Frequency time-varying testing device and method for ultra-narrow linewidth laser

Abstract

The invention provides an ultra-narrow linewidth laser frequency time-varying testing device and method. The device comprises a first 1 * 2 coupler, a self-heterodyne structure, a frequency etalon structure, a frequency drift signal processing module and a frequency noise signal processing module. A short-length delay optical fiber is adopted to form a self-heterodyne structure, and the self-heterodyne structure is matched with a frequency drift signal processing circuit to realize laser frequency drift (less than 100Hz) measurement; meanwhile, a photonic crystal resonant cavity is adopted as a frequency reference and is matched with a frequency noise signal processing circuit to realize laser frequency noise (greater than or equal to 100Hz) measurement. According to the invention, the rapid measurement of the optical frequency drift / noise of different frequency components can be realized, the test device is simple, the measurement precision is high, the measurement range is wide, and a foundation is laid for the frequency characteristic evaluation of the ultra-narrow linewidth laser.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and in particular to a device and method for testing the time-varying frequency of an ultra-narrow linewidth laser. Background Technology

[0002] Ultra-narrow linewidth lasers, due to their advantages such as high beam quality, long coherence length, and narrow spectral linewidth, are widely used in many fields, including laser gyroscopes, lidar, laser remote sensing, and coherent optical communication. In these applications, the laser not only requires single-mode output but also high frequency stability. However, due to the influence of various factors such as temperature and mechanical vibration, the operating frequency of ultra-narrow linewidth lasers varies over time, manifesting as frequency noise and drift.

[0003] Conventional laser frequency time-varying measurements require instruments such as spectrometers and wavelength meters, which are large, expensive, and limited by the minimum resolution of these instruments. Beat frequency methods require a frequency reference with extremely stable frequencies (frequency stability more than two orders of magnitude higher than that of the laser under test), which is also difficult to achieve. Etameter methods require optical frequency combs and FP cavities, but the measurement accuracy is easily affected by the environmental adaptability of the optical frequency comb and FP cavity. Furthermore, the test range is limited by the full width at half maximum (FWHM) of the etalon, making it difficult to apply to a wider range of frequency time-varying measurements. How to increase the measurement range of optical frequency drift and improve the measurement accuracy of frequency noise and drift without increasing testing costs is a pressing technical challenge. Summary of the Invention

[0004] This invention provides a device and method for testing the time-varying frequency of an ultra-narrow linewidth laser, which can solve the technical problems of large size, high cost, low resolution, and small measurement range in existing tests for the time-varying frequency of ultra-narrow linewidth lasers.

[0005] According to one aspect of the present invention, a frequency time-varying test device for an ultra-narrow linewidth laser is provided, comprising a first 1×2 coupler, a self-heterodyne structure, a frequency etalon structure, a frequency drift signal processing module, and a frequency noise signal processing module;

[0006] The first 1×2 coupler is used to receive the laser emitted by the laser under test and split the laser into two laser beams with equal intensity. One laser beam enters the self-heterodyne structure, and the other laser beam enters the frequency etalon structure.

[0007] The self-heterodyne structure includes a second 1×2 coupler, a first fiber arm, a second fiber arm, a 2×1 coupler, and a first detector; the length of the first fiber arm is greater than the length of the second fiber arm; the second 1×2 coupler splits the input laser into two laser beams of equal intensity, one of which enters the 2×1 coupler through the first fiber arm, and the other enters the 2×1 coupler through the second fiber arm; the 2×1 coupler performs beat frequency analysis on the two input laser beams to obtain a beat frequency optical signal, which is output to the first detector; the first detector performs beat difference analysis on the beat frequency optical signal to obtain a beat difference electrical signal, which is output to the frequency drift signal processing module;

[0008] The frequency drift signal processing module is used to generate a first clock signal, use the first clock signal as a sampling clock to sample the beat difference signal, use the sampled data as frequency drift data, use the first clock signal as the X-axis and the frequency drift data as the Y-axis to obtain a frequency drift curve, and obtain the frequency drift of the laser under test based on the frequency drift curve.

[0009] The frequency etalon structure includes a modulator, a 2×2 coupler, and a second detector. The modulator performs phase modulation on the input laser to obtain a modulated optical signal, which is then output to the 2×2 coupler. The 2×2 coupler acts as a photonic crystal resonator to resonate the modulated optical signal, resulting in a resonant optical signal that is output to the second detector. The second detector detects the intensity of the resonant optical signal and outputs a resonant electrical signal to the frequency noise signal processing module.

[0010] The frequency noise signal processing module is used to determine whether the resonant electrical signal is in or outside the valley. If it is outside the valley, the driving current of the laser under test is increased or decreased to change the frequency of the laser under test until the resonant electrical signal is in the valley. If it is in the valley, the resonant electrical signal is filtered to obtain a filtered resonant electrical signal. This is used to generate a second clock signal, which is used as a demodulation clock to demodulate the filtered resonant electrical signal to obtain voltage data reflecting the magnitude of the frequency noise. The second clock signal is used as the X-axis, and the voltage data reflecting the magnitude of the frequency noise is used as the Y-axis to obtain a frequency noise curve. The frequency noise of the laser under test is obtained based on the frequency noise curve.

[0011] Preferably, the middle part of the first fiber optic arm is provided with several annular extension lines for delay.

[0012] Preferably, the lengths of the first fiber arm and the second fiber arm satisfy the following formula:

[0013]

[0014] In the formula, U(t) is the voltage corresponding to the beat difference electrical signal output by the first detector at time t, L1 is the length of the first fiber arm, L2 is the length of the second fiber arm, E0 is the light field intensity of the laser received by the second 1×2 coupler, n is the refractive index of the fiber, f(t) is the frequency of the laser received by the second 1×2 coupler at time t, c is the speed of light in vacuum, ∝ indicates proportionality, and Δf1 is the frequency deviation corresponding to the beat frequency optical signal.

[0015] Preferably, the intensity of the resonant optical signal and the parameters of the photonic crystal resonator satisfy the following equation:

[0016]

[0017] Q = C bar (1-α L ) 1 / 2 (1-α c ) 1 / 2 e -2πδ

[0018] In the formula, I D Let α be the intensity of the resonant optical signal, Q be the broadcast transfer function, Δf2 be the deviation between the laser center frequency and the resonant frequency of the resonant cavity, I0 be the intensity of the laser light received by the modulator, FSR be the free spectral linewidth of the photonic crystal resonant cavity, and α be the intensity of the laser light received by the modulator. c For the loss of a 2×2 coupler, C bar α is the coupling coefficient of the through optical field of the 2×2 coupler. L δ represents the loss of the photonic crystal resonator, and δ represents the full width at half maximum (FWHM) of the photonic crystal resonator.

[0019] According to another aspect of the present invention, a method for testing the time-varying frequency of an ultra-narrow linewidth laser is provided, the method employing any of the aforementioned apparatus for testing, the method comprising:

[0020] The laser under test emits a laser beam to the first 1×2 coupler, which splits the laser beam into two beams of equal intensity. One beam enters the second 1×2 coupler, and the other beam enters the modulator.

[0021] The second 1×2 coupler splits the input laser into two laser beams of equal intensity. One laser beam enters the 2×1 coupler through the first fiber arm, and the other laser beam enters the 2×1 coupler through the second fiber arm. The 2×1 coupler performs beat frequency analysis on the two input laser beams to obtain a beat frequency optical signal, which is then output to the first detector. The first detector performs beat difference analysis on the beat frequency optical signal to obtain a beat difference electrical signal, which is then output to the frequency drift signal processing module.

[0022] The frequency drift signal processing module generates a first clock signal, uses the first clock signal as a sampling clock to sample the beat difference signal, uses the sampled data as frequency drift data, uses the first clock signal as the X-axis and the frequency drift data as the Y-axis to obtain the frequency drift curve, and obtains the frequency drift of the laser under test based on the frequency drift curve.

[0023] The modulator performs phase modulation on the input laser, and the modulated optical signal is output to the 2×2 coupler. The 2×2 coupler acts as a photonic crystal resonator to resonate the modulated optical signal, and the resonant optical signal is output to the second detector. The second detector detects the intensity of the resonant optical signal and the resonant electrical signal is output to the frequency noise signal processing module.

[0024] The frequency noise signal processing module determines whether the resonant electrical signal is in or outside a valley. If it is outside a valley, the driving current of the laser under test is increased or decreased to change the frequency of the laser under test until the resonant electrical signal is in a valley. If it is in a valley, the resonant electrical signal is filtered to obtain a filtered resonant electrical signal. A second clock signal is generated and used as a demodulation clock to demodulate the filtered resonant electrical signal to obtain voltage data reflecting the magnitude of the frequency noise. The second clock signal is used as the X-axis, and the voltage data reflecting the magnitude of the frequency noise is used as the Y-axis to obtain a frequency noise curve. The frequency noise of the laser under test is obtained based on the frequency noise curve.

[0025] By employing the technical solution of this invention, a short-length delay fiber is used to form a self-heterodyne structure, which, in conjunction with a frequency drift signal processing circuit, enables the measurement of laser frequency drift (<100Hz). Simultaneously, a photonic crystal resonator is used as a frequency reference, in conjunction with a frequency noise signal processing circuit, to achieve the measurement of laser frequency noise (≥100Hz). This invention enables rapid measurement of optical frequency drift / noise components at different frequencies. The testing device is simple, the measurement accuracy is high, and it has a large measurement range, laying the foundation for the evaluation of the frequency characteristics of ultra-narrow linewidth lasers. Attached Figure Description

[0026] The accompanying drawings, which form part of this specification, are provided to further illustrate embodiments of the invention and, together with the textual description, explain the principles of the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0027] Figure 1 A schematic diagram of a time-varying frequency testing device for an ultra-narrow linewidth laser according to an embodiment of the present invention is shown.

[0028] Figure 2 A diagram of a frequency drift test channel structure according to an embodiment of the present invention is shown;

[0029] Figure 3 A diagram of a frequency noise test channel structure according to an embodiment of the present invention is shown;

[0030] Figure 4 The diagram shows light intensity output curves with different resonant frequency deviations according to an embodiment of the present invention. Detailed Implementation

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0033] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the invention. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following figures denote similar items; therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0034] like Figure 1As shown, the present invention provides a frequency time-varying test device for an ultra-narrow linewidth laser, including a first 1×2 coupler, a self-heterodyne structure, a frequency etalon structure, a frequency drift signal processing module, and a frequency noise signal processing module;

[0035] The first 1×2 coupler is used to receive the laser emitted by the laser under test and split the laser into two laser beams with equal intensity. One laser beam enters the self-heterodyne structure, and the other laser beam enters the frequency etalon structure.

[0036] The self-heterodyne structure includes a second 1×2 coupler, a first fiber arm, a second fiber arm, a 2×1 coupler, and a first detector; the length of the first fiber arm is greater than the length of the second fiber arm; the second 1×2 coupler splits the input laser into two laser beams of equal intensity, one of which enters the 2×1 coupler through the first fiber arm, and the other enters the 2×1 coupler through the second fiber arm; the 2×1 coupler performs beat frequency analysis on the two input laser beams to obtain a beat frequency optical signal, which is output to the first detector; the first detector performs beat difference analysis on the beat frequency optical signal to obtain a beat difference electrical signal, which is output to the frequency drift signal processing module;

[0037] The frequency drift signal processing module is used to generate a first clock signal, use the first clock signal as a sampling clock to sample the beat difference signal, use the sampled data as frequency drift data, use the first clock signal as the X-axis and the frequency drift data as the Y-axis to obtain a frequency drift curve, and obtain the frequency drift of the laser under test based on the frequency drift curve.

[0038] The frequency etalon structure includes a modulator, a 2×2 coupler, and a second detector. The modulator performs phase modulation on the input laser light, obtaining a modulated optical signal which is output to the 2×2 coupler. The 2×2 coupler acts as a photonic crystal resonator to resonate with the modulated optical signal, obtaining a resonant optical signal which is output to the second detector. The second detector detects the intensity of the resonant optical signal, obtaining a resonant electrical signal which is output to the frequency noise signal processing module. The resonant optical signal is a signal that outputs light at a specific wavelength in a resonant valley manner.

[0039] The frequency noise signal processing module is used to determine whether the resonant electrical signal is in or outside the valley. If it is outside the valley, the driving current of the laser under test is increased or decreased to change the frequency of the laser under test until the resonant electrical signal is in the valley. If it is in the valley, the resonant electrical signal is filtered to obtain a filtered resonant electrical signal. This is used to generate a second clock signal, which is used as a demodulation clock to demodulate the filtered resonant electrical signal to obtain voltage data reflecting the magnitude of the frequency noise. The second clock signal is used as the X-axis, and the voltage data reflecting the magnitude of the frequency noise is used as the Y-axis to obtain a frequency noise curve. The frequency noise of the laser under test is obtained based on the frequency noise curve.

[0040] This invention also provides a method for testing the time-varying frequency of an ultra-narrow linewidth laser, wherein the method uses the aforementioned apparatus for testing, and the method includes:

[0041] The laser under test emits a laser beam to the first 1×2 coupler, which splits the laser beam into two beams of equal intensity. One beam enters the second 1×2 coupler, and the other beam enters the modulator.

[0042] The second 1×2 coupler splits the input laser into two laser beams of equal intensity. One laser beam enters the 2×1 coupler through the first fiber arm, and the other laser beam enters the 2×1 coupler through the second fiber arm. The 2×1 coupler performs beat frequency analysis on the two input laser beams to obtain a beat frequency optical signal, which is then output to the first detector. The first detector performs beat difference analysis on the beat frequency optical signal to obtain a beat difference electrical signal, which is then output to the frequency drift signal processing module.

[0043] The frequency drift signal processing module generates a first clock signal, uses the first clock signal as a sampling clock to sample the beat difference signal, uses the sampled data as frequency drift data, uses the first clock signal as the X-axis and the frequency drift data as the Y-axis to obtain the frequency drift curve, and obtains the frequency drift of the laser under test based on the frequency drift curve.

[0044] The modulator performs phase modulation on the input laser, and the modulated optical signal is output to the 2×2 coupler. The 2×2 coupler acts as a photonic crystal resonator to resonate the modulated optical signal, and the resonant optical signal is output to the second detector. The second detector detects the intensity of the resonant optical signal and the resonant electrical signal is output to the frequency noise signal processing module.

[0045] The frequency noise signal processing module determines whether the resonant electrical signal is in or outside a valley. If it is outside a valley, the driving current of the laser under test is increased or decreased to change the frequency of the laser under test until the resonant electrical signal is in a valley. If it is in a valley, the resonant electrical signal is filtered to obtain a filtered resonant electrical signal. A second clock signal is generated and used as a demodulation clock to demodulate the filtered resonant electrical signal, obtaining voltage data reflecting the magnitude of the frequency noise. The second clock signal is used as the X-axis, and the voltage data reflecting the magnitude of the frequency noise is used as the Y-axis to obtain a frequency noise curve. The frequency noise of the laser under test is obtained based on the frequency noise curve. Filtering the resonant electrical signal refers to removing low-frequency components.

[0046] To gain a further understanding of the present invention, the following description is provided. Figures 1-4 The present invention provides a detailed description of the time-varying frequency testing apparatus and method for ultra-narrow linewidth lasers.

[0047] like Figure 1 As shown, the laser emitted by the laser under test enters the first 1×2 coupler and is split into two laser beams of equal intensity after being split in a 1:1 ratio. One laser beam enters the frequency drift test channel composed of a self-heterodyne structure and a frequency drift signal processing module, while the other laser beam enters the frequency noise test channel composed of a frequency etalon structure and a frequency noise signal processing module. The laser frequency drift (<100Hz) characteristics and frequency noise (≥100Hz) are obtained respectively.

[0048] like Figure 2 The diagram shows the frequency drift test channel. The laser under test first enters the self-heterodyne optical path and is split into two beams by a 1×2 coupler. One beam is delayed by an optical fiber delay line, while the other beam is directly transmitted through an optical fiber as a non-delayed arm. Beat frequency occurs at the 2×1 coupler, and then the beat signal is output through detector 1. This electrical signal enters the drift signal processing circuit and then the high-speed signal acquisition circuit. The clock signal is used as the sampling clock, and the sampled data is used as typical frequency drift data as the Y-axis output. Simultaneously, the time signal is obtained by counting the clock signal and used as the X-axis output. A typical frequency drift curve is plotted, and the laser frequency drift per unit time can be obtained from the typical frequency drift curve.

[0049] The time-delay fiber is made of single-mode fiber, which has the advantage of low cost. Its length is designed as L1, and the length of the non-time-delay arm fiber is designed as L2. The voltage corresponding to the beat difference signal is U(t), and it satisfies the following relationship with the two fiber arms:

[0050]

[0051] The curve showing the relationship between arm length difference and frequency drift can be used to determine the minimum arm length difference. Combined with the low-cost advantage of this invention, the lengths of the fiber delay line and the non-delay arm can be designed. Based on the designed arm lengths, the frequency deviation corresponding to the single-cycle occurrence of the beat difference electrical signal output by detector 1 can be obtained, as shown below:

[0052]

[0053] In the formula, U(t) is the voltage corresponding to the beat difference electrical signal output by the first detector 1 at time t, L1 is the length of the first fiber arm, L2 is the length of the second fiber arm, E0 is the light field intensity of the laser received by the second 1×2 coupler, n is the refractive index of the fiber, f(t) is the frequency of the laser received by the second 1×2 coupler at time t, c is the speed of light in vacuum, ∝ indicates proportionality, and Δf1 is the frequency deviation corresponding to the beat frequency optical signal.

[0054] Since the short-term frequency drift of lasers is typically between 1-10MHz, in order to achieve frequency drift detection of around MHz, L1-L2 needs to be controlled between 20-200 meters. Taking this invention as an example, setting L1 = 200 meters and L2 = 20 meters, the frequency drift corresponding to the single-cycle occurrence of the beat signal output by detector 1 is approximately 1.15MHz. This can be achieved by... Figure 2 The frequency drift of the laser under test can be obtained by testing the typical frequency drift curve.

[0055] like Figure 3 The diagram shows the frequency noise test channel. The laser under test first enters the frequency etalon structure, then the phase modulator for square wave signal modulation, and subsequently the 2×2 coupler, which serves as a photonic crystal resonator. The photonic crystal resonator has frequency selectivity; only light satisfying a specific wavelength can be output in a resonant valley manner, and its intensity is detected on detector 2. The intensity I of the detected resonant light signal is measured. D The resonant cavity parameters satisfy the following relationship:

[0056]

[0057] Q = C bar (1-α L ) 1 / 2 (1-α c ) 1 / 2 e -2πδ

[0058] In the formula, I D Let α be the intensity of the resonant optical signal, Q be the broadcast transfer function, Δf2 be the deviation between the laser center frequency and the resonant frequency of the resonant cavity, I0 be the intensity of the laser light received by the modulator, FSR be the free spectral linewidth of the photonic crystal resonant cavity, and α be the intensity of the laser light received by the modulator. cFor the loss of a 2×2 coupler, C bar α is the coupling coefficient of the through optical field of the 2×2 coupler. L δ represents the loss of the photonic crystal resonator, and δ represents the full width at half maximum (FWHM) of the photonic crystal resonator.

[0059] From the above, it can be seen that when the deviation Δf2 between the laser center frequency and the resonant cavity frequency is minimal, I D The smallest value corresponds to the weakest output light intensity, which is the "resonance valley" (e.g., ...). Figure 4 As shown, different FSRs can be controlled by controlling the length of the resonant cavity. Once the length of the resonant cavity is determined, the laser frequency noise level can be obtained by observing the output light intensity in the resonant valley.

[0060] Since laser frequency noise disturbances are typically below 100kHz, the FSR must be at least 100 times larger. Since the FSR / FWHM ratio is usually between 20 and 50, to achieve a relatively ideal noise resolution, the FWHM needs to be at least 5 times larger than the noise disturbance. Therefore, a fiber loop size of 5-10m is more suitable. Taking this invention as an example, setting L = 5m, FSR = 40MHz, and 800kHz < FWHM < 2MHz facilitates effective detection of frequency noise below 100kHz.

[0061] In summary, this invention provides a time-varying frequency testing device and method for ultra-narrow linewidth lasers. It employs a short-length delay fiber to form a self-heterodyne structure, which, in conjunction with a frequency drift signal processing circuit, enables the measurement of laser frequency drift (<100Hz). Simultaneously, a photonic crystal resonator is used as a frequency reference, and in conjunction with a frequency noise signal processing circuit, the laser frequency noise (≥100Hz) is measured. This invention enables rapid measurement of optical frequency drift / noise components at different frequencies. The testing device is simple, has high measurement accuracy, and possesses a wide measurement range, laying the foundation for the evaluation of the frequency characteristics of ultra-narrow linewidth lasers.

[0062] For ease of description, spatial relative terms such as "above," "on top of," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation beyond the orientation of the device as described in the figures. For example, if the device in the figures were inverted, a device described as "above" or "on top of" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein will be interpreted accordingly.

[0063] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A time-varying frequency testing device for an ultra-narrow linewidth laser, characterized in that, It includes a first 1×2 coupler, a self-heterodyne structure, a frequency etalon structure, a frequency drift signal processing module, and a frequency noise signal processing module; The first 1×2 coupler is used to receive the laser emitted by the laser under test and split the laser into two laser beams with equal intensity. One laser beam enters the self-heterodyne structure, and the other laser beam enters the frequency etalon structure. The self-heterodyne structure includes a second 1×2 coupler, a first fiber arm, a second fiber arm, a 2×1 coupler, and a first detector; The length of the first fiber arm is greater than the length of the second fiber arm; the second 1×2 coupler splits the input laser into two laser beams of equal intensity, one of which enters the 2×1 coupler through the first fiber arm, and the other enters the 2×1 coupler through the second fiber arm. The 2×1 coupler performs beat frequency analysis on the two input laser beams to obtain a beat frequency optical signal, which is output to the first detector; the first detector performs beat difference analysis on the beat frequency optical signal to obtain a beat difference electrical signal, which is output to the frequency drift signal processing module. The frequency drift signal processing module is used to generate a first clock signal, use the first clock signal as a sampling clock to sample the beat difference signal, use the sampled data as frequency drift data, use the first clock signal as the X-axis and the frequency drift data as the Y-axis to obtain a frequency drift curve, and obtain the frequency drift of the laser under test based on the frequency drift curve. The frequency etalon structure includes a modulator, a 2×2 coupler, and a second detector. The modulator performs phase modulation on the input laser to obtain a modulated optical signal, which is then output to the 2×2 coupler. The 2×2 coupler acts as a photonic crystal resonator to resonate the modulated optical signal, resulting in a resonant optical signal that is output to the second detector. The second detector detects the intensity of the resonant optical signal and outputs a resonant electrical signal to the frequency noise signal processing module. The frequency noise signal processing module is used to determine whether the resonant electrical signal is in or outside the valley. If it is outside the valley, the driving current of the laser under test is increased or decreased to change the frequency of the laser under test until the resonant electrical signal is in the valley. If it is in the valley, the resonant electrical signal is filtered to obtain the filtered resonant electrical signal; The second clock signal is used to generate a second clock signal. The second clock signal is used as a demodulation clock to demodulate the filtered resonant electrical signal to obtain voltage data that reflects the magnitude of frequency noise. The second clock signal is used as the X-axis and the voltage data that reflects the magnitude of frequency noise is used as the Y-axis to obtain a frequency noise curve. The frequency noise of the laser under test is obtained based on the frequency noise curve.

2. The apparatus according to claim 1, characterized in that, The first fiber arm has several annular extension lines for delay in the middle.

3. The apparatus according to claim 1 or 2, characterized in that, The lengths of the first fiber arm and the second fiber arm satisfy the following formula: In the formula, U(t) is the voltage corresponding to the beat difference electrical signal output by the first detector at time t, L1 is the length of the first fiber arm, L2 is the length of the second fiber arm, E0 is the light field intensity of the laser received by the second 1×2 coupler, n is the refractive index of the fiber, f(t) is the frequency of the laser received by the second 1×2 coupler at time t, c is the speed of light in vacuum, ∝ indicates proportionality, and Δf1 is the frequency deviation corresponding to the beat frequency optical signal.

4. The apparatus according to any one of claims 1-3, characterized in that, The intensity of the resonant optical signal and the parameters of the photonic crystal resonator satisfy the following equation: Q=C bar (1-a L ) 1 / 2 (1-a c ) 1 / 2 e -2πδ In the formula, I D Let α be the intensity of the resonant optical signal, Q be the broadcast transfer function, Δf2 be the deviation between the laser center frequency and the resonant frequency of the resonant cavity, I0 be the intensity of the laser light received by the modulator, FSR be the free spectral linewidth of the photonic crystal resonant cavity, and α be the intensity of the laser light received by the modulator. c For the loss of a 2×2 coupler, C bar α is the coupling coefficient of the through optical field of the 2×2 coupler. L δ represents the loss of the photonic crystal resonator, and δ represents the full width at half maximum (FWHM) of the photonic crystal resonator.

5. A method for testing the time-varying frequency of an ultra-narrow linewidth laser, characterized in that, The method employs the apparatus described in any one of claims 1-4 for testing, and the method comprises: The laser under test emits a laser beam to the first 1×2 coupler, which splits the laser beam into two beams of equal intensity. One beam enters the second 1×2 coupler, and the other beam enters the modulator. The second 1×2 coupler splits the input laser into two laser beams of equal intensity. One laser beam enters the 2×1 coupler through the first fiber arm, and the other laser beam enters the 2×1 coupler through the second fiber arm. The 2×1 coupler performs beat frequency analysis on the two input laser beams to obtain a beat frequency optical signal, which is then output to the first detector. The first detector performs beat difference analysis on the beat frequency optical signal to obtain a beat difference electrical signal, which is then output to the frequency drift signal processing module. The frequency drift signal processing module generates a first clock signal, uses the first clock signal as a sampling clock to sample the beat difference signal, uses the sampled data as frequency drift data, uses the first clock signal as the X-axis and the frequency drift data as the Y-axis to obtain the frequency drift curve, and obtains the frequency drift of the laser under test based on the frequency drift curve. The modulator performs phase modulation on the input laser, and the modulated optical signal is output to the 2×2 coupler. The 2×2 coupler acts as a photonic crystal resonator to resonate the modulated optical signal, and the resonant optical signal is output to the second detector. The second detector detects the intensity of the resonant optical signal and the resonant electrical signal is output to the frequency noise signal processing module. The frequency noise signal processing module determines whether the resonant electrical signal is in or outside the valley. If it is outside the valley, the driving current of the laser under test is increased or decreased to change the frequency of the laser under test until the resonant electrical signal is in the valley. If it is in the valley, the resonant electrical signal is filtered to obtain the filtered resonant electrical signal. A second clock signal is generated and used as a demodulation clock to demodulate the filtered resonant electrical signal to obtain voltage data reflecting the magnitude of frequency noise. The second clock signal is used as the X-axis and the voltage data reflecting the magnitude of frequency noise is used as the Y-axis to obtain a frequency noise curve. The frequency noise of the laser under test is obtained based on the frequency noise curve.

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