A narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization

Abstract

The application discloses a kind of narrow line width fiber laser systems based on active phase compensation of fiber loop interferometer frequency stabilization, comprising: fiber laser, the laser is sequentially connected with first frequency shifter, Y type coupler, X type coupler and photoelectric detector by conducting optical fiber and constitutes loop interferometer loop.The photoelectric detector is connected with radio frequency power divider, and also be used to convert beat frequency optical signal output by fiber loop interference loop into radio frequency electric signal;The output end of the radio frequency power divider is respectively connected with first feedback calibration loop and second feedback calibration loop, to carry out the fast feedback calibration of laser frequency by first feedback calibration loop, and carry out the active compensation of delay optical fiber phase noise by second feedback calibration loop.And by second proportional integral amplifier for slow feedback calibration of laser frequency.The interferometer frequency stabilization system presented in the application has high detection sensitivity and large bandwidth, and has lower environmental sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of fiber lasers, specifically relating to a narrow-linewidth fiber laser system based on frequency stabilization using a fiber interferometer. Background Technology

[0002] Narrow-linewidth laser sources have broad application prospects in cutting-edge scientific and industrial fields such as gravitational wave detection, long-distance high-precision optical frequency transmission, and fiber optic sensing. The measurement accuracy and range of these application systems are directly limited by the linewidth of the laser source; therefore, improving the linewidth of the laser helps to enhance its practical value and broaden its application scope. With the development of important scientific engineering projects such as gravitational wave detection and high-precision ground-based time synchronization systems, research into portable narrow-linewidth lasers that meet engineering needs is extremely urgent, and their successful development can promote progress in the aforementioned related application fields.

[0003] Currently, narrow-linewidth lasers in the Hz range are mainly achieved through PDH frequency stabilization technology combined with ultra-stable optical reference cavities. With the development of fiber optic technology, tightly sealed fiber optic loops can stabilize the optical path difference of interferometers, thus ensuring high stability of the frequency discrimination signal. Therefore, interferometer frequency stabilization technology has become a research hotspot in the field of narrow-linewidth lasers. Compared with cavity-stabilized narrow-linewidth laser systems, using fiber optic interferometers for frequency discrimination to obtain the narrow linewidth of the laser system has advantages such as high frequency discrimination accuracy, easy integration, all-fiber operation, and low cost. In 1989, Chen first used an MZ fiber optic interferometer to narrow the laser linewidth. In the fiber optic interferometer frequency stabilization scheme, the larger the arm difference between the two arms, the greater the interferometer's detection sensitivity, and the higher the system's frequency discrimination accuracy. In 2018, Li Tang's team at the Shanghai Institute of Optics and Fine Mechanics, based on the above scheme, placed a Michelson interferometer with an arm difference of 5 km in a high-vacuum thermally shielded container and actively controlled its temperature, achieving a beat frequency linewidth of 0.2 Hz after closed-loop control. The above-mentioned solutions can effectively reduce the frequency noise of lasers and narrow the laser linewidth, but they all require thousands of meters of delay fiber to achieve high laser frequency discrimination accuracy. Furthermore, active temperature control and vacuum isolation of the fiber are also necessary. Therefore, these solutions do not possess the characteristics of being lightweight, miniaturized, and highly environmentally resistant, and are not suitable for widespread engineering applications.

[0004] In summary, existing technologies cannot simultaneously meet the requirements of high sensitivity and large bandwidth, and their frequency discrimination accuracy is low, noise is high, and application range is narrow. Summary of the Invention

[0005] To address the aforementioned problems in the existing technology, this invention provides a narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization. The technical problem to be solved by this invention is achieved through the following technical solution:

[0006] A narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization includes:

[0007] A fiber laser for generating a laser beam; a first frequency shifter connected to the fiber laser for shifting the laser beam to obtain a first frequency shift signal;

[0008] A Y-type coupler is provided, wherein the input end of the Y-type coupler is connected to the first frequency shifter, and the Y-type coupler includes a first output end and a second output end. The first output end is used to split the first frequency shift signal into light and output it as a linewidth laser source. The second output end splits the first frequency shift signal into light and outputs it to an optical fiber cyclic interference circuit to form a beat frequency optical signal and to a photodetector through an X-type coupler.

[0009] The photodetector is connected to the radio frequency power divider and is also used to convert the beat frequency optical signal output from the fiber optic cyclic interference circuit into a radio frequency electrical signal.

[0010] The output of the radio frequency power divider is connected to the first feedback calibration circuit and the second feedback calibration circuit, respectively, so as to perform rapid feedback calibration of the laser frequency through the first feedback calibration circuit and active compensation of the delayed fiber phase noise through the second feedback calibration circuit.

[0011] In one specific embodiment, the fiber optic cyclic interference loop includes a fiber optic amplifier, an optical bandpass filter, a second frequency shifter, a delay fiber loop, and a third frequency shifter connected in sequence, with the output of the third frequency shifter connected to the X-coupler.

[0012] In one specific embodiment, the first feedback calibration loop includes a first electrical bandpass filter, a mixer, and a first proportional-integral amplifier connected in sequence.

[0013] In one specific embodiment, a second feedback calibration loop is also included, which includes a second electrical bandpass filter, a frequency discriminator, a low-pass filter, and a third proportional-integral amplifier connected in sequence, wherein the output terminal of the third proportional-integral amplifier is connected to the third frequency shifter.

[0014] In one specific embodiment, a second proportional-integral amplifier is further included. The input of the second proportional-integral amplifier is connected to the output of the frequency discriminator, and the output is connected to the voltage modulation port of the fiber laser for slow feedback calibration of the laser frequency.

[0015] In one embodiment, the mixer is further included with a radio frequency signal generator connected to it.

[0016] The beneficial effects of this invention are:

[0017] 1. High frequency discrimination accuracy. Compared with traditional MZ and Michelson fiber interferometer frequency stabilization schemes, this device can improve the frequency discrimination accuracy of the system by at least one order of magnitude when using the same arm difference length.

[0018] 2. Balancing Interferometer Detection Sensitivity and Bandwidth. In traditional interferometer frequency stabilization schemes, the length of the long-arm delay fiber requires a trade-off between interferometer detection sensitivity and loop bandwidth. This device employs a cyclic interferometer scheme, eliminating the need for a composite interferometer. Different fiber detection sensitivities and loop bandwidths can be obtained simply by selecting the order of the beat frequency signal. The large bandwidth of a first-order beat frequency signal can be used for fast feedback control of the laser frequency, while higher-order beat frequency signals offer higher detection sensitivity and can be used for slow feedback control of the laser frequency.

[0019] 3. Low environmental sensitivity. Fiber optic active phase compensation technology is used to dynamically compensate for the effects of environmental coupling noise (such as airflow and temperature) on the interferometer in real time, solving the problem that fiber optic interferometers require a high-vacuum working environment to reduce their own noise.

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a graph showing the amplitude response of the fiber optic interferometer's transfer function in the frequency domain.

[0022] Figure 2 This is a schematic diagram of a narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization provided in an embodiment of the present invention. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0024] Example 1

[0025] To more clearly illustrate the embodiments of the invention, the basic principle of the fiber optic interferometer frequency-stabilized laser will first be explained.

[0026] It utilizes unequal-arm interferometer frequency discrimination technology to detect the frequency noise of the laser, and then corrects the laser frequency through a servo feedback circuit to achieve laser linewidth narrowing. The resolution capability of the fiber optic interferometer for laser frequency noise can be represented by a transfer function, which can be expressed in the frequency domain as:

[0027]

[0028] Where f is the Fourier frequency, L is the length of the long-arm delay fiber, τ = n·L / c is the delay difference between the two arms of the interferometer, c is the speed of light in vacuum, and n is the refractive index of the fiber. At low frequencies, the T(f) curve is flat, with an amplitude of 2πτ (proportional to the length of the long-arm fiber). As the frequency of the horizontal axis increases, the amplitude response of T(f) exhibits periodic zeros, and the positions of these zeros are integer multiples of 1 / τ. The position of the first zero (1 / τ) is the loop bandwidth of the interferometer. Figure 1 The figure shows the frequency domain amplitude response of T(f) simulated according to Formula 1, where the solid and dashed lines represent the simulated T(f) curves for delay fibers with arm differences of 200m and 2km, respectively. Figure 1 It can be seen that the loop bandwidth of the 2km unequal-arm interferometer is 100kHz, which is ten times smaller than that of the 200m unequal-arm interferometer represented by the dashed line; however, at low frequencies, the amplitude response of the 2km unequal-arm interferometer is ten times larger than that of the 200m interferometer. This shows that the longer the selected delay fiber, the greater the corresponding amplitude response (i.e., frequency noise detection sensitivity), but the smaller its corresponding loop bandwidth. Therefore, it is clear that traditional unequal-arm fiber interferometers cannot provide both high sensitivity and high loop bandwidth. This invention, however, does not require the construction of a composite interferometer; a single short-delay fiber loop can simultaneously obtain a series of beat frequency signals with different delay lengths, thus simultaneously obtaining a first-order beat frequency signal with a large control bandwidth and a high-order beat frequency signal with high detection sensitivity.

[0029] Please see Figure 2 , Figure 2 This is a schematic diagram of a narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization provided in an embodiment of the present invention, including:

[0030] Fiber laser 1 is used to generate a laser beam. In order to provide a high-quality laser signal, fiber laser 1 has a piezoelectric ceramic (PZT) voltage modulation port, and the center wavelength is generally selected from the fiber optic communication band. In this embodiment, 1550nm is preferred.

[0031] The first frequency shifter 2 is connected to the fiber laser 1 and is used to shift the frequency of the laser beam to obtain a first frequency shift signal. In this embodiment, the first frequency shifter 2 can perform frequency shifting and phase modulation on the laser beam. The specific frequency shift value can be adjusted according to specific experimental conditions. This embodiment does not impose specific limitations.

[0032] Y-coupler 3, the input end of which is connected to the first frequency shifter 2, the Y-coupler 3 includes a first output end and a second output end, wherein the first output end is used to split the first frequency shift signal and output it as a linewidth laser source, and the second output end splits the first frequency shift signal and outputs it through the b input port of X-coupler 4 to the fiber optic cyclic interference circuit 40 to form a beat frequency optical signal and to the photodetector 5 respectively;

[0033] It should be noted that the X-coupler 4 has two input terminals b and e and two output terminals c and d, where input terminal e and output terminal d are respectively connected to the fiber optic cyclic interference loop 40. The Y-coupler has a splitting ratio of 90:10, with the end with the larger splitting ratio serving as the linewidth laser source for output, and the end with the smaller splitting ratio outputting to input terminal b of the X-coupler 4. In this embodiment, the splitting ratio of the X-coupler can be selected as 50:50, that is, the optical power output to terminals c and d after splitting by the X-coupler is the same.

[0034] The photodetector 5 is connected to the radio frequency power divider 11 and is also used to convert the beat frequency optical signal output by the fiber optic cyclic interference circuit into a radio frequency electrical signal.

[0035] The output of the radio frequency power divider 11 is connected to the first feedback calibration circuit 50 and the second feedback calibration circuit 60, respectively, so as to perform rapid feedback calibration of the laser frequency through the first feedback calibration circuit 50 and active compensation of the delayed fiber phase noise through the second feedback calibration circuit 60.

[0036] In one specific embodiment, the fiber optic cyclic interference circuit 40 includes a fiber optic amplifier 6, an optical bandpass filter 7, a second frequency shifter 8, a delay fiber loop 9, and a third frequency shifter 10 connected in sequence. The output terminal of the third frequency shifter 10 is connected to the input terminal e of the X-type coupler 4 and then outputs through the output terminal c.

[0037] This embodiment explains the principle of the fiber optic cyclic interference loop 40. It should be noted that the more times the signal light circulates in the optical path, the longer the corresponding equivalent fiber delay. For example, when the signal light circulates N times in the loop, the delay is 0.5 × N μs (taking a delay fiber loop 9 with a length of 100m as an example), and the frequency shift is 5 × N MHz (taking the second frequency shifter as an acousto-optic frequency shifter with a frequency shift value of -ΩMHz, and the third frequency shifter as an acousto-optic frequency shifter with a frequency shift value of +(Ω+5)MHz). It should be noted that the number of cycles is determined by the amplification factor of the fiber amplifier 6 and the length of the delay fiber loop 9. The larger the amplification factor and the shorter the length of the delay fiber loop 9, the more cycles are required. In actual operation, adjustments can be made according to the specific circumstances.

[0038] In one specific embodiment, the first feedback calibration loop 50 includes a first electrical bandpass filter 12, a mixer 13, and a first proportional-integral amplifier 14 connected in sequence. The output of the first proportional-integral amplifier 14 is connected to a first frequency shifter 2 for fast feedback calibration of the laser frequency.

[0039] Specifically, a portion of the light input from port b of the X-coupler exits the loop from port c of the X-coupler and interferes with the reference light, beating at the same frequency. This beating light is then converted by photoelectric detector 5 to obtain a series of discrete, equally spaced beat signals. These beat signals are split into two outputs by RF power divider 11: one output passes through the first electrical bandpass filter 12 to obtain the first-order beat signal, which is then processed by mixer 13 and the first proportional-integral amplifier 14 to obtain a frequency discrimination signal with a large control bandwidth, and fed back to the RF modulation port of the first frequency shifter 2 for rapid feedback calibration of the laser frequency.

[0040] In one specific embodiment, a second feedback calibration loop 60 is also included. The second feedback calibration loop 60 includes a second electrical bandpass filter 15, a frequency discriminator 16, a low-pass filter 18, and a third proportional-integral amplifier 19 connected in sequence. The output terminal of the third proportional-integral amplifier 19 is connected to the third frequency shifter 10.

[0041] Specifically, the other beat frequency signal output from the RF power divider 11 is passed through the second electrical bandpass filter 15 to obtain the Nth order beat frequency signal. The equivalent fiber delay length corresponding to this Nth order beat frequency signal is 100×N m (taking the length of the delay fiber ring 9 as 100m as an example). For example, the detection sensitivity of a 2km (taking N=20 as an example, i.e., the signal light circulates 20 times in the fiber optic cyclic interference loop 40) unequal arm interferometer can be obtained with only 100m of experimental fiber.

[0042] The frequency discrimination accuracy of interferometer frequency stabilization technology depends on the stability of long-delay optical fibers; therefore, reducing the noise of long optical fibers is an important way to improve performance. Due to the presence of environmental coupling noise, the beat signal output by the interferometer not only comes from the laser noise but also from the environmental coupling noise of the interferometer. Ideally, the system feedback loop should reduce both types of noise simultaneously, but these two types of noise are superimposed and cannot be directly separated. Considering that laser noise (mainly white noise) exists across the entire Fourier frequency range, while the environmental noise coupled by the optical fiber mainly dominates at low Fourier frequencies, this embodiment employs active fiber phase compensation technology. Through a phase-locked loop (low-pass filter 18, third proportional-integral amplifier 19), the demodulated signal is fed back to the third frequency shifter 10 to compensate for the phase noise of the fiber interferometer caused by environmental noise. The main advantage of this technology is that it effectively reduces the influence of environmental coupling noise on the fiber interferometer, solves the interferometer's dependence on the vacuum cavity, and provides key technical support for achieving high performance, miniaturization, and lightweight design of the system.

[0043] In one specific embodiment, a second proportional-integral amplifier 17 is also included. The input of the second proportional-integral amplifier 17 is connected to the output of the frequency discriminator 16, and the output is connected to the voltage modulation port of the fiber laser 1 for slow feedback calibration of the laser frequency. The high-order beat frequency signal is demodulated by the frequency discriminator 16 to output a high-sensitivity discriminator signal. This discriminator signal is then applied to the voltage modulation port of the laser 1 by the second proportional-integral amplifier 17 for slow feedback calibration of the laser frequency, i.e., suppressing frequency noise at low laser frequencies.

[0044] In one specific embodiment, a radio frequency signal generator 20 is also included, connected to the mixer 13. It should be noted that in practical applications, the radio frequency signal generator 20 can be integrated into the frequency synthesizer.

[0045] The following describes the execution method of the narrow linewidth fiber laser system based on fiber interferometer frequency stabilization according to this embodiment (taking 20 loops as an example):

[0046] Process 1: Obtaining the beat frequency signal carrying laser noise using a fiber optic cyclic interferometer. The light output from the fiber laser is split into two parts by a Y-coupler via a first frequency shifter. One part is output as a narrow-linewidth laser source; the other part is used as frequency-stabilized light and enters the fiber optic cyclic interferometer. Part of the beam used for frequency stabilization is used as reference light and is directly output from end c of the X-coupler into the detector. The other part of the beam is used as signal light and enters the self-circulating optical path formed by connecting ends b and d of the X-coupler. After n cycles, part of the signal light is output from end c of the X-coupler and interferes with the reference light, resulting in a series of discrete, equally spaced beat frequency signals.

[0047] Process Two: Laser frequency noise suppression and active phase noise compensation for the delay fiber are achieved through servo feedback. The aforementioned beat frequency signal is split into two paths by a power divider. One path passes through a first bandpass filter to obtain a first-order beat frequency signal. This signal is processed by a mixer and a first proportional-integral amplifier to obtain a frequency discriminator signal with a large control bandwidth, which is then fed back to the electrical modulation port of the first frequency shifter for rapid feedback calibration of the laser frequency. The other beat frequency signal passes through a second bandpass filter to obtain a 20th-order beat frequency signal. This higher-order beat frequency signal is demodulated by a frequency discriminator to output a high-sensitivity frequency discriminator signal. This discriminator signal is split into two paths, which pass through the second and third proportional-integral amplifiers respectively, acting on the laser voltage modulation port and the electrical modulation port of the third frequency shifter, for slow laser frequency calibration and active phase compensation of the delay fiber. A 100Hz low-pass filter is connected between the frequency discriminator and the third proportional-integral amplifier to extract environmental noise from the coupling of the delay fiber.

[0048] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0049] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0050] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0051] Although this application has been described herein in conjunction with various embodiments, those skilled in the art, by reviewing the accompanying drawings, disclosure, and appended claims, will understand and implement other variations of the disclosed embodiments in carrying out the claimed application. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude a plurality. A single processor or other unit can implement several functions listed in the claims. While different dependent claims may recite certain measures, this does not mean that these measures cannot be combined to produce good results.

[0052] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization, characterized in that, include: Fiber lasers are used to generate laser beams; A first frequency shifter, connected to the fiber laser, is used to shift the frequency of the laser beam to obtain a first frequency shift signal; A Y-type coupler is provided, wherein the input end of the Y-type coupler is connected to the first frequency shifter, and the Y-type coupler includes a first output end and a second output end. The first output end is used to split the first frequency shift signal into light and output it as a linewidth laser source. The second output end splits the first frequency shift signal into light and outputs it to an optical fiber cyclic interference circuit to form a beat frequency optical signal and to a photodetector through an X-type coupler. The photodetector is connected to the radio frequency power divider and is also used to convert the beat frequency optical signal output from the fiber optic cyclic interference circuit into a radio frequency electrical signal. The output of the radio frequency power divider is connected to the first feedback calibration circuit and the second feedback calibration circuit respectively, so as to perform fast feedback calibration of the laser frequency through the first feedback calibration circuit and active compensation of the phase noise of the delayed fiber through the second feedback calibration circuit. The first feedback calibration loop includes a first electrical bandpass filter, a mixer, and a first proportional-integral amplifier connected in sequence. It also includes a second proportional-integral amplifier, the input of which is connected to the output of the frequency discriminator, and the output is connected to the voltage modulation port of the fiber laser for slow feedback calibration of the laser frequency. The fiber optic cyclic interference circuit includes a fiber amplifier, an optical bandpass filter, a second frequency shifter, a delay fiber loop, and a third frequency shifter connected in sequence, with the output of the third frequency shifter connected to the X-type coupler. It also includes a second feedback calibration loop, which includes a second electrical bandpass filter, a frequency discriminator, a low-pass filter, and a third proportional-integral amplifier connected in sequence. The output of the third proportional-integral amplifier is connected to the third frequency shifter. The demodulated signal is fed back to the third frequency shifter through the low-pass filter and the third proportional-integral amplifier to compensate for the phase noise of the fiber optic interferometer caused by environmental noise. Another beat frequency signal output from the RF power divider is passed through the second electrical bandpass filter to obtain the Nth order beat frequency signal. This Nth order beat frequency signal is demodulated by the frequency discriminator to output a high detection sensitivity frequency discriminator signal. This frequency discriminator signal is divided into two paths and passed through the second and third proportional-integral amplifiers respectively to the voltage modulation port of the laser and the electrical modulation port of the third frequency shifter, for slow calibration of the laser frequency and active phase compensation of the delay fiber.

2. The narrow-linewidth fiber laser system based on fiber interferometer frequency stabilization according to claim 1, characterized in that, It also includes a radio frequency signal generator connected to the mixer.

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