A single-frequency fiber laser with wide-range fine adjustment of line width

Abstract

This invention provides a single-frequency fiber laser with a wide range of finely adjustable linewidth, comprising a single-frequency fiber laser resonator, a pump source, a wavelength division multiplexer, an integrated fiber isolation coupling device, a monitoring-feedback module, and a phase modulation unit. The wavelength division multiplexer couples the pump light output from the pump source into the single-frequency fiber laser resonator and outputs the generated single-frequency fiber laser. The integrated fiber isolation coupling device splits the single-frequency fiber laser beam and inputs it to the main optical path and the monitoring-feedback module respectively. The monitoring-feedback module acquires the phase fluctuation information of the single-frequency fiber laser in real time and generates a phase modulation signal according to the preset linewidth control requirements. The phase modulation unit compensates for or superimposes the phase fluctuation of the single-frequency fiber laser transmitted on the main optical path according to the phase modulation signal output by the monitoring-feedback module, thereby achieving the effect of suppressing or increasing laser phase noise and realizing the function of finely controlling the linewidth of the fiber laser over a wide range.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and in particular to a single-frequency fiber laser with a wide range of finely adjustable linewidth. Background Technology

[0002] Single-frequency fiber lasers (SFLS) are widely used in fiber optic communication, distributed fiber optic sensing, lidar, and optical measurement due to their compact structure, high optical signal-to-noise ratio, and ease of fiber optic integration. Linewidth is a crucial performance indicator of SFLS, reflecting the laser's coherence and monochromaticity, and is a core parameter of concern for many laser application systems. For example, in fiber optic communication, phase shift keying (PSK) or higher-order quadrature amplitude modulation (QAM) techniques used to improve channel capacity are closely related to laser linewidth; similarly, in fiber optic sensing, phase-generated carrier technology used to enhance system detection sensitivity is also closely related to laser linewidth. However, studies on the correlation between linewidth and system performance are typically theoretical or based on comparisons of different light sources, lacking single-source light sources with finely tunable linewidth over a wide range for comparative experiments. Linewidth control is mainly divided into two dimensions: linewidth narrowing and linewidth broadening.

[0003] Common linewidth compression schemes mainly include external cavity feedback or self-injection locking, and high-precision reference cavity locking. External cavity feedback or self-injection locking utilizes a portion of the laser's output light, reflecting it back to the main cavity through fiber delay lines, filters, or resonant cavities to form external phase feedback, suppressing the laser's own phase noise and achieving linewidth compression. This type of scheme relies on the feedback loop formed by an external optical path (fiber delay lines, filters, etc.). The length and phase of the feedback path are highly susceptible to drift due to changes in ambient temperature and mechanical vibration. Even a slight change in the feedback conditions (such as feedback phase and intensity) caused by the external environment can easily disrupt the locking conditions, leading to mode hopping or direct locking failure, resulting in unstable output and making long-term reliable operation in engineering settings difficult. Furthermore, it typically only achieves a single compression function, making it difficult to dynamically switch or finely adjust the degree of compression within the same system. High-precision reference cavity locking technology typically uses high-quality Fabry-Perot cavities or ultra-stable optical resonators as phase-detection references. Error signals are extracted using methods such as Pound-Drever-Hall (PDH), and then driven by the laser's current, piezoelectric ceramics, or cavity length actuators to actively lock and narrow the linewidth. While high-precision reference cavity locking technology can achieve extremely narrow linewidths, the system structure is complex, requiring the introduction of free-space optical elements (lenses, mirrors, beam splitters, etc.) and precise temperature control and vibration isolation measures. This complexity and high cost limit its practicality in engineering applications.

[0004] Linear width broadening primarily falls into two categories: phase modulation broadening and nonlinear effect broadening. Phase modulation broadening typically involves connecting a high-speed electro-optic phase modulator in series after the seed laser. By injecting broadband noise or signals with specific spectral characteristics, random perturbations are generated in the optical field phase to obtain a definable broadening bandwidth and spectral shape. While rapid broadening can be achieved through noise injection, this strong phase modulation often introduces additional sidebands or frequency components, causing the output spectrum to deviate from strictly single-frequency characteristics, potentially impacting applications requiring extremely high spectral purity. Nonlinear effect broadening mainly relies on the four-wave mixing effect in the high-power amplifier link. Nonlinear effect broadening of laser output exhibits strong device and structural dependencies; once the system is built, its linewidth is essentially fixed, making real-time, dynamic adjustment of linewidth and spectral shape difficult. Furthermore, the broadened spectrum generated by the nonlinear process is usually a discrete multi-peak spectrum rather than a smooth single-peak spectrum, strictly deviating from the definition of a single-frequency laser and not a strictly single-frequency laser output.

[0005] For bidirectional linewidth modulation, South China University of Technology proposed a self-injection locking structure based on fiber stretcher noise injection in 2017, achieving single-frequency fiber laser linewidth modulation from 0.8 to 353 kHz. However, the linewidth compression capability of this scheme is strongly limited by the parameters of the self-injection locking structure; simultaneously, due to limitations in the bandwidth and amplitude of self-generated noise and the phase modulation capability of the fiber stretcher, further linewidth broadening is also difficult to achieve.

[0006] As can be seen from the above, the existing technology still lacks a method for achieving a wide dynamic range and continuously adjustable bidirectional linewidth fine control in the same system structure while maintaining single-frequency characteristics. Summary of the Invention

[0007] To address the problems existing in the prior art, the present invention provides a single-frequency fiber laser with a wide range of finely adjustable linewidth.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] On the one hand, a single-frequency fiber laser with a wide range of finely adjustable linewidth is provided, comprising: A single-frequency fiber laser resonator is used to generate single-frequency fiber lasers. A pump source is used to provide pump light to the single-frequency fiber laser resonator. A wavelength division multiplexer connects a pump source and a single-frequency fiber laser resonator. It is used to couple the pump light output from the pump source into the single-frequency fiber laser resonator and output the single-frequency fiber laser generated by the single-frequency fiber laser resonator. The fiber optic isolated coupling integrated device has an input end connected to the output end of a wavelength division multiplexer. The two output ends are a high-power output end and a low-power output end, which are used to separate single-frequency fiber lasers and input them to the main optical path and the monitoring-feedback module, respectively. The monitoring-feedback module receives the laser signal output from the low-power output terminal, acquires the phase fluctuation information of the single-frequency fiber laser in real time, and generates the corresponding phase modulation signal according to the preset linewidth control requirements. The main optical path is equipped with a phase modulation unit, which is connected to a high-power output terminal. The phase modulation unit is controlled by the monitoring-feedback module. Based on the phase modulation signal output by the monitoring-feedback module, the phase fluctuation of the single-frequency fiber laser transmitted on the main optical path is compensated or superimposed to achieve a wide range of fine control of the laser linewidth.

[0010] Furthermore, when the preset linewidth control requirement is linewidth narrowing mode, the monitoring-feedback module achieves linewidth narrowing through closed-loop suppression of single-frequency fiber laser phase noise, including: (1.1) Convert the phase fluctuation signal of the laser into a voltage error signal; (1.2) The signal processing unit uses a proportional-integral algorithm to generate a phase compensation signal based on the voltage error signal; (1.3) Apply the phase compensation signal to the photoelectric phase modulator, introduce the phase negative feedback signal, and after the closed-loop negative feedback control link, reduce the residual phase noise power spectral density within the feedback bandwidth to achieve laser linewidth narrowing.

[0011] Furthermore, when the preset linewidth control requirement is linewidth broadening mode, the monitoring-feedback module injects random noise signals into the photoelectric phase modulator, causing controlled random disturbances in the laser center frequency, thereby achieving active linewidth broadening, including the following steps: (2.1) Calculate the required white frequency noise power density based on the preset target Lorentz linewidth, and convert it into phase noise power spectral density by integration; (2.2) Generate Gaussian white noise samples in the digital domain; (2.3) Generate a phase-modulated signal based on the current gain factor and Gaussian white noise samples, wherein the initial gain factor is preset; (2.4) Apply the phase modulation signal to the photoelectric phase modulator; (2.5) By measuring the laser linewidth in real time and adjusting the gain factor, return to step (2.3) to achieve closed-loop calibration and stable control of the unfolded linewidth until the laser linewidth measured in real time meets the preset target Lorentz linewidth requirement.

[0012] Furthermore, the single-frequency fiber laser resonator is of the distributed Bragg reflection type or distributed feedback type, and is encapsulated in a temperature control unit to maintain a stable operating temperature. The high-reflection end of the resonator is cut into an 8-degree angle.

[0013] The technical effects that this invention can produce through the above technical solution are: Traditional single-frequency fiber lasers typically focus on minimizing linewidth, resulting in essentially fixed linewidth characteristics once manufactured. Demanding a wide linewidth source often requires replacing the laser or employing complex mechanical stretching / temperature tuning, which has a limited adjustment range and slow response. This invention provides a single-frequency fiber laser with a wide and finely tunable linewidth. Based on a monitoring-feedback module and a phase modulation unit, it achieves bidirectional control of the laser's phase frequency noise characteristics, enabling stable single-frequency laser output with a large dynamic range and high resolution, and continuously adjustable linewidth. This invention employs active feedback optical field phase noise manipulation technology, integrating linewidth compression and broadening into a single control device, improving the device's practicality while simplifying the laser system. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of a single-frequency fiber laser with a wide range of finely adjustable linewidth provided in one embodiment; Numbering on the map: 1. Single-frequency fiber laser resonator; 2. Temperature control unit; 3. Pump source; 4. Wavelength division multiplexer; 5. Fiber optic isolation coupling integrated device; 6. Phase detection unit; 7. Signal processing unit; 8. Signal amplifier; 9. Phase modulation unit. Detailed Implementation

[0016] The technical solution of the present invention will now be clearly and completely described through specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0017] In one embodiment, a single-frequency fiber laser with a wide range of finely adjustable linewidth is provided, comprising: Single-frequency fiber laser resonator 1, used to generate single-frequency fiber laser; Pump source 3 is used to provide pump light for the single-frequency fiber laser resonator 1; Wavelength division multiplexer 4 connects pump source 3 and single-frequency fiber laser resonator 1, and is used to couple the pump light output from pump source 3 into single-frequency fiber laser resonator 1 and output the single-frequency fiber laser generated by single-frequency fiber laser resonator 1. The fiber optic isolation coupling integrated device 5 has its input end connected to the output end of the wavelength division multiplexer 4. The two output ends are a high-power output end and a low-power output end, which are used to separate the single-frequency fiber laser and input it to the main optical path and the monitoring-feedback module, respectively. The monitoring-feedback module receives the laser signal output from the low-power output terminal, acquires the phase fluctuation information of the single-frequency fiber laser in real time, and generates the corresponding phase modulation signal according to the preset linewidth control requirements. The main optical path is equipped with a phase modulation unit, which is connected to a high-power output terminal. The phase modulation unit is controlled by the monitoring-feedback module. Based on the phase modulation signal output by the monitoring-feedback module, the phase fluctuation of the single-frequency fiber laser transmitted on the main optical path is compensated or superimposed to achieve a wide range of fine control of the laser linewidth.

[0018] In the above embodiment, the pump source 3 is a single-mode pump source, the output end of the pump source 3 is connected to the pump input end of the wavelength division multiplexer 4, the common end of the wavelength division multiplexer 4 is connected to the low reflection end of the single-frequency fiber laser resonator 1, and the output end of the wavelength division multiplexer 4 is used to output single-frequency fiber laser.

[0019] In the above embodiments, the monitoring-feedback module includes a phase detection unit 6, a signal processing unit 7, and a signal amplifier 8. The phase detection unit 6 is used to acquire the phase fluctuation information of the single-frequency fiber laser. The signal processing unit 7 is used to receive the phase fluctuation information of the single-frequency fiber laser and generate a corresponding phase modulation signal according to the preset linewidth adjustment requirements. The signal amplifier 8 is used to amplify the phase modulation signal and output it to the phase modulation unit 9. The phase detection unit 6 is any one of a Fabry-Perot resonator, an unbalanced fiber interferometer, an atomic / molecular absorption spectrum, or a high-Q microcavity. The phase detection unit 6 converts the optical field phase information of the single-frequency fiber laser into an electrical signal and simultaneously acquires the phase fluctuation signal of the single-frequency fiber laser. The signal processing unit 7 is any one of an analog control circuit, a digital control circuit, or an analog-digital hybrid control circuit. The phase modulation unit 9 is any one of an electro-optic phase modulator, an acousto-optic phase modulator, a fiber stretcher, or a liquid crystal phase modulator.

[0020] The preset linewidth adjustment requirements described in this invention include linewidth widening or linewidth narrowing: When the current preset linewidth control requirement is to narrow the linewidth, the signal processing unit 7 analyzes the phase fluctuation information of the single-frequency fiber laser and generates an error correction signal with the same amplitude but opposite phase as the phase fluctuation information as a phase modulation signal. The phase modulation signal is amplified by the signal amplifier and output to the phase modulation unit 9. The phase modulation unit 9 performs reverse compensation on the phase of the single-frequency fiber laser in the main optical path according to the phase modulation signal to cancel the inherent phase noise of the laser, thereby narrowing the laser linewidth. When the current preset linewidth adjustment requirement is linewidth widening, the signal processing unit 7 generates a phase modulation signal with the required statistical characteristics or spectral features based on the phase fluctuation information of the single-frequency fiber laser, or based on the internally generated pseudo-random noise sequence, or based on the internally preset frequency modulation waveform. The phase modulation signal is amplified by the signal amplifier and output to the phase modulation unit 9. The phase modulation unit 9 superimposes the phase modulation signal onto the phase of the single-frequency fiber laser on the main optical path, introducing controlled phase perturbation, thereby widening the laser linewidth.

[0021] One specific embodiment, based on Figure 1 The structure shown provides a single-frequency fiber laser with a wide range of finely adjustable linewidth, including a single-frequency fiber laser resonator 1, a temperature control unit 2, a pump source 3, a wavelength division multiplexer 4, an optical fiber isolation coupling integrated device 5, a phase detector unit 6, a signal processing unit 7, a signal amplifier 8, and a phase modulation unit 9.

[0022] The single-frequency fiber laser resonator 1 is a DBR short-cavity structure. The gain fiber is a rare-earth ion-doped fiber on the centimeter scale; in this embodiment, erbium-ytterbium co-doped fiber is used, with a fiber gain coefficient of 5.2 dB / cm. A polarization-maintaining narrowband fiber Bragg grating (PM-NB-FBG) and a broadband fiber Bragg grating (PM-WB-FBG) constitute the cavity mirror of the DBR resonator. In this embodiment, the PM-NB-FBG has a center wavelength of 1550 nm, a reflectivity of 60%, and a 3 dB bandwidth of approximately 0.06 nm; the PM-WB-FBG has a center wavelength of 1550 nm, a reflectivity of 99.9%, and a 3 dB bandwidth of approximately 0.4 nm. The gain fiber, the PM-NB-FBG, and the PM-WB-FBG are fused together to form the single-frequency fiber laser resonator. The pigtail of the PM-WB-FBG is cut at an 8-degree angle. DBR short cavities offer good single-mode selectivity and compactness, but they are often more sensitive to noise. This invention uses a monitoring-feedback module and a phase modulation unit to achieve feedback control, which effectively compensates for the shortcomings of short cavities in phase noise suppression, while retaining the advantages of short cavity structure, low threshold, and high efficiency.

[0023] The single-frequency fiber laser resonator 1 is encapsulated within a temperature control unit 2, which consists of an aluminum tank and a thermoelectric cooler (TEC). The TEC has a temperature control accuracy of 0.01℃. This high-precision temperature control device ensures the long-term stable operation of the seed source within a single longitudinal mode range, thus having a positive effect on the phase stability of the laser.

[0024] The pump source 3 is a single-mode pump source with a center wavelength of 974 nm, a maximum output power of 360 mW, and a maximum withstand current of 720 mA. The common terminal of the wavelength division multiplexer 4 operates in a wavelength range of 1555±20 nm, with a maximum insertion loss of 0.14 dB and a minimum isolation of 56 dB. The pump input terminal of the wavelength division multiplexer 4 operates in a wavelength range of 980±20 nm, with a maximum insertion loss of 0.42 dB and a minimum isolation of 37 dB. The output terminal of the pump source 3 is connected to the pump input terminal of the wavelength division multiplexer 4. The common terminal of the wavelength division multiplexer 4 is connected to the low-reflection terminal of the single-frequency fiber laser resonator 1, and the output terminal of the wavelength division multiplexer 4 is used to output single-frequency fiber laser light.

[0025] The fiber optic isolation coupling integrated device 5 integrates an isolator and a coupler, with a center wavelength of 1550 nm, a 1×2 configuration, a two-way output power ratio of 1:9 (output1-90%, output2-10%), maximum insertion losses of 0.52 dB and 10.23 dB respectively, and an isolation of 50 dB. The high-power output terminal of the fiber optic isolation coupling integrated device 5 is connected to the input terminal of the phase modulation unit 9, and the low-power output terminal of the fiber optic isolation coupling integrated device 5 is connected to the input terminal of the phase detection unit (6).

[0026] The phase detection unit 6 employs a Michelson unbalanced fiber interferometer with a short arm length of 1.0 m, an arm difference of 100 m, and an insertion loss of 3.63 dB. The fiber is an SMF28e single-mode fiber. The phase detection unit 6 is used to convert the optical field phase information into an electrical signal and simultaneously acquire the phase fluctuation signal of the laser.

[0027] The signal processing unit 7 adopts a PI-based integrated digital circuit module, which can generate corresponding positive or negative feedback phase modulation signals according to specific linewidth control requirements (including linewidth expansion or linewidth narrowing), thereby realizing a wide range of fine control of the laser linewidth.

[0028] The signal amplifier 8 operates at a frequency of 1 kHz to 10 GHz with a maximum gain of 30 dB. It is mainly used to enhance the strength of the phase modulation signal generated by the signal processing unit 7, thereby meeting the drive signal strength requirements of the phase modulation unit 9.

[0029] The phase modulation unit 9 employs an electro-optic phase modulator, which is a lithium niobate polarization-maintaining high-speed photoelectric phase modulator with a working wavelength of 1550 nm, a working bandwidth of DC to 200 MHz, a loss of less than 3.5 dB, and a half-wave voltage of less than 3.5 V. As an actuator for linewidth modulation, it receives signals carrying phase modulation information to achieve compensation or superposition manipulation of laser phase fluctuations.

[0030] The signal processing unit 7 incorporates different signal processing design algorithms, which can output different phase modulation signals according to the user's requirements for linewidth narrowing or widening. When linewidth narrowing is required, a compensation design for laser phase fluctuations is implemented, constructing a closed-loop negative feedback control link to achieve stable control of laser phase fluctuations (i.e., laser phase noise suppression), thereby achieving the technical effect of laser linewidth narrowing. When linewidth widening is required, superposition manipulation of laser phase fluctuations is implemented, constructing a closed-loop positive feedback control link to achieve enhanced manipulation of laser phase fluctuations (i.e., laser phase noise increase), thereby achieving the technical effect of laser linewidth widening.

[0031] When the preset linewidth adjustment requirement is to narrow the linewidth, the linewidth narrowing mode is activated. The monitoring-feedback module achieves linewidth narrowing by closed-loop suppression of single-frequency fiber laser phase noise. The specific steps are as follows: (1.1) Convert the phase fluctuation signal of the laser into a voltage error signal; (1.2) The signal processing unit uses a proportional-integral algorithm to generate a phase compensation signal based on the voltage error signal; (1.3) Apply the phase compensation signal to the photoelectric phase modulator, introduce the phase negative feedback signal, and after the closed-loop negative feedback control link, reduce the residual phase noise power spectral density within the feedback bandwidth to achieve laser linewidth narrowing.

[0032] In step (1.1) above, the definition is... Laser electric field intensity at time 1 for:

[0033] in For light field intensity, This refers to the random phase jitter of the light field. The imaginary unit, ω is the center angular frequency of the laser. Random phase jitter in the optical field reflects the instability of the laser phase and is the main factor leading to laser linewidth broadening.

[0034] The phase fluctuation of the optical field is converted into a voltage error signal using the phase detector unit (6). :

[0035] in, To improve phase detection sensitivity, The time delay introduced in the phase detection unit Random phase jitter of the light field The corresponding spectrum, For analysis of frequency, it refers to the frequency component of the phase noise of interest, used to describe the distribution characteristics of phase noise at different frequencies.

[0036] When the analysis frequency much smaller At this time, the phase detector unit is equivalent to a frequency discriminator, and the output signal is proportional to the laser frequency noise.

[0037] In step (1.2) above, the signal processing unit within the monitoring-feedback module generates signals using a proportional-integral (PI) algorithm. Phase compensation signal at time : After inverse Fourier transform, we obtain ; Phase compensation signal at time for:

[0038] in and These are the proportional coefficient and the integral coefficient, respectively. This is the time integration variable.

[0039] In step (1.3) above, the phase compensation signal is... Phase compensation signal applied to photoelectric phase modulator Drive the photoelectric phase modulator and introduce a phase negative feedback signal :

[0040] in This represents the gain of the photoelectric phase modulator.

[0041] By using a closed-loop negative feedback control link, the residual phase noise power spectral density is reduced within the feedback bandwidth, thereby narrowing the laser linewidth. Specifically, the residual phase noise power spectral density after closed-loop negative feedback control... Determined by the following formula:

[0042] in This represents the original phase noise power spectral density without closed-loop negative feedback control.

[0043] In summary, this invention adjusts the proportional coefficient through digital fine-tuning. and integral coefficient The negative phase feedback signal forms destructive interference with the original phase noise within the feedback bandwidth, reducing the power spectral density of the residual phase noise, thus achieving fine narrowing of the linewidth.

[0044] When the preset linewidth adjustment requirement is linewidth broadening, the linewidth broadening mode is activated. The monitoring-feedback module injects a pseudo-random noise signal with a specific power spectral density into the photoelectric phase modulator, causing a controlled random perturbation in the laser center frequency, thereby achieving active linewidth broadening. The specific steps include: (2.1) Calculate the required white frequency noise power density based on the preset target Lorentz linewidth, and convert it into phase noise power spectral density by integration; (2.2) Generate Gaussian white noise samples in the digital domain; (2.3) Generate a phase-modulated signal based on the current gain factor and Gaussian white noise samples, wherein the initial gain factor is preset; (2.4) Apply the phase modulation signal to the photoelectric phase modulator; (2.5) By measuring the laser linewidth in real time and adjusting the gain factor, return to step (2.3) to achieve closed-loop calibration and stable control of the unfolded linewidth until the laser linewidth measured in real time meets the preset target Lorentz linewidth requirement.

[0045] In step (2.1) above, the target Lorentz line width is determined according to the preset target width. Calculate the required white frequency noise power density value Both conditions are met:

[0046] Since the response of an optoelectronic phase modulator is a phase quantity, the white frequency noise needs to be converted through mathematical integration to obtain the phase noise power spectral density. for:

[0047] As can be seen from the above equation, in order to obtain the Lorentz curve, the system phase noise after positive feedback compensation should satisfy the following condition. Statistical characteristic distribution. Simultaneously, the output laser linewidth is measured in real time. The linewidth error signal defined below can be obtained. :

[0048] In step (2.2) above, two independent uniformly distributed random numbers are generated in the digital domain. Two Gaussian white noise samples were obtained through transformation. and :

[0049] Select one of the Gaussian white noise samples as the noise source. Its power spectral density Flat within the bandwidth:

[0050] in Sampling rate, Let Variance be the variance.

[0051] In step (2.3) above, the initial gain factor is set, and the phase modulation signal is generated as follows:

[0052] in This is the feedback coefficient, with a value between 0.999 and 1. For gain factor, The sample is Gaussian white noise. , The first The, the The phase modulation signal output at each sampling time. After passing through a digital-to-analog converter, the signal is applied to an optoelectronic phase modulator to apply controlled random perturbations to the laser phase, thereby achieving linewidth broadening.

[0053] The feedback coefficient is added here to avoid DC drift of the pure integrator and to ensure that the target bandwidth is satisfied. While maintaining these characteristics, it also prevents output saturation caused by the accuracy limitations of digital-to-analog converters.

[0054] In step (2.4) above, to ensure the accuracy of linewidth adjustment, a feedback correction loop based on slow measurement was established, and the gain factor was adjusted. Real-time positive feedback is applied to the laser phase fluctuations to ensure that the broadened linewidth remains stable at a preset value over a long period of time.

[0055] in Step size factor For slow calibration cycle index, , The first sequence Gain factor of +1 slow calibration cycle, For the first Linewidth error signal during the second slow calibration cycle , For the first The laser linewidth measured in real time during the next slow calibration cycle. The target Lorentz linewidth is the preset linewidth value, i.e., the desired linewidth value.

[0056] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0057] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

[0058] 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 single-frequency fiber laser with a wide range of finely adjustable linewidth, characterized in that, include: A single-frequency fiber laser resonator is used to generate single-frequency fiber lasers. A pump source is used to provide pump light to the single-frequency fiber laser resonator. A wavelength division multiplexer connects a pump source and a single-frequency fiber laser resonator. It is used to couple the pump light output from the pump source into the single-frequency fiber laser resonator and output the single-frequency fiber laser generated by the single-frequency fiber laser resonator. The fiber optic isolated coupling integrated device has an input end connected to the output end of a wavelength division multiplexer. The two output ends are a high-power output end and a low-power output end, which are used to separate single-frequency fiber lasers and input them to the main optical path and the monitoring-feedback module, respectively. The monitoring-feedback module receives the laser signal output from the low-power output terminal, acquires the phase fluctuation information of the single-frequency fiber laser in real time, and generates the corresponding phase modulation signal according to the preset linewidth control requirements. The main optical path is equipped with a phase modulation unit, which is connected to a high-power output terminal. The phase modulation unit is controlled by the monitoring-feedback module. Based on the phase modulation signal output by the monitoring-feedback module, the phase fluctuation of the single-frequency fiber laser transmitted on the main optical path is compensated or superimposed to achieve a wide range of fine control of the laser linewidth.

2. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 1, characterized in that, The pump source is a single-mode pump source. The output end of the pump source is connected to the pump input end of the wavelength division multiplexer. The common end of the wavelength division multiplexer is connected to the low-reflection end of the single-frequency fiber laser resonator. The output end of the wavelength division multiplexer is used to output single-frequency fiber laser.

3. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 1 or 2, characterized in that, The monitoring-feedback module includes a phase detection unit, a signal processing unit, and a signal amplifier. The phase detection unit is used to acquire the phase fluctuation information of the single-frequency fiber laser. The signal processing unit is used to receive the phase fluctuation information of the single-frequency fiber laser and generate a corresponding phase modulation signal according to the preset linewidth control requirements. The signal amplifier is used to amplify the phase modulation signal and output it to the phase modulation unit.

4. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 3, characterized in that, The preset line width adjustment requirements include line width widening or line width narrowing; When the current preset linewidth control requirement is to narrow the linewidth, the signal processing unit analyzes the phase fluctuation information of the single-frequency fiber laser and generates an error correction signal with the same amplitude but opposite phase as the phase fluctuation information as a phase modulation signal. The phase modulation signal is amplified by the signal amplifier and output to the phase modulation unit. The phase modulation unit performs reverse compensation on the phase of the single-frequency fiber laser in the main optical path according to the phase modulation signal to cancel the inherent phase noise of the laser, thereby narrowing the laser linewidth. When the current preset linewidth adjustment requirement is linewidth expansion, the signal processing unit generates a phase modulation signal with the required statistical characteristics or spectral features based on the phase fluctuation information of the single-frequency fiber laser, or based on the internally generated pseudo-random noise sequence, or based on the internally preset frequency modulation waveform. The phase modulation signal is amplified by a signal amplifier and then output to the phase modulation unit. The phase modulation unit superimposes the phase modulation signal onto the single-frequency fiber laser phase on the main optical path, introducing controlled phase perturbation, thereby broadening the laser linewidth.

5. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 3, characterized in that, The phase detection unit is one of the following: a Fabry-Perot resonator, an unbalanced fiber interferometer, an atomic / molecular absorption spectrum, or a high-Q microcavity; the phase detection unit converts the optical field phase information of the single-frequency fiber laser into an electrical signal, and simultaneously acquires the phase fluctuation signal of the single-frequency fiber laser.

6. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 3, characterized in that, The signal processing unit is one of an analog control circuit, a digital control circuit, or an analog-digital hybrid control circuit, and the phase modulation unit is one of an electro-optic phase modulator, an acousto-optic phase modulator, an optical fiber stretcher, or a liquid crystal phase modulator.

7. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 5 or 6, characterized in that, When the preset linewidth control requirement is linewidth narrowing mode, the monitoring-feedback module achieves linewidth narrowing through closed-loop suppression of single-frequency fiber laser phase noise, including: (1.1) Convert the phase fluctuation signal of the laser into a voltage error signal; (1.2) The signal processing unit uses a proportional-integral algorithm to generate a phase compensation signal based on the voltage error signal; (1.3) Apply the phase compensation signal to the photoelectric phase modulator, introduce the phase negative feedback signal, and after the closed-loop negative feedback control link, reduce the residual phase noise power spectral density within the feedback bandwidth to achieve laser linewidth narrowing.

8. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 5 or 6, characterized in that, When the preset linewidth adjustment requirement is linewidth broadening mode, the monitoring-feedback module injects random noise signals into the photoelectric phase modulator, causing controlled random perturbations in the laser center frequency, thereby achieving active linewidth broadening, including the following steps: (2.1) Calculate the required white frequency noise power density based on the preset target Lorentz linewidth, and convert it into phase noise power spectral density by integration; (2.2) Generate Gaussian white noise samples in the digital domain; (2.3) Generate a phase-modulated signal based on the current gain factor and Gaussian white noise samples, wherein the initial gain factor is preset; (2.4) Apply the phase modulation signal to the photoelectric phase modulator; (2.5) By measuring the laser linewidth in real time and adjusting the gain factor, return to step (2.3) to achieve closed-loop calibration and stable control of the unfolded linewidth until the laser linewidth measured in real time meets the preset target Lorentz linewidth requirement.

9. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 1, 2, 4, 5, or 6, characterized in that, It also includes a temperature control unit to stabilize the operating temperature of the single-frequency fiber laser resonator.

10. The single-frequency fiber laser with a wide range of finely adjustable linewidth according to claim 7, characterized in that, The single-frequency fiber laser resonator is of distributed Bragg reflection or distributed feedback type and is encapsulated in a temperature control unit to maintain a stable operating temperature. The high-reflection end of the resonator is cut at an 8-degree angle.

Images