A repetition frequency tunable all polarization-maintaining composite cavity mode-locked fiber laser

Abstract

This invention relates to the fields of ultrafast lasers and quantum optical technology, and provides a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency. It includes a pump source, a hybrid device of a polarization-maintaining fiber coupler and a wavelength division multiplexer, a polarization-maintaining erbium-doped fiber, a saturable absorber, a three-port polarization-maintaining circulator, a chirped Bragg fiber grating, and the polarization-maintaining fiber coupler connected in optical path order to form a seed source main resonant cavity; the three-port polarization-maintaining circulator, the chirped Bragg fiber grating, the fiber collimator, the half-wave plate, and the hybrid device of the fiber collimator and polarization-maintaining fiber coupler connected in optical path order to form a seed source secondary cavity; the fiber collimator, the half-wave plate, and the hybrid device of the fiber collimator and polarization-maintaining fiber coupler are all mounted on an electrically driven displacement stage, and the output mode-locked laser repetition frequency can be tunable by adjusting the relative distance between the fiber collimator and the hybrid device.

Description

Technical Field

[0001] This invention relates to the fields of ultrafast laser and quantum optical technology, and in particular to a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency. Background Technology

[0002] The statements in this section are merely background information related to this utility model and do not necessarily constitute prior art.

[0003] Fully polarization-maintaining fiber mode-locked lasers offer advantages such as high stability, good beam quality, ease of integration, and strong anti-interference capabilities. Erbium-doped fiber lasers operate in the traditional communication band of 1.5 μm with minimal transmission loss and have wide applications in fields such as optical coherence tomography (OCT), ultrafast laser micromachining, optical frequency comb generation, lidar, and precision detection. Compared to other mode-locking techniques such as nonlinear polarization rotation (NPR), nonlinear optical ring mirror (NOLM), and nonlinear amplifying ring mirror (NALM), fully polarization-maintaining fiber lasers with mode-locking via saturable absorbers (SA) have a simpler structure, are easier to integrate, and exhibit better stability.

[0004] Tunable repetition frequency technology for fiber lasers involves several approaches. These include incorporating optical components such as acousto-optic / electro-optic modulators or controllable fiber delay lines within the cavity; introducing tunable spatial optical paths within the resonant cavity; controlling the cavity length using piezoelectric ceramics; and controlling the effective cavity length by adjusting the refractive index through temperature. While using optical components like acousto-optic / electro-optic modulators, piezoelectric ceramics, or controllable fiber delay lines offers advantages such as ease of control and quantization, it suffers from high integration costs and stringent requirements for electrical regulation. Temperature-controlled cavity length methods are subject to stringent environmental and temperature control equipment requirements; furthermore, the tunable repetition frequency range of these methods is relatively small, limiting their practicality in industrial applications.

[0005] In addition, current mode-locked fiber lasers typically have many longitudinal mode oscillations inside the resonant cavity in order to obtain ultrashort pulse output, which affects the spectral purity in some applications. Utility Model Content

[0006] To address the lack of a mode-locked fiber laser with low spectral noise, stable output, compact structure, and tunable repetition rate in existing technologies, this invention provides a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition rate. This mode-locked laser has a continuously tunable repetition rate, which reduces spectral bandwidth and phase noise while maintaining a compact structure and stable output.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention provides a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency.

[0009] A fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency includes: a pump source, a hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer, a polarization-maintaining erbium-doped fiber, a saturable absorber, a three-port polarization-maintaining circulator, a chirped Bragg fiber grating, a polarization-maintaining fiber coupler, a fiber collimator, a half-wave plate, and a hybrid device of fiber collimator and polarization-maintaining fiber coupler.

[0010] The pump source, the hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer, the polarization-maintaining erbium-doped fiber, the saturable absorber, the three-port polarization-maintaining circulator, the chirped Bragg fiber grating, and the polarization-maintaining fiber coupler are connected in optical path order to form the seed source main resonant cavity.

[0011] The three-port polarization-maintaining circulator, chirped Bragg fiber grating, fiber collimator, half-wave plate, and hybrid device of fiber collimator and polarization-maintaining fiber coupler are connected in optical path sequence to form a seed source sub-cavity. The fiber collimator, half-wave plate, and hybrid device of fiber collimator and polarization-maintaining fiber coupler are all mounted on an electric displacement stage. By adjusting the relative distance between the fiber collimator and the hybrid device of polarization-maintaining fiber coupler, the repetition frequency of the output mode-locked laser can be tunable.

[0012] Furthermore, the pump source inputs the pump light into the main resonant cavity of the seed source through a hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer. After passing through the polarization-maintaining erbium-doped fiber, stimulated emission light is generated. Then, it passes through a saturable absorber, the a port and the b port of a three-port polarization-maintaining fiber circulator, and a chirped fiber Bragg grating. Half of the laser emission is transmitted and input into the polarization-maintaining fiber coupler. Finally, it is output through the output port of the hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer. The stimulated emission light circulates in the main resonant cavity of the seed source and passes through the saturable absorption effect of the saturable absorber to form a pulsed laser output with a fixed repetition frequency.

[0013] Furthermore, the other half of the laser is reflected and transmitted to port c via port b of the three-port polarization-maintaining circulator. The laser output from port c of the three-port polarization-maintaining circulator undergoes polarization rotation after entering the half-wave plate through the fiber collimator and is coupled into the hybrid device of fiber collimator and polarization-maintaining fiber coupler. The light from port a of the hybrid device is fed into the seed source main resonant cavity via the polarization-maintaining fiber coupler for the next cycle. The light from port b of the hybrid device enters the electronic control module.

[0014] Furthermore, the electronic control module is used to monitor the coupled optical power and adjust the rotation angle of the half-wave plate, as well as adjust the relative distance between the fiber collimator and the polarization-maintaining fiber coupler hybrid device on the electric displacement stage.

[0015] Furthermore, the pump source outputs continuous light with a center wavelength of 976nm and a maximum output power of 500mW.

[0016] Furthermore, the polarization-maintaining erbium-doped fiber has a length of 0.3m and a dispersion value at 1550nm. .

[0017] Furthermore, the modulation depth of the saturable absorber is 55%.

[0018] Furthermore, the transmittance / reflectance of the chirped fiber Bragg grating is 50 / 50.

[0019] Furthermore, the hybrid device of fiber collimator and polarization-maintaining fiber coupler includes output port a and output port b. Output port a is the 99.5% output end, which is connected to the polarization-maintaining fiber coupler through fiber fusion splicing. Output port b is the 0.5% output end, which is connected to the electronic control module.

[0020] Furthermore, the seed source sub-cavities are provided in multiple ways, and the seed source sub-cavities are connected in series or in parallel.

[0021] Compared with the prior art, the beneficial effects of this utility model are:

[0022] This invention provides a fiber laser with a composite cavity structure. By utilizing the characteristic of the seed source sub-cavity to control the longitudinal mode, a narrower spectral bandwidth can be obtained. At the same time, it has the advantages of compact structure, low noise, and tunable repetition frequency, which helps to improve the system performance and practicality in fields such as high-precision fiber optic sensing, optical frequency combs, and optical communication. Attached Figure Description

[0023] The accompanying drawings, which form part of this specification, are used to provide a further understanding of this utility model. The illustrative embodiments of this utility model and their descriptions are used to explain this utility model and do not constitute an improper limitation of this utility model.

[0024] Figure 1 This is a structural diagram of a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency, as shown in an embodiment of this utility model.

[0025] Figure 2 This invention relates to a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency, as shown in this embodiment. Figure 1 Diagrams of multiple secondary cavity structures evolved from the original structure;

[0026] Figure 3 This invention relates to a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency, as shown in this embodiment. Figure 1A diagram of another type of multiple-cavity structure evolved from the previous one;

[0027] Among them, 1 is the seed source main resonant cavity, 2 is the first seed source secondary cavity, 3 is the second seed source secondary cavity, 1 is the LD pump source, 2 is the hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer, 3 is the polarization-maintaining erbium-doped fiber, 4 is the saturable absorber, 5 is the first three-port polarization-maintaining circulator, 5' is the second three-port polarization-maintaining circulator, 6 is the first chirped Bragg fiber grating, 6' is the second chirped Bragg fiber grating, 7 is the 50 / 50 first polarization-maintaining fiber coupler, 7' is the 50 / 50 second polarization-maintaining fiber coupler, 8 is the first fiber collimator, 8' is the second fiber collimator, 9 is the first half-wave plate, 9' is the second half-wave plate, 10 is the hybrid device of the first fiber collimator and polarization-maintaining fiber coupler, 10' is the hybrid device of the second fiber collimator and polarization-maintaining fiber coupler, 11 is the first electric displacement stage, 11' is the second electric displacement stage, and 12 is the electronic control module. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0030] 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 the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, devices, components, and / or combinations thereof.

[0031] As described in the background section, there is a lack of a mode-locked fiber laser with low spectral noise, stable output, compact structure, and tunable repetition rate in the prior art. Therefore, this invention provides a fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition rate.

[0032] This repetition-frequency tunable, fully polarization-maintaining composite cavity mode-locked fiber laser includes: a seed source main resonant cavity I and multiple seed source secondary cavities II ( Figure 1 Only one embodiment of a seed source secondary cavity is shown in the figure.

[0033] The seed source main resonant cavity I is formed by sequentially connecting a 976nm LD pump source 1, a polarization-maintaining fiber coupler and wavelength division multiplexer hybrid device (TWDM) 2, a polarization-maintaining erbium-doped fiber (EDF) 3, a saturable absorber (SA) 4, a first three-port polarization-maintaining circulator 5, a first chirped Bragg fiber grating (CFBG) 6, and a 50 / 50 first polarization-maintaining fiber coupler 7 to form a ring main resonant cavity. A 976nm LD pump source 1 inputs pump light into the ring main resonant cavity via a polarization-maintaining fiber coupler and wavelength division multiplexer hybrid device 2. After passing through the polarization-maintaining erbium-doped fiber 3, stimulated emission light is generated. This light then passes successively through a saturable absorber 4, the a port and b port of the first three-port polarization-maintaining fiber circulator 5, and the first chirped fiber Bragg grating 6. 50% of the laser light is transmitted and input to the 50 / 50 first polarization-maintaining fiber coupler 7. Finally, it is output through the output port of the polarization-maintaining fiber coupler and wavelength division multiplexer hybrid device 2. The stimulated emission light circulates within the main resonant cavity, passing through the saturable absorption effect of the saturable absorber 4 to form a pulsed laser output with a fixed repetition frequency. The selected 976nm LD pump source 1 provides continuous light output with a center wavelength of 976nm and a maximum output power of 500mW. The polarization-maintaining erbium-doped fiber 3 is approximately 0.3m long and has a dispersion value of [value missing] at 1550nm. The saturable absorber 4 has a modulation depth of 55%; the first chirped fiber Bragg grating 6 has a transmittance / reflectance of 50 / 50 and a 3dB bandwidth of 50nm; the pigtails of other devices are all PM1550 polarization-maintaining fibers. The laser propagates in the main resonant cavity along the direction shown by the dashed line. Mode-locked laser output with a repetition frequency of less than 10MHz can be achieved by adjusting the length of the device pigtails and the pump power. Among them, the 976nm LD pump source 1, the polarization-maintaining erbium-doped fiber 3, the saturable absorber 4, and the first chirped fiber Bragg grating 6 are all commercially available.

[0034] The first seed source secondary cavity II includes: a first three-port polarization-maintaining circulator 5, a first chirped Bragg fiber grating (CFBG) 6, a first fiber collimator 8, a first half-wave plate 9, a hybrid device of the first fiber collimator and polarization-maintaining fiber coupler 10, a 50 / 50 first polarization-maintaining fiber coupler 7, a first electric displacement stage 11, and an electronic control module 12. The laser circulating in the seed source main resonant cavity I is reflected by 50% when it passes through the first chirped Bragg fiber grating 6. It is then transmitted to port c via port b of the first three-port polarization-maintaining circulator 5. Port c of the first three-port polarization-maintaining circulator 5 is connected to the first fiber collimator 8 via fiber fusion splicing. The laser output from the first fiber collimator 8 propagates in free space. After passing through the first half-wave plate 9, its polarization state is rotated, and it is coupled into the hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler. The light at output a of the hybrid device 10 is then fed back into the seed source main resonant cavity I via the first polarization-maintaining fiber coupler 7 for the next cycle (according to...). Figure 1 (As shown by the dashed line, the light is transmitted in a loop.) After passing through the output end b of the hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler, the light enters the electronic control module 12. This electronic control module 12 monitors the coupling power in real time and adjusts the angle of the first half-wave plate 9 in real time according to the monitored power until the maximum coupling efficiency is obtained. At the same time, this module can also change the cavity length of the secondary cavity by adjusting the first electric displacement stage 11, thereby realizing the tunability of the output mode-locked laser repetition frequency.

[0035] The first chirped fiber Bragg grating 6 has a transmittance / reflectance of 50 / 50 and a 3dB bandwidth of 50nm. The hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler has two output ports: port a is the 99.5% output port, which is connected to the 50 / 50 first polarization-maintaining fiber coupler 7 via fiber fusion splicing; port b is the 0.5% output port, which is connected to the electronic control module 12. The electronic control module 12 is used to monitor the coupled optical power, adjust the rotation angle of the first half-wave plate 9, and adjust the relative distance between the first fiber collimator 8 and the hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler on the first electric displacement stage 11.

[0036] The first chirped Bragg fiber grating 6 is a key component for balancing the main resonant cavity I and the secondary resonant cavity II of the seed source. It also acts as a filter, reducing the number of longitudinal modes within the cavity and preventing multi-mode competition, thereby achieving a narrower bandwidth and lower phase noise spectrum. By adjusting the relative distance between the first fiber collimator 8 and the hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler on the first electric displacement stage 11, a continuous tuning range of up to 5 MHz for the laser repetition frequency can be achieved.

[0037] Figure 2 and Figure 3 Shown in Figure 1 A seed source laser with dual cavities, extended from the existing technology.

[0038] in, Figure 2 The polarization-maintaining circulator in the middle is replaced by a four-port one instead of a three-port one; the transmission / reflection ratio of the first chirped Bragg fiber grating 6 can be selected as 40% / 60%, 30% / 70%, or 20% / 80% or other ratios; the 50 / 50 first polarization-maintaining fiber coupler 7 is replaced by a 1x3 polarization-maintaining fiber coupler with a ratio of 33.3% / 33.3% / 33.3%; at the same time, the end face of the first fiber collimator 8 is coated with a 50% reflective film to reflect 50% of the light passing through the collimator back to port c of the four-port collimator, and input to the second seed source sub-cavity III through port d. The second fiber collimator 8' in the second seed source sub-cavity III has no reflective coating. The second half-wave plate 9', the hybrid device 10' of the second fiber collimator and polarization-maintaining fiber coupler, and the second electric displacement stage 11' are all the same as the first half-wave plate 9, the hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler, and the first electric displacement stage 11.

[0039] Figure 3 This is another type of dual-cavity seed source laser, in which the transmission / reflection ratio of the first chirped Bragg fiber grating 6 and the second chirped Bragg fiber grating 6' can be selected as 40% / 60%, 30% / 70%, or 20% / 80%, etc. The other devices in the secondary cavity, such as the second three-port polarization-maintaining circulator 5', the 50 / 50 second polarization-maintaining fiber coupler 7', the second fiber collimator 8', the second half-wave plate 9', the hybrid device 10' of the second fiber collimator and polarization-maintaining fiber coupler, and the second electric displacement stage 11', are the same as the first three-port polarization-maintaining circulator 5, the 50 / 50 first polarization-maintaining fiber coupler 7, the first fiber collimator 8, the first half-wave plate 9, the hybrid device 10 of the first fiber collimator and polarization-maintaining fiber coupler, and the first electric displacement stage 11.

[0040] The above Figure 2 The two secondary cavities can be regarded as "parallel" optical paths, and more secondary cavities can be "parallel" based on this optical path. Figure 3 The two secondary cavities can be regarded as "series optical paths", and more secondary cavities can be "series" on the basis of this optical path.

[0041] This invention utilizes a composite cavity technology that adds one or more sub-cavities to a ring resonator. This effectively increases the longitudinal mode spacing, suppresses multimode oscillations, and helps achieve narrower spectral linewidths. Furthermore, the interference between the main cavity and the sub-cavities reduces phase noise and improves spectral purity. The sub-cavities also provide dynamic gain compensation, balancing gain variations in the main cavity to some extent. The composite cavity design also offers greater resistance to environmental temperature and mechanical vibration, thus improving long-term output stability. The composite cavity fiber laser designed in this invention utilizes a saturable absorber for mode locking, while also allowing for tunable repetition frequency.

[0042] In summary, the fully polarization-maintaining composite cavity mode-locked fiber laser provided by this invention has a continuously tunable repetition frequency, which reduces spectral bandwidth and phase noise while satisfying the requirements of compact structure and stable output.

[0043] The above description is merely a preferred embodiment of this utility model and is not intended to limit the utility model. Various modifications and variations can be made to this utility model by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model should be included within the protection scope of this utility model.

Claims

1. A fully polarization-maintaining composite cavity mode-locked fiber laser with tunable repetition frequency, characterized in that, include: Pump sources, hybrid devices of polarization-maintaining fiber couplers and wavelength division multiplexers, polarization-maintaining erbium-doped fiber, saturable absorbers, three-port polarization-maintaining circulators, chirped Bragg fiber gratings, polarization-maintaining fiber couplers, fiber collimators, half-wave plates, and hybrid devices of fiber collimators and polarization-maintaining fiber couplers. The pump source, the hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer, the polarization-maintaining erbium-doped fiber, the saturable absorber, the three-port polarization-maintaining circulator, the chirped Bragg fiber grating, and the polarization-maintaining fiber coupler are connected in optical path order to form the seed source main resonant cavity. The three-port polarization-maintaining circulator, chirped Bragg fiber grating, fiber collimator, half-wave plate, and hybrid device of fiber collimator and polarization-maintaining fiber coupler are connected in optical path sequence to form a seed source sub-cavity. The fiber collimator, half-wave plate, and hybrid device of fiber collimator and polarization-maintaining fiber coupler are all mounted on an electric displacement stage. By adjusting the relative distance between the fiber collimator and the hybrid device of polarization-maintaining fiber coupler, the repetition frequency of the output mode-locked laser can be tunable.

2. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 1, characterized in that, The pump source inputs pump light into the main resonant cavity of the seed source through a hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer. After passing through polarization-maintaining erbium-doped fiber, stimulated emission light is generated. Then, it passes through a saturable absorber, the a port and the b port of a three-port polarization-maintaining fiber circulator, and a chirped fiber Bragg grating. Half of the laser emission is transmitted and input into the polarization-maintaining fiber coupler. Finally, it is output through the output port of the hybrid device of polarization-maintaining fiber coupler and wavelength division multiplexer. The stimulated emission light circulates in the main resonant cavity of the seed source and passes through the saturable absorption effect of the saturable absorber to form a pulsed laser output with a fixed repetition frequency.

3. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 2, characterized in that, The other half of the laser is reflected and transmitted to port c via port b of the three-port polarization-maintaining circulator. The laser output from port c of the three-port polarization-maintaining circulator passes through the fiber collimator and enters the half-wave plate, where its polarization state rotates. It is then coupled into the hybrid device of fiber collimator and polarization-maintaining fiber coupler. The light from port a of the hybrid device is fed into the seed source main resonant cavity via the polarization-maintaining fiber coupler for the next cycle. The light from port b of the hybrid device enters the electronic control module.

4. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 3, characterized in that, The electronic control module is used to monitor the coupled optical power and adjust the rotation angle of the half-wave plate, as well as adjust the relative distance between the fiber collimator and the polarization-maintaining fiber coupler hybrid device on the electric displacement stage.

5. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 1, characterized in that, The pump source outputs continuous light with a center wavelength of 976nm and a maximum output power of 500mW.

6. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 1, characterized in that, The polarization-maintaining erbium-doped fiber has a length of 0.3m and a dispersion value at 1550nm.

7. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 1, characterized in that, The modulation depth of the saturable absorber is 55%.

8. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 2, characterized in that, The transmittance / reflectance of the chirped fiber Bragg grating is 50 / 50.

9. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 1, characterized in that, The hybrid device of fiber collimator and polarization-maintaining fiber coupler includes output port a and output port b. Output port a is the 99.5% output end and is connected to the polarization-maintaining fiber coupler through fiber fusion splicing. Output port b is the 0.5% output end and is connected to the electronic control module.

10. The repetition frequency tunable fully polarization-maintaining composite cavity mode-locked fiber laser according to claim 1, characterized in that, The seed source sub-cavities are provided in multiple ways, and the seed source sub-cavities are connected in series or in parallel.

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