shift, thereby retrieving the axial intensity distribution. By analyzing the standard deviation and gradient of this distribution, the uniformity of the intensity can be quantitatively assessed. This invention is the first to achieve a quantitative characterization of the axial intensity distribution within a hollow optical fiber, overcoming the limitations of traditional evaluation methods that rely on transmission loss and higher-order mode suppression ratio.

[0029] To illustrate the effectiveness of the method proposed in this invention, the following detailed description of the above technical solution is provided through a specific embodiment, such as... Figure 1 As shown, a method for evaluating the axial light intensity uniformity of hollow optical fibers based on microwave Raman spectroscopy is disclosed, and an evaluation device is used to evaluate the light intensity uniformity of hollow optical fibers.

[0030] like Figure 2 As shown, the evaluation device of the present invention includes a microwave gain antenna 2, a lens 3, a short-pass dichroic mirror 4, and an avalanche photodiode 5.

[0031] The hollow fiber 1 can be guided by the first guiding light 6 and the probe light 8. The first guiding light 6 and the probe light 8 are transmitted and emitted in the same direction in the hollow fiber 1, and then pass through the lens 3 and the short-wavelength dichroic mirror 4 to reach the avalanche photodiode 5.

[0032] The first guiding light 6 and the probe light 8 emitted from the hollow optical fiber 1 are directed perpendicularly to the center of the lens 3; the lens 3 is used to collimate the first guiding light 6 and the probe light 8.

[0033] After being collimated by lens 3, the first guide light 6 and the probe light 8 are incident at an angle of 45° towards the center of the short-pass dichroic mirror 4. The short-pass dichroic mirror 4 is used to separate the first guide light 6 and the probe light 8. The first guide light 6 is reflected by the short-pass dichroic mirror 4, and the probe light 8 is transmitted through the short-pass dichroic mirror 4.

[0034] The probe light 8, after being transmitted through the short-pass dichroic mirror 4, is directed perpendicularly to the center of the photosensitive area of ​​the avalanche photodiode 5; the avalanche photodiode 5 is used to measure the optical power of the probe light 8.

[0035] The second guiding light 7 is incident on the short-wavelength dichroic mirror 4 and reflected, then passes through the lens 3 and is incident on the hollow fiber 1.

[0036] The second guiding light 7, perpendicular to the propagation direction of the first guiding light 6 and the probe light 8, is incident at a 45° angle to the center of the short-pass dichroic mirror 4 and is reflected by the short-pass dichroic mirror 4. After reflection by the short-pass dichroic mirror 4, the second guiding light 7 follows the same propagation path as the first guiding light 6 and the probe light 8, but in the opposite direction.

[0037] The second guiding light 7, after being reflected by the short-wave dichroic mirror 4, is directed perpendicularly to the center of the lens 3 and converges.

[0038] The second guiding light 7, after being focused by lens 3, enters the hollow fiber 1, is transmitted in the hollow fiber 1, and exits from the other end face.

[0039] The first guiding light 6 and the second guiding light 7 are used to load and guide the free-space cold atom cluster 9 into the hollow optical fiber 1, so that it becomes the cold atom cluster 10 inside the optical fiber.

[0040] The first guiding light 6 and the second guiding light 7 have two guiding light configurations at different working stages: one is the "optical lattice configuration", in which the first guiding light 6 and the second guiding light 7 are turned on at the same time. These two beams of light propagate in the optical fiber and form an optical lattice, realizing the controllable transport of the cold atom cluster 10 in the optical fiber; the other is the "single beam configuration", in which only the first guiding light 6 or the second guiding light 7 is turned on, trapping the cold atom cluster 10 in the optical fiber and performing microwave Raman spectroscopy measurements.

[0041] The microwave gain antenna 2 is disposed outside the hollow optical fiber 1; the microwave gain antenna 2 is used to emit a microwave field to drive atoms to undergo magnetic dipole transitions, wherein the direction of the magnetic field component of the microwave field is parallel to the direction of the bias magnetic field that defines the quantization axis.

[0042] Free space cold atom cluster 9 and fiber optic cold atom cluster 10 represent the states of cold atom clusters at different times and in different spatial locations.

[0043] The free-space cold atom cluster 9 is located in the free space between the hollow fiber 1 and the lens 3; the center of the free-space cold atom cluster 9 coincides with the center of the first guiding light 6, the second guiding light 7 and the probe light 8.

[0044] The cold atom cluster 10 inside the optical fiber is a state in which the free-space cold atom cluster 9 is loaded and guided to any position inside the hollow optical fiber 1 by an optical lattice jointly formed by the first guiding light 6 and the second guiding light 7. The cold atom cluster 10 inside the optical fiber is within the effective and uniform operating region of the microwave gain antenna 2.

[0045] In some embodiments, the first guiding light 6 and the second guiding light 7 are two laser beams propagating in opposite directions within the hollow fiber, forming a standing wave field in their overlapping region, which is used to transport the free-space cold atom cluster 9 to any position within the hollow fiber 1.

[0046] In some embodiments, the wavelength of the probe light 8 is near resonance relative to a specific transition energy level of the atom.

[0047] by 87 Taking Rb atoms as an example, the typical wavelengths of the guide light and the probe light are 1064 nm and 780 nm, respectively. The passband of the hollow fiber 1 covers the first guide light 6, the second guide light 7, and the probe light 8; the working wavelength range of the lens 3 covers the first guide light 6, the second guide light 7, and the probe light 8; the cutoff wavelength of the short-pass dichroic mirror 4 is greater than the wavelength of the probe light 8, but less than the wavelengths of the first guide light 6 and the second guide light 7.

[0048] This invention uses the aforementioned device to evaluate the light intensity uniformity of hollow-core optical fibers. The specific implementation steps of the method for evaluating the axial light intensity uniformity of hollow-core optical fibers based on microwave Raman spectroscopy are as follows: Step S1: Prepare free-space cold atomic clusters; In this step, the present invention uses magneto-optical trap and polarization gradient cooling technology to prepare free-space cold atomic clusters.

[0049] Step S2: Release the free space cold atom cluster. Based on the first and second guiding light propagating in the hollow fiber, transport the free space cold atom cluster to the first position in the hollow fiber and turn it into a cold atom cluster inside the fiber. The evaluation device of the present invention includes a microwave gain antenna 2, a lens 3, a short-pass dichroic mirror 4, and an avalanche photodiode 5.

[0050] The present invention controls the first guiding light to be transmitted and emitted in the hollow optical fiber, and then passes through the lens 3 and the short-pass dichroic mirror 4 to reach the avalanche photodiode 5; the second guiding light is incident on the short-pass dichroic mirror 4 and reflected, and then passes through the lens 3 and enters the hollow optical fiber.

[0051] like Figure 3 As shown, simultaneously turning on the first and second guiding lights can set the guiding light configuration to an optical lattice configuration. In the optical lattice configuration, the two beams of light propagate within the optical fiber and form an optical lattice, enabling the controllable transport of cold atomic clusters.

[0052] In this step, the first guide light and the second guide light are turned on simultaneously to transport the free space cold atom cluster to the first position inside the hollow fiber, thus transforming it into a cold atom cluster inside the fiber.

[0053] This invention obtains the light intensity distribution by detecting the light intensity at consecutive different positions along the axis of a hollow optical fiber. The first position refers to the current position of the hollow optical fiber where the light intensity needs to be detected. This invention obtains the light intensity at different positions by continuously updating the first position in subsequent steps.

[0054] Step S3: Apply a bias magnetic field to the region where the hollow fiber is located; use pump light incident into the hollow fiber to pump atoms in the cold atom clusters inside the fiber to a magnetically insensitive state. In this step, the present invention first applies a bias magnetic field to the region where the hollow fiber is located. Specifically, a bias magnetic field can be generated and applied using a device such as a Helmholtz coil. The bias magnetic field of the present invention is used to define the direction of the quantization axis and to determine the magnetic field direction of the microwave field sent by the subsequent driving magnetic dipole transition.

[0055] In this step, pump light is used to pump atoms to a magnetically insensitive state, in order to 87 Taking Rb atoms as an example, using a beam Linearly polarized pump light pumps atoms into magnetically insensitive states. Specifically, a linearly polarized pump light beam whose propagation direction is parallel to the quantization axis can be decomposed into equal amounts of... and The superposition of polarized light drives the magneton energy level. Atom transition to excited state Excited state It will de-excite to the ground state through a spontaneous emission process. According to the transition selection rule, the magnetic quantum energy level The atoms no longer absorb linearly polarized pump light. After repeated stimulated absorption-spontaneous emission processes, they are eventually all pumped to a magnetically insensitive region. In this state, an initial state is formed that is insensitive to external magnetic field disturbances.

[0056] Step S4: Turn off either the first or second guiding light beam, obtain the current sweep frequency, and use a microwave gain antenna to apply a microwave π pulse of the current sweep frequency to the cold atom cluster in the optical fiber to drive the atoms to perform magnetic dipole transitions. by 87 Taking Rb atoms as an example, microwave gain antenna 2 transmits a beam with an angular frequency of... Microwave pulses drive the cold atomic clusters 10 within the optical fiber to undergo magnetic dipole transitions. Under resonance conditions, i.e. When, it is said to satisfy The microwave pulse is Pulse, at which time the cold atomic group 10 inside the optical fiber is in Normalized population of the state To reach its maximum. By scanning the frequency of microwave π pulses. It can plot the microwave Raman spectrum of cold atomic groups inside optical fibers.

[0057] This invention relates to microwave π pulse frequency Frequency sweep control is performed to obtain the normalized population corresponding to each frequency in subsequent steps.

[0058] In some embodiments, frequency can be progressively scanned and controlled by setting a predetermined sweep step size. First, the starting and ending values ​​of the frequency are determined. Then, following the set sweep step size, the frequency parameters are gradually changed, increasing or decreasing sequentially from the starting value. At each frequency value, the system performs corresponding measurements and data acquisition. This method enables precise and continuous frequency detuning scanning, ensuring the comprehensiveness and accuracy of subsequent data analysis.

[0059] Specifically, in this step, obtaining the current sweep frequency means determining the angular frequency of the currently transmitted microwave pulse. .

[0060] Then, a microwave π pulse at the current sweep frequency is applied to the cold atom cluster within the optical fiber using a microwave gain antenna. The microwave π pulse at the current sweep frequency in this invention refers to a pulse with a frequency equal to the current sweep frequency and satisfying the following conditions: microwave pulses, in which, Indicates the rabbi frequency. Indicates the pulse width of the microwave pulse. It is pi; by sending microwave π pulses, atoms in the cold atomic clusters inside the optical fiber are driven to undergo magnetic dipole transitions.

[0061] Step S5: Send three probe light pulses into the hollow fiber in succession. The avalanche photodiode obtains the optical power voltage value of the three probe light pulses after passing through the hollow fiber, and obtains the normalized population of the atom in the target transition state under the microwave π pulse at the current sweep frequency. In the above steps, by sending microwave π pulses, atoms in the cold atomic clusters within the optical fiber are driven to undergo magnetic dipole transitions. The normalized population of the cold atomic clusters in the target transition state is determined by... for:

[0062] in, This represents the normalized population in the target transition state. The amplitude factor is related to the efficiency of preparing atomically magnetically insensitive states. Indicates the rabbi frequency. This represents the angular frequency of a microwave pulse. This represents the angular frequency corresponding to the splitting of the hyperfine ground state energy level. This indicates the pulse width of the microwave pulse.

[0063] In this step, the normalized population at the target transition state is measured. This invention sends three probe light pulses sequentially into the hollow optical fiber.

[0064] In some embodiments, the atoms in the free-space cold atom cluster can be87 Rb atoms, 87 Taking the Rb atom as an example, the target transition state can specifically refer to... state.

[0065] The probe light 8 is transmitted and emitted in the hollow optical fiber along the same direction as the first guiding light 6, and passes through the lens 3 and the short-pass dichroic mirror 4 in sequence to reach the avalanche photodiode 5. Specifically, the probe light 8 is directed perpendicularly to the center of the lens 3, collimated by the lens 3 and directed at the center of the short-pass dichroic mirror 4 at an incident angle of 45°, and the probe light 8 is transmitted through the short-pass dichroic mirror 4 to reach the avalanche photodiode 5.

[0066] The optical powers of the three probe light pulses after passing through the hollow fiber correspond to the optical power P1 after absorption by the target transition state atoms, the optical power P2 after absorption by all atoms, and the optical power P0 without atomic absorption, respectively. The voltage values ​​after conversion by the avalanche photodiode 5 are respectively... , and .

[0067] The expression for the relationship between the normalized population of an atom in the target transition state and the voltage value after conversion by the avalanche photodiode 5 is as follows:

[0068] in, and These represent the optical depths corresponding to the first two probe light pulses, and are used to characterize the transmittance of the probe light pulses after atomic absorption within the optical fiber. This represents the voltage value after the light power absorbed by the target transition state atom is converted by the avalanche photodiode. This represents the voltage value after the light power absorbed by all atoms is converted by the avalanche photodiode. This represents the voltage value after the light power without atomic absorption is converted by the avalanche photodiode.

[0069] Through the above steps, the present invention obtains the normalized population by sending and measuring three probe light pulses.

[0070] Step S6: Update the sweep frequency and return to step S1 until the upper limit of the sweep frequency is reached; establish a microwave Raman spectrum based on each sweep frequency and the corresponding normalized population, obtain the differential optical frequency shift based on the microwave Raman spectrum, and obtain the light intensity based on the differential optical frequency shift. In this step, the frequency sweeping can be controlled by gradually scanning using a preset sweeping step size. The sweeping frequency parameters are updated sequentially, either increasing or decreasing from the initial value, according to the set sweeping step size.

[0071] Then, return to step S1 and repeat the process from S1 to S5 to obtain each sweep frequency and the corresponding normalized population. Use the sweep frequency as the horizontal axis and the normalized population as the vertical axis to establish the microwave Raman spectrum.

[0072] The guide light under far-infrared detuning conditions induces an AC Stark frequency shift in the atomic energy levels. In this invention, the optical potential well depth formed by the guide light is much higher than the temperature of the atomic cluster. At this point, the radial motion of the cold atomic cluster 10 within the fiber can be approximated as simple harmonic motion of the vibrational ground state. Therefore, the average differential optical frequency shift of all atoms at the same position in the hollow fiber 1 is considered uniform. Because the detuning amount of the guide light relative to the two hyperfine ground state energy levels of the atoms is different, the resulting frequency shifts in the ground state energy levels are also different, thus producing a differential optical frequency shift. .

[0073] In this step, the differential light frequency shift can be obtained based on the frequency shift of the resonance peak of the microwave Raman spectral line. The specific steps include: Obtain the frequency corresponding to the maximum normalized population in the microwave Raman spectrum. ,Will and The difference is used as the differential optical frequency shift .

[0074] The linewidth of the microwave Raman spectrum is determined by the generalized Rabi frequency. and pulse width Decision. To improve the average differential optical frequency shift. The measurement resolution can be selected using long pulse width microwaves. Pulse. Typical pulse width length The value is 10.0 ms, and the corresponding resonant pully frequency is... When the absolute value of the guide light detuning satisfies Conditions ( (Represents the angular frequency corresponding to the splitting of atomic fine energy levels), differential optical frequency shift. With guiding light intensity The relationship between them is satisfied:

[0075] in, Indicates differential optical frequency shift, The speed of light in a vacuum. The natural linewidth of an atom, To reduce Planck's constant, The resonant transition frequency of the atom. Indicates the guide light relative to The amount of detuning, Indicates the intensity of the guiding light. Indicates the intensity of the guiding light With differential optical frequency shift The scaling factor between them.

[0076] The above expression indicates that when the absolute value of the guide light detuning decreases, the absolute value of the scaling factor increases accordingly, thus affecting the same light intensity fluctuation. This produces a larger differential optical frequency shift. .

[0077] Based on the above expressions, this invention determines the differential optical frequency shift. With guiding light intensity The relationship between the two has been obtained in this step, where the differential optical frequency shift has been obtained. The light intensity at the current position of the hollow fiber can be obtained by inverse solving the above formula.

[0078] Step S7: Update the first position and return to step S1 until the position upper limit is reached; obtain the standard deviation of the axial light intensity distribution of the hollow fiber and the spatial gradient of light intensity based on the light intensity at each position.

[0079] In this step, the first position can be updated sequentially by increasing or decreasing the preset position update step size, thereby realizing the measurement of light intensity at various positions along the axial direction of the hollow fiber.

[0080] For the light intensity obtained at each location, this invention uses the standard deviation and gradient to quantitatively evaluate the uniformity of the distribution, as expressed by:

[0081]

[0082] in, The standard deviation of light intensity distribution This indicates the total number of measurement points for the axial position of the hollow fiber. Indicates measurement point Light intensity, This represents the average light intensity at all measurement points. Represents the spatial gradient of light intensity. denoted by , and z represents the axial coordinate of the hollow fiber.

[0083] Standard deviation of light intensity distribution This reflects the degree of dispersion of the light intensity value relative to the average value. A larger value indicates greater fluctuations in light intensity along the fiber axis and poorer overall uniformity; while the gradient... This represents the rate of change of light intensity in space. The larger the gradient value, the more intense the fluctuation of light intensity in the local area and the worse the local uniformity.

[0084] This invention utilizes microwave Raman spectroscopy to quantitatively evaluate the axial intensity uniformity of hollow-core optical fibers. The method measures the microwave Raman spectra at different locations within the fiber to obtain the corresponding differential optical frequency shift, thereby retrieving the axial intensity distribution. By analyzing the standard deviation and gradient of this distribution, the intensity uniformity can be quantitatively evaluated. This invention achieves, for the first time, a quantitative characterization of the axial intensity distribution within hollow-core optical fibers, overcoming the limitations of traditional evaluation methods that rely on transmission loss and higher-order mode suppression ratios. Furthermore, this invention offers significant advantages such as non-destructive in-situ measurement and ease of operation. It is of great significance for accurately quantifying the higher-order mode suppression capability of optical fibers, revealing the impact of intensity uniformity on atomic evolution, and advancing the field of strong light-atomic interactions within optical fibers.

[0085] While the specific embodiments of the present invention depict actions or steps in a particular order, this should be understood as requiring such actions or steps to be performed in the specific order shown or in sequential order, or requiring all illustrated actions or steps to be performed to achieve the desired result. In certain environments, multitasking and parallel processing may be advantageous. Similarly, although several specific implementation details are included in the above discussion, these should not be construed as limiting the scope of this disclosure. Certain features described in the context of individual embodiments may also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation may also be implemented individually or in any suitable sub-combination in multiple implementations.

[0086] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the axial light intensity uniformity of a hollow-core photonic fiber based on microwave Raman spectroscopy, characterized in that, The method comprises the following steps: Step S1, preparing a free-space cold atom group; Step S2, releasing the free-space cold atom group, transporting the free-space cold atom group to a first position in the hollow-core optical fiber based on first guide light and second guide light propagating in the hollow-core optical fiber, and changing into an in-fiber cold atom group; Step S3, applying a bias magnetic field in the region where the hollow-core optical fiber is located; and pumping atoms in the in-fiber cold atom group to a magnetic insensitivity state by using pump light incident into the hollow-core optical fiber; Step S4, turning off any one of the first guide light or the second guide light, obtaining a current sweep frequency, applying a microwave pi pulse of the current sweep frequency to the in-fiber cold atom group by using a microwave gain antenna, and driving atoms to perform a magnetic dipole transition; Step S5, sending three probe light pulses into the hollow-core optical fiber in sequence, obtaining light power voltage values of the three probe light pulses after passing through the hollow-core optical fiber by using an avalanche photodiode, and obtaining a normalized population of atoms in a target transition state under the microwave pi pulse of the current sweep frequency; Step S6, updating the sweep frequency, returning to step S1, and continuing until an upper limit of the sweep frequency is reached; establishing a microwave Raman spectrum based on each sweep frequency and a corresponding normalized population, obtaining a differential optical frequency shift based on the microwave Raman spectrum, and obtaining light intensity based on the differential optical frequency shift; Step S7, updating the first position, returning to step S1, and continuing until an upper limit of the position is reached; obtaining a standard deviation of an axial light intensity distribution and a light intensity spatial gradient of the hollow-core optical fiber based on light intensity of each position.

2. The method for evaluating the axial light intensity uniformity of the microwave-Raman spectrum based hollow-core photonic fiber according to claim 1, characterized in that, In step S2, the first guide light and the second guide light are emitted to an evaluation device, and the evaluation device comprises a microwave gain antenna (2), a lens (3), a short-wave-pass dichroic mirror (4), and an avalanche photodiode (5). The first guide light (6) is transmitted in the hollow-core optical fiber and is emitted, sequentially passes through the lens (3) and the short-wave-pass dichroic mirror (4), and reaches the avalanche photodiode (5). The second guide light (7) is incident to the short-wave-pass dichroic mirror (4), is reflected, passes through the lens (3), and is incident into the hollow-core optical fiber.

3. The method for evaluating the axial light intensity uniformity of the microwave-Raman spectrum based hollow-core photonic fiber according to claim 2, characterized in that, In step S4, the microwave gain antenna is arranged outside the hollow-core optical fiber (1); and a magnetic field component direction of a microwave field of the microwave pi pulse is parallel to a direction of the bias magnetic field. The microwave pi pulse is a microwave pulse satisfying wherein, denotes the Rabi frequency, denotes the pulse width of the microwave pulse, is the circle constant; The acquiring the current sweeping frequency specifically comprises: determining an angular frequency of a current transmitted microwave pulse .

4. The method for evaluating the axial light intensity uniformity of the microwave-Raman spectrum-based hollow-core fiber according to claim 3, characterized in that, In step S5, the three probe light pulses are respectively used to measure light power under the following three conditions: Light power after absorption by target transition state atoms Light power after absorption by all atoms Light power without atomic absorption ; , and The voltage values converted by the avalanche photodiode are , and ; Based on the voltage value , and The normalized population of the target transition state is obtained, expressed as: wherein, denotes the normalized population in the target transition state, and represent the optical depths corresponding to the first two probe light pulses, respectively.

5. The method of claim 4, wherein the method further comprises: In step S5, the probe light (8) is transmitted in the hollow-core optical fiber in the same direction as the first guide light (6) and is emitted, sequentially passes through the lens (3) and the short-wave-pass dichroic mirror (4), and reaches the avalanche photodiode (5). Specifically, the probe light (8) is vertically emitted to the center of the lens (3), is collimated by the lens (3) to be incident to the center of the short-wave-pass dichroic mirror (4) at an incident angle of 45°, and the probe light (8) is transmitted by the short-wave-pass dichroic mirror (4) to reach the avalanche photodiode (5).

6. The method of claim 5, wherein the method further comprises: In step S6, the step of updating the sweep frequency specifically comprises: updating the sweep frequency in a manner of sequentially increasing or decreasing from a starting value of the sweep frequency according to a preset sweep step; In step S6, the step of establishing the microwave Raman spectrum based on each sweep frequency and a corresponding normalized population specifically comprises: The microwave Raman spectrum is established by taking the sweep frequency as the horizontal axis and the normalized population in the target transition state as the vertical axis; The step of obtaining the differential optical frequency shift based on the microwave Raman spectrum in step S6 specifically includes: Obtaining the frequency corresponding to the maximum value of the normalized population in the target transition state in the microwave Raman spectrum , the difference between and is taken as the differential optical frequency shift , wherein represents the angular frequency corresponding to the hyperfine ground state energy level splitting.

7. The method of claim 6, wherein the method further comprises: The step of obtaining the light intensity based on the differential optical frequency shift in step S6 specifically includes: Determining differential optical frequency shift Relationship expression between guided light intensity and the intensity of the light wherein, represents a differential optical frequency shift, is the speed of light in vacuum, is the natural line width of the atom, is the reduced Planck constant, is the resonant transition frequency of the atom, represents the amount of detuning of the probe light with respect to the resonant transition frequency of the atom, represents the intensity of the probe light, represents the intensity of the probe light and the differential optical frequency shift ; According to the differential optical frequency shift The relationship expression between the guided light intensity The guided light intensity is inversely solved from the differential optical frequency shift The current position of the hollow core optical fiber.

8. The method of claim 7, wherein the method further comprises: determining the axial intensity uniformity of the microwave Raman spectrum based on the measured intensity of the microwave Raman spectrum. The step of updating the first position in step S7 specifically includes: The first position is updated in a manner of sequentially increasing or decreasing from a starting value of the first position according to a preset position update step; The step of obtaining the standard deviation and the light intensity gradient of the axial light intensity distribution of the hollow core optical fiber based on the light intensity of each position in step S7 specifically includes: The uniformity of the distribution is quantitatively evaluated by using the standard deviation and the gradient, and the expression is as follows: wherein represents the standard deviation of the light intensity distribution, represents the total number of axial position measurement points of the hollow core fiber, represents the light intensity at a measurement point represents the average value of the light intensity of all measurement points, represents the spatial gradient of the light intensity, represents the light intensity, z represents the axial coordinate of the hollow core fiber.​ 9. The method of claim 8, wherein the method further comprises: The atoms in the free-space cold atom ensemble are 87 Rb atoms; The target transition state is wherein, denotes the angular momentum quantum number, denotes the magnetic quantum number.

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